In vitro properties of an in situ forming gel for the parenteral delivery of macromolecular drugs.

The purpose of this research was to (i) formulate a solution of a water-insoluble interpolymeric complex (IPC) containing poly(methacrylic acid) (PMA), 15 kDa, and poly(ethylene glycol) (PEG), 20 kDa, in a biocompatible cosolvent system; (ii) demonstrate that the IPC solution can transform into a gel, in situ, at physiological pH; and (iii) determine the ability of the gel to entrap, protect, and control the release of macromolecular drugs such as proteins and oligonucleotides. Ternary phase diagrams were prepared to identify cosolvent composition containing N-methylpyrrolidone (NMP), ethanol, and water that dissolve the IPC. IPC solutions (40, 50, or 60% w/v) each containing 1 mg of either model proteins, fluorescein isothiocyanate (FITC)-insulin and FITC-albumin, or 24-mer phosphorothioate oligonucleotides, were placed in containers that were immersed in buffer, pH 7.4. Aliquots of the buffer were sampled periodically and analyzed for the macromolecular content. In addition, in vitro bioactivity of another model protein, alpha-amylase, contained in the IPC solution was also determined. The studies demonstrated that a cosolvent containing 1:1:2 ratio of NMP/ethanol/water was most suitable for dissolving the IPC. Concentrations > 30% w/v IPC were required to form the gel, however, those mixtures containing > 60% w/v IPC could not be easily injected via 18-22 gauge needle. The gel can entrap and control the release of the model macromolecules for up to 6 days, in vitro. In addition, the gel can maintain the bioactivity of the protein, alpha-amylase, for 6 days. Therefore, an IPC gel can entrap, protect, and control the release of macromolecular drugs over a period of 6 days, in vitro, and therefore can be considered for in vivo investigation.

In vivo properties of an in situ forming gel for parenteral delivery of macromolecular drugs.

PURPOSE: This study characterizes the in vivo properties of an in situ forming gel, comprising of IPC of water-soluble polymers, PMA and PEG, for sustained release of macromolecular drugs. METHODS: 40, 50, or 60% w/v formulations were injected subcutaneously in a rat model either alone, or containing model macromolecules, 3A2-ATG-psODN or REV-psODN, to (i) determine the approximate gelling and residence time of the gel at the site of injection (ii) assess the biological efficacy of the formulation using a MZ sleep time model and (iii) demonstrate specificity of the sequence and selectivity of the psODNs by measuring changes in microsomal enzyme levels and urine volumes. RESULTS: A sol to gel transition requires 15 min in vivo, and the 60% w/v IPC gel remains at the site of injection for up to 72 hr. The MZ sleep times and CYP3A2 expression due to 3A2-ATG-psODNs released from the gel are significantly different compared to that of REV-psODNs. CONCLUSIONS: The IPC solutions exhibit phase transformation in vivo. and demonstrate no evidence of toxicity. The pharmacological effects observed from the of release of 3A2-ATG-psODNs suggest that the formulation can entrap, protect, and sustain the delivery of macromolecules. .

Controlled release of substituted benzoic and naphthoic acids using Carbopol gels: measurement of drug concentration profiles and correlation to release rate kinetics.

PURPOSE: The objective of this study is to correlate drug release mechanism with measured drug concentration profiles in gel layers of Carbopol matrices containing mesalamine or benzoic acid. METHODS: Release rate experiments with Carbopol matrices were performed using a rotating disk apparatus. Matrices were frozen and the gel layer in the matrices was sliced using a microtome in a cryostat. Drug concentration profiles were determined by direct measurement of the concentration of the drug in the gel slices. The pH of the slices was measured using microelectrodes, and water content was measured by Karl Fisher titration. RESULTS: The concentration gradient in mesalamine matrices decreased over time and correlated with square root of time release rate kinetics. The concentration profiles of benzoic acid were unchanged over time and correlated with zero order release rate kinetics. Carbopol gel layers were highly hydrated (93-95% water). Gel layers in matrices with mesalamine had a more alkaline microenvironmental pH. This higher pH resulted in increased growth of the thickness of the gel layer and a reduction drug diffusivity in comparison to benzoic acid matrices. CONCLUSIONS: The release rate kinetics of mesalamine and benzoic acid correlated to the measured concentration profiles. The shape of the concentration profiles is determined by the rate of growth of the Carbopol gel layer and drug diffusivity.

