The expression of carbonic anhydrase-IX (CA-IX) in tumors can lead to a poor prognosis; thus, CA-IX has attracted much attention as a target molecule for cancer diagnosis and treatment. An 111 In-labeled imidazothiadiazole sulfonamide (IS) derivative, [ 111 In]In-DO3A-IS1, exhibited marked tumor accumulation but also marked renal accumulation, raising concerns about it producing a low signal/background ratio and a high radiation burden on the kidneys. In this study, four 111 In-labeled IS derivatives, IS-[ 111 In]In-DO2A-ALB1−4, which contained four different kinds of albumin binder (ALB) moieties, were designed and synthesized with the aim of improving the pharmacokinetics of [ 111 In]In-DO3A-IS1. Their utility for imaging tumors that strongly express CA-IX was evaluated in mice. An in vitro binding assay of cells that strongly expressed CA-IX (HT-29 cells) was performed using acetazolamide as a competitor against CA-IX, and IS-[ 111 In]In-DO2A-ALB1−4 did not exhibit reduced binding to HT-29 cells compared with [ 111 In]In-DO3A-IS1. In contrast, IS-[ 111 In]In-DO2A-ALB1−4 showed a greater ability to bind to human serum albumin than [ 111 In]In-DO3A-IS1 in vitro. In an in vivo biodistribution study, the introduction of an ALB moiety into the 111 Inlabeled IS derivative markedly decreased renal accumulation and increased HT-29 tumor accumulation and blood retention. The pharmacokinetics of the IS derivatives varied depending on the substituted group within the ALB moiety. Single-photon emission computed tomography imaging with IS-[ 111 In]In-DO2A-ALB1, which showed the highest tumor/kidney ratio in the biodistribution study, facilitated clear HT-29 tumor imaging, and no strong signals were observed in the normal organs. These results indicate that IS-[ 111 In]In-DO2A-ALB1 may be an effective CA-IX imaging probe and that the introduction of ALB moieties may improve the pharmacokinetics of CA-IX ligands.
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