Therapeutic targeting of folate receptor alpha (FRα) by antibody-polymer-drug conjugate in testicular germ cell tumors

Antibody-drug conjugates (ADCs) are an effective treatment for recurrent/refractory testicular germ cell tumors (rrTGCTs). In this exploratory study, we report disease stabilization achieved with the ADC Mirvetuximab soravtansine (MIRV) in a heavily pretreated patient with rrTGCT and compare this ADC with a novel antibody-polymer-drug conjugate (APDC) targeting folate receptor α (FRα), MIRV-P-MMAE. APDC MIRV-P-MMAE with a drug-to-antibody ratio ∼23, was synthesized using orthogonal attachment of a highly hydrophilic copolymer to the hinge region of anti-FRα antibody. Its efficacy was compared with MIRV in vitro in TGCT cells and in vivo in a metastatic model of human testicular choriocarcinoma (CHC). Furthermore, we evaluated FRα expression in TGCT and assessed efficacy of MIRV in an rrTGCT patient. Our investigations revealed the highest antiproliferative effect of MIRV-P-MMAE in FRα-positive CHC cells, JAR and JEG3, with IC50 values of 19.8 pM and 22.5 pM, respectively. These IC50 values are 210- and 70-fold lower than those of MIRV. Cytotoxicity was also confirmed in SuSa, PA1, and NCR-G1 cell lines. High efficacy was demonstrated in animals intravenously injected with JAR-luc cells, in which APDC treatment significantly prolonged survival (p = 0.0002) and completely cured 3 of 8 animals. Our data showed FRα membrane staining in 5,1% of TGCT samples, but confirmed cytoplasmic positivity in all 5 viable rrTGCT post-chemotherapy patient samples, suggesting that APDC with high DAR could be explored in rrTGCT patients with low expression of the target antigen. Our data warrant further exploration of FRα-targeting APDC as a novel approach for treating FRα-expressing rrTGCT.