Background: XTX101 is a tumor-selective, anti-CTLA-4 monoclonal antibody (mAb) engineered with covalently linked peptides that mask CTLA-4 antigen-binding regions. The masking peptides are cleaved by matrix metalloproteases that are more active in the tumor microenvironment (TME) compared with healthy tissues. XTX101 is designed with 2 Fc domain mutations (S239D and I332E) which enhance binding to Fcγ receptors and are intended to improve effector function, resulting in depletion of regulatory T-cells in the TME. In a murine model, XTX101 at 0.3 mg/kg achieved similar tumor growth inhibition as 3 mg/kg of an ipilimumab analog, suggesting higher relative potency of XTX101 over ipilimumab (Powderly SITC 2021). Methods: Interim results of first-in-human, Phase 1 portion of study XTX101-01/02-001 (NCT04896697) are reported here. Part 1A examines XTX101 monotherapy in 3+3 dose escalation. Part 1B is evaluating pharmacodynamic (PD) biomarkers. Results: As of January 27, 2023, 22 patients (pts) with advanced solid tumors were enrolled including colorectal (n = 3), pancreatic (n = 3), non-small cell lung (n = 3), squamous cell skin (n = 2), breast (n = 2), uterine (n = 2), cervical, prostate, Fallopian tube cancer, leiomyosarcoma and Merkel cell carcinoma in Part 1A, and melanoma and colorectal cancer in Part 1B. Pts had a median of 3 prior lines of therapy (1-10). Dose levels (DL) of 7, 20 and 60 mg every 3 weeks (Q3W) were cleared (1 dose limiting toxicity (DLT) of colitis among 8 patients treated at 60 mg Q3W), while 2 DLTs of colitis were reported among 6 pts at 180 mg Q3W. An alternative dosing schedule of 150 mg every 6 weeks (Q6W) was implemented based on clinical and pharmacokinetic (PK) data, and no DLTs were reported among 5 pts at this alternative schedule. There were no grade (G) 4 or 5 treatment-related (TR) adverse events (AEs) on the study. G3 TRAEs included colitis (n = 4), infusion reaction (IR, n = 3) and lymphopenia (n = 2). There were no skin, endocrine or liver-related TRAEs of any grade. One patient with squamous cell lung cancer treated at 150 mg Q6W dose had an unconfirmed partial response and continues on treatment. PK exposure with 150 mg Q6W schedule were comparable to 2 cycles of 60 mg Q3W. Among 8 pts treated at 60 mg Q3W and 5 treated at 150 mg Q6W, 2 out of 13 patients (15%) experienced G3 colitis, 1 pt G2 colitis and 1 pt G2 diarrhea. All colitis AEs improved with immunosuppression and resolved. There were 2 IRs at 60 mg Q3W, while no IR occurred with the Q6W schedule. Based on the totality of clinical and PK data, 150 mg Q6W is the recommended Phase 2 dose (RP2D), and enrollment in part 1B is ongoing at this dose. Conclusions: XTX101 is a Fc-engineered, masked, tumor-activated, anti-CTLA-4 mAb in development for advanced solid tumors. At RP2D, the preliminary safety profile is differentiated from systemically active anti-CTLA4 mAb. Updated clinical data, PK, immunogenicity and PD data from the study will be presented. Clinical trial information: NCT04896697.