"The active in vivo metabolite of NM283 was a triphosphate analog of cytosine, while the active in vivo metabolite of IDX184 is a triphosphate analog of guanosine."
i didn't realize that - but the important difference is one of potency/toxicity, not the fact that one competitively inhibits a different base substrate of the polymerase enzyme. bypassing the first phosphorylation step is simply the explanation for how 184 is more potent/less toxic than 283