Impact of pH modulation and NaDC micellular behavior on Amphotericin B solubility

Purpose:

Amphotericin B (AmB) has very poor water solubility (< 1 ug/mL), resulting poor oral absorption. Thus, AmB is only commercially produced in intravenous (IV) formulations. One AmB IV formulation, Fungizone® (FZ), uses sodium deoxycholate (NaDC), a surfactant, and sodium phosphate, a pH-modifying agent, to dissolve AmB. As per instructions, the powder is dissolved in water and then diluted lower than NaDC critical micelles concentration with 5% dextrose. To understand why a low concentration NaDC could dissolve AmB, we investigated the mechanism of AmB dissolution in FZ preparation.

Methods:

AmB solubility in aqueous buffer solutions:Aqueous pH buffers at pH 1.2, 4.5, 6.8, 7.8, and 11 were prepared. AmB powder was mixed with pH buffers at room temperature (RT) or 37C for 24 h and processed for AmB concentration measurement via HPLC.

Impact of pH on NaDC micelle formation:NaDC was mixed into pH buffer solutions at 4 mg/mL concentration and separately dissolved into pH 11 buffer at 0.08, 0.2, 0.4, 4, 10, and 20 mg/mL concentrations. The solutions were measured by Dynamic Light Scattering (DLS) to evaluate micelle formation.

NaDC micelle formation in the presence of AmB at pH 11:Micelle formation in 4 mg/mL NaDC-Buffer solution was measured by DLS. Next, NaDC was mixed in pH 11 buffer, DI water, and 5% dextrose solution at 0.08, 0.2, 0.4, 4, 10, and 20 mg/mL concentrations, and measured via DLS. Additionally, AmB was mixed into NaDC media solutions at 0.1 and 5 mg/ml and measured via DLS.

Enhancement of NaDC and pH 11 on AmB solubility:NaDC was dissolved in pH buffer at 4, 10, and 20 mg/mL. AmB powder then was added into the NaDC solution and mixed at RT. At 1, 12, and 24 h, samples were collected prepared for HPLC analysis.

Fungizone® Preparation and Characterization:Lab FZ and commercial FZ solutions were prepared. The lab FZ solution was prepared by mixing ingredients at RT until the solution was visibly clear. Commercial FZ powder was reconstituted with DI water. FZ solutions were diluted 1:50 with filtered 5% dextrose. Following dilution, the commercial FZ was characterized via pH measurements, DLS analysis, and HPLC measurements.

Results:

AmB is not stable at extreme pH with heat; therefore, solubility studies were performed at RT. AmB (pKa 5.5 and 10) had the highest solubility at pH 11 (216 ug/mL). NaDC has different micellular behavior in different pH conditions, generating lower CMCs at pH 7.8 and 11. Further, pH 11 was able to increase aqueous AmB solubility in the presence of NaDC.

Conclusion:

Both pH 11 and NaDC can increase AmB aqueous solubility; however, pH 11 combined with NaDC can synergistically enhance AmB solubility significantly. Additionally, pH 11 facilitated stable AmB-NaDC micelles formation below NaDC critical micelle concentration in water. Thus, the initial reconstitution of FZ in water generated a high pH facilitated AmB dissolution via AmB-NaDC micelles formation. Once the micelles formed, they were stable for further dilution with 5% dextrose below the CMC of NaDC.