Background: Tumor-targeted drug delivery technologies are urgently needed to overcome lack of tumor selectivity, a major drawback of conventional chemotherapy. The acidic intercellular microenvironment in solid tumors traps weak acid/base chemotherapy agents, preventing necessary intracellular concentrations in tumor cells. An alphalex conjugate, which contains a pH-low insertion peptide (pHLIP), a linker, and a payload, was designed to overcome these limitations. Unlike antibody drug conjugates, alphalex PDCs target tumors in an antigen-agnostic manner. At pH ≥7.0, the peptide is unstructured. In the low-pH tumor microenvironment the peptide forms an alpha helix and inserts directionally into the tumor cell membrane delivering the linker and payload intracellularly where the linker is cleaved releasing the payload in the cytosol. CBX-12 consists of the pH-sensitive peptide, a self-immolating linker, and the topoisomerase 1 (TOP1) inhibitor exatecan. Method: In a Phase 1 trial, cohorts of patients were treated with escalating doses of CBX-12 in a 3 + 3 design on three dosing schedules: Daily × 5 every 3 weeks (Schedule A), daily × 3 every 3 weeks (Schedule B) or once weekly (Schedule C). NCT04902872. The primary objectives are safety, tolerability, and to determine the MTD and/or RP2D. Secondary and exploratory objectives include evaluation of plasma PK and intratumoral levels of CBX-12 and exatecan, anti-tumor activity per RECIST v1.1, and measurement of anti-drug antibodies. Results: As of 28 June 2022, 24 patients have been treated in the following schedules/cohorts (dose: n): A1 (0.25 mg/kg: 1), A2 (0.50 mg/kg: 3), B1 (20 mg/m2: 3), B2 (30 mg/m2: 3), B3 (45 mg/m2: 7), B4 (60 mg/m2: 2), C1(20 mg/m2: 5). The most common treatment-related AEs (TRAEs) were nausea (11), diarrhea (8), vomiting (8), anemia (8), and WBC/ANC decrease and fatigue (7 each), dehydration (4), AST increased (3) and alopecia (3). The most common Gr3/4 TRAEs (Gr: n) were ANC decrease (Gr3:2, Gr4:3), anemia (Gr3:4), platelet (Plt) count decrease (Gr4:3), and WBC decrease (Gr3:1, Gr4:2). One patient in Cohort A2 had DLTs of ANC and Plt decrease. Two patients in Cohort B4 had DLTs including febrile neutropenia (2), ANC and Plt decrease (2) and sepsis (1). The best response for 14 response-evaluable patients were 1 CR (ovarian), 1 PR (breast) and 9 SD. The preliminary PR2D in Part B is 45 mg/m2. Dose escalation continues in Part C. Plasma CBX-12 and free exatecan concentrations increased in a dose-proportional manner. Intratumoral CBX-12 and exatecan levels will be presented. Conclusions: In this FIH study of a pH-targeting alphalex PDC, CBX-12 demonstrated good tolerability, single-agent anti-tumor activity including 2 confirmed responses, and an AE profile consistent with the TOP1 payload, exatecan. Phase 2 cohort expansions are planned in patients with ovarian and SCLC. Conflict of interest: Advisory Board: Funda Meric-Bernstam: Black Diamond, Biovica, Eisai, FogPharma, Immunomedics, Inflection Biosciences, Karyopharm Therapeutics, Loxo Oncology, Mersana Therapeutics, OnCusp Therapeutics, Puma Biotechnology Inc., Seattle Genetics, Sanofi, Silverback Therapeutics, Spectrum Pharmaceuticals, Zentalis. Corporate-sponsored Research: Funda Meric-Bernstam: Aileron Therapeutics, Inc. AstraZeneca, Bayer Healthcare Pharmaceutical, Calithera Biosciences Inc., Curis Inc., CytomX Therapeutics Inc., Daiichi Sankyo Co. Ltd., Debiopharm International, eFFECTOR