Merck’s fast-ascending kidney cancer drug hits a setback

Merck's Welireg Combination Fails Phase 3 Kidney Cancer Trial

Merck & Co.'s Welireg, in a three-drug combination with Keytruda and Lenvima, failed to meet its dual main objectives in a Phase 3 trial for newly diagnosed first-line clear cell renal cell carcinoma. The regimen did not significantly delay tumor progression or prolong survival when compared to Keytruda and Lenvima alone. This setback impacts Merck's strategy to offset future revenue losses from Keytruda's patent expiry and led to a 4% drop in Merck's shares. The failure also creates a potential opening for rival HIF-2 alpha inhibitors, such as Arcus Biosciences' casdatifan.

• The Phase 3 trial evaluated a triple combination of belzutifan (Welireg), pembrolizumab (Keytruda), and lenvatinib (Lenvima) against pembrolizumab and lenvatinib alone in patients with first-line clear cell renal cell carcinoma. The study missed its dual main objectives, indicating the triple therapy did not significantly improve tumor growth delay or overall survival compared to the dual therapy.
• This clinical failure significantly lowers the projected financial ceiling for belzutifan, a key asset in Merck's strategy to mitigate revenue losses post-Keytruda patent expiry. Analysts had previously projected belzutifan's global sales could reach $5.8 billion by 2030 with first-line success, but this is now reduced to an estimated $2.2 billion.
• The setback for belzutifan creates a strategic opportunity for competing HIF-2 alpha inhibitors. Arcus Biosciences, for instance, is developing casdatifan and plans to initiate a Phase 3 trial in the first-line kidney cancer setting by year-end, potentially benefiting from Merck's trial outcome.

Unpacking the Phase 3 Welireg Combination Trial Setback

Clear cell renal cell carcinoma clinical trials have evolved significantly in design complexity and endpoint sophistication over the past two decades. The landscape encompasses diverse study architectures ranging from early single-arm phase 2 investigations to large-scale randomized controlled phase 3 trials, with increasing emphasis on combination therapies and biomarker-driven patient stratification.

The Role of HIF-2 Alpha in Clear Cell RCC Progression

The von Hippel-Lindau (VHL) tumor suppressor gene represents the central genetic driver of clear cell renal cell carcinoma, with inactivation occurring in approximately 85% of sporadic cases. VHL loss leads to the accumulation and hyperactivation of hypoxia-inducible factors (HIFs), particularly HIF-2α, which subsequently activates hundreds of genes involved in oncogenic pathways including angiogenesis, cell growth, and metabolic reprogramming. This VHL-HIF axis also triggers HIF-independent mechanisms, including constitutive activation of nuclear factor kappa B (NF-κB) and c-jun N-terminal kinase (JNK), both of which drive tumor growth and epithelial-to-mesenchymal transition. Additionally, HIF-2α directly regulates target genes such as GAL3ST1, whose overexpression correlates with decreased survival and promotes immune evasion by protecting cancer cells against natural killer cell-mediated cytotoxicity.

Complementing VHL alterations, clear cell RCC progression involves dysregulation of multiple signaling cascades that converge on cellular proliferation and survival pathways. The PI3K-Akt-mTOR pathway becomes hyperactivated through various mechanisms, including abnormal mTOR activity that leads to MDM2-mediated degradation of the p53 tumor suppressor. The CDKL3 kinase functions as a critical adaptor protein that potentiates mTORC2-dependent Akt activation, creating a positive feedback loop that sustains oncogenic signaling. Mutations in chromatin remodeling genes, particularly PBRM1, SETD2, and BAP1, represent the most common somatic alterations after VHL and contribute to epigenetic dysregulation that facilitates tumor progression.

