RTK-dependent inducible degradation of mutant PI3Kα drives GDC-0077 (Inavolisib) efficacy.

PIK3CA is one of the most frequently mutated oncogenes; the p110α protein it encodes plays a central role in tumor cell proliferation. Small molecule inhibitors targeting the PI3K p110α catalytic subunit have entered clinical trials, with early-phase GDC-0077 studies showing anti-tumor activity and a manageable safety profile in patients with PIK3CA-mutant breast cancer. However, 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 mutant p110a degradation. Both are more effective than other PI3K inhibitors at maintaining prolonged pathway suppression. This study establishes a new strategy for identifying inhibitors that specifically target mutant tumors by selective degradation of the mutant oncoprotein and provide a strong rationale for pursuing PI3Kα degraders in patients with HER2-positive breast cancer.