Fibroblast Activation Protein-Targeted Photodynamic Therapy of Cancer-Associated Fibroblasts in Murine Models for Pancreatic Ductal Adenocarcinoma

Patients with pancreatic ductal adenocarcinoma (PDAC)have a dismal5 year survival of 9%. One important limiting factor for treatmentefficacy is the dense tumor-supporting stroma. The cancer-associatedfibroblasts in this stroma deposit excessive amounts of extracellularmatrix components and anti-inflammatory mediators, which hampers theefficacy of chemo- and immunotherapies. Systemic depletion of allactivated fibroblasts is, however, not feasible nor desirable andtherefore a local approach should be pursued. Here, we provide a proof-of-principleof using fibroblast activation protein (FAP)-targeted photodynamictherapy (tPDT) to treat PDAC. FAP-targeting antibody 28H1 and irrelevantcontrol antibody DP47GS were conjugated to the photosensitizer IRDye700DX(700DX) and the chelator diethylenetriaminepentaacetic acid. In vitrobinding and cytotoxicity were evaluated using the fibroblast cell-lineNIH-3T3 stably transfected with FAP. Biodistribution of In-111-labeled antibody-700DX constructs was determined in mice carryingsyngeneic tumors of the murine PDAC cell line PDAC299, and in a geneticallyengineered PDAC mouse model (CKP). Then, tPDT wasperformed by exposing the subcutaneous or the spontaneous PDAC tumorsto 690 nm light. Induction of apoptosis after treatment was assessedusing automated analyses of immunohistochemistry for cleaved caspase-3.28H1-700DX effectively bound to 3T3-FAP cells and induced cytotoxicityupon exposure to 690 nm light, whereas no binding or cytotoxic effectswere observed for DP47GS-700DX. Although both 28H1-700DX and DP47GS-700DXaccumulated in subcutaneous PDAC299 tumors, autoradiography demonstratedthat only 28H1-700DX reached the tumor core. On the contrary, controlantibody DP47GS-700DX was only present at the tumor rim. In CKP mice, both antibodies accumulated in the tumor, buttumor-to-blood ratios of 28H1-700DX were higher than that of the control.Notably, in vivo FAP-tPDT caused upregulation of cleaved caspase-3staining in both subcutaneous and in spontaneous tumors. In conclusion,we have shown that tPDT is a feasible approach for local depletionof FAP-expressing stromal cells in murine models for PDAC.