Robert A. Bellantone

A Method to Predict the Equilibrium Solubility of Drugs in Solid Polymers near Room Temperature Using Thermal Analysis

A method is presented for determining the equilibrium solubility of a drug in a solid polymer at or near room temperature, which represents a typical storage temperature. The method is based on a thermodynamic model to calculate the Gibbs energy change ΔG(SS) associated with forming a binary drug-polymer solid solution from the unmixed polymer and solid drug. The model includes contributions from heat capacity differences between the solid solution and the corresponding unmixed components, breaking up of the solid drug structure, and drug-polymer mixing. Calculation of ΔG(SS) from thermal analysis data is demonstrated, and it is shown that minima of plots of ΔG(SS) versus the dissolved drug concentration represent the equilibrium drug solubility in the polymer. Solid solutions were produced for drug-polymer systems (griseofulvin, indomethacin, itraconazole; PVP K30, Eudragit L100, Eudragit E100) in drug weight fractions up to ∼25%. At 25°C, it was seen that heat capacity effects were important in determining the drug solubility. It was concluded that drug solubilities in solid polymers can be determined using thermal analysis, and must include heat capacity effects when evaluated near room temperature.

In vitro evaluation of the release of albuterol sulfate from polymer gels: Effect of fatty acids on drug transport across biological membranes

ABSTRACT In this investigation, the diffusion of the beta2 agonist albuterol sulfate (ABS) across several membranes (cellulose, hairless mouse skin, human cadaver skin) from polymer gels was studied, and the effects of several fatty acids on drug permeation through skin were evaluated. The results were then used to predict whether transdermal delivery would be appropriate for ABS. All in vitro release studies were carried out at 37°C using modified Franz diffusion cells. In preliminary studies, ABS release through cellulose membranes was studied from two polymeric gels, Klucel® (hydroxypropylcellulose) and Methocel® (hydroxypropylmethylcellulose). Three polymer concentrations were used for each gel (0.5%, 1.0%, and 1.5%). From these experiments, Klucel 0.5% was selected as the optimal formulation to study ABS diffusion across hairless mouse skin. Experiments were conducted to evaluate the effects of capric acid, lauric acid, and myristic acid as penetration enhancers. The results suggested that lauric acid preferentially enhanced ABS diffusion compared to the other fatty acids studied, and follow-up studies were done to evaluate the release through human cadaver skin from a donor containing 2% ABS and lauric acid in 0.5% Klucel®. These experiments showed that a 2:1 (lauric acid:ABS) molar ratio gave the best ABS release rates. The release rate across human cadaver skin declined slowly over 24 hr, and an average flux over 24 hr of ˜0.09 mg/hr cm2 was measured. Using this value as a steady-state flux, extrapolations predicted that transdermal delivery can be used to maintain therapeutic ABS plasma levels (6–14 ng/mL) for extended periods. The results of this research suggest that ABS is a good candidate for transdermal drug delivery.

An improved method for the characterization of supersaturation and precipitation of poorly soluble drugs using pulsatile microdialysis (PMD).

In current pharmaceutical drug discovery, most candidates are poorly soluble in water, which can result in poor bioavailability. To overcome this problem, formulations that create supersaturation of the drug are a well-studied alternative. Characterizing the dissolution from these systems is challenging because conventional methods, such as sampling with a syringe then filtering with a 0.2-0.45 μm filter before an HPLC assay, can overestimate the concentration of dissolved drug by allowing nuclei or small precipitated particles to pass, which then dissolve in the HPLC mobile phase. Nuclei and small particles can also cause overestimation of the dissolved concentration when using optical methods. Such overestimations can lead to failure of in vivo prediction of drug bioavailability from supersaturated systems. This paper reports a novel method to determine the free dissolved drug concentration in a dissolution medium using pulsatile microdialysis (PMD). Ibuprofen was used as a model drug for determining precipitation and supersaturation. Supersaturation was induced chemically by changing pH, and also by dissolution/release from an in-house formulation. The adaptation of a previously developed PMD model is summarized, and experimental results comparing dissolved concentrations determined using PMD and direct sampling by syringe and filtering are presented.

