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Alpelisib in triple-negative breast cancer: EPIK-B3 ends without a signal

OncologyEnrollmentMay 17th, 2026·5 min read·10.5281/zenodo.20301169

Novartis terminated the Phase 3 EPIK-B3 study of alpelisib plus nab-paclitaxel in PIK3CA-mutant or PTEN-loss metastatic TNBC. The randomized part showed numerically higher PFS for alpelisib but overlapping confidence intervals. The single-arm cohort produced a 14 percent ORR, below historical nab-paclitaxel monotherapy.

Mechanism Risk Score

ComponentPoints
Phase-weighted failure burden15.7 / 40
Archetype severity2.0 / 25
Temporal recency4.3 / 15
Genetic evidence deficit3.7 / 15
Programmatic saturation2.5 / 5

For PIK3CA in Advanced Triple-Negative Breast Cancer (PIK3CA-mutant or PTEN-loss), the Mechanism Risk Score is 28/100 (yellow band). The score is a failure-burden index derived from Claidex post-mortems on this target–disease pair, not a probability of approval.

MRS 28/100 (YELLOW). 1 programs across PIK3CA have been documented for PIK3CA in Advanced Triple-Negative Breast Cancer (PIK3CA-mutant or PTEN-loss): 1 Phase 3, 0 Phase 2, 0 Phase 1 — of which 0 were efficacy failures, 0 safety, 0 biomarker, and 1 operational (enrollment, sponsor, or funding). The most informative failure on file is Alpelisib in triple-negative breast cancer: EPIK-B3 ends without a signal. This score quantifies the documented failure burden; the Open Targets association score of 0.75 provides strong human-genetic anchoring that partially offsets the documented failure record. The MRS is not a prediction of future trial outcomes — it is a structured summary of the empirical record, recomputed live from the Claidex claims table, and intended to flag mechanisms where any new program must explicitly resolve each prior failure mode before pursuit is justified.

This score does not predict whether the next trial will succeed. It flags how heavy the documented mechanistic failure record is before a new program is justified.

What was tried

EPIK-B3 was a Phase 3, randomized, double-blind, placebo-controlled study of alpelisib (BYL719) plus nab-paclitaxel in advanced triple-negative breast cancer with either a PIK3CA activating mutation or PTEN loss without a PIK3CA mutation. The trial had two parts. Part A randomized 102 patients to alpelisib plus nab-paclitaxel or placebo plus nab-paclitaxel, with PFS by investigator assessment as the primary endpoint. Part B1 was a single-arm cohort of 35 patients with PTEN loss receiving the active doublet, with ORR as the primary endpoint. Total enrollment was 137 patients across both segments. NCT04251533 terminated in May 2026 with the trial sponsor citing persistently slow enrollment, and full results posted to the registry.

The trial sat in a difficult position from the start. Alpelisib was approved in 2019 for HR-positive HER2-negative PIK3CA-mutant metastatic breast cancer based on SOLAR-1, and Novartis ran several Phase 3 expansion programs to test whether the PI3K-alpha signal would extend to other breast cancer subtypes and combinations. EPIK-B3 was the TNBC arm of that effort.

The biological hypothesis

The PI3K-AKT-mTOR pathway is the second-most-mutated signaling axis in breast cancer after the estrogen receptor pathway. PIK3CA encodes the p110-alpha catalytic subunit of class I PI3K. Hotspot mutations at E542K, E545K, and H1047R produce constitutive PI3K activity, increased PIP3 generation, and downstream AKT and mTORC1 activation. PTEN loss removes the lipid phosphatase that opposes PI3K, producing a phenocopy of PIK3CA activation. Martinez-Saez and colleagues catalogued PIK3CA mutation frequencies of 35 to 45 percent in HR-positive disease and 8 to 12 percent in TNBC, with PTEN loss adding another 10 to 15 percent of the TNBC population to the addressable pool.

The TNBC-specific rationale rested on Lehmann and Pietenpol's subtype work, which identified a luminal androgen receptor (LAR) subtype and a mesenchymal stem-like subtype with PI3K pathway activation. Preclinical models supported the strategy. Yuan and colleagues showed BYL719 synergy with the CDK4/6 inhibitor ribociclib in TNBC cell lines, and Boyd et al. found alpelisib activity in basal-like PDX models with PI3K pathway activation. The clinical hypothesis was that adding p110-alpha blockade to nab-paclitaxel, the standard first-line cytotoxic, would reverse the chemoresistance attributed to constitutive PI3K signaling.

