Microbicide product development has posed unique challenges to the field that stem from the simple fact that there are currently no regulatory approved microbicides on the market. This situation adds to the challenge of not only knowing which assays might be predictive of identifying safe and efficacious microbicides; it also means that most of the assays need to be developed or tailor-made to be as representative as possible of the physiological events of mucosal transmission of HIV. The following overview gives a brief summary of a sound microbicide product development strategy.
Twenty years ago, when the microbicide field began to evolve, the applicability of regulatory development guidelines to microbicides was uncertain. Today there are more comprehensive guidelines by the International Conference on Harmonisation (ICH) [1–3]. The US Food and Drug Administration (FDA) has even begun to recommend the use of specific guidelines in the microbicide development context. For example, the FDA has created a specific office within the Division of Antivirals to handle pre-investigational new drug (IND) consultations with microbicide clinical trial sponsors. That office has also recommended the use of particular guidance documents in the context of microbicide research . Use of this guidance will aid to streamline the approach to microbicide product development, while keeping in mind that a justifying rationale will be needed when deviation from the guidelines occur, which often happens.
A stepwise product development strategy should be embarked upon that lends itself to be harmonious with the categories of the ICH Common Technical Document: quality, safety, and efficacy. Although the development pathway evolves increasing complexity concurrent with conducting clinical development, a sound basic initial development profile needs to be established. This fact will enable microbicide developers to meet the basic requirements necessary for filing an IND application with the FDA, or similar application for foreign regulatory, which when approved will allow clinical testing to commence.
The key development parameters described below will map out a stepwise approach to establish sound product development profiles. The complexity of a microbicide will determine the extent to which development profiles need to be undertaken.
The quality of a microbicide affects the establishment of critical parameters for both the microbicide active pharmaceutical ingredient, or drug substance, and the actual manufactured microbicide, or drug product. The initial stage of quality establishment begins on the laboratory bench in the discovery stage with the chemistry of the drug substance and product. The parameters for quality profiles should include, minimumly, the source and process of manufacturing, control testing, reference standards, container closure systems, and stability [1,4–9]. The four key parameters – chemistry, manufacturing, control testing and stability – will be discussed below.
During the discovery process, chemistry profiles are initiated during the synthesis, extraction, or refinement of the drug substance. This task can be done by a research institution, a chemical manufacturer, small biotech nology company, or a large pharmaceutical company. The necessary steps to produce the drug substance, which include any critical and reprocessing steps, are determined, as well as identification of starting materials, reagents, solvents excipients and preservatives, characterization of degradants, and identifying and establishing acceptable limits for impurities. The level of impurities could differ between drug substance and product used in nonclinical testing from those used in clinical testing and commercial production. Prior to use in clinical testing the impurity profile will need to include how the limits are qualified. The parameters that constitute the chemical analysis and identification that allows for distinguishing the drug substance and product from any others will be utilized in the development of the pharmacopeia monograph [10,11].
Drug substance and drug product
Initial product development often starts with testing the drug substance that is produced from a single production lot of material. This initial testing should be conducted on a minimum of three different production lots of the drug substance as early as possible in order to establish the reproducibility of the manufacturing process. Similarly with the drug product, the initial process of manufacturing substance to product should be done a minimum of three times to establish production consistency and reproducibility. Once the manufacturing production is established, a validation of the process can be conducted using the different lots of drug substance to produce the drug product.
Early on in the development process the drug product is most probably manufactured in the laboratory setting. This fact may even hold true for the drug substance. All manufacturing should be carried out in accordance with current good laboratory practices (cGLP) . Once the product enters into clinical testing, manufacturing needs to be done in accordance with current good manufacturing practices (cGMP)  even if manufacturing occurs in the laboratory setting.
Ideally, there would be more than one source for the drug substance, which would allow for more competitive pricing; but quite often this is not the case, particularly for those substances that are under patent protection.
Source and process of manufacturing
Wherever the source for manufacture of the drug substance or drug product occurs, it is necessary to have all the steps of the process documented. Standard operating procedures (SOPs) should be developed early that include all starting materials, essential equipment, and a systematic description of the process. It is not uncommon that, over the course of development, changes might occur in the manufacturing procedures or even in the manufacturer. This situation is particularly true as production scale-up is necessary to meet the needs of clinical trial materials. This factor will require amendment of the SOP and documentation of the rationale for any changes. It is also extremely important to document all deviations that occur during the manufacturing process used for clinical study product production, as this manufacturing process must remain the same for commercial production.
