The Relationship Between Molecular Symmetry and Physicochemical Properties Involving Boiling and Melting of Organic Compounds.

OBJECTIVE AND METHODS: The reliable estimation of phase transition physicochemical properties such as boiling and melting points can be valuable when designing compounds with desired physicochemical properties. This study explores the role of external rotational symmetry in determining boiling and melting points of select organic compounds. Using experimental data from the literature, the entropies of boiling and fusion were obtained for 541 compounds. The statistical significance of external rotational symmetry number on entropies of phase change was determined by using multiple linear regression. In addition, a series of aliphatic hydrocarbons, polysubstituted benzenes, and di-substituted napthalenes are used as examples to demonstrate the role of external symmetry on transition temperature. RESULTS: The results reveal that symmetry is not well correlated with boiling point but is statistically significant in melting point. CONCLUSION: The lack of correlation between the boiling point and the symmetry number reflects the fact that molecules have a high degree of rotational freedom in both the liquid and the vapor. On the other hand, the strong relationship between symmetry and melting point reflects the fact that molecules are rotationally restricted in the crystal but not in the liquid. Since the symmetry number is equal to the number of ways that the molecule can be properly oriented for incorporation into the crystal lattice, it is a significant determinant of the melting point.

Effect of pH and Ionic Strength on the Solubility of Quinoline: Back-to-Basics.

The interest of quinoline as a contaminant agent and as scaffold for the development of new therapeutic agent warrants to revisit the pH-solubility behavior of quinoline (Q) and quinoline derivatives (Q-derivatives) with possible salting-out effect. Q is a weak base with potential hazard upon exposure that may be occupational by inhalation or ingestion of or dermal exposure to particulates in certain industries; or simply by inhalation of cigarette smoke. In contrast, quinoline and its derivatives are useful in diverse therapeutic applications such as anticancer, antiseptic, antipyretic, antiviral, and antimalarial. These claims have raised the possibility of using quinoline motif for the synthesis of new drugs; however, it may act as a pollutant on soil and water as ionizable organic compounds (IOC). The solubility and partitioning behavior of Q may be a critical factor in determining the extent of inhalation and oral absorption or sorption onto soil and water. Studies on the solubility of Q have been reported; however, due to Q-derivatives distinctive usage, it is necessary to revisit and evaluate the solubility profile of Q at different pH levels and ionic strengths. This study reports a simple analytical method for determining the solubility of nitrogen heterocyclic compounds and possible salting-out effect as a function of pH, buffer concentration, and ionic strength. This information can be of value when developing Q-derivatives and to enhance understanding of Q as well as its derivatives behavior in the gastrointestinal tract or when evaluating the presence of Q as an environmental contaminant.

A rule of unity for human intestinal absorption 3: Application to pharmaceuticals.

The rule of unity is based on a simple absorption parameter, Π, that can accurately predict whether or not an orally administered drug will be well absorbed or poorly absorbed. The intrinsic aqueous solubility and octanol-water partition coefficient, along with the drug dose are used to calculate Π. We show that a single delineator value for Π exist that can distinguish whether a drug is likely to be well absorbed (FA ≥ 0.5) or poorly absorbed (FA < 0.5) at any specified dose. The model is shown to give 82.5% correct predictions for over 938 pharmaceuticals. The maximum well-absorbed dose (i.e. the maximum dose that will be more than 50% absorbed) calculated using this model can be utilized as a guideline for drug design and synthesis.

Comments on prediction of the aqueous solubility using the general solubility equation (GSE) versus a genetic algorithm and a support vector machine model.

The general solubility equation (GSE) is the state-of-the-art method for estimating the aqueous solubilities of organic compounds. It is an extremely simple equation that expresses aqueous solubility as a function of only two inputs: the octanol-water partition coefficient calculated by readily available softwares like clogP and ACD/logP, and the commonly known melting point of the solute. Recently, Bahadori et al. proposed that their genetic algorithm support vector machine is a "better" predictor. This paper compares the use of the of Bahadori et al. model for the prediction of aqueous solubility to the existing GSE model.

Preformulation study of NSC-726796.

A stability-indicating high-performance liquid chromatography method to quantify 2-(2,4-difluorophenyl)-4,5,6,7-tetrafluoroisoindoline-1,3-dione (NSC-726796) and its three main degradation products was developed. This method was used to investigate its degradation kinetics and mechanism. The reaction follows first-order kinetics and appears to be base catalyzed with the maximum stability at pH 1. The products were identified as 2-(2,4-difluorophenylcarbamoyl)-3,4,5,6-tetrafluorobenzoic acid (NSC-749820), 2,4-difluoroaniline, and tetrafluorophthalic acid. The parent drug, NSC-726796, was also found to react with methanol and ethanol. NSC-726796 demonstrates antiangiogenic activity, however, when its degradant NSC749820 does not show antiangiogenic activity.

