NHC-triphosphate is an active phosphorylated intracellular metabolite of β-d-N4-Hydroxycytidine (NHC) as a triphosphate form[1]. NHC-triphosphate is a weak alternative substrate for the viral polymerase and can be incorporated into HCV replicon RNA[1][2].

In an intracellular metabolism assay, HCV replicon cells are treated with 10 μM 3H-labeled NHC, and intracellular nucleotide levels are determined after 1, 2 and 8 hours incubations. NHC is rapidly convered into the mono-, di-, and triphosphate forms, and NHC-TP reaches up to 71.12 pM after 8 hours[1].NHC-triphosphate (NHC-TP) (5-40 μM) absence leads to full-length polymerization products, it can be a weak alternative substrate. In addition, incorporation of NHC-TP instead of CTP increases the molecular weight of the polymerization product by 16 (one extra oxygen) for each event and an obvious electrophoretic shift is observed in cell-free HCV NS5B polymerization reactions[1].Huh-7 cells are incubated with (10-50 μM; 4 h) NHC or a McGuigan phosphoramidate prodrug of NHC. Intracellular levels of the parental compounds and phosphorylated metabolites are measured using LC-MS/MS. Small amounts of NHC-monophosphate (MP) and NHC-diphosphate (DP) can be observed, while NHC-triphosphate remains the most abundant metabolite[2].NHC-triphosphate (NHC-TP) metabolite may directly target the viral polymerase and behave as a nonobligate chain terminator. It plays a prominent role in inhibiting early negative-strand RNA synthesis, either through chain termination or mutagenesis, which may in turn interfere with correct replicase complex formation.

[1]. Stuyver LJ,et al. Ribonucleoside analogue that blocks replication of bovine viral diarrhea and hepatitis C viruses in culture.Antimicrob Agents Chemother. 2003 Jan;47(1):244-54. [2]. Maryam Ehteshami, et al. Characterization of β-d- N4-Hydroxycytidine as a Novel Inhibitor of Chikungunya Virus.