A novel differentiation method of vehicle models for topically applied drugs: application to a therapeutic timolol patch

Carbopol/chitosan based pH triggered in situ gelling system for ocular delivery of timolol maleate

The poor bioavailability and therapeutic response exhibited by conventional ophthalmic preparations due to rapid precorneal elimination, dilution and nasolacrimal drainage of the drug may be vanquished by the use of in situ gelling systems that are instilled as drops in to the eye and undergo a sol-gel transition in the cul-de-sac. Timolol eye drops may cause systemic side effects in glaucoma patients due to absorption of the drug into systemic circulation. In situ gelling system of this drug can provide localized effect with reduced contraindications, improved patient compliance and better therapeutic index. The present work describes the formulation and evaluation of an ophthalmic delivery system of an antiglaucoma drug, timolol maleate (TM) based on the concept of pH-triggered in situ gelation. Polyacrylic acid (carbopol) was used as the gelling agent in combination with chitosan (amine polysaccharide), which was acted as a viscosity-enhancing agent. Formulations were evaluated for pH, viscosity, gelling capacity and drug content. The 0.4% w/v carbopol/0.5% w/v chitosan based in situ gelling system was in liquid state at room temperature and at the pH formulated (pH 6.0) and underwent rapid transition into the viscous gel phase at the pH of the tear fluid (lacrimal fluid) (pH 7.4). The in vitro drug release and in vivo effects of the developed in situ gelling system were compared with that of Glucomol® (a 0.25% TM ophthalmic solution), 0.4% w/v carbopol solution as well as liposomal formulation. The results clearly demonstrated that developed carbopol-chitosan based formulation was therapeutically efficacious and showed a fickian (diffusion controlled) type of release behaviour over 24 h periods. The developed system is thus a viable alternative to conventional eye drops and can also prevent the rapid drainage as in case of liposomes.

Drug release from a suspension with a finite dissolution rate: theory and its application to a betamethasone 17-valerate patch

A random walk method for a diffusion equation is applied to the model for a suspension with a finite dissolution rate developed by Ayres and Lindstrom in 1977. In the method, the diffusion of dissolved drug and dissolution of crystal are calculated separately using a simple BASIC program. The random walk method strictly meets the principle of the conservation of mass as the drug amount in each sublayer rather than the concentration at each subinterval is concerned in the ointment. The model is used to analyze the release of betamethasone 17-valerate from a pressure-sensitive silicone adhesive into a sink. The drug release from the 1.50 mg/mL patch shows no substantial discrepancy from that predicted by the classic suspension model assuming an infinite dissolution rate. However, the classic model overestimates the release from the 3.08 and 5.88 mg/mL patches. The disagreement is lessened when the dissolution rate is assumed to be finite. However, the model does not give a perfect explanation because the drug release from the 3.08 and 5.88 mg/mL patches in the early phase is faster than the model predicts.

Lag time in the dual sorption model for percutaneous absorption with finite skin-receptor boundary clearance

The lag time in the dual sorption model for percutaneous drug absorption with finite skin-receptor boundary clearance is examined. The effect of the change in the perfusional resistance on the lag time in the dual sorption model is found to be similar to, but a little less prominent than, that in the linear model. It is suggested that the lag time is roughly proportional to the amount in the skin at the steady state. Two numerical methods are used in predictor-corrector combination to obtain a numerical solution of the parabolic partial differential equation, together with the associated initial and boundary conditions, which arise in a nonlinear mathematical model of percutaneous drug absorption. Numerical estimates are obtained for the drug concentration profiles, the cumulative amount of drug excreted into the blood, the flux in the transient and steady states, the lag time, and the amount of drug in the skin per unit area in the steady state.

Dual sorption model for the nonlinear percutaneous permeation kinetics of timolol

The nonlinear percutaneous permeation kinetics of timolol were studied in vitro with human cadaver skin. In the sorption isotherm (equilibrium) study, the plots of the amount of timolol per unit area of epidermis versus aqueous timolol concentration in equilibrium were curvilinear as the dual sorption model predicts. In the penetration study, several aqueous timolol concentrations were maintained in the donor cell for 25 h. The lag-time was prolonged as the timolol concentration in the donor cell was decreased. The change in the lag time was analyzed by a newly proposed method with the lag-time prolongation factor that is calculated from the parameter values obtained in the sorption isotherm study. The ratio of the lag-time to lag-time prolongation factor was of the same magnitude, indicating that the dual sorption model explains the nonlinear percutaneous permeation kinetics of timolol. Finally, the diffusion parameter for the mobile solute was estimated by fitting the data of the cumulative amount excreted into receptor cell versus time to the numerical solution of the dual sorption model. The observed data were roughly compatible with the values predicted by the dual sorption model.

