The views expressed in this article are those of the authors and do not reflect the official policy of the FDA. No official support or endorsement by the FDA is intended or should be inferred.
Patient variation in veterinary medicine – Part II – Influence of physiological variables
Article first published online: 18 NOV 2010
© 2010 Blackwell Publishing Ltd
Journal of Veterinary Pharmacology and Therapeutics
Volume 34, Issue 3, pages 209–223, June 2011
How to Cite
MODRIC, S. and MARTINEZ, M. (2011), Patient variation in veterinary medicine – Part II – Influence of physiological variables. Journal of Veterinary Pharmacology and Therapeutics, 34: 209–223. doi: 10.1111/j.1365-2885.2010.01249.x
- Issue published online: 14 APR 2011
- Article first published online: 18 NOV 2010
- (Paper received 31 January 2009; accepted for publication 18 September 2010)
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
Modric, S., Martinez, M. Patient variation in veterinary medicine – Part II – Influence of physiological variables. J. vet. Pharmacol. Therap.34, 209–223.
In veterinary medicine, the characterization of a drug’s pharmacokinetic properties is generally based upon data that are derived from studies that employ small groups of young healthy animals, often of a single breed. In Part I of the series, we focused on the potential influence of disease processes, stress, pregnancy and lactation on drug pharmacokinetics. In this Part II of the series, we consider other covariates, such as gender, heritable traits, age, body composition, and circadian rhythms. The impact of these factors with respect to predicting the relationship between dose and drug exposure characteristics within an animal population is illustrated through the use of Monte Carlo simulations. Ultimately, an appreciation of these potential influences will improve the prediction of situations when dose adjustments may be appropriate.
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
Although most of the data collected in veterinary species are derived from animals of similar physiological characteristics, there are readily identifiable factors that can influence drug clearance, bioavailability, and volume of distribution. These factors include genetic variability (e.g. breed effects), disease/stress, specific physiological conditions (such as pregnancy and lactation), hepatic and renal function, environment, food, gender, age, and circadian rhythms.
Within veterinary medicine, pharmacokinetic (PK) data are typically generated in small groups of normal healthy animals, and it is often assumed that these data will reflect the drug’s kinetic properties across the intended patient population. However, the population inferential value of these datasets is rarely assessed. In Part I of this series (Martinez & Modric, 2010), we explored published data describing the potential impact of altered physiological states, such as those associated with disease, stress, pregnancy, and lactation, on drug PK. In Part II, we continue this assessment with an evaluation of the influence of ‘normal’ physiological variables. These include such factors as gender, heritable traits, age, body composition, and circadian rhythms. Although none of these factors represent a pathological state or altered physiology, each has been shown to potentially influence the PK and/or pharmacodynamic (PD) characteristics of drugs both in human and in veterinary medicine. In the last section of this review, the results of Monte Carlo simulations are provided to illustrate how failure to identify the presence of subpopulations can distort expectations and render flawed population predictions.
As with Part I, we have included tables (Tables 1–4) which contain the results of PubMed searches conducted by including the variable of interest, the word ‘pharmacokinetics’ and the major veterinary species (dog, cat, swine, horse cattle) in the search line. For each factor, Tables include the number of hits collected from each search, and the number of relevant papers contained within these hits. A relevant paper is defined as a manuscript that addresses how these variables influence drug PK in that animal species. The citations for each of those relevant articles are provided. Inclusion of these papers is not intended to reflect our endorsement of their conclusions or of the experimental methodologies employed. Rather, it reflects our effort to provide as inclusive a list of references as possible. Relevant information derived from our search is provided in the body of this manuscript.
|Species||No. hits||No. relevant hits||Reference|
|Dog||45||7||Bruss et al. (2004)|
|Doursout et al. (1990)|
|Hay Kraus et al. (2000)|
|Izzat et al. (1989)|
|Lin et al. (1996)|
|Shiraga et al. (1995)|
|Vanapalli et al. (2002)|
|Cat||7||1||Lainesse et al. (2007)|
|Swine||16||2||Huxley et al. (2005)|
|Madej et al. (2005)|
|Species||No. hits||No. relevant hits||Reference|
|Dog||33||3||Kaiyala et al. (2000)|
|Lallemand et al. (2007)|
|Cat||3||1||Center et al. (2000)|
|Swine||11||3||Craven et al. (2001, 2002a,b)|
|Cattle||11||1||Lee et al. (2008)|
|Species||No. hits||No. relevant hits||Reference|
|Dog||24||3||Hardie et al. (1991), Rackley et al. (1988, 1991)|
|Swine||319||1||Prémaud et al. (2002)|
Whenever possible, veterinary examples are provided. However, in some cases veterinary examples were not available or human examples were more illustrative. Therefore, to provide a comprehensive discussion of each variable, both human and veterinary examples are explored. Finally, as emphasized in Part I, most of the discussions are addressed from the perspective of target animal safety and effectiveness evaluation, but the implications of altered PK on human food safety and withdrawal times should also be considered when reviewing the information in this article.
Physiological Variables That Can Influence Drug Kinetics
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
Variations in DNA sequence can have a major impact on how individuals respond to disease, environmental factors (including bacteria, viruses, toxins, and chemicals), and drugs. Single nucleotide polymorphisms (SNPs) are small variations in the DNA sequence that occur when a single nucleotide is altered. Although SNPs alone can neither explain total genetic diversity nor the genetic susceptibility to complex diseases and adverse drug reactions, by some accounts they are responsible for up to 90% of all human genetic variations (Human Genome Project, http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml).
Genetic-related variability in drug response often reflects population differences in the activity of drug transporters and/or metabolizing enzymes. Variation in the distribution and frequency of occurrence of mutated alleles of genes encoding for drug-metabolizing enzymes is known to alter the rate and extent of drug metabolism. The cytochrome P450 (CYP 450) enzymes, which are involved in the metabolism of >80% of all clinically used drugs (both in human and veterinary medicine), are known to be highly polymorphic, with the notable exception of CYP1A1 and CYP2E1 (Cropp et al., 2008).
Genetic diversity within veterinary species and breed-related differences in pharmacological responses to xenobiotics have been reported across a range of species, including cattle (Sallovitz et al., 2002), sheep (Ammoun et al., 2006), chickens (Opdycke & Menzer, 1984), pigs (Sutherland et al., 2005), and dogs (Paulson et al., 1999;. With regard to food-producing animals, Sallovitz et al. (2002) reported a significantly slower absorption and lower systemic availability of moxidectin in Aberdeen Angus as compared with Holstein calves. Depelchin et al. (1988) reported that antipyrine elimination in Friesian calves was twice that reported in the Blue White Belgian breed, suggesting a breed-related difference in hepatic microsomal oxidative activities. Ripoli et al. (2006) compared the polymorphism of the microsomal enzyme diacylglycerol O-acyltransferase (DGAT1) in 14 populations of cattle from Argentina, Bolivia, and Uruguay. DGAT1 catalyzes the triglyceride synthesis and has an important role in determination of the milk fat content. Previous work on the amino acid sequence for DGAT1 resulted in the identification of a SNP that has been associated with elevated milk fat content (Grisart et al., 2002). Based on a gene frequency analysis, Ripoli et al. (2006) reported significant differences among the breeds in the activity of the DGAT1 enzyme.
Because of the extensive selection pressure exerted by the restrictive breeding of dogs, genetic variability has been particularly accelerated in dogs. Analogous to the Human Genome Project, the Dog Genome Project at the National Human Genome Research Institute resulted in identification of over 2.5 million distinct SNPs mapped to the draft genome sequence, corresponding to an average density of approximately one SNP per kb (Lindblad-Toh et al., 2005). Recently published work has identified 155 genomic regions that present strong signatures of recent selection pressure in dogs (Akey et al., 2010) induced by domestication and carefully controlled breeding patterns.
An extensive summary of canine breed-related differences in drug metabolism and its influence on drug response has been reviewed elsewhere, with references to breed idiosyncrasies in the P450 enzyme system, thiopurine methyltransferase, N-acetyltransferase, as well as breed-related difference in physiology (Fleischer et al., 2008). For example, a significant pharmacogenetic variation in the gene encoding for CYP2D15 enzyme (counterpart to human CYP2D6) has been reported in purebred Beagles. Beagles that have the wild-type gene for CYP2D15, are extensive metabolizers, with a terminal elimination half-life (t½) of celecoxib ranging from 1.5 to 2 h vs. 5 h in poor metabolizers (Paulson et al., 1999). In terms of the breed effect on the PD response, the opioid dysphoria observed in certain dog breeds such as the Labrador Retriever, Alaskan Malamute and Siberian Husky may be attributable to a SNP in the canine μ-opioid receptor gene (Hawley & Wetmore, 2010).
With regard to safety concerns, a 4-bp deletion in the ABCB1 (formerly MDR-1) gene, which encodes for P-glycoprotein (P-gp), results in a nonfunctional gene (Mealey, 2001). This mutation affects 30–50% of the Collie population, as well as other divergent breeds (Neff et al., 2004; Mealey, 2006). Dogs that are homozygous recessive for this mutation exhibit neurotoxicity to the avermectin class of drugs, as well as to many other P-gp substrates. This is due to the inability of the efflux drugs to prevent these P-gp substrates from entering into the central nervous system (CNS). This efflux pump defect can also influence tissue drug concentrations, which may influence drug safety and effectiveness in these dogs (Martinez et al., 2008). In addition, there may be a different genetic mutation in some dogs of noncollie breed that, while not related to the mutation previously recognized in collies, leads to similar pump defects (Bissonnette et al., 2009). This includes subchronic signs of neurotoxicity to macrocyclic lactones, which is a typical clinical sign of the ABCB1 mutation.
