Targeting RET Solvent-Front Mutants with Alkynyl Nicotinamide-Based Inhibitors

Selpercatinib (LOXO292) and pralsetinib (BLU667) are RET protein tyrosine kinase inhibitors (TKIs) recently approved for treating RET-altered cancers. However, RET mutations that con-fer selpercatinib/pralsetinib resistance have been identified, neces-sitating development of next-generation RET TKIs. While acquired RET G810C/R/S/V mutations were reported in selpercatinib-treated patients, it was unclear whether all of these and other potential G810 mutants are resistant to selpercatinib and pralseti-nib. Here, we profiled selpercatinib and pralsetinib on all six possible G810 mutants derived from single nucleotide substitu-tion and developed novel alkynyl nicotinamide-based RET TKIs to inhibit selpercatinib/pralsetinib-resistant RET G810 mutants. Surprisingly, the G810V mutant found in a clinical study was not resistant to selpercatinib or pralsetinib. Besides G810C/R/S, G810D also conferred selpercatinib/pralsetinib resistance. Alkynyl nicotinamide compounds such as HSN608, HSL476, and HSL468 have better drug-like properties than alkynyl benzamides. Six of these compounds inhibited all six G810 solvent-front mutants and the V804M gatekeeper mutant with IC50 < 50 nmol/L in cell culture. Oral administration of HSN608 at a well-tolerated dose (30 mg/kg) gave plasma level > 30x the IC50s of inhibiting all G810 mutants in cell culture. In cell-derived xenograft tumors driven by KIF5B-RET (G810C) that contains the most frequently observed solvent-front mutant in selpercatinib-treated patients, HSN608, HSL476, and HSL468 significantly suppressed and caused regression of the selpercatinib-resistant tumors. This study clarifies the sensitivities of different RET solvent-front mu-tants to selpercatinib and pralsetinib and identifies novel alkylnyl nicotinamide-based RET TKIs for inhibiting selpercatinib/ pralsetinib-resistant G810 mutants.

Automated summary

Selpercatinib and pralsetinib are approved RET kinase inhibitors for treating RET-altered cancers, but resistance mutations have been identified. This study investigates the sensitivities of different G810 mutants to these inhibitors and identifies new compounds for inhibiting resistant mutations.