Pharmacokinetics and beta-blocking effects of transdermal timolol

The pharmacokinetic profiles of transdermal timolol 6 and 24 mg (as 5 and 20% w/v patches) was studied in four healthy young volunteers. To assess its bioavailability, the pharmacokinetics of an IV infusion of timolol maleate 5 mg was also determined in the same subjects. When the 20% (w/v) timolol patch was applied, the mean bioavailability was 74.4%. Plasma timolol concentrations were below the detection limit when a 5% patch was applied to the same skin area in all four subjects, except for one in whom the bioavailability was 23.6%. Weak erythema developed at the application site in all of the volunteers after application of the 20% (w/v) patch. However, erythema did not develop in any volunteer when the 5% patch was applied. The beta-blocking effect was determined by exercise testing. Similar plasma levels generated similar changes in exercise-induced heart rate after the transdermal and intravenous administration of timolol.

A random walk method for percutaneous drug absorption pharmacokinetics: application to repeated administration of a therapeutic timolol patch

A random walk method for predicting percutaneous drug absorption pharmacokinetics was proposed. The profiles predicted by this method were compatible with those predicted by the analytical method. The random walk method is particularly useful for predicting complex processes such as repeated topical application of a drug. The amount of a drug released into skin from four therapeutic timolol patches was measured when the patches were serially applied for 2.5 h each on the same site of six healthy male volunteers. On the average, 33.2, 23.4, 15.1, and 16.5% of the applied dose was released into skin from the first, second, third, and fourth patches, respectively. This pattern was comparable with the predicted profiles (43.9, 30.2, 24.4, and 21.1%) of amounts of drug which were expected to be released from the first to fourth patches into skin, respectively. The estimation method for the normalized skin-capillary boundary clearance is also described and applied in examining the percutaneous absorption of timolol. The estimated value for this parameter was much greater than the diffusion parameter, indicating that the removal process of timolol by the local circulation is much faster than the diffusion process through skin.

Skin irritation induced by topically applied timolol

1. Erythema induced by topically applied timolol, a non-selective beta-adrenoceptor blocker, was assessed in six male volunteers. Intensity of erythema developed on the inner surface of the left forearm where timolol free base was applied for 10 h was measured by a visual score, laser doppler flowmeter and reflectance spectrophotometry. Plasma timolol concentrations collected from the left and right arms were also measured. 2. The mean values for blood volume and blood flow per unit volume of tissue both of which were assessed by a laser doppler flowmeter, haemoglobin index measured by reflectance spectrophotometry, and magnitude of erythema graded by a visual score significantly increased after the application of an acrylic co-polymer adhesive patch containing 20% (w/v) timolol free base. 3. Plasma timolol concentrations collected from the left antecubital vein were 2.4 to 10.7 times greater than those from the right arm and had significant correlations (rs = 0.55 to 0.76) with the parameters indicating the extent of erythema developed where a patch containing timolol was applied. 4. The inter-individual variation of timolol was attributed to that of the diffusivity of timolol through skin rather than that of the skin reactivity to topically applied timolol because the plasma timolol concentrations drawn from the left arm in the subjects who did not develop erythema were very low.(ABSTRACT TRUNCATED AT 250 WORDS)

Finite dose percutaneous drug absorption: a BASIC program for the solution of the diffusion equation

A BASIC program to obtain the solution for the diffusion equation describing the finite dose percutaneous absorption pharmacokinetics is shown. Drug amount in the solution matrix, Av, that in the skin which is regarded as a simple diffusion membrane. As, and the drug flux from the skin which is provided by the local circulation per unit area, J, and the blood drug concentration. C, expressed as functions of time are obtained using Fourier series. Usually, a small number (less than 10) of series is enough to obtain the accurate values for Av, As, J, and C. It is also shown that the solution of the diffusion equation reported previously for the specific case where both the diffusion coefficient of a drug through the vehicle and skin-capillary clearance are very large is expressed as a limit.

Finite dose percutaneous drug absorption: theory and its application to in vitro timolol permeation

The finite dose in vitro percutaneous absorption kinetics of timolol, a beta-blocker, was studied. The flux of timolol across excised human abdominal cadaver skin was measured over a period of 72 h following application of a 40-microns thickness patch containing 5, 10, or 20% (w/v) timolol free base. Amounts of timolol in the patch and skin were also determined at 6, 12, 24, 48, and 72 h after the application of the 20% (w/v) patch. The mean diffusion and partition parameters were estimated to be 0.018 h-1 and 125.9 microns, respectively, using a newly developed theory. Diffusion and partition parameters were estimated using the values for amounts of a drug eventually partitioning into the perfusing water, as well as two newly proposed conceptual parameter values, AUCAv and AUCAs which are the AUCs of drug amounts in vehicle and skin, respectively. The dose-dependent skin-timolol interaction is also discussed.