Effect of pH, ionic strength and oxygen burden on the chemical stability of EPC/cholesterol liposomes under accelerated conditions: Part 1: Lipid hydrolysis

The accelerated stability of purified egg phosphatidylcholine (EPC)/cholesterol liposomes was studied under various formulation conditions using a 2³ factorial experimental design. The three factors included in the study were pH, ionic strength of the buffer and the headspace oxygen content in the container. The results showed that lipid hydrolysis followed pseudo first-order kinetics. Data analysis using factorial design revealed that pH of the buffer was the predominant factor influencing the rate of lipid hydrolysis. Neither the ionic strength of the buffer, nor the presence of oxygen in the headspace of the container significantly affected the EPC hydrolysis. The hydrolysis rate of EPC at pH 4.0 buffer was at least 1.75 times greater than that at pH 4.8. A prediction based on the Arrhenius equation suggests that the EPC/cholesterol liposomes should be formulated in a buffer with pH equal to or greater than 4.2 in order to have a shelf-life longer than 1 year at 5°C.     

A systematic study on the chemical stability of the novel indoloquinone antitumour agent EO9

The chemical stability of the new anticancer drug EO9 in aqueous solution has been investigated utilizing a stability-indicating reversed-phase high-performance liquid Chromatographie assay with ultraviolet detection and ultraviolet spectrophotornetry. The degradation kinetics of EO9 have been studied as a function of pH, buffer composition, ionic strength and temperature. A pH-rate profile, using rate constants extrapolated to zero buffer concentration, was constructed demonstrating that EO9 is most stable in the pH region 8–9. The degradation mechanism of EO9 in aqueous solution is discussed.     

A systematic study on the chemical stability of ifosfamide

The degradation kinetics of ifosfamide in aqueous solution have been investigated over the pH region 1-13 at 70 degrees C. A stability indicating high-performance liquid chromatographic assay with UV detection was used to separate degradation products from the parent compound. The degradation kinetics were studied as related to pH, buffer composition, ionic strength, temperature and drug concentration. A pH-rate profile at 70 degrees C, obtained from (pseudo) first-order kinetic plots, was constructed after corrections for buffer effects were made. The degradation reactions of ifosfamide were found to be largely independent of pH, although proton or hydroxyl catalysis occurs at extreme pH values. Ifosfamide shows maximum stability in the pH region 4-9, corresponding to a half-life of 20 h.     

Solution stability of ciclosidomine

The stability of N-cyclohexanecarbonyl-3-(4-morpholino)-sydnone imine hydrochloride (ciclosidomine) in solution was studied as a function of pH, temperature, ionic strength, and buffer species. The rate of hydrolysis in the absence of light was found to be apparent first order in drug and general acid- and base-catalyzed reactions. The pH rate profile at an ionic strength of 0.1 M at 60 degrees C had a minimum value near pH 6. Change in ionic strength in the range of 0.05 to 0.2 M did not affect the rate of degradation at pH 7 (carbonate buffer) or pH 2 (phosphate buffer) at 60 degrees C. Similar degradation rates were noticed in air or nitrogen in the dark at pH 3, 5, and 6. However, degradation in light was very rapid in either case at pH 3, 5, and 6, and, therefore, the protection of solutions from light was required during all studies. The time for 10% loss of drug in solution at pH 6 in dilute phosphate or citrate buffer at an ionic strength of 0.154 M was projected to be 9 months at 20 degrees C and 2.6 months at 30 degrees C.     

The effect of drug concentration on the thermal (dark) degradation of promethazine hydrochloride in aqueous solution

The thermal (dark) degradation of promethazine hydrochloride in aqueous solution presents a complex kinetic picture. The process is oxygen dependent and is modified by EDTA. In citrate buffer, pH 4·0, ionic strength 0·5m , containing 0·1 % EDTA, the thermal degradation at 90° can be fitted to first order rate plots at drug concentrations up to 1·56 × 10^?2m (0·5%) and to zero order rate plots at drug concentrations greater than 9·35 × 10^?2m (3·0%). At intermediate concentrations no simple equation can describe the data. These effects have been correlated with the formation of drug micelles and the rate data have been interpreted on the basis of a first order monomer process and a half order micellar process occurring simultaneously.     

