A Study to Evaluate the Pharmacokinetics, Safety and Tolerability of Retosiban (GSK221149) When Dosed With Efavirenz (EFZ)

Inclusion Criteria:

- Male/females aged between 18 and 45 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 subject with a clinical abnormality or laboratory parameter(s)
which is/are not specifically listed in the inclusion or exclusion criteria, outside
the reference range for the population being studied may be included only if the
Investigator in consultation with the GlaxoSmithKline (GSK) Medical Monitor agree and
document that the finding is unlikely to introduce additional risk factors and will
not interfere with the study procedures.

- Body weight >=50 kilogram (kg) and body mass index within the range 19-29.9 kg/m^2.

- A female subject is eligible to participate if she is of: Child-bearing potential
with negative pregnancy test as determined by serum human chorionic gonadotrophin
(hCG) test at screening or prior to dosing; AND Agrees to use the contraception
methods for an appropriate period of time (as determined by the product label or
investigator) prior to the start of dosing to sufficiently minimize the risk of
pregnancy at that point. Female subjects must agree to use contraception until the
follow-up visit; OR has only same-sex partners, when this is her preferred and usual
lifestyle.

- Capable of giving written informed consent, which includes compliance with the
requirements and restrictions listed in the consent form

- Alanine aminotransferase (ALT), alkaline phosphatase and bilirubin <=1.5 x upper
limit of normal (ULN) (isolated bilirubin >1.5xULN is acceptable if bilirubin is
fractionated and direct bilirubin <35%).

- Based on averaged corrected QT interval (QTc) values of triplicate electrocardiograms
obtained over a brief recording period: QTc < 450 milliseconds (msec); or QTc < 480
msec in subjects with Bundle Branch Block.

Exclusion Criteria:

- Current or chronic history of liver disease, or known hepatic or biliary
abnormalities (with the exception of Gilbert's syndrome or asymptomatic gallstones).

- History of regular alcohol consumption within 6 months of the study defined as: For
United States (US) sites: an average weekly intake of >14 drinks for males or >7
drinks for females. One drink is equivalent to 12 gram of alcohol: 12 ounces (360
milliliter [mL]) of beer, 5 ounces (150 mL) of wine or 1.5 ounces (45 mL) of 80 proof
distilled spirits.

- 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.

- Has active suicidal plan/intent or has had active suicidal thoughts in the past 6
months. Has history of suicide attempt in the last 2 years or more than 1 lifetime
suicide attempt.

- Subjects with a history of seizures will be excluded from this trial.

- Subjects with a history of severe or serious psychiatric disease requiring
hospitalization, history of social ideation or attempt, or on ongoing psychiatric
treatment will be excluded from this study.

- A positive pre-study Hepatitis B surface antigen or positive Hepatitis C antibody
result within 3 months of screening

- Urinary cotinine levels indicative of smoking or history or regular use of tobacco-
or nicotine-containing products within 6 months prior to screening.

- A positive pre-study drug/alcohol screen.

- A positive test for Human Immunodeficiency Virus (HIV) antibody.

- Pregnant females as determined by positive serum hCG test at screening or prior to
dosing.

- Where participation in the study would result in donation of blood or blood products
in excess of 500 mL within a 56 day period.

- Lactating females.

- 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.