What actually happened

Part A produced overlapping PFS distributions. Median PFS was 7.2 months (95% CI 3.71 to 8.57) on alpelisib plus nab-paclitaxel and 5.6 months (95% CI 3.75 to 12.68) on placebo plus nab-paclitaxel. The wide and overlapping confidence intervals reflect the small sample size and heterogeneous time-to-progression in TNBC. Part B1 single-arm activity was modest. Five of 35 patients (14.3 percent, 95% Wilson CI 4.8 to 30.3 percent) achieved an objective response. Serious adverse events occurred in 42 percent of alpelisib-treated patients in Part A versus 30 percent on placebo, driven by alpelisib's expected hyperglycemia, rash, and diarrhea profile.

PFS forest plot for alpelisib in TNBC versus SOLAR-1, with single-arm ORR placed against mTNBC benchmarks. Sources: NCT04251533 results, Andre 2019 NEJM SOLAR-1, Cortes 2022 NEJM KEYNOTE-355.

The numbers should be read against two benchmarks. Single-agent nab-paclitaxel in metastatic TNBC has historically produced ORRs of 25 to 40 percent in first and second line. Pembrolizumab plus chemotherapy in PD-L1-positive mTNBC (CPS at least 10), the regimen that became standard of care after KEYNOTE-355, produced ORRs in the 50 to 55 percent range. EPIK-B3 Part B1 activity was below the nab-paclitaxel monotherapy benchmark.

Failure mechanism, best guess

Three explanations carry weight.

First, the biomarker probably fails to identify pathway-dependent disease in TNBC the way it does in HR-positive disease. In HR-positive breast cancer, PIK3CA mutations are early oncogenic events on a relatively quiet genomic background, and the tumor remains addicted to PI3K signaling. TNBC carries a much higher tumor mutational burden, frequent TP53 loss, and frequent MYC amplification. PIK3CA mutations in this setting may be passengers or one of several parallel drivers. Patient-derived xenograft work by Boyd et al. found that PI3K-overactivated basal-like PDXs required combination partners, not single-agent PI3K inhibition, to produce regressions.

Second, the AKT axis has been hostile in TNBC for the same reason. The IPATunity130 and CO40016 Phase 3 programs for ipatasertib failed in TNBC despite a clean biomarker hypothesis. The FAIRLANE neoadjuvant biomarker analysis (Shi et al. 2022) showed that AKT-pathway protein activation in tumor tissue did not predict ipatasertib benefit, suggesting the predictive validity of PI3K/AKT pathway biomarkers in TNBC is weak.

Third, the enrollment problem itself is informative. KEYNOTE-355 shifted first-line mTNBC to immunotherapy plus chemotherapy for PD-L1-positive disease. Approximately 40 percent of metastatic TNBC is PD-L1 CPS at least 10, and the rest still typically receives chemotherapy with sacituzumab govitecan or trastuzumab deruxtecan in HER2-low disease. The slice of metastatic TNBC available for a PIK3CA-mut or PTEN-loss enrollment shrank during the trial. Alpelisib's hyperglycemia profile, which Rodon and colleagues quantified at a 30 to 40 percent grade 3 incidence, made the trial unattractive when easier-to-tolerate alternatives became available.

What this means for similar programs

The PI3K-alpha axis as a stand-alone target in TNBC is effectively closed. Inavolisib, which Novartis-affiliate Genentech progressed in combination with palbociclib and fulvestrant in HR-positive disease, has shown no TNBC signal of note. The more promising approach is combination. Batalini and colleagues reported a 36 percent ORR in PIK3CA-mutant TNBC with alpelisib plus olaparib, suggesting PARP plus PI3K-alpha may be the route forward for the genomically unstable fraction. CDK4/6 plus AKT inhibition has shown selective preclinical activity in the LAR subtype of TNBC (Chica-Parrado et al. 2024). Sponsors should design TNBC trials with combination intent, mandatory PD-L1 stratification, and consideration of biomarker overlap with the standard-of-care chemoimmunotherapy backbone.

Open questions

  • Does PIK3CA mutation status in TNBC predict response to any PI3K-axis intervention, or is the variant a passenger on a more chaotic driver landscape?
  • Is the LAR subtype the only TNBC population where PI3K and AKT inhibitors retain biology-driven benefit?
  • Can selective protein degradation of mutant p110-alpha (a Y110 mutant-selective approach) avoid the hyperglycemia ceiling that capped alpelisib's tolerability?
  • Does combining a PI3K-alpha inhibitor with sacituzumab govitecan or trastuzumab deruxtecan, the new TNBC second-line agents, recover the signal in a more contemporary patient population?

