pH control of nucleophilic/electrophilic oxidation.

Finding formulations that prevent degradation of the active pharmaceutical ingredient is an essential part of drug development. One of the major mechanisms of degradation is oxidation. Oxidative degradation is complex, and can occur via different mechanisms, such as autoxidation, nucleophilic/electrophilic addition, and electron transfer reactions. This paper uses three model compounds and determines the mechanisms of oxidation and strategies to reduce degradation. The mechanism of oxidation was established by comparing the results of different forced degradation experiments (radical initiation and peroxide addition), computational chemistry to those of formulated drug product stability. The model compounds chosen contained both oxidizable amine and sulfide functional groups. Although, both oxidative forced degradation conditions showed different impurity profiles the peroxide results mirrored those of the actual stability results of the drug product. The major degradation pathway of all compounds tested was nucleophilic/electrophilic oxidation of the amine to form N-oxide. Strategies to prevent this oxidation were explored by performing forced degradation experiments of the active pharmaceutical ingredient (API) in solution, in slurries containing standard excipient mixtures, and in solid formulation blends prepared by wet granulation. The reaction was significantly influenced by pH in solvent and excipient slurries, with 100% degradation occurring at basic pH values (>pH 8) and no degradation occurring at pH 2 upon exposure to 0.3% peroxide. Wet granulated blends were also stabilized by lowering the pH during granulation through the addition of citric acid prior to the solution of peroxide, resulting in little (0.02% maximum) or no degradation for the four different blends after 6 week storage at 40 degrees C/75%RH.

The development and stability assessment of extemporaneous pediatric formulations of Accupril.

Quinapril, the active ingredient in Accupril tablets, is an ACE inhibitor used to treat hypertension. Quinapril is unstable in aqueous solution and therefore the development of a liquid formulation is a significant challenge. Previous studies show the rate of degradation of quinapril into its two major degradants to be pH dependent, indicating the parent compound to be most stable in the narrow pH range of 5.5-6.5. Accupril (20 mg) and readily available pharmaceutical components were combined to generate three formulations that are stable for at least 28 days, possess acceptable appearance, and are palatable to pediatric patients. To combat the presence of magnesium carbonate in the Accupril tablets, which increase the pH of the solution above 6.5, several pharmaceutically available buffers were incorporated. Nine prototypes were developed and their characteristics evaluated after 1 week under stressed conditions. The three that most closely matched the stability criteria were chosen for a definitive stability study. A stability-indicating method was developed and validated for these studies. All three formulations met the following specifications when stored at 5 degrees C for 6 weeks; Quinapril remained >or=90% intact and the two known degradants did not reach values >or=3.0% individually or >or=5.0% combined.

Improving the detection of degradants and impurities in pharmaceutical drug products by applying mass spectral and chromatographic searching.

Liquid chromatography/mass spectrometry (LC/MS) and NMR are commonly used to identify metabolites, impurities and degradation products in the pharmaceutical industry. To more efficiently deal with the large volumes of data these techniques generate, software programs have been developed by various vendors to assist in the identification of these compounds through the use of spectral and chromatographic search algorithms. The feasibility of using such programs for detecting drug degradants and impurities is assessed. A number of compounds encompassing a wide range of both chemical and pharmaceutical properties were tested using LC/UV/MS and the spectral/chromatographic search algorithm MetaboLynx (Micromass UK Ltd.) to determine the feasibility of detecting analytes at low concentrations. In addition, drug product and stressed drug substance samples containing quinapril hydrochloride, the active ingredient in Accupril tablets, were determined by liquid chromatography with atmospheric pressure ionization-time-of-flight (API LC-TOF) and an API LC-quadrupole (Q) mass spectrometer, and the resulting data was processed using MetaboLynx. The ability of this program to detect and list a variety of analytes known to be present in the samples was evaluated. The combination of LC/UV, LC/MS and spectral/chromatographic searching is a valuable tool for the detection of impurities at low levels.

Early prediction of pharmaceutical oxidation pathways by computational chemistry and forced degradation.

PURPOSE: To show, using a model study, how electronic structure theory can be applied in combination with LC/UV/MS/MS for the prediction and identification of oxidative degradants. METHODS: The benzyloxazole 1, was used to represent an active pharmaceutical ingredient for oxidative forced degradation studies. Bond dissociation energies (BDEs) calculated at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level with isodesmic corrections were used to predict sites of autoxidation. In addition, frontier molecular orbital (FMO) theory at the Hartree-Fock level was used to predict sites of peroxide oxidation and electron transfer. Compound 1 was then subjected to autoxidation and H2O2 forced degradation as well as formal stability conditions. Samples were analyzed by LC/UV/MS/MS and degradation products proposed. RESULTS: The computational BDEs and FMO analysis of 1 was consistent with the LC/UV/MS/MS data and allowed for structural proposals, which were confirmed by LC/MS/NMR. The autoxidation conditions yielded a number of degradants not observed under peroxide degradation while formal stability conditions gave both peroxide and autoxidation degradants. CONCLUSIONS: Electronic structure methods were successfully applied in combination with LC/UV/MS/MS to predict degradation pathways and assist in spectral identification. The degradation and excipient stability studies highlight the importance of including both peroxide and autoxidation conditions in forced degradation studies.