Novel Long-Acting ProTides of Approved ARVs

Speaker: Benson Edagwa, Associate Professor of Pharmacology and
Experimental Neuroscience at University of Nebraska Medical Center

Shared progress using the prodrug approach to develop long-acting formulations of integrase inhibitors and Tenofovir alafenamide.

Overview of the prodrug approach and formulation advantages.

  • Creating a prodrug (ARV + promoiety) allows the active drug (ARV) to be formulated as a nanosuspension (prodrug nanocrystals). Once injected, the half-life of the ARV depends on the rate of prodrug release from the nanosuspension and subsequent slow hydrolysis (generates the ARV and a non-toxic promoiety).
  • ARV prodrug formulations are very stable (particle size, homogeneity and stability of API within the formulation and at various temperatures) – readily syringeable via a 28-gauge needle after several months of storage.

Preclinical studies suggest that novel integrase inhibitor prodrug formulations are well-tolerated locally and extend the half-life of the active drug (CAB, DTG and BIC) to potentially enable a once yearly injection. 

  • After a single injection, NM2CAB (45mg CAB eq/kg in rats), NM2DTG (45mg DTG eq/kg in rats) and NM2BIC (45mg BIC eq/kg in mice) sustained plasma ARV concentrations > PAIC90 for up to 12 months
  • High tissue levels of M2DTG (prodrug) and DTG were detected for up to 7 months following a single injection in monkeys – secondary storage in liver, lung, kidney, spleen, muscle and lymph nodes extends the half-life of the ARV (prodrug undergoes hydrolysis to sustain therapeutic concentrations of active drug in the tissues). 
  • Histology and imaging at the injection site (NM2DTG-injected vs saline injected vs uninjected) show normo-expected histiocytic infiltration followed by macrophages (carry drug from the injection site to peripheral tissues). 

Tenofovir alafenamide (TAF) is potent and inherently long-acting, yet unstable within the prodrug formulation (susceptible to hydrolysis) – applying the prodrug approach to tenofovir (TFV), creates a stable, long-acting ProTide that suppresses HBV infection for up to 3 months in a mouse model.

  • TFV ProTide formulations (NM1TFV and NM2TFV) are stable for months. 
  • Preclinical PK studies indicate that NM1TFV and NM2TFV readily convert to TFV-DP (active metabolite) in vivo, have no advantage over nanoformulated TAF (NTAF) in PBMCs, but lead to higher tissue levels of TFV-DP.   
    • A single injection of NM1TFV, NM2TFV or NTAF provides TFV-DP exposure > EC90 in PBMCs for 56 days. 
    • TFV-DP levels in rectal tissue and parenchymal cells differ by formulation administered – NM1TFV > NM2TFV > NTAF up to 2 months following a single injection (the study will continue for > 6 months to see how long the effect could last).  
  • A single injection of NM1TFV (lead candidate) suppressed HBV infection (HBV DNA) for up to 3 months in humanized mice infected with HBV, whereas NTAF was ineffective. 
    • The model was validated using serial human albumin levels (human hepatocytes remained stable) and liver histology (NM1TFV – liver cells only showed staining of human hepatocyte marker; NTAF and no treatment – liver cells showed staining of human hepatocyte marker and HBV markers, HBcAg and HBsAg). 

Summary and Next Steps.

  • Prodrug and formulation manufacture is scalable. 
  • Preclinical PK studies support the potential for once-yearly dosing of CAB, DTG and BIC prodrug nanocrystals.
  • TFV can be transformed into a long-acting ProTide that can suppress HBV replication for over 3 months in a mouse model. 
  • Exavir Therapeutics, Inc. has licensed the long-acting antiviral agents and has several pipeline programs to address HIV, HBV and HIV-HBV co-infection.
  • We continue to look for partners to accelerate development of prodrug formulations.
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