Abstract
241526
Introduction: Fibroblast activation protein (FAP) is overexpressed in cancer-associated fibroblasts (CAFs) in more than 90% of epithelial tumors, which is an ideal target for diagnosis and therapy of malignancy. [68Ga]Ga-FAPI-04 is one of the most widely studied FAP-targeted imaging agents with high binding affinity to FAP and rapid blood clearance. However, improving tumor uptake and retention may further broaden clinical applications for therapy. In this study, nitroimidazole was added to FAPI-04 to give DOTA-NI-FAPI-04 (compound 1) which led to excellent tumor accumulation and prolonged residence time after labeling with Ga-68 and Lu-177.
Methods: Compound 1 was synthesized and labeled with Ga-68 and Lu-177 at 95℃ for 10 min and 30 min, respectively. The radiochemical purity and in vitro stability of [68Ga]Ga/[177Lu]Lu-1 were determined by radio-HPLC and radio-TLC analysis. The cell uptake and internalization, competitive binding to FAP were studied with FAP-positive HT1080-FAP cells in vitro. Biodistribution and Micro-PET imaging were performed in U87MG tumor-bearing mice.
Results: [68Ga]Ga/[177Lu]Lu-1 showed high radiochemical purity (RCP > 95%, n > 5) and stability in saline and mouse serum in vitro (over 120 minutes for [68Ga]Ga-1, while [177Lu]Lu-1 for 6 days). The FAP binding affinity of compound 1 is 7.44 nM (IC50), comparable to FAPI-04 (3.94 nM) (Figure 1B). In HT1080-FAP cells, [68Ga]Ga/[177Lu]Lu-1 exhibited highly improved specific uptake and internalized fraction compared to [68Ga]Ga/[177Lu]Lu-FAPI-04, respectively (Figure 1C). Micro-PET imaging with [68Ga]Ga-1 showed higher specific tumor uptake and retention with rapid clearance from normal tissues (Figure 1D). In biodistribution study, [68Ga]Ga-1 displayed tumor uptake of 37.3 %ID/g at 60 min and 48.2 %ID/g at 120 min (for [68Ga]Ga-FAPI-04 the tumor uptake were 6.15 %ID/g at 60 min and 5.72 %ID/g at 120 min). [177Lu]Lu-1 also showed very high tumor uptake as compared to [177Lu]Lu-FAPI-04 (41.1 vs 17.9 and 50.8 vs 20.9 %ID/g at 60 and 120 min post-injection, respectively, n = 4).
Conclusions: [68Ga]Ga/[177Lu]Lu-1, containing nitroimidazole moiety, showed higher tumor uptake and retention compared to [68Ga]Ga/[177Lu]Lu-FAPI-04. Preliminary experiments indicate that [68Ga]Ga/[177Lu]Lu-1 holds promise as a novel FAP-targeting agent for both diagnostic and therapeutic purposes. Further clinical trials are currently underway.