The tumor microenvironment and inflammatory signaling networks play essential roles in clear cell RCC development and progression. Inflammatory pathways involving TNF-α, STAT, and TGF-β promote cell transformation and metastasis, while IL-6 mediates cross-talk between normal fibroblasts and cancer cells to enhance tumor cell migration through STAT3 activation. Immune dysfunction manifests through impaired natural killer cell function, with tumor-infiltrating NK cells displaying decreased expression of activating receptors and increased inhibitory markers. Select inflammatory cell populations, including tumor-associated macrophages, myeloid-derived suppressor cells, and neutrophils, actively promote immune evasion and create a permissive environment for tumor growth and metastatic spread.

The Shifting Competitive Landscape for First-Line HIF-2 Alpha Inhibitors

Several HIF-2α inhibitors beyond belzutifan are currently advancing through clinical development, representing an expanding therapeutic class for VHL-associated tumors and clear cell renal cell carcinoma. These emerging compounds—including casdatifan, NKT-2152, and DFF332—share belzutifan's mechanism of blocking HIF-2α and HIF-beta heterodimerization, thereby preventing expression of oncogenic target genes such as VEGF, PDGF, and CAIX.

The intervention models being explored across this drug class mirror those established for belzutifan, including monotherapy in advanced clear cell renal cell carcinoma, combinations with VEGFR tyrosine kinase inhibitors, combinations with immune checkpoint inhibitors, and combinations with cyclin-dependent kinase 4-6 inhibitors. Clinical trials are investigating these HIF-2α inhibitors across pheochromocytoma, hemangioblastoma, and pancreatic neuroendocrine tumors beyond the primary indications of RCC and VHL disease.

First-Line Triplet Failure: A Reality Check for HIF-2α in ccRCC

The recent announcement regarding Merck's Welireg (belzutifan) in a first-line triplet for clear cell renal cell carcinoma (ccRCC) sends a clear signal about the evolving landscape of advanced kidney cancer treatment. While belzutifan, a pioneering HIF-2α inhibitor, has demonstrated significant clinical activity and is approved for VHL-associated tumors and later-line ccRCC following immunotherapy and VEGF-TKI failure, its inability to significantly improve outcomes when added to Keytruda and Lenvima in the first-line setting is a notable event.

This outcome underscores the formidable challenge of improving upon established first-line regimens in advanced ccRCC. Combinations like pembrolizumab and lenvatinib have set a high bar, delivering substantial benefits to patients. Simply adding a third agent, even one with a validated mechanism of action in other settings, does not guarantee superior efficacy. The failure to meet dual primary endpoints for progression-free survival and overall survival suggests that the incremental benefit, if any, was not sufficient to justify the added complexity or potential toxicity of a triplet regimen in this specific context.

For Merck, this represents a strategic setback. The company has been actively exploring belzutifan's potential in earlier lines of therapy to broaden its market footprint and help mitigate the anticipated revenue decline from Keytruda's patent expiry. This particular trial's outcome will necessitate a re-evaluation of its first-line ccRCC development strategy for belzutifan, potentially shifting focus to other ongoing trials, such as LITESPARK-011 (belzutifan plus lenvatinib versus cabozantinib post-PD-1/PD-L1) or LITESPARK-012 (pembrolizumab plus lenvatinib with or without belzutifan as first-line), which are exploring different combinations or patient populations. The positive results from LITESPARK-022, showing belzutifan plus pembrolizumab as adjuvant treatment reduced recurrence risk, indicate that belzutifan's potential in other settings remains strong.

From a broader industry perspective, this event highlights the intense competition in the HIF-2α inhibitor space. While belzutifan was the first to market, the failure in this specific first-line triplet could create an opening for rival HIF-2α inhibitors to differentiate themselves. Companies developing next-generation HIF-2α inhibitors or exploring alternative combination strategies may now see an opportunity to carve out their niche. The scientific rationale for targeting HIF-2α in ccRCC remains robust, given its central role in tumorigenesis, but the optimal sequencing and combination partners are still being defined. Future research will likely focus on identifying predictive biomarkers to better select patients who would benefit most from HIF-2α inhibition, either as monotherapy or in specific combination regimens, to maximize efficacy while managing known on-target side effects like anemia and hypoxia.