Developing parameters for control testing is key to ensuring the quality integrity of the drug substance and product. For each parameter the acceptable limits need to be established, as do the analytical procedures and validation of the testing. In some instances the control testing simply involves applying the criteria set forth in the United States Pharmacopeia/The National Formulary (USP). Examples would be demonstrated either by using the procedures listed in the official monograph or by using the testing procedures, such as the USP antimicrobial preservatives effectiveness test for control testing, to ensure that a current preservative meets the standard specifications. In other instances, in which the parameters are unique to the substance or product, new assays may need to be developed or existing assays modified. Whichever might be the case, it is essential that assays are validated for all control testing.
Stability is a key criterion, second only to safety, in developing a microbicide that is accessible, competitively priced, and available over-the-counter (OTC). Of these, product pricing would be the most influenced by the stability profile. Even if the active ingredient is inexpensive, a short shelf life will drive up the final price due to the need for more frequent manufacturing and distribution. In addition, a microbicide that has a short shelf life will need to be more closely regulated once it is on the market, which is best achieved as a prescription drug. In addition, as the majority of new HIV infections occur through heterosexual intercourse in the developing world, a microbicide would need to withstand long, and often adverse, storage conditions in order to be beneficial to the most needed segment of the population.
Stability profiles should be set up to monitor the chemical interaction between product components that can cause degradation or inactivation of the drug substance. Stability testing should include evaluation of activity, appearance and chemical identity. Depending on the type of product, other parameters may include pH and viscosity for gels, and drug release rates for rings. A good stepwise approach to establish a stability profile would be to initiate a short-term evaluation under moderate storage condition together with testing conducted on a rather frequent basis. Subsequent evaluations should be for intermediate and accelerated assessment, which would involve longer duration of storage and under more adverse conditions. Promising products should be tested under stress conditions, such as increased higher temperatures and humidity, oxidation, hydrolysis, and photolysis. Eventually, long-term conditions should be monitored to establish a reasonable shelf life and storage conditions.
Developing a safe microbicide is paramount. Concerns over safety have been heightened in the past year when some microbicide trials have had a higher incidence of seroconversion in the product group than the placebo group. A less than exemplary safety profile would be a leading cause for product elimination from the development pipeline. Perhaps more than any other field of drug development, safety plays a major role in development. After all, the intended use of a microbicide will be people in general good health, conceivably with use on a frequent basis and over an extended period, and most women will be of child-bearing age. Unlike some prescription therapeutics, undesirable side effects would not be a concession for the beneficial effects received from this prevention technology.
Establishing a sound safety profile often constitutes the largest portion of development testing. Areas of evaluation include general parameters of pharmacodynamics and pharmacokinetics, and toxicology including local tolerance and reproductive and developmental toxicity.
Pharmacodynamics and pharmacokinetics
Pharmacodynamics evaluates what effect a drug has on the body, and pharmacokinetics evaluates the body's impact on the drug. These two safety parameters in many cases are very closely linked and one of the most essential aspects of any drug development. An example of how closely they are linked would be to evaluate the effect a microbicide might have on the naturally occurring vaginal flora, pharmacodynamics. Is the composition of the microbicide such that it would kill or inhibit the growth of the flora, or would the microbicide act as a substrate for the bacteria and cause it to proliferate? For evaluating the pharmacokinetics the question would become is the vaginal flora going to affect the microbicide in some way to inactivate or degrade the product? Because the two parameters are often so closely linked, or sometimes seemingly intertwined, they are often studied together as pharmacodynamics–pharmacokinetics.
This segment of safety evaluates what happens to the drug and to the body during the time from the point of introduction of drug into the body to elimination or clearance of drug from the body. The parameters focus on local or systemic absorption and distribution, metabolism and excretion. For microbicides that are not systemically absorbed, the extent of pharmacodynamics–pharmacokinetics will be considerably less, as well as reducing the risk of systemic side effects.
