Background: Activation of the GAS6/AXL pathway promotes tumor cell proliferation; high levels of GAS6 and AXL have been associated with drug resistance and decreased survival. Batiraxcept is an ultra-high affinity decoy protein that captures GAS6, preventing it from activating AXL signaling. Serum soluble AXL (sAXL)/GAS6 ratio correlates with response to batiraxcept in advanced ovarian cancer. Batiraxcept in combination with cabozantinib is being evaluated in patients (pts) with metastatic clear cell renal cell carcinoma (ccRCC) in this study. Prior studies of cabozantinib in similar pt populations have shown an overall response rate (ORR) ranging from 15 – 32.5% and progression-free survival (PFS) from 3.7-7.2 months (mo). Methods: Key eligibility criteria include previously treated ccRCC pts, excluding prior cabozantinib. Objectives were to determine the safety of batiraxcept plus cabozantinib 60 mg, the recommended phase 2 dose (RP2D), ORR, PFS, duration of response. Correlative endpoints included assessment of baseline serum sAXL/GAS6 with ORR and PFS. Results: In Phase 1b (P1b), 26 pts received at least one dose of batiraxcept with cabozantinib (16 pts - 15 mg/kg; 10 pts - 20 mg/kg). Median age was 60 (range 40-81), male 85% (n=22), IMDC intermediate/poor risk 77% (n=20). All pts received prior immuno-oncology (IO) therapy, 54% (n=14) also received VEGF-TKI; 85% (n=22) received 1 to 2 prior lines of therapy (range 1-6). Median follow-up time was 11.6 mo (range 3.7-18). The RP2D is 15 mg/kg. Related adverse events (AE) any grade and grade ≥ 3 were 100% and 39% (n=10), respectively. Cabozantinib dose reductions occurred in 54% (n=14). AEs of special interest were grade 1 or 2 infusion related reactions, 27% (n=7). Batiraxcept was discontinued due to disease progression 62% (n=16) or toxicity 12% (n=3). The table shows P1b efficacy, ORR of 42%, median PFS of 9.3 mo, and DCR of 84%. Twenty (77%) pts had high baseline sAXL/GAS6 levels; ORR was 55% (11/20) with mPFS of 9.3 mo, compared to 0% ORR and mPFS 6.4 mo in the low sAXL/GAS6 pts. Conclusions: Batiraxcept was well tolerated when combined with cabozantinib. No new safety signals were noted. Efficacy for the combination is encouraging compared to historical cabozantinib monotherapy data. Higher GAS6 levels were predictive for treatment response. The P2 accrual is complete and efficacy data are maturing. A P3 trial plans to evaluate the efficacy in ccRCC patients who have progressed on IO-based and VEGF-TKI therapies. Clinical trial information: NCT04300140.