This will be a randomized, placebo-controlled, single, oral dose, four-way Williams crossover study design in healthy male and female subjects. The study consists of screening (28 days), treatment (1 day/dosing session) and follow-up (7 to 14 days) period and the total duration of study participation for each subject will be approximately 9 weeks. Each subject will participate in 4 dosing sessions separated by a minimum 7-day washout period. All subjects will receive single doses of retosiban 100 mg, (treatment A) retosiban 800 mg (Treatment B), moxifloxacin 400 mg (Treatment C) and placebo (Treatment D) in one of the four treatment sequences (ABDC, BCAD, CDBA, DACB) following a Williams design Twelve-lead ECGs and continuous Holter monitoring, clinical laboratory safety tests, vital sign measurements, physical examinations, adverse event reports, and pharmacokinetic samples will be collected throughout the study. In each study period, cardiac conduction will be measured using a 24-hour continuous 12-lead Holter monitor from the morning of Day 1 (dosing) until the morning of Day 2.

Inclusion Criteria:

• - Male or female between 18 and 45 years of age inclusive, at the time of signing the informed consent
• A female subject is eligible to participate if she is of: child-bearing potential and agrees to use the contraception methods from time of consent until until 48 hours post last dose; Non-childbearing potential defined as pre-menopausal females with a documented tubal ligation or hysterectomy; or postmenopausal defined as 12 months of spontaneous amenorrhea. Females on hormone replacement therapy (HRT) and whose menopausal status is in doubt will be required to use the contraception methods if they wish to continue their HRT during the study. Otherwise, they must discontinue HRT to allow confirmation of post-menopausal status prior to study enrollment. For most forms of HRT, at least 2 to 4 weeks should elapse between the cessation of therapy and the blood draw; this interval depends on the type and dosage of HRT. Following confirmation of their post-menopausal status, they can resume use of HRT during the study without use of a contraceptive method.
• 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 subject with a clinical abnormality or laboratory parameters outside the reference range for the population being studied may be included only if the Investigator and the GlaxoSmithKline (GSK) Medical Monitor agree that the finding is unlikely to introduce additional risk factors and will not interfere with the study procedures.
• Body weight (greater than or equal to) >=50 kg and body mass index (BMI) within the range 19 to 29.9 kg/m^2
• Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form
• No significant abnormality on 12-lead ECG at screening, including the following specific requirements: ventricular rate >=40 beats per minute, PR interval less than or equal to (<=) 210 miliseconds (msec), Q waves less than (<)30 msec, QRS interval to be >=60 msec and <120 msec; the waveforms must enable the QT interval to be clearly defined; QTcB or QTcF interval must be <450 msec
• ALT, alkaline phosphatase and bilirubin <=1.5xULN (isolated bilirubin greater than (>) 1.5x upper limit of normal (ULN) is acceptable if bilirubin is fractionated and direct bilirubin <35%)

Exclusion Criteria:

• A positive pre-study Hepatitis B surface antigen or positive Hepatitis C antibody result within 3 months of screening.
• A supine blood pressure that is persistently higher than 140/90 millimeters of mercury (mmHg).
• A supine mean heart rate outside the range 40 to 90 beats per minute (bpm).
• 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 g of alcohol: 12 ounces (360 ml [milliliters]) of beer, 5 ounces (150 ml) of wine or 1.5 ounces (45 ml) of 80 proof distilled spirits.
• 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.
• History of sensitivity to any of the study medications, 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.
• 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 human chorionic gonadotropin (hCG) test at screening or prior to dosing. Lactating females.
• Unwillingness or inability to follow the procedures outlined in the protocol.
• Urinary cotinine levels indicative of smoking or history or regular use of tobacco- or nicotine-containing products within 6 months prior to screening.
• 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.
• History or presence of any medically significant disease, or any disorder that would introduce additional risk or interfere with the study procedures or outcome. In particular, a family history of QT prolongation, of early or sudden cardiac death or of early cardiovascular disease.
• Use of prescription or non-prescription drugs, including vitamins, herbal and dietary supplements from 14 days before screening until the follow-up visit unless in the opinion of the Investigator and sponsor the medication will not interfere with the study or compromise subject safety.
• Serum calcium, magnesium or potassium levels outside the reference range
• History of sensitivity to quinolone antibiotics 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.

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