Recent efforts to analyze genetic differences across equine breeds (Wade et al., 2009) will likely give rise to the identification of mutations that result in breed-related differences in disease and drug metabolism in horses.
Because of the tremendous increase in information on the influence of genetic attributes on breed idiosyncrasies, a detailed discussion of the relationship between heritable traits and drug exposure–response relationships would require volumes, not pages, of text. Nevertheless, although breed alone cannot explain all genetic variability reported for various animal species, it does provide a starting point for assessing the diversity that can influence drug safety and effectiveness.
In human medicine, women generally have lower total body weight and organ size, a higher percentage of body fat, lower glomerular filtration rate (GFR), and slower gastric motility when compared with men (Meibohm et al., 2002; Schwartz, 2003). In terms of gender differences in the PK response, men tend to have higher CYP1A2 and CYP2E1 activity. P-gp activity also appears to be greater in men as compared with women. Similar disparities have been observed for glucuronosyltransferases and sulfotransferases. On the contrary, women have higher levels of CYP2D6 activity.
Gender disparity in PK has been identified for numerous drugs; however, it is not common for these differences to result in a substantially different pharmacological response (Thürmann, 2007). Interestingly, a Bayesian statistical analysis of sex differences in the adverse events in human patients showed that, although the adverse events were reported at similar frequencies for men and women, those that are reported by women tended to be of a more serious nature (Miller, 2001).
Gender differences have also been occasionally observed in pharmacological response. For example, women tend to differ from men in their responses to pain therapy, glucose management, and arrhythmia susceptibility (Beierle et al., 1999). Gender differences in drug-induced QTc prolongation (Meibohm et al., 2002) can lead to a greater risk of drug-induced cardiotoxicity in females vs. males across both human and veterinary species. Other examples of PK- and PD-related gender effects in human medicine can be found elsewhere (Meibohm et al., 2002). Accordingly, within the human drug approval process, there is now greater attention given to potential gender differences in drug response. A discussion of observed gender differences in human drug approvals can be found at http://www.fda.gov/cder/reports/womens_health/women_clin_trials.htm.
A listing of the veterinary research articles published on the effect of gender on drug PK is summarized in Table 1. Although findings similar to those reported in human medicine have been observed in veterinary species, it should be noted that many domestic animals (both companion and food-producing) are castrated prior to reaching full maturity. The removal of sex organs results in a depletion of the sex hormones, thereby influencing the magnitude of observed gender effects. Nevertheless, castration does not completely eliminate gender effects, as shown by Hutson et al. (2008) in humans undergoing medical castration. These investigators hypothesized that therapeutic reduction in testosterone concentrations would affect the metabolism of other drugs, as testosterone is a substrate of the CYP3A4 drug-metabolizing enzyme. However, their study results showed that the decrease in testosterone concentrations did not lead to a significant change in the activity of the CYP3A4 enzyme.
Another major physiologic difference between humans and veterinary species is that, unlike humans, many domestic animals are seasonal breeders, with one or more estrous cycles occurring during certain periods of the year. Although seasonal breeders have dormant phases of reproductive cycle, which could suggest potential differences in the PK response due to the different levels of sex hormones, no studies have specifically compared the impact of gender on drug PK between seasonal breeders and continuous breeders.
Among veterinary species, gender-linked differences in PK and drug metabolism have been reported in cats (Erichsen et al., 1980; Lainesse et al., 2007), cattle (Dacasto et al., 2005), dogs (Hay Kraus et al., 2000; Bruss et al., 2004), ferrets (Court, 2001), and fish (Vega-Lopez et al., 2007). Janus and Antoszek (1999) reported marked sex-linked differences in plasma antipyrine clearance and urinary excretion of the main metabolites of antipyrine in cattle over 12 months of age, with females being the more active metabolizers. With respect to the PD-related gender differences, Doursout et al. (1990) reported a statistically significant difference in the response of male vs. female dogs to centrally administered angiotensin II. In male dogs, angiotensin II induced parallel pressor and dipsogenic responses, whereas no hypertension and no increase in fluid intake was observed in females, thus providing an evidence of the role of gender in the physiological properties of centrally administered angiotensin II.
Because the extent and direction of gender differences vary among the veterinary species (Witkamp et al., 1991), the interspecies extrapolation of gender effects should be performed with caution (Christian, 2001). This is especially true when attempting to extrapolate gender effect data derived from rodents. Rats in particular tend to express gender-related differences that do not necessarily translate to similar gender differences in other species (Niwa et al., 1995; Reinoso et al., 2001; Martinez, 2005). This species-specific gender effect has been linked to gender-related differences in the daily rhythm of rat hepatic enzymes and a secretion of the growth hormone (Furukawa et al., 1999; Czerniak, 2001).
Geriatrics. Changes in PK that occur with maturity and senescence are well recognized in both animals and humans, with the following three age-related physiologic processes having the most profound effect on drug PK: (i) decreased plasma protein binding, which affects drug distribution; (ii) declining liver function, which affects drug metabolism; and (iii) impaired kidney function, which delays drug elimination. In addition, age-related changes in body composition (less lean body mass and more fat tissue), as well as a decrease in gastrointestinal motility, further affect the fate of drugs in an aging body (Hilmer et al., 2007). The result is often an increased risk of drug toxicity. Moreover, the heterogeneity observed in older populations dramatically increases in whatever physiological variable is being studied. For this reason, the biggest shift that occurs between drug PK in young and old subjects is often a marked increase in the magnitude of PK variability rather than a substantial shift in the mean dose exposure–response relationship (Kinirons & O’Mahony, 2004).
A significant decrease in hepatic metabolizing capacity that occurs with aging, combined with decreased renal function, can markedly affect the ability of aging individuals to clear substances from the systemic circulation. In general, the t½ of drugs processed by the P450 enzyme system or via renal elimination is 50–75% longer in those human patients ages 65 and older as compared with their younger counterparts (Ginsberg et al., 2005). In addition, liver and kidney diseases are much more common in elderly individuals, further compromising organ and systemic clearance (Lam et al., 1997; Regev & Schiff, 2001).
Another organ system profoundly affected by the human aging process is the CNS, leading to greater risk of drug-related toxicities. With advancing age, the CNS undergoes a variety of changes, including neuronal loss, altered neurotransmitter and receptor levels, and a decreased ability to adapt to changes induced by xenobiotics. For example, a single therapeutic dose of flurazepam administered to people over the age of 60 caused measurable side effects in nearly half of the people, whereas only 5–10% of younger individuals experienced side effects (Liu & Christensen, 2002). Bartels et al. (2008) recently reported on a marked age-related decrease in the P-gp activity of the blood–brain barrier (BBB). The authors suggested that an age-related decrease in BBB functions could explain the increased risk of neurodegenerative diseases in the elderly.
In contrast to the age-related decrease in P-gp activity in the brain, both mouse and human data suggest an association between aging and the increase in P-gp expression in lymphocytes (Witkowski & Miller, 1993; Gupta, 1995). Because P-gp is expressed in a wide variety of tissues and cells, altered expression of P-gp with advancing age may underlie many drug interactions and altered drug effects in older people (Kinirons & O’Mahony, 2004).
In addition to its effect on PK, senescence can affect receptor numbers and affinities in both humans (McLean & Le Couteur, 2004) and animals (The Merck Manual, 2006). For example, the effect of verapamil on prolongation of the PR interval of ECG is significantly less in geriatric adults. However, the optimal verapamil dose tends to remain unchanged because of the reduced verapamil clearance observed in older people (Abernethy et al., 1993). Aging has also been linked to down-regulation of beta-adrenergic receptors, elevated plasma noradrenaline levels, and reduced cAMP response to adrenergic stimulation, possibly explaining the reduced bronchodilatory response of older people to beta-agonists (Scarpace et al., 1991).
As with research of other factors influencing drug PK and PD, we found that the systematic evaluation of effects of aging is lacking in veterinary species. Most of the information available to veterinarians is based on clinical experience with geriatric pets rather than on research findings. Therefore, it has been recommended that veterinarians prescribing drugs for geriatric dogs and cats rely on information from the human drug package insert as guidelines for dose adjustment (Dowling, 2005).
A common practice among veterinary practitioners is to use lower doses of drugs in geriatric patients based on clinical experience from either human or veterinary medicine, Yamashita et al. (2009) recently showed that the minimum alveolar concentration of sevoflurane needed in old dogs is approximately 17–20% lower than that needed in young dogs, confirming clinical observations of increased anesthetic potency with advanced age. This observation is consistent with the increase in CNS drug sensitivity in the human geriatric population. On the contrary, there is evidence suggesting that, despite similar histological changes observed in the glomerulus of aging dogs and humans, and a well characterized decrease in GFR in healthy aging humans (Hoang et al., 2003), only very limited data confirm a corresponding decrease in the canine GFR with age. Bexfield et al. (2008) compared GFR in 118 healthy dogs ages 0–14 years and showed that on the average, less than a 1% decrease in GFR/kg body weight or GFR/extracellular fluid volume occurred as a function of age. However, for dogs weighing 1.8–12. 4 kg, a small negative trend in renal function was observed with age. The authors hypothesized that the decrease in canine GFR associated with aging may not occur to the same extent as that seen in humans due to the dogs’ shorter life expectancy.
