A silicone elastomer vaginal ring for HIV prevention containing two microbicides with different mechanisms of action

Vaginal rings are currently being developed for the long-term (at least 30 days) continuous delivery of microbicides against human immunodeficiency virus (HIV). Research to date has mostly focused on devices containing a single antiretroviral compound, exemplified by the 25 mg dapivirine ring currently being evaluated in a Phase III clinical study. However, there is a strong clinical rationale for combining antiretrovirals with different mechanisms of action in a bid to increase breadth of protection and limit the emergence of resistant strains. Here we report the development of a combination antiretroviral silicone elastomer matrix-type vaginal ring for simultaneous controlled release of dapivirine, a non-nucleoside reverse transcriptase inhibitor, and maraviroc, a CCR5-targeted HIV-1 entry inhibitor. Vaginal rings loaded with 25 mg dapivirine and various quantities of maraviroc (50– 400 mg) were manufactured and in vitro release assessed. The 25 mg dapivirine and 100 mg maraviroc formulation was selected for further study. A 24-month pharmaceutical stability evaluation was conducted, indicating good product stability in terms of in vitro release, content assay, mechanical properties and related substances. This combination ring product has now progressed to Phase I clinical testing.

Modified Silicone Elastomer Vaginal Gels for Sustained Release of Antiretroviral HIV Microbicides

We previously reported nonaqueous silicone elastomer gels (SEGs) for sustained vaginal administration of the CCR5-targeted entry inhibitor maraviroc (MVC). Here, we describe chemically modified SEGs (h-SEGs) in which the hydrophobic cyclomethicone component was partially replaced with relatively hydrophilic silanol-terminated polydimethylsiloxanes (st-PDMS). MVC and emtricitabine (a nucleoside reverse transcriptase inhibitor), both currently under evaluation as topical microbicides to counter sexual transmission of human immunodeficiency virus type 1 (HIV-1), were used as model antiretroviral (ARV) drugs. Gel viscosity and in vitro ARV release were significantly influenced by st-PDMS molecular weight and concentration in the h-SEGs. Unexpectedly, gels prepared with lower molecular weight grades of st-PDMS showed higher viscosities. h-SEGs provided enhanced release over 24 h compared with aqueous hydroxyethylcellulose (HEC) gels, did not modify the pH of simulated vaginal fluid (SVF), and were shown to less cytotoxic than standard HEC vaginal gel. ARV solubility increased as st-PDMS molecular weight decreased (i.e., as percentage hydroxyl content increased), helping to explain the in vitro release trends. Dye ingression and SVF dilution studies confirmed the increased hydrophilicity of the h-SEGs. h-SEGs have potential for use in vaginal drug delivery, particularly for ARV-based HIV-1 microbicides.

Evaluation of the Factors Contributing to Levonorgestrel Binding in Addition Cure Silicone Elastomer Vaginal Rings

Background: Following progress of the dapivirine (DPV)-releasing silicone elastomer (SE) vaginal ring (VR) into Phase III clinical studies, there is now interest in developing next-generation rings that additionally provide contraception. Levonorgestrel (LNG) is a safe and effective progestin that is being widely considered for use as a hormonal contraceptive agent in future multipurpose prevention technology (MPT) products. Although LNG has previously been incorporated into various controlled release SE devices, minimal attention has focused on its propensity to irreversibly react with addition cure SE systems. Here, for the first time, we investigate this LNG binding phenomenon and outline strategies for overcoming it.
Methods: VRs containing various loadings of DPV and LNG were manufactured and in vitro release assessed. Different LNG-only SE samples were also prepared to assess the following parameters: (i) addition cure vs. condensation cure SEs; (ii) different types of addition cure SEs; (iii) mixing time, (iv) cure temperature, (v) cure time; and (vi) LNG particle size. After manufacture, the LNG-only samples were assayed for total drug content using a solvent extraction method. The SE curing reaction and the LNG binding reaction was probed using nuclear magnetic resonance (NMR) spectroscopy. Results:
Under certain drug/formulation/processing conditions, LNG was not recoverable from VRs. Further studies using non-ring samples showed that: (a) the phenomenon was only observed with addition cure SEs (and not condensation cure SEs); (b) the extent of binding was dependent upon the type of addition cure SE; (c) micronised LNG showed significantly greater binding than non-micronised LNG; (d) the extent of binding correlated with increased mixing time, cure time and cure temperature.
Conclusions: Careful control of the API characteristics, the SE composition, and the manufacturing conditions will be necessary to establish a practical VR formulation for controlled release of LNG.

Design of a silicone reservoir intravaginal ring for the delivery of oxybutynin.

