This study was completed at Colorado State University, Fort Collins, Colorado.
The Pharmacokinetics of Mirtazapine in Cats with Chronic Kidney Disease and In Age-Matched Control Cats
Article first published online: 30 AUG 2011
Copyright © 2011 by the American College of Veterinary Internal Medicine
Journal of Veterinary Internal Medicine
Volume 25, Issue 5, pages 985–989, September/October 2011
How to Cite
Quimby, J.M., Gustafson, D.L. and Lunn, K.F. (2011), The Pharmacokinetics of Mirtazapine in Cats with Chronic Kidney Disease and In Age-Matched Control Cats. Journal of Veterinary Internal Medicine, 25: 985–989. doi: 10.1111/j.1939-1676.2011.00780.x
This study was funded by a grant from the Winn Feline Foundation.
Presented in part as an abstract at the 2010 ACVIM Forum, Anaheim, CA.
- Issue published online: 20 SEP 2011
- Article first published online: 30 AUG 2011
- Manuscript Accepted: 6 JUL 2011
- Manuscript Revised: 27 MAY 2011
- Manuscript Received: 2 FEB 2011
Cats with chronic kidney disease (CKD) often experience inappetence, and may benefit from administration of mirtazapine, an appetite stimulant. The pharmacokinetics of mirtazapine in CKD cats is unknown.
CKD delays the clearance/bioavailability (CL/F) of mirtazapine.
Six CKD cats and 6 age-matched controls (AMC) were enrolled. Two CKD cats each from International Renal Interest Society (IRIS) stage II, III and IV were included.
Blood samples were collected before and 0.5, 1, 1.5, 2, 4, 8, 24, and 48 hours after a single PO dose of 1.88 mg of mirtazapine. Mirtazapine concentrations were measured by liquid chromatography coupled to tandem mass spectrometry. Non-compartmental pharmacokinetic modeling was performed.
Mean age was 11 years (CKD cats) and 10.8 years (AMC cats). Mean serum creatinine concentration ± standard deviation (SD) was 3.8 ± 1.6 mg/dL (CKD) and 1.3 ± 0.4 mg/dL (AMC). Mean half-life ± SD was 15.2 ± 4.2 hours (CKD) and 12.1 ± 1.1 hours (AMC). Mean area under the curve (AUC) ± SD was 770.6 ± 225.5 ng/mL•hr (CKD) and 555.5 ± 175.4 ng/mL•hr (AMC). Mean CL/F ± SD was 0.6 ± 0.1 L/hr/kg (CKD) and 0.8 ± 0.16 L/hr/kg (AMC). A Mann-Whitney test indicated statistically significant differences in AUC (P = 0.01) and CL/F (P = 0.04) between groups. Calculated accumulation factor for 48-hour dosing in CKD cats was 1.15.
CKD may delay the CL/F of mirtazapine. A single low dose of mirtazapine resulted in a half-life compatible with a 48-hour dosing interval in CKD cats.
area under the curve to infinity
area under the curve
chronic kidney disease
maximum serum concentration
International Renal Interest Society
liquid chromatography/tandem mass spectrometry
quality assurance/quality control
elimination half life
time to maximum serum concentration
volume of distribution
Chronic kidney disease (CKD) is common in geriatric cats. The kidneys are responsible for excretion of gastrin and as renal function deteriorates, gastrin concentrations may increase, leading to uremic gastritis. Other factors may contribute to lethargy and inappetence in these patients, including metabolic acidosis, anemia, and renal secondary hyperparathyroidism. As a result of these factors, cats with CKD frequently experience anorexia and vomiting. Inappetence can lead to negative energy balance with associated weight loss, muscle weakness, and poor quality of life. In addition, recent studies have documented the therapeutic value of specially formulated diets in the management of CKD.[3-5] These diets typically contain restricted amounts of high quality protein, adequate non-protein calories, and are restricted in phosphorus. Failure of the patient to eat negates the benefit of dietary management, and therefore a key therapeutic target for these patients is the maintenance of appetite and food intake. Current strategies to enhance appetite include the use of H2-receptor antagonists or proton pump inhibitors to manage uremic gastritis, and cyproheptadine as an appetite stimulant. Feeding tubes also may be used, but are not an acceptable option for many pet owners.
