The role of aggregation and ionization in the chemical instability of Amphotericin B in aqueous methanol.

Amphotericin B (AmB) is a potent antimicrobial agent used in clinical practice. Nevertheless, the mechanism of its aqueous instability remains not yet fully understood, especially the role that its aggregation state plays in this process. Therefore, the current study used an aqueous methanol media to evaluate the AmB instability as a function of pH-, organic solvent- and concentration-dependent ionization and aggregation. To reach this goal, the aggregation status and instability were determined using UV-vis spectroscopy, LC-MS and HPLC. Moreover, not only the hydrolytic degradation products were identified by UV-vis spectroscopy and LC-MS, but also, the degradation rate constants were estimated by nonlinear regression. The results indicated that monomeric AmB was the predominant species under pH conditions, wherein the substrate was cationic (pH < 4) or anionic (pH > 9). On the other hand, aggregated AmB form was the predominant species for the zwitterionic substrate (at methanol concentration < 30 %(v/v)). Anionic substrate degraded by specific base-catalyzed lactone hydrolysis. Oxidation accounted for the loss of zwitterionic substrate. Aggregated zwitterionic AmB exhibited lower stability than monomeric zwitterionic AmB under neutral pH conditions. These studies are a step forward in comprehending the degradation kinetics of AmB in aqueous medium. In fact, along with our previous research on AmB instability in oils, it leads to a better understanding of the AmB stability in complex systems with an oil-water interface, such as disperse lipid systems.

Unveiling the Amphotericin B Degradation Pathway and Its Kinetics in Lipid-Based Solutions.

The purpose of this work was to determine the degradation pathway of Amphotericin B (AmB) and its kinetics in lipid-based solutions. Mixtures of AmB in lipophilic solvent media were stored under different conditions, such as surface area, temperature, light exposure, presence of antioxidants and other co-solutes. AmB was quantified by HPLC and UV-Vis spectrometry. Empirical models were proposed, and degradation rate constants were estimated by nonlinear regression. The HPLC method was precise and accurate with linearity from 4.45 to 52.0 nM. Surface area studies revealed that adsorption to glass did not affect AmB loss. Unsaturated oils and methanol better preserved AmB compared to medium chain-triglyceride. Temperature increased AmB loss in a nonlinear behavior and the presence of antioxidants reduced its degradation. Under dark conditions, autoxidation was the predominant degradation pathway of AmB in oil, which undergoes a complex degradation. Under light exposure, photo-oxidation accounted for AmB loss, which appeared to be of pseudo-first order. AmB oily samples should be preferably stored in glass vials protected from light with the addition of antioxidants. Furthermore, this work encourages further investigation in other media for future complex modeling and estimation of AmB degradation and kinetics in lipid-based formulations.