Machine learning transition temperatures from 2D structure.

A priori knowledge of physicochemical properties such as melting and boiling could expedite materials discovery. However, theoretical modeling from first principles poses a challenge for efficient virtual screening of potential candidates. As an alternative, the tools of data science are becoming increasingly important for exploring chemical datasets and predicting material properties. Herein, we extend a molecular representation, or set of descriptors, first developed for quantitative structure-property relationship modeling by Yalkowsky and coworkers known as the Unified Physicochemical Property Estimation Relationships (UPPER). This molecular representation has group-constitutive and geometrical descriptors that map to enthalpy and entropy; two thermodynamic quantities that drive thermal phase transitions. We extend the UPPER representation to include additional information about sp2-bonded fragments. Additionally, instead of using the UPPER descriptors in a series of thermodynamically-inspired calculations, as per Yalkowsky, we use the descriptors to construct a vector representation for use with machine learning techniques. The concise and easy-to-compute representation, combined with a gradient-boosting decision tree model, provides an appealing framework for predicting experimental transition temperatures in a diverse chemical space. An application to energetic materials shows that the method is predictive, despite a relatively modest energetics reference dataset. We also report competitive results on diverse public datasets of melting points (i.e., OCHEM, Enamine, Bradley, and Bergström) comprised of over 47k structures. Open source software is available at https://github.com/USArmyResearchLab/ARL-UPPER.

Stability of 5-fluoro-2'-deoxycytidine and tetrahydrouridine in combination.

In vivo, the DNA methyltransferase inhibitor, 5-fluoro-2'-deoxycytidine (FdCyd, NSC-48006), is rapidly converted to its unwanted metabolites. Tetrahydrouridine (THU, NSC-112907), a cytidine deaminase inhibitor can block the first metabolic step in FdCyd catabolism. Clinical studies have shown that co-administration with THU can inhibit the metabolism of FdCyd. The National Cancer Institute is particularly interested in a 1:5 FdCyd/THU formulation. The purpose of this study was to investigate the in vitro pH stability of FdCyd and THU individually and in combination. A stability-indicating high-performance liquid chromatography method for the quantification of both compounds and their degradants was developed using a ZIC(R)-HILIC column. The effect of THU and FdCyd on the in vitro degradation of each other was studied as a function of pH from 1.0 to 7.4 in aqueous solutions at 37 degrees C. The degradation of FdCyd appears to be first-order and acid-catalyzed. THU equilibrates with at least one of its degradants. The combination of FdCyd and THU in solution does not affect the stability of either compound. The stability and compatibility of FdCyd and THU in the solid state at increased relative humidity and at various temperatures are also evaluated.

2-[N-(2,4-Difluoro-phen-yl)carbamo-yl]-3,4,5,6-tetra-fluoro-benzoic acid.

The title compound, C(14)H(5)F(6)NO(3), was synthesized by condensation of tetra-fluoro-phthalic anhydride and 2,4-difluoro-aniline. It was then recrystallized from hexane to give a nonmerohedral twin with two crystallographically unique mol-ecules in the asymmetric unit. The refined twin fraction is 0.460 (3). Torsional differences between the aryl rings and the central amide group account for the presence of two unique mol-ecules. The compound packs as double tapes formed by O-H⋯O and N-H⋯O hydrogen-bonding inter-actions between each unique mol-ecule and its symmetry equivalents.

Comparison of aqueous solubility estimation from AQUAFAC and the GSE.

The GSE (General Solubility Equation) and AQUAFAC (Aqueous Functional Group Activity Coefficients) are two empirical models for aqueous solubility prediction. This study compares the aqueous solubility estimation of a set of 1642 pharmaceutically and environmentally related compounds, using the two methods. The average absolute errors in the solubility prediction are 0.543 log units for AQUAFAC and 0.576 log units for the GSE. About 88.0% of the AQUAFAC solubilities and 83.0% of the GSE molar aqueous solubilities are predicted within one log unit of the observed values. The marginally greater accuracy of AQUAFAC is due to the fact that it utilizes fitted-parameters for many structural fragments and is based on experimental solubility data. The GSE on the other hand is a simpler, non-regression based equation which uses two parameters for solubility prediction.

Solubility improvement of drugs using N-methyl pyrrolidone.

