Synthesis and evaluation of novel radioiodinated anthranilate derivatives for in vivo imaging of vascular endothelial growth factor receptor with single-photon emission computed tomography

Angiogenesis facilitates tumor survival and promotes malignancy. The vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR) tyrosine kinase (TK) signaling pathway is a key factor mediating angiogenesis, suggesting that this pathway may be a target for diagnosis and therapy. In this study, we aimed to develop small molecule radioiodinated probes applicable for in vivo VEGFR imaging considering the versatility and usefulness of single-photon emission computed tomography (SPECT).

We designed and synthesized four radioiodinated anthranilate compounds (6a–d) based on the structure of an anticancer drug targeting VEGFR-TK. The inhibitory potencies of corresponding cold compounds 4a–d and in vitro stability of compounds 6a–d were assessed by cellular proliferation inhibition assays and radio thin-layer chromatography after incubation in neutral solution. In vivo biodistributions were evaluated by determining radioactivity in tissues of interest after intravenous injection of test compounds in tumor-bearing mice. In vitro and in vivo blocking experiments using a selective VEGFR-TK inhibitor and SPECT/computed tomography (CT) imaging were performed in tumor-bearing mice.

The radioiodinated compounds 6a–d were obtained with more than 68.0% radiochemical yield and more than 95% radiochemical purity. Because compounds 4a–d showed high inhibitory potencies and compounds 6c and 6d showed high in vitro stability, 6c ([¹²⁵I]m-NPAM) and 6d ([¹²⁵I]p-NPAM) were further evaluated. Analysis of the in vivo biodistribution revealed a tumor to blood radioactivity ratio of greater than 4 at 24 h after [¹²⁵I]p-NPAM administration. Accumulation of radioactivity in cultured tumor cells and tumor xenografts after [¹²⁵I]p-NPAM administration was significantly blocked by inhibitor pretreatment. Tumors were clearly imaged at 24 h after [¹²⁵I]p-NPAM injection with SPECT/CT in comparison to that in inhibitor-pretreated tumor-bearing mice.

[¹²⁵I]p-NPAM may have potential applications as a lead compound for future development of a clinically usable VEGFR imaging probe for SPECT.