* Peak annual sales seen at 600 mln Sfr
* Coveted "orphan" drug status means marketing exclusivity
* Shares rise 5.1 pct
ZURICH, May 28 (Reuters) - Swiss biotech group Basilea has clinched "orphan" drug status from U.S. health authorities for antifungal treatment isavuconazole, progress on a product that analysts estimate could be worth up to 150 million francs annually.
Sector analysts forecast potential peak annual sales of isavuconazole at 530-600 million Swiss francs ($623.47 million), of which Basilea could receive up to 25 percent in royalties.
Orphan status is granted to drugs that treat conditions or diseases that affect fewer than 200,000 people in the United States. The designation usually comes with a seven-year marketing exclusivity period if the drug is approved for sale.
"The granting of orphan designation for isavuconazole in the U.S. reflects the high medical need and is an important regulatory milestone for Basilea and our partner Astellas," chief Basilea medical officer Achim Kaufhold said in a statement.
Isavuconazole, which treats both mould and yeast infections such as candida infections, is being developed with Japan's Astellas.
Bank Sarasin called the drug backing a significant win for Basilea. The company's shares surged more than 6 percent in early trading. At 0731 GMT, the stock was 5.1 percent higher amid a 1 percent rise on the European healthcare sector.
The U.S. drug backing comes as Basel-based Basilea faces a year of waiting for regulatory approval on its main product, antibiotic drug ceftobiprole. The company is in the process of answering questions from European agencies on the drug and said any possible approval could come in the fourth quarter.
The firm has been hunting for a partner for ceftobiprole - which it hopes will make it to market to treat hospital and community-acquired pneumonia - for several years and said it was aiming for a partnership by the end of 2013.
The backing also follows a string of shake-ups at Basilea, including the departure of its chief financial officer just two months after that of the firm's founder and chief executive.