Cempra Inc., a clinical-stage drug company, saw its shares plummet Thursday after the U.S. Food and Drug Administration declined to approve its pneumonia antibiotic solithromycin.

The Chapel Hill-based company’s shares fell 57.38 percent to $2.60, the lowest price since the company went public at $6 a share in 2012. The company’s year-to-date losses total 91.5 percent.

In a Complete Response Letter, the FDA requested additional safety data, improvements to Cempra’s manufacturing facility and an additional clinical trial.

The FDA told Cempra that the original study of 920 patients who received solithromycin was too small a sample size to gauge the drug’s potential side effects on the liver. The agency recommended a study with 9,000 patients given the experimental drug.

Clinical studies aimed at determining drug safety are typically faster than studies determining drug efficacy. However, the company could not state the estimated cost of the FDA’s recommended study.

If the company declined to conduct the safety study, the drug might require a warning label for the risk of liver damage, which would limit its clinical use.

In a press release issued Thursday morning, Cempra attempted to assuage investor concerns by highlighting the company’s cash on hand, patent protection and alternative formulations of solithromycin.

Cempra has more than $225 million in cash, said newly-named CEO David Zaccardelli. Former CEO and co-founder Prabhavathi Fernandes retired earlier this month.

“We have flexibility to determine the best course forward for solithromycin and Cempra,” Zaccardelli said in the release.