In the past two weeks we’ve learned of a major advance in ongoing efforts to halt the spread of HIV, two separate clinical studies have reported that a daily regimen of a pill called Truvada as a pre-exposure prophylaxis (PrEP) is highly effective in preventing infection in high risk groups. This success is a result not just of the dedication of the clinicians who conducted these trials, but also of a series of pivotal studies conducted in non-human primates more than a decade ago that laid the scientific foundations for them.

In the first study of more than 600 high-risk individuals conducted at Kaiser Permanente in San Francisco, which was published in the journal Clinical Infectious Diseases, researchers found that Truvada – a combination of the anti-viral drugs tenofovir and emtricitabine – was 100% effective in preventing infection. In the 2nd study, called the PROUD study and published online this week in the Lancet, of more than 500 high-risk men undertaken in 13 sexual health clinics in England Truvada reduced infections by 86%.

These results have been greeted with enthusiasm in media reports, with headlines such as “Aids vanquished: A costly new pill promises to prevent HIV infection” , “A pill designed to prevent HIV is working even better than people thought” and “Truvada Protected 100 Percent Of Study Participants From HIV: This is exciting!”. It’s worth noting that these are not the only trials to show the potential for Truvada to block HIV infection, earlier trials in Kenya, Uganda and Botswana also showed that it could substantially reduce infection rates, including in heterosexual couples where one partner was HIV positive and the other was not. There has been some concern that those taking Truvada would be less likely to take other safe sex measures – such as using condoms – but the results of the PROUD study showed no difference in acquisition of other sexually transmitted infections between those who started Truvada treatment immediately and those who delayed for 1 year, suggesting that they did not engage in riskier behavior as a consequence of taking Truvada.

Thanks to a multi-pronged approach to preventing HIV infection, combining barrier methods such as condoms, Highly Active Antiretroviral Therapy (HAART) to lower viral load in infected individuals, and the use of antiviral medications to prevent mother-to-child transmission, the spread of HIV infection has slowed dramatically in many regions of the world, and pre-exposure prophylaxis with Truvada certainly has the potential to help reduce it further.

As we applaud the researchers who conducted these first real-world evaluations of Tenofovir in high-risk populations, it is also a good opportunity to remember the researchers whose work led us to this point. One of those pioneers is Dr. Koen Van Rompay, a virologist at the University of California at Davis who played a key role in the early development of Tenofovir and its evaluation in pre- and post- exposure phophylaxis in macaque models of HIV infection. In 2009 Dr Van Rompay wrote an article for Speaking of Research explaining how important animal research was to the early development of such HIV prophylaxis regimes, and how important it continues to be as scientists develop ever better treatments, which we share again today:

Contributions of nonhuman primate studies to the use of HIV drugs to prevent infection – Koen van Rompay

Since the early days of the HIV pandemic, as soon as it was clear that an effective HIV vaccine would still be years away, there has been considerable interest in using anti-HIV drugs to reduce the risk of infection following exposure to HIV (so-called prophylaxis). Animal models of HIV infection, especially the rhesus macaque, have played a major role in developing and testing these treatments.

The development of HIV drugs to treat HIV-infected persons has shown that many compounds that are effective in vitro (i.e., in tissue culture assays) fail to hold their promise when tested in humans, because of unfavorable pharmacokinetics, toxicity or insufficient antiviral efficacy. The same principles apply to the development of drugs to prevent HIV infection. The outcome of drug administration is determined by many complex interactions in vivo between the virus, the antiviral drug(s) and the host; with current knowledge, these interactions cannot be mimicked and predicted sufficiently by in vitro studies or computer models.

