Ironwood Pharmaceuticals Inc. reported a larger first-quarter loss Thursday as it prepared to file for approval of its constipation drug candidate linaclotide.

Ironwood posted a quarterly loss of $18.4 million, up from $15.9 million a year ago. Because its share count increased, its per-share loss decreased to 19 cents from 25 cents. A year earlier the company lost $15.9 million, or 25 cents per share. Revenue grew to $10.2 million from $8.8 million.

Analysts expected a loss of 16 cents per share and $11 million in revenue, according to FactSet.

The company had 99.1 million shares on the market during the first quarter, up from 64 million a year ago. Its research and development costs rose to $19.6 million from $17.5 million, and its general and administrative spending grew to $9.2 million from $5.8 million.

Ironwood and Forest Laboratories Inc. are developing the drug linaclotide as a treatment for irritable bowel syndrome with constipation and chronic constipation.

The companies plan to file for approval of linaclotide for both uses during the third quarter. Almirall SA of Spain has the rights to the drug in the European Union, and Ironwood said Almirall will file for approval of the drug as a treatment for IBS with constipation in the second half of 2011.

Linaclotide is in late-stage development, and Ironwood is also studying treatments for gastrointestinal diseases, pain and inflammation, respiratory illnesses, allergies, and heart disease.