Evaluating near‐infrared photoimmunotherapy for targeting fibroblast activation protein‐α expressing cells in vitro and in vivo

Photoimmunotherapy (PIT), carried out using an Ab conjugated to the near infrared dye IRDye700DX, is achieving significant success in target‐specific elimination of cells. Fibroblast activation protein alpha (FAP‐α) is an important target in cancer because of its expression by cancer‐associated fibroblasts (CAFs) as well as by some cancer cells. Cancer‐associated fibroblasts that express FAP‐α have protumorigenic and immune suppressive functions. Using immunohistochemistry of human breast cancer tissue microarrays, we identified an increase of FAP‐α(+) CAFs in invasive breast cancer tissue compared to adjacent normal tissue. We found FAP‐α expression increased in fibroblasts cocultured with cancer cells. In proof‐of‐principle studies, we engineered human FAP‐α overexpressing MDA‐MB‐231 and HT‐1080 cancer cells and murine FAP‐α overexpressing NIH‐3T3 fibroblasts to evaluate several anti‐FAP‐α Abs and selected AF3715 based on its high binding affinity with both human and mouse FAP‐α. After conjugation of AF3715 with the phthalocyanine dye IR700, the resultant Ab conjugate, FAP‐α‐IR700, was evaluated in cells and tumors for its specificity and effectiveness in eliminating FAP‐α expressing cell populations with PIT. Fibroblast activation protein‐α‐IR700‐PIT resulted in effective FAP‐α‐specific cell killing in the engineered cancer cells and in two patient‐derived CAFs in a dose‐dependent manner. Following an intravenous injection, FAP‐α‐IR700 retention was three‐fold higher than IgG‐IR700 in FAP‐α overexpressing tumors, and two‐fold higher compared to WT tumors. Fibroblast activation protein‐α‐IR700‐PIT resulted in significant growth inhibition of tumors derived from FAP‐α overexpressing human cancer cells. A reduction of endogenous FAP‐α(+) murine CAFs was identified at 7 days after FAP‐α‐IR700‐PIT. Fibroblast activation protein‐α‐targeted near infrared PIT presents a promising strategy to eliminate FAP‐α(+) CAFs.