Development of a PET-based Theranostic for Drug-Resistant BRAFV600E Melanoma

Targeted radiopharmaceutical therapy (TRT) involving systemic administration of a tumor-selective agent labeled with a radionuclide has demonstrated considerable promise for cancer treatment, gaining traction recently with FDA approval of two agents since 2018. TRT agents bind tumor targets or accumulate within them, reducing toxicity to healthy cells relative to traditional external beam radiation therapy and allowing dose delivery to distant metastases. The radionuclides generate emissions that are both cytotoxic and suitable for non-invasive nuclear medicine imaging and/or have a diagnostic imaging/therapy isotope partner (‘theranostic pair’), which allows pretherapy imaging to facilitate patient-individualized dosing. Current standard of care TRT is given with a fixed dosing regimen, despite evidence that individualization may improve outcomes. Combined TRT and immune checkpoint immunotherapy (ICI) has demonstrated remarkable responses even in poorly immunogenic tumors both preclinically and in patients. In ICI-resistant mouse models, an optimal low-dose TRT range was determined from subject-specific dosimetry for combined TRT + ICI that led to complete tumor regression in most mice. Here, I aimed to translate positron-emission tomography (PET) for image-guided TRT in a clinically relevant BRAF-mutant mouse melanoma model. This mutation is druggable, but resistance develops rapidly. Combining this therapy with ICI may provide only modest benefit, and patients experiencing tumor progression have no reliable alternatives. I labeled the very late antigen (VLA-4) targeted LLP2A tracer with copper-64 for PET imaging (64Cu-LLP2A) and observed target-selective binding and internalization in vitro, as well as robust tumor uptake and retention in vivo. From longitudinal 64Cu-LLP2A imaging in this model, subject- and timepoint-specific image segmentation of the tumor and other organs of interest were input into a Monte Carlo dosimetry software to predict absorbed doses with the corresponding therapy isotope, copper-67 (67Cu-LLP2A). The tumor was predicted to receive significantly more 67Cu dose than any other tissue, including the often dose-limiting kidneys. Predicted tumor dose per injected activity guided selection of two 67Cu-LLP2A dose tiers for a therapy study in combination with dual ICI. Although the higher TRT dose and dual ICI showed the greatest benefit, the paradoxical benefit of the saline control versus all other remaining treatments necessitates replication and further scrutiny