Forced Solid-State Oxidation Studies of Nifedipine-PVP Amorphous Solid Dispersion

In the present study, the oxidative degradation behavior of nifedipine (NIF) in amorphous solid dispersions (ASDs) prepared with poly(vinyl pyrrolidone) (PVP) with a short (K30) and a long (K90) chain length was investigated. The ASDs were prepared via dry ball-milling and analyzed using Fourier transform infrared (IR) spectroscopy, X-ray scattering, and differential scanning calorimetry. The ASDs were exposed to accelerated thermal-oxidative conditions using a pressurized oxygen headspace (120 °C for 1 day) and high temperatures at atmospheric pressure (60-120 °C for a period of 42 days). Additionally, solution-state oxidative degradation studies showed that pure NIF degrades to a greater extent than in the presence of PVP. Electronic structure calculations were performed to understand the impact of drug-polymer intermolecular interactions on the autoxidation of drugs. While no drug degradation was observed in freshly prepared ASD samples, alkyl free radicals were detected via electron paramagnetic resonance (EPR) spectroscopy. The free radicals were found to be consumed to a greater extent by PVP K30- than PVP K90-based ASDs upon exposure to high oxygen pressures. This was consistent with the greater solid-state oxidative degradation of NIF observed in ASDs with PVP K30 than with PVP K90. As no drug recrystallization occurred during this study period, the lower glass-transition temperature and presumed greater molecular mobility of PVP K30 and its ASD as compared to the PVP K90 system appear to contribute to the greater drug degradation in PVP-K30-based ASDs. The extent and the rate of oxidative degradation were higher in the case of PVP-K30-based ASD as compared to that in PVP-K90-based ASD, and the overall degradation increased with an increase in temperature. IR spectral analysis of drug-polymer interactions supports the electronic calculations of the oxidation process. We infer that, apart from the initial free radical content, the difference in the extent of drug-polymer intermolecular interactions in ASDs and amorphous stabilization during the forced oxidation experiments contribute to the observed differences in the autoxidative reactivity of the drug in ASDs with different PVP chain lengths. Overall, the chemical degradation of NIF in ASDs with two PVP chain lengths obtained from accelerated solid-state oxidation studies was in qualitative agreement with that obtained from long-term (3 years) storage under ambient conditions. The study highlights the ability of accelerated processes to determine the oxidative degradation behavior of polymeric ASDs and suggests that the polymer chain length could factor into chemical as well as physical stability considerations.

Determination and photodegradation of sertraline residues in aqueous environment

Sertraline is an antidepressant drug that has been frequently reported in the aquatic environment and biota. While the research has mostly dealt with its occurrence and toxicity, there is a lack of information pertaining to its environmental transformation. The present study aimed to fill in these gaps by giving an insight into mechanisms of sertraline phototransformation in surface waters, which was recognized as the main transformation pathway for this contaminant. We performed photodegradation experiments in presence of photosensitizers or reaction quenchers to determine rate constants and used them to predict sertraline phototransformation kinetics by "Aqueous Photochemistry of Environmentally occurring Xenobiotics" (APEX) software. It was established that sertraline degrades by pseudo-first order kinetics mostly dominated by direct photolysis, while the presence of certain reactive species including ^•OH, CO₃^-• and ³CDOM* further accelerate the compound's breakdown rate. To validate the predicted results, sertraline-spiked surface water was irradiated by sunlight, where the half-life of sertraline at around 1.4 days was estimated. While following the photodegradation kinetics, we also identified five transformation products, of which three were determined in Slovenian surface waters. According to the ECOSAR toxicity prediction, these transformation products will either have comparable or lower toxicity than their parent compound.

The use of microcalorimetry and HPLC for the determination of degradation kinetics and thermodynamic parameters of Perindopril Erbumine in aqueous solutions

Perindopril Erbumine (PER) is one of the newly used angiotensin-converting enzyme inhibitors (ACE inhibitors) and is used for the treatment of patients with hypertension and symptomatic heart failure. It has two main degradation pathways, i.e. the degradation by hydrolysis and the degradation by cyclization. An isothermal heat conduction microcalorimetry (MC) and high pressure liquid chromatography (HPLC) were used for the characterization of aqueous solutions of PER and its stability properties. The rates of heat evolved during degradation of perindopril were measured by MC as a function of temperature and pH and from these data rate constant and change in enthalpy of the reactions were determined. With the HPLC method the concentration of perindopril and its degradation products were measured as a function of time in aqueous solutions of different pH that were stored at different temperatures. We demonstrated that reactions of degradation of perindopril at observed conditions follow the first order kinetics. The Arrhenius equation for each pH was determined. At pH 6.8 only one degradation pathway is present, i.e. the degradation by hydrolysis. Degradation constants for this pathway calculated from MC data are in good agreement with those obtained from HPLC. MC as a non-specific technique was shown to be useful in studies of PER when one reaction was present in the sample and also when more chemical and physical processes were simultaneously running.

