Emerging therapies including chimeric antigen receptor (CAR) T therapy targeting B cell maturation antigen (BCMA) have shown promising clinical outcomes in relapsed and refractory multiple myeloma (RRMM). However, majority of patients eventually develop progressive disease due to heterogeneity in BCMA expression or loss of antigen during treatment. Therefore, new targets are required to overcome the problem of resistance encountered with these agents. The expression of GPRC5D (G-protein-coupled receptor class 5 member D) is high on MM cells and low in normal tissues. Although high levels of GPRC5D and BCMA were found in similar proportions of MM patients, they are independently expressed by MM cells. GPRC5DxCD3 bispecific T-cell-redirecting antibody is currently under clinical investigation. Preliminary results have shown early evidence in efficacy with tolerable safety profile. These findings identified GPRC5D as an ideal therapeutic antibody drug conjugate (ADC) target for the treatment of patients with RRMM. Therefore, the present study was aimed at developing a novel anti-GPRC5D ADC drug LM-305 and evaluating its preliminary efficacy by performing preclinical activity using in vitro and in vivo mouse models. The binding affinity of LM-305 was evaluated by flow cytometry in engineered GPRC5D over-expressing MM tumor cells and endogenous GPRC5D-expressing MM tumor cells. The internalization of LM-305 was assessed with pH-dependent dye in NCI-H929 and MM.1R cell lines. The cytotoxicity of LM-305 was assessed by cell viability assay in MM tumor cell lines. In vivo anti-tumor activity of multiple doses of LM-305 (1mg/kg, 3mg/kg, 10mg/kg) was evaluated in NCI-H929 and MM.1R tumor cell line derived xenograft (CDX) models. In vitro studies revealed that LM-305 binds with high affinity to GPRC5D over-expressing cell lines and GPRC5D endogenously expressing MM cells in a dose-dependent manner. It can be efficiently internalized by GPRC5D expressing cells and further lysosomal lysis was detected by pHrodo dye. LM-305 displayed potent cytotoxicity when co-cultured with MM tumor cells (NCI-H929 and MM.1R) with IC50 values ranging from 0.1 to 0.3 nM. In vivo tumor xenograft models suggested that treatment with LM-305 resulted in dose-dependent inhibition of tumor growth in tumor bearing mice. Additionally, LM-305 exhibited complete response (CR) in the GPRC5D high-expressing MM CDX models at a dose of 3mg/kg. Moreover, LM-305 showed good safety profile in the animal studies. In conclusion, this preclinical data suggested that LM-305 is a novel GPRC5D targeting ADC with best-in-class potential, and therefore it can be a promising therapeutic candidate for the treatment of RRMM patients expressing GPRC5D.

Citation Format: Wentao Huang, Jie Luo, Yifan Li, Da Fei, Xia Qin, Runsheng Li. Preclinical activity of LM-305 targeting G-protein-coupled receptor class 5 member D (GPRC5D) antibody drug conjugate for the treatment of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6020.