Abstract LB-227: Preclinical development and characterization of STI-6129, an anti-CD38 antibody-drug conjugate, as a new therapeutic agent for multiple myeloma
Authors: Li Lingna , Hau Andrew , Tong Wenyong , Lau Megan , Zhu Tong , Fells Katherine , Wei Angel , Li Xiaoqing , Deng Dylan , Sun Yingqing , Kovacs Ernest , Khasanov Alisher , Yan Zheng , Zhu Panyun , Zhou Heyue , Ji Henry , Li Hui , Zhang Hong 
Published: September 2020

Abstract
CD38 is a validated target for the treatment of CD38-overexpressing hematologic malignancies such as multiple myeloma (MM). Daratumumab is the first anti-CD38 mAb drug approved by the FDA for use as single agent and in combination with standard therapies for MM. However, resistance to Daratumumab in both primary and refractory MM patients has been reported. Here we report the preclinical development of STI-6129 (also CD38-077), a CD38-targeting antibody-drug conjugate (ADC), as a new therapeutic agent for MM. STI-6129 consists of a fully human anti-CD38 antibody, STI-5171, identified from the Sorrento G-MAB® antibody library, conjugated to the microtubule inhibitor duostatin-5.2 (Duo-5.2) via a non-polyethylene glycol linker using our site-specific conjugation proprietary C-LOCK technology. STI-5171 binds specifically to CD38-positive but not CD38-negative tumor cell lines with a KD ~ 1.72E-8 M. STI-6129 is internalized into CD38-positive cells at a rate comparable to that of the unconjugated antibody STI-5171. In cytotoxicity studies, STI-6129 exhibited CD38-dependent cytotoxic activity against a panel of CD38-expressing tumor cell lines with the range of EC50 from 5-100 µg/mL. Importantly, STI-6129 does not display cytotoxic activity (EC50 &gt; 150 μg/mL) against normal human PBMCs after 120 hr exposure. Mechanistic evaluation of STI-5171 compared to Daratumumab revealed that this antibody does not induce homotypic aggregation of tumor cells, a cell adhesion response that has been reported to be linked to drug resistance and increased metastatic potential. Further, the anti-tumor activity of STI-6129 showed broad, potent, and CD38-dependent in vivo efficacy in multiple xenograft animal models. Pharmacokinetic evaluation of STI-6129 in Daudi-Fluc tumor-bearing mice revealed that the ADC is stable with half-life (T1/2) of 7-11 days, comparable to that of the unconjugated antibody. The serum concentration of STI-6129 remained above the therapeutically effective level up to 1 week in mice after a single injection at 10 mg/kg. In cynomolgus monkey, the PK profiles of STI-6129 and STI-5171 were nearly identical, further confirming the stability of the ADC in monkey blood circulation.In summary, STI-6129 exhibits potent in vitro and in vivo anti-tumor activities in multiple CD38-positive hematological models. These results warrant further development of STI-6129, potentially as a better or alternative agent for treatment of multiple myeloma.
Citation Format: Lingna Li, Andrew Hau, Wenyong Tong, Megan Lau, Tong Zhu, Katherine Fells, Angel Wei, Xiaoqing Li, Dylan Deng, Yingqing Sun, Ernest Kovacs, Alisher Khasanov, Zheng Yan, Panyun Zhu, Heyue Zhou, Henry Ji, Hui Li, Hong Zhang. Preclinical development and characterization of STI-6129, an anti-CD38 antibody-drug conjugate, as a new therapeutic agent for multiple myeloma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-227.