Dissolution of Anecortave Acetate in a Cylindrical Flow Cell: Re‐Evaluation of Convective Diffusion/Drug Dissolution for Sparingly Soluble Drugs

Determination of Inherent Dissolution Performance of Drug Substances

The dissolution behavior of novel active pharmaceutical ingredients (API) is a crucial parameter in drug formulation since it frequently affects the drug release. Generally, a distinction is made between surface-reaction- and diffusion-controlled drug release. Therefore, dissolution studies such as the intrinsic dissolution test defined in the pharmacopeia have been performed for many years. In order to overcome the disadvantages of the common intrinsic dissolution test, a new experimental setup was developed within this study. Specifically, a flow channel was designed and tested for measuring the mass transfer from a flat, solid surface dissolving into a fluid flowing over the surface with well-defined flow conditions. A mathematical model was developed that distinguishes between surface-reaction- and diffusion-limited drug release based on experimental data. Three different drugs-benzocaine, theophylline and griseofulvin-were used to investigate the mass flux during dissolution due to surface reaction, diffusion and convection kinetics. This new technique shows potential to be a valuable tool for the identification of formulation strategies.

Influence of drug solubility and lipophilicity on transscleral retinal delivery of six corticosteroids

The influence of drug properties including solubility, lipophilicity, tissue partition coefficients, and in vitro transscleral permeability on ex vivo and in vivo transscleral delivery from corticosteroid suspensions was determined. Solubility, tissue/buffer partition coefficients for bovine sclera and choroid-retinal pigment epithelium (CRPE), and in vitro bovine sclera and sclera-choroid-retinal pigment epithelium (SCRPE) transscleral transport were determined at pH 7.4 for triamcinolone, prednisolone, dexamethasone, fluocinolone acetonide, triamcinolone acetonide, and budesonide in solution. Ex vivo and in vivo transscleral delivery was assessed in Brown Norway rats after posterior subconjunctival injection of a 1 mg/ml suspension of each corticosteroid. Corticosteroid solubility and partition coefficients ranged from ∼ 17 to 300 μg/ml and 3.0 to 11.4 for sclera and from 7.1 to 35.8 for CRPE, respectively, with the more lipophilic molecules partitioning more into both tissues. Transport across sclera and SCRPE was in the range of 3.9 to 10.7% and 0.3 to 1.8%, respectively, with the transport declining with an increase in lipophilicity. Ex vivo and in vivo transscleral delivery indicated tissue distribution in the order CRPE ≥ sclera > retina > vitreous. Tissue partitioning showed a positive correlation with drug lipophilicity (R(2) = 0.66-0.96). Ex vivo and in vivo sclera, CRPE, retina, and vitreous tissue levels of all corticosteroids showed strong positive correlation with drug solubility (R(2) = 0.91-1.0) but not lipophilicity (R(2) = 0.24-0.41) or tissue partitioning (R(2) = 0.24-0.46) when delivered as suspensions. In vivo delivery was lower in all eye tissues assessed than ex vivo delivery, with the in vivo/ex vivo ratios being the lowest in the vitreous (0.085-0.212). Upon exposure to corticosteroid suspensions ex vivo or in vivo, transscleral intraocular tissue distribution was primarily driven by the drug solubility.

Adaptation of USP Types II and IV Controlled Release Assays for Sparingly Soluble Compounds by Direct Eluent HPLC Analysis

Measurement of drug release of a sparingly soluble drug by conventional methods proceeds very slowly without the aid of surfactants. Two preliminary automated methods were developed that increase sensitivity and accelerate such studies by working at very small reservoir volumes. All high pressure liquid chromatography (HPLC) equipment components were commercially available. Results are presented for the drug release of a single pellet of the sparingly soluble drug Eliprodil in two types of drug release experiments using (1) a stirred cell and (2) a flow-through method. Release rates measured from each method were comparable.

Impact of Density Gradients on Flow-Through Dissolution in a Cylindrical Flow Cell

Measuring release rates for sparingly soluble compounds requires slow flow rates to achieve measurable eluent levels. However, flow rate cannot be reduced indefinitely because density gradients could measurably alter the drug release rate. Finite element simulations of convective diffusion/dissolution with an applied concentration-dependent density gradient reproduced the trends of the changes in dissolution rate on eluent flow rate which occur upon switching flow direction and sample cell orientation relative to gravitational field as observed in the literature employing a rectangular flow cell. The situation was experimentally reproduced using an implant obstruction placed centrally in a cylindrical flow cell.

Transscleral drug delivery to the posterior eye: prospects of pharmacokinetic modeling

Basic biological research has provided new approaches to treat severe diseases of the retina and choroid, such as age related macular degeneration. Although it is possible to deliver drugs from a subconjunctival drug depot to the retina and choroid, the barriers and kinetics of this route of drug administration are not well known. In this review we investigate the pharmacokinetic aspects of transscleral drug delivery into the posterior eye with emphasis on pharmacokinetic modeling. The existing simulation models related to the transscleral drug delivery are reviewed and future directions for the model development are discussed. In addition, a new simulation model for the transscleral drug delivery based on permeability data is introduced. This compartmental model contains several ocular tissues (sclera, choroid, retinal pigment epithelium and vitreous) and it takes into account the clearance of the drug via choroidal circulation. The model is used to simulate the vitreous delivery of macromolecules based on the available data on FITC-dextran 70 kDa.

Reexamination of Convective Diffusion/Drug Dissolution in a Laminar Flow Channel: Accurate Prediction of Dissolution Rate

PURPOSE

The convective diffusion/dissolution theory applied to flowthrough dissolution in a laminar channel was reexamined to evaluate how closely it can predict release rate for a model compound on an absolute basis--a comparison that was lacking from the original literature observations reported from this technique.

METHODS

The theory was extended to allow for a finite flux of dissolving material, replacing the fixed concentration by a flux condition on the dissolving surface. The derivation introduces a new parameter, k(s), an area-independent analog of the dissolution rate constant defined in the USP intrinsic dissolution procedure.

RESULTS

The release rate for ethyl-p-aminobenzoate originally observed fell within 10% of the absolute prediction assuming a solubility limited situation, and deviated from this prediction in a manner possibly consistent with a finite flux-limited condition, with k(s) approximately 10(-4) M s(-1). For materials exhibiting lower k(s) values, the derivation suggests that at high flow rates, a limit occurs where dissolution rate becomes independent of shear rate and merely a function of solubility and surface area.

CONCLUSIONS

The new parameter k(s) may be deduced from any set of geometric and flow conditions, provided the fluid velocity can be determined everywhere in the domain.