Affinity of antineoplastic amino acid drugs for the large neutral amino acid transporter of the blood-brain barrier

Abstract

The relative affinity of six anticancer amino acid drugs for the neutral amino acid carrier of the blood-brain barrier was examined in rats using an in situ brain perfusion technique. Affinity was evaluated from the concentration-dependent inhibition of L-[14C]-leucine uptake into rat brain during perfusion at tracer leucine concentrations and in the absence of competing amino acids. Of the six drugs tested, five, including melphalan, azaserine, acivicin, 6-diazo-5-oxo-L-norleucine, and buthionine sulfoximine, exhibited only low affinity for the carrier, displaying transport inhibition constants (Ki, concentrations producing 50% inhibition) ranging from 0.09 to 4.7 mM. However, one agent - D,L-2-amino-7-bis[(2-chloroethyl)amino]- 1,2,3,4-tetrahydro-2-naphthoic acid (D,L-NAM) - demonstrated remarkably high affinity for the carrier, showing a Ki value of approximately 0.2 microM. The relative affinity (1/Ki) of D,L-NAM was greater than 100-fold that of the other drugs and greater than 10-fold that of any compound previously tested. As the blood-brain barrier penetrability of most endogenous neutral amino acids is related to their carrier affinity, the results suggest that D,L-NAM may be a promising agent which may show enhanced uptake and distribution to brain tumors.