A model of transluminal flow of an anti-HIV microbicide vehicle: Combined elastic squeezing and gravitational sliding

Vaginal semisolid products: Technological performance considering physiologic parameters

Vaginal semisolid products are frequently used to treat vaginal infections and atrophy-related symptoms of menopause. Formulations composition and the methods for their characterization, especially those developed concerning the target epithelia, are key tools to predict in vivo results at early stages of product development. However, recent studies on this subject have been almost exclusively focused on anti-HIV preparations. The aim of this work consists on improving traditional characterization methods by using physiological parameters in order to construct predictive tools to characterize a new ideal vaginal semisolid formulation whatever target it may have. Ten vaginal antimicrobial and hormonal products already available in the market were studied (Gino-Canesten®, Sertopic®, Dermofix®, Gyno-pevaryl®, Lomexin®, Gino Travogen®, Dalacin V®, Ovestin®, Blissel®, Colpotrophine®). Furthermore, Universal Placebo gel and Replens® were used for comparison. Products were characterized in terms of: pH and buffering capacity in a vaginal fluid simulant (VFS); osmolality - directly and upon dilution in VFS; textural parameters (firmness, adhesiveness and bioadhesion) using vaginal ex vivo porcine epithelium; and viscosity (including VFS dilution at 37°C and after administration on an ex vivo model). Interestingly, the majority of the tested commercial vaginal formulations did not present technological characteristics close to the ideal ones when tested under target biological conditions. The inclusion of such methodologic adaptations is expected to optimize cost-efficiency of new formulations development by predicting efficacy and safety profiles at early stages of product development.

Coupled gel spreading and diffusive transport models describing microbicidal drug delivery

Gels are a drug delivery platform that is being evaluated for application of active pharmaceutical ingredients, termed microbicides, that act topically against vaginal and rectal mucosal infection by sexually transmitted HIV. Despite success in one Phase IIb trial of a vaginal gel delivering tenofovir, problems of user adherence to designed gel application scheduling have compromised results in two other trials. The microbicides field is responding to this dilemma by expanding behavioral analysis of the determinants of adherence while simultaneously improving the pharmacological, biochemical, and biophysical analyses of the determinants of microbicide drug delivery. The intent is to combine results of these two complementary perspectives on microbicide performance and epidemiological success to create an improved product design paradigm. Central to both user sensory perceptions and preferences, key factors that underlie adherence, and to vaginal gel mucosal drug delivery, that underlies anti-HIV efficacy, are gel properties (e.g. rheology) and volume. The specific engineering problem to be solved here is to develop a model for how gel rheology and volume, interacting with loaded drug concentration, govern the transport of the microbicide drug tenofovir into the vaginal mucosa to its stromal layer. These are factors that can be controlled in microbicide gel design. The analysis here builds upon our current understanding of vaginal gel deployment and drug delivery, incorporating key features of the gel's environment, the vaginal canal, fluid production and subsequent gel dilution, and vaginal wall elasticity. These have not previously been included in the modeling of drug delivery. We consider the microbicide drug tenofovir, which is the drug most completely studied for gels: in vitro, in animal studies in vivo, and in human clinical trials with both vaginal or rectal gel application. Our goal is to contribute to improved biophysical and pharmacological understanding of gel functionality, providing a computational tool that can be used in future vaginal microbicide gel design.

