The purpose of this study is to determine whether GSK1521498 attaches to sites in the brain called mu-opioid receptors that are involved in the liking reactions to palatable high fat and high sugar foods. We hope that blocking these receptors may modify certain behaviours that may lead to obesity.

This imaging study will be an open-label, non-randomised PET receptor occupancy study in healthy male volunteers. The degree and time course of μ-opioid receptor occupancy (RO) following single oral doses of GSK1521498 will be estimated by [11C]carfentanil displacement. Previous pre-clinical and human PET studies indicate that [11C]carfentanil is selective for the μ-opioid receptor and can be used to estimate μ-opioid receptor occupancy in vivo.

The PK/PD relationship between plasma concentrations of GSK1521498 and μ-opioid RO will be described. Potential relationships between μ-opioid RO and functional magnetic resonance imaging (fMRI) endpoints, measured in a food reward paradigm, will be assessed as an exploratory aim. Additionally, the PK/PD relationships between plasma concentrations of naltrexone (a generic μ-opioid receptor antagonist), μ-opioid receptor occupancy, and fMRI measures of reward processing will also be investigated.

Inclusion Criteria:

• Healthy male subjects aged between 25 and 65 years old inclusive.
• Body weight ≥ 50 kg and BMI within the range 18.5.0 - 30.0 kg/m2 (inclusive).
• Normal ECG.
• The subject is able to read, comprehend and record information.
• A signed and dated written informed consent is obtained from the subject.
• Compliance with birth control methods as described in the study protocol.

Exclusion Criteria:

A subject will not be eligible for inclusion in this study if any of the following criteria apply:

• The subject has a positive pre-study drug/alcohol screen.
• History of hepatitis B and /or C
• A positive result for an HIV test.
• Abnormal thyroid function
• Positive evaluation for depression.
• History of heavy alcohol use as described in the study protocol.
• The subject has participated in a clinical trial and has received an investigational product within: 90 days.
• Participation in other drug studies within a calendar year.
• Use of prohibited medications as described in the study protocol.
• History of sensitivity to any of the study medications.
• Where participation in the study would result in donation of blood or blood products in excess of 500 mL within a 56 day period
• Unwillingness or inability to follow the procedures outlined in the protocol.
• Past or present use of tobacco products.
• Consumption of red wine, seville oranges, grapefruit or grapefruit juice from 7 days prior to the first dose of study medication.
• Previous radiation dosages in excess of levels acceptable to take part in this study.
• History of claustrophobia or history of neurological conditions.
• Presence of a cardiac pacemaker.
• Works as a welder, metal worker or machinist

Patient-level data for this study will be made available through www.clinicalstudydatarequest.com following the timelines and process described on this site.