Synthesis and Structure−Activity Relationships of 3-Aryloxindoles:  A New Class of Calcium-Dependent, Large Conductance Potassium (Maxi-K) Channel Openers with Neuroprotective Properties

A series of 3-aryloxindole derivatives were synthesized and evaluated as activators of the cloned maxi-K channel mSlo expressed in Xenopus laevis oocytes using electrophysiological methods. The most promising maxi-K openers to emerge from this study were (+/-)-3-(5-chloro-2-hydroxyphenyl)-1,3-dihydro-3-hydroxy-6-(trifluoromethyl)-2H-indol-2-one ((+/-)-8c) and its 3-des-hydroxy analogue (+/-)-11b. The individual enantiomers of (+/-)-8c were synthesized, and the maxi-K channel-opening properties were shown to depend on the absolute configuration of the single stereogenic center with the efficacy of (-)-8c superior to that of both (+)-8c and the racemic mixture when evaluated at a concentration of 20 microM. Racemic 11b exhibited greater efficacy than either the racemic 8c or the more active enantiomer in the electrophysiological evaluation. In vitro metabolic stability studies conducted with (+/-)-8c and (+/-)-11b in rat liver S9 microsomal fractions revealed significant oxidative degradation with two hydroxylated metabolites observed by liquid chromatography/mass spectrometry for each compound in addition to the production of 8c from 11b. The pharmacokinetic properties of (+/-)-8c and (+/-)-11b were determined in rats as a prelude to evaluation in a rat model of stroke that involved permanent occlusion of the middle cerebral artery (MCAO model). In the MCAO model, conducted in the spontaneously hypertensive rat, the more polar 3-hydroxy derivative (+/-)-8c did not demonstrate a significant reduction in cortical infarct volume when administered intravenously at doses ranging from 0.1 to 10 mg/kg as a single bolus 2 h after middle cerebral artery occlusion when compared to vehicle-treated controls. In contrast, intravenous administration of (+/-)-11b at a dose of 0.03 mg/kg was found to reduce the measured cortical infarct volume by approximately 18% when compared to vehicle-treated control animals. Intraperitoneal administration of (+/-)-11b at a dose of 10 mg/kg 2 h following artery occlusion was shown to reduce infarct volume by 26% when compared to vehicle-treated controls. To further probe the effects of compounds (+/-)-8c and (+/-)-11b on neurotransmitter release in vitro, both compounds were examined for their ability to reduce electrically stimulated [3H]-glutamate release from rat hippocampal slices that had been preloaded with [3H]-glutamate. Only (+/-)-11b was able to demonstrate a significant inhibition [3H]-glutamate release in this assay at a concentration of 20 microM, providing concordance with the profile of these compounds in the MCAO model. Although (+/-)-11b showed some promise as a potential developmental candidate for the treatment of the sequelae of stroke based on its efficacy in the rat MCAO model, the pharmacokinetic profile of this compound was considered to be less than optimal and was not pursued in favor of derivatives with enhanced metabolic stability.