Background:Birinapant is a novel small molecule Smac-mimetic that targets members of the inhibitor of apoptosis proteins (cIAP1, cIAP2 and XIAP) involved in the blockade of apoptosis. A population PK model was developed to characterize the interpatient variability in birinapant PK and to evaluate the effect of multiple combination regimens on birinapant disposition and safety.
Methods:Birinapant was administered alone or in combination to 114 patients (55M/59F; 89% Caucasian) with advanced malignancies. Birinapant was administrated by a 30 min IV infusion QW alone (30 pats), or approx. 30 min. after chemotherapy with irinotecan (19 pats), docetaxel (20 pats), gemcitabine (17 pats), liposomal doxorubicin (13 pats), or paclitaxel/carboplatin (15 pats). Birinapant dose levels ranged from 0.18 to 35 mg/m2. Population PK modeling was performed to investigate the effect of the following patient covariates: [BW (38.5-127.5 kg), age (27.5-86.0 yrs), CrCL (36.4-219.2 ml/min), ALT (6-121 IU/L), and TBIL (0.1-1.7 mg/dL)].
Results:A 3-compartment PK model described the time course of birinapant disposition with predicted values for T1/2, CL, and Vd of 40 h, 21 L/h and 10.2 L, respectively. Birinapant displayed linear PK across the dose range with no significant accumulation in plasma following weekly dosing. Goodness of fit plots supported the model fit, with residual variability of 23%. The PK of birinapant remained unchanged when combined with irinotecan, docetaxel, gemcitabine and liposomal doxorubicin. Concomitant administration with paclitaxel/carboplatin resulted in a 2-fold increase in birinapant AUC possibly due to reduced OATP1B3 mediated tissue uptake.
Conclusions:These data show that birinapant possesses an excellent PK profile with dose proportional kinetics, a long terminal half-life for target coverage, low/moderate interpatient variability in CL and no significant accumulation following weekly dosing. Importantly, the PK of birinapant remained unchanged when combined with multiple chemotherapy regimens and the increased exposure with paclitaxel/carboplatin was not associated with any change in birinapant tolerability.