RTK-dependent inducible degradation of mutant PI3K alpha drives GDC-0077 (Inavolisib) efficacy

PIK3CA is one of the most frequently mutated oncogenes; the p110 protein it encodes plays a central role in tumor cell proliferation and survival. Small molecule inhibitors targeting the PI3K p110 catalytic subunit have entered clinical trials, with early-phase GDC-0077 (Inavolisib) studies showing anti-tumor activity and a manageable safety profile in patients with PIK3CA-mutant, hormone receptor-positive breast cancer as a single agent or in combination therapy. Despite this, preclinical studies have shown that PI3K pathway inhibition releases negative feedback and activates receptor tyrosine kinase signaling, reengaging the pathway and attenuating drug activity. Here we discover that GDC-0077 and taselisib more potently inhibit mutant PI3K pathway signaling and cell viability through unique HER2-dependent degradation. Both are more effective than other PI3K inhibitors at maintaining prolonged pathway suppression, resulting in enhanced apoptosis and greater efficacy. This unique mechanism against mutant p110 reveals a new strategy for creating inhibitors that specifically target mutant tumors with selective degradation of the mutant oncoprotein and also provide a strong rationale for pursuing PI3K degraders in patients with HER2-positive breast cancer.