Determination of equilibrium constant of amino carbamate adduct formation in sisomicin by a high pH based high performance liquid chromatography.

Amino carbamate adduct formation from the amino group of an aminoglycoside and carbon dioxide has been postulated as a mechanism for reducing nephrotoxicity in the aminoglycoside class compounds. In this study, sisomicin was used as a model compound for amino carbamate analysis. A high pH based reversed-phase high performance liquid chromatography (RP-HPLC) method is used to separate the amino carbamate from sisomicin. The carbamate is stable as the breakdown is inhibited at high pH and any reactive carbon dioxide is removed as the carbonate. The amino carbamate was quantified and the molar fraction of amine as the carbamate of sisomicin was obtained from the HPLC peak areas. The equilibrium constant of carbamate formation, Kc, was determined to be 3.3 × 10(-6) and it was used to predict the fraction of carbamate over the pH range in a typical biological systems. Based on these results, the fraction of amino carbamate at physiological pH values is less than 13%, and the postulated mechanism for nephrotoxicity protection is not valid. The same methodology is applicable for other aminoglycosides.

Effects of a citrate buffer system on the solid-state chemical stability of lyophilized quinapril preparations.

PURPOSE: The objective of this study was to examine the effect of a citric acid-citrate buffer system on the chemical instability of lyophilized amorphous samples of quinapril hydrochloride (QHCI). METHODS: Molecular dispersions of QHCI and citric acid were prepared by colyophilization from their corresponding aqueous solutions with a molar ratio of QHCI to citric acid from 1:1 to 6:1 and solution pH from 2.49 to 3.05. Solid samples were subjected to a temperature of 80 degrees C and were analyzed for degradation using high-performance liquid chromatography. The glass transition temperature, Tg, of all samples was measured by differential scanning calorimetry. RESULTS: Samples were first examined by varying the Tg and maintaining the initial solution pH constant. At pH 2.49 the rate of reaction was found to be less dependent on the sample Tg, whereas at pH > or = 2.75 the rate decreased with an increase in Tg. In a second set of experiments at a constant Tg of approximately 70 degrees C, the reaction rate increased as the pH increased. CONCLUSION: The overall solid-state chemical reactivity of amorphous quinapril depends on the relative amount of QHCI and Q+-, the zwitterionic form of quinapril. At high proportions of Q+- (higher pH values) the reaction rate seems to be strongly influenced by the Tg of the mixture, and hence the molecular mobility, whereas at higher proportions of QHCI (lower pH) the reaction rate is less sensitive to Tg, presumably because of different mechanistic rate determining steps for the two sets of conditions.

The solid-state stability of amorphous quinapril in the presence of beta-cyclodextrins.

The major objective of this study was to investigate the effects of beta-cyclodextrin (beta-CD) and hydroxypropyl-beta-cyclodextrin (HP-beta-CD) on the solid-state chemical reactivity of the drug, quinapril, when amorphous samples are prepared by colyophilization of quinapril and each of these beta-CDs. For comparison, a physical mixture with beta-CD and colyophilized mixtures with trehalose and dextran were also prepared and subjected to a similar chemical stability test at 80 degrees C followed by HPLC analysis. Significant inhibition of degradation was observed only for colyophilized miscible mixtures with beta-CD and HP-beta-CD at molar ratios in excess of 1:1. Colyophilized mixtures with trehalose and dextran, shown to have phase separated, and the physical mixture with beta-CD exhibited no inhibiting effects. This suggests that specific molecular complexation is responsible for the significant inhibition by the beta-CDs. The tendency of quinapril to form molecular complexes in solution with the beta-CDs was measured by (1)H solution NMR, by estimating complexation constants from the chemical shift of specific groups on quinapril. Supporting evidence for solid-state complexation was provided by FTIR analysis. DSC and TSC measurements indicated that the beta-CDs do not have high enough glass transition temperatures to reduce reactivity by reducing molecular mobility.