Immunosenescence, a gradual deterioration of the immune system associated with aging, has also been reported to affect the PD characteristics of a drug response. Immunosenescence has been studied in both animal models and in humans, and is considered by some to be a major contributory factor to the increased frequency of morbidity and mortality among elderly people. Both cellular- and humoral-mediated responses may be abnormal in the elderly, but the effect of aging on the immune response is highly variable. In their reviews of the effect of aging on host defenses, Scordamaglia et al. (1991) and Pawelec et al. (2002) focused on the role of T cells in aging, as they serve as a marker of the immune response and are also one of the main targets of many therapeutic drugs. Factors contributing to T cell immunosenescence may include an altered production of T cell progenitors, decreased levels of newly generated mature T cells, aging of resting immune cells, and disrupted activation pathways in immune cells. Similar processes have been reported in other types of immune cells, contributing to the increased incidence of morbidity and mortality from infectious disease, and possibly autoimmunity and cancer among elderly people (Mocchegiani & Malavolta, 2004).
Neonates and infants. As compared with the human adult, neonates have higher skin surface area per body weight, a greater percentage of body water, less body lipid, lower renal blood flow, and functionally immature hepatic and renal functions (Bartelink et al., 2006). In humans, the hepatic expression of CYP1A2, 2C, 2D6, 2E1, and 3A4 each develop gradually and at different rates in the postnatal period. Conversely, the expression of CYP3A7 progressively diminishes with maturity (Gow et al., 2001). Hepatic glucuronidation in the human neonate is relatively immature at birth, whereas hepatic sulfation activity is considerably more mature. Aside from immature hepatic and renal function, neonates and infants tend to have a prolonged gastric residence time, a higher gastric pH, slower intestinal motility, and in general, a slower oral absorption of compounds as compared with that observed in adults.
Because of developmental changes in absorptive capacity, first-pass metabolism, distribution, and elimination processes, administration of drugs on the mg/kg basis in young individuals can be inadequate to produce the desired clinical response in adults. Furthermore, because intestinal transit time and absorptive surface area are lower in young organisms as compared with mature ones, drug delivery systems that may be suitable for use in adults may not deliver the total dosage in children. Ultimately, in a manner similar to that seen in the elderly population, drug PK characteristics in infants and neonates are typically more variable than that seen in adults due to differences in rates of development of key physiologic and metabolizing systems.
There is only limited information on the differences that occur in adult vs. immature animals. A summary of studies comparing pharmacokinetics in adult vs. young animals or in various stages of maturation is provided in Table 2. Although these processes are analogous to the early development of human newborns, it should be noted that, in general, young animals are born much more precocious than humans and a rapid maturation of the major elimination organs occurs in the first 6 months of life.
The prolonged elimination t½ in early life, caused mainly by immature hepatic pathways or underdeveloped renal excretion processes, is one of the most pronounced PK differences between newborn and adult animals. Nouws (1992) reviewed the effect of age on half-lives of several antimicrobial drugs in young calves and pigs and reported a steady decrease in t½ for 17 out of 20 antibiotics when comparing 1–2 days of age, week 1, week 2–4, and >8 weeks of age. In addition to the slower elimination in infant animals, the absorption processes are also underdeveloped, including the gastrointestinal blood perfusion and motility. Moreover, the development of reticuloruminal activity takes place over several weeks, during which time a microbial population is established, which can affect the fate of drugs (Nouws, 1992).
Changes in body fluid composition are also responsible for age-related differences in drug PK. A greater proportion of aqueous fluids in the muscle interstitial tissues of newborns as compared with mature animals results in the enhanced spread of drugs at intramuscular injection sites, which in turn leads to an increase in the absorptive surface area and therefore enhanced drug absorption (Nouws et al., 1986). The greater total body water at birth leads to a greater drug volume of distribution for polar compounds (Short & Davis, 1970). In contrast, the amount of body fat is low at birth and increases progressively with maturity, which could lead to a lower volume of distribution for lipophilic compounds in neonates (Nouws, 1992).
While some enzymes are fully developed at birth, the microsomal oxidation activity of the P450 isoenzymes and glucuronide conjugation reactions are underdeveloped in the neonate. In rodents and pigs, the activities of oxidative and conjugative liver enzymes attain adult levels within the 2 months after birth (Short & Davis, 1970). Development of the oxidative metabolic pathways in calves is also a gradual process, achieving maximum capacity at 3–12 weeks of age (depending on the enzyme system) (Davis et al., 1973; Nouws, 1992). Galtier and Alvinerie (1996) studied the age-related changes in hepatic drug metabolizing activities of Lacaune ewes and found that hepatic total P450 concentrations reached maximum levels at 6 years of age, whereas the levels of CYP3A peak during the first 4 weeks of life. The activity of the Phase 2 enzymes also changes with age, with the glutathione S-transferase activity peaking around 11 months of age, and activity of glucuronyl transferase starting to decline at the same time. The results of this study clearly illustrate vast differences in the rate of maturation of various enzyme systems associated with drug elimination.
Age and maturation can also affect active transport mechanisms. For example, calcium absorption, which is characterized by both active and passive processes, varies as a function of maturation. The active component comprises a relatively more significant component of total calcium absorption in neonates, but decreases in its relative contribution to total calcium absorption as the dog matures (Tryfonidou et al., 2002). Several researchers have recently discussed the effect of aging on the expression and activity of the efflux transporter P-gp (Mangoni, 2007; Bartels et al., 2008). In both T-lymphocytes and natural killer cells, the highest levels of activity (as measured by the ability to prevent entry of a fluorescent marker) were highest in human cord blood and progressively declined with age (Machado et al., 2003; Giraud et al., 2009).
Body composition and exercise
Modification of drug dosages in obese human or veterinary patients has become a subject of major concern, particularly for drugs with a narrow therapeutic index. In general, the PK behavior of drugs characterized by low lipophilicity is rather predictable. As they are mostly distributed in lean tissues, dosage can be based on the ideal bodyweight (IBW) (Cheymol, 2000). Therefore, it has been suggested that lean body mass may be a better predictor of drug dosage than either total bodyweight or body surface area (BSA), for hydrophilic compounds (Morgan & Bray, 1994). However, as drug lipophilicity increases, dose adjustment for total body weight may be necessary, although it is the physico-chemical attributes of the drug rather than the degree of lipophilicity that determines the extent to which a drug will distribute into body fat (Cheymol, 2000; Hanley et al., 2010).
Fat is a rather stable reservoir because of a relatively low blood flow. Nevertheless, drug distribution into fat tissue may be rapid. For example, as much as 70% of thiopental is present in body fat only 3 h after administration (Buxton, 2006). Body composition has been shown to affect the PK of drugs, irrespective of the route of administration. Chan et al. (2003) demonstrated that the bioavailability of human chorionic gonadotropin, used to induce oocyte maturation during in vitro fertilization, is significantly affected by the body mass index: for both the intramuscular and subcutaneous routes, the bioavailability was markedly reduced in obese women.
Body condition has also been found to influence the PK of various drugs in veterinary species (as summarized in Table 3), especially highly lipophilic drugs. In the development of new veterinary compounds, where a persistent effect is desirable, the use of lipophilic actives or vehicles is a common method of extending drug availability and effectiveness (Hennessy, 1997). The long residence time, which allows for persistent effectiveness, is influenced by the route of administration, dose, formulation, and animal body composition. Because of their high lipophilicity, macrocyclic lactones are selectively distributed into the fat tissue from which they are gradually released, thus contributing to their prolonged persistent effects (McKellar & Benchaoui, 1996; Craven et al., 2002a,b).
Body condition score and body fat composition affect the rate and extend of exposure of topical eprinomectin in goats (Dupuy et al., 2001), subcutaneous ivermectin, doramectin and moxidectin in sheep (Echeverría et al., 2002; Barber et al., 2003), subcutaneous ivermectin and moxidectin in in pigs (Craven et al., 2002a,b), and oral moxidectin in dogs (Lallemand et al., 2007). These observations raise safety concerns about the use of these compounds in very thin animals, where low body fat can lead to high concentrations of drug in the blood. In addition to the safety risks in thin animals, moxidectin levels were found to be less persistent in thin as compared with fat animals, suggesting concerns about the product’s effectiveness, and possibly contributing to the emergence of resistance to these compounds. It is important to note that of the macrocyclic lactones used in veterinary medicine, moxidectin has higher lipophilicity than any of the other avermectins.
Similarly, amikacin, a molecule, that is sparingly soluble in water, has a substantially lower volume of distribution in Greyhounds as compared with Beagles (KuKanich & Coetzee, 2008), reflecting breed differences in lean body mass (Fleischer et al., 2008). Similar differences in the volume of distribution of Greyhounds vs. other canine breeds have been shown for several other lipophilic compounds (Robinson et al., 1986; Sams & Muir, 1988; Zoran et al., 1993).
