Specifically, this study will attempt to modify and reduce this permeability through the use of a substance called 'keratinocyte growth factor', or 'kgf'. KGF is a naturally occuring human protein, which is involved in stimulating the growth of cells lining the layers of the skin and gut, helping to repair damage and maintain their structure. It has been manufactured in a laboratory as the commercial compound 'Palifermin', which has already been used in humans to reduce damage to the lining of the mouth after chemotherapy. The study will see if Palifermin can similarly improve the lining of the lung in asthma patients and improve their symptoms. To date no treatments have been used in this area in asthma, and if successful the study will open up a whole new area of therapy