Pharmacokinetics of hedgehog pathway inhibitor vismodegib (GDC-0449) in patients with locally advanced or metastatic solid tumors: The role of alpha-1-acid glycoprotein binding

Purpose: In a phase I trial for patients with refractory solid tumors, hedgehog pathway inhibitor vismodegib (GDC-0449) showed little decline in plasma concentrations over 7 days after a single oral dose and nonlinearity with respect to dose and time after single and multiple dosing. We studied the role of GDC-0449 binding to plasma protein alpha-1-acid glycoprotein (AAG) to better understand these unusual pharmacokinetics. Experimental Design: Sixty-eight patients received GDC-0449 at 150 (n = 41), 270 (n = 23), or 540 (n =4) mg/d, with pharmacokinetic (PK) sampling at multiple time points. Total and unbound (dialyzed) GDC-0449 plasma concentrations were assessed by liquid chromatography/tandem mass spectrometry, binding kinetics by surface plasmon resonance-based microsensor, and AAG levels by ELISA. Results: A linear relationship between total GDC-0449 and AAG plasma concentrations was observed across dose groups (R² = 0.73). In several patients, GDC-0449 levels varied with fluctuations in AAG levels over time. Steady-state, unbound GDC-0449 levels were less than 1% of total, independent of dose or total plasma concentration. In vitro, GDC-0449 binds AAG strongly and reversibly (K ^D =13 μmol/L) and human serum albumin less strongly (K^D =120 μmol/L). Simulations from a derived mechanistic PK model suggest that GDC-0449 pharmacokinetics are mediated by AAG binding, solubility-limited absorption, and slow metabolic elimination. Conclusions: GDC-0449 levels strongly correlated with AAG levels, showing parallel fluctuations of AAG and total drug over time and consistently low, unbound drug levels, different from previously reported AAG-binding drugs. This PK profile is due to high-affinity, reversible binding to AAG and binding to albumin, in addition to solubility-limited absorption and slow metabolic elimination properties.