{"title":"Often neglected steps in transforming drug solubility from single measurement in pure water to physiologically-appropriate solubility-pH.","authors":"Alex Avdeef","doi":"10.5599/admet.2626","DOIUrl":null,"url":null,"abstract":"<p><strong>Background and purpose: </strong>The solubility of weakly-ionizable drugs in pure water, S<sub>w</sub>, is commonly measured. The pH-dependent properties of the saturated solutions can be surprisingly complex in subtle ways. This commentary examines the characteristics of such measurements through case studies of 32 free acids, bases, and ampholytes (including crocetin, glibenclamide, mellitic acid, quercetin, bedaquiline, brigatinib, imatinib, celecoxib, and lysine), using published water solubility data.</p><p><strong>Computational approach: </strong>Usually, in such saturated solutions, the ionic strength, <i>I</i> <sub>w</sub>, is close to zero. When the pH is adjusted away from pH<sub>w</sub>, the ionic strength increases, substantially in some cases (<i>e.g. I</i> <sub>w</sub> > 10 M at pH 7.4 for mellitic acid and lysine). This change in ionic strength alters the activities of the species in solution. The corresponding equilibrium constants used to calculate the concentrations of these species must be adjusted accordingly. Here, the Stokes-Robinson hydration theory, slightly modified with Setschenow 'salting-out' constants to account for solvent interactions with unionized drugs, was used to estimate activity coefficients. The calculations were performed with the pDISOL-X program.</p><p><strong>Key results: </strong>Given reliably-measured values of solubility in water (<i>S</i> <sub>w</sub>) and ionization constant (p<i>K</i> <sub>a</sub>) of the drugs and assuming that the Henderson-Hasselbalch equation is valid, a method is described for (i) adjusting the measured <i>S</i> <sub>w</sub> values at ionic strength, <i>I</i> <sub>w</sub> ~ 0 M, to values expected at reference ionic strength, <i>I</i> <sub>ref</sub> = 0.15 M (or at any other reasonable reference value), (ii) determining the water pH<sub>w</sub> in saturated solutions of added neutral-form drugs; (iii) determining the intrinsic solubility, <i>S</i> <sub>0</sub>, both at <i>I</i> <sub>w</sub> and <i>I</i> <sub>ref</sub>, and (iv) using analytic-continuation in the equilibrium mass action model to deduce the solubility values as a function of pH, harmonized to a selected <i>I</i> <sub>ref</sub>. For highly soluble drugs, whose <i>I</i> <sub>w</sub> exceeds 0.15 M, the intrinsic solubility values appear to depend on the amount of excess solid added.</p><p><strong>Conclusion: </strong>This commentary re-emphasizes that measured <i>S</i> <sub>w</sub> is not generally the same as <i>S</i> <sub>0</sub>. It is stressed that transforming measured drug solubility in pure water to an ionic strength level that is physiologically appropriate would better match the conditions found in biological media, potentially improving applications of solubility in pharmaceutical research and development.</p>","PeriodicalId":7259,"journal":{"name":"ADMET and DMPK","volume":"13 1","pages":"2626"},"PeriodicalIF":3.4000,"publicationDate":"2025-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11954141/pdf/","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"ADMET and DMPK","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.5599/admet.2626","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"2025/1/1 0:00:00","PubModel":"eCollection","JCR":"Q2","JCRName":"CHEMISTRY, MEDICINAL","Score":null,"Total":0}
引用次数: 0
Abstract
Background and purpose: The solubility of weakly-ionizable drugs in pure water, Sw, is commonly measured. The pH-dependent properties of the saturated solutions can be surprisingly complex in subtle ways. This commentary examines the characteristics of such measurements through case studies of 32 free acids, bases, and ampholytes (including crocetin, glibenclamide, mellitic acid, quercetin, bedaquiline, brigatinib, imatinib, celecoxib, and lysine), using published water solubility data.
Computational approach: Usually, in such saturated solutions, the ionic strength, Iw, is close to zero. When the pH is adjusted away from pHw, the ionic strength increases, substantially in some cases (e.g. Iw > 10 M at pH 7.4 for mellitic acid and lysine). This change in ionic strength alters the activities of the species in solution. The corresponding equilibrium constants used to calculate the concentrations of these species must be adjusted accordingly. Here, the Stokes-Robinson hydration theory, slightly modified with Setschenow 'salting-out' constants to account for solvent interactions with unionized drugs, was used to estimate activity coefficients. The calculations were performed with the pDISOL-X program.
Key results: Given reliably-measured values of solubility in water (Sw) and ionization constant (pKa) of the drugs and assuming that the Henderson-Hasselbalch equation is valid, a method is described for (i) adjusting the measured Sw values at ionic strength, Iw ~ 0 M, to values expected at reference ionic strength, Iref = 0.15 M (or at any other reasonable reference value), (ii) determining the water pHw in saturated solutions of added neutral-form drugs; (iii) determining the intrinsic solubility, S0, both at Iw and Iref, and (iv) using analytic-continuation in the equilibrium mass action model to deduce the solubility values as a function of pH, harmonized to a selected Iref. For highly soluble drugs, whose Iw exceeds 0.15 M, the intrinsic solubility values appear to depend on the amount of excess solid added.
Conclusion: This commentary re-emphasizes that measured Sw is not generally the same as S0. It is stressed that transforming measured drug solubility in pure water to an ionic strength level that is physiologically appropriate would better match the conditions found in biological media, potentially improving applications of solubility in pharmaceutical research and development.
期刊介绍:
ADMET and DMPK is an open access journal devoted to the rapid dissemination of new and original scientific results in all areas of absorption, distribution, metabolism, excretion, toxicology and pharmacokinetics of drugs. ADMET and DMPK publishes the following types of contributions: - Original research papers - Feature articles - Review articles - Short communications and Notes - Letters to Editors - Book reviews The scope of the Journal involves, but is not limited to, the following areas: - physico-chemical properties of drugs and methods of their determination - drug permeabilities - drug absorption - drug-drug, drug-protein, drug-membrane and drug-DNA interactions - chemical stability and degradations of drugs - instrumental methods in ADMET - drug metablic processes - routes of administration and excretion of drug - pharmacokinetic/pharmacodynamic study - quantitative structure activity/property relationship - ADME/PK modelling - Toxicology screening - Transporter identification and study