Modification of in situ gelling behavior of carbopol solutions by hydroxypropyl methylcellulose.

Aqueous solutions of Carbopol [polyacrylic acid (PAA)] are low viscosity acidic solutions that transform into gels upon an increase in the pH and, therefore, may be used as in situ gelling ophthalmic drug delivery systems. However, the amount of PAA required in the solution to form stiff gels upon installation in the eye is not easily neutralized by the buffering action of tear fluid. A reduction in the PAA concentration without comprising the in situ gelling properties as well as the overall rheological behavior of the system can be achieved by adding a suitable viscosity-enhancing polymer. The rheological properties of aqueous solutions containing PAA and hydroxypropyl methylcellulose (HPMC), a viscosity-enhancing polymer, evaluated as a function of temperature and pH, were similar to those of pure PAA solutions; that is, both form low viscosity liquids at pH 4.0 and transform into stiff gels with plastic rheological behavior and comparable viscosities upon increasing the pH to 7.4. In addition, HPMC-PAA gels show slow in vitro release of incorporated timolol maleate. Thus, the HPMC-PAA combination demonstrates properties suitable for formulation as a liquid ophthalmic delivery systems, which upon instillation into the cul-de-sac of the eye can undergo in situ phase transition to form gels capable of sustained drug release.

In situ-forming gels for ophthalmic drug delivery.

Poor bioavailability of ophthalmic solutions caused by dilution and drainage from the eye can be overcome by using in situ-forming ophthalmic drug delivery systems prepared from polymers that exhibit reversible phase transitions. Joshi et al. (1), have demonstrated that aqueous compositions that reversibly gel in response to simultaneous variations in at least two physical parameters, such as temperature, pH, and ionic strength, can be formed by appropriate combinations of macromolecular polymers which exhibit reversible gelation properties. In the present study, the rheological characterization of such a system, prepared by a combination of Carbopol (C) and methyl cellulose (MC), was carried out at two different pH (4.0 and 7.4) and temperatures (25 and 37 degrees C) by rotational cone and plate viscometry. The shear stress (tau) vs. shear rate (D) flow curves of the aqueous polymer solutions indicated a pseudoplastic behavior, with a yield point. An increase in pH from 4.0 to 7.4, or temperature from 25 to 37 degrees C, resulted in an increase in viscosity (eta), tau, and yield point, the magnitude of changes being highest when both the parameters were altered simultaneously. An increase in concentration of either C or MC, or an increase in MC molecular weight results in an increase in eta, tau, and yield point. Among the compositions studied, a solution containing 1.5% MC 0.3% C was found to have low eta, and formed a strong gel under simulated physiological conditions. Such a system can be formulated as drug containing liquid suitable for administration by instillation into the eye, which upon exposure to physiological conditions will shift to the gel (semi-solid) phase, thus increasing the precorneal residence time of the delivery system and enhancing ocular bioavailability.

Dissolution of ionizable drugs into unbuffered solution: a comprehensive model for mass transport and reaction in the rotating disk geometry.

A model has been developed to describe the mass transport and reaction of ionizable compounds where mass transfer is caused by convection and diffusion from a rotating disk. Dissolution rates of benzoic acid, 2-naphthoic acid, and indomethacin in aqueous solutions of high ionic strength (I = 0.5 with potassium chloride) at 25 degrees C were investigated. The model includes the effects of diffusion, convection, and simultaneous acid/base reaction at all points in the region adjacent to the dissolving solid. The solution of the transport equations is obtained numerically with an iterative algorithm which uses (a) closure of all material balances and (b) equilibria at the solid/liquid surface as constraints. The model solution yields both the flux of the dissolving acid and the concentration profile of each component. Reduced values of all reaction rate constants are used in the region adjacent to the dissolving surface to allow convergence of the computation. Although nonequilibrium concentration values are calculated, it is shown that the theoretical dissolution rate determined as the solution of the model is insensitive to the magnitude of the rate constants as their maximum useable values are approached. Comparisons of the model results with experimentally determined fluxes show close agreement and confirm that the transport mechanisms in the model formulation are consistent with the measured values. Further, the inclusion of convection allows accurate calculations without utilization of an arbitrary boundary layer thickness. Accurate dissolution rates can be determined using this technique under a wide range of conditions, except at low pH.

