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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 reversibility. Correlations of acidification- and alkalization-induced changes in recovery with chemical properties. (A) Ratios of acidic (pH = 6.0) vs. neutral (pH = 7.3) recovery are plotted against logP and (B) against pKa. In both panels classes are color-coded (as in Figure 2), three-letter codes identify individual drugs. (C) Acidic/neutral recovery ratios are color coded, and plotted on the logP against logN(pKa) plane, and (D) on the logP against polar surface area/molecular weight (PSA/MW) plane. Light and dark red indicates moderately and strongly decreased recovery, respectively; gray indicates no change; light and dark blue indicates moderately and strongly increased recovery, respectively. [See (D) for color codes. “R” stands for “ratio”). The size of the data points indicates the level of significance, as it is also shown in (D). (E) Neutral/alkalic recovery ratios are color coded and plotted on the logP against logN(pKa) plane. Light to dark blue indicates increasing ratios, red indicates minimal or no change. Levels of significance are coded by the size of data points. Codes are shown in the lower left corner.
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Figure 7: Chemical properties affecting pH-dependent reversibility. Correlations of acidification- and alkalization-induced changes in recovery with chemical properties. (A) Ratios of acidic (pH = 6.0) vs. neutral (pH = 7.3) recovery are plotted against logP and (B) against pKa. In both panels classes are color-coded (as in Figure 2), three-letter codes identify individual drugs. (C) Acidic/neutral recovery ratios are color coded, and plotted on the logP against logN(pKa) plane, and (D) on the logP against polar surface area/molecular weight (PSA/MW) plane. Light and dark red indicates moderately and strongly decreased recovery, respectively; gray indicates no change; light and dark blue indicates moderately and strongly increased recovery, respectively. [See (D) for color codes. “R” stands for “ratio”). The size of the data points indicates the level of significance, as it is also shown in (D). (E) Neutral/alkalic recovery ratios are color coded and plotted on the logP against logN(pKa) plane. Light to dark blue indicates increasing ratios, red indicates minimal or no change. Levels of significance are coded by the size of data points. Codes are shown in the lower left corner.

Mentions: Decreased recovery at acidic pH was not at all general even among the predominantly positively charged molecules. Only two compounds: venlafaxine and mirtazapine showed significantly (p < 0.01) decreased recovery at pH = 6.0, while the differences for nefazodone (p = 0.024), ambroxol (p = 0.036), lidocaine (p = 0.046), tolperisone (p = 0.057), and carbamazepine (p = 0.075) were non-significant at p < 0.01 level. The tendency was more frequent with drugs which are relatively hydrophilic (logP < ~3), and which have a substantial neutral fraction at pH = 7.3 (pKa < 9; N(pKa) > 1%; Figures 7A,B). We can also observe that (the predominantly charged) Class C compounds showed an opposite effect; acidic pH helped recovery from the inhibition by these drugs. In order to see the combined effects of lipophilicity and charged-neutral ratio, we plotted logP values against logN(pKa) (Figure 7C). Color code shows the ratio of acidic/neutral recovery values: light and dark red indicates moderately and strongly decreased recovery, respectively; gray indicates no change; light and dark blue indicates moderately and strongly increased recovery, respectively. The size of the data points indicates the level of significance. It seems that the compounds that are prone to be trapped at pH = 6.0 are more hydrophilic, and/or have a substantial neutral fraction at pH = 7.3 which becomes protonated at acidic pH (Figure 5, 4th row). Some of the “decreased-acidic-recovery” compounds, such as riluzole, lamotrigine, and carbamazepine, however, cannot be protonated even at pH = 6.0. All three compounds have, however, an exceptionally high number of hydrogen bond acceptors and donors, as well as polar surface compared to their size (Figure 7D). This suggests that the effect of external pH may go beyond changing the charged/neutral ratio of ligands, and may also affect protein-ligand interaction by other ways, possibly via interfering with hydrogen bonds or van der Waals forces. This chemical environment, which seems to be able to trap hydrophilic, polar or charged molecules must be different from the lipophilic trap mentioned above, we suggest that it is located within the inner vestibule of the channel.


