The pharmacokinetic-pharmacodynamic (PKPD) relationships of AZD3229, a novel and selective inhibitor of cKIT, in a range of mouse xenograft models of GIST.

Purpose: The emergence of secondary mutations is a cause of resistance to current KIT inhibitors used in the treatment of patients with gastrointestinal stromal tumors (GIST). AZD3229 is a selective inhibitor of wild-type KIT and a wide spectrum of primary and secondary mutations seen in GIST patients. The objective of this analysis is to establish the pharmacokinetic-pharmacodynamic (PKPD) relationship of AZD3229 in a range of mouse GIST tumor models harboring primary and secondary KIT mutations, and to benchmark AZD3229 against other KIT inhibitors. Experimental Design: A PKPD model was developed for AZD3229 linking plasma concentrations to inhibition of phosphorylated KIT using data generated from several invivo preclinical tumor models, and in vitro data generated in a panel of Ba/F3 cell-lines. Results: AZD3229 drives inhibition of phosphorylated KIT (pKIT) in an exposure-dependent manner, and optimal efficacy is observed when >90% inhibition of KIT phosphorylation is sustained over the dosing interval. Integrating the predicted human pharmacokinetics into the mouse PKPD model predicts that an oral twice daily human dose greater than 34 mg is required to ensure adequate coverage across the mutations investigated. Benchmarking shows that compared to SoC KIT inhibitors, AZD3229 has the potential to deliver the required target coverage across a wider spectrum of primary or secondary mutations. Conclusions: We demonstrate that AZD3229 warrants clinical investigation as a new treatment for GIST patients based on its ability to inhibit both ATP-binding and A-loop mutations of KIT at clinically relevant exposures.