BMS is shelling out $2.5 billion dollars for Inhibitex, a small pharma company with a Phase II molecule for treatment of Hepatitis C (HCV). The deal adds to the scramble for HCV assets in recent months, with Gilead agreeing to pay almost $11 billion for Pharmasset in November, and Roche’s recent purchase of Anadys. While much has been written about the merits (and price tags) of each deal, the Haystack thought it was worth taking a closer look at the chemical composition of the multi-million dollar molecules.
So what did BMS get for their money?
INX-089, Inhibitex’s lead molecule, has a common antiviral motif: a nucleoside core (the 5-membered ring sugar attached to a nitrogen heterocycle) with an amino acid based prodrug hanging off the left-hand side. Clinically-tested antivirals sharing this basic setup include IDX-184 and NM-283, both from Idenix, and PSI-352938, from Pharmasset (For an overview of the varied structures currently in development for HCV, see Lisa’s 2010 C&EN story).
INX-089 bears a close resemblance to Pharmasset’s lead nucleotide inhibitor PSI-7977. That’s not a mistake, believes ‘089 discoverer Chris McGuigan, of the Welsh School of Pharmacy. In a recent article (J. Med. Chem. 2010, 53, 4949), McGuigan himself comments “The Pharmasset nucleoside [is] rather parallel to our early work on anti-HIV ProTides.”
Wait, what are ProTides?
Both INX-089 and PSI-7977 aren’t themselves the active viral inhibitor, but phosphoramidate “ProTide” prodrugs: compounds broken down by the body into the active drug (Chem Note: PSI-7977 has single-enantiomer Sp chirality at phosphorus, while INX-189 is a mixture of diastereomers).
Once in the body, enzymes cleave the phosphoramidate group to a phosphate (PO42-). Kinases attach two more phosphate groups, and viruses let this dressed-up molecule inside, where the nucleotide warhead inhibits HCV by interfering with RNA replication (Antimicrob. Agents Chemother. 2011, 55, 1843).
A few comments on the drug itself: The similarity of the ProTide portion (left-hand side) of the molecule to PSI-7977 really is striking: swap in an isobutyl ester and a phenyl, and it’s the same beast! The more interesting switch comes on the upper-right (“eastern”) part of the structure: a protected guanosine ring. This ring harks back to guanine, one of the four common nucleic acids found in DNA.
PSI-7977, meanwhile, shows off a uracil, a nucleic acid found in RNA, not DNA.
Although it’s tempting to think such similar compounds all dock into the NS5B polymerase at the active site (in the yellow “palm” of the hand-shaped enzyme), don’t be too sure – a recent paper by Pharmasset scientists (J. Med. Chem. 2012, Just Accepted) shows quite a few “Finger,” “Palm,” and “Thumb” sites. It’s not yet clear whether all nucleoside drugs bind to the active site in the same way. The authors also remark that, due to fast replication and mutation, potentially resistant strains of HCV pop up daily.