Identification, characterization, and implications of species-dependent plasma protein binding for the oral Hedgehog...

Vismodegib (GDC-0449) is is an orally available selective Hedgehog pathway inhibitor in development for cancer treatment. The drug is ≥95% protein bound in plasma at clinically relevant concentrations and has an approximately 200-fold longer single dose half-life in humans than rats. We have identified a strong linear relationship between plasma drug concentrations and α-1-acid glycoprotein (AAG) in a phase I study. Biophysical and cellular techniques have been used to reveal that vismodegib strongly binds to human AAG (K(D) = 13 μM) and binds albumin with lower affinity (K(D) = 120 μM). Additionally, binding to rat AAG is reduced ∼20-fold relative to human, whereas the binding affinity to rat and human albumin was similar. Molecular docking studies reveal the reason for the signficiant species dependence on binding. These data highlight the utility of biophysical techniques in creating a comprehensive picture of protein binding across species.