Contact Line Instability of Gravity-Driven Flow of Power-Law Fluids

The moving contact line of a thin fluid film can often corrugate into fingers, which is also known as a fingering instability. Although the fingering instability of Newtonian fluids has been studied extensively, there are few studies published on contact line fingering instability of non-Newtonian fluids. In particular, it is still unknown how shear-thinning rheological properties can affect the formation, growth, and shape of a contact line instability. Our previous study (Hu and Kieweg, 2012) showed a decreased capillary ridge formation for more shear-thinning fluids in a 2D model (i.e. 1D thin film spreading within the scope of lubrication theory). Those results motivated this study's hypothesis: more shear-thinning fluids should have suppressed finger growth and longer finger wavelength, and this should be evident in linear stability analysis (LSA) and 3D (i.e. 2D spreading) numerical simulations. In this study, we developed a LSA model for the gravity-driven flow of shear-thinning films, and carried out a parametric study to investigate the impact of shear-thinning on the growth rate of the emerging fingering pattern. A fully 3D model was also developed to compare and verify the LSA results using single perturbations, and to explore the result of multiple-mode, randomly imposed perturbations. Both the LSA and 3D numerical results confirmed that the contact line fingers grow faster for Newtonian fluids than the shear-thinning fluids on both vertical and inclined planes. In addition, both the LSA and 3D model indicated that the Newtonian fluids form fingers with shorter wavelengths than the shear-thinning fluids when the plane is inclined; no difference in the most unstable (i.e. emerging) wavelength was observed at vertical. This study also showed that the distance between emerging fingers was smaller on a vertical plane than on a less-inclined plane for shear-thinning fluids, as previously shown for Newtonian fluids. For the first time for shear-thinning fluids, these results connect trends in capillary ridge and contact line finger formation in 2D models, LSA, and 3D simulations. The results can provide us insights on how to optimize non-Newtonian fluid properties to minimize a fingering instability in many industrial and biological applications.

Polymer-based nanocarriers for vaginal drug delivery

The vaginal delivery of various drugs is well described and its relevance established in current medical practice. Alongside recent advances and achievements in the fields of pharmaceutical nanotechnology and nanomedicine, there is an increasing interest in the potential use of different nanocarriers for the delivery of old and new pharmacologically active molecules with either therapeutic or prophylactic purposes. Nanosystems of polymeric nature in particular have been investigated over the last years and their interactions with mucosal fluids and tissues, as well as genital tract biodistribution upon vaginal administration, are now better understood. While different applications have been envisioned, most of the current research is focusing in the development of nano-formulations with the potential to inhibit the vaginal transmission of HIV upon sexual intercourse. The present work focuses its discussion on the potential and perils of polymer-based nanocarriers for the vaginal administration of different pharmacologically active molecules.

Vaginal drug distribution modeling

This review presents and applies fundamental mass transport theory describing the diffusion and convection driven mass transport of drugs to the vaginal environment. It considers sources of variability in the predictions of the models. It illustrates use of model predictions of microbicide drug concentration distribution (pharmacokinetics) to gain insights about drug effectiveness in preventing HIV infection (pharmacodynamics). The modeling compares vaginal drug distributions after different gel dosage regimens, and it evaluates consequences of changes in gel viscosity due to aging. It compares vaginal mucosal concentration distributions of drugs delivered by gels vs. intravaginal rings. Finally, the modeling approach is used to compare vaginal drug distributions across species with differing vaginal dimensions. Deterministic models of drug mass transport into and throughout the vaginal environment can provide critical insights about the mechanisms and determinants of such transport. This knowledge, and the methodology that obtains it, can be applied and translated to multiple applications, involving the scientific underpinnings of vaginal drug distribution and the performance evaluation and design of products, and their dosage regimens, that achieve it.

Modeling of Virion Collisions in Cervicovaginal Mucus Reveals Limits on Agglutination as the Protective Mechanism of Secretory Immunoglobulin A

Secretory immunoglobulin A (sIgA), a dimeric antibody found in high quantities in the gastrointestinal mucosa, is broadly associated with mucosal immune protection. A distinguishing feature of sIgA is its ability to crosslink pathogens, thereby creating pathogen/sIgA aggregates that are too large to traverse the dense matrix of mucin fibers in mucus layers overlying epithelial cells and consequently reducing infectivity. Here, we use modeling to investigate this mechanism of “immune exclusion” based on sIgA-mediated agglutination, in particular the potential use of sIgA to agglutinate HIV in cervicovaginal mucus (CVM) and prevent HIV transmission. Utilizing reported data on HIV diffusion in CVM and semen, we simulate HIV collision kinetics in physiologically-thick mucus layers–a necessary first step for sIgA-induced aggregation. We find that even at the median HIV load in semen of acutely infected individuals possessing high viral titers, over 99% of HIV virions will penetrate CVM and reach the vaginal epithelium without colliding with another virion. These findings imply that agglutination is unlikely to be the dominant mechanism of sIgA-mediated protection against HIV or other sexually transmitted pathogens. Rather, we surmise that agglutination is most effective against pathogens either present at exceedingly high concentrations or that possess motility mechanisms other than Brownian diffusion that significantly enhance encounter rates.

