Abstract CT104: MC1R imaging and histology in the targeted imaging of melanoma for alpha-particle radiotherapy (TIMAR1) trial

Purpose: Melanocortin 1 receptor (MC1R) has been shown in animal models to be a target for effective radionuclide therapy. This first-in-human clinical trial studied the safety and biodistribution of two novel MC1R-targeted imaging tracers designed to support the development of MC1R-targeted alpha particle therapy through image-based identification of MC1R and radiation dosimetry. Imaging was compared to MC1R expression by immunohistochemistry (IHC) of subjects’ melanoma tumors. Methods: Subjects with stage IV melanoma were randomized to receive either [203Pb]VMT01 SPECT/CT imaging followed by [68Ga]VMT02 PET/CT imaging approximately 1 month later, or vice versa in a cross-over design during their normal melanoma treatment course. Patient selection included a positive [18F]FDG PET/CT within 30 days of imaging tracer injection. 555 - 925 mBq of [203Pb]VMT01 was injected IV with imaging performed at 1, 4, and 24 hours. 74 - 277 mBq [68Ga]VMT02 was injected IV and imaging performed dynamically up to 1 hour, and at 2 and 3 hours. Due to an interruption in drug supply, only 3 subjects received [203Pb]VMT01 imaging. Key FDG-avid melanoma lesions were marked and presented to three experienced Radiologists serving as blinded reviewers. The blinded reviewers compared [18F]FDG PET/CT to experimental imaging and scored scans and key lesions on multiple metrics. Imaging positivity was defined as tumor uptake and retention of tracer in ≥ 1 melanoma tumors above background liver activity. Melanoma biopsy tissue was obtained and MC1R IHC performed as available. Results: 7 subjects were enrolled; 6 were imaged and 5 had available pathology. 3 of 6 imaged subjects had MC1R positive tumors via experimental imaging tracers. [68Ga]VMT02 PET/CT at 3 hours led to the best tumor to background ratio, and [203Pb]VMT01 SPECT/CT showed tumor retention at 24 hours. 4 of 5 subjects had positive MC1R staining by IHC. Of the 3 subjects with negative imaging, 2 had positive staining by IHC. Of the 3 subjects with positive imaging, 2 had available biopsies and all stained positive for MC1R by IHC. Conclusion: A subgroup of advanced melanoma patients were identified to express MC1R using new MC1R-targeted imaging agents. Full concordance between imaging and IHC of positive scans, but not with negative scans, suggests that IHC is a more sensitive biomarker detection method. Additional testing is needed to establish frequency of positive MC1R imaging in stage IV melanoma and its correlation to histological findings and treatment outcomes. Preferential tumor retention provides proof-of-concept that the MC1R-targeting therapy trial for [212Pb]VMT01 alpha-emitting therapy will deliver localized radiation to high MC1R expressing tumors as screened by [203Pb]VMT01 or [68Ga]VMT02. Imaging quality is sufficient for subject selection and dosimetry but may exclude subjects with MC1R receptor expression only detectable by IHC. Citation Format: Brenna M. Marks, Matthew Block, Geoffrey B. Johnson, Carrie B. Hruska, Mukesh K. Pandey, Andrew Paulsen, Michael K. Schultz, Michael A. McDonald, Edwin A. Sagastume, Frances L. Johnson. MC1R imaging and histology in the targeted imaging of melanoma for alpha-particle radiotherapy (TIMAR1) trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT104.