An improved in vitro method for measuring skin permeability that controls excess hydration of skin using modified Franz diffusion cells

When liquid donors/receivers are used for in vitro skin permeation studies, excess hydration can change skin properties compared to in vivo conditions. A novel in vitro method of determining the permeability of drugs through skin was developed that avoids exposing the membrane to dilute donor/receiver solutions. The drug is dissolved in an unstirred donor gel, and diffuses through a membrane into an unstirred gel receiver that can potentially be adjusted to mimic physiological conditions. Pulsatile microdialysis (PMD) was used to sample local concentrations in the receiver medium, and a model was developed to allow the determination of permeability. For Doxepin HCl, permeabilities through artificial membranes and human cadaver skin were determined using the new and previously reported methods. For artificial membranes that minimally hydrate, the new method gave consistent but slightly lower permeability values. For human cadaver skin, the permeability determined using the new method was 1/6 that of the fully hydrated skin. Limitations of the model, their relations to experimental design and data analysis were evaluated. It was concluded that this method can be applied to characterize membrane permeabilities using experiments that may avoid membrane breakdown and more closely mimic physiological conditions.

Permeability and Retention Studies of (-)Epicatechin Gel Formulations in Human Cadaver Skin

ABSTRACT (-)Epicatechin (EC) is a major antioxidant component of grape seed extract which has become increasingly popular in topical skin preparations. This study assessed the following: (1) the permeability through cellulose membranes of EC in three different gel formulations (Carbopol®940, Klucel®, and Ultrez™10); (2) the effect of three different antioxidants (butylated hydroxytoluene (BHT), alpha-tocopherol (VE), and ascorbic acid (AA)) on the stability and penetration properties of EC; and (3) the permeability and retention of EC in Ultrez™10 gels, supplemented with BHT or VE, on human cadaver skin. Permeability studies through cellulose membranes showed that different gelling agents do not significantly affect the permeability of EC (n = 7/gel; p > 0.05). BHT and VE have antioxidant properties superior to AA (p < 0.05) and preserve 100% of the initial content of EC for 28 days. Permeation studies on cadaver human skin, following application of two anhydrous gel formulations (0.5% EC in Ultrez™10 containing BHT or VE), showed that EC was not detectable in the receiving solution. However, the EC amount in viable skin increased with time, indicating that EC penetrated and was retained in the upper part of the skin for approximately 1% and 3% of the dose for the formulations containing BHT and VE, respectively.

Permeation study of five formulations of alpha-tocopherol acetate through human cadaver skin.

Alpha-tocopherol (AT) is the vitamin E homologue with the highest in vivo biological activity. AT protects against the carcinogenic and mutagenic activity of ionizing radiation and chemical agents, and possibly against UV-induced cutaneous damage. For stability consideration, alpha-tocopherol is usually used as its prodrug ester, alpha-tocopherol acetate (ATA), which once absorbed into the skin is hydrolyzed to alpha-tocopherol, the active form. The objective of this research was to characterize in vitro the permeation properties of ATA from various solutions and gel formulations. Permeation studies were conducted using modified Franz diffusion cells and human cadaver skin as the membrane. Specifically, 5% (w/w) alpha-tocopherol acetate was formulated in the following vehicles: ethanol, isopropyl myristate, light mineral oil, 1% Klucel gel in ethanol, and 3% Klucel gel in ethanol (w/w). The receiver temperature was 37 degrees C. Samples from the receiver were collected at 2, 4, 6, 8, 12, 24, 30, 36, and 48 hours and analyzed by HPLC for concentrations of alpha-tocopherol acetate and alpha-tocopherol. The permeabilities of ATA through human cadaver skin were 1.0x10(-4), 1.1x10(-2), 1.4x10(-4), 2.1x10(-4), and 4.7x10(-4) cm/h for the ethanol solution, isopropyl myristate solution, light mineral oil solution, 1% Klucel gel, and 3% Klucel gel, respectively. The results show that the formulation had relatively minor effects on the permeability coefficients of ATA through cadaver skin in all cases except for the isopropyl myristate solution.

