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Targeting Solute Carrier Transporters for Drug Delivery to the Central Nervous System

  • Author(s): Geier, Ethan George
  • Advisor(s): Giacomini, Kathleen M
  • et al.
Abstract

The blood-brain barrier (BBB) is a major reason that approximately 95% of small molecule drugs developed to target the central nervous system (CNS) fail. Thus, new approaches for delivering drugs across the BBB should be explored. The goal of this research was to identify solute carrier (SLC) influx transporters expressed at the BBB, and determine how SLCs can be exploited to deliver low CNS permeability platinum drugs across the BBB.

My first goal was to identify SLC transporters expressed at the human BBB. In human brain microvessels isolated from cerebral cortex, expression profiling of 359 SLC transporters, comparative expression with human kidney and liver samples, and immunoassays confirmed and identified BBB drug transporters, including SLC7A5, SLC22A3, SLC47A1, and SLC19A1. Furthermore, pharmacokinetic studies in mice with small molecule inhibitors of the reduced folate carrier (Rfc), established Rfc-mediated uptake of methotrexate across the BBB.

The large neutral amino acid transporter 1, LAT1 is one of several SLC transporters that mediate drug uptake across the BBB. The role of LAT1 in CNS drug delivery was characterized by (a) using LAT1 expressing cell lines to determine whether LAT1 can be targeted to deliver synthetic anti-cancer platinum analogs to the CNS; (b) creating and characterizing a mouse model with reduced function Lat1 as a tool for assessing LAT1 targeted drugs in vivo; and (c) using virtual screening of ~19,000 compounds against a comparative structural model of LAT1. One out of seven LAT1-targeted synthetic platinum analogs was a cytotoxic substrate of the transporter. Although Lat1 deletion appears to be embryonic lethal, Lat1 heterozygous mice have decreased Lat1 mRNA expression, and gabapentin and levodopa brain accumulation. Four novel LAT1 ligands were identified by comparative modeling, virtual screening, and experimental validation. These results provided a rationale for the enhanced brain permeability of two drugs. Two hits also inhibited proliferation of a cancer cell line by different mechanisms, providing useful chemical tools to characterize the role of LAT1 in cancer. The research presented here has important implications for LAT1-targeted CNS drug delivery and cancer therapy, and provide important tools to continue to address these implications in future studies.

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