CAMBRIDGE, Mass. ( TheStreet) -- Vertex Pharmaceuticals ( VRTX) is advancing one experimental hepatitis C drug into mid-stage clinical trials while shutting down work on a second drug due to poor efficacy, the company announced Tuesday. Both Vertex drugs, ALS-2200 and ALS-2158 belong to the class of hepatitis C drugs known as nucleotide (or nucleoside) polymerase inhibitors -- commonly referred to as "nucs" -- which have the potential to become the backbone of new, all-oral combination regimens. The current superstar of the "nuc" drug class is Gilead Sciences' ( GILD) GS-7977, but Bristol-Myers Squibb ( BMY) was forced to abandon its nuc BMS-094 because it caused heart failure. The fallout from Bristol's drug has since spread to Idenix Pharmaceuticals ( IDIX) and its nuc IDX-184, which is on clinical hold until U.S. regulators are assured the drug is safe. The combination of ALS-2200 plus ribavirin yielded a 4.2 log10 reduction in viral load after seven days of dosing in previously untreated, genotype 1 hepatitis C patients. Eight patients were treated, of which five were able to achieve undetectable levels of the hepatitis C virus, Vertex announced Tuesday. Vertex also disclosed that the "uridine" chemical structure of ALS-2200 is the same as Gilead's GS-7977. This is important since the recent safety issues related to nucs appear to be limited to those drugs with a "purine" chemical structure. Based on these results, Vertex plans to move ALS-2200 into two, phase II studies. One study will evaluate ALS-2200 in combination with ribavirin, the other study will combine ALS-2200 with Vertex's currently marketed hepatitis C drug Incivek. Vertex is discussing the new studies with regulatory agencies and hopes to begin enrollment before the end of the year. While Vertex is making progress towards developing an all-oral regimen for hepatitis C, the work being done by Gilead and others is still considerably in the lead. Vertex's ALS-2200 may also be less potent than GS-7977. Vertex's ALS-2158 will not be moving ahead into new studies due to insufficient antiviral activity, the company said. Vertex shares were down 1.5% to $57.86 in early Tuesday trading. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein.