Discovery of 3,4-dichloro-N-(1H-indol-5-yl)benzamide: A highly potent, selective, and competitive hMAO-B inhibitor with high BBB permeability profile and neuroprotective action.

Since there is no disease-modifying treatment discovered yet for Parkinson's disease (PD), there is still a vital need to develop novel selective monoamine oxidase B (MAO-B) inhibitors as promising therapeutically active candidates for PD patients. Herein, we report the design, synthesis, and full characterization of new twenty-six indole derivatives as potential human MAO-B (hMAO-B) selective inhibitors. Six compounds (2i, 3b-e, and 5) exhibited low micromolar to nanomolar inhibitory activities over hMAO-B; compared to our recently reported N-substituted indole-based lead compound VIII (hMAO-B IC50 = 777 nM), compound 5 (3,4-dichloro-N-(1H-indol-5-yl)benzamide) exhibited 18-fold increase in potency (IC50 = 42 nM). A selectivity study over hMAO-A revealed an excellent selectivity index of compound 5 (SI > 2375) with a 47-fold increase compared to rasagiline (II, a well-known MAO-B inhibitor, SI > 50). A further kinetic evaluation of compound 5 over hMAO-B showed a reversible and competitive mode of inhibition with Ki value of 7 nM. Highly effective permeability and high CNS bioavailability of compound 5 with Pe = 54.49 × 10-6 cm/s were demonstrated. Compound 5 also exhibited a low cytotoxicity profile and a promising neuroprotective effect against the 6-hydroxydopamine-induced neuronal cell damage in PC12 cells, which was more effective than that of rasagiline. Docking simulations on both hMAO-B and hMAO-A supported the in vitro data and served as further molecular evidence. Accordingly, we report the discovery of compound 5 as one of the most potent indole-based MAO-B inhibitors to date which is noteworthy to be further evaluated as a promising agent for PD treatment.

Rapid analysis of metabolic stability of dopamine receptor antagonists and detection of their metabolites by liquid chromatography/tandem mass spectrometry.

In vitro metabolic stability of dopamine D(3)/D(4) receptor antagonists and identification of their metabolites by high-performance liquid chromatography (HPLC) coupled with ion-trap mass spectrometry (ITMS) were assessed in rat liver microsomes. The compounds were divided into three cassette groups for rapid quantitative analysis of multiple drugs and simultaneous detection of their metabolites. The samples from incubation with rat liver microsomes were pooled into designed cassette groups and analyzed by HPLC/electrospray ITMS in full-scan mode. The metabolic stability of the drugs was determined by comparing their signals after incubation for 0 and for 30min. The metabolic stability of the examined dopamine receptor antagonists was in the range of 9.9-84.4%. In addition, the present cassette analysis allowed the simultaneous detection of metabolites formed during the same incubation without having to reanalyze the samples. The metabolites were first characterized by nominal mass measurement of the corresponding protonated molecules. Subsequent multistage tandem mass spectrometry on the ion-trap instrument allowed characterization of the structure of the detected metabolites. N,O-dealkylation and ring hydroxylation reactions were identified as major metabolic reactions in piperazinylalkylisoxazole derivatives. These results suggested that the present approach is useful for the rapid evaluation of metabolic stability and structural characterization of metabolites within a short period in new drug discovery.