Fialuridine
Fialuridine, or 1--5-iodouracil, is a nucleoside analogue that was investigated as a potential therapy for hepatitis B virus infection. In a 1993 clinical study at the NIH, unexpected toxicity led to the death of 5 out of 15 patients from liver failure alongside lactic acidosis; two further participants required liver transplantation. It is suspected that the toxicity of fialuridine was a result of mitochondrial damage caused by the incorporation of fialuridine into mitochondrial DNA via its 3'-hydroxyl moiety, leading to impaired DNA synthesis. This toxicity was unusual in that it was not predicted by animal studies.