Abstract 5085: Preclinical development of NaPi2b-PL2202, a novel camptothecin-based antibody-drug conjugate targeting solid tumors expressing NaPi2b
Authors: Horsley Elizabeth , Jabeen Asma , Veillard Nicolas , Havenith Karin , Janghra Narinder , Alves Pedro , Oblette Cecile , Kirby Ian , Hogg Paul W. , Zammarchi Francesca , de Haan Lolke , van Berkel Patrick 
Published: March 2024

Abstract
NaPi2b, encoded by the SLC34A2 gene, is a multi-transmembrane, sodium-dependent phosphate transporter expressed at high levels in tumors and low levels in normal tissues. Functionally, it is involved in trans-cellular flux of phosphate in the small intestine and in the synthesis of surfactant in lung alveoli. NaPi2b is reported to be expressed at a high frequency in ovarian carcinoma as well as in NSCLC, bladder, endometrial and papillary thyroid carcinoma1, and its expression is associated with poor prognosis 2, 3. The purpose of this study was to characterize the in vitro and in vivo anti-tumor activity and tolerability of NaPi2b-PL2202, an antibody-drug conjugate (ADC) composed of a humanized, Fc-silenced IgG1 antibody directed against human NaPi2b, to which PL2202, a novel payload containing a valine-alanine cleavable linker and a camptothecin warhead, has been site-specifically conjugated with a drug to antibody ratio of 6. In addition, the level of NaPi2b expression in selected human tumor specimens was evaluated by immunohistochemistry (IHC). In vitro, NaPi2b-PL2202 showed good binding affinity for mouse, rat, cynomolgus monkey and human NaPi2b, and displayed highly potent, targeted cytotoxicity against various NaPi2b-expressing tumor cell lines. NaPi2b-PL2202 also exhibited strong in vitro bystander activity. In vivo, intravenous (IV) administration of a single dose of NaPi2b-PL2202 at 6.6 mg/kg resulted in substantial tumor regression in the OVCAR-3 ovarian carcinoma xenograft model, with 3/10 partial responders (PR) and 7/10 complete responders (CR), and all CRs being tumor-free at the end of the study (Day 44). In the G-402 renal leiomyoblastoma xenograft model, administration of a single IV dose of NaPi2b-PL2202 at 3.3 mg/kg was associated with durable antitumor activity and resulted in 1/10 CR, with this animal remaining tumor-free by the end of the study (Day 42). With respect to toxicity, in single dose studies in male rats, NaPi2b-PL2202 was tolerated up to 300 mg/kg IV. Furthermore, in a dose-range finding study in male cynomolgus monkeys, NaPi2b-PL2202 was well tolerated up to 40 mg/kg IV given on Day 1 and 22. Finally, cell membranous NaPi2b expression was confirmed by IHC in a panel of tissue microarrays including ovarian, lung, bladder and endometrial cancers. In conclusion, NaPi2b-PL2202 demonstrated specific and potent in vitro and in vivo anti-tumor activity, and was tolerated at high doses in toxicity studies in rats and cynomolgus monkeys, indicating a good therapeutic index. Together, these observations support future clinical development of NaPi2b-PL2202 for the treatment of NaPi2b-expressing cancers. 1- Bodyak N. D. et al., 2021 2- Valsenkova R. et al., 2021 3- Hakim S. A. et al., 2021
Citation Format: Elizabeth Horsley, Asma Jabeen, Nicolas Veillard, Karin Havenith, Narinder Janghra, Pedro Alves, Cecile Oblette, Ian Kirby, Paul W. Hogg, Francesca Zammarchi, Lolke de Haan, Patrick van Berkel. Preclinical development of NaPi2b-PL2202, a novel camptothecin-based antibody-drug conjugate targeting solid tumors expressing NaPi2b [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5085.