Around 500 million cases of four of the major curable sexually transmitted infections (STIs), Chlamydia, gonorrhoea, syphilis and trichomoniasis, were recorded worldwide in 2008, an increase of 11% from 2005, confirming the need for improved prevention and treatment strategies. The chances of acquiring these infections are 8 times greater in women than men because of biological, social and cultural factors. Furthermore, the presence of these infections increases the chances of both acquisition and transmission of HIV/AIDS by causing genital ulcers and inflammation in the vagina.
The human vagina is considered to be a novel, non-invasive and safe route for drug delivery because of its rich blood supply, large surface area and low enzymatic activity. There are many vaginal preparations available in the market but conventional systems such as creams and gels are criticised because of messiness and leakage, and many vaginal formulations have the disadvantage of daily dosing. Intravaginal rings (IVRs) have potential advantages over other delivery systems, in that they can be used for prolonged periods of time, avoid messiness and sustained drug delivery is possible. Previous research has focussed on the use of IVRs for the prevention of HIV. This thesis describes the development of IVRs for the treatment of vaginal bacterial and fungal infections, including STIs.
The most important component of IVRs is the polymer used in their construction; currently IVRs are composed of silicone and polyethylene vinyl acetate but these require a high processing temperature and are suitable only for delivery of low molecular weight, hydrophobic drugs. In this thesis, the use of polycaprolactone (PCL) as a potential polymer for use in IVRs is investigated because of its perceived advantages over other polymers, such as low processing temperature and potential to deliver a wide range of drug molecules from hydrophobic to hydrophilic, and low-molecular to high molecular weight.
Following a review of the literature in Chapter 1, the delivery of metronidazole using PCL, which could be used for the treatment of bacterial vaginosis, is considered in Chapter 2. Delivery of metronidazole in an IVR could be a better option than the vaginal gel and oral tablet formulations that are currently available because an IVR would provide long term sustained delivery and would be expected to reduce the gastrointestinal side effects associated with oral delivery. PCL matrices loaded with different concentrations of metronidazole achieved an incorporation efficiency of 40-54%. The matrices were studied using a range of approaches including release into simulated vaginal fluid (SVF), morphological and drug distribution studies, thermal characterization and antibacterial activity against Gardnerella vaginalis (one of the main bacteria implicated in bacterial vaginosis). First day burst release occurred due to the presence of drug crystals at the surface of the PCL which was confirmed by scanning electron microscopy and Raman microscopy. Even considering the small decrease in antibacterial activity that was measured during the 14 day test period, drug release on each of the 14 days was greater than the minimum inhibitory concentration (MIC) against G. vaginalis. The effect of polyethylene glycol (PEG) addition on drug release and mechanical properties of the PCL was then investigated. Different concentrations of PEG were loaded into PCL matrices along with 10% w/w metronidazole. Increasing the concentration of PEG enhanced both drug loading and the amount of daily drug release, but this was associated with a negative effect on the mechanical properties of polymer and made it more soft and brittle. It was concluded that PCL has potential to be a useful polymer for use in IVR delivery of metronidazole but that there was still capacity to improve drug loading.
Chapter 3 describes experiments with PCL matrices loaded with doxycycline, which can be used for the treatment of gonorrhoea and Chlamydia. A slight alteration to the method of production of PCL matrices enabled 100% drug loading to be obtained. Following the same suite of tests described for Chapter 2, it was concluded that these PCL matrices can deliver doxycycline effectively for 14 days, and the concentrations released in vitro on each of the 14 days were greater than the minimum inhibitory concentration (MIC) against several pathogens that are sexually transmitted. Additionally, the toxicity of PCL leachates was tested on the vaginal cell line VK2/E6E7 and found to be safe for vaginal delivery. There are currently no commercial vaginal preparations of doxycycline so this study could be useful in introducing a new drug delivery system for doxycycline through the intravaginal route.
In Chapter 4, the ability of PCL to deliver a combination of drugs is investigated. Metronidazole and doxycycline are the combination involved, which are used together for the treatment of pelvic inflammatory disease. PCL matrices loaded with different concentrations of metronidazole and doxycycline were tested for in vitro drug release, morphological, thermal and antibacterial testing to investigate whether the combination compromised the effectiveness of the PCL matrix in drug delivery. Excellent drug loading of both drugs was obtained, and the concentrations released for each of 14 days were associated with a high level of antibacterial activity. This is the first investigation of the vaginal delivery of metronidazole and doxycycline in combination.
The delivery of a macromolecular protein using PCL matrices is investigated in Chapter 5. This study is based on the fact that vaginal vaccination is potentially a more effective means of eliciting a strong localized immune response against HIV, in comparison to conventional intramuscular or intranasal routes of administration, and most of these vaccines are either proteins or peptides in nature. Lactoferrin was used as a model protein for the study because of its reported activity against herpes simplex virus, Chlamydia trachomatis and HIV. PCL matrices proved suitable for the delivery of lactoferrin; the integrity of the protein was retained as shown using SDS-PAGE of the protein following loading into PCL and release into SVF.
These findings could provide a breakthrough in the field of vaginal drug delivery for the treatment of STIs and could also reduce the risk of HIV infection. Using PCL matrices in intravaginal rings for vaginal delivery of microbicides as a strategy to treat vaginal infections and STIs is expected to reduce the risks of drug resistance, treatment failure and gastrointestinal adverse effects associated with long-term oral drug delivery, by improving compliance with treatment duration and avoiding systemic adverse drug reactions. Future research should investigate the mechanical properties of PCL, which may limit the preparation of simple IVRs, and consider the use of drug-loaded PCL as inserts within a flexible inert IVR, or covering the drug-loaded PCL IVR with a flexible inert material with delivery windows that allow drug release. Mixing PCL with a second polymer such as polyethylene may also be considered but it would be important not to destroy the usefulness of PCL in terms of its low processing temperature.