Ribociclib Plus Letrozole Shows Potent Activity in Recurrent Low-Grade Serous Ovarian Carcinoma: Results from GOG 3026

Highlights

Targeted Synergy

The combination of the CDK4/6 inhibitor ribociclib and the aromatase inhibitor letrozole leverages the hormonal and molecular drivers of low-grade serous carcinoma (LGSOC), achieving an objective response rate of 30.6% and a clinical benefit rate of 84%.

Durable Disease Control

Unlike traditional chemotherapy, which often yields short-lived responses in this subtype, the median duration of response in this trial was an impressive 21.2 months, with a median progression-free survival of 14.5 months.

Manageable Safety Profile

The adverse event profile was consistent with known CDK4/6 inhibitor toxicities, primarily characterized by manageable neutropenia, confirming the regimen’s feasibility for long-term administration in the recurrent setting.

Introduction: The Challenge of Low-Grade Serous Ovarian Carcinoma

Low-grade serous carcinoma (LGSOC) of the ovary, fallopian tube, or peritoneum is a distinct clinical and molecular entity from the more common high-grade serous carcinoma (HGSOC). While HGSOC is characterized by ubiquitous TP53 mutations and high genomic instability, LGSOC typically features mutations in the MAPK pathway (KRAS, BRAF, or NRAS) and high expression of estrogen receptors (ER).Despite its relatively indolent growth compared to HGSOC, LGSOC is notoriously resistant to standard platinum-based chemotherapy. Response rates for recurrent LGSOC to cytotoxic agents often hover between 5% and 15%. While endocrine monotherapies, such as aromatase inhibitors or fulvestrant, have shown activity, there remains a critical need for more effective systemic therapies that target the underlying biology of the disease. The GOG 3026 trial was designed to address this by evaluating the combination of ribociclib and letrozole, a strategy that has already revolutionized the treatment of ER-positive, HER2-negative breast cancer.

Biological Rationale: Targeting the CDK4/6-p16-Rb Axis

The rationale for combining endocrine therapy with CDK4/6 inhibition in LGSOC is multifaceted. First, LGSOC is highly hormonally driven, with ER expression observed in up to 90% of cases. Second, genomic studies have shown that the CDK4/6-p16-Rb pathway is frequently dysregulated in this malignancy. Many LGSOC tumors exhibit loss of p16INK4A or amplification of CCND1 (cyclin D1), both of which lead to constitutive activation of CDK4/6 and subsequent phosphorylation of the Retinoblastoma (Rb) protein, driving the cell cycle from G1 to S phase.By inhibiting CDK4/6, ribociclib prevents Rb phosphorylation, inducing cell cycle arrest. When combined with letrozole, which depletes the estrogen levels that drive cyclin D expression, the two agents act synergistically to suppress tumor proliferation more effectively than either agent alone.

GOG 3026: Study Design and Methodology

GOG 3026 was an open-label, single-arm, multicenter phase II trial. The study enrolled women with measurable, recurrent LGSOC of the ovary, fallopian tube, or peritoneum. Key eligibility criteria included histological confirmation of LGSOC and evidence of measurable disease by RECIST 1.1.The treatment regimen consisted of:

Ribociclib:

600 mg orally, once daily, on days 1 through 21 of a 28-day cycle.

Letrozole:

2.5 mg orally, once daily, continuously.The primary end point was the investigator-assessed objective response rate (ORR) per RECIST 1.1. Secondary end points were comprehensive, including clinical benefit rate (CBR) at 24 weeks, duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety/tolerability.

Clinical Efficacy: Durable Responses in a Chemoresistant Disease

The trial results, published in the Journal of Clinical Oncology, represent a significant milestone in LGSOC research. Of the 49 treated patients, the confirmed ORR was 30.6% (90% CI, 19.9 to 43.2). This included one patient who achieved a complete response (CR) and 14 who achieved partial responses (PR).Perhaps more significant than the response rate itself was the durability of the clinical effect. The median duration of response (DOR) reached 21.2 months. The clinical benefit rate (CBR), defined as the percentage of patients achieving CR, PR, or stable disease (SD) for at least 24 weeks, was 84%.Other key survival metrics included:

Progression-Free Survival (PFS):

The median PFS was 14.5 months (90% CI, 10.1 to 28.8).

Overall Survival (OS):

The median OS was 44.5 months (90% CI, 31.8 to not reached).These data contrast sharply with historical data for chemotherapy in the recurrent setting, where PFS typically ranges from 7 to 13 months and response rates are significantly lower.

Safety and Tolerability Profile

The safety profile of ribociclib plus letrozole in this population was consistent with findings from the MONALEESA breast cancer trials. The most frequent adverse event (AE) was neutropenia, with grade 3 or 4 occurrences in 47% of patients. Importantly, neutropenia was generally managed through dose interruptions or reductions and did not frequently lead to treatment discontinuation.Other common AEs included fatigue, nausea, and anemia. Only 4% of patients discontinued treatment due to adverse events, suggesting that the regimen is well-tolerated for the extended durations required to treat this chronic malignancy. While three grade 5 events were reported, they were determined by investigators to be unrelated to the study drugs.

Expert Commentary and Clinical Implications

The findings of GOG 3026 provide strong evidence for the use of CDK4/6 inhibitors in LGSOC. Historically, clinicians have reached for letrozole monotherapy or MEK inhibitors (such as trametinib or binimetinib) in the recurrent setting. While the MEK inhibitor trametinib has shown a PFS benefit in the GOG 281 trial, its toxicity profile—including skin rash and gastrointestinal issues—can be challenging for some patients.The ribociclib-letrozole combination offers an alternative targeted strategy with a high CBR and manageable toxicity. The 84% CBR is particularly noteworthy for a population where disease stabilization is often a primary clinical goal.One area for further investigation is the identification of biomarkers of response. While ER expression is a prerequisite for the letrozole component, it remains unclear which specific genomic alterations (e.g., KRAS mutations vs. CCND1 amplification) best predict benefit from CDK4/6 inhibition. Future sub-analyses of GOG 3026 and subsequent trials may help refine patient selection.Furthermore, the success of this combination in the recurrent setting raises the question of whether it should be moved into the frontline maintenance setting. Given the chemoresistance of LGSOC, transitioning to a highly active, biologically targeted maintenance therapy following primary debulking surgery is a logical next step for clinical investigation.

Conclusion

The GOG 3026 trial successfully met its primary endpoint, demonstrating that ribociclib plus letrozole is a highly active and durable treatment for women with recurrent low-grade serous carcinoma. With an ORR of 30.6% and a median PFS of 14.5 months, this combination out-performs historical benchmarks for chemotherapy and provides a new evidence-based option for a patient population with limited therapeutic choices. As the field moves toward more personalized, molecularly-driven care in gynecologic oncology, GOG 3026 stands as a definitive proof of concept for the role of CDK4/6 inhibition in this rare and challenging disease.

Funding and ClinicalTrials.gov

This study was conducted by GOG Partners and supported by Novartis. ClinicalTrials.gov Identifier: NCT02730728.

References

Slomovitz BM, Weroha SJ, Deng W, et al. Phase II Trial of Ribociclib Plus Letrozole in Women With Recurrent Low-Grade Serous Carcinoma of the Ovary, Fallopian Tube, or Peritoneum: A GOG Partners Trial (GOG 3026). J Clin Oncol. 2026;44(3):153-163. doi:10.1200/JCO-25-01348.