Abstract
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Objectives: Mirvetuximab soravtansine (IMGN853), an antibody drug conjugate currently undergoing clinical trials in triple negative breast cancer (TNBC), ovarian cancer, and endometrial cancer patients, comprises the humanized folate receptor α (FRα)-binding M9346A antibody linked to the tubulin-disrupting maytansinoid, DM4. The clinical approach for patient screening is immunohistochemical assessment of archival tumor or biopsy samples, which may suffer from limitations of tumor heterogeneity and limited tissue collection. The goal of this study was to develop zirconium-89 (89Zr) radiolabeled M9346A antibody as a PET imaging tool to assess the expression of FRα in whole TNBC tumors and guide the treatment using IMGN853.
Methods: M9346A was conjugated to deferoxamine (DFO) for 89Zr radiolabeling and characterized with native mass spectrometry (MS). The FRαhigh HeLa cells and FRαlow OVCAR-3 cells were used to determine the binding specificity and immunoreactivity of 89Zr-M9346A.Biodistribution was performed in wild-type C57Bl/6 mice via tail vein injection at multiple time points post injection (p.i.). PET imaging was first performed in HeLa xenografts and then TNBC patient derived xenografts (PDXs) with various levels of FRα expression. Treatment studies were performed in a FRαhigh and a FRαlow PDX models treated with IMGN853, pemetrexed, IMGN853+pemetrexed, paclitaxel and saline, respectively. 89Zr-M9346A and 18F-FDG PET were performed during the treatment to determine the tracers’ tumor uptake. FOLR1 mRNA expression and histopathology of tumors were also performed.
Results: The MS characterization showed an average of 1.5 DFO conjugated to each M9346A, leading to specific activity of 6-8 µCi/µg of 89Zr-M9346A. Lindmo assay in HeLa cells demonstrated approximately 94% immunoreactivity and the EC50 for FRα binding was 4.81 ± 0.84 nM. Biodistribution of 89Zr-M9346A showed extended blood circulation (t1/2= 15.2 h, 17.8 ± 1.33 %ID/g at 144 h p.i., n=5) and gradually decreased liver accumulation (6.98 ± 0.59 %ID/g at day 6). PET imaging in HeLa tumors showed high tracer accumulation (SUV = 5.49 ± 1.92, n=3) at 72 h p.i. and the targeting specificity was confirmed by the competitive blocking studies (SUV = 1.87 ± 0.61, p≤0.05, n=3). In TNBC PDX models, 89Zr-M9346A PET showed more sensitive and specific detection of FRα in tumors than 18F-FDG, confirmed by RT-PCR. In the FRαhigh PDX model, 89Zr-M9346A uptake was approximately 3 times higher than that determined in FRαlow PDX model. In contrast to the chemotherapy drugs showing little therapeutic effect in both PDX models, IMGN853+pemetrexed and IMGN853 showed effective inhibition of tumor growth in the FRαhigh PDX model but not in the FRαlow PDX model during the therapy studies. The uptake of 89Zr-M9346A in FRαhigh tumors was significantly (SUV = 2.89 ± 0.31 vs. 0.66 ± 0.13, p≤0.0002, n=4) decreased following IMGN853 monotherapy targeted treatment, which was consistent with tumor mass variations during the treatment. Conclusion: 89Zr-M9346A demonstrated the FRα imaging sensitivity and specificity in PDX models. The effective treatment efficiency of IMGN853 in the FRαhigh TNBC PDX model suggests the potential of 89Zr-M9346A as a noninvasive imaging tool to prescreen patient for targeted treatment using IMGN853 in TNBC patients. Research Support: This research is sponsored by ImmunoGen, Inc. through the National Comprehensive Cancer Network (NCCN)