1465 Denosumab improves clinical benefit of PD1 inhibitors in metastatic melanoma (MM) presumably via a suppressive effect on immunoregulatory myeloid cell populations

<h3>Background</h3> RANKL blockade augments antitumor responses when combined with CTLA4±PD1 blockade in syngeneic melanoma-bearing mice. Single-cell RNA sequencing (scRNAseq) analysis of immune infiltrates in mouse B16 melanoma tumors revealed <i>RANK</i> expression in mature dendritic cells (DCs) —especially DCs enriched in immunoregulatory molecules— and M2 macrophages. <i>RANKL</i> mRNA was expressed in CD4/GATA3/CD40lg-expressing T-cells. We conducted a study of denosumab (D) induction followed by a combination with nivolumab (N) in PD1 inhibitor (PDi)-naïve IIIB-IV MM. The primary aim was to assess the safety, clinical benefit, and mechanism of action of D in MM. <h3>Methods</h3> This is a single-arm, phase 2 study of D alone (120mg sc on days 1,8 &amp; 21) followed on day 21 by D+N (480mg IV) q4w x12 in biopsiable MM. Patients must have available baseline tumor tissue and undergo a research biopsy on day 21. <h3>Results</h3> 25 patients were enrolled (median age, 60y; 14 males; 24 cutaneous; including 6 patients with stage IIIB-C and 4 patients with bone metastases). Immune-mediated fatigue and arthralgias attributed to D alone were seen in 11 and 5 patients, respectively. 1 patient developed colitis, and another developed pneumonitis; these adverse events (AEs) were attributed to N, which was permanently discontinued, but each re-developed grade 2 pneumonitis and colitis upon D rechallenge. Response rate (RECIST v1.1) was 48%. 2 patients had antitumor responses before N initiation. At a minimum f/u of 24 months, the median PFS was 18.1 months (range, 1.1–63.6), and the median OS was 31.7 months (range 7.2–68.9). As assessed by immunohistochemistry, RANK protein was highly expressed (H-score, 2+, 3+) across all tumor biopsies at baseline in melanoma and myeloid cells. In contrast, RANKL protein was highly expressed in both cell types in &gt;50% of tumors. Peripheral blood flow cytometry analysis showed a significant increase of non-exhausted (LAG3-PD1-) CD4+ and CD8+ cells, irrespective of TIGIT expression. ScRNAseq analysis of human melanoma specimens showed <i>RANK</i> expression in lymphatic endothelial cells, all melanoma cells, and tumor-associated macrophages (TAMs; <i>TREM2</i>/<i>GPNMB</i>/<i>SPP1</i>/<i>MITF-high</i> and <i>TNF</i>/<i>IL1β-low). RANKL</i> mRNA was expressed in CD4+ peripheral helper cells, <i>VGF</i>/<i>THBS2</i>-expressing melanoma cells, follicular DC subsets, medullary reticular cells, and lymph node-associated fibroblasts. <h3>Conclusions</h3> D alone has infrequent antitumor activity and induces mild immune-mediated AEs. D+N is well tolerated and has higher clinical benefit than prior studies with single-agent PD1i. We postulate that D restricts RANKL availability from immunoregulatory DCs and TAMs despite upregulation by small CD4+ cell subsets, expanding peripheral non-exhausted T cells. <h3>Trial Registration</h3> Clinicaltrials.gov NCT03620019. <h3>Ethics Approval</h3> The study obtained approval by the University of North Carolina at Chapel Hill Institutional Review Board (18-0798). Participants gave informed consent before taking part.