Biotech drug developer Replidyne Inc. and drugmaker Forest Laboratories Inc. said Monday the Food and Drug Administration won't approve their antibiotic faropenem medoxomil without further clinical trials.

The companies estimate that the required studies will add at least two years to the development of the antibiotic.

Replidyne and Forest, which partnered on the drug in February, are trying to get the antibiotic approved for acute bacterial sinusitis, community-acquired pneumonia, acute exacerbation of chronic bronchitis and uncomplicated skin infections.

While data from 11 late-stage clinical trials and more than 5,000 patients were submitted to the FDA, the companies believe that the FDA has recently started preferring superiority studies -- where a new drug has to be shown better than standard care -- for antibiotics.

The FDA recommended new clinical studies for all indications sought; there were no safety or manufacturing concerns.

Replidyne said it is in a strong financial position to continue developing the antibiotic with Forest. The companies plan to discuss development plans further with the FDA.

Forest shares fell $3.47, or 6.7 percent, to $48 in premarket activity on the INET electronic exchange, after closing Friday at $51.47 on the New York Stock Exchange. Replidyne shares, which trade on the Nasdaq, closed Friday were unchanged in premarket activity.