Transport of amantadine and rimantadine through the blood-brain barrier.

The unidirectional transport of amantadine and rimantadine across cerebral capillaries, the anatomical locus of the blood-brain barrier, was measured with an in situ rat brain perfusion technique. Both rimantadine and, to a lesser extent, amantadine were transported principally across the blood-brain barrier by a saturable transport system with a one-half saturation concentration of about 1.0 mM (either rimantadine or amantadine). The permeability surface area constants were 8.5 x 10(-2) sec-1 (rimantadine) and 1.1 x 10(-2) sec-1 (amantadine) with concentrations of less than 1.0 microM in the perfusate. The extraction of rimantadine and amantadine from the perfusate at low concentrations (less than 1.0 microM) was 88 and 26%, respectively, of the extraction of diazepam which is 100% extracted. Amantadine and rimantadine transport through the blood-brain barrier was significantly inhibited by weakly basic drugs (e.g., diphenhydramine) but not choline (10 mM), probenecid (1 mM) or leucine (1 mM). Inasmuch as both the pKa and percentage ionized at pH = 7.4 of rimantadine (10.4 and 99.9%, respectively) are much higher than that of amantadine (pKa = 9.0 and 97.5%, respectively), and inasmuch as rimantadine is transported into brain much more readily than amantadine, our results suggest that the carrier-mediated transport of the ionized moiety is the crucial process determining the penetration of amantadine and rimantadine through the blood-brain barrier.