Radiofluorinated probe for PET imaging of fatty acid binding protein 4 in cancer

Abstract

Introduction: Cancer-associated adipocytes metabolically interact with adjacent cancer cells to promote tumor proliferation and metastasis. Fatty acid binding protein 4 (FABP4) participates in this interaction, and is gathering attention as a therapeutic and diagnostic target. Positron emission tomography (PET) is a useful diagnostic method that enables noninvasive in vivo quantitative imaging of biofunctional molecules with probes labeled with positron-emitting radioisotopes. Here a novel (18)F labeled probe for PET FABP4 imaging developed through dedicated drug design from a radioiodinated probe we recently reported is evaluated in vitro and in vivo.

Methods: We designed the [(18)F]-labeled FTAP1 and FTAP3 probe, composed of a single or triple oxyethylene linker and a triazolopyrimidine scaffold derived from an FABP4 inhibitor. FABP4 binding affinities for chemically synthesized FTAP1 and FTAP3 were measured using FABP4 and 8-anilino-1-naphthalene sulfonic acid. Cell membrane permeability was measured using a commercially available plate assay system. After radiosynthesis, [(18)F]FTAP1 affinity and selectivity were evaluated using immobilized FABP3, FABP4, and FABP5. Cell uptake was investigated using differentiated adipocytes expressing FABP4 with inhibitor treatment. Following biodistribution studies in C6 glioblastoma-bearing mice, ex vivo autoradiography and immunohistochemistry were performed using thin sliced tumor sections. PET/CT imaging was then performed on C6 tumor bearing mice.

Results: FTAP1 showed high FABP4 affinity (Ki=68±8.9 nM) and adequate cell permeability. [(18)F]FTAP1 with ≥98% radiochemical purity was shown to selectively bind to FABP4 (16.3- and 9.3-fold higher than for FABP3 and FABP5, respectively). [(18)F]FTAP1 was taken up by FABP4 expressing cells, and this uptake could be blocked by an inhibitor, indicating very low non-specific cell binding. [(18)F]FTAP1 showed high tumor accumulation, which demonstrates its potential use for in vivo tumor PET imaging, and the intratumoral radioactivity distribution corresponded to the FABP4 expression profile.

Conclusion: [(18)F]FTAP1 is a promising PET probe to target FABP4.

Keywords: Fatty acid binding protein 4; Nuclear imaging; PET; Positron emission tomography; Radiolabeled probe.