Part 1 of the study will evaluate the safety, tolerability and pharmacokinetics of single, ascending doses of BCX4430 versus placebo in healthy subjects. Up to 6 ascending dose cohorts will be enrolled in a sequential manner. Eight subjects will be treated with a single dose of study drug per dose cohort: 6 subjects per cohort will receive BCX4430, and 2 subjects per cohort will receive matching placebo.