ABCB1‐1Δ Polymorphism Can Predict Hematologic Toxicity in Dogs Treated with Vincristine

Background: Dogs that harbor the naturally occurring ABCB1‐1Δ polymorphism experience increased susceptibility to avermectin‐induced neurological toxicosis as a result of deficient P‐glycoprotein function. Whether or not the ABCB1‐1Δ polymorphism affects susceptibility to toxicity of other P‐glycoprotein substrate drugs has not been studied. Hypothesis: Dogs that possess the ABCB1‐1Δ mutation are more likely to develop hematologic toxicity associated with vincristine than ABCB1 wild‐type dogs. Animals: Thirty‐four dogs diagnosed with lymphoma were included in this study. Methods: Cheek swab samples were obtained from dogs diagnosed with lymphoma that were to be treated with vincristine. DNA was extracted from cheek swabs and the ABCB1 genotype was determined. Hematologic adverse drug reactions were recorded for each dog and graded according to the Veterinary Comparative Oncology Group's criteria for adverse event reporting (Consensus Document). In order to avoid possible bias, ABCB1 genotype results for a particular patient were not disclosed to oncologists until an initial adverse event report had been submitted. Results: Dogs heterozygous or homozygous for the ABCB1‐1Δ mutation were significantly more likely to develop hematologic toxicity, specifically neutropenia ( P = .0005) and thrombocytopenia ( P = .0001), after treatment with vincristine than ABCB1 wild‐type dogs. Conclusions and Clinical Implications: At currently recommended dosages (0.5–0.7 mg/M 2 ), vincristine is likely to cause hematologic toxicity in dogs with the ABCB1‐1Δ mutation, resulting in treatment delays and unacceptable morbidity and mortality. Assessing the ABCB1‐1Δ genotype before vincristine administration and decreasing the dosage may prevent toxicity and treatment delays resulting from neutropenia or thrombocytopenia.