Subjects will participate in all three treatment periods and will be randomized to receive each of the following for 5 days: Treatment A: Placebo, Treatment B: Naltrexone (NTX) 50 mg once daily (25 mg once daily for the first two days) and Treatment C: GSK1521498 10 mg once daily. A washout period will be of at least 14 days between treatments. Subjects will return for a follow-up visit 7-10 days after the final treatment session washout period has been completed.