Vaginal deployment and tenofovir delivery by microbicide gels

Gels are one of the soft material platforms being evaluated to deliver topically acting anti-HIV drugs (microbicides) to the vaginal environment. For each drug, its loaded concentration, gel properties and applied volume, and frequency of dosing can be designed to optimize PK and, thence, PD. These factors also impact user sensory perceptions and acceptability. Deterministic compartmental modeling of vaginal deployment and drug delivery achieved by test gels can help delineate how multiple parameters characterizing drug, vehicle, vaginal environment, and dosing govern details of PK and PD and also gel leakage from the canal. Such microbicide delivery is a transport process combining convection, e.g., from gel spreading along the vaginal canal, with drug diffusion in multiple compartments, including gel, mucosal epithelium, and stroma. The present work builds upon prior models of gel coating flows and drug diffusion (without convection) in the vaginal environment. It combines and extends these initial approaches in several key ways, including: (1) linking convective drug transport due to gel spreading with drug diffusion and (2) accounting for natural variations in dimensions of the canal and the site of gel placement therein. Results are obtained for a leading microbicide drug, tenofovir, delivered by three prototype microbicide gels, with a range of rheological properties. The model includes phosphorylation of tenofovir to tenofovir diphosphate (which manifests reverse transcriptase activity in host cells), the stromal concentration distributions of which are related to reference prophylactic values against HIV. This yields a computed summary measure related to gel protection ("percent protected"). Analyses illustrate tradeoffs amongst gel properties, drug loading, volume and site of placement, and vaginal dimensions, in the time and space history of gel distribution and tenofovir transport to sites of its anti-HIV action and concentrations and potential prophylactic actions of tenofovir diphosphate therein.

Experimental and numerical models of three-dimensional gravity-driven flow of shear-thinning polymer solutions used in vaginal delivery of microbicides

HIV/AIDS is a growing global pandemic. A microbicide is a formulation of a pharmaceutical agent suspended in a delivery vehicle, and can be used by women to protect themselves against HIV infection during intercourse. We have developed a three-dimensional (3D) computational model of a shear-thinning power-law fluid spreading under the influence of gravity to represent the distribution of a microbicide gel over the vaginal epithelium. This model, accompanied by a new experimental methodology, is a step in developing a tool for optimizing a delivery vehicle's structure/function relationship for clinical application. We compare our model with experiments in order to identify critical considerations for simulating 3D free-surface flows of shear-thinning fluids. Here we found that neglecting lateral spreading, when modeling gravity-induced flow, resulted in up to 47% overestimation of the experimental axial spreading after 90 s. In contrast, the inclusion of lateral spreading in 3D computational models resulted in rms errors in axial spreading under 7%. In addition, the choice of the initial condition for shape in the numerical simulation influences the model's ability to describe early time spreading behavior. Finally, we present a parametric study and sensitivity analysis of the power-law parameters' influence on axial spreading, and to examine the impact of changing rheological properties as a result of dilution or formulation conditions. Both the shear-thinning index (n) and consistency (m) impacted the spreading length and deceleration of the moving front. The sensitivity analysis showed that gels with midrange m and n values (for the ranges in this study) would be most sensitive (over 8% changes in spreading length) to 10% changes (e.g., from dilution) in both rheological properties. This work is applicable to many industrial and geophysical thin-film flow applications of non-Newtonian fluids; in addition to biological applications in microbicide drug delivery.