Unlike highly lipophilic drugs, hydrophilic compound typically have a lower volume of distribution per kg body weight (Wright et al., 1991). For example, when gentamicin is dosed on the basis of total body weight to cats, there is a high risk of drug toxicity due to an over-estimation of the volume of distribution. Therefore, a more appropriate dosing for drugs such as cancer chemotherapeutics may be calculated on the basis of BSA rather than total body weight. However, such dosing strategies may not be equally applicable across all drug types and animal species. Frazier and Price (1998) concluded that dosing on the basis of BSA may be appropriate for drugs that are eliminated unchanged by glomerular filtration or are degraded via nonenzymatic processes. In contrast, it may be sub-optimal when administering drugs that are extensively metabolized (which tend to be more highly lipophilic). Therefore, it is not surprising that PK and pharmacological studies in humans do not support the global application of BSA dosing strategies for chemotherapeutic agents (Kouno et al., 2003; Kaestner & Sewell, 2007).
Because of their unique physiology, there have been reports on the impact of body composition (in terms of degree of hydration) on drug PK in camels. When tylosin tartrate was administered by i.v. injection to normal vs. water-deprived camels, serum tylosin concentrations in the water-deprived camels were significantly higher, the rate of drug elimination was slower, the volume of distribution was significantly smaller, and the total body clearance was significantly slower as compared with that observed in normal camels (Ziv et al., 1995). Similar results were reported in cattle after the administration of caffeine (Janus et al., 2001). It was suggested that even short-term (4-day) water or food deprivation can lead to an inhibition of the P450 system.
Finally, exercise-induced increases in muscular blood flow has a complex effect on drug PK, ranging from an increase in the binding of digoxin (leading to lower serum drug concentrations) to a decrease in hepatic blood flow which results in a decrease in the clearance of high extraction ratio drugs and consequently higher serum drug concentrations (e.g. propranolol) (Khazaeinia et al., 2000; Lenz et al., 2004). Exercise can also decrease the clearance of drugs that undergo renal elimination and can increase biliary-related drug clearance (Khazaeinia et al., 2000; Lenz et al., 2004). On the contrary, chronic exercise can increase drug absorption by increasing collateral blood flow and by changing gastrointestinal transit times. Chronic exercise can change drug distribution characteristics by increasing lean body mass, decreasing body fat, increasing plasma protein, and increasing plasma volume (Persky et al., 2003).
In most mammalian species, numerous physiological activities and organ systems are influenced, at least to some extent, by circadian rhythms, including heart rate, blood pressure, liver and renal plasma flows, bile and urine production, intestinal peristalsis, secretion of digestive enzymes into the GI tract, major endocrine functions, immune cell production, and metabolism (Levi & Schibler, 2007; Xu et al., 2008). It is, therefore, no surprise that the effect of circadian rhythms have been found to influence drug response (Beauchamp & Labrecque, 2007).
Chronopharmacology is the study of rhythmic, predictable-in-time differences in the effects and/or PK of drugs. Chronotoxicology describes the effect of the body’s endogenous circadian rhythms on drug toxicity. Daily variations in drug effectiveness and variations in PK across a wide range of therapeutic classes have been observed in humans (Lemmer, 1996). In fact, most of our knowledge with regard to chronopharmacology and chronotoxicology is built upon information gathered in people. For example, in a study of 179 patients with serious infection, Prins et al. (1997) determined that gentamicin- or tobramycin-induced nephrotoxicity was significantly greater when administered between midnight and 7:30 a.m. as compared with dosing at any other time of day. Other researchers have confirmed that the nephrotoxicity of aminoglycosides is minimized when they are administered in the early afternoon (Rougier et al., 2003; Beauchamp & Labrecque, 2007). Beauchamp and Labrecque (2007) suggested that possible mechanisms of the chronotoxicity of aminoglycosides include the daily variation in aminoglycoside pharmacokinetics, food intake, and the effect of diurnal variations in urine pH.
Chronotoxicity has been studied extensively in rats and mice. For example, it has been shown that approximately 75% of rodents studied survived the administration of 3′azido-3′-deoxythymidine when administered at the beginning of the resting period, but only 13% of treated rats survived when the drug was administered during the early phase of activity (Zhang et al., 1993). Similar results were collected when amphotericin B was administered to rats at resting vs. active periods (Skubitz et al., 1986; LeBrun et al., 1996).
Research on the effects of circadian rhythm on drug PK/PD is complicated by the circadian pattern associated with many diseases. In human medicine, disease areas for which chronotherapy has been identified as a major consideration include cancer, immune diseases, and cardiovascular diseases (Xu et al., 2008). With regard to pain control, Bruguerolle and Labrecque (2007) suggest that a primary reason that treatments fail to provide adequate pain control is a failure to recognize time-of-day patterns in pain intensity and the medication requirements based on the circadian rhythm of pain and PK. While rodent species exhibit their highest pain threshold at the end of the resting period and their lowest threshold at the end of the activity period (Frederickson et al., 1977; Kavaliers & Hirst, 1983; Gschossmann et al., 2001), human studies show a morning peak and/or an evening trough of pain (Bruguerolle & Labrecque, 2007). Bruguerolle & Labrecque further concluded that the impact of circadian rhythm is dependent upon the type of pain. For example, the perception of pain peaked in the morning in patients with angina pectoris, myocardial infarction, migraine, rheumatoid arthritis, and toothache, whereas it peaked in the evening/nighttime in patients with biliary colic, cancer, and intractable pain.
Circadian rhythms not only affect different types of pain differently, but also influence the selection of analgesics. For opioids, the circadian rhythmicity of pain perception seems to be related to the time-dependent variation in the brain levels of opioid peptides, which in humans are higher at the beginning of the day and lower in the evening (Labrecque & Vanier, 2003). Based on the recent review of pain and chronotherapy, animal and human studies are in agreement about the role of circadian variations in the PK and PD of NSAIDs, with bioavailability being greatest in the morning (Bruguerolle & Labrecque, 2007). For instance, Ohdo et al. (1995) demonstrated that the mortality induced by acetylsalicylic acid is highest when administered at the end of the resting period of rats; others have demonstrated that the effectiveness of NSAIDs varies by 40–50% according to the time of their administration (Labrecque et al., 1995).
With regard to veterinary species, as seen in Table 4, diurnal variability has been extensively studied in both food and companion animals. However, studies have focused primarily on such factors as endogenous substances (hormones), brain activity, sleep, exercise, physiological processes (including heart rate, blood pressure, body temperature, milk production), and reproduction. Only a handful of studies report chronopharmacology and chronotoxicology evaluations. In fasted dogs administered pentazocine in the morning (8 a.m.), or during the dark cycle (8–9 p.m.), there was a statistically significant difference in the estimates of volume of distribution and t½ (highest at night), although Cl and AUC remained unchanged (Ritschel et al., 1980). The time of cisplatin intravenous administration also affected drug PK and renal toxicity in dogs, where renal clearance was highest at night but nephrotoxicosis and free cisplatin drug concentrations were greatest in the morning (Hardie et al., 1991). In pigs, methotrexate (but not vinorelbine) exhibited very marked circadian rhythms in the plasma drug concentration–time profiles, with the highest concentrations generally occurring around midnight (Prémaud et al., 2002).
Part of the problem with studying diurnal rhythms in animals is that under real-life conditions, the inherent variability (particularly in companion animals) is confounded by human interactions. Diurnal pH changes in the duodenum of the conscious dogs are linked to digestion (Itoh et al., 1980). Similarly, studies to track canine diurnal changes in plasma concentrations of adrenal corticotrophic hormone (ACTH) and cortisol appear to be confounded by human activity (Castillo et al., 2009). However, dogs do show marked diurnal variation in theophylline plasma drug concentrations: in a study following constant aminophylline infusion for 48 h, peak concentrations occurred between 24 and 30 h after administration and troughs at approximately 36–42 h after the start of drug administration (Rackley et al., 1988). In one dog, whose infusion was initiated in the p.m. rather than the a.m., times of peaks and troughs relative to the time of start of infusion were reversed, indicating that it was more the time of day rather than time relative to the start of infusion that determined the time of peak and trough concentrations. Interestingly, this variation may be, at least in part, related to food intake because despite its intravenous administration, the major route of theophylline elimination (the kidney) is affected by food. This link appears to be a consequence of food-induced changes in urine pH, resulting in a greater proportion of the drug existing in its ionized state and therefore an increase in its urinary elimination (Rackley et al., 1991). Thus, at least in part, food consumption had an important role in promoting the chronopharmacokinetics of this compound. Similar results on the crucial importance of the food effect in the temporal variation of renal toxicity were reported for amphotericin B in rats (LeBrun et al., 1999).
In cats, inherent diurnal patterns in cardiovascular variables and motor activity appeared to be negated by the effects of human interactions (Brown et al., 1997). Hormonal releases (e.g. melatonin and prolactin) are regulated primarily by the duration of the light/dark cycles (Leyva et al., 1984). Therefore, when evaluating chronopharmacology and chronotoxicology in domesticated animals, there is a fine line between evaluating the inherent patterns that might exist in the wild vs. the patterns that are likely to due to human–animal interactions.
Despite these challenges, physiological information in veterinary species and chronopharmacology and chronotoxicology information derived in humans and rodents confirm that for some compounds, circadian rhythms can influence drug exposure, safety, and effectiveness. Accordingly, this phenomenon should not be ignored as a potential source of variability in veterinary medicine.
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
Identifying variables can markedly influence drug exposure characteristics is an important step in predicting the relationship between drug physico-chemical characteristics, its elimination pathways, and the variability likely to occur when that drug is used under clinical field conditions. This is particularly important when considering the very small clinical trials generally conducted in veterinary medicine. The small sample size associated with the clinical trial population renders it unlikely that the study will have the power to identify covariates that may necessitate dose adjustments. Data from clinical trials are generally pooled together and analyses generated with an assumption that all of the data are collected from a single normal (log-normal) distribution. With this practice in mind, we considered it important to demonstrate how the typical practice of pooling data can lead to large errors in population predictions.
