• Proportion of subjects in each arm with Grade 3 or greater treatment emergent adverse events (AE)
  
• Plasma Cmax, Cmin, tmax, AUC0→τ, AUC0→∞, t1/2, CL/F of DAPD and AZT in each group at Days 1 and 10
  
• Intracellular Cmax, Cmin, tmax, AUC0→τ, AUC0→∞, and t1/2 of DXG-TP, -MP and -DP and AZT-TP, -MP, and -DP at Days 1 and 10
  

• Change in viral load (plasma HIV-1 RNA) from Baseline through Day 10
  
• Correlate intracellular DXG-TP and AZT-TP with viral response, as measured by plasma HIV-1 RNA
  
• Quantitate DXG, AZT, and GAZT in urine with and without DAPD
  
• Characterize viral rebound (plasma HIV-1 RNA) following drug discontinuation for 48 hr
  
• Measure CD4+ count changes from Baseline to Day 10
  

By 2008, market data suggest that the most commonly prescribed initial treatment regimen for HIV-1 will consist of Truvada® (FTC and tenofovir disoproxil fumarate (TDF)) and Sustiva® (efavirenz (EFV)). A newly formulated drug called Atripla™ containing all three active ingredient has now been approved by the US FDA. Therefore, second line treatments that are currently in development should provide activity against resultant mutations, primarily M184V/I (17%) and much less commonly K65R (0 to 5%), and ideally prevent or be effective against mutations that may occur during second line therapy.

The goal of our program is to identify an AZT/DAPD co-formulation with reduced resistance development and an improved safety profile for the treatment of HIV infections. DAPD has increased sensitivity to M184V/I strains and is active against thymidine analog mutations (TAMs) that may have occurred during previous antiretroviral regimens. AZT offers anti-K65R activity which is believed to be conferred by the 3'-azido moiety containing pseudo-sugar structure and base components of AZT. Hence, AZT could potentially be incorporated to prevent the emergence of the K65R mutation that could limit the long-term benefit of DAPD.

Since nucleoside reverse transcriptase inhibitors (NRTIs) require intracellular phosphorylation to form their active triphosphates, it is preferable to combine NRTIs with different critical kinases. Thymidine kinase-1 is the critical enzyme for the phosphorylation of AZT to its monophosphates. The enzyme involved in the initial phosphorylation of the active metabolite of DAPD, DXG, is guanosine kinase. Nucleotide competition studies conducted using activated human peripheral blood mononuclear (PBM) cells performed with DAPD and AZT with concentrations between 0.1-10 µM demonstrated no competitive inhibition of DXG-triphosphate formation.

The approved dose for AZT is 300 mg bid, and AZT is available as a 300 mg tablet or 100 mg capsule. A previous study in 10 HIV seropositive individuals comparing cellular AZT-mono, di- and tri-phosphate nucleotides at normal and reduced doses, 100 mg tid versus 300 mg bid, demonstrated a significant decrease in plasma AZT and intracellular AZT-monophosphate (AZT-MP) levels, (AZT-MP is associated with toxicity), without significant changes in AZT-triphosphate (AZT-TP) levels in activated PBM cells or antiviral activity. These findings on the effect of AZT dose on intracellular AZT-TP are supported by computer simulations. Maximal predicted cellular levels of AZT-MP and AZT-TP at steady-state, following 200 mg bid and 300 mg bid dosing in 3,000 simulated individuals demonstrated a high overlap between the AZT-TP histograms (> 85 %), suggesting similar efficacies for the 200 and 300 mg bid doses. The low degree of overlap between the AZT-MP histograms (< 8 %) for the two dose regimens, suggests that there may be fewer toxicities with the 200 mg bid dose, supporting our hypothesis that a zidovudine dose of 200 mg bid could reduce the AZT-MP levels without compromising the AZT-TP levels that would be obtained with a 300 mg bid dose.

This 10 day, proof of principle, pharmacokinetic study will provide important information about reduced AZT dosing to support the development of an AZT/DAPD co-formulation which may prevent the emergence of K65R mutations.