(-)-[3R,4S]-Chromanol 293B

The KCNQ1-KCNE2 K(+) channel is required for adequate thyroid I(-) uptake.[Pubmed:22549510]

FASEB J. 2012 Aug;26(8):3252-9.

The KCNQ1 alpha subunit and the KCNE2 beta subunit form a potassium channel in thyroid epithelial cells. Genetic disruption of KCNQ1-KCNE2 causes hypothyroidism in mice, resulting in cardiac hypertrophy, dwarfism, alopecia, and prenatal mortality. Here, we investigated the mechanistic requirement for KCNQ1-KCNE2 in thyroid hormone biosynthesis, utilizing whole-animal dynamic positron emission tomography. The KCNQ1-specific antagonist (-)-[3R,4S]-Chromanol 293B (C293B) significantly impaired thyroid cell I(-) uptake, which is mediated by the Na(+)/I(-) symporter (NIS), in vivo (dSUV/dt: vehicle, 0.028 +/- 0.004 min(-1); 10 mg/kg C293B, 0.009 +/- 0.006 min(-1)) and in vitro (EC(50): 99 +/- 10 muM C293B). Na(+)-dependent nicotinate uptake by SMCT, however, was unaffected. Kcne2 deletion did not alter the balance of free vs. thyroglobulin-bound I(-) in the thyroid (distinguished using ClO(4)(-), a competitive inhibitor of NIS), indicating that KCNQ1-KCNE2 is not required for Duox/TPO-mediated I(-) organification. However, Kcne2 deletion doubled the rate of free I(-) efflux from the thyroid following ClO(4)(-) injection, a NIS-independent process. Thus, KCNQ1-KCNE2 is necessary for adequate thyroid cell I(-) uptake, the most likely explanation being that it is prerequisite for adequate NIS activity.

Molecular impact of MinK on the enantiospecific block of I(Ks) by chromanols.[Pubmed:11139424]

Br J Pharmacol. 2000 Dec;131(8):1503-6.

Slowly activating I:(Ks) (KCNQ1/MinK) channels were expressed in Xenopous: oocytes and their sensitivity to chromanols was compared to homomeric KCNQ1 channels. To elucidate the contribution of the ss-subunit MinK on chromanol block, a formerly described chromanol HMR 1556 and its enantiomer S5557 were tested for enantio-specificity in blocking I:(Ks) and KCNQ1 as shown for the single enantiomers of chromanol 293B. Both enantiomers blocked homomeric KCNQ1 channels to a lesser extent than heteromeric I:(Ks) channels. Furthermore, we expressed both WT and mutant MinK subunits to examine the involvement of particular MinK protein regions in channel block by chromanols. Through a broad variety of MinK deletion and point mutants, we could not identify amino acids or regions where sensitivity was abolished or strikingly diminished (>2.5 fold). This could indicate that MinK does not directly take part in chromanol binding but acts allosterically to facilitate drug binding to the principal subunit KCNQ1.

A kinetic study on the stereospecific inhibition of KCNQ1 and I(Ks) by the chromanol 293B.[Pubmed:11739240]

Br J Pharmacol. 2001 Dec;134(8):1647-54.

1. Recently we and others have demonstrated a stereoselective inhibition of slowly activating human I(Ks) (KCNQ1/MinK) and homomeric KCNQ1 potassium channels by the enantiomers of the chromanol 293B. Here, we further characterized the mechanism of the 293B block and studied the influence of the 293B enantiomers on the gating kinetics of both channels after their heterologous expression in Xenopus oocytes. 2. Kinetic analysis of currents partially blocked with 10 microM of each 293B enantiomer revealed that only 3R,4S-293B but not 3S,4R-293B exhibited a time-dependent block of I(Ks) and KCNQ1 currents, indicating preferential open channel block activity. 3. Inhibition of both KCNQ1 and I(Ks) channels by 3R,4S-293B but not by 3S,4R-293B increased during a 2 Hz train of stimuli. 4. At high extracellular potassium concentrations the inhibition of KCNQ1 by 3R,4S-293B and 3S,4R-293B was unaffected. Drug inhibition of KCNQ1 and I(Ks) by both enantiomers also did not display a significant voltage-dependence, indicating that 293B does not strongly interact with permeant ions in the pore. 5. The inhibitory properties of 3R,4S-293B on I(Ks)-channels but not those of 3S,4R-293B fulfill the theoretical requirements for a novel class III antiarrhythmic drug, i.e. positive use-dependency. This enantiomer therefore represents a valuable pharmacological tool to evaluate the therapeutic efficiency of I(Ks)blockade.

Stereoselective interactions of the enantiomers of chromanol 293B with human voltage-gated potassium channels.[Pubmed:10945846]

J Pharmacol Exp Ther. 2000 Sep;294(3):955-62.

Selective inhibitors of the slow component of the cardiac delayed rectifier K(+) current, I(Ks), are of interest as novel class III antiarrhythmic agents and as tools for studying the physiologic roles of the I(Ks) current. Racemic chromanol 293B is an inhibitor of both native I(Ks) and its putative molecular counterpart, the KvLQT1+minK ion channel complex. We synthesized the (+)-[3S,4R] and (-)-[3R,4S] enantiomers of chromanol 293B using chiral intermediates of known absolute configuration and determined their relative potency to block recombinant human K(+) channels that form the basis for the major repolarizing K(+) currents in human heart, including KvLQT1+minK, human ether-a-go-go-related gene product (hERG), Kv1.5, and Kv4.3, corresponding to the slow (I(Ks)), rapid (I(Kr)), and ultrarapid (I(Kur)) delayed rectifier currents and the transient outward current (I(To)), respectively. K(+) channels were expressed in mammalian cells and currents were recorded using the whole-cell patch-clamp technique. We found that the physicochemical properties and relative potency of the enantiomers differed from those reported previously, with (-)-[3R,4S]293B nearly 7-fold more potent in block of KvLQT1+minK than (+)-[3S,4R]293B, indicating that the original stereochemical assignments were reversed. K(+) current inhibition by (-)-293B was selective for KvLQT1+minK over hERG, whereas the stereospecificity of block for KvLQT1+minK and Kv1.5 was preserved, with (-)-293B more potent than (+)-293B for both channel complexes. We conclude that the (-)-[3R,4S] enantiomer of chromanol 293B is a selective inhibitor of KvLQT1+minK and therefore a useful tool for studying I(Ks).