Heroin: Stability and formulation approaches

A rapid, selective and sensitive high-performance liquid Chromatographic stability indicating assay for heroin and its hydrolysis products was applied to study the stability and kinetics of heroin hydrolysis at its optimal pH in the presence of various pharmaceutical excipients. Heroin hydrolysis involved a two-step sequential mechanism. The pseudo-first-order rate constants have been determined. Buffers catalyzed heroin degradation, while sodium chloride, mannitol or povidone (40,000) had no effect. The physical and chemical stabilities of lyophilized dosage forms containing heroin alone or with excipients: buffer (optimal pH), mannitol, lactose and dextrose were also studied at various temperatures. Phosphate buffer enhanced heroin degradation, while heroin or heroin plus lactose provided stable formulations even at elevated temperatures.     

Hydrolysis of Partially Saturated Egg Phosphatidylcholine in Aqueous Liposome Dispersions and the Effect of Cholesterol Incorporation on Hydrolysis Kinetics

Abstract— Hydrolysis kinetics of partially hydrogenated egg phosphatidylcholine (PHEPC) were studied as a function of pH, temperature, buffer concentration, ionic strength, and the effect of cholesterol incorporation. Results showed that PHEPC has a maximum stability at around pH 6·5. General acid base catalysis was observed for acetate, HEPES and Tris buffers. Increasing the ionic strength of the buffer solutions did not influence the hydrolysis kinetics. The relationship between the observed hydrolysis rate constants and the temperature could adequately be described by the Arrhenius equation. Incorporation of cholesterol did not affect the hydrolysis kinetics. This result indicates that the hydrolysis kinetics of PHEPC do not depend on the changes in bilayer rigidity induced by cholesterol incorporation. Cholesterol is stable under the experimental conditions used in this study; no changes were observed in cholesterol concentration over the experimental time interval.     

The effect of drug concentration on the thermal (dark) degradation of promethazin hydrochloride in aqueous solution.

The thermal (dark) degradation of promethazine hydrochloride in aqueous solution presents a complex kinetic picture. The process is oxygen dependent and is modified by EDTA. In citrate buffer, pH 4.0, ionic strength 0.5M, containing 0.1% EDTA, the thermal degradation at 90 degrees can be fitted to first order rate plots at drug concentrations up to 1.56 x 10.27 (0.5%) and to zero order rate plots at drug concentrations greater than 9.35 x 10.2M (3.0%). At intermediate concentrations no simple equation can describe the data. These effects have been correlated with the formation of drug micelles and the rate date have been interpreted on the basis of a first order monomer process and a half order micellar process occurring simultaneously.     

A Systematic Degradation Kinetics Study of Dalbavancin Hydrochloride Injection Solutions

The degradation kinetics of the glycopeptide antibiotic dalbavancin in solution are systematically evaluated over the pH range 1–12 at 70°C. The decomposition rate of dalbavancin was measured as a function of pH, buffer composition, temperature, ionic strength, and drug concentration. A pH-rate profile was constructed using pseudo first-order kinetics at 70°C after correcting for buffer effects; the observed pH-rate profile could be fitted with standard pseudo first order rate laws. The degradation reactions of dalbavancin were found to be strongly dependent on pH and were catalyzed by protons or hydroxyl groups at extreme pH values. Dalbavancin shows maximum stability in the pH region 4–5. Based on the Arrhenius equation, dalbavancin solution at pH 4.5 is predicted to have a maximum stability of thirteen years under refrigerated conditions, eight months at room temperature and one month at 40°C. Mannosyl Aglycone (MAG), the major thermal and acid degradation product, and DB-R6, an additional acid degradation product, were formed in dalbavancin solutions at 70°C due to hydrolytic cleavage at the anomeric carbons of the sugars. Through deamination and hydrolytic cleavage of dalbavancin, a small amount of DB-Iso-DP2 (RRT-1.22) degradation product was also formed under thermal stress at 70°C. A greater amount of the base degradation product DB-R2 forms under basic conditions at 70°C due to epimerization of the alpha carbon of phenylglycine residue 3.     

Alkylation of DNA by 1,3-dialkyl-3-acyltriazenes: correlation of biological activity with chemical behavior.

