Phase 1/2 study of pepinemab, an inhibitor of semaphorin 4D, in combination with pembrolizumab as first-line treatment of recurrent or metastatic head and neck cancer (KEYNOTE-B84).

e18033 Background: Immunosuppressive myeloid cells in the tumor microenvironment (TME) limit the efficacy of immune checkpoint inhibitors (ICIs) in head and neck squamous cell carcinoma (HNSCC). Preclinical and clinical studies demonstrated that antibody blockade of semaphorin 4D (SEMA4D) promotes tumor infiltration and activation of DCs and CD8+ T cells and reverses immunosuppression, including attenuation of MDSC recruitment and function, leading to enhanced efficacy of ICIs. Pepinemab, a SEMA4D blocking antibody, in combination (combo) with avelumab provided clinical benefit in some patients with ICI-resistant and PD-L1-low NSCLC. Pembrolizumab (pembro) is approved as monotherapy or in combo with chemo for the first-line treatment of recurrent or metastatic (R/M) HNSCC, representative of a class of tumors that express higher levels of SEMA4D and MDSC. The primary hypothesis of this proof-of-concept study is that pepinemab in combo with pembro will yield increased clinical benefit compared to the reported activity for pembro monotherapy in R/M HNSCC. Methods: KEYNOTE-B84 (NCT04815720) is a single-arm open-label study evaluating the safety, efficacy, and PK/PD of pepinemab in combo with pembro as first-line treatment of R/M HNSCC. Subjects with measurable disease per RECIST1.1, ECOG 0 or 1, are eligible; subjects who have received prior ICIs or other systemic treatment for R/M HNSCC are excluded. The study includes a safety run-in (n = 3) and dose-expansion phase (maximum n = 62). Pepinemab, previously found to be well-tolerated in combo with other ICIs, will be evaluated initially at the highest intended dose of 20 mg/kg, in combo with 200 mg pembro, both administered i.v. Q3W. The dose expansion phase plans to include nearly equal distribution of subjects who have tumor PD-L1 combined positive scores (CPS) of < 20 and ≥20. The primary efficacy endpoint is ORR, and the secondary endpoints are PFS, DoR, and OS, as well as exploratory biomarker analyses. Pre- and on-treatment biopsies will be collected for evaluation of immune contexture in TME. Results: The safety run-in phase (n = 3) is complete, and the combo appeared to be well-tolerated with no DLTs observed; SMC approved continuation to dose expansion. Strikingly, two of the three patients have been observed to experience a complete response (CR). Biomarker analysis revealed that tumors in both responders expressed low levels of PD-L1 (CPS < 20). The third patient progressed prior to first scan, suffering several unrelated SAEs attributed to a pre-existing co-morbidity. Conclusions: The ongoing KEYNOTE-B84 study completed the initial safety run-in phase, with two of three subjects achieving CR. Phase 2 will evaluate pepinemab, a SEMA4D inhibitor, as a novel strategy to potentially overcome resistance and enhance activity of pembro in R/M HNSCC. Clinical trial information: NCT04815720.