To illustrate how failure to identify the presence of subpopulations can greatly distort expectations, we simulated a population of 10 000 dogs, where 70% of the dogs were extensive metabolizers (EM) and 30% of the dogs were poor metabolizers (PM). The relationship between the clearances of the two subpopulations (clearance of EM group = 2 × clearance of PM group) and the between-animal variability associated with each of the clearance estimates (25% CV) was based upon the relationship between PM and EM reported by Paulson et al. (1999), where they examined polymorphism in the metabolism of celecoxib in dogs. The results of this simulation are provided in Fig. 1. A summary of how to interpret the symbols used in boxplots is provided in the appendix.
The important point to note is that a failure to recognize the presence of two subpopulations (fast and slow metabolizers) can introduce substantial bias into our dose-exposure evaluations. When the EM and PM groups are separated, the AUC values collected in the EM group are markedly lower than are those collected in the PM group. Accordingly, for one or both groups, dose adjustments may be needed to achieve the expected safety and effectiveness. The divergence in exposure that can occur within the patient population would be overlooked if we simply pooled data from all 10 000 simulated animals and ignored the pharmacokinetic differences.
While this is simply a simulated example, the hope is that it will underscore the importance of considering the numerous variables that can affect the dose exposure–response relationship in veterinary medicine, as described in this manuscript and in Part I of this series.
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
Numerous factors can influence drug PK and the response to pharmacological therapy. Identifying these variables is an important step in predicting the relationship between drug physico-chemical characteristics, the elimination pathways, and the variability likely to occur when a drug is used in the target population. Considering the small number of animals generally included in veterinary clinical field trials, potentially important sources of population variability in the PK and PD response may be undetected. For that reason, it is our ultimate hope that this series on patient variation will challenge veterinary pharmacologists to consider these factors across a range of target animal species. Understanding and appreciating the impact of patient variability will ultimately allow us to develop more reliable predictions of drug safety and effectiveness under the clinical conditions of use.
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
- 1993) Stereoselective verapamil disposition and dynamics in aging during racemic verapamil administration. Journal of Pharmacology and Experimental Therapeutics, 266, 904–911. , , & (
- 1991) Influence of age on the disposition kinetics of chloramphenicol in equine neonates. American Journal of Veterinary Research, 52, 426–431. , , , & (
- 1992) Pharmacokinetics of felbamate in pediatric and adult beagle dogs. Epilepsia, 33, 955–960. , , , & (
- 1993) Pharmacokinetics of the new antiasthma and antiallergy drug, azelastine, in pediatric and adult beagle dogs. Biopharmaceutics and Drug Disposition, 14, 233–244. , , , & (
- 2010) Tracking footprints of artificial selection in the dog genome. Proceedings of the National Academy of Sciences of the United States of America, 107, 1160–1165. , , , , , , & (
- 1993) Effect of age on theophylline pharmacokinetics in dogs. American Journal of Veterinary Research, 54, 1112–1115. , , & (
- 2006) Effects of breed on kinetics of ovine FSH and ovarian response in superovulated sheep. Theriogenology, 66, 896–905. , , , , , , , & (
- 1985) Age-related changes in the pharmacodynamics of verapamil. American Heart Journal, 110, 981–985. , & (
- 1994) Neomycin metabolism in calves. Journal of Animal Science, 72, 683–689. & (
- 2003) The comparative serum disposition kinetics of subcutaneous administration of doramectin, ivermectin and moxidectin in the Australian Merino sheep. Journal of Veterinary Pharmacology and Therapeutics, 26, 343–348. , , & (
- 2008) Effects of age on the pharmacokinetics of single dose sulfamethazine after intravenous administration in cattle. Veterinary Research Communications, 32, 509–519. , , , , , & (
- 2006) Guidelines on paediatric dosing on the basis of developmental physiology and pharmacokinetic considerations. Clinical Pharmacokinetics, 45, 1077–1097. , , & (
- 2008) Blood-brain barrier P-glycoprotein function decreases in specific brain regions with aging: a possible role in progressive neurodegeneration. Neurobiology of Aging, 30, 1818–1824. , , , , , , & (
- 2007) Chronobiology and chronotoxicology of antibiotics and aminoglycosides. Advanced Drug Delivery Reviews, 59, 896–903. & (
- 1999) Gender differences in pharmacokinetics and pharmacodynamics. International Journal of Clinical Pharmacology and Therapeutics, 37, 529–547. , & (
- 2008) Glomerular filtration rate estimated by 3-sample plasma clearance of iohexol in 118 healthy dogs. Journal of Veterinary Internal Medicine, 22, 66–73. , , , , , & (
- 1996) Changes in the concentration of fructose in the blood of piglets of different ages after doses of fructose, fructose plus glucose, and sucrose. British Journal of Nutrition, 76, 399–407. & (
- 2009) The ABCB1-1Δ mutation is not responsible for subchronic neurotoxicity seen in dogs of non-collie breeds following macrocyclic lactone treatment for generalized demodicosis. Veterinary Dermatology, 20, 60–66. , , & (
- 1996) Effects of age on the pharmacokinetics of single dose ceftiofur sodium administered intramuscularly or intravenously to cattle. Journal of Veterinary Pharmacology and Therapeutics, 19, 32–38. , & (
- 1997) Effects of certain vasoactive agents on the long-term pattern of blood pressure, heart rate, and motor activity in cats. American Journal of Veterinary Research, 58, 647–652. , & (
- 1983) Skeletal retention and distribution of 226Ra and 239Pu in beagles injected at ages ranging from 2 days to 5 years. Health Physiology, 44, 513–527. , , , , & (
- 1991) The influence of age at time of exposure to 226Ra or 239Pu on distribution, retention, postinjection survival, and tumor induction in beagle dogs. Radiation Research, 125, 248–256. , & (
- 2007) Rhythmic pattern in pain and their chronotherapy. Advanced Drug Delivery Reviews, 59, 883–895. & (
- 2004) Pharmacokinetics of orally administered pirfenidone in male and female beagles. Journal of Veterinary Pharmacology and Therapeutics, 27, 361–367. , & (
- 1992) Rifampin disposition in the horse: effects of age and method of oral administration. Journal of Veterinary Pharmacology and Therapeutics, 15, 124–132. , , , & (
- 2006) Pharmacokinetics and pharmacodynamics: the dynamics of drug absorption, distribution, action, and elimination. In Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 11th edn. Ed. Brunton, L.L., pp. 1–40. McGraw-Hill Professional Publishing, New York. (
- 2005) Disposition of orally administered cefpodoxime proxetil in foals and adult horses and minimum inhibitory concentration of the drug against common bacterial pathogens of horses. American Journal of Veterinary Research, 66, 30–35. , , , , & (
- 2009) Diurnal ACTH and plasma cortisol variations in healthy dogs and in those with pituitary-dependent Cushing’s syndrome before and after treatment with retinoic acid. Research in Veterinary Science, 86, 223–229. , , , , & (
- 2000) The clinical and metabolic effects of rapid weight loss in obese pet cats and the influence of supplemental oral L-carnitine. Journal of Veterinary Internal Medicine, 14, 598–608. , , , , , , , , & (
- 2003) Bioavailability of hCG after intramuscular or subcutaneous injection in obese and non-obese women. Human Reproduction, 18, 2294–2307. , , , , , & (
- 2000) Effects of obesity on pharmacokinetics implications for drug therapy. Clinical Pharmacokinetics, 39, 215–231. (
- 2001) Introduction/overview: gender-based differences in pharmacologic and toxicologic responses. International Journal of Toxicology, 20, 145–148. (
- 2005) Functional interactions between P-glycoprotein and CYP3A in drug metabolism. Expert Opinion on Drug Metabolism & Toxicology, 1, 641–654. , & (
- 1992) Pharmacokinetics of gentamicin and antipyrine in the horse-effect of advancing age. Journal of Veterinary Pharmacology and Therapeutics, 15, 309–313. , & (
- 2001) Acetaminophen UDP-glucuronosyltransferase in ferrets: species and gender differences, and sequence analysis of ferret UGT1A6. Journal of Veterinary Pharmacology and Therapeutics, 24, 415–422. (
- 2001) Pharmacokinetics of moxidectin and ivermectin following intravenous injection in pigs with different body compositions. Journal of Veterinary Pharmacology and Therapeutics, 24, 99–104. , , , & (
- 2002a) The effects of body composition on the pharmacokinetics of subcutaneously injected ivermectin and moxidectin in pigs. Journal of Veterinary Pharmacology and Therapeutics, 25, 227–232. , , & (
