Inhaled insulin--does it become reality?

After more than 80 years of history the American and European Drug Agencies (FDA and EMEA) approved the first pulmonary delivered version of insulin (Exubera) from Pfizer/Nektar early 2006. However, in October 2007, Pfizer announced it would be taking Exubera off the market, citing that the drug had failed to gain market acceptance. Since 1924 various attempts have been made to get away from injectable insulin. Three alternative delivery methods where always discussed: Delivery to the upper nasal airways or the deep lungs, and through the stomach. From these, the delivery through the deep lungs is the most promising, because the physiological barriers for the uptake are the smallest, the inspired aerosol is deposited on a large area and the absorption into the blood happens through the extremely thin alveolar membrane. However, there is concern about the long-term effects of inhaling a growth protein into the lungs. It was assumed that the large surface area over which the insulin is spread out would minimize negative effects. But recent news indicates that, at least in smokers, the bronchial tumour rate under inhaled insulin seems to be increased. These findings, despite the fact that they are not yet statistical significant and in no case found in a non-smoker, give additional arguments to stop marketing this approach. Several companies worked on providing inhalable insulin and the insulin powder inhalation system Exubera was the most advanced technology. Treatment has been approved for adults only and patients with pulmonary diseases (e.g., asthma, emphysema, COPD) and smokers (current smokers and individuals who recently quitted smoking) were excluded from this therapy. Pharmacokinetics and pharmacodynamics of Exubera are similar to those found with short-acting subcutaneous human insulin or insulin analogs. It is thus possible to use Exubera as a substitute for short-acting human insulin or insulin analogs. Typical side effects of inhaled insulin were coughing, shortness of breath, sore throat and dry mouth. Physical exercise increases the transport of inhaled insulin into the circulation and in consequence the likelihood of hypoglycemia. Other problems were the inability to deliver precise insulin doses, because the smallest blister pack available contained the equivalent of 3 U of regular insulin and this dose would make it difficult for many people using insulin to achieve accurate control, which is the real goal of any insulin therapy. For example, someone on 60 U of insulin per day would lower the blood glucose about 90 mg/dl (5 mmol) per 3 U pack, while someone on 30 U a day would drop 180 mg/dl (10 mmol) per pack. Precise control was not possible, especially compared with an insulin pump that can deliver one twentieth of a unit with precision. Another disadvantage was the size of the device. The Exubera inhaler, when closed, was about the size of a 200 ml water glass. It opened to about twice the size for delivery. To our information also other companies (Eli Lilly in cooperation with ALKERMES, Novo Nordisk (AERx, Liquid), Andaris (Powder)) stopped further development and it is unclear whether an inhaled form of insulin will ever be marketed, because of the problems that have occurred. Only Mannkind (Technosphere, Powder) is still working on a Phase III trial. However, our review will briefly summarize the experience regarding inhalant administration of insulin and will describe potential future developments for this type of therapy focussing on the lung.