Dynamic, Nonsink Method for the Simultaneous Determination of Drug Permeability and Binding Coefficients in Liposomes

Abstract

Drug release from liposomal formulations is governed by a complex interplay of kinetic (i.e., drug permeability) and thermodynamic factors (i.e., drug partitioning to the bilayer surface). Release studies under sink conditions that attempt to mimic physiological conditions are insufficient to decipher these separate contributions. The present study explores release studies performed under nonsink conditions coupled with appropriate mathematical models to describe both the release kinetics and the conditions in which equilibrium is established. Liposomal release profiles for a model anticancer agent, topotecan, under nonsink conditions provided values for both the first-order rate constant for drug release and the bilayer/water partition coefficient. These findings were validated by conducting release studies under sink conditions via dynamic dialysis at the same temperature and buffer pH. A nearly identical rate constant for drug release could be obtained from dynamic dialysis data when appropriate volume corrections were applied and a mechanism-based mathematical model was employed to account for lipid bilayer binding and dialysis membrane transport. The usefulness of the nonsink method combined with mathematical modeling was further explored by demonstrating the effects of topotecan dimerization and bilayer surface charge potential on the bilayer/water partition coefficient at varying suspension concentrations of lipid and drug.

Mechanistic model and analysis of doxorubicin release from liposomal formulations

“…A compartmental model capable of accurately describing DXR uptake kinetics into the dialysis membrane was developed and implemented in the evaluation of DXR liposomal release kinetics. However, simulations of liposomal DXR release using a single DXR rate constant for dialysis membrane transport (not shown) as used in previous work [11,19,27] showed very minor differences in DXR release profiles. These minor differences would have a negligible effect on the overall release kinetics for the relatively slow release formulations examined here.…”

“…[17] DXR release rates were measured using dynamic dialysis in a manner similar to previous studies with other liposomal formulations. [11,15,18,19,26] Briefly, aliquots of the commercial liposomal suspensions resembling DOXIL  were diluted (8, 16, or 24-fold) with buffer (50 mM phosphate, 170 mM NaCl) containing NH 4 Cl (0, 50, or 100 mM) and adjusted to either pH 7.0, 7.4, or 7.8. The osmolality of these buffers was measured with a Fiske® One-Ten TM Osmometer (Advanced Instruments, Inc., MA).…”

“…[11,15,16,19] Drug transport kinetics across the dialysis membrane are likely to vary with the drug of interest in the study, either by having an increased or decreased rate of transport across the membrane, or because of processes associated with the equilibria and kinetics of drug binding to the dialysis membrane. The present study considered a mechanism in line with the latter scenario to describe DXR dialysis membrane transport.…”

Release, Partitioning, and Conjugation Stability of Doxorubicin in Polymer Micelles Determined by Mechanistic Modeling

“…A secondary set of drug release experiments employed a previously developed ultrafiltration method to monitor drug release under non-sink conditions (30). Micelle suspensions (3 mL, 0.5 mg/mL) were transferred to scintillation vials that were placed in a 37°C incubator and gently shaken.…”

“…This possible instability was explored and later confirmed experimentally. Additionally, a non-sink release method previously developed to simultaneously determine release kinetics and drug partitioning in liposomal formulations (30) was employed to determine the extent of free Dox partitioning in the HYD micelle formulations and further validate the mechanistic model. Parameter values generated from simultaneous fitting of experimental data to the model served to assist in identifying the factors governing the pH-sensitive release of Dox from these micelle formulations.…”