Biomedical Engineering – From Theory to Applications

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32 Biomedical Engineering – From Theory to Applications prodrug form of tenofovir, a nucleotide reverse transcriptase inhibitor which prevents elongation of the transcribed HIV DNA by interpolation into the growing chain in place of adenosine 5’-monophosphate (Gilead, 2010). Emtricitabine functions in a similar manner and is the (-) enantiomer of a thio analog of cytidine (Gilead, 2008). Phase IIb and III Trial Products Population Countries Parallel comparison of Tenofovir and Emtricitabine/Tenofovir PrEP to prevent HIV-1 acquisition within HIV-1 discordant TDF/FTC: Oral, TDF: Oral couples (Partners PrEP) Safety and efficacy of daily tenofovir to prevent HIV infection (Bangkok Tenofovir TDF: Oral Study Safety and effectiveness study of tenofovir TDF/FTC:Oral 1% gel, tenofovir disoproxil fumarate tablet TDF: Oral and emtricitabine/tenofovir disoproxil TDF: Gel fumarate tablet for the prevention of HIV infection in women (VOICE/MTN 003) Phase I and II 4700 discordant heterosexual couples 2400 IV drug users 5000 women Kenya, Uganda Thailand South Africa, Uganda, Zimbabwe Extended safety trial (PrEP in young MSM) TDF: Oral 99 young MSM United States Safety and efficacy of daily and oral antiretroviral use for the prevention of HIV infection in heterosexually active young adults (TDF2) IAVI E001 & E002 Phase I/II TDF/FTC: Oral 1200 heterosexual Botswana men and women TDF/FTC: Oral 150 daily and coitally serodiscordant dependent couples Kenya, Uganda Table adapted from Alliance for AIDS Vaccine Advocacy Coalition website at www.avac.org accessed 22 February 2011. TDF/FTC: tenofovir disoproxil fumarate/emtricitabine; TDF: tenofovir disoproxil fumarate Table 3. Oral pre-exposure prevention candidates in clinical trials The results of the iPrEx trial of oral TDF/FTC showed that oral PrEP may be able to reduce HIV incidence among men who have sex with men (Grant et al., 2010). However, the effectiveness of the treatment is likely to be heavily influenced by the adherence of the persons taking the medication – low adherence among participants in the iPrEx trial may have affected the level of protection afforded by the study drugs (Grant et al., 2010). If consensus emerges that PrEP is safe and effective, there will be several challenges for wide-spread roll-out of the method to the population that needs it, including identifying the appropriate target population, monitoring of adherence and side-effects, and development of potential drug resistance. PrEP is unlikely to be 100% effective, so monitoring of adherence and risk compensation will be critical and will require behavioural intervention.
Biomedical HIV Prevention 2.6 Treatment of Sexually Transmitted Infections Many observational studies have suggested that sexually transmitted infections (STIs), including HSV2, enhance HIV acquisition (Wasserheit, 1992; del Mar Pujades Rodríguez et al., 2002; Freeman et al., 2006). The mechanism by which infection is thought to be facilitated varies depending on the particular STI. Rebbapragada and Kaul (Rebbapragada & Kaul, 2007) summarise five mechanisms by which STIs might enhance susceptibility to HIV: macro or micro-scale breach of the genital epithelium; alteration of the levels of innate immune proteins and/or mucosal environment; enhancing susceptibility to other genital infections; attraction of activated HIV target cells to the site of HIV exposure; and by increasing inflammation which in turn enhances HIV replication. Ulcerative STIs such as HSV2 and syphilis may facilitate HIV infection directly through discontinuities in the genital epithelium (Wasserheit, 1992), but may also have mechanisms in common with other non-ulcerative STIs such as gonorrhoea and Chlamydia trachomatis which are thought to facilitate infection both through micro-ulcerations and by increasing inflammation within the genital tract with subsequent proliferation of HIV target cells (Rebbapragada & Kaul, 2007). Treatment of STIs for prevention of HIV can include antibiotic or viral suppressive medications, or a combination of both. Randomised controlled trials of STI treatment have found a significant reduction in HIV in only one instance – for a community randomised trial of syndromic treatment of bacterial infections in Mwanza, Tanzania (Grosskurth et al., 1995). Monthly provision of 1g of azithromycin to female commercial sex workers in a RCT in Nairobi, Kenya did not result in a reduction in subsequent acquisition of HIV, although significant reductions in gonorrhoea, Chlamydia and Trichomonas vaginalis infections were noted (Kaul et al., 2004). A large scale community intervention with three arms (including an STI treatment arm) carried out near Masaka, Uganda, found that although the trial activities may have positively influenced sexual behaviour and incidence of certain STIs, no detectable influence on HIV was noted (Kamali et al., 2003). In a trial comparing the effects of intensive versus standard STI care and treatment, a lower but not significantly different HIV seroconversion rate was noted in participants randomized to the intensive care regimen (Ghys et al., 2001). Two recent largescale trials showed no evidence for an effect of HSV2 suppressive therapy on HIV acquisition (Celum et al., 2008; Watson-Jones et al., 2008). HSV2 suppressive therapy was also unable to prevent transmission of HIV from infected to uninfected partners, despite a reduction in their HIV viral load (Celum et al., 2010). It has been proposed that STI treatment may only be effective in concentrated epidemics, and that the effectiveness of such interventions decreases with increasing generalization of the epidemic (Grosskurth et al., 2000). Despite overwhelming biological evidence for STI treatment as a potential HIV prevention option, efficacy has not been demonstrated in trials due to limitations in treatment approach, adherence and acceptability (Grosskurth et al., 2000; Lagakos & Gable, 2008). This should not preclude the inclusion of effective STI treatment in HIV prevention plans, as this approach is important for preventing other significant causes of morbidity, and may also have the effect of normalising the need for safe sexual behaviour. 2.7 Harm reduction for intravenous drug users Although the primary mode of HIV transmission in the world is heterosexual sex, there remain concentrated HIV epidemics among intravenous drug users (IDUs) in parts of Asia 33
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180 16. Acknowledgments Biomedical
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190 R* M M M M MR*M O O O O M O R*
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