Assessment of Amino Acid/Drug Transporters for Renal Transport of [(18)F]Fluciclovine (anti-[(18)F]FACBC) in Vitro

[(18)F]Fluciclovine (trans-1-amino-3-[(18)F]fluorocyclobutanecarboxylic acid; anti-[(18)F]FACBC), a positron emission tomography tracer used for the diagnosis of recurrent prostate cancer, is transported via amino acid transporters (AATs) with high affinity (K(m): 97–230 μM). However, the mechanism underlying urinary excretion is unknown. In this study, we investigated the involvement of AATs and drug transporters in renal [(18)F]fluciclovine reuptake. [(14)C]Fluciclovine (trans-1-amino-3-fluoro[1-(14)C]cyclobutanecarboxylic acid) was used because of its long half-life. The involvement of AATs in [(14)C]fluciclovine transport was measured by apical-to-basal transport using an LLC-PK1 monolayer as model for renal proximal tubules. The contribution of drug transporters herein was assessed using vesicles/cells expressing the drug transporters P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), multidrug resistance-associated protein 4 (MRP4), organic anion transporter 1 (OAT1), organic anion transporter 3 (OAT3) , organic cation transporter 2 (OCT2), organic anion transporting polypeptide 1B1 (OATP1B1), and organic anion transporting polypeptide 1B3 (OATP1B3). The apical-to-basal transport of [(14)C]fluciclovine was attenuated by l-threonine, the substrate for system alanine-serine-cysteine (ASC) AATs. [(14)C]Fluciclovine uptake by drug transporter-expressing vesicles/cells was not significantly different from that of control vesicles/cells. Fluciclovine inhibited P-gp, MRP4, OAT1, OCT2, and OATP1B1 (IC(50) > 2.95 mM). Therefore, system ASC AATs may be partly involved in the renal reuptake of [(18)F]fluciclovine. Further, given that [(18)F]fluciclovine is recognized as an inhibitor with millimolar affinity for the tested drug transporters, slow urinary excretion of [(18)F]fluciclovine may be mediated by system ASC AATs, but not by drug transporters.