Trial Design and Patient Population
The VIKTORIA-1 trial was a Phase 3, open-label, randomized, global study conducted across 147 sites in 23 countries between December 5, 2022, and January 23, 2025. The trial enrolled 392 patients with PIK3CA wild-type status who had a diagnosis of HR+/HER2- metastatic or locally advanced breast cancer and had experienced documented progression on prior treatment with a CDK4/6 inhibitor combined with a nonsteroidal aromatase inhibitor. Eligible patients were age 18 years or older, and the study employed a 1:1:1 randomization strategy across three treatment arms, each with 28-day treatment cycles.
The study population represents a clinically important cohort: patients with hormone receptor-positive, HER2-negative disease who have exhausted one of the most effective endocrine therapy combinations currently available. Prior CDK4/6 inhibitor failure indicates progression through a cycle of active therapy, making these patients candidates for alternative mechanistic approaches to target residual disease.
Treatment Arms and Rationale
| Arm | Regimen | Schedule |
|---|---|---|
| Arm A (Triplet) | Gedatolisib 180 mg IV weekly + Palbociclib 125 mg oral daily + Fulvestrant 500 mg IM | 3 weeks on/1 week off cycles |
| Arm B (Doublet) | Gedatolisib 180 mg IV weekly + Fulvestrant 500 mg IM | 3 weeks on/1 week off cycles |
| Arm C (Control) | Fulvestrant 500 mg IM monotherapy | Standard dosing |
Gedatolisib, the investigational agent, is a first-in-class, pan-class I PI3K/mTOR inhibitor. As an ATP-competitive, reversible dual inhibitor, it targets all four class I phosphatidylinositol 3-kinase (PI3K) isoforms as well as mechanistic target of rapamycin (mTORC1) and mTORC2 complexes. The rationale for triplet and doublet combinations reflects the biology of PIK3CA wild-type disease: while these tumors lack direct PIK3CA mutations, many exhibit dysregulation of the PI3K/AKT/mTOR pathway through alternative mechanisms, making pathway-level inhibition potentially therapeutic.
A critical supportive measure was mandated for all gedatolisib-treated patients: prophylactic steroid-containing “swish-and-spit” regimens to mitigate oral toxicity. The control arm included a crossover provision, permitting patients randomized to fulvestrant monotherapy to receive gedatolisib upon documented progression, a design feature that acknowledges the trial’s positive hypothesis while maintaining equipoise during enrollment.
Primary Efficacy Endpoints
“VIKTORIA-1 is the first Phase 3 study to show a significant improvement in median progression-free survival with inhibition of the PI3K/AKT/mTOR pathway in patients with PIK3CA wild-type HR+/HER2- advanced breast cancer who previously received a CDK4/6 inhibitor.”
— Sara A. Hurvitz, MD, Fred Hutchinson Cancer Center, University of Washington
The primary efficacy analysis employed blinded independent central review using RECIST v1.1 criteria for progression-free survival assessment. The median follow-up duration at the time of analysis was 10.1 months. Both gedatolisib-containing regimens demonstrated statistically significant and clinically meaningful improvements over fulvestrant monotherapy, with the triplet achieving a particularly robust 76% hazard reduction (100% minus the HR 0.24 percentage).
Secondary Efficacy Outcomes
Response rates and duration of response support the durability of the benefit observed in progression-free survival. The triplet arm demonstrated an objective response rate (ORR) of 31.5% (including 1 complete response), with a median duration of response (DOR) of 17.5 months. The doublet arm achieved an ORR of 28.3% with a median DOR of 12.0 months, whereas the control arm recorded only 1% ORR with fulvestrant monotherapy. Overall survival data remain immature, with fewer than 50% of required events reported at the time of analysis; a preliminary trend favoring gedatolisib was observed, but longer follow-up is required for definitive conclusions.
“The magnitude of difference between the gedatolisib triplet and fulvestrant is the largest reported in a contemporary phase 3 trial in this patient population.”
— Sara A. Hurvitz, MD
These secondary endpoints underscore the robustness of the primary PFS finding. The median duration of response in the triplet arm—17.5 months—is particularly notable, indicating that responses are not merely early transient events but are sustained over an extended interval. The response rate of approximately 30% in both gedatolisib arms, compared to 1% in the control arm, demonstrates that the drug achieves direct tumor cytoreduction in a substantial minority of patients, a marker of on-target activity.
Safety Profile
Gedatolisib was associated with an increased incidence of treatment-related adverse events (TRAEs), with the most common Grade ≥3 events reflecting both class effects of PI3K/mTOR inhibition and expected toxicities of combination therapy. The safety data guide clinical management and help define the risk-benefit profile for individual patient populations.