Sources

  1. André F, Ciruelos E, Rubovszky G, Campone M, Loibl S, Rugo HS, et al.. Alpelisib for PIK3CA-Mutated, Hormone Receptor-Positive Advanced Breast Cancer. N Engl J Med. 2019;380(20);1929-1940. PMID: 31091374.

  2. André F, Ciruelos EM, Juric D, Loibl S, Campone M, Mayer IA, et al.. Alpelisib plus fulvestrant for PIK3CA-mutated, hormone receptor-positive, human epidermal growth factor receptor-2-negative advanced breast cancer: final overall survival results from SOLAR-1. Ann Oncol. 2021;32(2);208-217. PMID: 33246021.

  3. Rugo HS, André F, Yamashita T, Cerda H, Toledano I, Stemmer SM, et al.. Time course and management of key adverse events during the randomized phase III SOLAR-1 study of PI3K inhibitor alpelisib plus fulvestrant in patients with HR-positive advanced breast cancer. Ann Oncol. 2020;31(8);1001-1010. PMID: 32416251.

  4. Rodón J, Demanse D, Rugo HS, Burris HA, Simó R, Farooki A, et al.. A risk analysis of alpelisib-induced hyperglycemia in patients with advanced solid tumors and breast cancer. Breast Cancer Res. 2024;26(1);36. PMID: 38439079.

  5. Martínez-Sáez O, Chic N, Pascual T, Adamo B, Vidal M, González-Farré B, et al.. Frequency and spectrum of PIK3CA somatic mutations in breast cancer. Breast Cancer Res. 2020;22(1);45. PMID: 32404150.

  6. Lehmann BD, Bauer JA, Chen X, Sanders ME, Chakravarthy AB, Shyr Y, et al.. Identification of human triple-negative breast cancer subtypes and preclinical models for selection of targeted therapies. J Clin Invest. 2011;121(7);2750-2767. PMID: 21633166.

  7. Yuan Y, Wen W, Yost SE, Xing Q, Yan J, Han ES, et al.. Combination therapy with BYL719 and LEE011 is synergistic and causes a greater suppression of p-S6 in triple negative breast cancer. Sci Rep. 2019;9(1);7509. PMID: 31101835.

  8. Boyd DC, Zboril EK, Olex AL, Leftwich TJ, Hairr NS, Byers HA, et al.. Discovering Synergistic Compounds with BYL-719 in PI3K Overactivated Basal-like PDXs. Cancers (Basel). 2023;15(5). PMID: 36900375.

  9. Batalini F, Xiong N, Tayob N, Polak M, Eismann J, Cantley LC, et al.. Phase 1b Clinical Trial with Alpelisib plus Olaparib for Patients with Advanced Triple-Negative Breast Cancer. Clin Cancer Res. 2022;28(8);1493-1499. PMID: 35149538.

  10. Cortes J, Rugo HS, Cescon DW, Im SA, Yusof MM, Gallardo C, et al.. Pembrolizumab plus Chemotherapy in Advanced Triple-Negative Breast Cancer. N Engl J Med. 2022;387(3);217-226. PMID: 35857659.

  11. Schmid P, Turner NC, Barrios CH, Isakoff SJ, Kim SB, Sablin MP, et al.. First-Line Ipatasertib, Atezolizumab, and Taxane Triplet for Metastatic Triple-Negative Breast Cancer: Clinical and Biomarker Results. Clin Cancer Res. 2024;30(4);767-778. PMID: 38060199.

  12. Shi Z, Wulfkuhle J, Nowicka M, Gallagher RI, Saura C, Nuciforo PG, et al.. Functional Mapping of AKT Signaling and Biomarkers of Response from the FAIRLANE Trial of Neoadjuvant Ipatasertib plus Paclitaxel for Triple-Negative Breast Cancer. Clin Cancer Res. 2022;28(5);993-1003. PMID: 34907082.

  13. Chica-Parrado MR, Kim GM, Uemoto Y, Napolitano F, Lin CC, Ye D, et al.. Combined inhibition of CDK4/6 and AKT is highly effective against the luminal androgen receptor (LAR) subtype of triple negative breast cancer. Cancer Lett. 2024;604;217219. PMID: 39244005.

  14. ClinicalTrials.gov. NCT04251533. A Phase III Study to Assess the Efficacy and Safety of Alpelisib in Combination With Nab-paclitaxel in Subjects With Advanced TNBC Who Carry Either a PIK3CA Mutation or Have PTEN Loss. Terminated 2026. https://clinicaltrials.gov/study/NCT04251533.

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