Preclinical Evaluation of 89Zr-Df-IAB22M2C PET as a Potential Imaging Biomarker in Targeted Immuno-Therapeutic Drug Development of a GUCY2C-CD3 bispecific antibody PF-07062119.

408 Introduction: A specific and sensitive imaging biomarker to monitor immune activation and quantify pharmacodynamic (PD) responses would be a useful tool in drug development for immunomodulating anti-cancer agents. PF-07062119 is a T-cell engaging CD3 bispecific antibody targeting GUCY2C (GUCY2C-CD3) which is over expressed widely across colorectal cancer (CRC) and other gastrointestinal malignancies. Previous data has demonstrated the efficacy of PF-0762119 as a targeted immuno-therapeutic candidate in human CRC xenograft mouse models with adoptive transfer of human T cells (ref). 89Zr-Df-IAB22M2C, an 89Zr-labeled human CD8 specific minibody developed by ImaginAb, is currently under testing in a phase 2 clinical study (ClinicalTrials.gov Identifier: NCT03802123) in patients with metastatic solid tumors to measure CD8 T-cell tumor infiltrates. In this study we investigated the ability of 89Zr-Df-IAB22M2C to track responses to interventions with varying doses of PF-07062119 in a human CRC adoptive transfer mouse model, as well as, correlation of the radiotracer to CD8 immunohistochemical (IHC) staining. Materials & Method: NOD scid gamma (NSG) female mice (total 70; n = 5-12 per group) bearing human CRC LS1034 tumors were used in this study. Following a pilot study, animals were treated with PF-07062119 in four different dose groups (0.03 mg/kg, 0.06 mg/kg, 0.1 mg/kg or 1 mg/kg), or with a non-targeted CD3 bispecific control at a dose of 1 mg/kg, on study Day-0 and injected with activated T-cells on Day-1. Different imaging time points, Day-4, Day 6, or Day-9 were assessed with 89Zr-Df-IAB22M2C intravenously injected (200 µL, with radioactivity of approximately 65 µCi) on Day-3, Day-5, and Day-8, respectively. All animals underwent a 20-minute static PET scan at 22h post 89Zr-Df-IAB22M2C injection, followed by CT scan for anatomical registration. For Day-9 imaging group animals, a second treatment of PF-07062119 was conducted on Day-7. Following PET/CT imaging, mice were euthanized under anesthesia and dissected for ex vivo distribution analysis of tissues [whole blood, brain, heart, kidneys, large intestine (emptied), liver, lungs, skeletal muscle (quadriceps), skin, small intestine (emptied), spleen, tail, and tumor]. Tissues were collected, weighed and gamma counted for their radioactivity. Data analyzed and reported as %ID/g and SUV for both in vivo imaging and ex vivo tissue distribution. In addition, tumor tissues were pretreated with 10% neutral buffered formalin (NBF) prior to the gamma counting for upcoming tissue section and CD8 IHC staining. Results & Conclusions: The results demonstrated substantial mean uptake in treated tumors, up to 13.0 %ID/g (2.7 S.D.) in highest dosed animals (ex vivo gamma counting), with significant differences in comparison to isotype controls. The data also showed a dose-dependent response for both time and treatment: significantly higher radioactivity concentration was observed at the higher dose levels of PF-0762119 (0.1 and 1 mg/kg) on Day-9 compared to Day-4 (p