Infection with human immunodeficiency virus (HIV) remains a global public health concern and is particularly serious in low- and middle-income countries. Widespread sexual violence and poverty, among other factors, increase the risk of infection in women, while currently available prevention methods are outside the control of most. This has driven the study of vaginal microbicides to prevent sexual transmission of HIV from men to women in recent decades. The first microbicides evaluated were formulated as gels for daily use and contained different substances such as surfactants, acidifiers and monoclonal antibodies, which failed to demonstrate efficacy in clinical trials. A gel containing the reverse transcriptase inhibitor tenofovir showed protective efficacy in women. However, the lack of adherence by patients led to the search for dosage forms capable of releasing the active principle for longer periods, and hence to the emergence of the vaginal ring loaded with dapivirine, which requires a monthly application and is able to reduce the sexual transmission of HIV. The future of vaginal microbicides will feature the use of alternative dosage forms, nanosystems for drug release and probiotics, which have emerged as potential microbicides but are still in the early stages of development. Protecting women with vaginal microbicide formulations would, therefore, be a valuable tool for avoiding sexual transmission of HIV.

According to UNAIDS, as there is still no effective vaccine against HIV, pre-exposure prophylasis (PrEP) is necessary to reduce its incidence. Sexual transmission rate is higher from men to women in developing countries and vertical transmission may also occur from mother to child. Hence, vaginal formulations are an interesting proposal for the protection of women, preventing the virus from infecting vagina through different mechanisms. Several drugs, such as Dapivirine, Tenofovir or Maraviroc, have been assessed and showed to be effective in this field. These microbicides are included in different dosage forms able to release the drug once in contact with the vaginal medium. Innovative excipients are being employed for the development of different systems trying to get an easier posology through control release and high comfortability, thus leading to a better compliance. In this line, several formulations have been developed and tested, such as rings, tablets, gels or films. Some of them are nowadays in clinical trials, such as a Tenofovir gel or a Dapivirine vaginal ring. The aim of this chapter is to synthetize the research and findings in the field of the development and assessment of vaginal formulations in the PrEP of HIV sexual transmission.