Abstract 7531: Mechanisms of response to anti-PD1 (PD1) combined with Ipilimumab (IPI) in patients (pts) with PD1-resistant metastatic melanoma (MM)

Abstract Background: Approximately 1 in 3 pts with PD1-resistant MM will benefit from IPI+PD1, but the role of adding IPI in this context is unclear. We sought to (a) identify biomarkers and (b) determine mechanisms of response to IPI+PD1 in PD1 resistant pts. Methods: Pts with PD1-resistant MM subsequently treated with IPI+PD1 were included. We performed multiplex immunofluorescence (mIF), imaging mass cytometry (IMC) and RNA sequencing (RNAseq) on melanoma samples, and cytometry by time of flight (CYTOF) on peripheral blood mononuclear cells (PBMCs), at 3 timepoints (before PD1 [T1], before IPI+PD1 [T2], and after IPI+PD1 [T3]). Results: 34 pts with PD1-resistant MM (82% innate resistance) who received IPI+PD1 were included: 16 (47%) responders and 18 (53%) non-responders to IPI+PD1. Median age was 66yo (37-86) and 19 (56%) were male. Six pts (18%) were ECOG PS ≥1, 18 (53%) were stage M1C/D and 11 (32%) had elevated LDH at the start of IPI+PD1.Gene expression analysis (RNAseq) on T1 and T2 tumor samples revealed higher expression of cancer testis antigens (CTA; MAGE-A3 and MAGE-C1; p<0.05) and immune regulatory genes (ILR2; p<0.2), but lower expression of fibrosis regulation genes (FABP3 and MMP7; p < 0.2) in responders vs non-responders. We then examined whether the addition of IPI led to expansion of CTA-specific T cells, and CyTOF analysis of MAGE-A3 tetramers on PBMCs showed a decrease in peripheral MAGE-A3-specific CD8+ T cells with PD1 (T1 to T2; p=0.03); and expansion of this T-cell subset with subsequent IPI+PD1 (T2 to T3) was seen in 7/12 HLA-A2+ pts. Further analysis on PBMC longitudinal samples showed an increase in regulatory T cells (Tregs) in responders (p=0.04), between T1 and T3, and a decrease in % of TCF7+ CD8+ & CD4+ T cells in non-responders (p<0.05) with the addition of IPI (T2 to T3). On baseline (T1) tumor samples (mIF), IPI+PD1 responders had a higher % of stem cell-like TCF7+ CD8+ T cells (p=0.0004), lower % of well-differentiated Tbet+ CD8+ T cells (p=0.07), and higher % of CD4+ T cells (p=0.006), compared with non-responders; but no difference in Tregs. On longitudinal tumor samples, there was a decrease in the % of stem cell-like TCF7+ CD8+ T cells but an increase in the % of tumor-reactive CD39+ CD103+ CD8+ T cells with PD1 (T1 to T2; p<0.05), mainly seen in responders (p=0.06). There were only 3 T2 to T3 IHC paired samples, precluding further analysis. IMC analysis on longitudinal samples (T1, T2 and T3) is underway to characterize the interaction between Tregs and T effector cells. Conclusions: We identified potential mechanisms of response to IPI+PD1 in PD1-resistant MM pts. Our data suggests that response to IPI+PD1 requires baseline stem cell-like TCF7+ T cells with the potential to differentiate to an effector phase (CD39+ CD103+ CD8+ T cells), and that IPI facilitated expansion of antigen-specific T cells in a CTA-expression context, that PD1 alone is unable to do. Citation Format: Ines Pires da Silva, Jordan Conway, Jarem Edwards, Felix Marsh-Wakefield, Camelia Quek, Angela Ferguson, Peter Johnansson, Matteo Carlino, Alexander Menzies, James Wilmott, Umaimainthan Palendira, Richard Scolyer, Georgina Long. Mechanisms of response to anti-PD1 (PD1) combined with Ipilimumab (IPI) in patients (pts) with PD1-resistant metastatic melanoma (MM) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7531.