Development of a first-in-class camptothecin-based antibody-drug conjugate targeting ALPP/ALPPL2 with potent antitumor activity and excellent tolerability

Introduction: ALPP (alkaline phosphatase, placental) and ALPPL2 (alkaline phosphatase, placental-like 2) emerge as promising targets due to their expression across multiple solid tumors, including ovarian (60%), endometrial (50%), pancreatic (30%), and gastric cancer (15%) along with minimal expression in normal adult tissues. We developed a novel ALPP/ALPPL2-directed antibody-drug conjugate (ADC) incorporated a proprietary camptothecin-derived payload linked through a protease-cleavable linker to a high-affinity monoclonal antibody. Preclinical studies demonstrate robust antitumor activity with favorable safety, supporting its potential to benefit patients with refractory advanced solid tumors. Methods: Parental antibody binding to ALPP/ALPPL2-positive cells was evaluated by flow cytometry. Selectivity against related phosphatases (hALPL and hALPI) was confirmed via enzyme-linked immunosorbent assay. ALPP/ALPPL2-ADC cytotoxicity was assessed using CellTiter-Glo (CTG) in ALPPL2-positive cells (HEP2, NCI-H1651). Bystander killing was quantified in co-cultures of ALPPL2-positive HEP2 cells and Jurkat controls by measuring Jurkat viability with CTG assay. In vivo efficacy was examined in Capan-1 pancreatic adenocarcinoma cell line-derived xenograft (CDX) model and LD1-0017-200702 gastric patient-derived xenograft (PDX) models. Pharmacokinetics and toxicology were characterized in cynomolgus monkeys at 20 and 40 mg/kg, administered every three weeks for three cycles. Results: The parental ALPP/ALPPL2 antibody demonstrated high specificity for human and cynomolgus ALPP/ALPPL2, with no cross-reactivity to related phosphatases (hALPL and hALPI). The ALPP/ALPPL2-ADC (drug antibody ratio of 8), incorporating a proprietary camptothecin-based linker-payload, exhibited target mediated cytotoxicity in HEP2 (ALPP/ALPPL2 high ) and NCI-H1651 (ALPP/ALPPL2 medium ) cells in vitro . The ADC also showed a 50-fold greater bystander killing effect compared with a deruxtecan-based ADC. In vivo , a single 1 mg/kg dose induced profound tumor regression in the Capan-1 CDX model, while in the gastric adenocarcinoma PDX model refractory to an ADC with an MMAE payload, a single 8 mg/kg dose achieved deep tumor remission. In non-human primates, the ALPP/ALPPL2 showed linear pharmacokinetics and excellent tolerability (highest non severely toxic dose ≥40 mg/kg). Conclusion: ALPP/ALPPL2-ADC is a promising therapeutic candidate for solid tumors, particularly gynecologic cancers. As a first-in-class camptothecin-based ADC targeting ALPP/ALPPL2, it combines high antibody specificity with robust antitumor efficacy and favorable tolerability, supporting advancement into clinical development.