Testing different compounds in human clinical trials is logistically difficult, time-consuming and expensive, so only a very limited number of candidates can be explored in a given time. Fortunately, the development of antiviral strategies can be accelerated by efficient and predictive animal models capable of screening and selecting the most promising compounds. No animal model is perfect and each model has its limitations, but the simian immunodeficiency virus (SIV) of macaques is currently considered the best animal model for HIV infection because of the many similarities of the host, the virus and the disease. Non-human primates are phylogenetically the closest to humans, and have similar immunology and physiology (including drug metabolism, placenta formation, fetal and infant development). In addition, SIV, a virus closely related to HIV-1, can infect macaques and causes a disease that resembles HIV infection and AIDS in humans, and the same markers are used to monitor the disease course. For these reasons, SIV infection of macaques has become an important animal model to test antiviral drugs to prevent or treat infection.

Different nonhuman primate models have been developed based on the selection of the macaque species, the particular SIV strain and the inoculation route (e.g. IV injection, vaginal exposure) used (reviewed in (33)). These models have been improved and refined during the past two decades. For example, SIV-HIV chimeric viruses have been engineered to contain portions of HIV-1, such as the enzyme reverse transcriptase (“RT-SHIV”) that the virus requires in order to multiply or the envelope protein (“env-SHIV”) that the virus needs if it is to escape from a cell and infect other cells, to allow these models to also test drugs that are specific for HIV-1 reverse transcriptase or envelope (28, 35).

Many studies in non-human primates have investigated whether the administration of anti-HIV drugs prior to or just after exposure to virus can prevent infection. The earliest studies indicated that drugs such as the reverse transcriptase inhibitor zidovudine (AZT), the first approved drug treatment for HIV, were not very effective in preventing infection, but a likely reason for this was the combination of a high-dose viral inoculums used, the direct intravenous route of virus inoculation, and the relative weak potency of drugs at that time (2, 4, 13, 19, 20, 36). The proof-of-concept that HIV drugs can prevent infection was demonstrated in 1992 when a 6-weeks zidovudine regimen, started 2 hours before an intravenous low-dose virus inoculation that more accurately represented HIV infection in humans, protected infant macaques against infection (29). These results were predictive of a subsequent clinical trial (Pediatric AIDS Clinical Trials Group Protocol 076), which demonstrated that zidovudine administration to HIV-infected pregnant women beginning at 14 to 34 weeks of gestation, and continuing to their newborns during the first 6 weeks of life reduced the rate of viral transmission by two-thirds (10).

Since then, a growing number of studies have been performed in macaques to identify more effective and simpler prophylactic drug regimens. These studies generally used lower virus doses, sometimes combined with a mucosal route of virus inoculation that mimics vaginal or anal exposure responsible for the majority of human HIV infections. These studies demonstrated that administration of some newer anti-HIV drugs, including the reverse transcriptase inhibitors adefovir (PMEA), tenofovir (PMPA), and emtricitabine (FTC) that prevent the virus from multiplying in the infected cell, and the CCR5 inhibitor CMPD167 that stops the virus from binding the CCR5 receptor on the cell surface and entering a cell in the first place, starting prior to, or at the time of virus inoculation, was able to prevent infection, though with varying success rates (3, 4, 16, 24, 25, 31, 34, 35). Only very few compounds such as the reverse transcriptase inhibitors tenofovir, BEA-005 and GW420867, and the CCR5 inhibitor CMPD167, were able to reduce infection rates when treatment was started after virus inoculation. For those drugs that were successful in post-exposure prophylaxis studies, a combination of the timing and duration of drug administration was found to determine the success rate, because a delay in the start, a shorter duration, or interruption of the treatment regimen all reduced the prophylactic efficacy (5, 11, 21, 22, 26, 27, 31) , information that has guided the design of subsequent clinical trials.

While some of the compounds such as GW420867 that showed prophylactic efficacy in the macaque model are no longer in clinical development (e.g., due to toxicity or pharmacokinetic problems discovered later in pre-clinical testing), the very promising results achieved with tenofovir have sparked further studies aimed at simplifying the prophylactic regimen. Several studies in infant and adult macaques have demonstrated that short or intermittent regimens of tenofovir (with or without coadministration of emtricitabine) consisting of one dose before and one dose after each virus inoculation were highly effective in reducing SIV infection rates (15, 30, 32).