The solubilization efficiency of N-methyl pyrrolidone (NMP) has been determined and compared to that of ethanol and propylene glycol for 13 poorly soluble drugs. NMP is found to be a more efficient solubilizer for all the drugs studied. The solubility enhancement as high as about 800-fold is obtained in 20% v/v NMP solution as compared to water. The mechanism of drug solubilization by NMP has also been investigated. It is proposed that NMP enhances drug solubility by simultaneously acting as a cosolvent and a complexing agent. A mathematical model is used to estimate the drug solubility in NMP-water mixture, according to which the total solubility enhancement is a sum of the two effects. This model describes the experimental data well and is more accurate than other models. A large and uniform reduction in the surface tension of water as a function of NMP concentration demonstrates its cosolvent effect. The complexation is supported by the fact that it's strength is affected by the temperature and the polarity of the medium. A strong correlation exists between log K (ow) of the drugs and the cosolvency coefficients. The correlation between log K (ow) and the complexation coefficients is weak suggesting that factors such as molecular shape and aromaticity of the drug molecule are significant in determining the complexation strength. This has been confirmed by the absence of a significant complexation between NMP and linear drug-like solutes.

Estimation of Melting Points of Organics.

Unified physicochemical property estimation relationships is a system of empirical and theoretical relationships that relate 20 physicochemical properties of organic molecules to each other and to chemical structure. Melting point is a key parameter in the unified physicochemical property estimation relationships scheme because it is a determinant of several other properties including vapor pressure, and solubility. This review describes the first-principals calculation of the melting points of organic compounds from structure. The calculation is based on the fact that the melting point, Tm, is equal to the ratio of the heat of melting, ΔHm, to the entropy of melting, ΔSm. The heat of melting is shown to be an additive constitutive property. However, the entropy of melting is not entirely group additive. It is primarily dependent on molecular geometry, including parameters which reflect the degree of restriction of molecular motion in the crystal to that of the liquid. Symmetry, eccentricity, chirality, flexibility, and hydrogen bonding, each affect molecular freedom in different ways and thus make different contributions to the total entropy of fusion. The relationships of these entropy determining parameters to chemical structure are used to develop a reasonably accurate means of predicting the melting points over 2000 compounds.

Estimating the Physicochemical Properties of Polysubstituted Aromatic Compounds Using UPPER.

The UPPER model (Unified Physicochemical Property Estimation Relationships) has been used to predict 9 essential physicochemical properties of pure compounds. It was developed almost 25 years ago and has been validated by the Yalkowsky group for almost 2000 aliphatic, aromatic, and polyhalogenated hydrocarbons. UPPER is based on a group of additive and nonadditive descriptors along with a series of well-accepted thermodynamic relationships. In this model, the 2-dimensional chemical structure is the only input needed. This work extends the applicability of UPPER to hydrogen bonding and non-hydrogen bonding aromatic compounds with several functional groups such as alcohol, aldehyde, ketone, carboxylic acid, carbonate, carbamate, amine, amide, nitrile as well as aceto, and nitro compounds. The total data set includes almost 3000 compounds. Aside from the enthalpies and entropies of melting and boiling, no training set is used for the calculation of the properties. The results show that UPPER enables a reasonable estimation of all the considered properties.

Overcoming the Challenges of Low Drug Solubility in the Intravenous Formulation of Solithromycin.

Solithromycin is a fluoro-ketolide (a fourth-generation macrolide) antibiotic that has been undergoing clinical trials for the treatment of community-acquired bacterial pneumonia. In this study, development of the tri-amino acid-buffered solithromycin intravenous (IV) formulation was performed to minimize the occurrence of infusion-associated local adverse events (infusion-site pain or phlebitis) observed in patients who received the tartaric acid-buffered IV formulation with a lower buffered capacity during phase I clinical trials. Development of the tri-amino acids-buffered solithromycin IV formulation was achieved using a dynamic in vitro precipitation model. Computational modeling also supports the superiority of the amino acid-buffered formulation over the tartaric aid-buffered formulation.

Solubilization of poorly soluble compounds using 2-pyrrolidone.

The solubilization of nine poorly soluble compounds in aqueous solution by 2-pyrrolidone has been studied. Solubility enhancement as high as 500-fold is achieved using 20% 2-pyrrolidone. A comparison shows that 2-pyrrolidone is a better solubilizer than glycerin, propylene glycol, polyethylene glycol 400 or ethanol. The observed solubilization curves are deconvoluted into components representing complexation and cosolvency. A clear linear relationship exists between the cosolvency solubilization power (sigma) of 2-pyrrolidone and the partition coefficient (log K(ow)) of the drug (R(2)=0.96) extending over three orders of magnitude. The stability constants for the formation of 1:1 complex (K(1:1)) involving 2-pyrrolidone and the drugs have been calculated. A weaker correlation (R(2)=0.74) is observed between the complexation constants and the partition coefficients of respective drugs. This study indicates that 2-pyrrolidone, like NMP, can act as a complexant at low concentrations and as a cosolvent at high concentrations and that both these properties are affected by the partition coefficient of the solute.

Stacking complexation by nicotinamide: a useful way of enhancing drug solubility.

