Drug developer

Theravance

(THRX)

saw its shares sink in premarket trading Wednesday despite positive results from a clinical trial of its antibiotic telavancin.

According to two trials involving 1,867 patients, the fast-acting injectable antibiotic for skin infections reached its goal of being just as effective as vancomycin, the standard treatment for

Staphylococcus aureus

infections that are resistant to another antibiotic, methicillin.

The company also said trials of telavancin showed numerically greater bacteria death and a higher cure rate with its drug than with vancomycin. Shares fell $1.53, or 6.1%, to $23.75.

"This positive outcome represents a landmark event for our company and the team that helped make it happen," said Rick Winningham, Theravance's CEO. "Telavancin is a Theravance-discovered medicine, and these data demonstrate that our innovative approach to drug discovery works and can have an impact on combating serious infections."

"With the incidence of methicillin-resistant Staphylococcus aureus infections on the rise, it is imperative that clinicians have effective alternatives in the treatment of these difficult infections," said Dr. Sef Kurstjens, head of research and development at Astellas Pharma, Theravance's marketing partner for the drug.