A kinetic method for measuring functional delivery of amphotericin B by drug delivery systems

Amphotericin B release rate is the link between drug status in the liposomal bilayer and toxicity

Amphotericin B (AmB) is an amphiphilic drug commonly formulated in liposomes and administered intravenously to treat systemic fungal infections. Recent studies on the liposomal drug product have shed light on the AmB aggregation status in the bilayer, which heat treatment (curing) modifies. Although toxicity was found related to aggregation status - loose aggregates significantly more toxic than tight aggregates - the precise mechanism linking aggregation and toxicity was not well understood. This study directly measured drug release rate from various AmB liposomal preparations made with modified curing protocols to evaluate correlations among drug aggregation state, drug release, and in vitro toxicity. UV–Vis spectroscopy of these products detected unique curing-induced changes in the UV spectral features: a ∼25 nm blue-shift of the main absorption peak (λ_max) in aqueous buffer and a decrease in the OD₃₄₆/OD₃₂₂ ratio upon thermal curing, reflecting tighter aggregation. In vitro release testing (IVRT) data showed, by applying and fitting first-order release kinetic models for one or two pools, that curing impacts two significant changes: a 3–5-fold drop in the overall drug release rate and a ten-fold decrease in the ratio between the loosely aggregated and the tightly aggregated, more thermodynamically stable drug pool. The kinetic data thus corroborated the trend independently deduced from the UV–Vis spectral data. The in vitro toxicity assay indicated a decreased toxicity with curing, as shown by the significantly increased concentration, causing half-maximal potassium release (TC₅₀). The data suggest that the release of AmB requires dissociation of the tight complexes within the bilayer and that the reduced toxicity relates to this slower rate of dissociation. This study demonstrates the relationship between AmB aggregation status within the lipid bilayer and drug release (directly measured rate constants), providing a mechanistic link between aggregation status and in vitro toxicity in the liposomal formulations.

Osmotic stress sensitizes sterol-free phospholipid bilayers to the action of Amphotericin B

We have tested the ability of Amphotericin B to form ion channels/defects in osmotically stressed large unilamellar vesicles (LUV) using pyranine fluorescence detected ion/H+ exchange. We found that sterol-free LUV exhibit greatly increased sensitivity to AmB channel formation in the soluble oligomer state (> 0.5 microM) under modestly hypoosmotic conditions (< 100 delta mosM). These vesicles are completely insensitive under isoosmotic conditions. The related antibiotics, Amphotericin B methyl ester and Nystatin showed almost no activity under hypoosmotic conditions in the absence of sterol. This difference may be attributable to differences in solution oligomeric states. Experiments with KCl and CaCl2 internal buffers demonstrate that these sterol-free AmB membrane disruptions are highly selective for monovalent cations (K+) over anions (Cl-), ruling out massive lysis or unselective membrane defects caused by osmotic pressure. Thus, AmB seems to be acting as a 'molecular harpoon', an expression coined to describe substances which can selectively target osmotically stressed, strained or highly curved membranes. These results may provide a rationale for AmB's reported anti-HIV activity and reported activity against sterol-free small unilamellar vesicles (highly curved membranes) as well as the reduced activity of liposomal drug delivery systems toward cholesterol-containing and sterol-free membranes (fewer soluble oligomers).

Temperature effects on the aggregation state and activity of amphotericin B

Amphotericin B (AmB) in aqueous solution becomes less aggregated as temperature is increased as measured by CD spectroscopy. Concomitantly, AmB's ability to induce K+ leakage from cholesterol-containing large unilamellar vesicles (LUV) decreases 4-fold per 10 degrees C increase. In contrast, ergosterol-containing vesicles show no clear trend in their temperature sensitivity to AmB action. These data strongly support a model of AmB action where drug aggregates and not monomers are the major channel-inducing species against cholesterol- but not ergosterol-containing membranes.