This is a double blind, randomised, placebo controlled, single dose escalation, first-time-in-human, 5-way crossover study to assess the safety, tolerability, pharmacokinetic and pharmacodynamic parameters of ascending doses of GSK189075A in healthy subjects (Part A). Single doses will be given to 10 healthy subjects. The planned doses are 20mg, 50mg, 150mg, 500mg, and 1000mg. Each subject will receive 4 of the 5 active doses and a placebo separated by a washout of 5-10 days. A second cohort of 6 subjects with type 2 diabetes mellitus will receive 2 active doses and a placebo along with oral glucose in a three-way, randomised, placebo controlled, crossover design to assess glucose lowering following GSK189075A administration (Part B). Blood samples will be taken throughout the study day for pharmacokinetic analysis of prodrug and metabolites. Safety will be assessed by measurement of blood glucose, blood pressure, heart rate, ECGs, laboratory safety screens, and collection of adverse events.