A phase 8 and pharmacologic trial of two schedules of the proteasome inhibitor, PS-341 (bortezomib ,velcade), in patients with advanced cancer

Purpose: To define the toxicities, pharmacodynamics, and clinical activity of the proteasome inhibitor, PS-341 (bortezomib), inpatients with advanced malignancies. Patients and Methods: Twenty-eight patients (14 male and 14 female) received PS-341 twice weekly for 4 of 6 weeks (schedule I). Because toxicity necessitated dose omissions on this schedule, 16 additional patients (12 male and 4 female) received PS-341 twice weekly for 2 of every 3 weeks (schedule II). A total of 73 courses of treatment was given (median, 2; range, 1-4). Inhibition of 20S proteasome activity in peripheral blood mononuclear cells (PBMC) and accumulation of proteasome-targeted polypeptides in tumor tissue were evaluated as pharmacodynamic markers of PS-341 activity. Results: The most common toxicity was thrombocytopenia, which was dose limiting at 1.7 mg/m(2) (schedule I) and 1.6 mg/m2 (schedule II), respectively. Sensory neuropathy was dose-limiting in a patient in schedule I. Grade >= 3 toxicities for schedule I were constipation, fatigue, myalgia, and sensory neuropathy. Grade >= 3 toxicities for schedule II were dehydration resulting from diarrhea, nausea and vomiting, fatigue, hypoglycemia, and hypotension. The maximum tolerated dose was 1.5 mg/m(2) for both schedules. Reversible dose-dependent decreases in 20S proteasome activity in PBMCs were observed, with 36% inhibition at 0.5 mg/m(2), 52% at 0.9 mg/m(2), and 75% at 1.25 mg/m(2). Accumulation of proteasome-targeted polypeptides was detected in tumor samples after treatment with PS-341. A patient with multiple myeloma had a partial response. Conclusion: PS-341 given 1.5 mg/m(2) twice weekly for 2 of every 3 weeks is well tolerated and should be further studied.