Precorneal sampling techniques for ophthalmic gels.

Drug-cornea contact time is a critical issue in ocular drug delivery. Existing methods for its experimental determination are developed mainly for eye drops and ointments, and have not been reported for ophthalmic gels. The present study evaluated two tear film sampling techniques (capillary tubes and Schirmer strips) and one recovery technique (cotton swab) for their suitability for the determination of precorneal drug concentration as a function of time for ophthalmic gels. The study was conducted using the rabbit eye model, and the gel studied was a commercial polyacrylate-based gel containing pilocarpine HCl. The three techniques explored yield similar results with respect to drug-cornea contact time, about one hour for the gel studied. The strip method suffers from a gel-carry-over problem at the early time points; therefore it is not recommended for tear sampling until most of the gel is cleared from the cul-de-sac. Successful tear sampling was accomplished using capillary tubes. Drug concentration in the tear film as a function of time determined using this technique reveals not only the duration of contact between the drug and the cornea, but also demonstrates a nonuniform drug distribution in the tear film at the early time points (10 and 30 minutes). Finally the cotton swab technique is gentle, easy, and nondestructive. It recovers total drug remaining in the cul-de-sac but does not yield information for the tear film.

Experimental determinations of diffusion coefficients in dilute aqueous solution using the method of hydrodynamic stability.

Diffusion coefficients were experimentally determined in dilute aqueous solution at 25 +/- 0.1 degrees C, ionic strength 0.5 M, using Taylor's method of hydrodynamic stability. The methodology described is accurate enough to show significant differences in diffusion coefficients between the various ionic forms of the same species as a function of degree of ionization. In Taylor's method, diffusion coefficients were measured by allowing two solutions of differing solute concentration to contact in a capillary tube, forming a stable, measurable concentration gradient. The solute diffusion coefficient is a function of the gradient, the solution viscosity, the solution density, and some capillary dimensions. Viscosity was maintained constant across experiments and values of sufficient accuracy were available in the literature. Solution densities were measured with a tuning fork densimeter. Compounds studied were o-aminobenzoic acid, benzoate anion, the four forms of phosphate and citrate, and the zwitterionic forms of glycine, diglycine, and triglycine. Based on the results for the four forms of phosphate and citrate, experimental diffusivity values vary with the ionic state of the diffusant, presumably because of the altered state of hydration as charge varies. For the glycine series, the diffusivity showed an unexpected dependency on molecular weight (size).

Dissolution of carboxylic acids. III: The effect of polyionizable buffers.

The dissolution behavior of three carboxylic acids of variable aqueous solubility but with approximately equal pKa values into aqueous buffered solutions has been studied as a function of pH and of buffer properties. The dissolution from constant-surface-area compressed disks of benzoic acid, 2-naphthoic acid, and indomethacin into solutions of constant ionic strength (mu = 0.5 with potassium chloride) and constant pH (maintained by pH stat) at 25 degrees C using a rotating disk apparatus was evaluated. Models for dissolution of these weak acids into diprotic and triprotic buffering media are developed to predict the flux of the acid as a function of bulk solution pH and the physical and chemical properties of the buffer and acid. The models assume that mass transfer can be represented by a single second order diffusive term and that instantaneous equilibrium between all reactive species exists. Values of flux and pH at the solid-liquid interface are calculated and the fluxes compared to experimentally determined values. Reasonable correlation was found between values predicted by the models and experimental flux values. Major influences on model accuracy are the Ka and physical properties of the buffer.

Poly(ortho ester) biodegradable polymer systems.