Oxybutynin, a drug of choice in the treatment of urinary incontinence, has low oral bioavailability due to extensive first-pass metabolism. A toxic metabolite, N-desethyloxybutynin, has been linked to adverse reactions to oral oxybutynin. This study, therefore, reports on the design of an oxybutynin intravaginal ring (IVR) of reservoir design, comprising an oxybutynin silicone elastomer core encased in a non-medicated silicone sheath, manufactured by reaction injection moulding at 50oC. An unusually high initial burst release of oxybutynin (42.7 mg in 24 h) was observed in vitro with a full length core (100 mg drug loading), with subsequent non-zero order drug release. Use of fractional segment cores substantially reduced the burst effect, yielding linear cumulative drug release versus time plots from days 2 to 14. Thus, a 1/8 fractional segment core gave a 24 h burst of 11.28 mg oxybutynin and, thereafter, zero order release at the target dose of 5 mg/day over 14 days. Two oxybutynin cores, each 1/16 of full length, gave a greater release than a single 1/8 core, due to core segment end effects resulting in an increased surface area for release. The burst release was investigated by determining drug solubilities in the propan-1-ol product of elastomer condensation cure (390 mg/ml) and in the elastomer itself (13.9-20.21 mg/ml, by direct extraction and indirect thermal methods). These high oxybutynin solubilities were considered the major contributors to the burst effect. It was concluded that use of a fractional segment core would allow development of a suitable oxybutynin reservoir IVR.

High speed DSC (hyper-DSC) as a tool to measure the solubility of a drug within a solid or semi-solid matrix

Conventional differential scanning calorimetry (DSC) techniques are commonly used to quantify the solubility of drugs within polymeric-controlled delivery systems. However, the nature of the DSC experiment, and in particular the relatively slow heating rates employed, limit its use to the measurement of drug solubility at the drug's melting temperature. Here, we describe the application of hyper-DSC (HDSC), a variant of DSC involving extremely rapid heating rates, to the calculation of the solubility of a model drug, metronidazole, in silicone elastomer, and demonstrate that the faster heating rates permit the solubility to be calculated under non-equilibrium conditions such that the solubility better approximates that at the temperature of use. At a heating rate of 400 degrees C/min (HDSC), metronidazole solubility was calculated to be 2.16 mg/g compared with 6.16 mg/g at 20 degrees C/min. (C) 2005 Elsevier B.V. All rights reserved.

Development and evaluation of a vaginal ring device for sustained delivery of HIV microbicides to non-human primate

Background There is considerable interest in developing coitally indepen- dent, sustained release formulations for long-term administration of HIV microbicides. Vaginal ring devices are at the forefront of this formulation strategy. Methods Non-medicated silicone elastomer vaginal rings were prepared having a range of appropriate dimensions for testing vaginal ?t in pig- tailed and Chinese rhesus macaques. Cervicovaginal proin?ammatory markers were evaluated. Compression testing was performed to compare the relative ?exibility of various macaque and commercial human rings. Results All rings remained in place during the study period and no tissue irritation or signi?cant induction of cervicovaginal proin?ammatory mark- ers or signs of physical discomfort were observed during the 8-week study period. Conclusions Qualitative evaluation suggests that the 25 · 5-mm ring pro- vided optimal ?t in both macaque species. Based on the results presented here, low-consistency silicone elastomers do not cause irritation in maca-

Persistence of antimicrobial activity through sustained release of triclosan from pegylated silicone elastomers

Microbial adhesion to silicone elastomer biomaterials is a major problem often resulting in infection and medical device failure. Several strategies have been employed to modulate eukaryotic cell adhesion and to hamper bacterial adherence to polymeric biomaterials. Chemical modification of the surface by grafting of polyethylene glycol (PEG) chains or the incorporation of non-antibiotic antimicrobial agents such as triclosan into the biomaterial matrix may reduce bacterial adhesion. Here, such strategies are simultaneously applied to the preparation of both condensation-cure and addition-cure silicone elastomer systems, seeking a sustained release antimicrobial device biomaterial. The influence of triclosan incorporation and degree of pegylation on antimicrobial release, surface microbial adherence and persistence (Escherichia coli and Staphylococcus epidermidis) were evaluated in vitro. Non-pegylated silicone elastomers provided an increased percentage release of triclosan extending over a relatively short duration (99% release by day 64) compared with their pegylated (4% w/w) counterparts (65% and 72% release by day 64, for condensation and addition-cure systems respectively). Viable E. coli adherence to a non-pegylated silicone elastomer containing 1% w/w triclosan was reduced by over 99% after 24 h compared to the non-pegylated silicone elastomer containing no triclosan. No viable S. epidermidis adhered to any of the triclosan-loaded (>0.1% w/w) formulations other than the control. Persistence of the antimicrobial activity of the triclosan-loaded pegylated silicone elastomers continued for at least 70 days compared to the triclosan-loaded non-pegylated elastomers (at least 49 days). Understanding how PEG affects the release of triclosan from silicone elastomers may prove useful in the development of a biomaterial providing prolonged, effective antimicrobial activity.

