Fibroblast activation protein (FAP) is a promising target for radiopharmaceuticals due to its high expression in a variety of cancers. Alpha particle radiotherapy has emerged as a promising treatment by delivering radiation with a high linear energy transfer to tumor sites. Lead-212 (²¹²Pb; alpha-particle therapy; half-life 11 h) and ²⁰³Pb (SPECT imaging; half-life 52 h) are an elementally identical isotope pair for targeted-image-guided alpha-particle radioligand therapies. A novel cyclic peptide targeting human FAP was discovered via phage display screening and optimized  into final candidate PSV-359 consisting of  a molecular linker to a lead specific chelator. The objective of this study is the in vitro and in vivo evaluation of [^203/212Pb]Pb-PSV359.

Radiolabeling of ²¹²Pb and ²⁰³Pb with PSV359 was achieved using in-house protocols. Superior binding affinity and specificity of PSV359 against hFAP was observed via surface plasmon resonance (SPR) and enzyme inhibition assays (K_d=1.8 nM, K_i=0.4 nM). No binding of PSV359 to prolyl endopeptidase or dipeptidyl peptidase IV was observed, indicating excellent binding specificity. In vivo biodistribution of [²⁰³Pb]Pb-PSV359 in athymic nude mice bearing HT1080-hFAP xenografts (n=3), revealed strong tumor uptake (1.5 h: 20% ID/g, 24 h: 14% ID/g), and fast blood clearance (0.2%ID/g at 3 h) via the renal system (4.7%ID/g at 24 h). Preclinical planar imaging of [²⁰³Pb]Pb-PSV359 and [²¹²Pb]Pb-PSV359 in athymic nude mice bearing HT1080-FAP tumors also exhibited strong tumor uptake and retention, with low background in off-target tissues. Biodistribution of [²⁰³Pb]Pb-PSV359 in U87MG bearing athymic nude mice (n=3) showed high uptake (2 h: 11% ID/g, 6 h: 15% ID/g), despite low endogenous hFAP expression in U87MG cells. This indicates expression of mFAP in the U87MG tumor microenvironment from recruitment of stromal cells.

Alpha-particle radiotherapy of [²¹²Pb]Pb-PSV359 was performed in athymic nude mice bearing HT1080-hFAP or U87MG xenografts with 100 mm³ initial tumor volume. Body weight and tumor volume was determined twice per week during the studies, and upon conclusion blood samples were collected to perform CBC and serum chemistry against age-matched naïve mice. In the HT1080-hFAP model, 4.9 MBq [²¹²Pb]Pb-PSV359 was administered via IV injection over 3 fractions (n=10) at two-week intervals. An additional dose of 1.9 MBq was given when tumor recurrence was observed. In the HT1080-hFAP model, 100% survival was observed in treated mice upon conclusion of the 90-day study including 80% tumor remission. In U87MG xenograft model, 4.8 MBq [²¹²Pb]Pb-PSV359 was administered over 3 fractions (n=9) at two-week intervals. The nude mice bearing U87MG xenografts treated with [²¹²Pb]Pb-PSV-359 had an 89% survival rate upon conclusion of the 60-day study, among which 50% had minimal to no residual tumor. No significant change was found in CBC and serum chemistry analysis compared with naïve mice in both efficacy studies.

PSV-359 is a novel cyclic peptide that exhibits superior binding affinity, binding specificity, and in vivo tumor targeting in a preclinical setting. Strong anti-tumor efficacy of [²¹²Pb]Pb-PSV-359 was found in both HT1080-hFAP (FAP on cancer cells) and U87MG (FAP in stromal tissues) xenograft models.

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