This Study Will Examine the Effects of GSK256066 to Protect Mild Steroid-naive Asthmatics Against an Antigen Challenge

Inclusion criteria

Inclusion: (All must Apply) * Males and females aged 18 to 55 years inclusive. * Documented history of bronchial asthma, first diagnosed at least 6 months prior to the screening visit and currently being treated only with intermittent short-acting beta -agonist therapy by inhalation. * Pre-bronchodilator FEV1 \>75% of predicted at screening. * Documented sensitivity to AMP with a provocative concentration of AMP resulting in a 20% fall in FEV1 (PC20 AMP) of \<80 mg/ml at the screening visit. * Demonstrate stable bronchoconstriction in response to inhaled AMP at the run-in visit. 'Stable bronchoconstriction' is a term to define a population of asthmatics who have a repeatable and reproducible response to a challenge agent that induces bronchoconstriction. The run-in PC20 needs to be within 1.25DD of the screening PC20. * Non-smoking status as verified by urinary cotinine levels below 300 ng/mL cotinine at the screening visit. This can include ex-smokers who have given up smoking for \>6 months and have less than a 5 pack-year smoking history. * Able and willing to give written informed consent to take part in the study. * Available to complete all study measurements. Exclusion: * Pregnant or nursing females. * Past or present disease, which as judged by the investigator, may affect the outcome of this study. * Subject has known history of hypertension or is hypertensive at screening. * Respiratory tract infection and/or exacerbation of asthma within 4 weeks prior to the first dose of study medication. * History of life-threatening asthma, defined as an asthma episode that required intubation and/or was associated with hypercapnoea, respiratory arrest and/or hypoxic seizures. * Unable to abstain from prescription medication, other than short acting inhaled beta-agonists and paracetamol for the treatment of minor ailments eg headache from 48h before the first dose until the follow-up visit. * The subject has participated in a clinical trial during the previous 3 months. * History of blood donation (450 mL) within 2 months of starting the clinical study. * The subject regularly drinks more than 28 units of alcohol in a week if male, or 21 units per week if female. * The subject has tested positive for hepatitis C antibody or hepatitis B surface antigen or HIV antibodies. * The subject has positive drugs of abuse test. * Subjects weighing less than 50kg are to be excluded from participating in the study.