A two compartment in vitro release model for the testing of HIV microbicide formulations

BACKGROUND: In vitro release testing of vaginal formulations is usually performed in a one-compartment model (OCM) where the release medium, usually comprising pH-adjusted water, an aqueous surfactant solution or a solvent-water solution, provides sink conditions throughout the release experiment. Although this model is useful in evaluating the effect of formulation parameters upon release, it rarely reflects in vivo conditions. Here we report use of a two-compartment diffusion cell model (TCM, comprising a small volume donor, a large volume receptor, and separated by a model epithelial membrane) to more closely mimic in vivo vaginal release and tissue absorption following administration of a UC781 vaginal ring.

METHODS: Macaque-sized matrix silicone elastomer vaginal rings containing 100mg UC781 were prepared by injection molding, and in vitro release testing performed using both OCM (20mL simulated vaginal fluid, SVF) and TCM (5mL SVF in donor cell and variable quantities of Tween 80; silicone elastomer membrane; 100mL 3:2 ethanol/water in receptor cell). In the TCM, drug levels were measured by HPLC in both donor and receptor cells, representing fluid and tissue levels respectively. Rings containing 100mg UC781 and 10% w/w Tween 80 were also manufactured and tested.

RESULTS: The amount of UC781 released from rings was significantly influenced by the choice of release model. Greatest release (56mg/14 days) was measured in the ethanol/water OCM, compared with no measurable release into SVF only. Increasing the concentration of Tween 80 in the SVF medium (1, 3 and 5% w/w) led to increased UC781 release (11, 16 and 18mg, respectively), demonstrating that vaginal fluid solubility of UC781 may be rate-determining in vivo. In the TCM, UC781 accumulates in the receptor cell medium over time, despite not being measured in the donor medium containing the ring device. Incorporation of Tween 80 directly into the ring provided enhanced release in both donor and receptor cells.

CONCLUSIONS: Release of UC781 was influenced by the choice of release medium and the inclusion of Tween 80 in the ring. Although use of SVF-only in the OCM indicated no measurable UC781 release from rings, data from the TCM confirms that UC781 is not only released but is also capable of penetrating across the model epithelial membrane. The TCM may therefore provide a more representative in vitro release model for mimicking in vivo absorption.

Pre-treatment with Depo-Provera modifies the pharmacokinetics of CMPD167 in rhesus macaques following vaginal ring administration

BACKGROUND: Vaginal ring devices are being developed to provide sustained release of HIV microbicides. To date, only limited pharmacokinetic data is available from animal or human studies. Here we report the effect of Depo-Provera (DP) pre- treatment, commonly used to thin the vaginal epithelium in challenge experiments, on the pharmacokinetic profile of CMPD167 (a small molecule CCR5 co-receptor antagonist) in rhesus macaques following vaginal ring administration.

METHODS: A single 400mg CMPD167 silicone elastomer vaginal ring was inserted into each of twelve female rhesus macaques. Six macaques were treated with (DP) 30 days before ring placement; the other six macaques were untreated. Blood, vaginal fluid and vaginal biopsies were collected prior to and at various times during 28 days of ring placement and assayed for CMPD167 levels by HPLC. Rings were assayed for residual CMPD167 at the end of the study and the calculated amount of CMPD167 released in vivo compared with in vitro release data.

RESULTS: Vaginal fluid, plasma and tissue levels of CMPD167 were detectable throughout ring placement. Significant differences were observed in mean daily vaginal fluid levels between the DP-treated (16–56 mcg/mL) and untreated groups (48–181 mcg/mL). Plasma CMPD167 levels were significantly higher peaking at 4 ng/mL and maintaining levels of 1–2 nM throughout the 14 days of testing in animals pre-treated with DP compared to non DP-treated macaques (<1 ng/mL maintained). Tissue levels were varied between 2–10 g/mL CMPD167 with no significant difference between the DP-treated and untreated macaques.

CONCLUSIONS: The study demonstrates that clinically relevant, and possibly protective doses of CMPD167 are released in the vaginal vault of rhesus macaques from vaginal rings through 28 days duration. DP is known to induce vaginal epithelial thinning and lower vaginal fluid levels, which accounts for the increased plasma levels of CMPD167. In contrast, macaques not treated with DP had minimal absorption into plasma compartments and significantly higher levels of CMPD167 in the vagina, similar to those previously shown to be protective against vaginal challenge.

