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 μM. 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 μM, providing