Selective monoamine oxidase-B (MAO-B) inhibition is an attractive subject for the treatment of Parkinson's disease (PD). In the current study, we synthesized some selected derivatives of methylthiosemicarbazones and investigated their MAOs and acetylcholinesterase (AChE) inhibitory activities. Among the series synthesized, compoundsSM5,SM4, andSM9most inhibited MAO-B with IC(50)values of 5.48, 7.06, and 8.03 mu M, respectively. All compounds tested weakly inhibited MAO-A at 10 mu M with the residual activities of >50%. CompoundSM5had the highest selectivity index (SI) value for MAO-B (>7.30), followed bySM4(>5.67). Kinetic experiments revealed thatSM5competitively inhibited MAO-B, with a meanK(i)value of 2.39 +/- 0.15 mu M. Reversibility experiments showed thatSM5reversibly inhibited MAO-B, and 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) assays revealed thatSM5was not toxic to Vero cells (IC50= 198.96 mu g/mL). TheSM5/MAO-B interaction was ascertained by molecular docking and dynamics studies. The study shows thatSM5competitively inhibits MAO-B in a reversible, moderate selective manner, and that it is non-toxic to Vero cells. Communicated by Ramaswamy H. Sarma