PIP: 1 approach to developing bioerodible drug delivery systems is to prepare hydrolytically labile polymers into which a drug can be physically dispersed under relatively mild conditions and in which release of the drug is controlled by the hydrolytic erosion of the matrix. Focus is on devices in which the hydrolytic erosion process is confined to the surface of a solid device. In such devices, and for a constant rate of polymer hydrolysis, rate of drug release is directly proportional to drug loading, and lifetime of the device is directly proportional to the physical dimensions of the device. Due to the fact that the rate of drug release is directly proportional to the total surface area of the device, as the physical dimensions of the device decrease because of the erosion process, the rate of drug release also will decrease. There are 2 possible approaches to the achievement of controlled surface erosion: the interior of the matrix is stabilized so that only the outer layers can erode; and development of a highly hydrophobic polymer that contains linkages in the polymer backbone whose rates of hydrolysis are pH dependent and to incorporate into the polymer excipients that, in contact with water, produce a pH that induces the desired polymer hydrolysis rate. 1 such polymer system is the poly(ortho esters). This chapter describes some work done on polymer synthesis and the use of various excipients to achieve controlled drug release of various therapeutic agents physically dispersed in these polymers. It contains a review of drug release studies, which covers the use of basic or neutral water-soluble excipients, the use of acidic excipients, the use of calcium lactate, and the use of anhydrides. Poly(ortho ester) bioerodible polymers are suitable materials for the topical administration of a wide variety of therapeutic agents; varying the nature and amounts of excipients physically incorporated into the polymer will vary the erosion rates from a few hours to many months. With this range of control, delivery systems for short-term applications such as oral 12-24 hour tablets, intermediate ophthalmic products lasting 1 week, or implants lasting as long as 1 year can be produced. No extensive toxicological studies have been performed as yet. In vivo release rate experiments performed over many months have not shown any adverse tissue reactions beyond the expected encapsulation.^ieng

Hydrolysis of some poly(ortho-ester)s in homogeneous solutions.

The hydrolysis of poly(ortho-ester)s and a monomeric model compound, 3,9-dibenzyloxy-3,9-diethyl-2,4,8,10-tetraoxaspiro[5,5]un decane, was carried out in dioxane-d8-dioxane and followed by 1H-NMR and HPLC, respectively. Experimental results suggested that the polymer degradation proceeds to a large extent via random scission. The hydrolysis was catalyzed by the acid; the catalytic rate constant increased predictably with decreasing aqueous pKa of the acid. The reaction is first order with respect to the catalyst concentration and the number of ortho-ester linkages present, and it is independent of water in the concentration range studied. Strain at the ortho-ester bond may be a factor influencing the hydrolysis rate.

Modelling of drug release kinetics from a laminated device having an erodible drug reservoir.

The purpose of this communication is to present a preliminary analysis to demonstrate the effect of laminating a drug-containing erodible polymer matrix with a second barrier membrane. A mathematical model for the diffusive release of the drug from an erodible polymer device undergoing surface erosion has been extended to similar devices with a secondary membrane to allow a comparison of the results. The results indicate that the constant rate of release characteristic of erodible devices is not sacrificed with the addition of the secondary membrane; moreover, the membrane provides additional controllable parameters at the disposal of the device designer.

Drug delivery from catalysed erodible polymeric matrices of poly(ortho ester)s.

The incorporation of small amounts of acid anhydrides into hydrophobic poly(ortho ester)s can facilitate the erosion and drug release from delivery systems. Since the reaction can be controlled by the amount of anhydride employed, the reaction is confined to a small reaction zone near the surface and constant delivery rates can be achieved. The catalytic activity is negatively correlated with the pKa of the corresponding acid of the anhydride.

Quantitative evaluation of topically applied pilocarpine in the precorneal area.

The bioavailability of topically applied pilocarpine is poor due to various loss mechanisms that serve to lessen delivery of the drug to the aqueous humor. Precorneal drainage and other routes of loss have been investigated qualitatively as well as quantitatively. The mechanistic models proposed to date suffer from the drawback of being dependent on our understanding of the mechanism of drug transport through the corneal membrane. This report quantifies the initial disposition of topically applied drugs and their availability for systemic and local absorption. The rate constant for the conjunctival absorption is determined and is independent of the mechanism of transcorneal transport.