Gravity-Driven Thin Film Flow of an Ellis Fluid

The thin film lubrication approximation has been studied extensively for moving contact lines of Newtonian fluids. However, many industrial and biological applications of the thin film equation involve shear-thinning fluids, which often also exhibit a Newtonian plateau at low shear. This study presents new numerical simulations of the three-dimensional (i.e. two-dimensional spreading), constant-volume, gravity-driven, free surface flow of an Ellis fluid. The numerical solution was validated with a new similarity solution, compared to previous experiments, and then used in a parametric study. The parametric study centered around rheological data for an example biological application of thin film flow: topical drug delivery of anti-HIV microbicide formulations, e.g. hydroxyethylcellulose (HEC) polymer solutions. The parametric study evaluated how spreading length and front velocity saturation depend on Ellis parameters. A lower concentration polymer solution with smaller zero shear viscosity (η₀), τ_1/2, and λ values spread further. However, when comparing any two fluids with any possible combinations of Ellis parameters, the impact of changing one parameter on spreading length depends on the direction and magnitude of changes in the other two parameters. In addition, the isolated effect of the shear-thinning parameter, λ, on the front velocity saturation depended on τ_1/2. This study highlighted the relative effects of the individual Ellis parameters, and showed that the shear rates in this flow were in both the shear-thinning and plateau regions of rheological behavior, emphasizing the importance of characterizing the full range of shear-rates in rheological measurements. The validated numerical model and parametric study provides a useful tool for future steps to optimize flow of a fluid with rheological behavior well-described by the Ellis constitutive model, in a range of industrial and biological applications.

Transient swelling, spreading, and drug delivery by a dissolved anti-HIV microbicide-bearing film

There is a widespread agreement that more effective drug delivery vehicles with more alternatives, as well as better active pharmaceutical ingredients (APIs), must be developed to improve the efficacy of microbicide products. For instance, in tropical regions, films are more appropriate than gels due to better stability of drugs at extremes of moisture and temperature. Here, we apply fundamental fluid mechanical and physicochemical transport theory to help better understand how successful microbicide API delivery depends upon properties of a film and the human reproductive tract environment. Several critical components of successful drug delivery are addressed. Among these are: elastohydrodynamic flow of a dissolved non-Newtonian film; mass transfer due to inhomogeneous dilution of the film by vaginal fluid contacting it along a moving boundary (the locally deforming vaginal epithelial surface); and drug absorption by the epithelium. Local rheological properties of the film are dependent on local volume fraction of the vaginal fluid. We evaluated this experimentally, delineating the way that constitutive parameters of a shear-thinning dissolved film are modified by dilution. To develop the mathematical model, we integrate the Reynolds lubrication equation with a mass conservation equation to model diluting fluid movement across the moving vaginal epithelial surface and into the film. This is a complex physicochemical phenomenon that is not well understood. We explore time- and space-varying boundary flux model based upon osmotic gradients. Results show that the model produces fluxes that are comparable to experimental data. Further experimental characterization of the vaginal wall is required for a more precise set of parameters and a more sophisticated theoretical treatment of epithelium.

Transient spreading and swelling behavior of a gel deploying an anti-HIV topical microbicide