The reactions of calf thymus DNA with four 1,3-dialkyl-3-acyltriazenes were studied alone or in the presence of pig liver esterase in pH 7.4 phosphate buffer for varying lengths of time. The best alkylating agent in the absence of esterase was determined to be 1,3-dimethyl-3-carbethoxytriazene (DMC), followed in order by 1-(2-hydroxyethyl)-3-methyl-3-carbethoxytriazene (HMC), 1-(2-hydroxyethyl)-3-methyl-3-acetyltriazene (HMA), and 1-(2- chloroethyl)-3-methyl-3-carbethoxytriazene (CMC). This order is the same as that for the rate of decomposition of the various acyltriazenes in pH 7.5 phosphate buffer. The extent of calf thymus DNA alkylation by CMC was found to be dependent on both the reaction buffer and the ionic strength of the medium. Alkylation by CMC alone in low ionic strength glycine buffer produced large quantities of 7-(2-chloroethyl)guanine and 7-(2-hydroxyethyl)guanine. The products of DNA alkylation observed at neutral pH are consistent with N(2)-N(3) heterolysis of the triazene, resulting in the N(1) alkyldiazonium ion as the sole alkylating species. In the presence of esterase, CMC showed an enhanced rate of product formation. Furthermore, the product distribution shifted dramatically from mainly hydroxyethylation to predominantly methylation. CMC is postulated to undergo initial enzymatic deacylation, leading to two different alkyldiazonium ions which competitively alkylate DNA. HMC, on the other hand, was little affected by the esterase. The enzyme-catalyzed reaction showed a small increase in methylation and a smaller decrease in hydroxyethylation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetics of degradation in aqueous solution of Abbott-79175, a potent second generation 5-lipoxygenase inhibitor

The degradation kinetics of Abbott-79175 in aqueous solution have been studied as a function of pH. Concentration/time plots indicated a pseudo-first order nature of reactions throughout the pH range studied. Additionally, the effects of temperature, ionic strength, and buffer concentration have been examined. From multiple temperature experiments, Arrhenius and activation parameters were calculated. Furthermore, it was determined that upon ionization, Abbott-79175 degradation proceeded independently of ionic strength. These data in addition to the plateau-like nature of the pH-rate constant profile above pH 10 suggest a lack of participation of hydroxide ion during the reaction. This behavior in the neutral and alkaline regions was qualitatively very similar to that of zileuton, a 5-lipoxygenase inhibitor in phase III clinical trials. In addition to allowing the determination of the buffer independent rate constants, kinetic studies as a function of buffer concentration allowed in some of the systems the deduction of which buffer species were catalytic. A multi-parameter model was fitted to the pH buffer independent rate constant data using non-linear regression. This modeling yielded parameters such as the microscopic rate constants and the pK_a under the aforementioned conditions. From the pH-rate constant profile, Abbott-79175 was found to be more labile than zileuton throughout the pH range studied. This difference was greater than three orders of magnitude at pH 1. Such acid lability produced a pH profile which had a much narrower region of maximum stability.     

Degradation kinetics and mechanisms of a new cephalosporin, cefixime, in aqueous solution

Hydrolysis of cefixime in buffer solutions (pH 1-9) at 25 degrees C and a constant ionic strength of 0.3 was investigated using ion-pair reversed-phase HPLC. Hydrolysis rates followed pseudo first-order kinetics; the rate of hydrolysis of cefixime was very slow at pH 4-7, slightly faster at lower pH, and quite rapid at higher pH. In the early stages of hydrolysis, six major degradation products were isolated and identified: a beta-lactam ring-opened product and a 7-epimer (basic conditions), three lactones derived from intramolecular cyclization between the 2-carboxyl and 3-vinyl groups (acidic conditions), and an aldehyde derivative involving a 7-acyl moiety (neutral conditions). Principal degradation pathways for cefixime were found to involve initial cleavage of the beta-lactam ring.     

Effects of pH and phosphate on the oxidation of iron in aqueous solution

Studies have been initiated to evaluate the catalytic effect of monohydrogen phosphate ions on the oxidation of ferrous (Fe²⁺) to ferric (Fe³⁺) ions in an aqueous solution under atmospheric oxygen conditions. The reactions were performed with an initial concentration of 1 × 10^?4 M ferrous sulfate in solutions containing varying concentrations of phosphate buffer (0.005–0.0175 M) over the pH range of 6.6–7.1. The final ionic strength of the solutions were adjusted to 0.1 M with sodium chloride and the temperature was kept constant at 25 ± 0.5 °C. The rates of oxidation reactions were measured by following the increase in UV absorbance due to the formation of ferric ion in solution. The reactions appeared to follow pseudo-first-order kinetics and were very prone to catalysis by monohydrogen phosphate at any given pH. H₂PO₄^? seemed to have no effect on the reaction. HPO_s^2? was the sole catalytic species with a second-order rate constant of 116.74 M^?1 · min^?1. The buffer independent pH-rate profile showed a sigmoidal behavior with the pseudo-first-order rate constant increasing with increasing pH. The sigmoidal nature of the experimental pH-rate profile could possibly suggest a change in the reactivity of the oxidizing species which might follow complex kinetics. The effects of ionic strength and temperature on the reaction rates were also evaluated.     