The safety profile reflects a manageable yet clinically significant burden. The triplet’s higher neutropenia rate (62.3%) is attributable to the combination with palbociclib, a CDK4/6 inhibitor known to suppress myelopoiesis; this adverse event was anticipated and can be addressed through dose modification or growth factor support. The stomatitis signal, while higher with gedatolisib than control, was substantially mitigated by the oral prophylaxis regimen, demonstrating that proactive management strategies can reduce the clinical impact of this class effect. Notably, treatment discontinuation rates remained low, suggesting that most patients could tolerate their assigned regimen with appropriate supportive care.
Clinical Interpretation and Practice Implications
“The efficacy data from the VIKTORIA-1 PIK3CA wild-type cohort represent an important addition to the clinical evidence in HR-positive, HER2-negative, PIK3CA wild-type advanced breast cancer. Importantly, these findings are potentially practice changing for patients with limited options.”
— Igor Gorbatchevsky, MD, Chief Medical Officer, Celcuity
The VIKTORIA-1 trial addresses a significant unmet clinical need. Patients with HR+/HER2- metastatic breast cancer who progress on CDK4/6 inhibitor plus aromatase inhibitor therapy have historically faced limited options: fulvestrant monotherapy, clinical trial enrollment, or chemotherapy. The trial demonstrates that PI3K/mTOR pathway inhibition, via the first-in-class dual inhibitor gedatolisib, offers a meaningful improvement in progression-free survival. The reduction in progression risk—a 76% decrease with the triplet (HR 0.24)—is among the largest observed in contemporary Phase 3 trials in this population, positioning gedatolisib as a potential standard-of-care option.
The efficacy of both triplet and doublet regimens suggests flexibility in treatment selection. The triplet (gedatolisib + palbociclib + fulvestrant) offers superior PFS (9.3 vs. 7.4 months) but carries higher neutropenia burden; the doublet (gedatolisib + fulvestrant) provides nearly equivalent benefit with reduced hematologic toxicity. Clinicians may select the regimen based on baseline cytopenias, patient fitness, and individual risk tolerance.
The PIK3CA wild-type restriction is noteworthy: this cohort, unlike patients with PIK3CA-mutant disease, cannot access targeted PIK3CA inhibitors such as alpelisib. The positive VIKTORIA-1 results suggest that pathway-level inhibition via dual PI3K/mTOR targeting is effective even in the absence of activating PIK3CA mutations, possibly because these tumors harbor upstream pathway dysregulation (e.g., PTEN loss, RTK activation, or AKT amplification) that sensitizes them to pathway-level intervention.
Outstanding Questions and Future Directions
Several questions remain to guide future investigation and clinical application. First, the preliminary OS hazard ratio of 0.69 is encouraging but requires mature data to confirm whether the PFS benefit translates to improved overall survival. Second, biomarker enrichment strategies may help identify which patients within the PIK3CA wild-type population derive the greatest benefit: for example, tumors with PTEN loss, PIK3CA alteration (despite “wild-type” coding-sequence status), or elevated AKT phosphorylation might represent particularly responsive subsets. Third, the role of gedatolisib earlier in the treatment course—for example, as first-line therapy or in combination with other pathway inhibitors—warrants exploration.
Additionally, comparative effectiveness data are needed to contextualize gedatolisib’s position relative to other emerging agents in this space, including other PI3K/mTOR inhibitors and next-generation endocrine therapies. The durability of responses and long-term safety profile will be critical as longer follow-up accumulates. Finally, investigation of de novo resistance mechanisms and strategies to overcome progression in responders will be essential to maximize the clinical utility of this new therapeutic option.
Conclusion
The VIKTORIA-1 trial represents a landmark advance in the treatment of PIK3CA wild-type HR+/HER2- metastatic breast cancer. Gedatolisib, a first-in-class pan-class I PI3K/mTOR inhibitor, demonstrates substantial and statistically significant improvements in progression-free survival when combined with fulvestrant, with or without palbociclib, compared to fulvestrant alone. The triplet regimen achieved a median PFS of 9.3 months with a hazard ratio of 0.24—among the largest benefits observed in contemporary trials of this patient population—and a response rate of 31.5%. The safety profile is manageable, with neutropenia and stomatitis as the primary Grade ≥3 concerns, both amenable to supportive care strategies.
These findings establish PI3K/mTOR pathway inhibition as a viable therapeutic strategy for patients with PIK3CA wild-type disease and provide a potentially practice-changing option for patients with limited alternatives after CDK4/6 inhibitor failure. As clinicians integrate these results into practice, attention to patient selection, toxicity management, and biomarker discovery will be essential to optimizing outcomes and further refining treatment paradigms in HR+/HER2- advanced breast cancer.