Development of a Radioiodinated Triazolopyrimidine Probe for Nuclear Medical Imaging of Fatty Acid Binding Protein 4

Fatty acid binding protein 4 (FABP4) is the most well-characterized FABP isoform. FABP4 regulates inflammatory pathways in adipocytes and macrophages and is involved in both inflammatory diseases and tumor formation. FABP4 expression was recently reported for glioblastoma, where it may participate in disease malignancy. While FABP4 is a potential molecular imaging target, with the exception of a tritium labeled probe there are no reports of other nuclear imaging probes that target this protein. Here we designed and synthesized a nuclear imaging probe, ¹²³I]TAP1, and evaluated its potential as a FABP4 targeting probe in in vitro and in vivo assays. We focused on the unique structure of a triazolopyrimidine scaffold that lacks a carboxylic acid to design the TAP1 probe that can undergo facilitated delivery across cell membranes. The affinity of synthesized TAP1 was measured using FABP4 and 8-anilino-1-naphthalene sulfonic acid. ¹²⁵I]TAP1 was synthesized by iododestannylation of a precursor, followed by affinity and selectivity measurements using immobilized FABPs. Biodistributions in normal and C6 glioblastoma-bearing mice were evaluated, and excised tumors were subjected to autoradiography and immunohistochemistry. TAP1 and ¹²⁵I]TAP1 showed high affinity for FABP4 (K _i = 44.5±9.8 nM, K _d = 69.1±12.3 nM). The FABP4 binding affinity of ¹²⁵I]TAP1 was 11.5- and 35.5-fold higher than for FABP3 and FABP5, respectively. In an in vivo study ¹²⁵I]TAP1 displayed high stability against deiodination and degradation, and moderate radioactivity accumulation in C6 tumors (1.37±0.24% dose/g 3 hr after injection). The radioactivity distribution profile in tumors partially corresponded to the FABP4 positive area and was also affected by perfusion. The results indicate that ¹²⁵I]TAP1 could detect FABP4 in vitro and partly in vivo. As such, ¹²⁵I]TAP1 is a promising lead compound for further refinement for use in in vivo FABP4 imaging.