3139 Background: Immune checkpoint inhibitor (ICI) therapy has proved efficacy in MSI colorectal (CRC) and endometrial cancer (EC), but its use in MSS tumors remains a challenge. As the mechanisms of action of CTLA-4 and PD-1 are non-redundant, targeting both pathways may have additive or synergistic activity in comparison with single PD-1/PD-L1 inhibition in MSS and MSI CRC and EC tumors, while single cell characterization of persistent cells upon treatment would enable to describe cell populations and build a detailed single-cell map of the immune, tumor and stromal cell composition. A single dose of tremelimumab (Trem) 300mg, while maintaining a similar overall exposure, has 3- to 4-fold higher Cmax compared to 4 doses of Trem 1 mg/kg, which may have the potential for better anti-tumor activity while potentially avoiding cumulative toxicity. Here we present the results of patients (pts) with refractory ICI-naïve MSS EC and CRC treated with Dur plus Trem and the characterization of immune and persistent cells with longitudinal single-cell analyses. Methods: Pts with advanced MSS/pMMR CRC and EC that have progressed during or after standard therapy with measurable disease per RECIST 1.1, ECOG ≤1, were treated with Dur 1500mg plus Trem 300mg at Week 0, followed by Dur 1500 mg Q4W. Response rate was correlated with transcriptome and T cell repertoire dynamics measured at single-cell level from freshly processed biopsies at three different timepoints: baseline, upon treatment and at progression. Results: 24 samples for single-cell analysis were obtained (4 MSS CRC and 4 MSS EC). The response rate (RR) was 0%, while disease control rate was 12.5%. Despite the lack of response, we found a massive shift in the immune tumor microenvironments of all patients. Following treatment, immune cell composition shifted to increased frequency of effector-memory, proliferating CD8 + cells and CD4 + cells, surprisingly, with enriched T helper phenotypes. Both lineages showed strong signals of exhaustion. The integration of matched T cell receptor (TCR) data pointed to an infiltration of new clonotypes, rather than the expansion of pre-existing tumor-resident T cells, with distinct degrees of dysfunctional phenotypes between the clones. Intriguingly, expanded clonotypes could be detected and monitored in peripheral blood cells using ultra-deep TCR sequencing. Conclusions: Dual treatment with Dur and Trem does not result in an improved RR in patients with refractory MSS/pMMR CRC and EC, despite profound dynamics in the tumor infiltrating immune cells landscape. The infiltration of CD4 + T helper cells provides mechanistic insights of immune evasion, a potential avenue to treat MSS/pMMR tumors. Clinical trial information: 2021-004061-13 .

Single cell characterization of persistent cells upon immunotherapy treatment in colorectal and endometrial tumors: The SERPENTINE trial.