The demonstration at the beginning of the 1990’s that anti-HIV drugs can prevent infection in macaques has provided the rationale to administer these compounds to humans to reduce the likelihood of infection in several clinical settings. Antiviral drugs are now recommended, usually as a combination of several drugs, to reduce the risk of HIV infection after occupational exposure (e.g., needle-stick accidents of health care workers) and non-occupational exposure (e.g. sex or injection-drug use) (6, 7). As mentioned previously, drug regimens containing zidovudine and more recently also more potent drugs such as nevirapine have proven to be highly effective in reducing the rate of mother-to-infant transmission of HIV, including in developing countries (10, 14, 17), and save many thousands of lives every year . Because the short nevirapine regimen that is given to pregnant HIV-infected women at the onset of labor frequently induces drug resistance mutations in the mother that may compromise future treatment (12), tenofovir’s high prophylactic success in the infant macaque model has sparked clinical trials in which a short tenofovir-containing regimen was added to existing perinatal drug regimens to reduce the occurrence of resistance mutations and/or further lower the transmission rate (8, 9, 18, 30, 32).

Because an efficacious HIV vaccine has so far not been identified, the concept of using pre-exposure prophylaxis also as a possible HIV prevention strategy in adults has gained rapid momentum in recent years. The promising prophylactic data of tenofovir (with or without emtricitabine) in the macaque model (23, 32, 35, 37) combined with the favorable pharmacokinetics, safety profile, drug resistance pattern and therapeutic efficacy of these drugs in HIV-infected people, have pushed these compounds into front-runner position in ongoing clinical trials that investigate whether uninfected adults who engage in high-risk behavior will have a lower infection rate by taking a once daily tablet of tenofovir or tenofovir plus emtricitabine. The results of these ongoing trials are highly anticipated. An overview of the design, status and challenges of these trials which are currently underway at several international sites and target different high-risk populations can be found on the website of the AIDS Vaccine Advicacy Coalition (1, 23).

In conclusion, nonhuman primate models of HIV infection have played an important role in guiding the development of pre- and post-exposure prophylaxis strategies. Ongoing comparison of results obtained in these models with those observed in human studies will allow further validation and refinement of these animal models so they can continue to provide a solid foundation to advance our scientific knowledge and to guide clinical trials.

Koen van Rompay DVM Ph.D. is a research virologist at the California National Primate Research Center at UC Davis.

Cited literature

1. AIDS Vaccine Advocacy Coalition. August 2008, posting date. Anticipating the results of PrEP trials. http://avac.org/prep08.pdf

2. Black, R. J. 1997. Animal studies of prophylaxis. Am. J. Med. 102 (5B):39-43.

3. Böttiger, D., P. Putkonen, and B. Öberg. 1992. Prevention of HIV-2 and SIV infections in cynomolgus macaques by prophylactic treatment with 3′-fluorothymidine. AIDS Res. Hum. Retrovir. 8:1235-1238.

4. Böttiger, D., L. Vrang, and B. Öberg. 1992. Influence of the infectious dose of simian immunodeficiency virus on the acute infection in cynomolgus monkeys and on the effect of treatment with 3′-fluorothymidine. Antivir. Chem. Chemother. 3:267-271.

5. Böttiger, D., N. G. Johansson, B. Samuelsson, H. Zhang, P. Putkonen, L. Vrang, and B. Öberg. 1997. Prevention of simian immunodeficiency virus, SIVsm, or HIV-2 infection in cynomolgus monkeys by pre- and postexposure administration of BEA-005. AIDS 11:157-162.

6. Centers for Disease Control and Prevention. 1996. Update: provisional Public Health Service recommendations for chemoprophylaxis after occupational exposure to HIV. MMRW 45:468-472.

7. Centers for Disease Control and Prevention. 2005. Antiretroviral postexposure prophylaxis after sexual, injection-drug use, or other nonoccupational exposure to HIV in the United States: recommendations from the U.S. Department of Health and Human Services. MMWR 54:1-19.