- 2002b) Does the rate of fat deposition influence the pharmacokinetic disposition of subcutaneously administered moxidectin and ivermectin in pigs? Journal of Veterinary Pharmacology and Therapeutics, 25, 351–357. , & (
- 2008) Genetic variation in drug transporters in ethnic populations. Clinical Pharmacology and Therapeutics, 84, 412–416. , & (
- 2008) The legacy of domestication: accumulation of deleterious mutations in the dog genome. Molecular Biology and Evolution, 25, 2331–2336. , & (
- 1990) Pharmacokinetics of gentamicin in newborn to 30-day-old foals. American Journal of Veterinary Research, 51, 1988–1992. , , & (
- 2001) Gender-based differences in pharmacokinetics in laboratory animal models. International Journal of Toxicology, 20, 161–163. (
- 2005) Effect of breed and gender on bovine liver cytochrome P450 3A (CYP3A) expression and inter-species comparison with other domestic ruminants. Veterinary Research, 36, 179–190. , , , , , , , & (
- 1973) Biotransformation and pharmacokinetics of salicylate in newborn animals. American Journal of Veterinary Research, 34, 1105–1108. , & (
- 1988) Effects of age, sex and breed on antipyrine disposition in calves. Research in Veterinary Science, 44, 135–139. , , & (
- 1990) Effect of gender in centrally induced angiotensin II hypertension in dogs. Hypertension, 15, I117–1120. , , , , & (
- 2005) Geriatric pharmacology. Veterinary Clinics of North America: Small Animal Practice, 35, 557–569. (
- 1997) The pharmacokinetics of cefadroxil over a range of oral doses and animal ages in the foal. Journal of Veterinary Pharmacology and Therapeutics, 20, 427–433. , & (
- 2001) Eprinomectin in dairy goats: dose influence on plasma levels and excretion in milk. Parasitology Research, 87, 294–298. , , & (
- 2002) Comparative pharmacokinetics of ivermectin after its subcutaneous administration in healthy sheep and sheep infected with mange. Journal of Veterinary Pharmacology and Therapeutics, 25, 159–60. , & (
- 1988) Drug disposition and biotransformation in the developing beagle dog. Fundamental and Applied Toxicology, 11, 29–37. , & (
- 1994) Oral bioavailability of pivampicillin in foals at different ages. Veterinary Quarterly, 16, S113–S116. , , , & (
- 1980) Therapeutic and toxic plasma concentrations of digoxin in the cat. American Journal of Veterinary Research, 41, 2049–2058. , & (
- 1993) Age-related changes in secretion rate and post-secretory metabolism of growth hormone in swine. Domestic Animal Endocrinology, 10, 249–255. , , & (
- FDA’s Guidance for Industry: Pharmacokinetics in Patients with Impaired Renal Function – Study Design, Data Analysis, and Impact on Dosing and Labeling (May 1998). http://www.fda.gov/cder/guidance/1449fnl.pdf
- FDA’s Guidance for Industry: Pharmacogenomic Data Submissions (March 2005). http://www.fda.gov/cder/guidance/6400fnl.pdf
- FDA’s Guidance for Industry: International Conference on Harmonisation; Guidance on E15 Pharmacogenomics Definitions and Sample Coding (April 2008). http://www.fda.gov/cder/guidance/8083fnl.pdf
- 2008) Pharmacogenetic and metabolic differences between dog breeds: their impact on canine medicine and the use of the dog as a preclinical animal model. American Association of Pharmaceutical Scientists Journal, 10, 110–119. , , , & (
- 1998) Use of body surface area to calculate chemotherapeutic drug dose in dogs: II. Limitations imposed by pharmacokinetic factors. Journal of Veterinary Internal Medicine, 12, 272–278. & (
- 1977) Hyperalgesia induced by naloxone follows diurnal rhythm in responsivity to painful stimuli. Science, 198, 756–758. , & (
- 1981) Postnatal development of renal function in piglets: changes in excretory pattern of sulphachlorpyridazine. Acta Pharmacologica et Toxicologica (Copenh), 48, 409–417. (
- 1984) Pharmacokinetics and metabolism of sulphadiazine in neonatal and young pigs. Acta Pharmacologica et Toxicologica (Copenh), 54, 321–326. , , , & (
- 1999) Sex difference in the daily rhythm of hepatic P450 monooxygenase activities in rats is regulated by growth hormone release. Toxicology and Applied Pharmacology, 161, 219–224. , , , & (
- 1996) Pharmacological basis for hepatic drug metabolism in sheep. Veterinary Research, 27, 363–372. & (
- 2003) Pharmacokinetic considerations in the treatment of childhood epilepsy. Paediatric Drugs, 5, 267–277. , & (
- 2005) Pharmacokinetic and pharmacodynamic factors that can affect sensitivity to neurotoxic sequelae in elderly individuals. Environmental Health Perspectives, 113, 1243–1249. , , & (
- 2009) High levels of P-glycoprotein activity in human lymphocytes in the first 6 months of life. Clinical Pharmacology and Therapeutics, 85, 289–95. , , , , , , , , , , , & (
- 1994) Effect of route of administration and age on the pharmacokinetics of amikacin administered by the intravenous and intraosseous routes to 3 and 5-day-old foals. Equine Veterinary Journal, 26, 367–373. , , , , & (
- 2001) Neonatal hepatic drug elimination. Pharmacology & Toxicology, 88, 3–15. , , , & (
- 2002) Positional candidate cloning of a QTL in dairy cattle: identification of a missense mutation in the bovine DGAT1 gene with major effect on milk yield and composition. Genome Research, 12, 222–231. , , , , , , , , , , , & (
- 2001) Diurnal variation of abdominal motor responses to colorectal distension and plasma cortisol levels in rats. Neurogastroenterology and Motility, 13, 585–589. , , , , , , & (
- 1995) P-glycoprotein expression and regulation. Drugs and Aging, 7, 19–29. (
- 1984) Metabolism of trimethoprim in neonatal and young pigs: comparative in vivo and in vitro studies. Acta Pharmacologica et Toxicologica (Copenh), 55, 402–409. , , & (
- 2010) Effect of obesity on the pharmacokinetics of drugs in humans. Clinical Pharmacokinetics., 49, 71–87. , & (
- 1991) Effect of time of cisplatin administration on its toxicity and pharmacokinetics in dogs. American Journal of Veterinary Research, 52, 1821–1825. , , & (
- 1985) The influence of age on plasma lignocaine levels following tracheal spray in young dogs. Anaesthesiology and Intensive Care, 13, 392–394. , , & (
- 1986) The influence of age on lignocaine pharmacokinetics in young puppies. Anaesthesiology and Intensive Care, 14, 135–139. , , & (
- 2010) Identification of single nucleotide polymorphisms within exon 1 of the canine mu-opiod receptor gene. Veterinary Anaesthesia and Analgesia, 377, 79–82. & (
- 2000) Evidence for propofol hydroxylation by cytochrome P4502B11 in canine liver microsomes: breed and gender differences. Xenobiotica, 30, 575–588. , , & (
- 1997) Modifying the formulation or delivery mechanism to increase the activity of anthelmintic compounds. Veterinary Parasitology, 72, 367–382. (
- 2007) Clinical pharmacology in the geriatric patient. Fundamental & Clinical Pharmacology, 21, 217–230. , & (
- 2003) Determinants of glomerular hypofiltration in aging humans. Kidney International, 64, 1417–1424. , , , , , , & (
- 2007) Comparison and reproducibility of plasma clearance of exogenous creatinine, exo-iohexol, endo-iohexol, and 51Cr-EDTA in young adult and aged healthy cats. Journal of Veterinary Internal Medicine, 21, 950–958. , , , , , , & (
- Human Genome Project, http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml
- 2008) Effect of medical castration on CYP3A4 enzyme activity using the erythromycin breath test. Cancer Chemotherapy and Pharmacology, 62, 373–377. , , , , , , , & (
- 2005) Sexual dimorphism in the permeability response of coronary microvessels to adenosine. American Journal of Physiology. Heart and Circulatory Physiology, 288, H2006–H2013. , & (
- 2002) Chiral inversion of (R)-ketoprofen: influence of age and differing physiological status in dairy cattle. Veterinary Research Communications, 26, 29–37. , , & (
- 2004) Some pharmacokinetic parameters of R-(-)- and S-(+)-ketoprofen: the influence of age and differing physiological status in dairy cattle. Veterinary Research Communications, 28, 81–87. , , & (
- 2006) Pharmacokinetic parameters of (R)-(-) and (S)-(+)-flurbiprofen in dairy bovines. Veterinary Research Communications, 30, 513–522. , , & (
- 1980) Diurnal pH changes in duodenum of conscious dogs. American Journal of Physiology, 238, G91–G96. , & (
- 1989) Renal function in conscious dogs: potential effect of gender on measurement. Research in Experimental Medicine (Berlin), 189, 371–379. & (
- 1999) The effect of sex on antipyrine metabolism in cattle at different ages. Journal of Veterinary Pharmacology and Therapeutics, 22, 163–169. & (
- 2000) The effect of sex and age on caffeine pharmacokinetics in cattle. Research in Veterinary Science, 69, 33–37. & (
- 1996) Effect of age on the pharmacokinetics of antipyrine in calves. Research in Veterinary Science, 60, 234–237. & (
- 1992) Pharmacokinetics of antipyrine in calves during first 35 days of life. Archives of Veterinary Medicine (Pol.), 32, 75–81. , , , & (
- 2001) The effect of short-term starvation or water deprivation on caffeine pharmacokinetics in calves. Research in Veterinary Science, 70, 109–113. , & (
