A New Triple Therapy in Breast Cancer: Zovegalisib–Atirmociclib–Fulvestrant
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Publication date: 27 April 2026 · Data cutoff: 13 April 2026
Study name: First-in-human study of mutant-selective PI3Kα inhibitor RLY-2608 (zovegalisib) + endocrine therapy ± CDK4/6 or CDK4 inhibitor
Target patient group: PIK3CA-mutant, HR+, HER2−, advanced/metastatic breast cancer (previously treated with a CDK4/6 inhibitor)
In the treatment landscape of hormone receptor-positive (HR+), HER2-negative metastatic breast cancer, the PI3Kα pathway has long been an important but challenging target. Although approximately 30-40% of patients have a PIK3CA mutation , drugs targeting this pathway have struggled to provide an acceptable tolerability profile — particularly regarding hyperglycemia and gastrointestinal adverse effects. Now, new data from Relay Therapeutics bring a new generation onto the stage: zovegalisib (RLY-2608), a mutant-selective PI3Kα inhibitor; atirmociclib (PF-07220060), a selective CDK4 inhibitor; and fulvestrant or an aromatase inhibitor. In the dose-finding phase of the ReDiscover phase 1 study, a response rate of 44% and tumor shrinkage rate of 85% were reported in 34 predominantly pretreated patients. More strikingly, although 47% of patients were prediabetic , no grade 3 hyperglycemia was observed. These findings provide the main motivation for a phase 3 study planned to begin in 2027.
Why Is the PI3Kα Pathway So Challenging? The Clinical Meaning of a PIK3CA Mutation
PIK3CA mutations are among the most common driver mutations in HR+/HER2− metastatic breast cancer. They contribute to tumor cells developing resistance to endocrine therapy and to cancer progression. PI3Kα inhibitors such as alpelisib (Piqray) have previously received approval, but hyperglycemia, diarrhea, and rash prevented a substantial proportion of patients from tolerating or continuing treatment. Consequently, frontline positioning of PI3Kα-targeted therapy remained limited.
THE CONCEPT OF MUTANT-SELECTIVE INHIBITION
The critical difference in zovegalisib's design is that it is mutant-selective . Conventional PI3Kα inhibitors block both mutant and wild-type PI3Kα enzyme; this is the principal source of adverse effects on glucose metabolism in normal tissues (muscle, liver, fat). A mutant-selective drug preferentially targets mutated PI3Kα in tumor cells, relatively sparing normal tissues — the basic mechanistic explanation for reduced hyperglycemia. Unlike conventional CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib), atirmociclib selectively targets only CDK4; by sparing CDK6, the neutropenia profile remains milder. This triplet represents a shift from the traditional philosophy of “broad-spectrum targeted therapy” to a “selective multitargeted” approach.
The ReDiscover Study: Design and Patient Profile
ReDiscover (NCT05216432) is an open-label, multicenter, first-in-human phase 1 study evaluating the safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary efficacy of zovegalisib in patients aged 18 years and older with advanced solid tumors. The breast cancer patients enrolled in the dose-finding cohort had a clinically very challenging profile:
- All patients had previously received a CDK4/6 inhibitor
- All patients had received at least 1 endocrine therapy
- Median: 2 previous lines of treatment
- 21% had received 3 or more metastatic treatments
- 63% had visceral disease (liver, lung)
- 29% had previously received chemotherapy
- 47% were prediabetic
- ECOG performance status 0 or 1
- Primary: Zovegalisib MTD/RP2D dose
- Primary: Overall safety profile
- Secondary: ORR (overall response rate)
- Secondary: DOR (duration of response)
- Secondary: DCR and clinical benefit rate
- Secondary: Plasma ctDNA and tissue PIK3CA status
- Secondary: Pharmacokinetic parameters
- Data cutoff: 13 April 2026
Zovegalisib + atirmociclib + fulvestrant triplet in 34 patients previously exposed to a CDK4/6 inhibitor
Efficacy: Response Regardless of Mutation Type
In the dose-finding phase, 34 patients were eligible for response assessment. At a median follow-up of 7.4 months:
An important subgroup analysis finding: response rates were similar between kinase-domain and non-kinase-domain PIK3CA mutations . This is a clinically valuable observation because some earlier PI3Kα inhibitors showed better activity only in patients with “hot-spot” mutations (such as E545K, H1047R). Zovegalisib's potential activity across a broader range of mutations may simplify patient selection in clinical practice.
