INBRX-106 Phase 2 Signal Is Real, But OS Absence and Phase 2 Ceiling Cap the Verdict

Inhibrx's INBRX-106 Shows Strong Efficacy in Phase 2 HNSCC

Inhibrx Biosciences announced positive primary endpoint results from the randomized Phase 2 HexAgon study of INBRX-106, a hexavalent OX40 agonist, combined with pembrolizumab, versus pembrolizumab monotherapy, in first-line PD-L1 positive metastatic or unresectable recurrent HNSCC. The combination arm achieved a confirmed objective response rate (cORR) of 48.3% compared to 26.5% for monotherapy, with a median progression-free survival (PFS) of 9.6 months versus 4.9 months. Efficacy was particularly strong in HPV+ patients, showing an 80.0% cORR and 90.0% six-month PFS rate for the combination. These results support expanding the Phase 2 study in HPV+ patients for a potential accelerated approval pathway.

• The HexAgon study demonstrated a significant improvement in objective response with INBRX-106 plus pembrolizumab, achieving a cORR of 48.3% compared to 26.5% for pembrolizumab alone. Notably, 13.8% of patients in the combination arm experienced a complete response, whereas no complete responses were observed in the control arm. The median PFS for the combination was 9.6 months, nearly double the 4.9 months seen with monotherapy, indicating enhanced disease control and durability.
• The efficacy of INBRX-106 was particularly striking in HPV+ HNSCC patients. In this subpopulation, the combination therapy yielded an 80.0% cORR versus 33.3% for pembrolizumab alone, with a remarkable 30.0% complete response rate in the combination arm. Furthermore, 90.0% of HPV+ patients on the combination remained progression-free at six months, significantly outperforming the 33.0% in the control arm, highlighting a profound and durable clinical benefit.
• The combination of INBRX-106 and pembrolizumab exhibited a generally manageable safety profile, with common treatment-related adverse events being predominantly low-grade rash, fatigue, and diarrhea. Based on these compelling results, Inhibrx plans to expand the Phase 2 HexAgon trial with approximately 50 additional HPV+ OPSCC patients, aiming for a potential accelerated regulatory pathway. The company also intends to explore INBRX-106 in other highly immunogenic tumors and in combination with cancer vaccines.

HexAgon Study: INBRX-106's Efficacy and Safety in HNSCC

Several recent clinical studies have evaluated novel and established interventions in recurrent/metastatic (R/M) head and neck squamous cell carcinoma (HNSCC), reporting meaningful efficacy signals alongside manageable safety profiles. The studies below span bispecific antibody therapy, combination chemotherapy-immunotherapy, and single-agent immunotherapy approaches.

• OrigAMI-4 (Amivantamab): This phase 1b/2 study evaluated subcutaneous amivantamab — an EGFR-MET bispecific antibody — in 102 participants with R/M HNSCC after prior PD-(L)1 inhibitor and platinum-based chemotherapy. Blinded independent central review-assessed objective response rate (ORR) was 42% (95% CI, 32–52), with a complete response rate of 15%. Median duration of response (DoR) was not reached (95% CI, 6.9–NR); 56% of responses lasted ≥6 months. Median progression-free survival (PFS) was 6.8 months (95% CI, 5.2–8.3) and median overall survival (OS) was 12.5 months (95% CI, 10.2–16.8) at a median follow-up of 11.8 months. Treatment-related discontinuations were low at 8%, with no new safety signals identified.

• Phase 2 Trial of Nab-Paclitaxel and Nivolumab (NCT04831320): This single-arm phase 2 trial enrolled 46 patients with PD-1 inhibitor-refractory R/M HNSCC receiving nanoparticle albumin-bound (nab)-paclitaxel combined with nivolumab in 28-day cycles. ORR was 46.7% (95% CI: 33.8–59.9; vs. null hypothesis, p = 0.0073) in 45 evaluable patients, including 4 complete responses and 17 partial responses. Median DoR was 6.1 months (95% CI: 2.8–9.4), median PFS was 5.5 months (95% CI: 3.9–7.8), and median OS was 13.9 months (95% CI: 9.0–18.9) at a median follow-up of 14.1 months. No treatment-related deaths occurred.