Fundamentals of Amorphous Systems: Thermodynamic Aspects

Drugs that are poorly soluble in water present a major challenge to the pharmaceutical industry because they tend to show poor oral bioavailability. To overcome this problem, much attention has been given to delivering drugs in formulations that supersaturate in the gastrointestinal (GI) fluids. One approach to achieve the supersaturation is to formulate drugs in amorphous forms. This chapter considers amorphous forms of the pure active pharmaceutical ingredients (APIs), also known as neat amorphous APIs. The relationship between supersaturation and absorption is discussed, along with the thermodynamics of solubility of supersaturated solutions. The relevant properties of neat amorphous APIs are discussed and explained in terms of thermodynamics and microscopic models, and their relationships to improved solubility/dissolution and physical stability are considered. In addition, the relationship between neat amorphous forms and glasses, especially as presented in the literature, is reviewed.

Faster determination of membrane permeabilities without using the lag time method

A new method of data analysis is presented that allows the determination of membrane permeabilities. The method is applicable to data obtained from a common experimental setup, in which drug dissolved in an inert donor gel diffuses through a membrane, initially void of drug, into a receiver for which sink conditions are maintained. The equations developed can also be used to predict the release of drug from these systems. Ficks Laws are solved, and the early time behavior of the mathematical solution is used to develop the analysis methods. Limitations of the model and their relations to experimental design are determined, and the method of application to experimental data is presented. The method is tested numerically using simulated data generated by a 1-d finite difference program that was used to numerically solve Ficks Laws, and also applied to in vitro human cadaver skin transdermal data for the drugs doxepin, imipramine and amitriptyline. It is concluded that this method can be applied to determine membrane permeabilities and diffusion coefficients with accuracy comparable to other experimental setups, such as lag time experiments and steady state experiments, but requiring experiments that can be significantly shorter.

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 μM−1) nM1(636 ± 5.23 μM), Ka2(0.46 ± 0.0083 μM−1), and nM2(80.9 ± 0.389 μM). 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/μg-min), k−2(1.59 ± 0.0358 min−1), and k2(3.1 ± 0.035 ml/μg-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.

An in situ method to quantitatively determine dissolved free drug concentrations in vitro in the presence of polymer excipients using pulsatile microdialysis (PMD)

In drug formulations containing polymer excipients, the effects of the polymer on the dissolved free drug concentration and resulting dissolution or release can be important, especially for poorly soluble drugs. In this study, an in vitro method based on pulsatile microdialysis (PMD) was developed to quantitatively determine dissolved free concentrations of drugs in the presence of polymers in aqueous media in situ (e.g., in place within the system being characterized). Formulations were made by dissolving various ratios of the drug griseofulvin and polymer PVP K30 in water and allowing the mix to equilibrate. A PMD probe was immersed in each mixture and the dissolved free drug concentrations were determined in the PMD samples. The experimental procedure and the equations used for data analysis are presented. To assess the consistency of data, a binding model was fit to the data obtained using PMD by calculating the dissolved free drug fraction fD for each drug-polymer ratio in solution, and obtaining the product of the binding stoichiometry and binding constant (νK per mole of polymer) from the slope of a plot of (1-fD)/fD vs. the molar polymer concentration. For comparison, equilibrium binding experiments were also performed at 23C, and the determined value of νK was similar to the value found using PMD. Experiments were performed at three temperatures, and a plot of ln (νK) vs. 1/T was linear and a binding enthalpy of -110.9±4.4J/mol of monomer was calculated from its slope. It was concluded that PMD can be used to determine the dissolved free drug concentrations in situ, which allows characterization of the drug-polymer interaction, even for low drug concentrations. This information may be important in modeling the dissolution or release of drugs from formulations containing polymers.