Synthesis and Evaluation of Novel In-Labeled Picolinic Acid-Based Radioligands Containing an Albumin Binder for Development of a Radiotheranostic Platform.

Picolinic acid-based metallic chelators, e.g., neunpa and octapa, have attracted much attention as promising scaffolds for radiotheranostic agents, particularly those containing larger α-emitting radiometals. Furthermore, albumin binder (ALB) moieties, which noncovalently bind to albumin, have been utilized to improve the pharmacokinetics of radioligands targeting various biomolecules. In this study, we designed and synthesized novel neunpa and octapa derivatives (Neunpa-2 and Octapa-2, respectively), which contained a prostate-specific membrane antigen (PSMA)-binding moiety (model targeting vector) and an ALB moiety. We evaluated the fundamental properties of these derivatives as radiotheranostic agents using In. In a cell-binding assay using LNCaP (PSMA-positive) cells, [In]In-Neunpa-2 and [In]In-Octapa-2 specifically bound to the LNCaP cells. In addition, a human serum albumin (HSA)-binding assay revealed that [In]In-Neunpa-2 and [In]In-Octapa-2 exhibited greater binding to HSA than their non-ALB-conjugated counterparts ([In]In-Neunpa-1 and [In]In-Octapa-1, respectively). A biodistribution assay conducted in LNCaP tumor-bearing mice showed that the introduction of the ALB moiety into the In-labeled neunpa and octapa derivatives resulted in markedly enhanced tumor uptake and retention of the radioligands. Furthermore, single-photon emission computed tomography imaging of LNCaP tumor-bearing mice with [In]In-Octapa-2 produced tumor images. These results indicate that [In]In-Octapa-2 may be a useful PSMA imaging probe and that picolinic acid-based ALB-conjugated radiometallic complexes may be promising candidates as radiotheranostic agents.