Development

Regeneron's fianlimab and cemiplimab combination misses primary endpoint in Phase III melanoma trial

Edited by Robert McTiernan··4 min read

Regeneron Pharmaceuticals (Nasdaq: REGN) reported that its Phase III trial for the LAG-3 antibody fianlimab in melanoma did not meet its primary endpoint. The combination of fianlimab and cemiplimab failed to achieve a statistically significant improvement in progression-free survival over Merck's Keytruda (pembrolizumab) monotherapy in first-line unresectable or metastatic melanoma, despite a numeric difference of 5.1 months in median PFS favoring the high-dose combination.

Trial specifics

The Phase III study is a randomized, double-blind trial enrolling 1,546 patients aged 12 years or older with unresectable locally advanced or metastatic melanoma who had not received prior systemic treatment for advanced disease. Patients were assigned to one of four arms: high-dose fianlimab (1,600 mg) plus cemiplimab (350 mg) every three weeks; low-dose fianlimab (400 mg) plus cemiplimab (350 mg) every three weeks; pembrolizumab (200 mg) monotherapy every three weeks; or cemiplimab monotherapy, which served as a component-contribution arm and was not included in the formal statistical comparisons.

The high-dose fianlimab combination produced a median PFS of 11.5 months (95% CI: 6.3–16.8) compared with 6.4 months (95% CI: 4.4–11.1) for pembrolizumab monotherapy, yielding a hazard ratio of 0.845 (95% CI: 0.709–1.008) and a p-value of 0.0627 — falling short of statistical significance. The low-dose combination produced a median PFS of 9.6 months against a concurrently randomized pembrolizumab subset (n=421), with a hazard ratio of 0.931 (95% CI: 0.773–1.122) and a p-value of 0.4661. No secondary endpoint data were disclosed in the announcement. No new safety signals were identified with either fianlimab combination. Regeneron said detailed results would be presented at an upcoming medical meeting.

Context and positioning

Fianlimab is a monoclonal antibody targeting LAG-3, a co-inhibitory receptor expressed on exhausted T cells. When combined with cemiplimab, a PD-1 inhibitor, the regimen is designed to provide dual checkpoint blockade — releasing both LAG-3- and PD-1-mediated suppression of antitumor T-cell responses. The rationale mirrors that of Bristol Myers Squibb's Opdualag (nivolumab and relatlimab-rmbw), the first approved LAG-3/PD-1 combination, which received US FDA approval in March 2022 based on RELATIVITY-047 data showing improved PFS over nivolumab monotherapy in first-line metastatic melanoma. The fianlimab LAG-3 melanoma trial was designed to test whether a distinct LAG-3 antibody paired with a different PD-1 backbone could improve upon single-agent PD-1 blockade — specifically pembrolizumab, the dominant first-line standard.

The numeric PFS difference observed with the high-dose combination — 11.5 months versus 6.4 months — is directionally consistent with the hypothesis that dual checkpoint blockade can extend disease control beyond PD-1 inhibition alone. However, the wide confidence intervals and a p-value that narrowly missed the threshold for significance mean that the trial does not provide the confirmatory evidence needed to support a regulatory filing on this endpoint. Cross-trial comparisons are limited by differences in patient populations, study designs, and follow-up duration, but the PFS figure for pembrolizumab in this trial (6.4 months) appears lower than has been observed in some prior pembrolizumab studies, which may have affected the statistical landscape.

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The first-line metastatic melanoma space is well-populated. Beyond pembrolizumab and nivolumab as PD-1 monotherapies, the combination of nivolumab plus Bristol Myers Squibb's Yervoy (ipilimumab) established dual checkpoint blockade as a high-efficacy but toxicity-intensive option. Opdualag's approval introduced LAG-3 inhibition as a clinically validated approach with a more manageable safety profile than the PD-1/CTLA-4 combination. In BRAF V600-mutant disease, targeted therapy combinations — including Novartis's Tafinlar plus Mekinist (dabrafenib and trametinib) and Pfizer's Braftovi plus Mektovi (encorafenib and binimetinib) — remain standard options. The fianlimab program was positioned to compete primarily in the immunotherapy segment, where the LAG-3/PD-1 axis has emerged as the leading dual-checkpoint strategy.

The result is a setback for Regeneron's efforts to position fianlimab as a first-line melanoma treatment via superiority over pembrolizumab, the trial's active comparator. The 5.1-month numeric improvement in median PFS for the high-dose combination is not trivial in absolute terms, but without statistical significance it cannot anchor a regulatory submission based on this endpoint alone. Whether overall survival data or additional analyses from the trial could support a different regulatory pathway remains unclear; the company has not disclosed overall survival results or response rate data.

The more consequential near-term read for the fianlimab program may come from the separate ongoing Phase III Harmony trial, which evaluates the high-dose fianlimab plus cemiplimab combination in a direct head-to-head comparison against nivolumab and relatlimab — the approved Opdualag regimen. That trial, which is currently recruiting, would test whether fianlimab's LAG-3 inhibition offers a meaningful advantage over the only approved LAG-3/PD-1 combination, rather than over PD-1 monotherapy. A positive result in that study would carry different strategic weight, potentially differentiating fianlimab within the dual-checkpoint class rather than against a single-agent standard.

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