8. Chi, B. H., M. Sinkala, F. Mbewe, R. A. Cantrell, G. Kruse, N. Chintu, G. M. Aldrovandi, E. M. Stringer, C. Kankasa, J. T. Safrit, and J. S. Stringer. 2007. Single-dose tenofovir and emtricitabine for reduction of viral resistance to non-nucleoside reverse transcriptase inhibitor drugs in women given intrapartum nevirapine for perinatal HIV prevention: an open-label randomised trial. Lancet 370:1698-705.

9. Chi, B. H., N. Chintu, R. A. Cantrell, C. Kankasa, G. Kruse, F. Mbewe, M. Sinkala, P. J. Smith, E. M. Stringer, and J. S. Stringer. 2008. Addition of single-dose tenofovir and emtricitabine to intrapartum nevirapine to reduce perinatal HIV transmission. J. Acquir. Immune Defic. Syndr. 48:220-3.

10. Connor, E. M., R. S. Sperling, R. Gelber, P. Kiselev, G. Scott, M. J. O’Sullivan, R. VanDyke, M. Bey, W. Shearer, R. L. Jacobson, E. Jiminez, E. O’Neill, B. Bazin, J.-F. Delfraissy, M. Culnane, R. Coombs, M. Elkins, J. Moye, P. Stratton, J. Balsley, and for the Pediatric AIDS Clinical Trials Group Protocol 076 Study Group. 1994. Reduction of maternal-infant transmission of human immunodeficiency virus type 1 with zidovudine treatment. N. Engl. J. Med. 331:1173-1180.

11. Emau, P., Y. Jiang, M. B. Agy, B. Tian, G. Bekele, and C. C. Tsai. 2006. Post-exposure prophylaxis for SIV revisited: Animal model for HIV prevention. AIDS Res. Ther. 3:29.

12. Eshleman, S. H., M. Mracna, L. A. Guay, M. Deseyve, S. Cunningham, M. Mirochnick, P. Musoke, T. Fleming, M. G. Fowler, L. M. Mofenson, F. Mmiro, and J. B. Jackson. 2001. Selection and fading of resistance mutations in women and infants receiving nevirapine to prevent HIV-1 vertical transmission (HIVNET012). AIDS 15:1951-1957.

13. Fazely, F., W. A. Haseltine, R. F. Rodger, and R. M. Ruprecht. 1991. Postexposure chemoprophylaxis with ZDV or ZDV combined with interferon-a: failure after inoculating rhesus monkeys with a high dose of SIV. J. Acquir. Immune Defic. Syndr. 4:1093-1097.

14. Gaillard, P., M.-G. Fowler, F. Dabis, H. Coovadia, C. van der Horst, K. Van Rompay, A. Ruff, T. Taha, T. Thomas, I. de Vicenzi, M.-L. Newell, and for the Ghent IAS Working Group on HIV in Women and Children. 2004. Use of antiretroviral drugs to prevent HIV-1 transmission through breastfeeding: from animal studies to randomized clinical trials. J. Acquired Immune Defic. Syndr. 35:178-187.

15. Garcia-Lerma, J. G., R. A. Otten, S. H. Qari, E. Jackson, M. E. Cong, S. Masciotra, W. Luo, C. Kim, D. R. Adams, M. Monsour, J. Lipscomb, J. A. Johnson, D. Delinsky, R. F. Schinazi, R. Janssen, T. M. Folks, and W. Heneine. 2008. Prevention of rectal SHIV transmission in macaques by daily or intermittent prophylaxis with emtricitabine and tenofovir. PLoS Med. 5:e28.

16. Grob, P. M., Y. Cao, E. Muchmore, D. D. Ho, S. Norris, J. W. Pav, C.-K. Shih, and J. Adams. 1997. Prophylaxis against HIV-1 infection in chimpanzees by nevirapine, a nonnucleoside inhibitor of reverse transcriptase. Nature Med. 3:665-670.