A key pharmacokinetics parameter is the time that a microbicide remains active in the body. Ideally a microbicide would be active upon insertion and remain active over a long period of time. This fact would give the user more flexibility, which conceivably would make the product more acceptable. In addition, if a microbicide was not coital dependent, adherence of product use may increase, which could be a confounder in demonstrating product effectiveness in clinical trials.
Local tolerance and toxicology
Testing drug substance and product in an array of assays to determine local tolerance is essential. One mode of HIV infection is through micro-ulcerations that can be caused by irritation. Therefore, it is imperative that a microbicide be nonirritating, especially when considering that it may be used frequently.
There are many in-vitro assays that can be used in initial evaluations to screen candidate drug substances and products. Selected assays might begin with an evaluation of toxicity to cervical, vaginal, and colorectal cells. Once initial toxicity results are favorable further analysis should include, but not be limited to, cell proliferation, DNA incorporation, effect of forward progression and motility of spermatozoa, and organisms of the naturally occurring vaginal flora. Mutagenicity and carcinogenicity studies should also be undertaken.
After establishing a sound toxicity profile based on in-vitro testing, the next evaluation should include in-vivo analysis for effects on vaginal epithelium. This study is most commonly conducted in rabbits. Although this approach might not be the most representative model, it is the one most commonly used in the pharmaceutical industry and will allow for early stage clinical testing to commence.
An essential component in microbicide product development is evaluation of reproductive and developmental toxicology. Regardless of whether or not a microbicide is also a contraceptive it is imperative to conduct this testing. Not only are the majority of women of child-bearing age using a microbicide, many women may be unaware of their pregnancy status prior to using a product or may unknowingly conceive during product use. There are an array of mammalian reproductive function tests established in accordance with regulatory guidelines, prenatal development, one and two generation, repeat dosing, and teratology in rat and rabbit, as well as evaluation of specific development stages, premating to conception, conception to implantation, to close of hard palate, end of pregnancy, postnatal, weaning, through to puberty.
Because this area of product development entails extensive testing and can be exceedingly expensive a strategic development plan needs to be mapped out that will ensure a comprehensive analysis of reproductive and developmental toxicology and that is within realistic confines of development. Although it would be valuable to have clinical trial data on product use in pregnant women, it might not be a cost-effective approach to establish the necessary profile that would allow inclusion of pregnant women prior to establishing proof-of-concept for a microbicide's effectiveness.
The nonclinical efficacy component of microbicide product development will be comprised of in-vitro testing and in-vivo animal evaluation. (Because two other reviews, ‘In-vitro testing of new agents: cell/explant models’ by Robin Shattock, pp. 000–000; and ‘Safety/efficacy studies in animals: macaques and small animals’ by Ron Veazey, pp. 000–000, will discuss the various types of assays available for evaluation no further description will be included here.) Until a proof-of-concept is established for microbicide efficacy there is no way of telling which of these assays might predict the physiological events in humans. Therefore, the microbicide field needs to be cautious when interpreting the results generated by these assays and ensure that one assay is not weighted more heavily than another and, more importantly, no given assay becomes the ‘gatekeeper’ for moving forward in development.
The key criterion for an efficacy profile is that sufficient analysis has been conducted to establish a sound rationale for moving forward with clinical development.
Microbicide product development has made tremendous strides over the past 20 years. Initial research and development challenges have shown the importance of devising a development strategy at the earliest stages of research. A comprehensive development strategy is one that considers trends and changes in drug regulation. This strategy also requires a stepwise approach to the development of product profiles that thoroughly describe quality, safety and efficacy. The four key quality parameters that must be considered are chemistry, manufacturing, control testing and stability. Establishing a safety profile requires evaluation of a product's pharmacokinetics, pharmacodynamics, and toxicology, including local tolerance and reproductive and developmental toxicity. Finally, while it is currently unclear whether existing in-vitro and in-vivo models can adequately predict product efficacy, it is critical that preclinical testing aimed at establishing a sound rationale for moving forward with clinical development be done for each potential microbicide.
United States Agency for International Development; Bill and Melinda Gates Foundation; Swedish Ministry of Foreign Affairs; National Institutes of Allergy and Infectious Diseases.