L-type amino acid transporter 1 (LAT1) is selectively expressed in the blood-brain barrier (BBB) and brain parenchyma. This transporter can facilitate brain delivery of neuroprotective agents and additionally give opportunity to minimize systemic exposure. Here, we investigated structure-pharmacokinetics relationship of five newly synthesized LAT1-utilizing prodrugs of the cyclooxygenase inhibitor, ketoprofen, in order to identify beneficial structural features of prodrugs to achieve both targeted brain delivery and low peripheral distribution of the parent drug. Besides, we studied whether pharmacokinetics and bioconversion of LAT1-utilizing prodrugs in vivo can be predicted in early stage experiments. To achieve these goals, we compared the in vitro brain uptake mechanism of prodrugs, rate of BBB permeation of compounds using in situ perfusion technique, their systemic pharmacokinetics and release of parent drug in brain, plasma and liver of mice. The results revealed that both excellent LAT1-binding ability and transporter utilization in vitro can be achieved by conjugating the parent drug to aromatic amino acids such as phenylalanine in comparison to prodrugs with an aliphatic promoiety. The presence of an aromatic promoiety directly conjugated in meta- or para-position to ketoprofen led to LAT1-utilizing prodrugs capable of delivering the parent drug into the brain with higher unbound brain to plasma ratio and reduced liver exposure than with ketoprofen itself. In contrast, the prodrugs with aliphatic promoieties and with an additional carbon attached between the parent drug and phenylalanine aromatic ring did not enhance brain delivery of ketoprofen. Furthermore, we have devised a screening strategy to pinpoint successful candidates at an early stage of development of LAT1-utilizing prodrugs. The screening approach demonstrated that early stage experiments could not replace pharmacokinetic studies in vivo due to the lack of prediction of the intra-brain/systemic distribution of the prodrugs as well as the release of the parent drug. Overall, this study provides essential knowledge required for improvement of targeted brain delivery and reduction of systemic exposure of drugs via the LAT1-mediated prodrug approach.

Our growing understanding of membrane transporters and their substrate specificity has opened a new avenue in the field of targeted drug delivery. The L-type amino acid transporter 1 (LAT1) has been one of the most extensively investigated transporters for delivering drugs across biological barriers. The transporter is predominantly expressed in cerebral cortex, blood-brain barrier, blood-retina barrier, testis, placenta, bone marrow and several types of cancer. Its physiological function is to mediate Na+ and pH independent exchange of essential amino acids: leucine, phenylalanine, etc. Several drugs and prodrugs designed as LAT1 substrates have been developed to improve targeted delivery into the brain and cancer cells. Thus, the anti-parkinsonian drug, L-Dopa, the anti-cancer drug, melphalan and the anti-epileptic drug gabapentin, all used in clinical practice, utilize LAT1 to reach their target site. These examples provide supporting evidence for the utility of the LAT1-mediated targeted delivery of the (pro)drug. This review comprehensively summarizes recent advances in LAT1-mediated targeted drug delivery. In addition, the use of LAT1 is critically evaluated and limitations of the approach are discussed.

The l-type amino acid transporter 1 (LAT1) is a transmembrane protein carrying bulky and neutral amino acids into cells. LAT1 is overexpressed in several types of tumors, and its inhibition can result in reduced cancer cell growth. However, known LAT1 inhibitors lack selectivity over other transporters. In the present study, we designed and synthesized a novel selective LAT1 inhibitor (1), which inhibited the uptake of LAT1 substrate, l-leucin as well as cell growth. It also significantly potentiated the efficacy of bestatin and cisplatin even at low concentrations (25 μM). Inhibition was slowly reversible, as the inhibitor was able to be detached from the cell surface and blood-brain barrier. Moreover, the inhibitor was metabolically stable and selective toward LAT1. Since the inhibitor was readily accumulated into the prostate after intraperitoneal injection to the healthy mice, this compound may be a promising agent or adjuvant especially for the treatment of prostate cancer.

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