Mechanism of hydrolytic degradation of poly(L-lactide) microcapsules: effects of pH, ionic strength and buffer concentration

The hydrolytic degradation rate of poly(L-lactide) molecules constituting the microcapsule membrane was estimated at different pH, ionic strength and buffer concentration. Poly(L-lactide) microcapsules were observed to be hydrolytically degraded rapidly in a strongly alkaline solution to lactic acid as the final product. The degradation was accelerated when the poly(L-lactide) microcapsules were immersed in solutions of high ionic strength. The effect of pH and ionic strength of the bulk solution is interpreted in terms of the electric potential distribution in the membrane. It is suggested that the concentration of OH- in the membrane has an important role in the hydrolysis of poly(L-lactide) microcapsules, when the microcapsules are dispersed in solutions where the zeta potential of the microcapsules is negative. On the other hand, when the zeta potential is positive, the concentration of H+ in the membrane has a predominant effect on the degradation. The degradation was also found to be affected by the salt concentration in buffered solutions, suggesting that the cleavage reaction of the polymer ester bonds is accelerated by conversion of the acidic degradation products into neutral salts.     

Modulation of melphalan uptake in murine L5178Y lymphoblasts in vitro by changes in ionic environment.

The alkylating agent melphalan is actively transported in mammalian cells by two amino acid transport carriers: the sodium-dependent carrier with substrate preference for alanine-serine-cysteine (system ASC), and a sodium-independent carrier with preference for leucine (system L). The effect of altering the ionic environment of murine L5178Y lymphoblasts was investigated in order to determine not only the direct effects of hydrogen and calcium ions on these transport systems, but also the indirect effects of agents or modulators known to alter intracellular calcium. Melphalan transport followed a bell-shaped distribution curve over a pH range from 3 to 9 with a pH optimum of 4.3 and 4.6 for transport by systems ASC and L, respectively. Those agents that could cause a decrease in cytosolic calcium such as the calcium channel blockers verapamil, diltiazem and nitrendipine, the calcium chelator (ethyleneglycol-bis-(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) and reduction of pH were found to augment melphalan uptake, whereas conditions that would elevate intracellular calcium such as the calcium ionophore A23187, the calcium channel agonist (-) Bay K 8644, elevation of extracellular calcium and the calcium pump inhibitor trifluoperazine were all found to decrease melphalan uptake. These findings suggest that modification of ionic environment directly or indirectly by agents known to alter intracellular calcium can modulate melphalan uptake.     

Preformulation study of epigallocatechin gallate, a promising antioxidant for topical skin cancer prevention.

Epigallocatechin gallate (EGCG) is a potent polyphenolic antioxidant extracted from green tea. Due to its antimutagenic and antitumor activities, it is a promising candidate for use in topical formulations for skin cancer prevention. The overall goal of this study was therefore to determine the influence of several factors on the stability of EGCG in solution to obtain information that would facilitate the subsequent development of topical formulations. Our first objective was to determine the influence of pH, temperature, and ionic strength on the aqueous stability of EGCG. A second objective was to determine the stability of EGCG in various solvents in the presence and absence of different antioxidants. A simple and rapid stability indicating high-performance liquid chromatography assay for EGCG was developed. Stability studies were performed in 0.05 M aqueous buffers at pH 3, 5, 7, and 9 at 4, 25, and 50 degrees C. The effect of ionic strength on EGCG stability was evaluated in 0.05 M acetate buffer, pH 5, adjusted to the desired ionic strength with sodium chloride. An accelerated stability study of EGCG was performed at 50 degrees C in the organic solvents glycerin and Transcutol P in the presence of antioxidants. The degradation of EGCG increased rapidly as temperature and solution pH were increased. Ionic strength increases also caused an accelerated degradation. The solution stability of EGCG was prolonged in glycerin and Transcutol P compared with an aqueous environment. The addition of 0.1% concentrations of several antioxidants in combination with 0.025% EDTA caused variable effects on EGCG stability. Butylated hydroxytoluene in glycerin produced the greatest stability improvement for EGCG. The t(90) (time for 10% degradation to occur) was 76.1 days at 50 degrees C. It can be concluded that glycerin-based vehicles are suitable for stabilizing EGCG.     

