2623 Background: PIVOT-02 is an ongoing phase 1/2 study of bempegaldesleukin (NKTR-214), a CD122-preferential IL-2 pathway agonist, plus nivolumab in patients with advanced solid tumors. Bempegaldesleukin (NKTR-214) increases proliferative tumor infiltrating lymphocytes (TIL) and cell surface PD-1 on immune cells and PD-L1 on tumor cells, demonstrating potential synergy with anti-PD-1 therapy. Pre-treatment tumor biopsies from metastatic 1L melanoma (MEL) and urothelial carcinoma (UC) patients were analyzed to correlate baseline immune phenotype to response. Methods: Pre-treatment TIL (CD8+ T cells/mm2 and %CD3+ by IHC; 29 MEL; 22 UC) were measured and divided into high and low groups based on median values. PD-L1 (% PD-L1 on tumor cells by IHC [28-8 PharmDx]; 33 MEL; 23 UC) was scored negative (<1%) or positive (≥1%). Interferon gamma gene score (IFNG; 11 MEL) was scored as high or low based on median p value of <0.1 for 15 genes (EdgeSeq). High and low groups were correlated with responses per RECIST 1.1. Results: Baseline demographics and prognostic factors were balanced in the biomarker subgroups. Response rates for response evaluable MEL and UC were 53% (SITC 2018) and 48% (ASCO-GU 2019), respectively. In MEL, median values of CD3-TIL and CD8-TIL were 19% and 203 cells/mm2, respectively. Response rate correlations were 67% and 20% with IFNG high and low, 79% and 29% with CD3-TIL high and low, 79% and 33% with CD8-TIL high and low, and 68% and 43% with PD-L1 positive and negative. Most importantly, responses were observed in patients with the least favorable tumor microenvironment, characterized as both PD-L1 negative and TIL low, with responses of 17% (1/6 CD8-TIL), and 25% (2/8 CD3-TIL), respectively. Similar correlative trends were observed in UC, with 50% (4/8 CD8-TIL) and 38% (3/8 CD3-TIL) responses in patients with least favorable microenvironment. Conclusions: The biomarker program included in PIVOT-02 identified baseline immune signatures correlated with response for MEL and UC. The response rates observed in both the favorable and unfavorable tumor microenvironments indicate the potential of this combination and support its broad development. Clinical trial information: NCT02983045.