Glass formation in barbiturates and solid dispersion systems of barbiturates with citric acid

Triazine-based molecular glasses frustrate the crystallization of barbiturates

Hydrogen bonding is a key element in crystal engineering to direct crystal packing. Here, hydrogen bonding with molecular glasses is rather exploited to thwart crystallization.

Pharmaceutical Dispersion Techniques for Dissolution and Bioavailability Enhancement of Poorly Water-Soluble Drugs

Over the past decades, a large number of drugs as well as drug candidates with poor dissolution characteristics have been witnessed, which invokes great interest in enabling formulation of these active ingredients. Poorly water-soluble drugs, especially biopharmaceutical classification system (BCS) II ones, are preferably designed as oral dosage forms if the dissolution limit can be broken through. Minimizing a drug’s size is an effective means to increase its dissolution and hence the bioavailability, which can be achieved by specialized dispersion techniques. This article reviews the most commonly used dispersion techniques for pharmaceutical processing that can practically enhance the dissolution and bioavailability of poorly water-soluble drugs. Major interests focus on solid dispersion, lipid-based dispersion (nanoencapsulation), and liquisolid dispersion (drug solubilized in a non-volatile solvent and dispersed in suitable solid excipients for tableting or capsulizing), covering the formulation development, preparative technique and potential applications for oral drug delivery. Otherwise, some other techniques that can increase the dispersibility of a drug such as co-precipitation, concomitant crystallization and inclusion complexation are also discussed. Various dispersion techniques provide a productive platform for addressing the formulation challenge of poorly water-soluble drugs. Solid dispersion and liquisolid dispersion are most likely to be successful in developing oral dosage forms. Lipid-based dispersion represents a promising approach to surmounting the bioavailability of low-permeable drugs, though the technique needs to traverse the obstacle from liquid to solid transformation. Novel dispersion techniques are highly encouraged to develop for formulation of poorly water-soluble drugs.

Studies on dissolution enhancement and mathematical modeling of drug release of a poorly water-soluble drug using water-soluble carriers

Role of various water-soluble carriers was studied for dissolution enhancement of a poorly soluble model drug, rofecoxib, using solid dispersion approach. Diverse carriers viz. polyethylene glycols (PEG 4000 and 6000), polyglycolized fatty acid ester (Gelucire 44/14), polyvinylpyrollidone K25 (PVP), poloxamers (Lutrol F127 and F68), polyols (mannitol, sorbitol), organic acid (citric acid) and hydrotropes (urea, nicotinamide) were investigated for the purpose. Phase-solubility studies revealed AL type of curves for each carrier, indicating linear increase in drug solubility with carrier concentration. The sign and magnitude of the thermodynamic parameter, Gibbs free energy of transfer, indicated spontaneity of solubilization process. All the solid dispersions showed dissolution improvement vis-à-vis pure drug to varying degrees, with citric acid, PVP and poloxamers as the most promising carriers. Mathematical modeling of in vitro dissolution data indicated the best fitting with Korsemeyer-Peppas model and the drug release kinetics primarily as Fickian diffusion. Solid state characterization of the drug-poloxamer binary system using XRD, FTIR, DSC and SEM techniques revealed distinct loss of drug crystallinity in the formulation, ostensibly accounting for enhancement in dissolution rate.

Stability prediction of amorphous benzodiazepines by calculation of the mean relaxation time constant using the Williams-Watts decay function

The enthalpic relaxation of three amorphous benzodiazepines, diazepam, temazepam and triazolam was studied using differential scanning calorimetry for ageing temperatures which were below the glass transition temperature, and ageing times up to 16 h. Experimental determination of the relaxation enthalpy and the heat capacity change, both accompanying the glass transition, enabled us to calculate the extent of relaxation of the amorphous drugs at specific ageing conditions. Fitting of the relaxation function to the Williams-Watts two parameter decay function led to calculation of the mean relaxation time constant tau and the molecular relaxation time distribution parameter beta. The mean relaxation time constants for the three drugs increased from approximately ten h at the glass transition temperature with more than eight orders of magnitude at 66 K below the glass transition temperature. It was found that the benzodiazepines exhibited significant molecular mobility until approximately 50 K below the glass transition temperature; below this temperature molecular mobility becomes unimportant with respect to the shelf life stability. Hence the presented procedure provides the formulation scientist with a tool to set storage conditions for amorphous drugs and glassy pharmaceutical products.

Amorphous spray-dried hydroflumethiazide-polyvinylpyrrolidone systems: physiochemical properties

Hydroflumethiazide was spray-dried with polyvinylpyrrolidone (PVP) to produce products containing 0-30% PVP. These systems were amorphous and differed from previously prepared coprecipitates of similar composition. Differential scanning calorimetry (DSC) suggested that at low PVP weight fractions both amorphous drug and an amorphous drug-PVP complex can be present in spray-dried systems. The apparent solubility of hydroflumethiazide in spray-dried products increased with increasing PVP content reaching a plateau value approximately four times that of the pure crystalline drug. The estimated free energy and entropy of the spray-dried drug were greater than that of crystalline drug and also increased with increasing PVP content. Dissolution studies with compressed discs supported the apparent solubility data. The results suggest that amorphous phases having different orders of organization are formed in spray-dried systems with increasing PVP content.