Arena Pharmaceuticals Inc. on Wednesday said its experimental obesity drug was effective in helping patients lose weight in a short-term, mid-stage clinical trial, sending its shares up more than 22 percent.

Patients taking a 15-milligram dose of Arena’s oral drug, known as APD356, lost an average of 2.9 pounds (1.3 kg) after 28 days of treatment, compared with a loss of 0.7 pounds (0.3 kg) by obese patients taking a placebo.

The San Diego-based biotechnology company called the result highly statistically significant.

Shares of Arena were up $1.29 at $7.26 in early trade on the Nasdaq, after rising earlier to an 18-month high of $7.49.

The drug is designed to stimulate receptors in the hypothalamus, a part of the brain known to play an important role in regulating food intake and metabolism.

The phase II trial of 352 obese patients also tested the drug at 1-mg and 5-mg doses, but no statistically significant weight loss occurred at those strengths, the company said.

Arena said it next planned to study the drug over a three-month period in another mid-stage trial involving 300 to 400 patients.

“These top-line results should strongly support our partnering efforts and further validate our research and development capabilities,” Arena Chief Executive Jack Lief said in a statement.

Patients in the trial were instructed to maintain their normal diet and activity, but were told to abstain from consuming alcohol.

There were no serious side effects reported, with the most common complaint being headaches. The company said there was no apparent drug effect on the heart as assessed by echocardiograms done upon completion of the 28-day treatment period.