Doxorubicin is one of the most clinically effective and widely used antineoplastic drugs in humans, dogs, and cats.[1-3] However, its therapeutic potential has not been explored extensively in horses. Dose-limiting toxicoses (DLT) of doxorubicin and target organs have been determined in a Phase I trial conducted in 17 horses with 34 localized or multicentric advanced tumors.1 DLT included reversible drug hypersensitivity and neutropenia. Other minor toxicoses included reversible hair loss and skin reactions including multifocal dermatitis, neutrophilic necrotizing, and ulcerative epidermitis and patchy crusting and scaling of the skin in areas of alopecia. Cardiac, gastrointestinal, and renal toxicoses common in other species were not observed. Cardiac toxicosis was monitored by sequential echocardiograms, serum CK, SDH, AST activities, and cTnI concentrations. Gastrointestinal toxicosis was monitored by weight loss, anorexia, diarrhea, gastrointestinal (GI) bleeding assessed by occult blood test, and BUN : Cr ratio. Renal toxicity was evaluated by serum concentrations of creatinine and BUN, and urinalysis.
For cytotoxic chemotherapy, the therapeutic margin is narrow and it is important to treat close to the MTD to evaluate efficacy. Maximum tolerated dose and safe dosage of doxorubicin have been established for use in tumor-bearing horses in a previous dose-ranging study. However, the wide range of doxorubicin dosage (40–85 mg/m2) and dosage escalation protocol did not allow evaluation of efficacy at the determined recommended dosage.1
The large quantity (1,127–2,900 mg)1 of doxorubicin administered to horses for treatment prompted consideration of health risks and environmental hazards associated with drug residues in body fluids or excreta. Guidelines on doxorubicin use and protection in horses1 have been based on protocols for handling cytotoxic drugs and related wastes in small animal medicine. However, drug and metabolites residue concentrations in body fluids and feces depend on their pattern of distribution, metabolism, and elimination, which differs among species. Because the pattern of distribution and elimination of doxorubicin in dogs and humans[6, 7] are different, it seems inappropriate to rely on protocols used in dogs and humans for handling body fluids and wastes in horses.
The hypothesis of this study was that doxorubicin is effective in horses with tumors, and an appropriate use protocol could minimize a potential biohazard risk. The goals of this pilot study were to explore antitumor activity in horses treated with an established safe dosage and provide preliminary information on pharmacokinetic variables and drug residues to assess potential occupational and environmental hazards.
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- Materials and Methods
The main aim of phase II cancer clinical trials is to evaluate the antitumor effect of a treatment, screening out agents that are insufficiently active and selecting active agents for future studies. Because of the proven efficacy of doxorubicin in veterinary[17, 18] and human medicine, a 2-stage Fleming Phase II design was used to minimize expected sample sizes by stopping accrual for lack of efficacy, but also for sufficient activity. The number of patients in the first screening stage was minimized according to Bayesian decision theory. The study design allowed for simultaneously testing the hypothesis that the response rate was less than or equal to the specified ineffective treatment response rate set at 10% and the alternative hypothesis that the response rate was greater than the effective response rate set at 50%. This approach saves drug development time, brings useful treatments into clinical practice more quickly, and minimizes the number of patients used.
In this study, doxorubicin was found to be very active against lymphomas and carcinomas. The true response rate for these tumors is likely >50%. Doxorubicin appeared active against fast-growing fibroblastic sarcoids. However, the drug was less active on slow-growing verrucous sarcoids and resulted in growth control rather than tumor reduction. Stable disease in 4 verrucous sarcoids for 14 months was far greater than what would be expected if the horse had not been treated. As a result, the overall response rate for sarcoid tumors is likely to be between 10 and 50%. Further accrual of patients with these tumor types will be necessary to better estimate doxorubicin efficacy. Doxorubicin was not found active against melanomas and additional trials are not warranted.
The dose of doxorubicin as single-agent chemotherapy used in this study ranged from 2,171 to 2,906 mg (mean, 2,603 mg; median, 2,576 mg) based on a dosage of 70 mg/m2. On an mg/m2 basis, cumulative dosage ranged from 410 to 435 mg/m2. On mg/kg basis, dosages ranged from 0.84 to 0.96. This dose was comparable to that used in humans, both as determined on an mg/m2 and mg/kg basis.[7, 20] Although the sample size in this study did not allow accurate determination of the true efficacy of single-agent doxorubicin, the response rate confirms the efficacy of doxorubicin against lymphomas, carcinomas, and sarcomas in horses. On the basis of the observed efficacy for horses with gross disease, doxorubicin is likely to have superior results in an adjuvant setting.
