In Vitro Characterization of the Erythrocyte Distribution of Methazolamide: A Model of Erythrocyte Transport and Binding Kinetics

The rate and extent of binding of methazolamide to human erythrocytes was studied in vitro. All experiments were carried out at physiological temperature (37C) and pH (7.4). Methazolamide (MTZ) buffer concentrations were analyzed by HPLC. Distributional equilibrium between buffer and washed red blood cells was achieved after 1 hr. Results of equilibrium studies were consistent with two classes of binding sites for MTZ within the erythrocyte: a low affinity, high capacity site (CA-I) and a high affinity, low capacity site (CA-II). A two-binding site model was fitted to experimental data generating estimates for binding parameters Ka₁(0.0017 ± 0.00022 M^–1) nM₁(636 ± 5.23 M), Ka₂(0.46 ± 0.0083 M^–1), and nM₂(80.9 ± 0.389 M). Based upon these findings, kinetic studies were performed in order to characterize the rate of drug distribution. The rate of erythrocyte uptake of MTZ was mathematically modeled using a series of differential equations describing drug diffusion across the red blood cell membrane and subsequent complexation with intracellular binding sites. The model assumed that penetration of MTZ into the red blood cells was passive but drug binding to the carbonic anhydrase isozymes was not instantaneous. Using a novel curve fitting technique, parameter estimates of RBC membrane permeability (0.0102 ± 0.000618 cm/min), and binding rate constants k_–1(0.254 ± 0.0213 min^–1), k₁(0.0022 ± 0.00020 ml/g-min), k_–2(1.59 ± 0.0358 min^–1), and k₂(3.1 ± 0.035 ml/g-min) were obtained. The model characterized the observed biphasic decline of MTZ buffer concentrations over time and may help explain the prolonged residence of MTZ in vivo.