A microbicide is a chemical compound designed to block the sexual transmission of HIV by killing or inactivating the virus, blocking the entry of the virus into target cells or interrupting the viral life cycle once it has entered target cells. Microbicides are being developed chiefly for vaginal use, although research on products for rectal use is also under way. The drive behind microbicide development is the urge to provide women with an HIV prevention technology that does not depend on male cooperation to the same extent as the male, or even the female, condom. The earliest randomized trials were conducted with nonoxynol-9, which has been used as a spermicide but has also shown anti-HIV activity in vitro. Altogether there were five rigorous randomized controlled trials conducted in a number of countries, chiefly among female commercial sex workers; these studies found no evidence that nonoxynol-9 protects against vaginal acquisition of HIV infection (106, 107). There was some evidence that it may, in fact, cause harm by increasing genital lesions. For example, the large multisite study of COL-1492 vaginal gel, which contains nonoxynol-9, found a significant increase in HIV incidence among female sex workers who used the gel compared with those who did not (106,107).
There are more than 60 products in preclinical and clinical development, targeting a number of different mechanisms of action and different phases of the HIV life-cycle (108). In early 2006, there were about 10 compounds in phase I trials, 6 in phase II trials, and 4 in phase III trials (109). The difficulties of developing and testing microbicides are similar to those of vaccines, including scientific uncertainties, the complexity of trial designs and the costs (3). Also, as in the case of vaccines, it is unlikely that any microbicide will be 100% effective. Yet, it is estimated that the introduction of even a partially effective microbicide could prevent as many as 2.5 million new HIV infections over 3 years (110).
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