DETERMINATION OF SERUM AND BRAIN CONCENTRATIONS OF NEUROPROTECTIVE AND NON-NEUROPROTECTIVE DOSES OF MK-801

We developed a sensitive and reliable gas chromatographic (GC) technique for the quantitative analysis of MK-801 in brain and serum and applied the technique to investigate tissue concentrations of neuroprotective and non-neuroprotective doses of MK-801 in a neonatal rat model of hypoxic-ischemia. Brain concentrations of MK-801 were a linear function of dose over 4 orders of magnitude. After administration of a neuroprotective dose of MK-801 (29.6 mumol/kg) to control rats, both serum and brain concentrations rose rapidly to almost-equal-to 300 nM and almost-equal-to 2000 nM, respectively, within 30 min. Approximately 60% of serum and 90% of brain MK-801 were bound leaving the free concentrations in both blood and brain at almost-equal-to 100-200 nM. After hypoxic-ischemia, serum MK-801 concentrations were not different from controls but brain concentrations were lowered by 32%. Free brain concentrations of MK-801 after hypoxic-ischemic were 124 nM after 29.6 mumol/kg while after 8.9 mumol/kg (the non-neuroprotective dose) they were 39 nM. In view of the slow kinetics of MK-801 channel blockade, which never reaches equilibrium during the current experiment, this difference in concentration would be expected to result in an 80-fold (or greater) difference in the current flux through NMDA receptor-operated ion-channels as the rate of NMDA receptor-operated ion-channel blockade is concentration dependent. These results suggest that the lower dose of MK-801 may not block the ion-channel quickly or completely enough to prevent a toxic influx of Ca2+, and that the brain concentrations needed for effective post-hoc therapy of neuronal insults may be higher than equilibrium kinetics would indicate.