⁸⁹Zr-M9346A immuno-PET imaging folate receptor alpha in triple negative breast cancer for image-guided intervention with mirvetuximab soravtansine

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 FRα^high HeLa cells and FRα^low 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 FRα^high and a FRα^low 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 (t_1/2= 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 FRα^high PDX model, ⁸⁹Zr-M9346A uptake was approximately 3 times higher than that determined in FRα^low 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 FRα^high PDX model but not in the FRα^low PDX model during the therapy studies. The uptake of ⁸⁹Zr-M9346A in FRα^high 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 FRα^high 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)