Plasma PIK3CA Mutation Testing and the Selection of PI3K–AKT Pathway Inhibitors in HR+ Breast Cancer

The diagnostic and clinical question

Hormone receptor–positive, HER2-negative (HR+/HER2−) breast cancer is the most common form of the disease, and a substantial fraction of these tumors carry activating mutations in PIK3CA, the gene encoding the p110α catalytic subunit of phosphatidylinositol 3-kinase (PI3K). Mutations in this gene drive signaling through the PI3K–AKT–mTOR axis, a pathway implicated in endocrine therapy resistance. Approximately 40% of HR+ patients in the SOLAR-1 program carried a PIK3CA mutation [1].

The clinical question is a predictive one: can we identify, from a patient's tumor, a molecular alteration that makes them more likely to benefit from a drug that targets this pathway? A blood-based (plasma) test that detects PIK3CA mutations offers a minimally invasive route to that answer, and it is this predictive use — eligibility for a class of pathway inhibitors — that this article examines. Nothing here constitutes treatment advice; decisions about any individual regimen rest with the treating team.

What the assay is and how it works

The analyte in this setting is circulating tumor DNA (ctDNA) — fragments of tumor-derived DNA shed into the bloodstream and recovered from the cell-free DNA (cfDNA) compartment of plasma. Because tumor DNA is mixed with far larger quantities of DNA from normal cells, the assays must be sensitive enough to detect a mutant allele present at a low fraction of total cfDNA. Methods include allele-specific PCR and next-generation sequencing panels designed to interrogate the recurrent "hotspot" PIK3CA mutations that characterize breast cancer.

A plasma genotyping result can establish the presence of a specific PIK3CA mutation when ctDNA is detectable and the variant is above the assay's limit of detection. What it cannot reliably do is exclude the mutation: a negative plasma result may reflect a genuinely wild-type tumor, or it may reflect low tumor DNA shedding — some tumors release little ctDNA, particularly in lower-burden disease. For that reason, plasma and tissue testing are generally regarded as complementary rather than interchangeable, and a negative plasma test often prompts tissue confirmation. The clinical trials that established the role of these inhibitors permitted PIK3CA status to be ascertained from tissue and, in the AKT-inhibitor program, incorporated plasma testing into eligibility and biomarker analyses.

The evidence

SOLAR-1 (alpelisib). SOLAR-1 was a phase 3 randomized trial of alpelisib, an α-specific PI3K inhibitor, combined with fulvestrant versus fulvestrant plus placebo in men and postmenopausal women with HR+/HER2− advanced breast cancer previously treated with endocrine therapy [1]. The trial prospectively defined a PIK3CA-mutated cohort (n = 341) and a cohort without mutation. In the mutated cohort, median progression-free survival (PFS) was 11.0 months with alpelisib–fulvestrant versus 5.7 months with placebo–fulvestrant (hazard ratio for progression or death 0.65; 95% CI, 0.50–0.85; P<0.001) [1]. Importantly, benefit was confined to the mutated population — in the cohort without PIK3CA mutation there was no meaningful PFS advantage — which is precisely what makes PIK3CA status a predictive, not merely prognostic, biomarker [1]. SOLAR-1 also analyzed ctDNA: PIK3CA mutations could be identified in plasma, and the treatment benefit was observed among patients whose mutation was detectable in circulating tumor DNA, supporting the feasibility of plasma-based selection [1]. On the safety side, hyperglycemia and rash were notable toxicities, reflecting on-target effects of α-selective PI3K inhibition [1].

