Shares in Basilea are slipping this morning after the Swiss biotech revealed that its investigational late-stage antifungal isavuconazole, partnered with Astellas Pharma will not hit the market any time soon.

The Japanese drugmaker has taken over the development programme for isavuconazole, having teamed up with Basilea in February. However Astellas has decided to expand the Phase III programme to include more patients.

Isavuconazole is being investigated in three Phase III studies, one targeting yeast infections (candidaemia and other invasive Candida infections), one mould infections (invasive aspergillosis) and a third study including rare molds infections and renally impaired patients with aspergillosis. However, Astellas and Basilea have implemented “certain study protocol amendments” to the invasive aspergillosis study in order to account for “changes in clinical practice, the evolving regulatory environment and ongoing advancements in clinical diagnostics”.

This means that the treatment of first new patients in the Phase III programme is anticipated in the third quarter of this year and recruitment is anticipated to be completed in 2012. The first trial results from are expected in 2013, between 12-18 months later than planned.

Achim Kaufhold, Basilea’s chief medical officer, said that “despite the change to the initially anticipated timelines”, the moves will “further strengthen the competitive positioning of isavuconazole in an evolving medical field”. Analysts believe that the changes increase the likelihood of getting regulatory approval, sometime in the middle of 2014 and it could have peak sales pushing $700 million.

However investors seem concerned and at 10.50am (UK time), Basilea shares had fallen 5.9% to 67.35 Swiss francs.