Mirtazapine, an antidepressant used in humans, has gained popularity in veterinary medicine because of its anti-emetic and appetite-stimulating properties.[6, 7] These effects appear to be a result of antagonism of the 5-HT3 receptor, which is important in the physiology of emesis. A recent placebo-controlled crossover study in young normal cats demonstrated that mirtazapine is an effective appetite stimulant in this species. The efficacy of commonly administered doses was examined and although in both 1.88 mg/cat and 3.75 mg/cat resulted in increased food consumption as compared to placebo, more undesirable effects (increased vocalization, activity, and socialization) were seen at the higher dose. Other reported adverse effects in cats are mild and dose dependent, and include hyperexcitability and muscle tremors.
Previously, mirtazapine doses for cats and dogs were extrapolated from human medicine and adjusted based on clinical observations. A recent study in young normal cats indicated that the half-life of the drug was shorter than that previously suspected. In addition, the drug does not appear to display linear pharmacokinetics in cats, and larger doses may result in a longer half-life. Pharmacokinetic data from humans have demonstrated that a number of factors affect the metabolism of mirtazapine, including sex and age, and hepatic or renal impairment. The latter is likely because the drug undergoes hepatic metabolism and renal excretion. The purpose of this study was to determine the pharmacokinetics of mirtazapine in cats with CKD and in age-matched controls (AMC) to investigate the effects of renal impairment in this species and the potential for dose modification.
Materials and Methods
In this prospective pharmacokinetic study, 6 client-owned stable CKD cats, 2 each from International Renal Interest Society (IRIS) stages II, III and IV, and 6 age-matched (within 6 months) healthy geriatric control cats (AMC) were enrolled by stratified convenience sampling. There were 4 spayed females and 2 neutered males in the CKD group and 3 spayed females and 3 neutered males in the AMC group. Cats were considered to have stable CKD if serum creatinine concentration had not changed by more than 10% on at least 2 measurements in the previous 60 days. Diagnostic tests required before enrollment included a minimum database consisting of serum biochemistry profile, CBC, urinalysis, urine culture, blood pressure, and serum total thyroxine concentration. Healthy control cats were defined as those with no clinical abnormalities, normal laboratory test results including serum creatinine <1.8 mg/dL and urine specific gravity >1.035. Exclusion criteria included other systemic illnesses, complications of CKD such as hypertension, pyelonephritis or ureteral obstruction, or decompensation of CKD requiring hospitalization and IV fluid therapy. The project was approved by the Institutional Animal Care and Use Committee at Colorado State University, and all owners gave written informed consent before participation.
Commercially available generic 15 mg mirtazapine1 tablets were compounded into 1.88 mg doses by the pharmacy at the Colorado State University Veterinary Medical Center according to the Professional Compounding Centers of America® protocol as previously described. The method used is guaranteed to produce accurate compounding to within 10% of the target dose. Analysis of random compounded capsules for mirtazapine content using liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) showed accuracies of 94.5 ± 4.6% to the intended content and stability of at least 6 months as formulated. Mirtazapine capsules were compounded within 1 month of use and stored at room temperature.
The cats were fasted for 12 hours before beginning the study. A jugular catheter was placed under ketamine2 (20 mg per cat IV) and butorphanol2 (0.1 mg/kg IV) sedation 3 hours before mirtazapine administration, to allow for ease of sample collection. A capsule containing 1.88 mg of mirtazapine was administered PO once, followed by 3 mL of water administered by syringe. Blood samples (1.0 mL) were obtained before, and 0.5, 1, 1.5, 2, 4, 8, 24, and 48 hours after mirtazapine administration. Samples were centrifuged within 10 minutes of collection and serum was harvested and stored at −80°C until analysis.