Effect of self-association on rate of penicillin G degradation in concentrated aqueous solutions.

The apparent rate of degradation of penicillin G potassium micellar solutions of 500,000 units/ml, a concentration commonly encountered in vials reconstituted for storage in the refrigerator, was investigated and compared to that of nonmicellar solutions of 8000 units/ml at 25 degrees, ionic strength of 1.1 M, and pH range from 5.0 to 9.5. In the micellar solutions the apparent rate of the H+-catalyzed degradation was increased twofold but that of water- and OH minus-catalyzed hydrolysis was decreased two- to three-fold. Consequently, the pH-rate profile of the micellar solutions was shifted to higher pH values and the pH of minimum degradation was found to be at 7.0 compared to 6.5 for the nonmicellar solution of the same ionic strength. Compared at their respective pH-rate profile minima, micellar penicillin G is 2.5 times as stable as the nonmicellar solution under the conditions of constant pH and ionic strength.     

Stability Studies of Gabapentin in Aqueous Solutions

Gabapentin is a -aminobutyric acid analogue, which has been shown to be an effective antiepileptic. The solution stability of gabapentin in buffered systems was studied in order to facilitate the formulation of a liquid product. The degradation of the drug was followed as a function of pH, buffer concentration, ionic strength, and temperature. The results indicated that the rate of degradation was proportional to the buffer concentration and temperature. The pH–rate profile of gabapentin degradation showed that the rate of degradation was minimum at an approximate pH of 6.0. Further, the data suggested a slower solvent-catalyzed degradation rate for the zwitterionic species compared to the cationic or anionic species in the pH range of 4.5 to 7.0. There was no influence of ionic strength on the rate of degradation. Arrhenius plots of the data indicated that a shelf life of 2 years or more at room temperature may be obtained in an aqueous solution at a pH value of 6.0.     

Stability of batanopride hydrochloride in aqueous solutions.

The degradation of batanopride hydrochloride, an investigational antiemetic drug, was studied in aqueous buffer solutions (pH 2-10; ionic strength, 0.5; 56 degrees C) in an attempt to improve drug stability for parenteral administration. Degradation occurs by two different mechanisms depending on the pH of the solution. In acidic media (pH 2-6), the predominant reaction was intramolecular cyclization followed by dehydration to form a 2,3-dimethylbenzofuran. There was no kinetic or analytical (high-performance liquid chromatography) evidence for the formation of an intermediate; therefore, the rate of dehydration must have been very rapid compared with the rate of cyclization. In alkaline media (pH 8-10), the primary route of degradation was cleavage of the C-O alkyl ether bond. In the intermediate pH range (pH 6-8), both reactions contributed to the overall degradation. Both degradation reactions followed apparent first-order kinetics. The pH-rate profile suggests that batanopride hydrochloride attains its optimal stability at pH 4.5-5.5. Citrate buffer was catalytic at pH 3 and 5, and phosphate buffer was catalytic at pH 8. No catalytic effect was observed for the borate buffer at pH 9-10.     

HPLC法测定阿奇霉素在水溶液中的降解动力学

目的研究阿奇霉素在水溶液中的降解动力学,为阿奇霉素液体制剂的开发提供参考。方法通过经典恒温试验,应用HPLC法测定阿奇霉素在不同pH值、不同温度、不同离子强度、不同缓冲液条件下的降解动力学参数。结果阿奇霉素在水溶液中的降解呈现一级动力学特征,其最稳定pH值(pHm)为6.41;随着离子强度和温度的增加,阿奇霉素的降解加快;阿奇霉素在磷酸盐缓冲液中比在醋酸盐、枸橼酸盐缓冲液中相对稳定。结论阿奇霉素降解速率与溶液pH值、缓冲液种类、离子强度以及温度有关;溶液pH值与温度对阿奇霉素降解作用的影响较为明显。