Drug delivery of topical microbicidal molecules against HIV offers promise as a modality to prevent sexual transmission of the virus. Success of any microbicide product depends, in an interactive way, upon its drug (the microbicide active pharmaceutical ingredient, API) and its delivery system (e.g. a gel, film or intravaginal ring). There is a widespread agreement that more effective drug delivery vehicles, as well as better APIs, must be developed to improve the efficacy of microbicide products. Non-Newtonian gels are primary microbicide vehicles, but those to date have been created with limited understanding of how their properties govern their spreading and retention in the vagina, which, in turn, govern successful drug delivery. Here, we apply fundamental fluid mechanical and physicochemical transport theory to help better understand how successful microbicide API delivery depends upon properties of a gel and the vaginal environment. We address several critical components of this complex process, including: elastohydrodynamic flow of the bolus of a non-Newtonian fluid; and mass transfer due to inhomogeneous dilution of the gel by vaginal fluid contacting it along a moving boundary (the locally deforming vaginal epithelial surface). Local dilution of gel alters local rheological properties. We evaluated this experimentally, delin-eating the way that constitutive parameters of a shear-thinning gel are modified by dilution. We supplement the Reynolds lubrication equation with a mass conservation equation to model diluting fluid movement across the moving vaginal epithelial surface and into the gel bolus. This is a physicochemically complex phenomenon that is not well understood. We implement a boundary flux model based upon the elevated hydrodynamic pressures in the cells. Results show that this model produces fluxes that lie within the range of mean values that have been reported. Further experimental characterization of the vaginal wall is required for a more precise set of parameters and a more sophisticated theoretical treatment of epithelium.

Zinc acetate/carrageenan gels exhibit potent activity in vivo against high-dose herpes simplex virus 2 vaginal and rectal challenge

Topical microbicides that block the sexual transmission of HIV and herpes simplex virus 2 (HSV-2) are desperately needed to reduce the incidence of HIV infections worldwide. Previously we completed phase 3 testing of the carrageenan-based gel Carraguard. Although the trial did not show that Carraguard is effective in preventing HIV transmission during vaginal sex, it did show that Carraguard is safe when used weekly for up to 2 years. Moreover, Carraguard has in vitro activity against human papillomavirus (HPV) and HSV-2 and favorable physical and rheological properties, which makes it a useful vehicle to deliver antiviral agents such as zinc acetate. To that end, we previously reported that a prototype zinc acetate carrageenan gel protects macaques against vaginal challenge with combined simian-human immunodeficiency virus reverse transcriptase (SHIV-RT). Herein, we report the safety and efficacy of a series of zinc acetate and/or carrageenan gels. The gels protected mice (75 to 85% survival; P < 0.001) against high-dose (10(6)-PFU) HSV-2 vaginal or rectal challenge. In contrast, zinc acetate formulated in HEC (hydroxyethylcellulose; or the Universal Placebo) failed to protect mice against the high-dose vaginal HSV-2 challenge (similar to aqueous zinc acetate solution and the placebo controls). The gels were found to be effective spreading gels, exhibited limited toxicity in vitro, caused minimal damage to the architecture of the cervicovaginal and rectal mucosae in vivo, and induced no increased susceptibility to HSV-2 infection in a mouse model. Our results provide a strong rationale to further optimize and evaluate the zinc acetate/carrageenan gels for their ability to block the sexual transmission of HIV and HSV-2.

The consequences of yield stress on deployment of a non-Newtonian anti-HIV microbicide gel

A recent study in South Africa has confirmed, for the first time, that a vaginal gel formulation of the antiretroviral drug Tenofovir, when applied topically, significantly inhibits sexual HIV transmission to women [10]. However the gel for this drug, and anti-HIV microbicide gels in general, have not been designed using full understanding of how gel spreading and retention in the vagina govern successful drug delivery. Elastohydrodynamic lubrication theory can be applied to model such spreading of microbicide gels, which are inherently non-Newtonian [13,15]. A yield stress is emerging as one of the important properties of microbicide gel vehicle deployment, as this may improve retention within the vaginal canal. On the other hand, a yield stress may decrease the initial extent of the coating flow. Here, we first explain a certain yield stress paradox observed generally in many lubrication flows. Four conditions are determined, via scaling analysis, which mitigate the inconsistency in the use of lubrication theory to analyze the specific problem of elastic wall squeezing flow of yield stress fluid. Parameters characterizing these conditions are obtained experimentally for a test gel. Using them, it is shown that the lubrication approximation may be applied to the elastic wall-squeezing problem for this gel.

