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Different pH-sensitivity patterns of 30 sodium channel inhibitors suggest chemically different pools along the access pathway.

Lazar A, Lenkey N, Pesti K, Fodor L, Mike A - Front Pharmacol (2015)

Bottom Line: One-way to probe this is to modify the pH of the extracellular fluid, which alters the ratio of charged vs. uncharged forms of some compounds, thereby changing their interaction with the membrane.We recorded the pH-dependence of potency, reversibility, as well as onset/offset kinetics.Unexpectedly, however, the pH-dependence of reversibility or kinetics showed diverse patterns, not simple correlation.

View Article: PubMed Central - PubMed

Affiliation: Intensive Care Unit, University of Medicine and Pharmacy Tirgu Mures, Romania.

ABSTRACT
The major drug binding site of sodium channels is inaccessible from the extracellular side, drug molecules can only access it either from the membrane phase, or from the intracellular aqueous phase. For this reason, ligand-membrane interactions are as important determinants of inhibitor properties, as ligand-protein interactions. One-way to probe this is to modify the pH of the extracellular fluid, which alters the ratio of charged vs. uncharged forms of some compounds, thereby changing their interaction with the membrane. In this electrophysiology study we used three different pH values: 6.0, 7.3, and 8.6 to test the significance of the protonation-deprotonation equilibrium in drug access and affinity. We investigated drugs of several different indications: carbamazepine, lamotrigine, phenytoin, lidocaine, bupivacaine, mexiletine, flecainide, ranolazine, riluzole, memantine, ritanserin, tolperisone, silperisone, ambroxol, haloperidol, chlorpromazine, clozapine, fluoxetine, sertraline, paroxetine, amitriptyline, imipramine, desipramine, maprotiline, nisoxetine, mianserin, mirtazapine, venlafaxine, nefazodone, and trazodone. We recorded the pH-dependence of potency, reversibility, as well as onset/offset kinetics. As expected, we observed a strong correlation between the acidic dissociation constant (pKa) of drugs and the pH-dependence of their potency. Unexpectedly, however, the pH-dependence of reversibility or kinetics showed diverse patterns, not simple correlation. Our data are best explained by a model where drug molecules can be trapped in at least two chemically different environments: A hydrophilic trap (which may be the aqueous cavity within the inner vestibule), which favors polar and less lipophilic compounds, and a lipophilic trap (which may be the membrane phase itself, and/or lipophilic binding sites on the channel). Rescue from the hydrophilic and lipophilic traps can be promoted by alkalic and acidic extracellular pH, respectively.

No MeSH data available.


Related in: MedlinePlus

Chemical properties affecting pH-dependent affinity and onset kinetics. Correlations of the extent of pH-dependence with chemical properties. Classes are color-coded (same colors as in Figure 2), three-letter codes identify individual drugs. (A) Ratios of apparent affinities measured at alkalic vs. neutral solution, plotted against calculated pKa values of the drugs. (B) The same correlation could be made close to linear by mathematical transformations: The logarithm of the alkalic/neutral apparent affinity ratio, plotted against the logarithm of the percentage of neutral form at pH = 7.3 [logN(pKa)] (neutral fraction was calculated from pKa values using the Henderson–Hasselbalch equation—see Methods). (C) pH-dependent acceleration/deceleration of onset as a function of aromatic atom count (AAC). (D) pH-dependent acceleration/deceleration of onset as a function of logD(7.3).
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Figure 6: Chemical properties affecting pH-dependent affinity and onset kinetics. Correlations of the extent of pH-dependence with chemical properties. Classes are color-coded (same colors as in Figure 2), three-letter codes identify individual drugs. (A) Ratios of apparent affinities measured at alkalic vs. neutral solution, plotted against calculated pKa values of the drugs. (B) The same correlation could be made close to linear by mathematical transformations: The logarithm of the alkalic/neutral apparent affinity ratio, plotted against the logarithm of the percentage of neutral form at pH = 7.3 [logN(pKa)] (neutral fraction was calculated from pKa values using the Henderson–Hasselbalch equation—see Methods). (C) pH-dependent acceleration/deceleration of onset as a function of aromatic atom count (AAC). (D) pH-dependent acceleration/deceleration of onset as a function of logD(7.3).

Mentions: For all predominantly positively charged molecules the affinity was increased at alkalic pH, as it was expected. When the apparent affinity ratios [IC50 (pH = 6.0)/IC50 (pH = 8.6)] were plotted against pKa values, an abrupt increase was seen at about pKa = 8 (Figure 6A). Seven compounds out of the 30 had pKa values below 7.7, and none of them was significantly more potent at pH = 8.6 (the maximal difference was 1.43-fold). On the other hand, all compounds with pKa value above 8.2 (18 out of the 30) had apparent affinity ratios above 2 (range: 2.17–12.48). The affinity ratio - pKa correlation was evidently not linear, therefore, we also plotted the affinity ratios against the logarithm of the percentage of neutral form at pH = 7.3 (logN(pKa); calculated using the Henderson-Hasselbalch equation as described in Methods). This plot gave a significant correlation (R2 = 0.49; p < 0.001). The correlation coefficient was even higher (R2 = 0.61; p < 0.001) when the logarithm of the apparent affinity ratios was plotted against logN(pKa) (Figure 6B). These results suggest that the apparent affinity is manifestly determined by the ability of drug molecules to populate an intramembranous pool, which is only accessible for their neutral form (Figure 5, 1st row).