- 1990) Effect of age and training status on pharmacokinetics of flunixin meglumine in thoroughbreds. American Journal of Veterinary Research, 51, 591–594. , & (
- 1997) Pharmacokinetics of enrofloxacin in newborn and one-week-old calves. Journal of Veterinary Pharmacology and Therapeutics, 20, 479–82. , , & (
- 2007) Chemotherapy dosing part I: scientific basis for current practice and use of body surface area. Clinical Oncology, 19, 23–37. & (
- 2000) Obesity induced by a high-fat diet is associated with reduced brain insulin transport in dogs. Diabetes, 49, 1525–1533. , , , & (
- 1983) Daily rhythms of analgesia in mice: effects of age and photoperiod. Brain Research, 279, 387–393. & (
- 2000) The effects of exercise on the pharmacokinetics of drugs. Journal of Pharmacy & Pharmaceutical Sciences, 3, 292–302. , & (
- 2004) Drug metabolism and ageing. British Journal of Clinical Pharmacology, 57, 540–544. & (
- 1995) Impact of age-related alteration of plasma alpha 1-acid glycoprotein concentration on erythromycin pharmacokinetics in pigs. American Journal of Veterinary Research, 56, 362–365. , , & (
- 2003) Standardization of the body surface area (BSA) formula to calculate the dose of anticancer agents in Japan. Japanese Journal of Clinical Oncology, 33, 309–313. , , , & (
- 2008) Comparative pharmacokinetics of amikacin in Greyhound and Beagle dogs. Journal of Veterinary Pharmacology and Therapeutics, 31, 102–107. & (
- 2003) Rhythms, pain and pain management. In Chronotherapeutics. Ed. Redfern, P., pp. 212–233. Pharmaceutical Press, London. & (
- 1995) Biological rhythms in the inflammatory response and in the effects of non-steroidal anti-inflammatory drugs. Pharmacology & Therapeutics, 66, 285–300. , & (
- 2007) Effects of physiological covariables on pharmacokinetic parameters of clomipramine in a large population of cats after a single oral administration. Journal of Veterinary Pharmacology and Therapeutics, 30, 116–126. , , & (
- 2007) Estimation of absolute oral bioavailability of moxidectin in dogs using a semi-simultaneous method: influence of lipid co-administration. Journal of Veterinary Pharmacology and Therapeutics, 30, 375–380. , , , & (
- 1997) Principles of drug administration in renal insufficiency. Clinical Pharmacokinetics, 32, 30–57. , , & (
- 1996) Nephrotoxicity of amphotericin B in rats: effects of the time of administration. Life Sciences, 58, 869–876. , , , , & (
- 2008) The effect of body condition on serum concentrations of two teratogenic alkaloids (anagyrine and ammodendrine) from lupines (Lupinus species) that cause crooked calf disease. Journal of Animal Science, 86, 2771–2778. , , , & (
- 1985) Pharmacokinetics of phenylbutazone in two age groups of ponies: a preliminary study. Veterinary Research, 116, 229–232. , & (
- 1996) The clinical relevance of chronopharmacology in therapeutics. Pharmacological Research, 33, 107–115. (
- 2004) Potential interactions between exercise and drug therapy. Sports Medicine, 34, 293–306. , & (
- 2007) Circadian rhythms: mechanisms and therapeutic implications. Annual Review of Pharmacology and Toxicology, 47, 593–628. & (
- 1984) The effect of different photoperiods on plasma concentrations of melatonin, prolactin, and cortisol in the domestic cat. Endocrinology, 115, 1729–1736. , & (
- 2006) Emerging evidence for the interrelationship of xenobiotic exposure and circadian rhythms: a review. Xenobiotica, 36, 1140–1151. , , & (
- 1996) Sex-dependent pharmacokinetics of indinavir: in vivo and in vitro evidence. Drug Metabolism and Disposition, 24, 1298–1306. , , , & (
- 2005) Genome sequence, comparative analysis and haplotype structure of the domestic dog. Nature, 438, 803–819. , , et al. (
- 2002) The continuing challenge of inappropriate prescribing in the elderly: an update of the evidence. Journal of the American Pharmaceutical Association (Washington), 42, 847–857. & (
- 2003) Age-related changes of the multidrug resistance P-glycoprotein function in normal human peripheral blood T lymphocytes. Brazilian Journal of Medical and Biological Research, 36, 1653–1657. , , & (
- 2005) Effect of ACTH and CRH on plasma levels of cortisol and prostaglandin F2alpha metabolite in cycling gilts and castrated boars. Acta Veterinaria Scandinavica, 46, 249–256. , , & (
- 2007) The impact of advancing age on P-glycoprotein expression and activity: current knowledge and future directions. Expert Opinion on Drug Metabolism & Toxicology, 3, 315–3120. (
- 2005) Interspecies differences in physiology and pharmacology: extrapolating preclinical data to human populations. In Preclinical Drug Development (Drugs and the Pharmaceutical Sciences, Vol. 152). Eds. Rogge, M.C. & Taft, D.R. pp. 11–67. Taylor & Francis Group, LLC, Boca Raton, FL. (
- 2010) Patient Variation in Veterinary Medicine. Part I. Influence of Altered Physiological States. Journal of Veterinary Pharmacology and Therapeutics., 33, 213–216. & (
- 2008) The pharmacogenomics of P-glycoprotein (P-gp) and its role in veterinary medicine. Journal of Veterinary Pharmacology and Therapeutics, 31, 285–300. , , , , & (
- 1996) Avermectins and milbemycins. Journal of Veterinary Pharmacology and Therapeutics, 19, 331–351. & (
- 2004) Aging biology and geriatric clinical pharmacology. Pharmacological Reviews, 56, 163–184. & (
- 2006) Adverse drug reactions in Herding-breed dogs: the role of P-gp. Compendium, 28, 23–33. (
- 2001) Ivermectin sensitivity in Collies is associated with a deletion mutation of the mdr1 gene. Pharmacogenetics, 11, 727–733. , , & (
- 2002) How important are gender differences in pharmacokinetics? Clinical Pharmacokinetics, 41, 329–342. , & (
- 2001) Gender-based differences in the toxicity of pharmaceuticals–the Food and Drug Administration’s perspective. International Journal of Toxicology, 20, 149–152. (
- 2004) NK and NKT cell functions in immunosenescence. Aging Cell, 3, 177–184. & (
- 1994) Lean body mass as a predictor of drug dosage. Implications for drug therapy. Clinical Pharmacokinetics, 26, 292–307. & (
- 1993) Release and disposition of 3H-noradrenaline in the saphenous vein of neonate and adult dogs. Naunyn Schmiedeberg’s Archives of Pharmacology, 347, 186–191. , , , & (
- 1987) Age-related digoxin effects in an intact canine model. American Heart Journal, 114, 583–588. , , , , & (
- 1992) Pharmacokinetics of cefprozil in infant and adult beagle dogs. Japanese Journal of Antibiotics, 45, 1469–1473. , , & (
- 2004) Breed distribution and history of canine mdr1-1Delta, a pharmacogenetic mutation that marks the emergence of breeds from the collie lineage. Proceedings of the National Academy of Sciences of the United States of America, 101, 11725–11730. , , , , , , & (
- 1991) Disposition of parathion in neonatal and young pigs. Pharmacology and Toxicology, 69, 233–237. , , & (
- 1995) Species and sex differences of testosterone and nifedipine oxidation in liver microsomes of rat, dog and monkey. Xenobiotica, 25, 1041–1049. , , , , , , & (
- 1997) Age-related changes in the pharmacokinetic disposition of diazepam in foals. American Journal of Veterinary Research, 58, 878–880. , & (
- 1992) Pharmacokinetics in immature animals: a review. Journal of Animal Science, 70, 3627–3634. (
- 1986) Age difference in pharmacokinetics of an amoxycillin trihydrate-15% formulation administered intramuscularly to ruminants. Veterinary Quarterly, 8, 339–342. , , & (
- 1989) Pharmacokinetics, metabolism and renal clearance of sulphatroxazole in calves and cows. Journal of Veterinary Pharmacology and Therapeutics, 12, 50–57. , , & (
- 1991) Pharmacokinetics of sulphamethoxazole in calves and cows. Veterinary Quarterly, 13, 10–15. , , & (
- 1995) Chronopharmacological study of acetylsalicylic acid in mice. European Journal of Pharmacology, 293, 151–157. , & (
- 1984) Pharmacokinetics of diflubenzuron in two types of chickens. Journal of Toxicology and Environmental Health, 13, 721–733. & (
- 1999) Evidence for polymorphism in the canine metabolism of the cyclooxygenase inhibitor, celecoxib. Drug Metabolism and Disposition, 27, 1133–1142. , , , , , , & (
- 2002) T cells and aging. Frontiers in Bioscience, 7, D1056–D1183. , , , , , , , , , , , & (
- 1994) Comparison of theophylline pharmacokinetics in yearling and 4-year-old horses. Journal of Veterinary Pharmacology and Therapeutics, 17, 473–476. , , , & (
- 2003) A review of the effects of chronic exercise and physical fitness level on resting pharmacokinetics. International Journal of Clinical Pharmacology and Therapeutics, 41, 504–516. , & (
- 2002) An animal model for the study of chronopharmacokinetics of drugs and application to methotrexate and vinorelbine. Toxicology and Applied Pharmacology, 183, 189–197. , , , , , & (
- 1997) Circadian variations in serum levels and the renal toxicity of aminoglycosides in patients. Clinical Pharmacology and Therapeutics, 62, 106–111. , , & (
- 1988) Circadian rhythm in theophylline disposition during a constant-rate intravenous infusion of aminophyllline in the dog. Journal of Pharmaceutical Sciences, 77, 658–661. , & (
- 1991) Circadian rhythm in theophylline disposition: simulations and observations in the dog. Journal of Pharmaceutical Sciences, 80, 824–829. , & (