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 reversibility. Correlations of acidification- and alkalization-induced changes in recovery with chemical properties. (A) Ratios of acidic (pH = 6.0) vs. neutral (pH = 7.3) recovery are plotted against logP and (B) against pKa. In both panels classes are color-coded (as in Figure 2), three-letter codes identify individual drugs. (C) Acidic/neutral recovery ratios are color coded, and plotted on the logP against logN(pKa) plane, and (D) on the logP against polar surface area/molecular weight (PSA/MW) plane. Light and dark red indicates moderately and strongly decreased recovery, respectively; gray indicates no change; light and dark blue indicates moderately and strongly increased recovery, respectively. [See (D) for color codes. “R” stands for “ratio”). The size of the data points indicates the level of significance, as it is also shown in (D). (E) Neutral/alkalic recovery ratios are color coded and plotted on the logP against logN(pKa) plane. Light to dark blue indicates increasing ratios, red indicates minimal or no change. Levels of significance are coded by the size of data points. Codes are shown in the lower left corner.
© Copyright Policy
Related In: Results  -  Collection

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Figure 7: Chemical properties affecting pH-dependent reversibility. Correlations of acidification- and alkalization-induced changes in recovery with chemical properties. (A) Ratios of acidic (pH = 6.0) vs. neutral (pH = 7.3) recovery are plotted against logP and (B) against pKa. In both panels classes are color-coded (as in Figure 2), three-letter codes identify individual drugs. (C) Acidic/neutral recovery ratios are color coded, and plotted on the logP against logN(pKa) plane, and (D) on the logP against polar surface area/molecular weight (PSA/MW) plane. Light and dark red indicates moderately and strongly decreased recovery, respectively; gray indicates no change; light and dark blue indicates moderately and strongly increased recovery, respectively. [See (D) for color codes. “R” stands for “ratio”). The size of the data points indicates the level of significance, as it is also shown in (D). (E) Neutral/alkalic recovery ratios are color coded and plotted on the logP against logN(pKa) plane. Light to dark blue indicates increasing ratios, red indicates minimal or no change. Levels of significance are coded by the size of data points. Codes are shown in the lower left corner.
Mentions: Decreased recovery at acidic pH was not at all general even among the predominantly positively charged molecules. Only two compounds: venlafaxine and mirtazapine showed significantly (p < 0.01) decreased recovery at pH = 6.0, while the differences for nefazodone (p = 0.024), ambroxol (p = 0.036), lidocaine (p = 0.046), tolperisone (p = 0.057), and carbamazepine (p = 0.075) were non-significant at p < 0.01 level. The tendency was more frequent with drugs which are relatively hydrophilic (logP < ~3), and which have a substantial neutral fraction at pH = 7.3 (pKa < 9; N(pKa) > 1%; Figures 7A,B). We can also observe that (the predominantly charged) Class C compounds showed an opposite effect; acidic pH helped recovery from the inhibition by these drugs. In order to see the combined effects of lipophilicity and charged-neutral ratio, we plotted logP values against logN(pKa) (Figure 7C). Color code shows the ratio of acidic/neutral recovery values: light and dark red indicates moderately and strongly decreased recovery, respectively; gray indicates no change; light and dark blue indicates moderately and strongly increased recovery, respectively. The size of the data points indicates the level of significance. It seems that the compounds that are prone to be trapped at pH = 6.0 are more hydrophilic, and/or have a substantial neutral fraction at pH = 7.3 which becomes protonated at acidic pH (Figure 5, 4th row). Some of the “decreased-acidic-recovery” compounds, such as riluzole, lamotrigine, and carbamazepine, however, cannot be protonated even at pH = 6.0. All three compounds have, however, an exceptionally high number of hydrogen bond acceptors and donors, as well as polar surface compared to their size (Figure 7D). This suggests that the effect of external pH may go beyond changing the charged/neutral ratio of ligands, and may also affect protein-ligand interaction by other ways, possibly via interfering with hydrogen bonds or van der Waals forces. This chemical environment, which seems to be able to trap hydrophilic, polar or charged molecules must be different from the lipophilic trap mentioned above, we suggest that it is located within the inner vestibule of the channel.

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