Evaluation of silicone-based pressure-sensitive adhesives for transdermal drug delivery. I. Effect of penetrant hydrophilicity

A cascade computer model for mocrobicide diffusivity from mucoadhesive formulations

Background

The cascade computer model (CCM) was designed as a machine-learning feature platform for prediction of drug diffusivity from the mucoadhesive formulations. Three basic models (the statistical regression model, the K nearest neighbor model and the modified version of the back propagation neural network) in CCM operate sequentially in close collaboration with each other, employing the estimated value obtained from the afore-positioned base model as an input value to the next-positioned base model in the cascade.

The effects of various parameters on the pharmacological efficacy of a female controlled drug delivery system (FcDDS) intended for prevention of women from HIV-1 infection were evaluated using an in vitro apparatus “Simulant Vaginal System” (SVS). We used computer simulations to explicitly examine the changes in drug diffusivity from FcDDS and determine the prognostic potency of each variable for in vivo prediction of formulation efficacy. The results obtained using the CCM approach were compared with those from individual multiple regression model.

Results

CCM significantly lowered the percentage mean error (PME) and enhanced r² values as compared with those from the multiple regression models. It was noted that CCM generated the PME value of 21.82 at 48169 epoch iterations, which is significantly improved from the PME value of 29.91 % at 118344 epochs by the back propagation network model. The results of this study indicated that the sequential ensemble of the classifiers allowed for an accurate prediction of the domain with significantly lowered variance and considerably reduces the time required for training phase.

Conclusion

CCM is accurate, easy to operate, time and cost-effective, and thus, can serve as a valuable tool for prediction of drug diffusivity from mucoadhesive formulations. CCM may yield new insights into understanding how drugs are diffused from the carrier systems and exert their efficacies under various clinical conditions.

Increased water transport in PDMS silicone films by addition of excipients

The development of new adhesive wound care products intended for an application over a prolonged time requires good water transporting properties of the adhesive for the maintenance of a suitable environment around the wound. The ability of polydimethylsiloxane (PDMS)-based silicone films to transport water has led to its use in skin pressure-sensitive adhesives and it would be advantageous to find ways for controlling or increasing water transport across PDMS films in order to be able to develop improved skin adhesives. In this study we present a way to increase water transport in such films by the addition of hydrophilic excipients. Three hydrophilic additives, highly water-soluble sucrose and the two superabsorbent polymers (SAP) Carbopol® and Pemulen™, were investigated. The effect of the excipients was characterized by water transport studies, swelling tests, scanning electron microscopy imaging and confocal microscopy. The cross-linked polymers, primarily Pemulen™, were efficient water transport enhancers, whereas sucrose did not show any effect. The effect of the additives seemed to correlate with their water binding capacity. For SAPs the formation of a percolating structure by swollen polymer was also suggested, which enhances water penetration by the higher volume fraction of areas with a higher diffusion constant (swollen SAP), leading to a faster transport through the entire film.

Sustained release of protein from poly(ethylene glycol) incorporated amphiphilic comb like polymers

Amphiphilic comb like macromonomer containing hydrophilic poly(ethylene glycol) groups covalently linked to poly(hydromethyl siloxane) (PHMS) were prepared by hydrosilylation reaction. The epoxy reacting sites were introduced to this amphiphilic system by the reaction with allyl epoxy propyl ether (AEPE). Bovine serum albumin (BSA), a model protein drug was loaded to the PEG-PDMS system and very thin membranes were made from this macromonomer adopting solution casting technique. The in vitro protein release studies at various pH conditions showed a controlled release profile without exhibiting any initial burst. The control of the initial burst might be due to the strong linkages of the protein with the membrane and the aggregation of the protein at the surface. The morphology of the membrane before and after the protein release, and the mechanical strength were evaluated. The surface properties of the membrane were studied using the contact angle measurements.

Effects of intrinsic variables on release of sodium dodecyl sulfate from a female controlled drug delivery system

The release profile of sodium dodecyl sulfate (SDS), a potent microbicide, from a female controlled drug delivery system (FcDDS) made of Carbopol 934P and hydroxypropyl methylcellulose (HPMC) was evaluated using a newly developed in vitro Simulant Vaginal System (SVS). The major parameters involved in the release profiles of SDS were categorized as: (1) formulation variables (total loading weight of intravaginal delivery systems, SDS loading doses in intravaginal delivery systems); (2) intrinsic variables (vaginal fluid secretion rate, vaginal fluid pH); and (3) extrinsic variables (inserting position). In most conditions, about 70% of the loading dose of SDS was released from FcDDS within 6h of application. The release profile showed that concentrations needed for complete human papilloma virus (HPV) inactivation could be obtained within 10 min after the application. It was demonstrated that intrinsic variables (i.e., the rate and pH of vaginal fluid) played an integral role in determining the release profile of SDS, while loading dose of SDS in FcDDS did not significantly affect the percentage of the total amount of SDS released. It can be concluded that FcDDS can be exploited as a controlled delivery device for prevention against sexually transmitted diseases.

