Abstract LB045: Exploiting biparatopic binding to deliver novel, differentiated ALPP/ALPPL2 targeting protein drug conjugates

Abstract The placental alkaline phosphatases ALPP and ALPPL2 are attractive targets for antibody and protein-drug conjugate (ADC and PDC) approaches. These GPI-anchored cell-surface proteins, which share very high sequence identity, are over-expressed in a number of different solid tumor indications including testicular, ovarian, endometrial, gastric, pancreatic and non-small cell lung cancers but have very little expression on normal adult tissues. Consequently, there is significant interest in exploiting ALPP/ALPPL2 for the development of new targeted oncology therapeutics, and an ALPP/ALPPL2 ADC, utilizing a classical full-length IgG antibody, is currently in Phase I clinical trials. The exploitation of biparatopic binding is an attractive approach for improving the therapeutic index of ADCs. This mode of target engagement increases the effective number of binding sites on the tumor cells and can promote internalisation and trafficking, thereby delivering increased efficacy for the same payload bolus and opening up patient populations with lower target expression. This can lead to a widening of the therapeutic window, by improving the efficacy achievable for a given amount of payload administered (often defined as the “platform” toxicity). For the development of biparatopic conjugates, the use of small protein domain binders offers advantages over the use of full-length antibodies, as they can readily be reformatted into a variety of biparatopic architectures to deliver homogeneous drug conjugates in high yield. Additionally, smaller protein formats have the capacity to penetrate deeper into solid tumors than large antibodies. We have identified a series of highly selective, high affinity ALPP/ALPPL2 single domain VHH binders, which target distinct epitopes on the protein and which, importantly, show no binding to the closely related ALPI or ALPL isoforms (which have high normal tissue expression). Using protein engineering approaches, a series of homogenous site-specifically labelled biparatopic PDCs have been developed employing the clinically validated vcMMAE (vedotin) payload. In pre-clinical studies, these PDC agents demonstrate excellent anti-tumor efficacy in vivo in ALPP/ALPPL2 positive cell-line derived xenograft models of gastric, pancreatic and ovarian cancer displaying a range of target expression. In benchmarking studies, lead biparatopic PDCs show increased internalisation and target-mediated in vitro cell kill potency compared to an ALPP/ALPPL2 targeting ADC. Additionally, in multiple CDX models, lead biparatopic PDCs demonstrate improved efficacy over an ALPP/ALPPL2 ADC both on a mg/kg and on a payload equivalents basis, in both single dose and multi-dose studies, providing complete regressions. Based on this excellent efficacy profile and favourable developability characteristics, preclinical development activities have been initiated. Citation Format: Graham Cotton, Estelle G. McLean, Paul Trumper, Mark Wappett, Stephanie Gatdula, Stacey Bell, Greg Papadakos, Kieron Lucas, Stephanie Burton, Chiara Saladino, Georgiana Parau, Aidan McCann, Jennifer Thom, Aaron N. Cranston, Tim Harrison. Exploiting biparatopic binding to deliver novel, differentiated ALPP/ALPPL2 targeting protein drug conjugates abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr LB045.