Solvency and hydrogen bonding interactions in nonaqueous systems

Development of micro-fibrous solid dispersions of poorly water-soluble drugs in sucrose using temperature-controlled centrifugal spinning

Solid dispersion technology represents a successful approach to addressing the bioavailability issues caused by the low aqueous solubility of many Biopharmaceutics Classification System (BCS) Class II drugs. In this study, the use of high-yield manufacture of fiber-based dispersion is explored as an alternative approach to monolith production methods. A temperature-controlled solvent-free centrifugal spinning process was used to produce sucrose-based microfibers containing the poorly water-soluble drugs olanzapine and piroxicam (both BCS Class II); these were successfully incorporated into the microfibers and the basic characteristics of fiber diameter, glassy behavior, drug loading capacity and drug-sucrose interaction assessment were measured. Scanning electron microscopy revealed that bead-free drug-loaded microfibers with homogenous morphology and diameter in the range of a few micrometers were prepared using our process. Differential scanning calorimetric and X-ray diffraction analyses showed that both drug and carrier were present in the amorphous state in the microfibers, although in the case of piroxicam-loaded microfibers, the presence of small amounts of crystalline drug was observed under polarized light microscopy and in Fourier transform infrared spectra. Drug dissolution performance was evaluated under both sink and non-sink conditions and was found to be significantly enhanced compared to the corresponding crystalline physical mixtures and pure drugs, with evidence of supersaturation behavior noted under non-sink conditions. This study has demonstrated that microfiber-based dispersions may be manufactured by the centrifugal spinning process and may possess characteristics that are favorable for the enhanced dissolution and oral absorption of drugs.

Solubility behaviour of griseofulvin in fatty acids

The solubility of griseofulvin in the straight-chain alkanoic acids from C2 to C22 and in the C4 and C5 alkanoic acids branched at C-2 was measured at various temperatures. The enthalpy of fusion of griseofulvin, measured by differential scanning calorimetry, was 39.39 kJ mol−1 at the melting point (495.15 K) and 36.95 kJ mol−1 at 373.15 K. The standard Gibbs free energy (ΔG°), standard enthalpy (ΔH°) and standard entropy (ΔS°) of solution were calculated at 373.15 K, from the van't Hoff plot of the temperature dependence of the mole fraction solubility in terms of the pure supercooled liquid solute as the standard state. With increasing chain length of the alkanoic acid solvents, ΔG° increased in parallel with the increase in pKa of the acids in water, suggesting that the solubility behaviour involves specific solute-solvent proton interactions, while ΔH° and ΔS° fluctuated but tended to decrease in parallel with the corresponding decreases in their enthalpies and entropies of ionization, respectively. The fluctuations in ΔH° and ΔS° may be attributed to the different solid adducts containing griseofulvin and the solvent. An observed non-linear (logarithmic) decrease in solubility with decreasing molality of the carboxyl group in the liquid solvent on ascending the homologous series is attributed to the disturbing influence of the hydrocarbon chains on the specific solute-solvent hydrogen bonding. Chain-branching of the solvent at C-2 gave a reduced solubility of griseofulvin and higher ΔG°, ΔH° and ΔS° values compared with the corresponding straight chain acid.

From Physical Pharmacy to Clinical Pharmacology

Research works on molecular interactions in solutions were carried out at School of Pharmacy, the University of Wisconsin under the direction of Prof. T. Higuchi and at Faculty of Pharmaceutical Sciences, Kyoto University under the direction of Prof. H. Sezaki. Studies on permeation of drugs through polymer membranes were carried out at Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Canada and at Pharmaceutical Chemistry Research Laboratories at Food and Drug Directorate, Department of Health and Welfare, Canada. Studies on modification of delivery patterns by means of pharmaceutical approaches were carried out at Faculty of Pharmaceutical Sciences, Hokkaido University. Topics related to modification of drug delivery patterns include employment of amorphous forms such as ground mixture with micro-crystalline cellulose and coprecipitate with polyvinylpyrrolidone, use of biodegradable polymers such as polylactic acid and polycarbonates, gel-forming materials such as konjac, agar and hydroxypropylcellulose, and physicochemical systems such as complexation. Works related to drug delivery and disposition of drugs in humans were carried out at Department of Pharmacy, Kumamoto University Hospital. Topics related to drug delivery in humans include injections containing anticancer drugs for intra-arterial administration, lidocaine gels for dermal anesthesia, glucagon solution for nasal administration. Topics related to disposition of drugs in humans include clinical pharmacokinetic studies in infants and elderly and medical uses of adsorbents.