17. Guay, L. A., P. Musoke, T. Fleming, D. Bagenda, M. Allen, C. Nakabiito, J. Sherman, P. Bakaki, C. Ducar, M. Deseyve, L. Emel, M. Mirochnick, M. G. Fowler, L. Mofenson, P. Miotti, K. Dransfield, D. Bray, F. Mmiro, and J. B. Jackson. 1999. Intrapartum and neonatal single-dose nevirapine compared with zidovudine for prevention of mother-to-child transmission of HIV-1 in Kampala, Uganda: HIVNET 012 randomized trial. Lancet 354:795-802.

18. Hirt, D., S. Urien, D. K. Ekouevi, E. Rey, E. Arrive, S. Blanche, C. Amani-Bosse, E. Nerrienet, G. Gray, M. Kone, S. K. Leang, J. McIntyre, F. Dabis, and J. M. Treluyer. 2009. Population pharmacokinetics of tenofovir in HIV-1-infected pregnant women and their neonates (ANRS 12109). Clin. Pharmacol. Ther. 85:182-9.

19. Lundgren, B., D. Bottiger, E. Ljungdahl-Ståhle, E. Norrby, L. Ståhle, B. Wahren, and B. Öberg. 1991. Antiviral effects of 3′-fluorothymidine and 3′-azidothymidine in cynomolgus monkeys infected with simian immunodeficiency virus. J. Acquir. Immune Defic. Syndr. 4:489-498.

20. McClure, H. M., D. C. Anderson, A. A. Ansari, P. N. Fultz, S. A. Klumpp, and R. F. Schinazi. 1990. Nonhuman primate models for evaluation of AIDS therapy. Ann. N. Y. Acad. Sci. 616:287-298.

21. Mori, K., Y. Yasumoti, S. Sawada, F. Villinger, K. Sugama, B. Rosenwirth, J. L. Heeney, K. Überla, S. Yamazaki, A. A. Ansari, and H. Rübsammen-Waigmann. 2000. Suppression of acute viremia by short-term postexposure prophylaxis of simian/human immunodeficiency virus SHIV-RT-infected monkeys with a novel reverse transcriptase inhibitor (GW420867) allows for development of potent antiviral immune responses resulting in efficient containment of infection. J. Virol. 74:5747-5753.

22. Otten, R. A., D. K. Smith, D. R. Adams, J. K. Pullium, E. Jackson, C. N. Kim, H. Jaffe, R. Janssen, S. Butera, and T. M. Folks. 2000. Efficacy of postexposure prophylaxis after intravaginal exposure of pig-tailed macaques to a human-derived retrovirus (human immunodeficiency virus type 2). J Virol 74:9771-5.

23. PrEP Watch, http://www.prepwatch.org/

24. Subbarao, S., R. A. Otten, A. Ramos, C. Kim, E. Jackson, M. Monsour, D. R. Adams, S. Bashirian, J. Johnson, V. Soriano, A. Rendon, M. G. Hudgens, S. Butera, R. Janssen, L. Paxton, A. E. Greenberg, and T. M. Folks. 2006. Chemoprophylaxis with Tenofovir Disoproxil Fumarate Provided Partial Protection against Infection with Simian Human Immunodeficiency Virus in Macaques Given Multiple Virus Challenges. J. Infect. Dis. 194:904-11.

25. Tsai, C.-C., K. E. Follis, A. Sabo, R. F. Grant, C. Bartz, R. E. Nolte, R. E. Benveniste, and N. Bischofberger. 1994. Preexposure prophylaxis with 9-(-2-phosphonylmethoxyethyl)adenine against simian immunodeficiency virus infection in macaques. J. Infect. Dis. 169:260-266.

26. Tsai, C.-C., K. E. Follis, T. W. Beck, A. Sabo, R. F. Grant, N. Bischofberger, and R. E. Benveniste. 1995. Prevention of simian immunodeficiency virus infection in macaques by 9-(2-phosphonylmethoxypropyl)adenine (PMPA). Science 270:1197-1199.