Mirtazapine was measured using LC/MS/MS. Analysis was carried out in the Pharmacology Core at the Colorado State University Veterinary Medical Center using a previously developed and validated LC/MS/MS- based assay for the analysis of mirtazapine in cat serum. Assay performance for each batch was assessed utilizing at least 10% quality assurance, quality control (QA/QC) samples dispersed among unknown samples at low (1 ng/mL), mid (10 ng/mL) and high (100 ng/mL) ranges of the standard curve (0.5–500 ng/mL) with batches failing if >25% of the QA/QC samples were outside of the accepted level of 85% accuracy. Accuracy of QA/QC samples among the batches analyzed for this study ranged from 94.5 ± 4.6% to 92.2 ± 6.8%. The lower limit of quantitation (LLOQ) for this assay was based on the level of detection with >85% accuracy and a coefficient of variation (%) <15%, and was determined to be 0.5 ng/mL. Assay performance was linear to >500 ng/mL, but 500 ng/mL was used as the upper limit of the assay as utilized because of a lack of samples exceeding this concentration.
Pharmacokinetic analysis was performed using a non-compartmental method. Area under the curve to infinity (AUC∞), disappearance half-life (t1/2λ), time to maximum serum concentration (Tmax), and maximum serum concentration (Cmax) were calculated. Because mirtazapine was administered by an extravascular route, absorbed dose is equal to D (dose) × bioavailability (F). Thus, parameters in which the calculation is based on the assumption that 100% of the dose reaches the systemic circulation (clearance [CL] and volume of distribution [Vd]) are expressed as CL corrected for bioavailability or clearance/bioavailability (CL/F) and volume of distribution/bioavailability (Vd/F). Using the term CL/F and comparing it between the 2 treatment groups assumes that F, or bioavailability, is not different between the 2 treatment groups. The accumulation factor at steady state after multiple doses was estimated from the pharmacokinetic data using the equation: Accumulation Factor = 1/(1-e-Kel*T). The terminal elimination rate was used for estimating the accumulation factor as the applicable kel, and the dosing interval (T) was set at 24 or 48 hours.
Comparison of pharmacokinetic parameters between the CKD and AMC cat groups was performed using a Mann-Whitney U-test. Prism software3 was used for all analyses.
Descriptive statistics for the 12 cats enrolled, including age, dosage (mg/kg), and serum creatinine concentration, are presented in Table 1. Two cats in IRIS Stage II (serum creatinine concentrations 2.4 and 2.5 mg/dL), 2 cats in IRIS Stage III (serum creatinine concentrations 2.9 and 3.3 mg/dL), and 2 cats in IRIS Stage IV (serum creatinine concentrations 5.7 and 6.1 mg/dL) were enrolled in the CKD group. Statistically significant differences were not detected in age or mg/kg dosage between the 2 groups. There was a statistically significant difference in serum creatinine concentration between the 2 groups (P = .002).
|Pharmacokinetic Parameter||Healthy Geriatric||Chronic Kidney Disease|
|Median||Range||Mean ± SD||Median||Range||Mean ± SD|
|Age (years)||10.7||7.8–13.8||10.8 ± 2.3||11||8.3–13.7||11 ± 2.2|
|Mg/kg dose||0.43||0.33–0.58||0.44 ± 0.08||0.45||0.4–0.78||0.51 ± 0.15|
|Creatinine (mg/dL)||1.4||0.7–1.8||1.3 ± 0.4||3.1||2.4–6.1||3.8 ± 1.6|
Pharmacokinetic parameters are shown in Table 2. There was a statistically significant difference in AUC∞ and CL/F between the AMC cats and CKD cats. Graphical representations of drug concentration curves for AMC cats and CKD cats are illustrated in Figure 1.
