Development of Fibroblast Activation Protein Inhibitor-Based Dimeric Radiotracers with Improved Tumor Retention and Antitumor Efficacy.

Fibroblast activation protein (FAP), a fundamental component of the tumor stroma, is overexpressed in cancer-associated fibroblasts (CAFs). As a promising theranostic probe, we evaluated whether the FAP inhibitor (FAPI) dimer (DOTA-2P[FAPI]) is more effective than its monomeric analogs for FAP-targeted radionuclide therapy. [Ga]Ga/[Lu]Lu-DOTA-2P(FAPI) were assayed in a stability study, small-animal positron emission tomography (PET) and single-photon emission computed tomography (SPECT), biodistribution, and radionuclide therapy to comprehensively evaluate their preclinical pharmacokinetics. The pharmacokinetics of [Ga]Ga-DOTA-2P(FAPI) and [Lu]Lu-DOTA-2P(FAPI) were determined in FAP-positive hepatocellular carcinoma patient-derived xenografts (PDXs) and HT-1080-FAP cell-derived xenografts (CDXs). [Ga]Ga-DOTA-2P(FAPI) and [Lu]Lu-DOTA-2P(FAPI) were stable in phosphate-buffered saline for 4 h. The tumor retention of [Ga]Ga-DOTA-2P(FAPI) was better than that of [Ga]Ga-FAPI-46 in HT-1080-FAP CDXs, while healthy organs showed low tracer uptake and fast body clearance. In single-photon emission computed tomography, [Lu]Lu-DOTA-2P(FAPI) showed a higher uptake and longer retention for tumors in both PDXs and CDXs from 1-48 h. [Lu]Lu-DOTA-2P(FAPI) showed the best inhibition of tumor growth in PDXs and CDXs. DOTA-2P(FAPI) has increased tumor uptake and retention properties compared to FAPI-46, which significantly improves the use of FAPI-based vectors for PET imaging and radionuclide therapy. [Lu]Lu-DOTA-2P(FAPI) may be safe and effective for the treatment of FAP-positive malignant tumors.