How Pre-Exposure Prophylaxis (PrEP) Works
PrEP prevents HIV infection by maintaining therapeutic concentrations of antiretroviral drugs in blood and mucosal tissues before viral exposure, blocking HIV reverse transcriptase and preventing the virus from establishing infection in target cells. 1
Mechanism of Action
PrEP uses antiretroviral medications—specifically nucleoside reverse transcriptase inhibitors—that must be present in sufficient concentrations in tissues before HIV exposure occurs. The drugs work by:
- Blocking viral replication at the earliest stage: When HIV attempts to infect CD4+ T cells and other target cells in mucosal tissues, the antiretroviral agents inhibit the reverse transcriptase enzyme that HIV needs to convert its RNA into DNA 2
- Exploiting early viral vulnerabilities: The drugs take advantage of the narrow window during initial mucosal transmission when the virus is most susceptible to intervention 2
- Preventing establishment of infection: By blocking reverse transcription, PrEP stops HIV from integrating into host cell DNA and establishing a permanent infection 3
Critical Pharmacologic Considerations
Daily dosing is required for optimal protection, particularly for women, because tenofovir concentrates at 10-fold lower levels in vaginal tissue compared to rectal tissue, and clearance is faster 1. This tissue distribution difference explains why adherence is even more critical for cisgender women and why 6 of 7 doses per week are needed for adequate protection in this population 4.
For men who have sex with men (MSM), protective drug levels in rectal tissue are achieved more rapidly and maintained with 4 of 7 doses per week 4.
Available Regimens
Oral PrEP: TDF/emtricitabine (tenofovir disoproxil fumarate/emtricitabine) is the recommended agent 1. The combination provides dual inhibition of reverse transcriptase, with tenofovir and emtricitabine working synergistically.
Injectable PrEP: Long-acting cabotegravir (600 mg intramuscular every 8 weeks after initial 4-week interval) works as an integrase inhibitor, blocking a different step in the viral replication cycle 4.
On-demand PrEP ("2-1-1"): For MSM with infrequent sexual exposures, taking 2 doses 2-24 hours before sex, then 1 dose at 24 hours and another at 48 hours after the initial double dose reduces HIV acquisition by 86% 1. This approach is not recommended for women due to inadequate tissue drug concentrations with intermittent dosing.
Why PrEP Can Fail
PrEP is not fail-safe. Seroconversion despite excellent adherence has occurred in cases of 1:
- High viral inoculum exposure
- Transmission of drug-resistant virus
- Insufficient tissue drug levels (particularly in women with suboptimal adherence)
The most common reason for PrEP failure is poor medication adherence 4. When breakthrough infections occur, they typically involve wild-type virus in non-adherent individuals, though rare cases of M184V resistance mutations have been documented when PrEP was initiated during undiagnosed acute HIV infection 1.
Important Caveats
- PrEP does not prevent other sexually transmitted infections—condom use should still be encouraged 1
- 3 to 6 months of viral suppression may be required before achieving full protective effect 1
- Transfeminine individuals using estrogens or androgen blockers may have up to 27-30% reduction in tenofovir concentrations, requiring additional adherence support 4
- Unanticipated interruptions in PrEP delivery (insurance lapses, incarceration, relocation) have been associated with seroconversions 1