 Subjects in Cohort 1 n4 will receive a single 1mg IV dose given as a 15 minute infusion of GW856553. Safety, tolerability and PK exposures will be reviewed, and in an optional cohort Cohort 2 n4, the dose may be adjusted upward or downward based on data from Cohort 1. Based on the results from Cohort 1 and Cohort 2 if required, Cohort 3 n12 will be dosed appropriately to receive a single IV 15 minute infusion of GW856553. 