Release and Skin Permeation of Scopolamine From Thin Polymer Films in Relation to Thermodynamic Activity

Cyclodextrins and drug membrane permeation: Thermodynamic considerations

Cyclodextrins are hydrophilic oligosaccharides that can increase aqueous solubility of lipophilic drugs through formation of water-soluble drug/cyclodextrin complexes. Although the complexes are hydrophilic, and as such do not permeate biological membranes, the complexes are known to enhance drug permeation through lipophilic membranes and improve drug bioavailability after, for example, oral administration. However, it is not clear how cyclodextrins enhance the permeation. An artificial biomembrane (PermeaPad®) was used to study the effect of donor medium composition on drug permeation. It was observed that in aqueous solutions the hydrophilic cyclodextrins behave not like disperse systems but rather like organic cosolvents such as ethanol, increasing the solubility without having significant effect on the molecular mobility and ability of lipophilic drug molecules to partition into the lipophilic membrane. Also, that partition of dissolved drug molecules from the aqueous exterior into the membrane is at its maximum when their thermodynamic activity is at its maximum. In other words, that drug flux from aqueous cyclodextrin solutions through lipophilic membranes depends on both the concentration and the thermodynamic activity of dissolved drug. Maximum flux is obtained when both the drug concentration and thermodynamic activity of the dissolved drug molecules are at their maximum value.

Topical drug delivery to the posterior segment of the eye: Thermodynamic considerations

Almost all studies on non-invasive topical drug delivery to the eye have emphasized the importance of biological barriers, static membrane barriers such as the cornea and the conjunctiva/sclera and dynamic barriers such as the lacrimal drainage. Hardly any have discussed the importance of the thermodynamic activity of the permeating drug molecules. Most drugs permeate from the eye surface into the eye by passive diffusion where, according to Fick's first law, the drug concentration gradient over the various permeation barriers (e.g., the tear fluid and the lipophilic membrane barriers) is the driving force. At the barrier interphases the dissolved drug molecules must partition from one barrier to another. For example, at the tear-cornea interphase the drug molecules must partition from the aqueous exterior into the lipophilic membrane. The drug partition coefficient between two phases is commonly defined as the equilibrium concentration ratio. However, these are only approximations. The actual driving force in Fick's first law is the gradient of the chemical potential and the equilibrium between two phases is attained when the chemical potential of the drug in one phase is equal to that in the other phase. Here the importance of thermodynamic considerations in topical drug delivery to the eye is reviewed.

Dissipative Particle Dynamics Investigation of the Transport of Salicylic Acid through a Simulated In Vitro Skin Permeation Model

Permeation models are often used to determine diffusion properties of a drug through a membrane as it is released from a delivery system. In order to circumvent problematic in vivo studies, diffusion studies can be performed in vitro, using (semi-)synthetic membranes. In this study salicylic acid permeation was studied, employing a nitrocellulose membrane. Both saturated and unsaturated salicylic acid solutions were studied. Additionally, the transport of salicylic acid through the nitrocellulose membrane was simulated by computational modelling. Experimental observations could be explained by the transport mechanism that was revealed by dissipative particle dynamics (DPD) simulations. The DPD model was developed with the aid of atomistic scale molecular dynamics (AA-MD). The choice of a suitable model membrane can therefore, be predicted by AA-MD and DPD simulations. Additionally, the difference in the magnitude of release from saturated and unsaturated salicylic acid and solutions could also be observed with DPD. Moreover, computational studies can reveal hidden variables such as membrane-permeant interaction that cannot be measured experimentally. A recommendation is made for the development of future model permeation membranes is to incorporate computational modelling to aid the choice of model.

Effect of neat and binary vehicle systems on the solubility and cutaneous delivery of piperine

Vitiligo is a skin disease characterized by depigmentation disorders due to lack of melanin production. Piperine, an alkaloid extracted from black piper, is active in melanocytes proliferation. To achieve this, the drug has to reach the melanocytes which exist in the deep layer of the epidermis. Higher drug concentration can be obtained after application of optimized formulation to skin. Accordingly, the aim of this work is to investigate the effect of vehicles on skin penetration of piperine as the first step in development of optimized formulation. The tested vehicles include ethanol (Eth), propylene glycol (PG), polyethylene glycol 400 (PEG), and oleic acid (OA) and their combinations. Water was used as the control and skin permeation was monitored using rabbit ear model skin. The highest piperine solubility (48.6 mg/ml) and flux (40.8 μg/cm² h) was achieved by Eth and the lowest piperine flux (1.17 μg/cm² h) was reported for PEG. PG and OA showed piperine flux values comparable to that of the control. Among different combination systems, Eth-OA (75:25) binary system had the highest piperine flux (59.3 μg/cm² h) followed by Eth-OA (50:50) (32.3 μg/cm² h) and PG-OA (90:10) (22.7 μg/cm² h). The study thus introduced a vehicle system as the first step in the development of topical formulation of piperine.

