Biopharmaceutic Profiling of Salts to Improve Absorption of Poorly Soluble Basic Drugs

Improving Oral Absorption of a Rapidly Crystallizing Parent Drug Using Prodrug Strategy: Comparison of Phosphate Versus Glycine Based Prodrugs

With an increasing number of Biopharmaceutical Classification System (BCS) II/IV pipeline compounds, solubilizing and supersaturating formulation strategies are becoming prevalent. Beyond formulation and solid form strategies, prodrugs are also employed to overcome solubility-limited absorption of poorly water-soluble compounds. Prodrugs can potentially yield supersaturated systems upon conversion to the parent drug intraluminally and thus enhance absorption. However, supersaturation also increases the driving force for crystallization, resulting in low solution concentrations, which can potentially negate the advantage of prodrugs. In this work, two unique solubility-enhancing prodrugs, phosphate and glycine esters, were investigated for a rapidly crystallizing parent drug. Ex vivo absorption studies using rat tissue and in vivo studies in dogs were performed. Conversion rate of the phosphate prodrug to the parent was dependent on the milieu and increased ∼24-fold in the presence of intestinal contents as medium and tissue relative to neat buffer. In contrast, conversion of the glycine prodrug was minimal under any conditions tested, suggesting that the conversion occurs after absorption into the enterocytes. Phosphate prodrug showed a non-linear increase in parent drug absorptive flux across rat intestinal tissue with concentration when intestinal contents were used as donor media. This was attributed to rapid conversion and high supersaturation of the parent drug which subsequently resulted in crystallization at high doses in the donor chamber. Glycine prodrug did not undergo complete conversion at high doses and was absorbed unchanged on the basolateral side, indicating saturation of the converting enzymes in the enterocytes. The combined flux (parent drug and glycine) showed a linear increase with dose and crystallization was not observed. Under physiological conditions, glycine prodrug that is absorbed unchanged from the intestine can potentially undergo complete conversion in hepatocytes after absorption and make the parent drug systemically available. Thus, glycine prodrug provided overall higher absorption compared to phosphate prodrug. The observed flux levels for both the prodrugs were higher compared to the parent drug alone, highlighting an advantage to use of a prodrug strategy to improve absorption of such compounds. Oral dosing in a dog PK study revealed that the bioavailability using the phosphate prodrug was ∼50% whereas, it was ∼100% with glycine prodrug, supporting the in vitro observations.

Supersaturated formulations of poorly soluble weak acid drugs evaluated in rodents; a case study

In the present study we describe a way of working to overcome oral administration challenges in an early preclinical project. As candidate drugs were obtained, the preclinical delivery route was replaced by the intended route of the product and resources were allocated to optimize the oral absorption. Two main approaches were followed in order to formulate a selected weak acid, AZ'403, for oral administration in large scale toxicological studies and the early clinical phases. Both approaches relies on the suppression of precipitation from obtained supersaturated solutions achieved either by amorphous solid dispersions (using hydroxypropyl methylcellulose acetate succinate, HPMC-AS) or crystalline salts (sodium and potassium salts). In vivo studies in rodents were performed to evaluate oral AZ'403 absorption from amorphous and crystalline formulations, using nano- and micro crystalline particles of the neutral form, as references. The oral absorption of AZ'403 formulated using both approaches was significantly higher compared with the references. The improvements in overall exposures were 7-100 times during the investigated conditions. The pharmacokinetic profiles implied that both solid dispersions and crystalline salts of AZ'403 generated supersaturation in the small intestine in rodents and indicated that both approaches may be ways forward for subsequent late stage product development.

Fast-Dissolving Solid Dispersions for the Controlled Release of Poorly Watersoluble Drugs

Solid dispersions offer many advantages for oral drug delivery of poorly water-soluble drugs over other systems, including an increase in drug solubility and drug dissolution. An improvement in drug absorption and the higher bioavailability of active pharmaceutical ingredients in the gastrointestinal tract have been reported in various studies. In certain circumstances, a rapid pharmacological effect is required for patients. Fastdissolving solid dispersions provide an ideal formulation in such cases. This report will provide an overview of current studies on fast-dissolving solid dispersions, including not only solid dispersion powders with fast dissolution rates but also specific dose form for the controlled release of poorly water-soluble drugs. Specifically, the applications of fast-dissolving solid dispersions will be described in every specific case. Moreover, pharmaceutical approaches and the utilization of polymers will be summarized. The classification and analysis of fastdissolving solid dispersions could provide insight into strategies and potential applications in future drug delivery developments.

Preformulation investigation and challenges; salt formation, salt disproportionation and hepatic recirculation

A compound, which is a selective peroxisome proliferator activated receptor (PPAR) agonist, was investigated. The aim of the presented studies was to evaluate the potential of the further development of the compound. Fundamental physicochemical properties and stability of the compound were characterized in solution by liquid chromatography and NMR and in solid-state by various techniques. The drug itself is a lipophilic acid with tendency to form aggregates in solution. The neutral form was only obtained in amorphous form with a glass-transition temperature of approximately 0°C. The intrinsic solubility at room temperature was determined to 0.03mg/mL. Chemical stability studies of the compound in aqueous solutions showed good stability for at least two weeks at room temperature, except at pH1, where a slight degradation was already observed after one day. The chemical stability in the amorphous solid-state was investigated during a period of three months. At 25°C/60% relative humidity (RH) and 40°C/75% RH no significant degradation was observed. At 80°C, however, some degradation was observed after four weeks and approximately 3% after three months. In an accelerated photostability study, degradation of approximately 4% was observed. Attempts to identify a crystalline form of the neutral compound were unsuccessful, however, salt formation with tert-butylamine, resulted in crystalline material. Results from stability tests of the presented crystalline salt form indicated improved chemical stability at conditions whereas the amorphous neutral form degraded. However, the salt form of the drug dissociated under certain conditions. The drug was administered both per oral and intravenously, as amorphous nanoparticles, to conscious dogs. Plasma profiles showed curves with secondary absorption peaks, indicating hepatic recirculation following both administration routes. A similar behavior was observed in rats after oral administration of a pH-adjusted solution. The observed double peaks in plasma exposure and the dissociation tendency of the salt form, were properties that contributed to make further development of the candidate drug challenging. Options for development of solid dosage forms of both amorphous and crystalline material of the compound are discussed.