Use of microcalorimetry in determination of stability of enalapril maleate and enalapril maleate tablet formulations

The stability properties of enalapril maleate (EM) and of different tablet formulations including EM were studied by isothermal microcalorimetry and by high performance liquid chromatography (HPLC). It was shown that water content of the sample and elevated temperature have a high impact on stability properties of the substance itself and of the formulations including this substance. The degradation is more extensive at higher water content and at elevated temperature. The type of the tablet formulation (5 or 20mg EM tablet formulation) also has an impact: the 5 EM tablet formulation is the less stable one. The heat output of individual tablet formulations was used to evaluate the enthalpy changes and to calculate the difference in the amount of degraded EM between various samples. These results agreed satisfactorily with those obtained by HPLC. Isothermal microcalorimetry proved to be a fast and predictive method that could be used in preformulation studies to accelerate the pharmaceutical development and shorten the time before launching the product to the market.

Evaluation of the moisture sorption behaviour of several excipients by BET, GAB and microcalorimetric approaches

Moisture content of solid-state pharmaceutical products is one of the main factors that affect drug stability, therefore suitable sorption studies need to be performed to assure drug quality throughout their shelf life. The Brunauer-Emmett-Teller (BET) and the Guggenheim-Anderson-de Boer (GAB) models are usually used for this purpose. Using gravimetrically obtained data, both methods were applied in the present work to evaluate the sorption characteristics of several excipients. Microcalorimetric analysis was also performed in order to evaluate the interaction between water and the substances. The results of these experiments show excellent agreement between data and the BET model up to 55% RH and the GAB model over the entire humidity range, confirmed by high values of the statistical determination coefficients. Furthermore, microcalorimetric measurements suggested that the hygroscopicity of solid materials could be estimated approximately using this approach.

Determination of main low molecular weight antioxidants in urinary bladder wall using HPLC with electrochemical detector

The aim of the present work was to develop validated HPLC method using electrochemical detector for simultaneous detection of low molecular weight antioxidants (LMWA) in urinary bladder. Furthermore, the method was applied to study the distribution of LMWA in urinary bladder wall. The ascorbic acid (AA), glutathione in reduced (GSH) and oxidized (GSSG) form and uric acid (UA) were resolved by isocratic elution from C18 reversed-phase column. The bladder tissue sample preparation involved extraction with meta-phosphoric acid solution for LMWA stabilization. The AA, GSH and UA tissue peak was identified by different approaches. The obtained method validation parameters were in acceptable range: intra-day precision (<4.4%), intra-day accuracy (<8.4%), inter-day precision (<9.4%) and inter-day accuracy (<15.6%). Additionally, the method provided good linearity (r2>0.99) and recoveries (98.9-112.6%). The distribution of LMWA in urinary bladder was determined by measuring their concentration in bladder wall layers: urothelium, lamina propria, muscularis and serosa. The validated method was able to quantify the reduced form of all three LMWA in all four bladder wall layers. The LMWA concentrations were decreasing from urothelium to serosa except of UA. The developed HPLC method with electrochemical detection of LMWA is simple, fast and can be used for simultaneous quantification of LMWA in tissues, which contain low concentrations of antioxidants.

HPLC determination of tramadol in human breast milk

Tramadol is a centrally acting analgesic used for prevention and treatment of moderate to severe pain. It is estimated that 0.1% of the administered dose passes into breast milk causing potentially unwanted effects in nursing babies. Pharmacokinetically, breast milk is supposed to be a separate compartment into which the drug is excreted-mainly by passive diffusion. Due to a complex composition of breast milk, a suitable sample preparation procedure is needed with a subsequent chromatographic analysis for drug determination. Among several sample cleanup procedures tested we chose the liquid-liquid extraction procedure using n-hexane as an organic phase with back extraction into aqueous phase since it was considered the most suitable and the most compatible with the subsequent HPLC analysis. The precision and the reproducibility of the method were improved approximately two times by using metoprolol as an internal standard thus making the method also more robust with regard to a variable composition of milk samples. These characteristics, together with low detection limit and short analysis time, proved that the developed method is suitable for monitoring of tramadol in human breast milk.

Liquid chromatographic determination of diclofenac in human synovial fluid

A simple, rapid and sensitive HPLC method for the determination of diclofenac in synovial fluid is described. Special attention was paid to the procedure of sample preparation since gel formation may sometimes occur in synovial samples. With a one-step extraction procedure good and reproducible recovery of diclofenac was obtained. A subsequent HPLC assay was adjusted so as to achieve adequate sensitivity and precision needed for analysis of true samples. The results obtained by the described procedure proved the method to be suitable for monitoring concentrations of diclofenac in synovial fluid.