The solubility enhancement of 11 poorly soluble drugs by complexation using nicotinamide has been studied. The solubilization efficiency of nicotinamide has been compared to that of hydroxypropyl-beta-cyclodextrin and sulfobutylether-beta-cyclodextrin. Solubility enhancements as high as 4000-fold are observed in 20% (w/v) nicotinamide solution. Furthermore, nicotinamide is more effective than cyclodextrins for solubilizing some of the drugs. The mechanism of drug solubilization by nicotinamide is investigated by studying the effects of nicotinamide concentration on the surface tension and the conductivity of water. A slight break in both, the surface tension and conductivity is noticed at around 10% (w/v), suggesting self-association at higher concentrations. Corresponding breaks in the solubility profiles of estrone and griseofulvin at similar concentrations support self-association. Based on this observation it appears that at low concentrations, one molecule of nicotinamide undergoes complexation with one drug molecule to form a 1:1 complex. At higher concentrations, two molecules of nicotinamide undergo complexation with one drug molecule forming a 1:2 complex. The complexation constants have been calculated for all the drugs and the data are well described by this model. Expectedly, increasing the temperature reduces the complexation constants.

Estimation of melting points of organic compounds-II.

A model for calculation of melting points of organic compounds from structure is described. The model utilizes additive, constitutive and nonadditive, constitutive molecular properties to calculate the enthalpy of melting and the entropy of melting, respectively. Application of the model to over 2200 compounds, including a number of drugs with complex structures, gives an average absolute error of 30.1 degrees.

Solubilization of two structurally related anticancer drugs: XK-469 and PPA.

The efficiency of a solubilization technique is determined by the physical-chemical properties of the drug. This study investigates the solubilization on two structurally related anticancer drugs, XK-469 and PPA. XK-469 is much less polar than PPA with an intrinsic solubility of 0.000274 mg/mL, which is about 10,000 fold less than that of PPA. Fortunately, its physical-chemical properties make it much more formulatable. An ionizable drug can be solubilized by pH adjustment with cosolvency, micellization, or complexation. Both XK-469 and PPA are weak acids with pKa values of 2.7 and 2.9, respectively. Thus, they can be solubilized by pH adjustment. At pH 4.55, neither cosolvency, micellization nor complexation has much effect on the solubility of PPA. However, these techniques can significantly increase the solubility of XK-469. In fact, the solubility of XK-469 in 20% HPbetaCD at pH 4.55 is 5.85 mg/mL, which is more than 20,000 times greater than its intrinsic solubility. With the solubilization descriptors obtained from the experimental data for both unionized and ionized drug species at pH 1.0 and pH 4.55, the solubility of each drug at any pH and excipient concentration can be estimated. Then, a solubilization technique can be chosen for preparing a desired final drug concentration.

Solubilization of monovalent weak electrolytes by micellization or complexation.

In order to prepare a liquid formulation for a weak electrolyte, micellization or complexation is often applied with the solution pH controlled to have some of the drug molecules ionized. The efficiency of the micellization is evaluated by either the micellar solubilization capacities, kappau, and kappai or the micellar partition coefficients, Ku(m) and Ki(m), for the unionized and ionized drug species. Similarly, the efficiency of complexation is evaluated by either the complex solubilization capacities, tauu and taui or the drug-ligand binding constants,Ku(1:1) and Ki(1:1). In this study, the experimental values of these descriptors were generated for seven ionizable drugs. The relationships of the logarithms of each descriptor to the logarithm of the octanol-water partition coefficient of the unionized drug (log Pu) and ionized drug species (log Pi) were evaluated. Although kappa and tau cannot be predicted, this study shows that Km and K1:1 are dependent on log P for both the unionized and ionized drug species. Thus, the total drug solubility for a weak electrolyte solubilized by micellization or complexation can be predicted at any pH.

Estimation of the aqueous solubility of weak electrolytes.

Jain and Yalkowsky [Jain, N., Yalkowsky, S.H., 2001. Estimation of the aqueous solubility. I. Application to organic non-electrolytes. J. Pharm. Sci. 90, 234-252.] demonstrated that the general solubility equation (GSE) can be used to estimate the aqueous solubility of organic non-electrolytes. In this study the applicability of the GSE was extended to weak electrolytes. It is demonstrated that the GSE estimates the aqueous solubility of 949 compounds, including 367 weak electrolytes with an AAE of 0.58. It is also shown that the intrinsic solubilities of weak acids for which the pK(a)+log S(w)

Solubility prediction in octanol: a technical note.

The purpose of this work was to derive an equation for the rapid estimation of octanol solubilities of organic compounds. Solubilities ranging over 4 orders of magnitude were predicted with an average absolute error of 0.39 logarithmic units using melting point alone. The greatest error in prediction occurred for strongly bonded compounds.