Ravidasvir (RDV, ASC16) is a second‐generation, pan‐genotypic non‐structural (NS) 5A inhibitor, which inhibits viral replication and assembly. Ravidasvir exhibits high antiviral potency with EC50 0.04–1.14 nM for HCV GT1–GT6. The pharmacokinetics results indicated that steady status achieved quickly after the first dose. Metabolism studies utilizing human clinical samples showed that Ravidasvir was very stable, with only modest (~2%) metabolite formation. Biliary excretion of Ravidasvir appears to be the primary route of elimination of the absorbed dose, while renal excretion of the intact drug appears to be negligible. In clinical trans twelve-week Ravidasvir and ritonavir-boosted Danoprevir in combination with ribavirinfor 12 weeks achieve the sustained virologic response rate of 100% in treatment-naïve non-cirrhotic Asian patients with HCV GT1 infection. Ravidasvir for treatment‐naïve, non‐cirrhotic HCV GT1 patients was safe and well tolerated. There was no death, treatment‐related serious adverse events, and discontinued cases due to adverse events.

Mericitabine is a specific inhibitor of hepatitis C virus (HCV) replication that target NS5B polymerase. Mericitabine, a prodrug, is hydrolyzed in vivo to produce PSI-6130. It had been studied in phase II clinical trials for the treatment of chronic hepatitis C. However, while it showed a good safety profile in clinical trials, it was not sufficiently effective to be used as a standalone agent.

Fozivudine tidoxil is a thioether lipid–Zidovudine (ZDV) conjugate. After intake it is split intracellularly into the lipid moiety and ZDV-monophosphate, which is subsequently phosphorylated to the active metabolite ZDV-triphosphate. The rationale behind the development of fozivudine (FZD) was to take advantage of the high cleavage activity in mononuclear cells and other organs resulting in increased amounts of intracellular ZDV available for phosphorylation to the active metabolite, and a very low activity in red blood and stem cells, which should result in reduced haematologic toxicity. It is member of the family of nucleoside reverse transcriptase (RT) inhibitors. Fozivudine tidoxil has been in Phase II clinical trials for the treatment of HIV infection. There were three adverse events possibly related to fozivudine: urine abnormality, gastrointestinal pain and abnormal dreams.

Torcitabine is the beta-L-enantiomer of the natural nucleoside D-cytidine. The drug was under development as an antiviral agent for the treatment of chronic hepatitis B virus infection. Torcitabine has poor oral bioavailability, but its 3’,5’-derivative ester (val-L-dC) and the 3’-monovaline ester, valtorcitabine dihydrochloride, have excellent oral bioavailability and consequently the torcitabine prodrug, valtorcitabine, has replaced torcitabine in clinical development. Torcitabine is active against hepadnaviruses, specifically human hepatitis B virus (HBV), duck hepatitis virus (DHBV) and woodchuck hepatitis virus (WHV). Torcitabine triphosphate is a selective inhibitor of the polymerase enzyme of HBV.

Celgosivir is a butanoyl ester derivative of castanospermine, a compound derived from the Australian chestnut with activity against hepatitis C virus. Celgosivir rapidly converts to castanospermine in the body, where it is a potent inhibitor of alpha-glucosidase I, a host enzyme required for viral assembly, release, and infectivity.

Bevirimat (3-O-(3',3'-dimethylsuccinyl) betulinic acid or MPC-4326 or PA-457) potently inhibits replication of both WT and drug-resistant HIV-1 isolates and demonstrate that the compound acts by disrupting a late step in Gag processing involving conversion of the capsid precursor (p25) to mature capsid protein (p24). Bevirimat inhibits replication of both wild-type and drug-resistant HIV-1 isolates in vitro, achieving similar 50% inhibitory concentration values with both categories. Serial drug passage studies have identified six single amino acid substitutions that independently confer bevirimat resistance. These resistance mutations occur at or near the CA-SP1 cleavage site, which is not a known target for resistance to other antiretroviral drugs. Bevirimat has been in phase 2 trial for the treatment of HIV infections. Bevirimat has demonstrated a consistent pharmacokinetic profile in healthy volunteers and HIV-infected patients. The demonstration of an antiviral effect following a single oral dose of bevirimat validates maturation inhibition as a potential target for antiretroviral therapeutics in humans.

Tiviciclovir (A188) is a nucleoside analog used in the synthesis of antiviral drugs. This acyclic guanosine analog has a potential for the treatment of hepatitis B virus. However, as with penciclovir and the related compounds acyclovir and ganciclovir, the intestinal absorption of Tiviciclovir is limited. One approach to overcome this is through synthesis of the 6-deoxy prodrug version of AM188, i.e. AM365, which has improved intestinal absorption. Following absorption, the 6-deoxyguanosine analog AM365 is converted to the corresponding anti-virally active guanosine analog, AM188, probably by the liver molybdenum hydroxylases, aldehyde oxidase and xanthine oxidase. Renal tubular secretion of AM188 involves organic anion and cation transport systems.

R1479 (4′-azidocytidine) is a specific inhibitor of hepatitis C virus (HCV) replication. 4′-azidocytidine triphosphate is an inhibitor of native HCV replicase and recombinant HCV polymerase (NS5B). Balapiravir (R1626) is the tri-isobutyrate ester prodrug of R1479 under clinical development to improve exposure of R1479 upon oral administration.

R1479 (4′-azidocytidine) is a specific inhibitor of hepatitis C virus (HCV) replication. 4′-azidocytidine triphosphate is an inhibitor of native HCV replicase and recombinant HCV polymerase (NS5B). Balapiravir (R1626) is the tri-isobutyrate ester prodrug of R1479 under clinical development to improve exposure of R1479 upon oral administration.

Droxinavir is hydroxyethylurea human immunodeficiency virus type 1 (HIV-1) protease inhibitor. Position 88 of HIV-1 protease gene plays a key role in the interaction between droxinavir and the protease molecule. A mutation in this position, located outside the active site, confers resistance to the hydroxyethylurea inhibitor droxinavir. The V82A and N88S substitutions conferred droxinavir resistance on HIV-1 recombinant variants. Positions 82 and 88 are reported to be variable in natural populations isolated from patients who have not been treated with protease inhibitors. Droxinavir had been in preclinical phase for the treatment of HIV-1 infection. However, this study was discontinued.