The effect of aggregation state on the degradation kinetics of Amphotericin B in aqueous solution

Amphotericin B (AmB) is an antifungal agent that has a propensity to self-aggregate. It is insoluble in most of the solvents and unstable in the presence of heat, pH extremes and oxygen. The objective of this project is to study the effect of substrate aggregation on the degradation kinetics of aqueous AmB. The presence of AmB aggregates and monomer were determined from UV spectra in accordance with the methods reported in the literature. Monomeric form of AmB was the predominant form in acidic and alkaline pH region and in solution containing methanol higher than 35% whereas aggregated AmB was predominant in neutral pH region. HPLC, UV and LCMS methods were developed for quantification and identification of substrate and degradation products. Degradation studies were conducted in the pH range 2 – 12 using several buffers, HCl and NaOH solutions. The kinetics of substrate loss suggested AmB was susceptible to autoxidation reaction in the pH range 2 – 9 whereas first-order substrate loss was observed in very alkaline conditions (pH > 9). An unstable intermediate and the ring-opened product of lactone hydrolysis were observed in the pH range 9 – 12. The kinetic model was successfully used to describe substrate loss over the entire pH range. The results from methanol effect study suggested that aggregated AmB was more susceptible to oxidative degradation than monomeric AmB.