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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 acidification-induced recovery. (A) Correlation between recovery caused by alkalic-to-neutral and neutral-to-acidic solution exchange. (B) Acidification-induced recovery plotted against logP and (C) pKa. The sum of the logarithms of the two recovery steps (neutral-to-acidic and alkalic-to-neutral) was used as a measure of acidification-induced recovery.
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Figure 8: Chemical properties affecting acidification-induced recovery. (A) Correlation between recovery caused by alkalic-to-neutral and neutral-to-acidic solution exchange. (B) Acidification-induced recovery plotted against logP and (C) pKa. The sum of the logarithms of the two recovery steps (neutral-to-acidic and alkalic-to-neutral) was used as a measure of acidification-induced recovery.

Mentions: When an inhibitor had previously been perfused at neutral pH, and the recovery was not complete (mostly Class C, F, and G compounds), neutral-to-acidic solution exchange evoked a component of recovery. We quantified this by calculating the ratio of current amplitudes evoked at the end of this section vs. the amplitude of the first current evoked in acidic solution (circled in the inset shown in the 6th row of Figure 5). The explanation is essentially the same as in the case of “decreased recovery at alkalic pH” (see above), except that in this case the recovery must be compromised even at neutral pH. Lowering the pH then helps the protonation of the compounds, which shifts the “protonated-in-the-outer-membrane-layer-pool” vs. “deprotonated-within-the-membrane-pool” equilibrium, and also increases aqueous solubility, helping partitioning into the aqueous phase, and therefore wash-out is accelerated. Most, but not all compounds, which showed enhanced recovery upon neutral-to-acidic exchange reacted similarly to alkalic-to-neutral exchange (both are shown by red arrows in the 6th row of Figure 5); there was a significant correlation (p < 0.001) between the reactions of compounds to these two pH decreasing steps (Figure 8A; Supplemental Figure 1). Decreasing the pH must have helped escape from the lipophilic trap, therefore this phenomenon would be most evident for predominantly charged, lipophilic compounds. Indeed, logP had a significant correlation (p < 0.01) with the extent of acidification-induced recovery (Figure 8B), and there was a definite optimum of pKa for it between 8.8 and 10.4 (Figure 8C). In these Figures 8B,C acidification-induced recovery was calculated by the sum of the logarithms of the two recovery steps. We can conclude that as we have expected, essentially the same chemical properties predispose compounds both to “decreased recovery at alkalic pH” and to “acidification induced recovery.”


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 acidification-induced recovery. (A) Correlation between recovery caused by alkalic-to-neutral and neutral-to-acidic solution exchange. (B) Acidification-induced recovery plotted against logP and (C) pKa. The sum of the logarithms of the two recovery steps (neutral-to-acidic and alkalic-to-neutral) was used as a measure of acidification-induced recovery.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 8: Chemical properties affecting acidification-induced recovery. (A) Correlation between recovery caused by alkalic-to-neutral and neutral-to-acidic solution exchange. (B) Acidification-induced recovery plotted against logP and (C) pKa. The sum of the logarithms of the two recovery steps (neutral-to-acidic and alkalic-to-neutral) was used as a measure of acidification-induced recovery.
Mentions: When an inhibitor had previously been perfused at neutral pH, and the recovery was not complete (mostly Class C, F, and G compounds), neutral-to-acidic solution exchange evoked a component of recovery. We quantified this by calculating the ratio of current amplitudes evoked at the end of this section vs. the amplitude of the first current evoked in acidic solution (circled in the inset shown in the 6th row of Figure 5). The explanation is essentially the same as in the case of “decreased recovery at alkalic pH” (see above), except that in this case the recovery must be compromised even at neutral pH. Lowering the pH then helps the protonation of the compounds, which shifts the “protonated-in-the-outer-membrane-layer-pool” vs. “deprotonated-within-the-membrane-pool” equilibrium, and also increases aqueous solubility, helping partitioning into the aqueous phase, and therefore wash-out is accelerated. Most, but not all compounds, which showed enhanced recovery upon neutral-to-acidic exchange reacted similarly to alkalic-to-neutral exchange (both are shown by red arrows in the 6th row of Figure 5); there was a significant correlation (p < 0.001) between the reactions of compounds to these two pH decreasing steps (Figure 8A; Supplemental Figure 1). Decreasing the pH must have helped escape from the lipophilic trap, therefore this phenomenon would be most evident for predominantly charged, lipophilic compounds. Indeed, logP had a significant correlation (p < 0.01) with the extent of acidification-induced recovery (Figure 8B), and there was a definite optimum of pKa for it between 8.8 and 10.4 (Figure 8C). In these Figures 8B,C acidification-induced recovery was calculated by the sum of the logarithms of the two recovery steps. We can conclude that as we have expected, essentially the same chemical properties predispose compounds both to “decreased recovery at alkalic pH” and to “acidification induced recovery.”

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