Tolerability Profile: An Unexpected Success in PI3Kα Inhibition
Perhaps the most remarkable result of this triplet is its safety profile. Recall the characteristics of the 62 patients evaluable for safety: almost half (%47) were already prediabetic. With conventional PI3Kα inhibitors, this group faces a high risk of hyperglycemia. Yet in ReDiscover:
✅ UNEXPECTED GOOD NEWS ON HYPERGLYCEMIA
Although 47% of patients were prediabetic, no grade 3 (severe) hyperglycemia was observed. This clearly demonstrates the clinical translation of mutant-selective PI3Kα inhibition. In conventional alpelisib studies, grade 3+ hyperglycemia rates could reach 30-40%.
✅ LOW DISCONTINUATION AND DOSE-REDUCTION RATES
Zovegalisib dose reduction due to TRAEs occurred in %10, while permanent treatment discontinuation occurred in only %3 of patients. This indicates a remarkably manageable profile in a predominantly pretreated population.
🟡 MAIN GRADE 3+ TRAE: NEUTROPENIA
In 40% of patients , grade 3 or higher treatment-related adverse events occurred; most were neutropenia. However, no febrile neutropenia was reported — a critical safety indicator. Atirmociclib targeting only CDK4 while sparing CDK6 may have limited neutropenia.
An Interesting Pharmacokinetic Interaction
Another important study finding is the pharmacokinetic interaction between the two drugs: adding atirmociclib more than doubled zovegalisib exposure, regardless of the atirmociclib dose. In contrast, zovegalisib did not affect atirmociclib exposure. This asymmetric interaction created an issue to consider in drug design; the phase 3 dose was established as zovegalisib 150 mg twice daily .
The HR+/HER2− Metastatic Breast Cancer Landscape: Where Does the Triplet Fit?
The treatment landscape for HR+/HER2− metastatic breast cancer has changed dramatically over the past decade. Current frontline approaches combine a CDK4/6 inhibitor (palbociclib, ribociclib, abemaciclib) with endocrine therapy (an aromatase inhibitor or fulvestrant); these doublets provide median PFS of around 25-28 months and response rates of approximately 53-55%. In subsequent lines, options include alpelisib + fulvestrant for patients with a PIK3CA mutation (SOLAR-1 study) and elacestrant for those with an ESR1 mutation.
The critical question is this: if the zovegalisib + atirmociclib + AI triplet demonstrates superiority to the CDK4/6 inhibitor + AI doublet in the phase 3 study beginning in 2027, a new frontline standard for PIK3CA-mutant HR+/HER2− patients will emerge. This means mutation-based personalized first-line treatment — an important paradigm shift in breast cancer practice.
The Planned Phase 3 Study: Toward 2027
📅 2027 PHASE 3 STUDY DESIGN
Patient population: Endocrine-sensitive, HR+/HER2−, PIK3CA-mutant advanced/metastatic breast cancer (frontline)
Comparison: Zovegalisib + atirmociclib + AI (experimental arm) versus investigator-selected CDK4/6 inhibitor + AI (control arm)
Primary endpoint: PFS (progression-free survival)
Key secondary endpoint: OS (overall survival)
The clinical meaning of this design is important: for years, CDK4/6 inhibitor + AI has been the “single standard” in frontline HR+/HER2− metastatic breast cancer. Success in the phase 3 study has the potential to change this regimen's position in the PIK3CA-mutant subgroup — molecular stratification would thus enter frontline clinical decision-making as well.
Critical Appraisal: Keeping Enthusiasm Grounded in Hard Data
Although the findings are promising, several limitations need careful consideration:
🔴 SMALL SAMPLE, PHASE 1, UNCONTROLLED
The efficacy analysis rests on only 34 patients. This is a dose-finding study, not a randomized trial. Although an ORR of 44% looks impressive, the figure carries wide confidence intervals in a small sample and may not be replicated in phase 3.