Rheological Evaluation of the Isothermal Cure Characteristics of Medical Grade Silicone Elastomers

Silicone elastomer systems have been shown to offer potential for the fabrication of medical devices and sustained release drug delivery devices comprising low molecular weight drugs and protein therapeutics. For drug delivery systems in particular, there is often no clear rationale for selection of the silicone elastomer grade, particularly in respect of optimizing the manufacturing conditions to ensure thermal stability of the active agent and short cycle times. In this study, the cure characteristics of a range of addition-cure and condensation-cure, low-consistency, implant-grade silicone elastomers, either as supplied or loaded with the model protein bovine serum albumin (BSA) and the model hydrophilic excipient glycine, were investigated using oscillatory rheology with a view to better understanding the isothermal cure characteristics. The results demonstrate the influence of elastomer type, cure temperature, protein loading, and glycine loading on isothermal cure properties. By measuring the cure time required to achieve tan delta values representative of early and late-stage cure conditions, a ratio t(1)/t(2) was defined that allowed the cure characteristics of the various systems to be compared. Sustained in vitro release of BSA from glycine-loaded silicone elastomer covered rod devices was also demonstrated over 14 days. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2320-2327, 2010

Pharmacokinetics of UC781-loaded intravaginal ring segments in rabbits: a comparison of polymer matrices

UC781 is a potent and poorly water-soluble nonnucleoside reverse transcriptase inhibitor being investi- gated as a potential microbicide for preventing sexual transmission of HIV-1. This study was designed to evaluate the in vivo release and pharmacokinetics of UC781 delivered from matrix-type intravaginal ring segments in rabbits. Three polymer matrices (polyurethane, ethylene vinyl acetate copolymer, and silicone elastomer) and two drug loadings (5 and 15 mg/segment) were evaluated in at least one of two independent studies for up to 28 days in vivo. Inter-study comparison of in vivo release, vaginal tissue, and plasma concentrations for similar formulations demonstrated good reproducibility of the animal model. Mean estimates for a 28-day in vivo release ranged from 0.35 to 3.17 mg UC781 per segment. Mean proximal vaginal tissue levels (adjacent to the IVR segment) were 8– 410 ng/g and did not change significantly with time for most formulations. Distal vaginal tissue levels of UC781 were 6- to 49-fold lower than proximal tissue levels. Mean UC781 plasma levels were low for all formulations, at 0.09–0.58 ng/mL. All formulations resulted in similar UC781 concentrations in vaginal tissue and plasma, except the low loading polyurethane group which provided significantly lower levels. Loading dependent release and pharmacokinetics were only clearly observed for the polyurethane matrix. Based on these results, intravaginal ring segments loaded with UC781 led to vaginal tissue concen- trations ranging from below to approximately two orders of magnitude higher than UC781’s EC50 under in vitro conditions (2.8 ng/mL), with little influence by polymer matrix or UC781 loading. Moreover, these findings support the use of rabbit vaginal pharmacokinetic studies in preclinical testing of microbicide intravaginal rings.

Sustained Release of the CCR5 Inhibitors CMPD167 and Maraviroc from Vaginal Rings in Rhesus Macaques

Antiretroviral entry inhibitors are now being considered as vaginally administered microbicide candidates for prevention of sexual transmission of human immunodeficiency virus. Previous studies testing the entry inhibitors maraviroc and CMPD167 in aqueous gel formulations showed efficacy in the macaque challenge model, although protection was highly dependent on the time period between initial gel application and subsequent challenge. In this paper, we describe the sustained release of the entry inhibitors maraviroc and CMPD167 from matrix-type silicone elastomer vaginal rings both in vitro and in vivo. Both inhibitors were released continuously over 28 days from rings in vitro, at rates of 100-2500 µg/day. In 28-day pharmacokinetic studies in rhesus macaques, the compounds were measured in the vaginal fluid and vaginal tissue; steady state fluid concentrations were ~106 fold greater than IC50 values for SHIV-162P3 inhibition in macaque lymphocytes in vitro. Plasma concentrations for both compounds were very low. Pretreatment of macaques with Depo-Provera® (DP), as commonly used in macaque challenge studies, was shown to significantly modify the bio-distribution of the inhibitors, but not the overall amount released. Vaginal fluid and tissue concentrations were significantly decreased while plasma levels increased with DP pretreatment. These observations have implications for designing macaque challenge experiments, and also for ring performance during the human female menstrual cycle. Copyright © 2012, American Society for Microbiology. All Rights Reserved.