|Median||Range||Mean ± SD||Median||Range||Mean ± SD|
|Cmax (ng/mL)||83.6||50.2–103||79.6 ± 21.7||109.5||79.1–164||110.6 ± 30.8|
|Cmax/Dose (ng/mL)/(mg/kg)||179.1||132.3–210.1||180.3 ± 44.3||221.6||154.4–278.3||219.6 ± 44.7|
|Tmax (hr)||1||1–4||2 ± 1.5||1||0.5–1.5||1 ± 0.3|
|Half life (hr)||12.0||10.1–15.4||12.1 ± 1.1||15.8||10.8–24.8||15.2 ± 4.2|
|Area under the curve to infinity (AUC∞)a (ng/mL · hr)||560.8||400.4–941.2||589.8 ± 185.3||828.4||597.1–1253.6||866.5 ± 257.9|
|AUC∞/Dosea (ng/mL · hr)/(mg/kg)||1375.6||926.5–1612.2||1320.4 ± 236.0||1676.8||1355.1–2229.3||1701.2 ± 301.3|
|CL/Fa (L/hr/kg)||0.73||0.62–1.1||0.79 ± 0.16||0.6||0.45–0.74||0.61 ± 0.1|
|Vdz/F (L)||14.4||10.1–16.3||13.9 ± 3.2||13.4||11.5–16.1||13.6 ± 2.0|
Assessment of Accumulation
Drug accumulation was calculated for 24- and 48-hour dosing intervals for both CKD and AMC groups. For the CKD cats, an accumulation factor of 1.57 was calculated for 24-hour dosing and an accumulation factor of 1.15 was calculated for 48-hour dosing. For the AMC cats, an accumulation factor of 1.35 was calculated for 24-hour dosing and an accumulation factor of 1.07 was calculated for 48-hour dosing.
In the present study, the pharmacokinetics of mirtazapine in CKD cats and AMC were explored. A significant difference in drug exposure (AUC) and CL/F of mirtazapine was found between CKD cats and AMC cats. Mirtazapine is a 5-HT3 receptor antagonist with appetite-stimulating properties. We have previously demonstrated that a dose of 1.88 mg significantly stimulates appetite in young normal cats. This dose was associated with a half-life of approximately 10 hours, thus allowing daily dosing with little drug accumulation. In comparison to our previously reported mean half-life in normal cats (10 hours), the mean half-life of mirtazapine in this study was approximately 12 hours for AMC cats and 15 hours for CKD cats. The mean AUC in normal cats previously was reported to be 397 ng/mL/hr, in comparison to 523.9 ng/mL/hr in AMC cats and 686.5 ng/mL/hr in CKD cats in the present study. Although there was no significant difference in mirtazapine dose between the CKD and AMC cats, the statistical power of the comparison between doses in each group is limited attributable to sample size. Therefore, AUC and Cmax also were calculated with dose adjustment (Table 2) to decrease the possible effect of difference in dose between the 2 groups. Even with this adjustment, AUC still was significantly different between the AMC and CKD groups. The mean CL/F previously was found to be 1.1 L/hr/kg in young healthy cats, in comparison to 0.79 L/hr/kg in AMC cats and 0.61 L/hr/kg in CKD cats in the present study. From this information, we suggest that although age appears to have some influence on the metabolism of the drug, it cannot entirely account for the difference between CKD and young normal cats. Therefore, we conclude that CKD delays the CL/F of mirtazapine in cats.