CAPItello-291 (capivasertib). CAPItello-291 tested capivasertib, an inhibitor of AKT (the kinase immediately downstream of PI3K), added to fulvestrant in HR+/HER2− advanced breast cancer that had progressed during or after aromatase-inhibitor therapy [2]. The trial enrolled an overall population and a prospectively defined subgroup with AKT-pathway alterations — tumors harboring alterations in PIK3CA, AKT1, or PTEN [2]. In the overall population (708 patients), median PFS was 7.2 months with capivasertib–fulvestrant versus 3.6 months with placebo–fulvestrant (hazard ratio 0.60; 95% CI, 0.51–0.71; P<0.001) [2]. In the biomarker-altered population (289 patients), median PFS was 7.3 versus 3.1 months (hazard ratio 0.50; 95% CI, 0.38–0.65; P<0.001) [2]. Diarrhea and rash were common adverse events [2]. CAPItello-291 is notable for broadening the relevant biomarker beyond PIK3CA alone to the composite PI3K–AKT–PTEN pathway, and for its use of tissue and circulating tumor DNA to determine alteration status [2].

INAVO120 (inavolisib). INAVO120 evaluated inavolisib, a highly selective PI3Kα inhibitor, in a different clinical setting: patients with PIK3CA-mutated, HR+/HER2− advanced breast cancer whose disease had recurred during or within 12 months of completing adjuvant endocrine therapy — a population with aggressive, endocrine-resistant biology [3]. Inavolisib was combined with palbociclib (a CDK4/6 inhibitor) and fulvestrant and compared against placebo plus palbociclib and fulvestrant [3]. Among 325 randomized patients, median PFS was 15.0 months in the inavolisib arm versus 7.3 months in the placebo arm (hazard ratio for progression or death 0.43; 95% CI, 0.32–0.59; P<0.001) [3]. PIK3CA mutation status for eligibility could be determined from tumor tissue or ctDNA [3]. As with the other agents, hyperglycemia, stomatitis, and diarrhea featured among adverse events [3].

Taken together, these three randomized phase 3 trials consistently show that targeting the PI3K–AKT pathway improves PFS in biomarker-selected HR+/HER2− advanced breast cancer, and that the biomarker — PIK3CA mutation (SOLAR-1, INAVO120) or the broader PIK3CA/AKT1/PTEN alteration set (CAPItello-291) — defines the population expected to benefit [1,2,3].

Where it stands

The evidence supports plasma (ctDNA) detection of PIK3CA mutations as a legitimate route to identify patients eligible for PI3K–AKT pathway inhibitor classes in HR+/HER2− advanced breast cancer, with all three pivotal trials incorporating circulating tumor DNA into their biomarker frameworks [1,2,3]. The key practical limitation is the asymmetry of a plasma result: a detected mutation is informative and actionable for eligibility, whereas a non-detected mutation does not exclude a tissue-detectable mutation, because tumors differ in how much DNA they shed into blood. Tissue testing therefore remains an important confirmatory pathway when plasma is negative.

Several open questions remain. The optimal first-line versus later-line positioning of these agents differs across the trials — SOLAR-1 and CAPItello-291 enrolled endocrine-pretreated advanced disease, while INAVO120 targeted early-relapsing disease with a three-drug combination [1,2,3]. The biomarker definitions are not identical: alpelisib and inavolisib were studied in PIK3CA-mutated disease, whereas capivasertib used a composite PI3K–AKT–PTEN alteration panel [1,2,3]. Clinicians interpreting a plasma report must therefore match the specific alteration detected to the specific agent's evidence base. Finally, the metabolic and dermatologic toxicities associated with on-target pathway inhibition underscore that biomarker positivity establishes eligibility, not an individualized recommendation — a distinction that belongs at the center of any clinical conversation.

References

1. André F, et al. Alpelisib for PIK3CA-Mutated, Hormone Receptor–Positive Advanced Breast Cancer (SOLAR-1). N Engl J Med. 2019. PMID 31091374; DOI 10.1056/NEJMoa1813904.
2. Turner NC, et al. Capivasertib in Hormone Receptor–Positive Advanced Breast Cancer (CAPItello-291). N Engl J Med. 2023. PMID 37256976; DOI 10.1056/NEJMoa2214861.
3. Turner NC, et al. Inavolisib-Based Therapy in PIK3CA-Mutated Advanced Breast Cancer (INAVO120). N Engl J Med. 2024. PMID 39476340; DOI 10.1056/NEJMoa2404625.