Trans-urocanic acid enhances tenofovir alafenamide stability for long-acting HIV applications

Int J Pharm. 2020 Sep 25;587:119623. doi: 10.1016/j.ijpharm.2020.119623. Epub 2020 Jul 11.


Long-acting (LA) pre-exposure prophylaxis (PrEP) for HIV prevention is poised to address non-adherence and implementation challenges by alleviating the burden of user-dependent dosing. Due to its potency, tenofovir alafenamide (TAF) is a viable candidate for LA PrEP. However, the inherent hydrolytic instability of TAF presents a challenge for application in LA systems. In this work, we examined the mechanism of TAF hydrolysis in a reservoir-based implant system and characterized TAF degradation kinetics as a function of the solution pH. We determined a pH "stability window" between pH 4.8 - 5.8 in which TAF degradation is substantially mitigated, with minimal degradation at pH 5.3. In a pursuit of a TAF formulation suitable for LA PrEP, we studied trans-urocanic acid (UA) as a buffer excipient. Here we show that UA can maintain the pH of TAF free base (TAFfb) solution inside a surrogate implant model at approximately pH 5.4. Through in vitro analysis, we demonstrated preservation of released TAF purity above 90% for over 9 months. Further, we performed an in vivo assessment of TAFfb-UA formulation in a reservoir-based nanofluidic implant inserted subcutaneously in non-human primates. Preventive levels of tenofovir diphosphate above 100 fmol/106 peripheral blood mononuclear cells were achieved in 2 days and sustained over 35 days. Fluid retrieved from implants after 60 days of implantation showed that UA preserved the aqueous phase in the implant at ~ pH 5.5, effectively counteracting the neutralizing action of interstitial fluids. Moreover, residual TAF in the implants maintained > 98% purity. Overall, TAF-UA represents a viable formulation applicable for LA HIV PrEP.

Keywords: Drug formulation; HIV PrEP; Implants; Long-acting drug delivery.

MeSH terms

  • Adenine / analogs & derivatives
  • Alanine
  • Animals
  • Anti-HIV Agents*
  • HIV Infections* / drug therapy
  • HIV Infections* / prevention & control
  • Leukocytes, Mononuclear
  • Tenofovir / analogs & derivatives
  • Urocanic Acid* / therapeutic use


  • Anti-HIV Agents
  • Tenofovir
  • tenofovir alafenamide
  • Urocanic Acid
  • Adenine
  • Alanine