27. Tsai, C.-C., P. Emau, K. E. Follis, T. W. Beck, R. E. Benveniste, N. Bischofberger, J. D. Lifson, and W. R. Morton. 1998. Effectiveness of postinoculation (R)-9-(2-phosphonylmethoxypropyl)adenine treatment for prevention of persistent simian immunodeficiency virus SIVmne infection depends critically on timing of initiation and duration of treatment. J. Virol. 72:4265-4273.

28. Uberla, K., C. Stahl-Hennig, D. Böttiger, K. Mätz-Rensing, F. J. Kaup, J. Li, W. A. Haseltine, B. Fleckenstein, G. Hunsmann, B. Öberg, and J. Sodroski. 1995. Animal model for the therapy of acquired immunodefiency syndrome with reverse transcriptase inhibitors. Proc. Natl. Acad. Sci. U.S.A. 92:8210-8214.

29. Van Rompay, K. K. A., M. L. Marthas, R. A. Ramos, C. P. Mandell, E. K. McGowan, S. M. Joye, and N. C. Pedersen. 1992. Simian immunodeficiency virus (SIV) infection of infant rhesus macaques as a model to test antiretroviral drug prophylaxis and therapy: oral 3′-azido-3′-deoxythymidine prevents SIV infection. Antimicrob. Agents Chemother. 36:2381-2386.

30. Van Rompay, K. K. A., C. J. Berardi, N. L. Aguirre, N. Bischofberger, P. S. Lietman, N. C. Pedersen, and M. L. Marthas. 1998. Two doses of PMPA protect newborn macaques against oral simian immunodeficiency virus infection. AIDS 12:F79-F83.

31. Van Rompay, K. K. A., M. L. Marthas, J. D. Lifson, C. J. Berardi, G. M. Vasquez, E. Agatep, Z. A. Dehqanzada, K. C. Cundy, N. Bischofberger, and N. C. Pedersen. 1998. Administration of 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) for prevention of perinatal simian immunodeficiency virus infection in rhesus macaques. AIDS Res. Hum. Retroviruses 14:761-773.

32. Van Rompay, K. K. A., M. B. McChesney, N. L. Aguirre, K. A. Schmidt, N. Bischofberger, and M. L. Marthas. 2001. Two low doses of tenofovir protect newborn macaques against oral simian immunodeficiency virus infection. J. Infect. Dis. 184:429-438.

33. Van Rompay, K. K. A. 2005. Antiretroviral drug studies in non-human primates: a valid animal model for innovative drug efficacy and pathogenesis studies. AIDS Reviews 7:67-83.

34. Van Rompay, K. K. A., B. P. Kearney, J. J. Sexton, R. Colón, J. R. Lawson, E. J. Blackwood, W. A. Lee, N. Bischofberger, and M. L. Marthas. 2006. Evaluation of oral tenofovir disoproxyl fumarate and topical tenofovir GS-7340 to protect infant macaques against repeated oral challenges with virulent simian immunodeficiency virus. J. Acquir. Immune Defic. Syndr. 43:6-14.

35. Veazey, R. S., M. S. Springer, P. A. Marx, J. Dufour, P. J. Klasse, and J. P. Moore. 2005. Protection of macaques from vaginal SHIV challenge by an orally delivered CCR5 inhibitor. Nat Med.

36. Wyand, M. S. 1992. The use of SIV-infected rhesus monkeys for the preclinical evaluation of AIDS drugs and vaccines. AIDS Res. Hum. Retrovir. 8:349-356.

37. García-Lerma J. G., Otten R. A., Qari S. H., Jackson E., Cong M. E., Masciotra S., Luo W., Kim C., Adams D. R., Monsour M., Lipscomb J., Johnson J. A., Delinsky D., Schinazi R. F., Janssen R , Folks T. M., Heneine W. Prevention of rectal SHIV transmission in macaques by daily or intermittent prophylaxis with emtricitabine and tenofovir. PLoS Med. 2008 Feb;5(2):e28