Adverse drug reactions (grade 3) were a dose-limiting toxicity after 6 cycles. Mild bone marrow suppression was common at 10–12 days after each treatment and a self-limiting adverse effect of chemotherapy with doxorubicin. Double nadir was not observed. Grade 3 drug reactions were seen after 6 cycles (<10% of cycles) and resulted in dosage reduction to 65 mg/m2. After a cycle at 65 mg/m2 without dose-limiting toxicity, dosage at 70 mg/m2 was resumed without grade 3 drug reactions. These results confirm the validity of the study design used in a previous phase I study to determine a safe dosage.1
The low Cmax and AUC and large clearance and volume of distribution calculated from plasma concentrations in 3 horses in this study indicated extensive drug distribution into tissues. Plasma drug concentrations are affected by the rate at which drug is administered, the volume in which it distributes, and its clearance. When compared with dogs receiving 30 mg/m2 administered IV over 20 minutes, Cmax and AUC were approximately 10 times lower in the 3 horses in this study receiving 60–75 mg/m2. Conversely, Vss was approximately 3 times higher and CL more than 10 times higher in horses. Because the study was not designed to determine drug pharmacokinetics in horses, but rather provide safety guidelines, the pharmacokinetic parameters and failure to account for the total administered drug must be interpreted with caution. When compared with dogs and humans, the striking differences in pharmacokinetic variables may reflect a poor correlation between drug pharmacokinetics and drug dosages expressed in mg/m2 (Table 1). Finally, the preliminary pharmacokinetic findings may have reflected an inadequate formula of surface area in horses, resulting in an inaccurate dosage on mg/m2 basis.
The lack of doxorubicin-induced cardiotoxicity in this study and in a previous Phase I study may be a reflection of the low plasma concentration of doxorubicin and metabolites in horses. Doxorubicin-induced cardiotoxicity has been shown in humans to be dependent on Cmax. The low Cmax of doxorubicin in the 3 horses evaluated relative to dogs and humans[6, 7] may explain the lack of cardiotoxicity.
The anticancer activity of doxorubicin has been reported in humans to be dependent on the AUC. Two (2 lymphomas) of the 3 horses that were evaluable for tumor response and had pharmacokinetics data had CR. These 2 horses had lower Cmax and AUC relative to dogs and humans,[6, 7] but substantially more extensive tissue drug distribution into tissues potentially including tumor tissues. These findings may provide preliminary evidence that drug distribution may contribute more to the anticancer effects than the AUC and warrant additional studies.
The increased use of chemotherapeutics and large amounts of drugs used in equine medicine prompted consideration of health risks and environmental hazards associated with residues of these drugs in patient excretions. The present studies indicated that residues of doxorubicin and its metabolite, doxorubicinol, in serum, urine, and feces of treated horses were substantially lower than those in treated dogs or humans. There were no detectable residues in plasma 48 hours after administration. As a result, the risk of occupational hazards for veterinarians and laboratory personnel by collecting and handling blood samples ≥2 days after administration is minimal. There were no detectable residues in fecal samples at any time point after administration. This also has been reported in humans despite the fact that biliary excretion is an important route of plasma clearance. This could have resulted from extensive metabolism of the drug and metabolites by the intestinal flora in horses, poor recovery from the feces, or efficient reabsorption from the intestine during a process of enterohepatic circulation. As a result, the risk of exposure to owner and personnel handling feces with no direct contact appears to be negligible.
In this study, urine doxorubicin concentrations in horses receiving dosages up to 75 mg/m2 were below the range reported in dogs receiving 30 mg/m2. Residues in urine were not detectable in 2 horses (at dosages of 60 and 70 mg/m2) 2 days after treatment, whereas 1 horse (75 mg/m2) had detectable residues up to 3 days after treatment. Although the concentrations of doxorubicin and doxorubicinol in this horse were low and below urine concentrations in dogs receiving 30 mg/m2, the large urine output may have resulted in a potential contamination risk. However, this risk is minimized by the rapid degradation of doxorubicin in urine when exposed to daylight (complete degradation in 4 hours) and artificial light (50% degradation in 3.2 hours). In addition, owners and staff are not likely to come into close contact with a horse's excreta after treatment. As a result, a 2-day period of risk to owner, personnel, and public for doxorubicin and metabolites in equine urine appears reasonable. At any time during that period, daily excreta or soiled bedding should be kept on site for an additional 24 hours before disposal to allow drug degradation to nondetectable concentrations.
In summary, systemic chemotherapy with doxorubicin offers a potentially highly effective, affordable, and practical treatment option for horses with advanced cancers. Horse owners are aware of progress made in cancer treatment and are seeking treatment options for their horses when diagnosed with advanced or metastatic cancers. This study establishes a step toward developing effective chemotherapy regimens in equine oncology. The drug residue findings in this study provide information on which guidelines can be developed for safe handling and avoiding exposure risk for veterinary personnel, public, and the environment.