NEW YORK (AP) — Merrimack Pharmaceuticals said Thursday that patients treated with its experimental pancreatic cancer drug lived longer than patients who received only standard chemotherapy.

The company's shares surged $2.60, or 59 percent, to close at $6.99. Earlier Thursday they reached an 18-month high of $7.65.

Merrimack said patients who received the drug, called MM-398, and a chemotherapy regimen in the clinical trial lived an average of 6.1 months after treatment. Patients who were given just the chemotherapy regimen lived 4.2 months on average. Patients who were treated with MM-398 without other drugs alone did not live longer than the chemotherapy patients.

Pancreatic cancer is one of the most deadly forms of cancer because it is often not caught until it has spread to other parts of the body.

MM-398 is a new form of the chemotherapy drug irinotecan. Merrimack says its formulation of the drug is designed to stay in the bloodstream longer, allowing it to more effectively build up in tumors while sparing other cells. The company says it believes the drug could be effective against other types of cancer.

The late-stage clinical trial involved 417 patients whose cancer had spread to other parts of their bodies and who had previously been treated with Gemzar, an older cancer drug. The Cambridge, Massachusetts-based company said the most common serious side effects of the combination treatment included low white blood cell counts, fatigue, diarrhea and vomiting.

Merrimack Pharmaceuticals Inc. does not have any approved drugs, and it plans to file for marketing approval of MM-398 later this year.