Abstract 2835: Fibroblast activation protein (FAP)-targeted radiotherapy increases tumor CD8+ T cell infiltration and enhances response to PD-1 immune checkpoint blockade

Background: FAP is a membrane-bound protease under investigation as a pan-cancer target, given its high expression in tumors but limited expression in normal tissues. FAP-2286 is a FAP-targeted radiopharmaceutical in clinical development for multiple solid tumors that consists of two functional elements: a FAP-targeting peptide and a chelator used to attach radioisotopes for imaging and therapeutic use. Preclinically, we evaluated the potential modulation of the immune response and antitumor efficacy of FAP-2287, a murine surrogate for FAP-2286, conjugated to the β-emitting radionuclide lutetium-177 (177Lu) as a monotherapy and in combination with a programmed cell death protein 1-targeting antibody (anti-PD-1). Methods: C57BL/6 mice bearing MCA205 fibrosarcoma mouse FAP-expressing tumors (MCA205-mFAP) were treated with 177Lu-FAP-2287, anti-PD-1, or both. Tumor uptake of 177Lu-FAP-2287 was assessed by SPECT/CT scanning, while therapeutic efficacy was measured by tumor volume. Immune profiling of tumor infiltrates was evaluated through flow cytometry, RNA expression, and immunohistochemistry analyses. Results: 177Lu-FAP-2287 accumulated in MCA205-mFAP tumors with 8.2 %ID/g uptake at 3 hours post-injection (pi), which was maintained at 2.9 %ID/g by 24 hours pi. Treatment with 177Lu-FAP-2287 demonstrated significant tumor growth inhibition (TGI, [74%; 267 mm3; P < 0.01]) vs vehicle control (750 mm3). Significant TGI was also observed from anti-PD-1 (57%; 373 mm³; P < 0.01) and the combination (92%; 145 mm³; P < 0.001) vs vehicle control. No significant differences in TGI were observed between the three treatment groups. In flow cytometry analysis of tumors, 177Lu-FAP-2287 increased CD8+ T cell infiltration by 2-fold on day 8 pi, alone or in combination with anti-PD-1 (16.9% and 18.0% of leukocytes, respectively), compared with 7.5% for vehicle control (P < 0.01). In contrast, anti-PD-1 alone had a nonsignificant increase in CD8+ T cell infiltration of 10.8% vs vehicle control (P > 0.05). On day 13 pi, the combination had durable higher levels of CD8+ T cells, with 16.3% of leukocytes vs 5.0% for vehicle control (P < 0.01). Monotherapy 177Lu-FAP-2287 also displayed a trend towards higher levels of CD8+ T cells at Day 13 vs vehicle (11.9% of leukocytes; P > 0.05). The increase in CD8+ T cells was accompanied by changes in co-stimulatory molecules such as CD86, resulting in greater activation by antigen-presenting cells. Additional immune cell characterization w