Design of a Topically Applied Gel Spray Formulation with Ivermectin Using a Novel Low Molecular Weight Gelling Agent, Palmitoyl-Glycine-Histidine, to Treat Scabies

Palmitoyl-glycine-histidine (Pal-GH) is a new low molecular weight gelling agent. It exhibits thixotropic behavior, low viscosity, and high dissolving properties for a wide range of hydrophilic to lipophilic drugs. Orally administered ivermectin (IVM) is used to treat scabies. However, this treatment is associated with well-known side effects, thus a study is awaited to search for alternative routes of administration. Although a topical formulation of IVM could be a candidate, it requires whole body application except the head and face for several hours on a daily basis. Therefore, in this study, we prepared a gel spray formulation containing IVM as an approach for application to large skin areas with a single spray application without further contact with the applied formulation. Pal-GH gel spray formulations were prepared from its aqueous solution by a heating and cooling method. Rheological behavior and physical appearance (spraying, spreading ability, volume of spraying, and homogeneity) of the prepared formulations were evaluated. Pal-GH gel with propylene glycol demonstrated impressive rheological properties (typical thixotropic behavior) with high hysteresis area among all the tested Pal-GH gels and spreading ability. The obtained IVM concentration in the skin after topical application of 0.1% IVM-containing Pal-GH formulation onto hairless rats was much higher than the reported therapeutic concentration obtained from oral administration in humans. These results suggested that topical application of IVM using a Pal-GH gel spray formulation could be an alternative to the conventional oral forms for the scabies treatment.

Reflectometric monitoring of the dissolution process of thin polymeric films

Pharmaceutical thin films are versatile drug-delivery platforms i.e. allowing transdermal, oral, sublingual and buccal administration. However, dissolution testing of thin films is challenging since the commonly used dissolution tests for conventional dosage forms correspond rather poorly to the physiological conditions at the site of administration. Here we introduce a traditional optical reflection method for monitoring the dissolution behavior of thin polymeric films. The substances, e.g. drug molecules, released from the film generate an increase in the refractive index in the liquid medium which can be detected by reflectance monitoring. Thin EUDRAGIT^® RL PO poly(ethyl acrylate-co-methyl methacrylate-co trimethylammonioethyl methacrylate chloride) (RLPO) films containing the model drug perphenazine (PPZ) were prepared by spraying on a glass substrate. The glass substrates were placed inside the flow cell in the reflectometer which was then filled with phosphate buffer solution. Dissolution was monitored by measuring the reflectance of the buffer liquid. The method was able to detect the distinctive dissolution characteristics of different film formulations and measured relatively small drug concentrations. In conclusion, it was demonstrated that a traditional optical reflection method can provide valuable information about the dissolution characteristics of thin polymeric films in low liquid volume surroundings.

Tortuosity of Aligned Channels in Alumina Membranes Produced by Vacuum-Induced Surface Directional Freezing

Vacuum-induced surface freezing of colloidal alumina was used to produce membranes that have elongated, aligned channels and, hence, are tortuous in the direction perpendicular to ice crystal growth. The effective tortuosity of the membranes was measured by steady-state diffusion of a solute, methylene blue. The resulting diffusion profiles show an initial step-increase in amount of dye reaching the acceptor that is caused by capillarity drawing the donor solution through any non-wetted channels in the membrane. This is followed by a linear steady-state phase whose flux is proportional to dye concentration in the donor and inversely proportional to the colloid's volume fraction of dispersed phase. From the steady-state flux, the effective tortuosity, τ* = (α/τ)^-1, was calculated. This is the reciprocal quotient of the reduced available area for diffusion within the membrane, α = A*/A, where A* is the available area and A is the cross-sectional area of the membrane, and the increased mean diffusional path length, i.e., tortuosity = L * / L , where L* is the mean path length and L is the membrane thickness. The values of τ* lie in the range of 2-38 and increase as the volume fraction of dispersed phase is larger. This latter effect indicates that τ* > 1 results, to a larger extent, from the reduced available diffusion area, α, than from the lengthened pathway, τ, in these aligned porous membranes.