This is a single-center, randomized, open-label, 3 parallel treatment study in healthy adult subjects to assess the relative bioavailability of new formulations of GSK1265744 LAP 400 mg intra muscular compared to the current GSK1265744 LAP 400 mg nanomilled formulation. This study will evaluate LAP formulations of GSK1265744 with different particle sizes.

Following a 14 day lead in period with oral GSK1265744, forty-five subjects will receive 400 mg of one of three GSK1265744 formulations which vary in particle size from 200 nm to 5 um by intramuscular injection. Samples for determination of GSK1265744 concentrations will be collected for 12 weeks post-injection. Safety will be evaluated by adverse event recording and laboratory values at frequent intervals throughout the trial. A subgroup of 12 subjects will receive a 3 mg dose of oral midazolam at baseline on Day-29 and then again on the last day of the oral GSK1265744 lead in period to evaluate the effect of GSK1265744 on CYP3A enzymes.

The subjects will undergo follow-up evaluations for a minimum of 12 weeks.

Inclusion Criteria

• Male or female between 18 and 65 years of age inclusive, at the time of signing the informed consent.
• Healthy as determined by a responsible and experienced physician, based on a medical evaluation including medical history, physical examination, laboratory tests and cardiac monitoring.
• A female subject is eligible to participate if she is of: non-childbearing potential. Female subjects of child bearing potential must agree to use contraception for at least 6 months after the final dose of study drug and should understand that drug concentrations may be measurable for up to one year after final dose.
• Male subjects with female partners of child-bearing potential must agree to use one of the required contraception methods noted in the protocol.
• Body weight >=50 Kilograms (kg) for men and >=45 kg for women and body mass index (BMI) within the range 18.5-31.0 kg/m^2 (inclusive).
• Alanine aminotransferase (ALT), alkaline phosphatase and bilirubin <= 1.5xupper limit of normal (ULN) (isolated bilirubin >1.5xULN is acceptable if bilirubin is fractionated and direct bilirubin <35%).
• Single QTcB <450 msec.

Exclusion Criteria:

• A positive pre-study Hepatitis B surface antigen or positive Hepatitis C antibody result within 3 months of screening
• Current or chronic history of liver disease, or known hepatic or biliary abnormalities (with the exception of Gilbert's syndrome or asymptomatic gallstones).
• A positive pre-study drug/alcohol screen.
• A positive test for Human Immunodeficiency Virus (HIV) antibody.
• History of regular alcohol consumption within 6 months of the study defined as:

An average weekly intake of >14 drinks for males or >7 drinks for females. One drink is equivalent to 12 grams (g) of alcohol: 12 ounces (360 milliliters [mL]) of beer, 5 ounces (150 mL) of wine or 1.5 ounces (45 mL) of 80 proof distilled spirits.

• History or regular use of tobacco-or nicotine-containing products within 3 months prior to screening.
• The subject has participated in a clinical trial and has received an investigational product within the following time period prior to the first dosing day in the current study: 30 days, 5 half-lives or twice the duration of the biological effect of the investigational product (whichever is longer).
• Exposure to more than four new chemical entities within 12 months prior to the first dosing day.
• Unable to refrain from the use of prescription or non-prescription drugs, including vitamins, herbal and dietary supplements (including St John's Wort) within 7 days (or 14 days if the drug is a potential enzyme inducer) or 5 half-lives (whichever is longer) prior to the first dose of study medication, unless in the opinion of the Investigator and GSK Medical Monitor the medication will not interfere with the study procedures or compromise subject safety.
• History of sensitivity to any of the study medications, including midazolam and flumazenil, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GSK Medical Monitor, contraindicates their participation.
• History or presence of allergy or intolerance to the study drugs or their components or drugs of their class.
• Where participation in the study would result in donation of blood or blood products in excess of 500 mL within a 56 day period.
• Pregnant females as determined by positive serum or urine hCG test at screening or prior to dosing.
• Lactating females.
• Unwillingness or inability to follow the procedures outlined in the protocol.
• Subject is mentally or legally incapacitated.
• History of sensitivity to heparin or heparin-induced thrombocytopenia.
• Unable to refrain from consumption of red wine, seville oranges, grapefruit or grapefruit juice and/or pummelos, exotic citrus fruits, grapefruit hybrids or fruit juices from 7 days prior to the first dose of study medication.
• The subject's systolic blood pressure is outside the range of 90-140mmHg, or diastolic blood pressure is outside the range of 45-90mmHg.
• History of clinically significant cardiovascular disease.
• Any significant arrhythmia which, in the opinion of the principal Investigator and GSK Medical Monitor, will interfere with the safety for the individual subject. Non-sustained (>=3 consecutive ventricular ectopic beats) or sustained ventricular tachycardia.