Theravance Inc. said the Food and Drug Administration has asked for additional study data and some label revisions on its injectable experimental antibiotic for serious skin infections but it would not require additional clinical trials for FDA approval. 

Theravance CEO Rick Winningham said the company will respond to the FDA's demands "promptly" adding that it could take another six months before telavancin would face approval.

The drug in question, telavancin is already being tested as a treatment for hospital-acquired pneumonia HAP, was submitted in December 2006 for the agency's review. The label revisions include a boxed warning for pregnant women, monitoring of renal function and cautionary language regarding the use of telavancin in patients with certain renal risk factors, but the FDA's "complete response letter" doesn't require more expensive tests or a stronger "black box" warning.

The FDA in a complete response letter to the company asked for data on patients with certain renal risk factors from the complicated skin and skin structure infections study and HAP studies. The company said the FDA had asked for data to ensure that the drug's benefits outweigh the risk of it causing malformations of an embryo or fetus. Theravance will be required to maintain a registry of pregnant women exposed to the drug after the FDA's approval.

Referring to the FDA's letter to the company, Theravance CFO Michael Aguiar said, "There was nothing really surprising in here. There's a relatively clear path forward."

Theravance stock on Thursday went down about 9 % to close at $13.13 and in after-hours trading, it fell another $1.13 to trading at $12 after the bell.