A pilot phase II study of the efficacy and biosafety of doxorubicin chemotherapy in tumor-bearing equidae

Background: The efficacy and biosafety of a previously established tolerable dosage of doxorubicin have not been established in horses. Objectives: To provide preliminary evidence of the efficacy of doxorubicin in tumor-bearing horses, explore drug pharmacokinetics profile, and estimate period of risk of exposure to drug residues. Animals: Twelve horses with 37 tumors. Procedures: Treatment protocol included 6 treatments at 3-week intervals. Eight horses were uniformly treated at a dosage of 70 mg/m² and 4 horses received 4 of 6 treatment cycles at 70 mg/m². Clinical signs, tumor responses, and toxicoses were evaluat Drug residue concentrations were quantitated in 3 horses receiving of 65, 70, and 75 mg/m² by high-performance liquid chromatography with ultraviolet detection (plasma, feces) and liquid chromatography and tandem mass spectrometry (urine). Results: Thirty tumors, including lymphomas, carcinomas, sarcoids, and melanoma, were evaluated for efficacy. The overall response rate was 47% (95% CI, 28-65%). Doxorubicin was not found to be effective against melanomas. Lymphomas and carcinomas were most responsive. Pooled serum Cmax and half-life of doxorubicin were 121.3 ng/mL and 12.9 hours, respectively. There were no detectable residues in fecal samples up to 3 weeks after treatment and in plasma and urine after 2 and 3 days, respectively. Conclusion and Clinical Relevance: This study provides preliminary evidence that single-agent doxorubicin at a dosage of 70 mg/m² has a broad spectrum of activity. The risk of exposure to drug residues in plasma and feces was low. Direct contact with urine-contaminated wastes should be avoided for 2 days after treatment.