Study of Favezelimab Coformulated With Pembrolizumab (MK-4280A) in Participants With Selected Solid Tumors (MK-4280A-010)

This is a Phase 2, signal-seeking study with response-based primary endpoints across two defined cohorts, which modestly favors a positive outcome. Clinical benefit rate/pathologic response in cutaneous squamous cell carcinoma and ORR in endometrial cancer are earlier, more achievable efficacy readouts than survival endpoints, so a study like this does not need a dramatic effect to generate a positive narrative. The populations are at least disease-specific rather than a diffuse all-comer basket, which should improve interpretability. The regimen design also leans on pembrolizumab-based therapy, and one cohort adds lenvatinib, so the trial is not relying on a completely unproven treatment concept. The main negatives are complexity and disclosure risk: two cohorts, different endpoints, and multiple regimens raise the chance of mixed data that are scientifically interesting but not clearly "positive" for market resolution. The endometrial cohort is also a more difficult setting, and the cSCC endpoint structure is less straightforward than a single hard endpoint. Net, I see modestly better-than-even odds of a positive readout.

Merck discontinued the entire favezelimab clinical development program in December 2024, citing a thorough evaluation of data across all favezelimab trials. The KEYFORM-007 Phase 3 trial in CRC failed its primary OS endpoint. The KEYFORM-008 Phase 3 in cHL had enrollment stopped. This broad program termination strongly signals that favezelimab does not add meaningful benefit to pembrolizumab. MK-4280A-010 is a Phase 2 basket study testing whether favezelimab adds benefit over pembrolizumab alone (Cohort A: cSCC) or over pembro+lenvatinib (Cohort B: pMMR EC). With the program already abandoned due to lack of efficacy signals across multiple tumor types, the likelihood that this trial demonstrates clear positive results for favezelimab's incremental contribution is low. Both cohorts use pembrolizumab-containing control arms, so the bar is showing favezelimab adds something — which the broader program suggests it does not. Small Phase 2 sample sizes (~80 per cohort) add noise. Estimated 25% chance of positive results.

This is a Phase 2 trial from Merck evaluating favezelimab/pembrolizumab (a LAG-3 + PD-1 combination) in cutaneous squamous cell carcinoma and endometrial cancer. The trial has two cohorts with different endpoints (pCR for cSCC, ORR for endometrial cancer), introducing complexity. Multiple treatment arms (combination therapy vs monotherapy) further dilute the signal. While checkpoint inhibitor combinations have shown promise in oncology, Phase 2 trials carry inherent uncertainty, and the pCR endpoint in Cohort A is stringent. Merck's resources support execution quality, but the multi-arm design and novel combination make positive outcomes far from guaranteed. The trial is completed as of the analysis date, so results are likely available. Given the Phase 2 risk profile and complex design, I estimate intrinsic YES probability modestly above even odds.

This Phase 2 trial for favezelimab/pembrolizumab faces significant challenges. The primary endpoints differ by cohort: Cohort A uses a composite Clinical Benefit Rate (CBR) including major pathologic response or pCR in cSCC, while Cohort B uses investigator-assessed ORR in pMMR endometrial cancer. The endpoints are demanding, especially the pathologic response in cSCC. pMMR endometrial cancer is typically less responsive to immunotherapy than dMMR tumors. The study completed early, but no results are disclosed, suggesting a lack of immediate positive data release. While Merck has strong immuno-oncology experience, the combination's novelty and the tough patient populations create substantial uncertainty. The early completion without announcement leans slightly negative.

Phase 2 multi-cohort trial tests favezelimab/pembrolizumab coform vs pembrolizumab ± lenvatinib in neoadjuvant cSCC (Cohort A: CBR = mPR/pCR/cCR per BICR pathology up to 22mo) and advanced pMMR EC (Cohort B: inv-assessed ORR per RECIST 1.1). Endpoints strong surrogates—pathologic response predicts EFS/OS in neoadjuvant; ORR validates Phase 2 signals. cSCC pop immunogenic/resectable (likely n~30-50/arm); pMMR EC refractory post-platinum. Historical: pembro neoadjuvant cSCC pCR~50%; pMMR EC ORR~15%, len+pembro~38%. Favezelimab (LAG-3) adds uncertain value—class shows modest pembro synergy but misses (e.g., melanoma OS). Merck operational excellence, on-time completion. Disclosure risk elevated: 21 days post-primary completion (2026-03-12), full status 'Completed', no topline implies no blowout positivity. Positive resolution requires CBR/ORR exceeding thresholds/superiority; modest/null likely. Intrinsic YES 42%.

The trial evaluates favezelimab/pembrolizumab in cSCC (Cohort A) and endometrial cancer (Cohort B). For Cohort A, pembrolizumab is already standard of care for recurrent/metastatic cSCC, and the co-formulation with LAG-3 inhibitor favezelimab is expected to show non-inferiority or improvement. In Cohort B, the combination of lenvatinib and pembrolizumab is supported by data from KEYNOTE-775/9, showing improved PFS in EC. The primary endpoint for Cohort A (pCR/pCR) is achievable given the known activity of checkpoint inhibitors in cSCC. The likelihood of a positive result is high, estimated at 65%, driven by the established efficacy of pembrolizumab and the synergistic potential of LAG-3 inhibition.

Phase 2 study of MK-4280A (favezelimab/pembrolizumab coformulation) in solid tumors completed primary completion 21 days ago. Cohort A (cSCC) uses pCR/mPR endpoints with surgery—pathologic responses are harder endpoints less susceptible to bias. Cohort B (pMMR endometrial cancer) uses ORR with lenvatinib combination; this is a validated active backbone in endometrial cancer. Merck's execution record is strong. The coformulation strategy targets PD-1/LAG-3 dual blockade, mechanistically plausible. Disclosure risk is elevated given completion date passed, but no public results yet. Prior pembrolizumab data in cSCC shows activity; adding LAG-3 blockade could enhance responses. Primary endpoint structure across two cohorts provides multiple shots on goal. No safety red flags in brief summary.

Merck officially discontinued the entire favezelimab (anti-LAG-3) clinical development program in December 2024 after a thorough evaluation of data showing a lack of efficacy across multiple trials. Furthermore, the specific results for Cohort A of this trial (KeyForm-010 / MK-4280A-010) were presented in November 2025. They demonstrated that the favezelimab/pembrolizumab combination arm had a major clinical benefit rate of 59%, which was numerically inferior to the 64% observed in the pembrolizumab monotherapy control arm. The data explicitly concluded that adding favezelimab resulted in higher adverse event incidence without providing any additional efficacy over pembrolizumab alone in the cSCC population. With the sponsor permanently abandoning the investigational drug and the primary efficacy readouts explicitly failing to show any clinical benefit over the control arm, there is virtually no path for this trial to be interpreted as positive.

The trial is in Phase 2, investigating a combination therapy for selected solid tumors. The primary endpoint includes clinical benefit rate and objective response rate. Given the early stage and the specific patient populations, there's a moderate chance of positive results, but the complexity of cancer treatments and variability in patient responses suggest a cautious outlook. The probability of a positive outcome is slightly below 50% due to these factors.