Question 1

What is a PIK3CA mutation?

Accepted Answer

PIK3CA encodes the p110α catalytic subunit of PI3Kα. Hotspot mutations at H1047R, E542K, and E545K constitutively activate lipid kinase activity, generating PIP3 and driving AKT/mTOR signalling without upstream receptor input. These are gain-of-function oncogenic mutations.

Question 2

In which cancers are PIK3CA mutations common?

Accepted Answer

PIK3CA hotspot mutations occur in ~30% of HR+ breast cancers, ~20% of endometrial cancers, ~20% of head and neck cancers, and ~15% of colorectal cancers. They are among the most common oncogenic mutations across solid tumours.

Question 3

Can PIK3CA mutations be targeted with drugs?

Accepted Answer

Alpelisib is a PI3Kα-selective inhibitor approved in combination with endocrine therapy for PIK3CA-mutant HR+/HER2− breast cancer, improving progression-free survival in a biomarker-selected population. Resistance typically develops through KRAS mutations or receptor tyrosine kinase amplification.

Question 4

What is the structural difference between H1047R and E545K PIK3CA mutations?

Accepted Answer

H1047R is located in the kinase domain C-lobe (exon 20) and activates p110α by increasing membrane binding affinity — the arginine residue enhances electrostatic interaction with the negatively charged phospholipid bilayer, positioning p110α for more efficient PIP2 phosphorylation. E545K and E542K are in the helical domain (exon 9) and activate p110α by disrupting an inhibitory electrostatic interaction between the helical domain and the nSH2 domain of p85 regulatory subunit — the substitution of lysine for glutamate removes the charge-charge repulsion that normally keeps p110α partially inhibited. Both mutation types constitutively activate lipid kinase activity but through distinct structural mechanisms, which may have implications for inhibitor sensitivity profiles.

Question 5

Why does alpelisib cause hyperglycaemia and how is it managed?

Accepted Answer

Hyperglycaemia is an on-target mechanism-based toxicity of alpelisib: PI3Kα is essential for insulin receptor signalling in skeletal muscle and adipose tissue, where insulin normally activates PI3K→AKT→GLUT4 translocation to drive glucose uptake. Alpelisib inhibits this pathway in normal insulin-responsive tissues simultaneously with inhibiting it in PIK3CA-mutant tumour cells, reducing peripheral glucose uptake and increasing hepatic glucose output. The result is glucose elevations in ~64% of patients, with grade 3–4 hyperglycaemia in ~37%. Management includes pre-treatment fasting glucose assessment, prophylactic metformin (reduces severity and discontinuation rates), dietary carbohydrate restriction, and insulin/oral hypoglycaemic agents if needed. Hyperglycaemia is the primary dose-limiting toxicity driving treatment discontinuation.

Question 6

What combination strategies are being tested to overcome PIK3CA inhibitor resistance?

Accepted Answer

Several approaches address the primary resistance mechanisms to alpelisib: (1) Gedatolisib (pan-PI3K/mTOR inhibitor) + palbociclib + endocrine therapy — the VIKTORIA-1 phase III trial showed superior PFS by simultaneously blocking PI3K, mTOR, and the CDK4/6-driven G1 bypass that emerges during PI3K pathway inhibition. (2) CDK4/6 inhibitors (palbociclib, ribociclib) + alpelisib — combining PI3K inhibition with cell cycle blockade to prevent G1 re-entry from compensatory cyclin D1 stabilisation. (3) AKT inhibitors (capivasertib) for patients with concurrent PTEN loss who may not respond optimally to PI3Kα-selective agents. (4) EGFR/HER2 targeted therapy for resistance mediated by RTK amplification-driven compensatory PI3K activation.

PIK3CA Gene Function

Overview

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