- 2001) Liver disease in the elderly. Gastroenterology Clinics of North America, 30, 547–563. & (
- 2001) Pharmacokinetics of E-6087, a new anti-inflammatory agent, in rats and dogs. Biopharmaceutics & Drug Disposition, 22, 231–242. , , , & (
- 1992) The influence of age on the pharmacokinetics of aditoprim in pigs after intravenous and oral administration. Veterinary Research Communications, 16, 355–364. , & (
- 2006) Analysis of a polymorphism in the DGAT1 gene in 14 cattle breeds through PCR-SSCP methods. Research in Veterinary Science, 80, 287–290. , & (
- 1993) Disposition characteristics of coumarin as a function of age in the beagle dog model. Arzneimittel-Forschung, 43, 963–966. & (
- 1980) Chronopharmacokinetics of pentazocine in the beagle dog. Arzneimittel-Forschung, 30, 1535–1538. , , & (
- 1988) Effect of age on the pharmacokinetics of coumarin. Arzneimittel-Forschung, 38, 1466–1468. , , , & (
- 1991) Pharmacokinetics of papaverine HCl upon intravenous route of administration in old and young beagle dogs. Methods and Findings in Experimental and Clinical Pharmacology, 13, 51–55. , , & (
- 1986) Barbiturate anesthesia in greyhound and mixed-breed dogs: comparative cardiopulmonary effects, anesthetic effects, and recovery rates. American Journal of Veterinary Research, 47, 2105–2112. , & (
- 2003) Aminoglycoside nephrotoxicity: modeling, simulation, and control. Antimicrobial Agents and Chemotherapy, 47, 1010–1016. , , , , , , & (
- 2005) A SAS/IML program for simulating pharmacokinetic data. Computer Methods and Programs in Biomedicine, 78, 39–60. , & (
- 2002) Breed differences on the plasma availability of moxidectin administered pour-on to calves. Veterinary Journal, 164, 47–53. , , , , & (
- 1988) Effects of phenobarbital on thiopental pharmacokinetics in greyhounds. American Journal of Veterinary Research, 49, 245–249. & (
- 1991) Beta-adrenergic function in aging. Basic mechanisms and clinical implications. Drugs and Aging, 1, 116–129. , & (
- 2003) The influence of sex on pharmacokinetics. Clinical Pharmacokinetics, 42, 107–121. (
- 1991) The effect of aging on host defenses. Implications for therapy. Drugs and Aging, 1, 303–316. , , & (
- 2004) Pharmacokinetics of enrofloxacin in neonatal kittens. American Journal of Veterinary Research, 65, 350–356. , , , & (
- 2008) The success of the genome-wide association approach: a brief story of a long struggle. European Journal of Human Genetics, 16, 554–564. & (
- 1995) Species- and gender-related differences in amine, alcohol and phenol sulphoconjugations. Xenobiotica, 25, 1063–1071. , , , , , , & (
- 1987) The effect of age and diet on sulfadiazine/trimethoprim disposition following oral and subcutaneous administration to calves. Journal of Veterinary Pharmacology and Therapeutics, 10, 331–345. , & (
- 1989) Pharmacokinetics of sulfadiazine/trimethoprim in neonatal male calves: effect of age and penetration into cerebrospinal fluid. American Journal of Veterinary Research, 50, 396–403. , & (
- 1970) Perinatal development of drug-metabolizing enzyme activity in swine. The Journal of Pharmacology and Experimental Therapeutics, 174, 185–196. & (
- 1984) Clearance of penicillin G in the newborn calf. Journal of Veterinary Pharmacology and Therapeutics, 7, 45–48. , , , & (
- 1978) Age dependant factors influencing digoxin pharmacokinetics in the postnatal puppy. Research Communications in Chemical Pathology and Pharmacology, 21, 87–101. , & (
- 1986) Timing of amphotericin B therapy is a critical determinant of toxicity. Annual Review of Chronopharmacology, 3, 183–186. , , , , , , & (
- 1987) Age-dependent oral bioavailability of erythromycin thiocyanate in calves. Zentralblatt Veterinarmedizin. Reihe A, 34, 102–107. , , & (
- 2005) Breed and age affect baseline immune traits, cortisol, and performance in growing pigs. Journal of Animal Science, 83, 2087–2095. , , & (
- 1993) Comparative pharmacokinetics of aditoprim in milk-fed and conventionally fed calves of different ages. Research in Veterinary Science, 54, 86–93. , & (
- 1976) Pharmacokinetics of hexobarbital, sulphadimidine and chloramphencoliin neonatal and young pigs. Acta Veterinaria Scandinavica, 17, 1–14. (
- 1990) Age-related differences in pharmacokinetics of phosphonoformate in cats. Antimicrobial Agents and Chemotherapy, 34, 871–874. , , , & (
- 1994) Alpha 1-acid glycoprotein-binding as a factor in age-related changes in the pharmacokinetics of trimethoprim in piglets. Veterinary Quarterly, 16, 13–17. , , & (
- 2007) [Sex-specific differences in drug treatment] [Article in German] Therapeutische Umschau, 64, 325–329. (
- 2002) Intestinal calcium absorption in growing dogs is influenced by calcium intake and age but not by growth rate. Journal of Nutrition, 132, 3363–3368. , , & (
- 2002) Pharmacokinetics and dose proportionality of oral moxidectin in beagle dogs. Biopharmaceutics and Drug Disposition, 23, 263–272. , , , & (
- 2007) Gender related differences in the oxidative stress response to PCB exposure in an endangered goodeid fish (Girardinichthys viviparus). Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 146, 672–678. , , , & (
- 1990) Age-dependent pharmacokinetics of phenylbutazone in calves. Veterinary Quarterly, 12, 98–102. , , , & (
- Broad Institute Genome Sequencing Platform; Broad Institute Whole Genome Assembly Team, & (2009) Genome sequence, comparative analysis, and population genetics of the domestic horse. Science, 326, 865–867. , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,
- 2002) Influence of age and body size on gastrointestinal transit time of radiopaque markers in healthy dogs. American Journal of Veterinary Research, 63, 677–682. , , , , & (
- 2008) How does obesity affect residence time dispersion and the shape of drug disposition curves? Thiopental as an example. Journal of Pharmacokinetics and Pharmacodynamics, 35, 325–336. (
- 1989) Decrease in intestinal permeability to polyethylene glycol 1000 during development in the pig. Journal of Developmental Physiology, 11, 83–87. , , , & (
- 1989) Effect of age on absorption and immune responses to weaning or introduction of novel dietary antigens in pigs. Research in Veterinary Science, 46, 180–186. , & (
- 1991) Species- and sex-related differences in the plasma clearance and metabolite formation of antipyrine. A comparative study in four animal species: cattle, goat, rat and rabbit. Xenobiotica., 21, 1483–1492. , , , , & (
- 1993) Increased function of P-glycoprotein in 7 lymphocyte subsets of ageing mice. Journal of Immunology, 150, 1296–1306. & (
- 1991) Pharmacokinetics of gentamicin after intravenous and subcutaneous injection in obese cats. Journal of Veterinary Pharmacology and Therapeutics, 14, 96–100. , , , & (
- 2008) Assessment of the impact of dosing time on the pharmacokinetics/pharmacodynamics of prednisolone. American Association of Pharmaceutical Scientists Journal, 10, 331–341. , , & (
- 2009) Effect of age on minimum alveolar concentration (MAC) of sevoflurane in dogs. Journal of Veterinary Medical Science, 71, 1509–1512. , , & (
- 1992) Felbamate metabolism in pediatric and adult beagle dogs. Drug Metabolism and Disposition, 20, 84–88. , , , & (
- 1998) Pharmacokinetic disposition and arthropathic potential of oral ofloxacin in dogs. Journal of Veterinary Pharmacology and Therapeutics, 21, 128–132. , , , , , , & (
- 1993) The time of administration of 3’-azido-3’-deoxythymidine (AZT) determines its host toxicity with possible relevance to AZT chemotherapy. Antimicrobial Agents Chemotherapy, 37, 1771–1776. , , , & (
- 1995) Disposition kinetics of tylosin tartrate administered intravenously and intramuscularly to normal and water-deprived camels. Journal of Veterinary Pharmacology and Therapeutics, 18, 299–305. , , , & (
- 1993) Pharmacokinetics of propofol in mixed-breed dogs and greyhounds. American Journal of Veterinary Research, 54, 755–760. , & (
- Top of page
- Physiological Variables That Can Influence Drug Kinetics
- Population Predictions
Interpretation of a boxplot graph
A boxplot provides a graphical presentation of descriptive statistics such as the mean, lowest 25% (first quartile, or Q1), upper 75% (third quartile, or Q3), and median values. The box itself contains the middle 50% of the data and is called the interquartile range (IQR). It represents values associated with the middle 50% of the population (i.e. Q3–Q1). Within this box, the median is represented by a line and the mean is represented by a plus sign (+). If the median line within the box is not equidistant from the edges, then the data are skewed. Any data observation which is less than the Q1 or higher than the Q3 (but is greater than Q1 − 1.5 × IQR and less than Q3 + 1.5 × IQR) is connected to the box with a vertical line (‘whisker’). Extreme outliers are more than three times the IQR from Q1 and Q3. These values are indicated by an asterisk. Values that are more than 1.5 times the IQR (but less than three times the IQR) from Q1 and Q3 are indicated by an open circle (○).