PET Imaging of Solid Tumors by a Novel Tracer, 68Ga-FAPI

Malignant tumors exceeding 1-2 mm in size require formation of a supporting stroma, which includes vascular cells, inflammatory cells and fibroblasts . Several organs in the upper gastro-intestinal tract are known to develop tumors with strong desmoplastic reaction characterized by pervasive growth of tumor stroma. The pancreas, stomach, bile ducts and ovaries are all organs with this property. Within tumor stroma, a subpopulation of fibroblasts called cancer-associated fibroblasts (CAFs) are known to be involved in growth, migration and progression of the tumor. The Fibroblast Activation Protein (FAP) is one of the more prominent stroma markers and was the focus in the development of an agent for imaging and, eventually, even targeted radionuclide therapy. FAP is a type II membrane bound glycoprotein absent or only expressed at insignificant levels, in normal tissues in adults. The FAP inhibitor, FAPI, gets selectively enriched in tissues where its target protein is expressed and there is no or very limited FAPI uptake in all normal organs. This opens new possibilities for the detection of malignant lesions with higher stromal content based on the high contrast positron emission tomography (PET) images obtained with a 68-Gallium (68Ga) radiolabeled - FAPI compound. As cancers in pancreas, stomach, bile ducts and ovaries are all characterized by abundant desmoplasia that constitutes up to 90% of the total tumor volume and contains extracellular matrix, immune cells, vasculature and CAFs, it would be suitable for targeted imaging with FAPI. Preliminary studies show elevated FAPI uptake in many tumors rich in fibroblasts along with low background uptake. The main objective of this prospective study is to improve non-invasive diagnostics of malignancy in tumors of pancreas, stomach, bile ducts and ovaries, all known for a strong desmoplastic reaction by evaluating the diagnostic accuracy of PET/CT with a novel radiotracer, FAPI in the primary diagnosis and staging of such cancers.