Influence of silicone elastomer solubility and diffusivity on the in vitro release of drugs from intravaginal rings

The in vitro release characteristics of eight low-molecular-weight drugs (clindamycin, 17beta-estradiol, 17beta-estradiol-3-acetate, 17beta-estradiol diacetate, metronidazole, norethisterone, norethisterone acetate and oxybutynin) from silicone matrix-type intravaginal rings of various drug loadings have been evaluated under sink conditions. Through modelling of the release data using the Higuchi equation, and determination of the silicone solubility of the drugs, the apparent silicone elastomer diffusion coefficients of the drugs have been calculated. Furthermore, in an attempt to develop a quantitative model for predicting release rates of new drug substances from these vaginal ring devices, it has been observed that linear relationships exist between the log of the silicone solubility of the drug (mg x ml(-1)) and the reciprocal of its melting point (K(-1)) (y=3.558x-9.620, R=0.77), and also between the log of the diffusion coefficient (cm(2) s(-1)) and the molecular weight of the drug molecule (g x mol(-1)) (y=-0.0068x-4.0738, R=0.95). Given that the silicone solubility and silicone diffusion coefficient are the major parameters influencing the permeation of drugs through silicone elastomers, it is now possible to predict through use of the appropriate mathematical equations both matrix-type and reservoir-type intravaginal ring release rates simply from a knowledge of drug melting temperature and molecular weight.

Effect of several factors on the mechanical properties of pressure-sensitive adhesives used in transdermal therapeutic systems

The effects of coating thickness, type of adhesive, and type and concentration of enhancer on the mechanical properties of two acrylic pressure-sensitive adhesives (PSAs) were investigated using a 2(4) factorial design and an optimization technique. Sixteen formulations containing 0% or 10% of either caprylic acid or methyl laurate in two different PSAs, namely Duro-Tak 87-2196 and Duro-Tak 87-2097, were prepared. The adhesive properties of these laminates were evaluated by applying the 90 degrees Dynamic Adhesive Strength Peel Test (90 degrees DASPT) and 1800 Release Liner Peel Test (180 degrees RLPT). Coating thickness, concentration of enhancer, and type of adhesive did affect the 90 degrees DASPT. For the 180 degrees RLPT, the most significant factors were coating thickness and concentration of enhancer, with a strong interaction observed between the two. Coating thickness and concentration of enhancer were also used to create mathematical models that correlated these factors with the mechanical properties of the PSAs. For this purpose, the optimization technique 3(2) was applied. It was found that the correlation of the above factors can be adequately described with polynomial equations, which can be used for predicting the mechanical properties of the laminates containing the above PSAs and methyl laurate (0%-10%).

Methoxy-polyethoxy side-chain silastomers as materials controlling drug delivery by diffusion flux

The density of a diffusion medium and the solubility of the diffusant in this material are predominant parameters which control the diffusion flux. Side-chain silastomers can be structurally modified in such a manner that density and polarity are widely varied and adjustable to distinct conditions. The synthesis of cross-linked methoxy-polyethoxy side-chain polysiloxanes is performed by reaction of alpha, omega-bis-(trimethyl-silyloxy)-poly-(methyl-hydrogen-siloxane), alpha, omega-divinyl-poly-(dimethyl-siloxane), and 4-propenyloxy-(4'-methoxy-polyethoxy)-biphenyl. Silastomer foils were obtained with side-chains having up to 9 ethoxy groups. Various silastomer types were characterized by DSC-measurements, polarization microscopy, uptake of water and salicylic acid (as model drug), and by permeation measurements. The polarity of the polymers depends on their contents of ethoxy groups influencing the uptake of polar substances. Polymers with penta-ethoxy and hepta-ethoxy groups at the side-chains take up water under swelling. The solubility and the distribution coefficients of salicylic acid are linearly correlated to the weight fractions of the methoxy-polyethoxy groups in all silastomer types. The temperature dependence of the distribution coefficients of the penta-ethoxy and hepta-ethoxy polymer types shows deviations from the Arrhenius equation. As the side-chains occupy considerable volumes, the density of the molecular packing within the cross-linked polysiloxane matrices is high. The arrangement of the side-chain domains, therefore, depends on the chain length of the cross-linker; the diffusion coefficients are influenced by this parameter. Evidence for the existence of a lyotropic liquid-crystalline phase was observed for one type of these polymers only. Methoxy-polyethoxy side-chain silastomers as membranes or matrices are suited for controlled drug delivery. Drug liberation rates and swelling by uptake of water can be widely altered by variations of the side-chain structures.

In vitro permeation study of a mucoadhesive drug delivery system for controlled delivery of nonoxynol-9

A carbopol 934P-based drug delivery system (AmDDS) was developed to achieve the dual-controlled delivery of Nonoxynol-9 (N-9), a spermicidal agent, and EDTA, a potentiator of spermicidal activity. This gel-type system made intimate contact with vaginal mucosa and maintained an effective drug concentration within the vagina for a prolonged period of time. An existing mathematical model, based on a unilayer diffusion membrane, was applied for describing permeation of N-9 through vaginal mucosa. Vaginal permeation of N-9 from AmDDS was negligible over the initial 5 hr, then increased in a Q versus t pattern, but remained low. A relationship was observed between the permeation rate (Js) of drug through vaginal mucosa and the release flux (Q/t1/2) from AmDDS, which showed that the permeation rate (Js) increased with increased square of the release flux [(Q/t1/2)2] in a hyperbolic manner. This result indicated that release of N-9 from AmDDS is still the rate-limiting step at doses within the tested range. EDTA, at concentration of up to 0.32%, did not change the permeation rate of N-9 through vaginal mucosa. It is thus concluded that the developed AmDDS can control the intravaginal delivery of N-9 as well as its permeation through vaginal mucosa.