Short-acting novel MAO inhibitors: In vitro evidence for the reversibility of MAO inhibition by moclobemide and Ro 16-6491

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Abstract
The inhibition of monoamine oxidase (MAO) in rat liver and brain by the short-acting MAO-A inhibitors moclobemide (Ro 11-1163 = p-chloro-N-[2-morpholinoethyl]benzamide) and brofaremine and by the short-acting MAO-13 inhibitors Ro 16-6491 (N-[2-aminoethyl]-p-chlorobenzamide) and almoxatone, administered p. o. at roughly equieffective doses 2 h before decapitation, was investigated for its reversibility under various in vitro conditions. MAO A activity in liver homogenates, inhibited by moclobemide (300 μmol/kg) to approx. 15% of control, time dependently recovered during 0.5 to 2 h of incubation at 37°C, irrespective of whether the homogenates were prepared and incubated in distilled water or Krebs-Ringer buffer (KRB). Dialysis of such homogenates for 4 h in distilled water at 37°C (but not at 13°C) led to a complete return of the MAO activity. In liver homogenates from rats pretreated with brofaremine (30 μmol/kg), dialysis for 4 h at 37°C against distilled water caused only little recovery of the MAO activity. Likewise, MAO-B inhibited by Ro 16–6491 (30 μmol/kg) to approx. 4% of control returned to almost control activity after 4 h of dialysis at 37°C, while inhibition induced by almoxatone (30 μmol/kg) was little or not reversed at all. In brain homogenates prepared in, and dialysed against, distilled water or KRB at 37°C (but not at 13°C), MAO-A inhibited by moclobemide (100–300 μmol/kg) to approx. 15% of control, partially (KRB) or almost completely (dist. water) returned to control activity after 4 h of dialysis. From rats pretreated with Ro 16–6491 (30 μmol/kg), MAO-B in brain homogenates prepared in KRB was reduced to 12% of control and returned to control value upon dialysis for 4 h in KRB at 37°C; in homogenates prepared in H2O, MAO-B was reduced to only 60% of control and completely recovered by dialysis against dest. water even at 13°C. In all of these conditions, recovery of the enzyme activity was small after brofaremine and almoxatone. Analogous results were obtained with brain slices (0.2 × 0.2 × 1.5 mm) in KRB at 37°C, whereby time dependent recovery of MAO activity during incubation was achieved, and superfusion was somewhat more effective than incubation in restoring enzyme activity. In the experiments with incubated or superfused brain slices, inhibition of MAO-A and -B by the irreversible inhibitors clorgyline and selegiline (l-deprenyl), resp., could not be reversed at all. Tyramine (0.3 mmol/l) clearly enhanced the recovery of MAO-A in KRB-prepared liver homogenates and brain slices of moclobemide-pretreated rats but not in brain slices of brofaremine- and clorgyline-pretreated rats. Thus, the reversibility of MAO inhibition in vitro could be convincingly demonstrated for moclobemide and Ro 16–6491 but not for the other novel, short-acting MAO inhibitors studied.