• CheckMate 141 (Nivolumab — Long-term Subgroup Analysis): This randomized phase 3 trial evaluated nivolumab versus investigator's choice (IC) of chemotherapy in R/M squamous cell carcinoma of the head and neck (SCCHN) post-platinum therapy. In the first-line treatment subgroup long-term follow-up analysis, OS benefit with nivolumab (n = 50) versus IC (n = 26) was maintained at 2 years, with median OS of 7.7 months versus 3.3 months (hazard ratio: 0.56; 95% confidence interval, 0.34–0.94). No new safety signals were identified.

• Cetuximab + Paclitaxel (Cmab + PTX) Retrospective Study: This retrospective study assessed weekly cetuximab and paclitaxel in 18 patients with R/M HNSCC who progressed after immune checkpoint inhibitor therapy. ORR was 44.4%, disease control rate (DCR) was 72.2%, median PFS was 3.8 months, and median OS was 9.6 months. Grade 3 adverse events included neutropenia (3 patients), anemia, paronychia, asthenia, and peripheral neuropathy (1 patient each). There were no treatment-related deaths.

INBRX-106: A First-in-Class OX40 Agonist for HNSCC

BMS-986178 is another OX40 agonist antibody that has been evaluated in clinical trials for advanced solid tumors — the same broad indication targeted by INBRX-106 — and shares the same mechanism of action: agonism of the OX40 (CD134) receptor to enhance T-cell activation, proliferation, survival, and effector function. BMS-986178 was studied as monotherapy and in combination with the checkpoint inhibitors nivolumab and/or ipilimumab in a phase I/IIa study (NCT02737475).

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INBRX-106: A Pivotal Advance in First-Line HNSCC Immunotherapy

The landscape of head and neck squamous cell carcinoma (HNSCC) treatment, particularly in the recurrent or metastatic setting, has seen significant shifts with the advent of immune checkpoint inhibitors. However, despite the benefits of agents like pembrolizumab, many patients still face disease progression, underscoring a persistent unmet need for more effective first-line strategies. The recent positive Phase 2 results for INBRX-106, a novel hexavalent OX40 agonist, in combination with pembrolizumab, signal a potentially transformative development.

INBRX-106 is designed to overcome the limitations of earlier OX40 agonists by inducing superior receptor clustering and robust T-cell activation, a mechanism that research indicates is crucial for potent antitumor responses. This innovative approach appears to be paying dividends, with the combination therapy demonstrating a confirmed objective response rate of 48.3% and a median progression-free survival of 9.6 months, significantly outperforming pembrolizumab monotherapy.

Crucially, the data revealed an exceptional response in HPV-positive HNSCC patients, achieving an 80.0% cORR and a 90.0% six-month PFS rate. This striking efficacy in a specific patient subset could pave the way for an accelerated approval pathway, rapidly introducing a new, highly effective option for these patients. Such a development would not only establish a new standard of care for HPV-positive HNSCC but also validate the company's differentiated approach to costimulatory immunotherapy.

However, several considerations remain. While the HPV-positive data is compelling, the generalizability of these profound benefits to the broader PD-L1 positive HNSCC population requires further investigation. Additionally, as with any novel immunotherapy, long-term safety and durability of response will be critical to fully characterize, especially given past challenges with other immune-modulating agents. The regulatory path for accelerated approval, while promising, will undoubtedly involve rigorous scrutiny of the Phase 2 data and the need for robust confirmatory evidence. Nevertheless, these results position INBRX-106 as a significant contender, potentially reshaping the treatment paradigm for first-line HNSCC, particularly for those with HPV-positive disease.