Development of a leucine-rich repeat-containing protein 15-targeted radio-immunotheranostic approach to deplete pro-tumorigenic mechanisms and immunotherapy resistance.

Leucine-rich repeat containing 15 (LRRC15) has emerged as an attractive biomarker and target for cancer therapy. Transforming growth factor-β (TGFβ) induces the expression of this plasma membrane protein specifically in aggressive and treatment resistant tumor cells derived from mesenchymal stem cells, with minimal expression observed in non-neoplastic tissues. We have developed a humanized monoclonal antibody, DUNP19, that specifically binds with high affinity to a phylogenetically conserved LRRC15 epitope and is rapidly internalized upon LRRC15 binding. In multiple subcutaneous and orthotopic tumor xenograft mouse models, Lutetium-177 labeled DUNP19 ([177Lu]Lu-DUNP19) enabled non-invasive imaging and molecularly precise radiotherapy to LRRC15-expressing cancer cells and murine cancer-associated fibroblasts, effectively halting tumor progression and prolonging survival with minimal toxicity. Transcriptomic analyses of [177Lu]Lu-DUNP19-treated tumors reveal a loss of pro-tumorigenic mechanisms, including a previously reported TGFβ-induced LRRC15+ signature associated with immunotherapy resistance. In a syngeneic tumor model, administration of [177Lu]Lu-DUNP19 significantly potentiated checkpoint-blockade therapy, yielding durable complete responses. Together, these results demonstrate that radio-theranostic targeting of LRRC15 with DUNP19 is a compelling precision medicine platform for image-guided diagnosis, eradication, and reprogramming of LRRC15+ tumor tissue that drives immuno-resistance and disease aggressiveness in a wide range of currently untreatable malignancies.