The company is hoping to raise between £20 million (€30m) and £25 million through the placing on the Alternative Investments Market, which would give it a market value of up to £85 million.
The proceeds will be used to develop Vectura's range of inhaled drugs, including the phosphodiesterase-5 inhibitor VR004 for erectile dysfunction and a new product for chronic obstructive pulmonary disease (COPD), developed in tandem with fellow UK firm Arakis.
The cash would allow it to take the projects through Phase II trials, Vectura said, adding that it would then try to find licensing partners for the projects.
Vectura claimed that inhaled VR004 is delivered into the bloodstream in around eight minutes - faster than the orally active drugs that can take half an hour or more to act. This would provide the rapidity achievable with older injectable therapies, but without the obvious dosing disadvantages.
The company is 30 per cent owned by venture capital group Merlin.
More than 50 million men in Europe and the US are thought to be affected by erectile dysfunction, and the market for drugs to treat the condition is currently valued at around €2 billion a year.