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

Degradation of L-Ascorbic Acid in the Amorphous Solid State

Ascorbic acid degradation in amorphous solid dispersions was compared to its degradation in the crystalline state. Physical blends and lyophiles of ascorbic acid and polymers (pectins and polyvinylpyrrolidone [PVP]) were prepared initially at 50:50 (w/w), with further studies using the polymer that best inhibited ascorbic acid crystallization in the lyophiles in 14 vitamin : PVP ratios. Samples were stored in controlled environments (25 to 60 °C, 0% to 23% RH) for 1 mo and analyzed periodically to track the physical appearance, change in moisture content, physical state (powder x-ray diffraction and polarized light microscopy), and vitamin loss (high performance liquid chromatography) over time. The glass transition temperatures of select samples were determined using differential scanning calorimetry, and moisture sorption profiles were generated. Ascorbic acid in the amorphous form, even in the glassy amorphous state, was more labile than in the crystalline form in some formulations at the highest storage temperature. Lyophiles stored at 25 and 40 °C and those in which ascorbic acid had crystallized at 60 °C (≥70% ascorbic acid : PVP) had no significant difference in vitamin loss (P > 0.05) relative to physical blend controls, and the length of storage had little effect. At 60 °C, amorphous ascorbic acid lyophiles (≤60% ascorbic acid : PVP) lost significantly more vitamin (P < 0.05) relative to physical blend controls after 1 wk, and vitamin loss significantly increased over time. In these lyophiles, vitamin degradation also significantly increased (P < 0.05) at lower proportions of ascorbic acid, a scenario likely encountered in foods wherein vitamins are naturally present or added at low concentrations and production practices may promote amorphization of the vitamin.

PRACTICAL APPLICATION

Vitamin C is one of the most unstable vitamins in foods. This study documents that amorphous ascorbic acid is less stable than crystalline ascorbic acid in some environments (for example, higher temperatures within 1 wk), especially when the vitamin is present at low concentrations in a product. These findings increase the understanding of how material science properties influence the stability of vitamin C.

Tris(hydroxymethyl) aminomethane salt of ramipril: synthesis, structural characterization from X-ray powder diffraction and stability studies

Tris(hydroxymethyl) aminomethane (tris) salt of API ramipril was synthesized, and characterized by FTIR, TG-DSC and ab initio X-ray powder structure analysis. The compound, ramipril-tris (II), crystallizes in the monoclinic space group P2(1) with a=24.3341(15), b=6.4645(5), c=9.5357(7) Å, β=96.917(3)° and V=1489.1(3) Å(3). The crystal structure has been determined from laboratory X-ray powder diffraction data using direct space global optimization strategy (simulated annealing) followed by the Rietveld refinement. A network of intermolecular OH…O, CH…N and CH…O hydrogen bonds between the ramipril-ramipril, tris-tris and ramipril-tris components in the compound generates a two-dimensional molecular assembly in (110) plane. A comparative study of solid-state stabilities of ramipril-tris (II) with that of ramipril (I) and ramipril-erbumine (III) indicates that ramipril-tris (II) is the most stable one among the three, and the conversion to impurity D after 72 h at 80 °C is only 1.5%. The solution phase analysis at different pH values also reveals a greater stability of ramipril-tris (II) over ramipril (I).

Determination of solid-state acidity of chitin-metal silicates and their effect on the degradation of cephalosporin antibiotics

It was of interest to determine the solid-state acidity of chitin-metal silicate coprocessed excipients and to correlate this acidity to the chemical stability of cefotaxime sodium in the presence of the aforementioned excipients. The solid-state acidities of chitin aluminum silicate, chitin magnesium silicate, and chitin calcium silicate were determined by reflectance spectroscopy using structurally different dye molecules. The chemical stability of cefotaxime sodium was assessed at 50 °C in a 4% (w/v) slurry system in the pH range 6.6-10.5 and in the solid-state in the Hammett acidity range 6.1-7.8. The solid-state acidity was found to be reproducible because one or more structurally different dye molecules gave reliable solid-state acidity values. A significant discrepancy in pH stability profile of cefotaxime sodium between the solid-state and the slurry system was observed. Furthermore, chitin aluminum silicate showed minimum drug stability in the solid-state, close to where the maximum drug stability in the slurry was observed. This unexpected effect might be ascribed to the catalytic properties of chitin aluminum silicate. The slurry method was not able to predict efficiently the solid-state surface acidity and stability of cefotaxime sodium. Moreover, the solid-state chemical stability might be influenced by factors other than the solid-state acidity.

