Twenty-eight day safety, antiviral activity, and pharmacokinetics of tenofovir alafenamide for treatment of chronic hepatitis B infection

Background & Aims Tenofovir alafenamide, a phosphonate prodrug of tenofovir with greater plasma stability than tenofovir disoproxil fumarate, provides efficient delivery of active drug to hepatocytes at reduced systemic tenofovir exposures. Methods Non-cirrhotic, treatment-naïve subjects with chronic hepatitis B were randomized (1:1:1:1:1) to receive tenofovir alafenamide 8, 25, 40, or 120 mg, or tenofovir disoproxil fumarate 300 mg for 28 days and assessed for safety, antiviral response, and pharmacokinetics, followed-up by off-treatment for 4 weeks. Results 51 subjects were randomized and all completed study treatment. Groups were generally well matched (67% male, 57% Asian, 53% HBeAg-negative, mean HBV DNA approximately 6.0 log₁₀ IU/ml) with HBV genotypes reflective of the population. No subject experienced an adverse event that was serious or severe (grade 3/4). Across the tenofovir alafenamide groups, similar mean changes in serum HBV DNA were found at Week 4 (-2.81, -2.55, -2.19, and -2.76 log₁₀ IU/ml for the 8, 25, 40, and 120 mg groups, respectively) which were also comparable to the control (-2.68 log₁₀ IU/ml for tenofovir disoproxil fumarate 300 mg). Kinetics of viral decline were also similar among groups. Tenofovir alafenamide pharmacokinetics were linear and proportional to the dose; doses ≤25 mg were associated with ≥92% reductions in mean tenofovir area under the curve relative to tenofovir disoproxil fumarate 300 mg. Conclusions Tenofovir alafenamide was safe and well tolerated; declines in HBV DNA were similar to tenofovir disoproxil fumarate at all doses evaluated. Tenofovir alafenamide 25 mg has been selected for further hepatitis B clinical development.