EFFECTS OF VINCRISTINE AND PREDNISONE ON PLATELET NUMBERS AND FUNCTION IN CLINICALLY NORMAL DOGS

Effects of a single Iv administered therapeutic dose of vincristine su lfate on platelet numbers and function were evaluated in 16 clinically normal dog; over the 2 weeks after drug administration. Results were statistically compared with those of a previous control study in which the same 16 dogs were administered saline solution (IV), instead of v incristine. Of the 16 dogs, 8 were orally administered daily immunosup pressive doses of prednisone concurrently throughout the saline-contro l and vincristine study periods. Platelet numbers and mean platelet vo lume were measured, using an automated hematology analyzer. Platelet f unction was evaluated by turbidimetric measurement of platelet aggrega tion in response to collagen, platelet-activating factor, and adenosin e diphosphate (ADP), and by dot retraction (diluted whole-blood method ) and buccal mucosa bleeding time. Vincristine had a significant (P < 0.05) effect on circulating platelet numbers. Vincristine induced a tr ansient mild decrease in platelet numbers, followed by a moderate incr ease in numbers, with peak platelet count observed 8 days after drug a dministration. Mean platelet volume was not significantly affected by administration of vincristine. Vincristine had no significant effects on platelet aggregation in response to collagen, low or high doses of platelet-activating factor, and a high dose of ADP. The maximal degree of platelet aggregation attained in response to a low dose of ADP was not significantly affected by prior administration of vincristine. Th e maximal rate of platelet aggregation induced by a low dose of ADP af ter vincristine administration, however, was significantly (P < 0.05) lower than the rate of aggregation induced by a similar dose of ADP in the previous control study. Vincristine had no significant effects on clot retraction and bleeding time. Prednisone did not significantly a ffect platelet numbers and function, and did not modify vincristine's effects on the same variables.