Inhibition mechanism exploration of investigational drug TAK-441 as inhibitor against Vismodegib-resistant Smoothened mutant

Hedgehog signaling is a driving force in medulloblastoma and basal cell carcinoma (BCC), making it an attractive therapeutic target. Vismodegib recently received FDA approval for the treatment of inoperable BCC, but a drug-resistant Smoothened (Smo) mutant (D473H) was identified in a clinical study. TAK-441 is a pyrrolo[3,2-c]pyridine-4-one derivative that potently inhibits Hh signal transduction and is currently under investigation in clinical trials. We demonstrated that TAK-441 inhibits reporter activity in D473H-transfected cells with an IC50 of 79 nM, while Vismodegib showed an IC50=7100 nM. In order to investigate the mode of inhibition, we evaluated the Smo inhibitors with three different binding assays, such as [3H]–TAK-441 membrane binding assay, affinity selection-MS detection assay, and bodipy-cylopamine whole cell assay. In three different assays, Vismodegib and cyclopamine showed lower affinity for the D473H mutant in comparison with wild-type Smo. On the other hand, TAK-441 showed almost equal binding affinity for the D473H mutant compared with wild-type Smo in the binding assays, although TAK-441 binds to the same binding site as two other well-known inhibitors. These in vitro findings suggest that TAK-441 has the potential for clinical use in cancers that are dependent on Hedgehog signaling, including wild-type tumors and Vismodegib-resistant D473H mutants.

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