Solubility enhancement of phenol and phenol derivatives in perfluorooctyl bromide

Perfluorinated solvents are gaining popularity as pulmonary ventilation fluids, but they suffer from poor solvent quality in concurrent drug delivery applications. The present study examines the use of a hydrophobic solubilizing agent capable of interacting with model drug solutes by hydrogen bonding with the purpose of enhancing solubility in perfluorooctyl bromide (PFOB). A series of solubilizing agents containing a ketone carbonyl to act as a hydrogen bond acceptor and a perfluoroalkyl chain to maintain the solubility of the putative complex in PFOB are investigated. The solubility of phenol in PFOB is enhanced to the greatest extent by 1-(4-perfluorobutyl phenyl)-1-hexanone (III) where the ketone carbonyl is protected from the electron withdrawing effects of the perfluorobutyl chain by a phenyl ring. Experiments with solubilizers lacking the ketone group suggest that pi-pi bond interactions of III with phenol do not significantly enhance solubility. For a series of phenol derivatives, a rank-order correlation exists between the magnitude of solubility enhancement by III, as reflected by the calculated association constants, and the Hammett sigma parameter of the phenols. Because the O-methyl-substituted phenols do not have the ability to hydrogen bond, their solubility is not enhanced by the presence of III. The results of the present study indicate that solubility of model drug hydrogen bond donating compounds can be enhanced in PFOB by the presence of fluorocarbon-soluble hydrogen bond acceptors.

Hydrogen-bond patterns of dialkylpyridone iron chelators and their 1:1 formic acid solvates: description, prediction, and role in crystal packing

The crystal structures (determined from single-crystal X-ray diffraction studies) and hydrogen-bond patterns of three crystalline 1,2-dialkyl-3-hydroxy-4-pyridones and their 1:1 formic acid solvates are elucidated. The primary hydrogen-bond connectivities observed are explained by a model that predicts that the best donor bonds to the best acceptor. Relative hydrogen-bond donating and accepting abilities of the functional groups observed in these compounds are evaluated by a combination of pKas, energy calculations, resonance arguments, and crystallographic evidence. The primary (O--H ... O) hydrogen-bond patterns are described by graph set notation, and brief explanations of the graph set assignments are also included. A total of 17 secondary (C--H ... O) hydrogen bonds are also observed in these six structures. Correlations are drawn between the observed C--H ... O hydrogen bonds and the molecular packing. The possible role of these secondary hydrogen bonds in influencing molecular packing is discussed.

Development of a preformulation lipophilicity screen utilizing a C-18-derivatized polystyrene-divinylbenzene high-performance liquid chromatographic (HPLC) column

Alkane/water partition coefficients have been predicted from the retention times of solutes using a C-18-derivatized polystyrene-divinylbenzene HPLC column (Act-I). Several classes of compounds, with molecular weights from 78 to 379 and partition coefficients ranging over several orders of magnitude, were included in the present study. A high correlation coefficient (0.953) was obtained from log-log plots of alkane/water partition coefficients versus capacity factor. A poor correlation was observed for octanol/water partition coefficient, presumably due to the hydrogen-bonding capability of octanol. The alkane/water correlation suggests that the system is devoid of significant specific solute-stationary phase interactions which are known to impart anomalous retention behavior to traditional reverse phase columns. Deviations of calculated alkane/water partition coefficients (and Hansch II alkane coefficients) from observed values could not be explained in terms of solute (or substituent) polarizability, dipole moment, sigma para, or pKHB values, further suggesting that specific interactions between the stationary phase and the solute are not significant. A molecular weight dependence that was independent of lipophilicity was observed. Thermodynamic and extra-thermodynamic parameters of retention were obtained in order to investigate retention mechanisms for the Act-I column. The molecular weight dependence does not appear to be due to size exclusion or entropic expulsion of the solute from the stationary phase. Hansch II substituent coefficients calculated from retention times were found to be similar for benzene and steroid derivatives. Thus, the Act-I column may be utilized as a rapid lipophilicity screen for drug candidates of similar molecular weight.

Solubility and complexation behavior of griseofulvin in fatty acid-isooctane mixtures

The influence of complex formation on the solubility behavior of griseofulvin in the straight-chain fatty acids was investigated by using phase solubility analysis in isooctane (2,2,4-trimethylpentane) at 25 degrees C. The apparent molar solubility of the proton acceptor griseofulvin ([A]t) was determined spectrophotometrically in the presence of various total molar concentrations ([D]t) of each of the proton donors (acetic, propanoic, butanoic, hexanoic, and octanoic acids). Increasing [D]t caused a pronounced increase in [A]t according to a biphasic log-log relationship, suggesting the formation of two complexes, ADm and ADn. The data are in close agreement with a simple mathematical model which assumes that two complexes, ADm and ADn, are formed and that [D]t approximately equal to 2[D2], where D2 refers to the fatty acid dimer. Linear regression analysis showed that the data best fit the complexation models with n = 5 or 6 and m = 0, 1, or 2, depending on the fatty acid. Assuming values of the dimerization constants of the fatty acids as reported in the literature, the stability constants of the complexes, Kn and Km, were calculated and found to decrease with increasing chain length of the fatty acids. The proposed model was critically appraised. An alternative model, which takes into full account the fatty acid monomer while assuming that only one complex is formed, leads to unacceptable conclusions.

