Early-onset thrombocytopenia during combination chemotherapy in testicular cancer is induced by vinblastine.

Platelet kinetics were studied in 70 patients with testicular cancer to elicit the agent responsible for chemotherapy-induced transient early-onset thrombocytopenia; 204 treatment courses were analyzed using three different therapy protocols, which contained vinblastine and bleomycin either alone or in combination with cisplatin. Platelet count decreased significantly from the start of vinblastine administration reaching its nadir on the third day of therapy in each of the three treatment groups. Between day 4 and day 10 of the treatment cycle, platelet counts steadily increased even in patients still receiving continuous bleomycin infusions. The conclusion that the observed early-onset thrombocytopenia was caused by vinblastine was substantiated by the outcome of two additional examinations. Platelet half-life was significantly shortened 24 hours after vinblastine administration and electron microscopy revealed a dissolution of cytoplasmatic microtubules with loss of the typical discoid shape of platelets within 15 minutes after the start of therapy. Both findings occurred irrespective of the specific treatment protocol, i.e., even after nothing but vinblastine had been given. These results strongly suggest that vinblastine is the main cytostatic agent responsible for the transient early-onset thrombocytopenia observed during chemotherapy of testicular cancer.