Cloned blood-brain barrier adenosine transporter is identical to the rat concentrative Na+ nucleoside cotransporter CNT2

Site-directed mutagenesis of rabbit LAT1 at amino acids 219 and 234

The availability of amino acids in the brain is regulated by the blood-brain barrier (BBB) large neutral amino acid transporter type 1 (LAT1) isoform, which is characterized by a high affinity (low Km) for substrate large neutral amino acids. The hypothesis that brain amino acid transport activity can be altered with single nucleotide polymorphisms was tested in the present studies with site-directed mutagenesis of the BBB LAT1. The rabbit has a high Km LAT1 large neutral amino acid transporter, as compared to the low Km neutral amino acid transporter at the human or rat BBB. The rabbit LAT1 was cloned from a rabbit brain capillary cDNA library. Alignment of the amino acid sequences of rabbit, human, and rat LAT1 revealed two radical amino acid residues that differ in the rabbit relative to the rat or human LAT1. The G219D mutation had a modest effect on the Km and Vmax of tryptophan transport via cloned rabbit LAT1 in frog oocytes, but the W234L variant reduced the Km by 64% and the Vmax by 96%. Conversely, LAT1 transport of either tryptophan or phenylalanine was nearly normalized when the double mutation W234L/G219D variant was produced. These studies show that marked changes in the affinity and capacity of the LAT1 are caused by single nucleotide polymorphisms and that phenotype can be restored with a double mutation.

Current prodrug strategies via membrane transporters/receptors

Prodrug design strategies have been employed to improve the delivery of drugs with undesirable pharmacokinetic properties such as chemical stability and lack of specificity. Targeted prodrug design represents a new strategy for site-directed and efficient drug delivery. Targeting of drugs to transporters and receptors to aid in site-specific carrier-mediated absorption is emerging as a novel and clinically significant approach. Various prodrugs have been successful in achieving the goals of enhanced bioavailability and are, therefore, considered to be an important tool in biopharmaceutics. This review highlights the advances in prodrug design targeted towards membrane transporters/receptors in the past few years.