Stabilization of quinapril by incorporating hydrogen bonding interactions

In the present study stability of various known solvates of quinapril hydrochloride has been compared with nitromethane solvate. Nitromethane solvate was found to be more stable compared to other known solvates. Single crystal X-ray diffraction analysis of quinapril nitromethane solvate shows intermolecular hydrogen bonding between quinapril molecule and nitromethane. Stabilization of quinapril by forming strong hydrogen bonding network as in case of co-crystals was further studied by forming co-crystal with tris(hydroxymethyl)amino methane. Quinapril free base forms a stable salt with tris(hydroxymethyl)amino methane not reported earlier. Quinapril tris(hydroxymethyl)amino methane salt found to be stable even at 80° for 72 h i.e. hardly any formation of diketopiperazine and diacid impurity. As expected single crystal X-ray diffraction analysis reveals tris(hydroxymethyl)amino methane salt of quinapril shows complex hydrogen bonding network between the two entities along with ionic bond. The properties of this stable salt - stable in solid as well as solution phase, might lead to an alternate highly stable formulation.

Influence of solid-state acidity on the decomposition of sucrose in amorphous systems. I

It was of interest to develop a method for solid-state acidity measurements using pH indicators and to correlate this method to the degradation rate of sucrose. Amorphous samples containing lactose 100mg/ml, sucrose 10mg/ml, citrate buffer (1-50mM) and sodium chloride (to adjust the ionic strength) were prepared by freeze-drying. The lyophiles were characterized using powder X-ray diffraction, differential scanning calorimetry and Karl Fischer titremetry. The solid-state acidity of all lyophiles was measured using diffuse reflectance spectroscopy and suitable indicators (thymol blue or bromophenol blue). The prepared lyophiles were subjected to a temperature of 60 degrees C and were analyzed for degradation using the Trinder kit. The results obtained from this study have shown that the solid-state acidity depends mainly on the molar ratio of the salt and the acid used in buffer preparation and not on the initial pH of the solution. The degradation of sucrose in the lyophiles is extremely sensitive to the solid-state acidity and the ionic strength. Reasonable correlation was obtained between the Hammett acidity function and sucrose degradation rate. The use of cosolvents (in the calibration plots) can provide good correlations with the rate of an acid-catalyzed reaction, sucrose inversion, in amorphous lyophiles.

Correlations between molecular mobility and chemical stability during storage of amorphous pharmaceuticals

Recent studies have demonstrated that molecular mobility is an important factor affecting the chemical stability of amorphous pharmaceuticals, including small-molecular-weight drugs, peptides and proteins. However, quantitative correlations between molecular mobility and chemical stability have not yet been elucidated. The purpose of this article is to review literature describing the effect of molecular mobility on chemical stability during storage of amorphous pharmaceuticals, and to seek a better understanding of the relative significance of molecular mobility and other factors for chemical reactivity. We first consider the feature of chemical stability often observed for amorphous pharmaceuticals; changes in temperature dependence of chemical stability around matrix glass transition temperature (Tg), and greater stability associated with higher Tg. Secondly, we review papers which quantitatively studied the effects of the global mobility (often referred to as structural relaxation or -relaxation) of amorphous pharmaceuticals on chemical stability, and discuss correlations between chemical stability and global mobility using various equations that have thus far been proposed. Thirdly, the significance of local mobility of drug and excipient molecules in chemical reactivity is discussed in comparison with that of global mobility. Furthermore, we review literature reports which show no relationship between chemical stability and molecular mobility. The lack of apparent relationship is discussed in terms of the effects of the contribution of excipient molecules as reactants, the specific effects of water molecules, the heterogeneity of the matrix, and so on. The following summary has been obtained; the chemical stability of amorphous pharmaceuticals is affected by global mobility and/or local mobility, depending on the length scale of molecular mobility responsible for the chemical reactivity. In some cases, when activation energy for degradation processes is high and when other factors such as the specific effects of water and/or excipients contribute the degradation rate, stability seems to be largely independent of molecular mobility.

Acid–Base Characteristics of Bromophenol Blue–Citrate Buffer Systems in the Amorphous State

In this study, we have examined the acid-base characteristics of various citrate buffer systems alone and in the presence of the pH indicator dye, bromophenol blue, in aqueous solution, and after lyophilization to produce amorphous material. Fourier transform Raman and solid-state nuclear magnetic resonance spectroscopy have been used to monitor the ratio of ionized to un-ionized citric acid under various conditions, as a function of initial pH in the range of 2.65-4.28. Ultraviolet-visible spectrophotometry was used to probe the extent of proton transfer of bromophenol blue in the citrate buffer systems in solution and the amorphous state. Spectroscopic studies indicated greater ionization of citric acid and bromophenol blue in solution and the solid state with increasing initial solution pH, as expected. Fourier transform Raman measurements indicated the same ratio of ionized to un-ionized citrate species in solution, frozen solution, and the amorphous state. It is shown that the ratio of species at any particular initial pH is primarily determined by the amount of sodium ion present so as to maintain electroneutrality and not necessarily to the fact that pH and pK(a) remain unchanged during freezing and freeze drying. Indeed, for bromophenol blue, the relative ultraviolet-visible intensities for ionized and un-ionized species in the amorphous sample were different from those in solution indicating that the extent of protonation of bromophenol blue was significantly lower in the solid samples. It is concluded that under certain conditions there can be significant differences in the apparent hydrogen activity of molecules in amorphous systems.