🔴 COMPARISON COHORTS DIFFER
The press release's claim of “approaching historical ORRs of 53-55%” may be misleading. The historical figures come from patients receiving 1st-line treatment , whereas ReDiscover patients had received a median of 2 previous lines of treatment and were later-line patients. A genuine comparison therefore cannot be made; this is a “hypothetical” approximation.
🟡 IMMATURE PFS DATA
At a median follow-up of 7.4 months, median PFS could still not be calculated (77% of patients remained on study). Although this suggests responses may be prolonged, there are no data yet on actual durability. Some patients may develop resistance after an early response.
🟡 NEUTROPENIA 40% — REQUIRES ATTENTION
The absence of febrile neutropenia is good news, but a 40% rate of grade 3+ TRAEs cannot be ignored. In practice, complete blood count monitoring, dose adjustments, and G-CSF support when necessary will be required.
🟡 UNPROVEN FIRST-LINE CLAIM
The phase 3 study will start in 2027; results are unlikely to mature before the 2030s. The claim that it “could become a frontline standard” is currently only a hypothesis — evidence is not yet available.
🟡 INDUSTRY-SOURCED DATA
The data were shared through company press releases before being presented at a major medical conference (such as ASCO or SABCS). A full scientific assessment will be possible after independent peer review.
Implications for Practice in Turkey and the Access Problem
These study results have several dimensions for oncology practice in Turkey. First, under current Turkish standards, in first-line HR+/HER2− metastatic breast cancer, palbociclib, ribociclib, or abemaciclib + AI/fulvestrant are reimbursed by SGK; for patients with a PIK3CA mutation, in the second line, alpelisib + fulvestrant is accessible under certain criteria, although use has remained limited by tolerability. Approval of the new triplet could reshape this landscape.
Second, PIK3CA mutation testing has become standardized in Turkey but depends on major centers and private insurance — broad access remains debated. If the phase 3 study succeeds, “PIK3CA mutation analysis for every patient with HR+/HER2− metastatic breast cancer” may become routine practice. This again highlights the importance of comprehensive molecular testing infrastructure.
Third, patient access to clinical trials is strategically critical. With the phase 3 study starting in 2027, preparations by major breast cancer centers in Turkey (including Memorial Göztepe) to participate in these studies are important for our patients' early access to innovative treatments. Strengthening clinical research infrastructure is not only a scientific but also an ethical necessity — Turkish patients should be active participants in international drug development, not merely observers.
Fourth, if the drug is approved, cost and reimbursement will become a major discussion. The triple oral regimen will be considerably more expensive than current doublets; SGK's decision-making mechanism should be ready to address this issue when the phase 3 results mature. Accessibility must also be explained realistically when informing patients.
Finally, in communication with patients, it is important that the message “wait for this new drug” does not create exaggerated hopes. Until phase 3 results mature (probably in the 2030s), existing effective standard treatments — CDK4/6 inhibitor + AI/fulvestrant — are effective, reimbursed options with a strong evidence base and should not be delayed.
🎓 DROZDOGAN Academy — Commentary and Discussion
PIK3CA · MUTANT-SELECTIVE INHIBITION · CDK4 · COMBINATION THERAPY · FRONTLINE BREAST CANCER
Why This Study Matters
The past decade has seen a dramatic transformation in the HR+/HER2− metastatic breast cancer landscape with the arrival of CDK4/6 inhibitors. Yet this success was limited: in patients with a PIK3CA mutation, progression was inevitable after the best available treatment, the CDK4/6 inhibitor + AI doublet. Although PI3Kα inhibition was conceptually attractive, hyperglycemia and other toxicities prevented frontline use. ReDiscover shows the boldness to overcome this obstacle by combining mutant-selective PI3Kα inhibition with selective CDK4 inhibition. The absence of grade 3 hyperglycemia in a population in which 47% were prediabetic provides concrete evidence of the clinical promise of mutant-selective design. Viewed more broadly, this study could be a starting point for bringing the concept of a targeted triple oral combination into metastatic breast cancer practice — combinations based on molecular stratification that require no intravenous administration and can be taken at home. If phase 3 succeeds, this could deliver important gains for both patient quality of life and the healthcare system.
