Abuse liability assessment of neuroprotectants

There has been considerable interest in the potential of N-methyl-D-aspartate (NMDA) receptor antagonists in the treatment of a diverse group of neurological disorders including cerebral ischemia and neurodegeneration. The amino acids L-glutamate and L-aspartate have been shown to possibly mediate excitatory synaptic transmission in the central nervous system (CNS) via selective excitatory amino acid receptors. Competitive and noncompetitive antagonists acting at the NMDA receptors have been shown to possess relevant activity. However, NMDA antagonists can produce a variety of adverse neurobehavioral effects in both animals and humans. These adverse events are particularly pronounced with NMDA antagonists (phencyclidine (PCP), ketamine, and MK-801) that have dissociative anesthetic properties and block NMDA receptor-mediated responses by binding to the cation channel of the NMDA receptor complex. When a new pharmaceutical product demonstrates structural similarity and/or a similar pharmacological profile with a known drug of abuse, the characterization of its abuse potential is needed by the FDA for scientific review. The abuse liability assessment is based upon an evaluation of data on the chemistry, pharmacology (preclinical and clinical), pharmacokinetics, and pharmacodynamic profiles of the drug, and the adverse events/effects reported in clinical trials. The evaluation of the drug's abuse potential is determined relative to pharmacologically similar drugs. This includes determination of the drug's receptor binding efficacy, preclinical pharmacology, reinforcing efficacy, discriminative stimulus effects, dependence-producing potential, pharmacokinetics, and assessment of the clinical efficacy-safety database relative to abuse and clinical abuse liability studies. It has been well established that high-affinity noncompetitive NMDA antagonists have reinforcing efficacy and can serve as discriminative stimuli in operant procedures. In a variety of species in drug discrimination studies, each antagonist is capable of generalizing to the others, and it is believed that these effects may be mediated through the NMDA blockade. The generalization of each substance for another suggests production of common subjective effects in humans.