"In our view, it is important to note that PSI-938 has a different active species (2-fluoro guanosine triphosphate) than INX-189 (2-methyl guanosine triphosphate) and that the compounds are not similar outside of the guanosine base - looking at the compounds as a whole, we believe INX-189 is more structurally similar to PSI-7977 than PSI-938. We also note that there are guanosine drugs marketed such as acyclovir, and therefore we do not believe that guanosine is the reason for the PSI-938 toxicity." Deutstch Bank analyst Robyn Karnauskas held a call for clients Friday morning with an Inhibitex executive and a medicinal chemist who has worked on INX-189 and is familiar with other hepatitis C drugs. On the call, Inhibitex re-affirmed the absence of liver-related toxicity in clinical studies of INX-189 to date at doses up to 100 mg. In one ongoing study, four patients treated with 100 mg of INX-189 have gone through 10 weeks of treatments, with two of those patients completing treatment. Not all are convinced that INX-189 is in the clear. "If you compare the activated forms of each (which happens pretty fast) the 2 structures are much much closer: specifically, varying only in that 938 has a F at C-2' in and 189 has a OH (F is a chemical isostere of OH)...The 938 result does not necessarily mean the same will hold for 189, but as I see it, it increases odds," said one buy-side analyst, referring to a side-by-side diagram of the chemical structures of INX-198 and PSI-939. This analyst had a short position in Inhibitex and was using Friday's slide to partially cover. Idenix Pharmaceuticals is developing hepatitis C drugs in the same class as Inhibitex and Pharmasset, so naturally, shares were down 10% to $7 Friday. Perhaps, the only people chuckling Friday were executives at Vertex Pharmaceuticals ( VRTX), which was supposed to be a hepatitis C also ran due to the looming entrant of drugs like those from Pharmasset and Inhibitex. Perhaps Vertex isn't out of the hepatitis C game quite yet. Vertex shares were up 5% to $33.20. --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. >To submit a news tip, send an email to: firstname.lastname@example.org. Follow TheStreet on Twitter and become a fan on Facebook.