The effects of inhomogeneous boundary dilution on the coating flow of an anti-HIV microbicide vehicle

A recent study in South Africa has confirmed, for the first time, that a vaginal gel formulation of the antiretroviral drug Tenofovir, when topically applied, significantly inhibits sexual HIV transmission to women [Karim et al., Science 329, 1168 (2010)]. However, the gel for this drug and anti-HIV microbicide gels in general have not been designed using an understanding of how gel spreading and retention in the vagina govern successful drug delivery. Elastohydrodynamic lubrication theory can be applied to model spreading of microbicide gels [Szeri et al., Phys. Fluids 20, 083101 (2008)]. This should incorporate the full rheological behavior of a gel, including how rheological properties change due to contact with, and dilution by, ambient vaginal fluids. Here, we extend our initial analysis, incorporating the effects of gel dilution due to contact with vaginal fluid produced at the gel-tissue interface. Our original model is supplemented with a convective-diffusive transport equation to characterize water transport into the gel and, thus, local gel dilution. The problem is solved using a multi-step scheme in a moving domain. The association between local dilution of gel and rheological properties is obtained experimentally, delineating the way constitutive parameters of a shear-thinning gel are modified by dilution. Results show that dilution accelerates the coating flow by creating a slippery region near the vaginal wall akin to a dilution boundary layer, especially if the boundary flux exceeds a certain value. On the other hand, if the diffusion coefficient of boundary fluid is increased, the slippery region diminishes in extent and the overall rate of gel spreading decreases.

Using modeling to help understand vaginal microbicide functionality and create better products

A summary is presented of a range of mathematical models that relate to topical microbicidal molecules, applied vaginally to inhibit HIV transmission. These models contribute to the fundamental understanding of the functioning of those molecules, as introduced in different delivery systems. They also provide computational tools that can be employed in the practical design and evaluation of vaginal microbicide products. Mathematical modeling can be implemented, using stochastic principles, to understand the probability of infection by sexually transmitted HIV virions. This provides a frame of reference for the deterministic models of the various processes that underlie HIV transmission and its inhibition, including: the temporal and spatial history of HIV migration from semen to vaginal epithelial surfaces and thence to the underlying stroma; the time and spatial distribution of microbicidal drugs as delivered by various vehicles (e.g., gels, rings, films, and tablets)-this is central to understanding microbicide product pharmacokinetics; and the time and space history of the drug interactions with HIV directly and with host cells for HIV within the vaginal environment-this informs the understanding of microbicide pharmacodynamics. Models that characterize microbicide functionality and performance should and can interface with both in vitro and in vivo experimental studies. They can serve as a rapidly applied, inexpensive tool, to facilitate microbicide R&D, in advance of more costly and time consuming clinical trials.

Clinical evaluation of microbicide formulations

The clinical evaluation of microbicide formulations presents variable and interacting challenges. Specific domains of acceptability, pharmacokinetics, and toxicity interact with each other to potentially inhibit or enhance a microbicide's efficacy. Each of these is further influenced by application and use parameters in the relative presence (or absence) of vaginal and/or seminal fluids, ultimately impacting effectiveness. Historically, acceptability of formulation and use parameters, and their concomitant behavioral influences, have been considered separately from pharmacokinetics and toxicity. While independent evaluation of these elements is necessary in some respects, we must acknowledge that this approach is not sufficient for the successful development of microbicides. Each needs to be considered in an integrated clinical evaluation strategy. This article presents the rationale for such an approach. This article forms part of a special supplement covering two presentations on clinical evaluation of microbicides from the symposium on "Recent Trends in Microbicide Formulations" held on 25 and 26 January 2010, Arlington, VA.

Increasing the effectiveness of vaginal microbicides: a biophysical framework to rethink behavioral acceptability

BACKGROUND

Microbicide candidates delivered via gel vehicles are intended to coat the vaginal epithelium after application. The coating process depends on intrinsic biophysical properties of the gel texture, which restricts the potential choices for an effective product: the gel first must be physically synthesizable, then acceptable to the user, and finally applied in a manner promoting timely adequate coating, so that the user adherence is optimized. We present a conceptual framework anchoring microbicide behavioral acceptability within the fulfillment of the product biophysical requirements.

METHODS

We conducted a semi-qualitative/quantitative study targeting women aged 18-55 in Northern California to assess user preferences for microbicide gel attributes. Attributes included: (i) the wait time between application and intercourse, (ii) the gel texture and (iii) the trade-off between wait time and gel texture. Wait times were assessed using a mathematical model determining coating rates depending upon the gel's physical attributes.

RESULTS

71 women participated. Results suggest that women would independently prefer a gel spreading rapidly, in 2 to 15 minutes (P<0.0001), as well as one that is thick or slippery (P<0.02). Clearly, thick gels do not spread rapidly; hence the motivation to study the trade-off. When asked the same question 'constrained' by the biophysical reality, women indicated no significant preference for a particular gel thickness (and therefore waiting time) (P>0.10) for use with a steady partner, a preference for a watery gel spreading rapidly rather than one having intermediate properties for use with a casual partner (P = 0.024).

CONCLUSIONS

Biophysical constraints alter women's preferences regarding acceptable microbicide attributes. Product developers should offer a range of formulations in order to address all preferences. We designed a conceptual framework to rethink behavioral acceptability in terms of biophysical requirements that can help improve adherence in microbicide use ultimately enhancing microbicide effectiveness.

Design of a semisolid vaginal microbicide gel by relating composition to properties and performance

PURPOSE

Develop a preclinical in vitro algorithm enabling de novo design of semisolid vaginal drug delivery gels, by using biomechanical modeling of gel spreading in the vaginal canal and empirically relating gel composition to mechanical properties and predicted performance.

METHODS

Gel performance was defined through a multivariate objective function constructed from gels' mechanical properties and selected performance criteria for gel spreading within the vaginal canal. Mixture design of experiment was used to establish a semi-empirical relationship linking composition-property and property-performance relationships for gels with varying concentrations of hydroxyethylcellulose and Carbopol 974P. This permits definition of a local optimum for gel composition and volume of administration, within a defined gel composition space.

RESULTS

Rheological behavior and, consequently, the value of the objective function varied broadly with composition. The algorithm indicated a 3.0 wt% HEC gel as the near optimal composition for a 3.5 mL applied volume for gels designed to spread throughout the vagina.

CONCLUSIONS

The algorithm introduced herein is a novel tool that facilitates an understanding of the composition-property-performance relationship for vaginal semisolid drug delivery gels. This approach has promise as a scientific methodology for evaluation and optimization of vaginal gels prior to in vivo investigations.

Transport theory for HIV diffusion through in vivo distributions of topical microbicide gels

Topical microbicide products are being developed for the prevention of sexually transmitted infections. These include vaginally-applied gels that deliver anti-HIV molecules. Gels may also provide partial barriers that slow virion diffusion from semen to vulnerable epithelium, increasing the time during which anti-HIV molecules can act. To explore the barrier function of microbicide gels, we developed a deterministic mathematical model for HIV diffusion through realistic gel distributions. We applied the model to experimental data for in vivo coating distributions of two vaginal gels in women. Time required for a threshold number of virions to reach the tissue surface was used as a metric for comparing different scenarios. Results delineated how time to threshold increased with increasing gel layer thickness and with decreasing diffusion coefficient. We note that for gel layers with average thickness > approximately 100 microm, the fractional area coated, rather than the gel layer thickness, was the primary determinant of time to threshold. For gel layers < approximately 100 microm, time to threshold was brief, regardless of fractional area coated. Application of the model to vaginal coating data showed little difference in time to threshold between the two gels tested. However, the protocol after gel application (i.e., with or without simulated coitus) had a much more significant effect. This study suggests that gel distribution in layers of thickness >100 microm and fractional area coated >0.8 is critical in determining the ability of the gel to serve as a barrier to HIV diffusion.