Solubility behavior of phenolic compounds in hexane-ethyl acetate, hexane-ethyl myristate, and hexane-ethyl pivalate cosolvent systems

Interactions of phenolic compounds 4-hexylresorcinol and 3,4-dimethylphenol with esters were studied using hexane-ester cosolvent systems by both phase solubility and partitioning methods. The data obtained by the phase solubility method were variable and could not be analyzed by any mathematical model. The data obtained by the partitioning method, however, strongly suggest that 4-hexylresorcinol forms 1:1 and 1:2 complexes with the esters in hexane, while 3,4-dimethylphenol forms only 1:1 complexes with the same esters.

Some solute-solvent complexes involving testosterone and testosterone propionate

Solute-solvent interactions have been studied by observing the influences of carbon tetrachloride, carbon disulphide, ethyl oleate, isopropyl myristate and octanol on the solubilities of testosterone and testosterone propionate in an inert solvent (cyclohexane). Complexation with testosterone was detected, but there was no evidence of complexation between testosterone propionate and these solvents. The solvent-induced infrared shifts were due to non-specific solvent effects. The existence of solute-solvent complexes between the two solutes and chloroform, previously detected by infrared absorption, was confirmed. The solubility technique, which has previously only been used with solid complexing agents, is adaptable to liquid complexing agents, but has limitations.

Solubility of polar organic solutes in nonaqueous systems: role of specific interactions

The changes in solubility of several polar organic solutes when polar organic solvents are added to a relatively inert solvent such as isooctane were determined. The relative changes in solubility predicted from regular solution theory using solubility parameters often did not agree with the observed results. However, the solubilities could be rationalized mathematically by assuming the formation of specific solute-solvent complexes. Agreement of the thermodynamic data reported here with such models provides further evidence that specific interactions, when they occur, are more important than the bulk properties of the pure components in determining drug solubilities in nonaqueous systems. Specific examples demonstrate the relationship between the solubility and molecular structure of the solute and solvent. For example, solubility can be related to the hydrogen-donating and hydrogen-accepting abilities of the solute and solvent. Steric factors also appear to play a role in solubility, while structural modifications in a solute or solvent molecule far removed from the interactive functional group have little influence on molar solubility changes with the added polar cosolvent.

Improvements of dissolution characteristics and chemical stability of 16,16-dimethyl-trans-delta 2-prostaglandin E1 methyl ester by cyclodextrin complexation

Inclusion complexation of 16,16-dimethyl-trans-delta 2-prostaglandin E1 methyl ester (I), which is effective in early pregnancy termination, with cyclodextrins in water was ascertained by a solubility study. A solid complex of I-beta-cyclodextrin in a 1:2 molar ratio was obtained, and its dissolution behavior and chemical stability were examined. The results indicated that the complex may have great utility as a rapidly dissolving form of I with prolonged storage time.

Nucleophilic addition of bisulfite ion to prostaglandins E2 and A2: implication in aqueous stability

Evidence is presented to indicate that the bisulfite ion (HSO3-) adds across the C-9 carbonyl group of dinoprostone (prostaglandin E2) and across the delta 10,11- bond of prostaglandin A2. At room temperature, the apparent equilibrium constant, determined by phase solubility analysis, circular dichroism, UV spectroscopy, and partitioning, for the formation of the bisulfite adduct of dinoprostone is about 7.5 M-1 at neutral pH. From this result and a free energy relationship reported in the literature for the thermodynamics of nucleophilic addition to carbonyl groups, it is concluded that the chemical reactivity of the C-9 carbonyl group of dinoprostone is not high enough to improve aqueous stability through reversible one-step nucleophilic reactions. However, from a series of kinetic experiments, it is concluded that the equilibrium is extremely favorable for the formation of the bisulfite adduct of prostaglandin A2 over pH 4-8 at room temperature. The second-order rate constant for the attack of sulfite ion (SO3 2-) to prostaglandin A2 is 1.75 sec-1 M-1.

Determination of the thermodynamics of the methyl group in solutions of drug molecules

The contribution of the CH3 group to the solution thermodynamics of drug molecules is dependent on whether it is attached to a ring system or is in the terminal position in an aliphatic chain. In the former case group contributions for CH3 are very similar to those found for the CH2 group. For instance, in partition, the CH3 group contribution (log Fch3) is in the range 0.65 to 0.28 (ΔG = 2.303 RT log Fch3) and is dependent on the polarity of the organic solvent. The contribution for the terminal aliphatic CH3 is not equivalent to the mid chain CH2 and a CH3 correction factor or 1.14 to 1.34 kcalmol−1 (4.77 to 5.61 kJmol−1), has been calculated from alkane solubility data and partition studies. The additivity of group contributions and the correct choice of reference state are also discussed.