Strengths
The study's methodological strengths are multifaceted. First, enrolling 69 breast cancer patients in a phase 1 dose-finding study is impressive by phase 1 standards. Second, the population is clinically challenging — a median of 2 previous treatment lines, 63% visceral disease, and 47% prediabetes. Demonstrating activity nevertheless increases confidence in the reality of the signal. Third, achieving 44% ORR in a predominantly pretreated population (high for later lines), along with activity independent of mutation type (kinase versus non-kinase), matters for the drug's broad applicability. Fourth, the safety profile is truly notable — no grade 3 hyperglycemia, no febrile neutropenia, and a 3% permanent discontinuation rate. Fifth, the pharmacokinetic interaction analysis was performed rigorously, establishing the phase 3 dose (150 mg b.i.d.) on a scientific basis. Sixth, the phase 3 design is planned directly against the competing standard treatment with PFS as the primary endpoint — a gold-standard design. This shows that the sponsor (Relay Therapeutics) trusts its own data — the aim is direct superiority rather than comfort against a competitor.
Limitations and Open Questions
Several important points emerge in a critical appraisal. First and foremost, this is a small, uncontrolled phase 1 study — efficacy findings carry wide confidence intervals and may not be replicated in phase 3. Second, the press release's message of “approaching historical ORRs of 53-55%” may be misleading — those figures come from 1st-line patients, whereas ReDiscover patients are in later lines; the comparison is categorically flawed. Third, median PFS is not yet mature — at 7.4 months of follow-up, 77 percent of patients remain on study, but it is unknown whether this reflects deep, long-term responses or simply that progression has not yet been observed because follow-up is early. Fourth, the 40% rate of grade 3+ TRAEs (mostly neutropenia) cannot be ignored and creates management challenges in real practice — particularly in older, frail patients or those with comorbidities. Fifth, the study is industry-sponsored , and the data were shared through a company press release before being presented at a scientific congress following independent peer review; full methodological transparency is not yet possible. Sixth, the population's ethnic and geographic diversity is unknown — whether activity and tolerability will be the same in Turkish or other Mediterranean/Asian populations is uncertain. Seventh, with existing PI3K/AKT pathway inhibitors such as alpelisib + fulvestrant or capivasertib + fulvestrant, there is no direct comparison; a claim of “better tolerated” requires a comparative study. Eighth, atirmociclib is a CDK4 inhibitor that is not yet approved; evidence for the triplet's individual components as monotherapy or in combination is also limited. Ninth, an overall survival benefit (OS) is unknown — in CDK4/6 inhibitor studies, PFS benefit has not always translated into OS benefit; this is a critical concern in metastatic breast cancer. Finally, because the phase 3 study will begin in 2027 and take years to mature, integration into clinical practice will be in the 2030s at the earliest — the message that things will “change soon” may therefore be exaggerated.
Implications for Clinical Practice
The concrete messages for oncology practitioners in Turkey are as follows. First, this triplet is not yet an approved treatment — clinical use requires regulatory authorization, phase 3 results, and SGK reimbursement. This framework must be explained clearly when speaking with patients. Second, maintaining current Turkish standards — in frontline HR+/HER2− metastatic breast cancer, a CDK4/6 inhibitor (palbociclib, ribociclib, abemaciclib) + AI or fulvestrantremains the best choice as an effective treatment with a strong evidence base. Third, PIK3CA mutation testing should become routine; it is already performed for the alpelisib indication but will also be needed for zovegalisib and other PI3K/AKT inhibitors in the future. Fourth, patients who could benefit from this triplet (PIK3CA-mutant, progression after a CDK4/6 inhibitor) need to be actively assessed for clinical trial opportunities ; continued participation by centers such as Memorial Göztepe in international phase 2/3 trials is important. Fifth, optimizing hyperglycemia management in patients receiving alpelisib + fulvestrant — consultation with a diabetes specialist, glucose monitoring, and starting metformin when necessary — should be integrated into current standard practice; managing adverse effects of PI3Kα inhibition is becoming an increasingly refined field. Sixth, capivasertib + fulvestrant (the AKT inhibitor approved through CAPItello-291) is already developing as an alternative PI3K/AKT pathway-targeted approach; discussion is beginning about access in Turkey and its substitution for alpelisib in clinical decision-making. Seventh, it is important to communicate the message that “promising new treatments are coming” in a balanced way — conveying the field's active development without creating exaggerated hopes. Finally, the triplet being entirely oral (applicable to the AI version, since fulvestrant is an intramuscular injection) is an important advantage for patient quality of life; it could become an attractive option for patients wishing to avoid intravenous treatments. Reminder: zovegalisib and atirmociclib, discussed in this article, are not yet licensed in Turkey or worldwide.
