Background:  The objectives of the Bangkok Tenofovir Study (BTS) are to determine whether tenofovir (TDF) is safe and if it prevents HIV infection among injecting drug users (IDU).

Methods:  BTS, an ongoing phase II/III, randomized, double-blind, placebo-controlled trial, is being conducted in 17 Bangkok Metropolitan Administration (BMA) drug treatment clinics. A total of 2000 eligible IDU are being randomized (1:1) to receive daily TDF 300 mg or placebo and followed until all complete at least 48 weeks on study. Participant demographics are assessed at enrollment; risk behavior (using audio-computer-assisted self-interview), blood chemistry and hematology at enrollment and every 3 months; and adherence (using audio-computer-assisted self-interview) every month during follow-up. Interim safety and efficacy analyses are conducted by an independent data and safety monitoring board.

Results:  From June 2005 through August 2007, 3131 IDU were screened and 1810 (58%) enrolled. The most common reasons for screen failure were HIV infection (11%), elevated alanine aminotransferase (ALT) or aspartate aminotransferase (AST) (8%), and chronic hepatitis B infection (6%). The median age of enrolled participants was 31 years, 79% were male, and 86% completed primary school or more. At enrollment, 45% of participants reported injecting methamphetamine, 41% midazolam, and 38% heroin. Using audio-computer-assisted self-interview, participants reported taking study medication the day before their visit at 94% of monthly assessments and taking study medication 6 or 7 days in the week before their visit at 92% of monthly assessments. A total of 917 participants had completed 12 months follow-up. At enrollment, 599 (65%) of these 917 participants reported injecting drugs and 130 (14%) sharing needles in the 3 months before enrollment. The proportion of participants reporting injecting decreased to 32% and sharing to 4% at month 3 (both p <0.001) and remained steady through month 12. The data and safety monitoring board advised trial continuation following safety reviews in July 2006 and September 2007.

Conclusions:  The BTS is 90% enrolled; methamphetamine, midazolam, and heroin use is common; reported adherence to study medication is good; and risk behaviors have declined. Although participants have reported decreased risk behavior during trial follow-up, they remain at risk for HIV infection and there is an urgent need for effective interventions to prevent HIV infection. Final results are expected in late 2009.