Diffusion of phenol in the presence of a complexing agent, tetrahydrofuran.

The effect of a complexing agent, tetrahydrofuran, on the diffusion of phenol across a stagnant fluid layer has been studied. At a given activity of free phenol, the steady-state flux of phenol appearing in an acceptor phase was greatly enhanced. However, as the fraction of phenol associated with the complexing agent increased, the flux of phenol decreased, since the transport was then controlled by the diffusion of the complex, a larger structure. A mathematical model of simultaneous association and diffusion was derived to determine whether the diffusional behavior of two associating species could be accounted for in terms of the association equilibrium constant and Fick's second law. Experimental results supported the model. It was concluded that the presence of a complexing agent tends to reduce the rate of diffusion, the effect being more pronounced at high concentrations of complexing agent.

Simultaneous self-association and diffusion of phenol in isooctane.

The impact of self-association on mass transport was studied. The model system chosen was the diffusion of phenol through an immobilized layer of isooctane. In the theoretical development, self-associated phenol contributed to diffusion, with the fluxes being interdependent because of the self-association equilibrium. Predictions from theory were then compared with experimental results. It was shown that self-association can significantly affect flux of diffusing species.

A physiologically based pharmacokinetic model for the intraocular distribution of pilocarpine in rabbits.

This report presents a mathematical model which has been developed to describe the intraocular disposition of pilocarpine following topical dosing in rabbits. The model uses experimentally determined parameters such as rates of tissue uptake of drug and equilibrium distribution coefficients. Differential mass balance equations for pilocarpine in the cornea, aqueous humor, iris-ciliary body, and lens were written and solved numerically. Measured tear concentrations, following topical dosing wih pilocarpine, were fit by a monoexponential curve and used as the forcing function of the model. By using a combination of known physiological and experimentally determined parameters, predictions of intraocular tissue levels of pilocarpine were made. These predictions were then compared to experimentally determined concentration-time profiles.

Dissolution kinetics of phenylbutazone.

This study investigated the possible effects of simultaneous, noninstantaneous, reversible chemical ionization of carbon acids on the dissolution of a typical pharmaceutical carbon acid, phenylbutazone, and its deutero analog. The dissolution rate versus pH profile for phenylbutazone was consistent with phenylbutazone acting as if it were an acid where the ionization can be considered instantaneous. In view of the dissolution behavior of phenylbutazone under various conditions, it is unlikely that the noninstantaneous ionization kinetics demonstrated for this compound play a major role in determining the dissolution rate, either in vitro or in vivo, since the average residence time in a typical aqueous diffusion layer for phenylbutazone dissolution is longer than the reaction time for its ionization. Slowing the reaction time with a primary isotope effect by deuterium substitution for the ionizable proton caused significant deviation from classical behavior for d-phenylbutazone.

Pharmacokinetics of 2-butanol and its metabolites in the rat.

A pharmacokinetic model is presented to describe the biotransformation of 2-butanol (2-OL) and its metabolites (2-butanone, 3-hydroxy-2-butanone, and 2, 3-butanediol) using in vivo experimental blood concentration. A flow limited model is developed to simulate 2-OL, 2-butanone (2-ONE), 3-hydroxy-2-butanone (3H-2B), and 2,3-butanediol (2,3-RD) blood concentrations in rats after oral administration of 2-OL. Assuming the only important site of 2-OL biotransformation is the liver, the tissues included are the liver and a volume of distribution, essentially body water in the case of 2-OL and its metabolites. A distribution coefficient is found to be necessary to describe the low concentration of 3H-2B in blood after administration of 2-OL. The need for this coefficient may be due to partitioning, binding, or altered transport rates from the liver. Inhibition of 2-ONE metabolism to 3H-2B by 2-OL has been included to explain a time delay in the appearance of 3H-2B after administration of 2-OL. Subsequent experimental verification confirms the mixed function oxidase inhibitory properties of 2-OL. The model is able to simulate blood concentrations and elimination of all four compounds after the oral administration of 2-OL. Additionally, the model also simulates the results obtained after i.v. administration of 3H-3B and 2,3-BD.