Pharmacokinetic and pharmacodynamic study of two doses of bortezomib in patients with relapsed multiple myeloma

Purpose: Characterize bortezomib pharmacokinetics/pharmacodynamics in relapsed myeloma patients after single and repeat intravenous administration at two doses. Methods: Forty-two patients were randomized to receive bortezomib 1.0 or 1.3 mg/m², days 1, 4, 8, 11, for up to eight 21-day treatment cycles (n = 21, each dose group). Serial blood samples for pharmacokinetic/ pharmacodynamic analysis were taken on days 1 and 11, cycles 1 and 3. Observational efficacy and safety data were collected. Results: Twelve patients in each dose group were evaluable for pharmacokinetics/pharmacodynamics. Plasma clearance decreased with repeat dosing (102-112 L/h for first dose; 15-32 L/h following repeat dosing), with associated increases in systemic exposure and terminal half-life. Systemic exposures of bortezomib were similar between dose groups considering the relatively narrow dose range and the observed pharmacokinetic variability, although there was no readily apparent deviation from dose-proportionality. Blood 20S proteasome inhibition profiles were similar between groups with mean maximum inhibition ranging from 70 to 84% and decreasing toward baseline over the dosing interval. Response rate (all 42 patients) was 50%, including 7% complete responses. The safety profile was consistent with the predictable and manageable profile previously established; data suggested milder toxicity in the 1.0 mg/m² group. Conclusions: Bortezomib pharmacokinetics change with repeat dose administration, characterized by a reduction in plasma clearance and associated increase in systemic exposure. Bortezomib is pharmacodynamically active and tolerable at 1.0 and 1.3 mg/m² doses, with recovery toward baseline blood proteasome activity over the dosing interval following repeat dose administration, supporting the current clinical dosing regimen.