Different pH-sensitivity patterns of 30 sodium channel inhibitors suggest chemically different pools along the access pathway.

Lazar A, Lenkey N, Pesti K, Fodor L, Mike A - Front Pharmacol (2015)

Chemical properties affecting pH-dependent affinity and onset kinetics. Correlations of the extent of pH-dependence with chemical properties. Classes are color-coded (same colors as in Figure 2), three-letter codes identify individual drugs. (A) Ratios of apparent affinities measured at alkalic vs. neutral solution, plotted against calculated pKa values of the drugs. (B) The same correlation could be made close to linear by mathematical transformations: The logarithm of the alkalic/neutral apparent affinity ratio, plotted against the logarithm of the percentage of neutral form at pH = 7.3 [logN(pKa)] (neutral fraction was calculated from pKa values using the Henderson–Hasselbalch equation—see Methods). (C) pH-dependent acceleration/deceleration of onset as a function of aromatic atom count (AAC). (D) pH-dependent acceleration/deceleration of onset as a function of logD(7.3).
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4585259&req=5

Figure 6: Chemical properties affecting pH-dependent affinity and onset kinetics. Correlations of the extent of pH-dependence with chemical properties. Classes are color-coded (same colors as in Figure 2), three-letter codes identify individual drugs. (A) Ratios of apparent affinities measured at alkalic vs. neutral solution, plotted against calculated pKa values of the drugs. (B) The same correlation could be made close to linear by mathematical transformations: The logarithm of the alkalic/neutral apparent affinity ratio, plotted against the logarithm of the percentage of neutral form at pH = 7.3 [logN(pKa)] (neutral fraction was calculated from pKa values using the Henderson–Hasselbalch equation—see Methods). (C) pH-dependent acceleration/deceleration of onset as a function of aromatic atom count (AAC). (D) pH-dependent acceleration/deceleration of onset as a function of logD(7.3).
Mentions: For all predominantly positively charged molecules the affinity was increased at alkalic pH, as it was expected. When the apparent affinity ratios [IC50 (pH = 6.0)/IC50 (pH = 8.6)] were plotted against pKa values, an abrupt increase was seen at about pKa = 8 (Figure 6A). Seven compounds out of the 30 had pKa values below 7.7, and none of them was significantly more potent at pH = 8.6 (the maximal difference was 1.43-fold). On the other hand, all compounds with pKa value above 8.2 (18 out of the 30) had apparent affinity ratios above 2 (range: 2.17–12.48). The affinity ratio - pKa correlation was evidently not linear, therefore, we also plotted the affinity ratios against the logarithm of the percentage of neutral form at pH = 7.3 (logN(pKa); calculated using the Henderson-Hasselbalch equation as described in Methods). This plot gave a significant correlation (R2 = 0.49; p < 0.001). The correlation coefficient was even higher (R2 = 0.61; p < 0.001) when the logarithm of the apparent affinity ratios was plotted against logN(pKa) (Figure 6B). These results suggest that the apparent affinity is manifestly determined by the ability of drug molecules to populate an intramembranous pool, which is only accessible for their neutral form (Figure 5, 1st row).

Bottom Line: One-way to probe this is to modify the pH of the extracellular fluid, which alters the ratio of charged vs. uncharged forms of some compounds, thereby changing their interaction with the membrane.We recorded the pH-dependence of potency, reversibility, as well as onset/offset kinetics.Unexpectedly, however, the pH-dependence of reversibility or kinetics showed diverse patterns, not simple correlation.

View Article: PubMed Central - PubMed

Affiliation: Intensive Care Unit, University of Medicine and Pharmacy Tirgu Mures, Romania.

ABSTRACT
The major drug binding site of sodium channels is inaccessible from the extracellular side, drug molecules can only access it either from the membrane phase, or from the intracellular aqueous phase. For this reason, ligand-membrane interactions are as important determinants of inhibitor properties, as ligand-protein interactions. One-way to probe this is to modify the pH of the extracellular fluid, which alters the ratio of charged vs. uncharged forms of some compounds, thereby changing their interaction with the membrane. In this electrophysiology study we used three different pH values: 6.0, 7.3, and 8.6 to test the significance of the protonation-deprotonation equilibrium in drug access and affinity. We investigated drugs of several different indications: carbamazepine, lamotrigine, phenytoin, lidocaine, bupivacaine, mexiletine, flecainide, ranolazine, riluzole, memantine, ritanserin, tolperisone, silperisone, ambroxol, haloperidol, chlorpromazine, clozapine, fluoxetine, sertraline, paroxetine, amitriptyline, imipramine, desipramine, maprotiline, nisoxetine, mianserin, mirtazapine, venlafaxine, nefazodone, and trazodone. We recorded the pH-dependence of potency, reversibility, as well as onset/offset kinetics. As expected, we observed a strong correlation between the acidic dissociation constant (pKa) of drugs and the pH-dependence of their potency. Unexpectedly, however, the pH-dependence of reversibility or kinetics showed diverse patterns, not simple correlation. Our data are best explained by a model where drug molecules can be trapped in at least two chemically different environments: A hydrophilic trap (which may be the aqueous cavity within the inner vestibule), which favors polar and less lipophilic compounds, and a lipophilic trap (which may be the membrane phase itself, and/or lipophilic binding sites on the channel). Rescue from the hydrophilic and lipophilic traps can be promoted by alkalic and acidic extracellular pH, respectively.

No MeSH data available.


Related in: MedlinePlus