In the human medical literature, moderate to severe renal disease is reported to increase mirtazapine exposure (AUC) caused by a decrease in drug CL. A similar relation may exist between renal disease and mirtazapine CL/F in cats, based on the data presented herein and that in a previous study. When the data from the 2 studies were combined, there was a significant negative correlation between serum creatinine concentration and CL of mirtazapine (r = −0.69 with P = .0024) when data from young normal cats, normal geriatric cats, and CKD cats were analyzed using Spearman rank correlation. Elimination half-life of mirtazapine is unaffected by the severity of renal disease in humans. In this study, although a significant difference in half-life was not detected between AMC and CKD cats, perhaps attributable to small sample size, there is a difference between these data and that reported in normal cats. It is unknown to what extent nonlinear pharmacokinetics may play a role in this observation, because half-life would be expected to be prolonged with increased exposure. As in humans, differences in metabolism attributable to age also may play a role. In this data set, pharmacokinetic parameters for the AMC group, particularly AUC, vary notably from those reported for young normal cats. Two cats from each IRIS stage were included in the present study to represent the range of renal function encountered in clinical practice. This likely contributed to greater standard deviation (SD) for some parameters, including half-life, and potentially affected our ability to find a significant difference between groups. If cats from only 1 IRIS stage had been studied, variability may have been decreased, and it may have been possible to demonstrate a difference between half-life in AMC and CKD cats.
In humans, sex is known to affect mirtazapine pharmacokinetics, with shorter half-life and lower AUC in young men compared to women. Thus, an effort was made to control for this factor by having approximately equal numbers of both sexes in each group. Unfortunately, complete age and sex matching was not possible because of difficulty in recruiting CKD patients without concurrent illness, and this potentially could have affected results. Study participant numbers were too small to determine if there was a significant effect of sex on metabolism of mirtazapine in cats.
A limitation of this study is the inability to determine true CL because the drug was not given IV. This study was performed in client-owned animals and the authors were reluctant to administer a medication IV with no prior experience with this route, and with no knowledge of possible adverse effects. Secondly, an aim of this study was to make comparisons with a previously published study in which the medication was administered PO to normal young cats. In addition, as the previously published pharmacodynamic study demonstrated a clinical effect of the PO dose used in the present study, we did not believe administering the medication IV would provide clinically useful information. Thus, the alternate measurement of CL/F was used instead. This measurement operates under the assumption that bioavailability will not change between groups. Because a subjective nonstatistically significant difference in Cmax was noted between groups, implying that absorption may have differed between the groups, this assumption may not be valid and results should be interpreted accordingly.
Another potential limitation of this study is the measurement of renal function by serum creatinine concentration. The use of serum creatinine concentration instead of glomerular filtration rate may have affected interpretation of the correlation between renal function and CL of mirtazapine. Glomerular filtration rate is a more accurate assessment of renal function but is not practical for clinical assessment. Serum creatinine concentration was used in this study to provide a clinically applicable tool on which to base a decision about mirtazapine administration. As a result, however, cats with subclinical kidney disease may have been inadvertently included in the AMC group. Because muscle wasting is known to decrease serum creatinine concentration, cats selected for the study were of normal body condition and did not have marked muscle wasting.
The pharmacokinetic information obtained in this study can be used to help determine dose intervals for cats with CKD. Calculation of an accumulation factor for daily dosing compared to every other day dosing was performed. Although no evidence of drug accumulation was seen with the 48-hour dose interval (accumulation factor = 1.15), accumulation potentially is possible with daily dosing in CKD cats (accumulation factor = 1.57). This is in contrast to young normal cats, where no evidence of drug accumulation was found with daily dosing (accumulation factor = 1.2). However, because concentration may not reflect clinical effect, to fully understand the pharmacokinetic and pharmacodynamic implications of this dosing regimen in CKD cats, a clinical trial with repeated dosing every 48 hours should be performed. In conclusion, the results of this study indicate that CKD in cats results in higher drug exposure and appears to slow the CL/F of mirtazapine. Age, subclinical kidney disease, or both also may affect the metabolism of mirtazapine in cats. This information should be considered when clinicians are determining dosing regimens for their patients.
The authors are grateful to Paul Lunghofer for technical assistance.
Aurobindo Pharma USA, Inc., Dayton, NJ
Fort Dodge, Fort Dodge, IA
GraphPad Software, Inc, La Jolla, CA
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