This low potency arose in part because the enzymatic addition of the first of the three phosphate groups of the triphosphate is slow. The design of sofosbuvir, based on the ProTide approach, avoids this slow step by building the first phosphate group into the structure of the drug during synthesis. Additional groups are attached to the phosphorus to temporarily mask the two negative charges of the phosphate group, thereby facilitating entry of the drug into the infected cell. The NS5B protein is a RNA-dependent RNA polymerase critical for the viral reproduction cycle. History
Sofosbuvir was discovered in 2007 by Michael Sofia, a scientist at Pharmasset, and the drug was first tested in people in 2010. In 2011, Gilead Sciences bought Pharmasset for about $11 billion. Gilead submitted the New Drug Application for sofosbuvir in combination with ribavirin in April 2013, and in October 2013 it received the FDAs Breakthrough Therapy Designation. In December 2013, the FDA approved sofosbuvir in combination with ribavirin for oral dual therapy of HCV genotypes 2 and 3, and for triple therapy with injected pegylated interferon (pegIFN) and RBV for treatment-naive people with HCV genotypes 1 and 4. Two months before, the FDA had approved another drug, simeprevir, as a hepatitis C treatment.In 2014, the fixed dose combination drug sofosbuvir/ledipasvir, the latter a viral NS5A inhibitor, was approved; it had also been granted breakthrough status.Prior to the availability of sofosbuvir, hepatitis C treatments involved 6 to 12 months of treatment with an interferon-based regimen.
CPNP belongs to the nitrosamine class of compounds, some of which are classified as probable or possible human carcinogens (substances that could cause cancer), based on laboratory tests such as rodent carcinogenicity studies. Although there are no data available to directly evaluate the carcinogenic potential of CPNP, information available on closely related nitrosamine compounds was used to calculate lifetime exposure limits for CPNP.As of January 2021, the FDA continues to investigate the presence of 1-methyl-4-nitrosopiperazine (MNP) in rifampin or 1-cyclopentyl-4-nitrosopiperazine (CPNP) in rifapentine approved for sale in the US. See also
Rifampicin
Isoniazid
Rifamycin
Rifabutin

References
External links
"Rifapentine". Drug Information Portal. U.S. National Library of Medicine.