Future Questions
The critical questions for the coming period are these: Can the phase 3 study (starting in 2027), with its randomized, controlled design, demonstrate superiority to the CDK4/6 inhibitor + AI doublet, and can PFS benefit translate into OS benefit? When the results mature (in the 2030s), can it obtain FDA and EMA approval, and if so, which subgroup will be suitable for frontline use? Could it also work in HR+/HER2− patients without a PIK3CA mutation — or will the regimen remain an option based solely on molecular stratification? Will direct comparative studies be conducted against other PI3K/AKT inhibitors (alpelisib, capivasertib)? Can atirmociclib independently demonstrate superiority to conventional palbociclib/ribociclib/abemaciclib — or will it remain only a combination partner? Will studies reflecting ethnic diversity (Asian, Mediterranean, Latin populations) provide sufficient safety and efficacy signals? Can Turkey's SGK reimbursement system cover the cost of this triple oral regimen, and under what criteria? Can new-generation mutant-selective targeted therapies (such as zovegalisib for PI3Kα, followed perhaps by ESR1-mutant-selective and AKT-mutant-selective drugs) create modular combinations and usher in an era of “molecular cocktails” in breast cancer treatment? Will molecular profiling after resistance to CDK4/6 inhibitors become routine? Finally, the patient-centered dimension: can entirely oral regimens with manageable adverse-effect profiles deliver concrete improvements in quality of life (lifestyle, working life, time with family), and will this be systematically incorporated into the evidence base? The answers will shape the next decade not just for zovegalisib but for the development of all targeted oncology treatments.
Scientific References
- Relay Therapeutics. Relay Therapeutics announces clinical data for zovegalisib plus atirmociclib triplet combination supportive of further development in frontline metastatic breast cancer. Press release, 27 April 2026.
- ClinicalTrials.gov. First-in-human study of mutant-selective PI3Kα inhibitor RLY-2608 (zovegalisib) as a single agent in patients with advanced solid tumors and in combination with endocrine therapy ± a CDK4/6 or CDK4 inhibitor (ReDiscover, NCT05216432). Updated: 22 September 2025.
- André F, Ciruelos E, Rubovszky G et al. (SOLAR-1). Alpelisib for PIK3CA-Mutated, Hormone Receptor-Positive Advanced Breast Cancer. N Engl J Med 2019;380:1929–1940.
- Turner NC, Oliveira M, Howell SJ et al. (CAPItello-291). Capivasertib in Hormone Receptor-Positive Advanced Breast Cancer. N Engl J Med 2023;388:2058–2070.
- Slamon DJ, Neven P, Chia S et al. (MONALEESA-3). Overall Survival with Ribociclib plus Fulvestrant in Advanced Breast Cancer. N Engl J Med 2020;382:514–524.
- Finn RS, Martin M, Rugo HS et al. (PALOMA-2). Palbociclib and Letrozole in Advanced Breast Cancer. N Engl J Med 2016;375:1925–1936.
- Goetz MP, Toi M, Campone M et al. (MONARCH 3). Monarch 3: Abemaciclib as Initial Therapy for Advanced Breast Cancer. J Clin Oncol 2017;35:3638–3646.
- Bidard FC, Kaklamani VG, Neven P et al. (EMERALD). Elacestrant (oral selective estrogen receptor degrader) Versus Standard Endocrine Therapy for Estrogen Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Advanced Breast Cancer. J Clin Oncol 2022;40:3246–3256.
This article was created using various editorial tools, including artificial intelligence, as part of the process. It was reviewed by human editors before publication. Zovegalisib (RLY-2608) and atirmociclib (PF-07220060), discussed here, are investigational drugs that have not yet received regulatory approval anywhere in the world. This triple combination is not currently accessible in Turkey. Current standard treatments for HR+/HER2− metastatic breast cancer (CDK4/6 inhibitor + AI or fulvestrant) are approved, reimbursed, and effective options.
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