FAP Targeted Imaging Agents

Fibroblast Activation Protein (FAP) is a type II transmembrane serine protease that is highly expressed in the tumor microenvironment in the form of activated fibroblasts or cancer-associated fibroblasts (CAFs). Fibroblasts are involved in multiple interactions with stromal tissues and extracellular components, and as part of the tumor microenvironment, they play an active role in the proliferation and migration of the cancer cells. Since CAFs are negligibly expressed in normal tissues but overexpressed in 90% of the epithelial tumors, FAP could be a promising target for the development of novel drug molecules, both for diagnostic and therapeutic applications. Recently, several small-molecule-based FAP-targeting drugs (including FAPI-46 and FAPI-72) and a peptide, FAP-2286, have emerged as powerful drug molecules for diagnostic imaging and therapeutic applications. The diagnostic imaging using Fluorine-18 and Gallium-68 labeled compounds allows visualization of a wide range of tumors, whereas the radionuclide therapy using Lu-177 or Ac-225 labeled drugs has shown enhanced efficacy when used as therapy. Both FAPI-46 and FAPI-72, and the peptide FAP-2286, have shown exceptional specificity to FAP expressing tumors in pre-clinical models, and are under evaluation to investigate their diagnostic and therapeutic role in the clinic.

Through this research proposal, we want to explore triazine-based difluoropyrrolidine-2-carbonitrile derivatives as next-generation FAP-inhibitors. Our work will focus on incorporating the difluoropyrrolidine-2-carbonitrile (a small organic molecule known for its exceptional selectivity to CAFs) with s-triazine and other anticancer moieties, including pyrazole, morpholine, and piperidine. Reason to choose triazine as a building block is, triazine will allow molecular diversification to synthesize dimers and bi-specific molecules in addition to the attachment point for albumin binders, which will enhance blood circulation and plasma stability of novel drug molecules. On successful synthesis, the best molecule will be radiolabeled with Gallium-68, Copper-64, and/or Fluorine-18 (any of the three PET radionuclides) to perform biodistribution and in vivo evaluation using PET imaging. The novel molecules may have improved pharmacokinetic properties and can lead to the discovery of novel radiotracers as future candidates for imaging and therapy of fibroblast-associated cancers, including breast, colorectal, pancreatic, and lung cancer.