This is a two-part study. Part 1 is designed to find the best modified release formulation of GW423753; Part 2 is designed to use the selected formulation(s) from Part 1 to find out if food affects the way the medication is processed by the body.

Inclusion Criteria:

• Healthy males & females
• Part 1 ages 18-60
• Part 2 ages 18-50 & 65-80
• Within normal weight range given your height
• Negative urine drug and alcohol test
• Willing to follow all study procedures

Exclusion Criteria:

• Any clinically relevant abnormality identified on the screening physical examination, clinical laboratory tests, 12-lead ECG, or any other medical condition or circumstance making the subject unsuitable for participation in the study based on the Investigator's assessment.
• Subjects with either a blood pressure measurement > 150/90 mmHg, or a history of coronary disease AND a blood pressure > 140/90 mmHg, at screening.
• History of drug or other allergy, which, in the opinion of the Investigator, contraindicates their participation.
• Regular alcohol consumption averaging >/7 drinks/week for women or >/ 14 drinks/week for men (1 drink = 5 ounces of wine or 12 ounces of beer or 1.5 ounces of hard liquor) within 6 months of the first dose of study medication.
• Positive urine drug, alcohol or serum pregnancy test at screening and prior to dosing in each study session.
• Positive for hepatitis C antibody, hepatitis B surface antigen or HIV at screening.
• Treatment with an investigational drug within 30 days or 5 half-lives (whichever is longer) prior to administration of study medication.
• Use of prescription or non-prescription drugs within 7 days or 5 half-lives (whichever is longer) prior to administration of study medication and during the study. By exception, use of acetaminophen at doses of ≤ 2 grams per day, the use of contraceptives (oral, depots, patches, etc.) and anti-hypertensive medications will be permitted.
• Use of vitamins or herbal/dietary supplements within 7 days prior to administration of study medication and during the study.
• Subject is unable and/or unwilling to adhere to Lifestyle Guidelines
• Subjects who have donated more than 500 mL of blood or plasma within 56 days prior to administration of study medication.
• An unwillingness on the part of male volunteers to abstain from sexual intercourse with pregnant or lactating women; or an unwillingness to use a condom and another form of contraception (e.g., IUD, birth control pills taken by female partner, diaphragm with spermicide) if engaging in sexual intercourse with a woman who could become pregnant from the time of the first dose of study medication until completion of follow-up procedures.
• Pregnant or lactating woman. A pregnancy test will be performed for all women at screening and prior to each dosing session to confirm eligibility.
• The subject has a known hypersensitivity or idiosyncratic reaction to any drug chemical related to this study.