Therapeutics, Genentech Inc., Guardant Health Inc., Klus Pharma, Takeda Pharmaceutical, N ovartis, Puma Biotechnology Inc., Taiho Pharmaceutical Co. Andrea Vandross: Mabwell (Shanghai), Abbvie, Astra Zeneca, Aminex Therapeutics, Ascentage Pharma, Asana Bio, BioOneCure, BJ Bioscience Inc, Elpiscience Biopharma, Nanjing Immunophage Biotech, Chugai Pharmaceuticals, Lyvgen Biopharma, NGMBio, Zhuhai Yufan Biotechnologies, siRNAomics, Sorrento therapeutics, Exelixis, Xilio, ZielBio. Anthony Tolcher: AbbVie Inc, ABL Bio Inc, Adagene Inc, ADC Therapeutics SA, Agenus Inc, Aminex Therapeutics Inc, Amphivena Therapeutics Inc, Apros Therapeutics Inc, Arcellx Inc, ARMO Biosciences, Arrys Therapeutics Inc, Artios Pharma Limited, Asana BioSciences LLC, Ascentage Pharma Group Inc, Astex Pharmaceuticals, Basilea Pharmaceutica International Ltd, BioInvent International AB, BioNTech RNA Pharmaceuticals GmbH, Birdie Biopharmaceuticals HK Ltd, Boehringer Ingelheim Pharmaceutical, Inc., Boston Biomedical Inc, Calgent Biotechnology Co Ltd, Codiak BioSciences Inc, CStone Pharmaceuticals (Suzhou) Co., Ltd., Cybrexa Therapeutics, Inc., Daiichi Sankyo Inc., Deciphera Pharmaceuticals, LLC, eFFECTOR Therapeutics, Inc, Eli Lilly and Company, EMD Serono, Gilead Sciences, Inc., GlaxoSmithKline Research & Development Limited, Haihe Biopharma Co., Ltd., Heat Biologics, IDEAYA Biosciences, ImmuneOncia Therapeutics, Inc., IMPACT Therapeutics, Inc., Inhibrx, Inc., Innate Pharma SA, Janssen Research & Development, K-Group Beta, Inc., KeChow Pharma, Inc., Kirilys Therapeutics, Kiromic Biopharma, Inc, Mabspace Biosciences (Suzhou) Co., Limited, Merck Sharp & Dohme Corp., a subsidiary of Merck & Co. Inc., Mersana Therapeutics, Inc., Mirati Therapeutics, Inc., NatureWise Biotech & Medicals Corporation, Navire Pharma Inc., NBE-Therapeutics AG, NextCure, Inc, Nitto BioPharma, Inc., Odonate Therapeutics, Inc., ORIC Pharmaceuticals, Pelican Therapeutics, Inc., Petra Pharma, Pfizer, Inc., Pieris Pharmaceuticals, Inc., PMV Pharmaceuticals, Inc., Qilu Puget Sound Biotherapeutics Corporation, Samumed, LLC, Seattle Genetics, Inc., Spring Bank Pharmaceuticals, Inc., Sunshine Guojian Pharmaceutical (Shanghai) Co., Ltd., Symphogen A/S, Syndax Pharmaceuticals Inc., Synthorx, Inc., Takeda, Tizona Therapeutics, VRISE Therapeutics, Zymeworks Inc. Other Substantive Relationships: Funda Meric-Bernstam (consultant): AbbVie, Aduro BioTech Inc., Alkermes, AstraZeneca, Daiichi Sankyo Co. Ltd., DebioPharm, eFFECTOR Therapeutics, F. Hoffman-La Roche Ltd., Genentech Inc., Harbinger Health, IBM Watson, Infinity Pharmaceuticals, Jackson Laboratory, Kolon Life Science, Lengo Therapeutics, OrigiMed, PACT Pharma, Parexel International, Pfizer Inc., Protai Bio Ltd, Samsung Bioepis, Seattle Genetics Inc., Tallac Therapeutics, Tyra Biosciences, Xencor, Zymeworks. J. Paul Eder (consultant): Parthenon, Artis, Hillstream, Roche Diagnostic. Michelle Gara (consultant): Cybrexa Therapeutics Inc, Heat Biologics, Pyramid. Paul Pearson (consultant): Acurastem, Ariagen Pharmaceuticals, Blueprint Medicines, Cybrexa Therapeutics, Epizon, Erasca, ESSA Pharmaceuticals, Evommune, Inipharm, National Institute of Health (NINDS), NIDO Biosciences, Lengo Therapeutics, Monopteros, Tetra Therapeutics. Arthur DeCillis (consultant): Ariagen, Bright Peak, Codiak, Genocea, Lumeda, Monopteros, Osmol, Pyramid. Arthur DeCillis (employee): Cybrexa Therapeutics Inc. Sophia Gayle (employee): Cybrexa Therapeutics Inc.