CAMBRIDGE, Mass. (AP) — Shares of Merrimack Pharmaceuticals Inc. tanked Wednesday after the drug developer said a potential treatment for an advanced form of ovarian cancer missed a mid-stage study’s main goal when compared to patients who did not receive the drug.

THE SPARK: The Cambridge, Mass., company said patients who received the treatment, labeled MM-121, in combination with the chemotherapy drug paclitaxel had the same progression-free survival as a group that just received paclitaxel.

However, the company also said a subgroup of patients showed more than double the progression-free survival. This measures the time from the start of treatment until a patient’s cancer begins advancing again or the patient dies.

The company also saw more instances of gastrointestinal side effects like diarrhea and vomiting in the group taking MM-121 and a higher rate of pulmonary embolisms. Merrimack officials said researchers did not connect the embolisms to the drug.

The company released initial results on Wednesday and said it would provide more details at a conference next year.

THE BACKGROUND: Merrimack is developing MM-121 with French pharmaceutical giant Sanofi. The drug developers also are studying it as a potential treatment for forms of breast and lung cancer.

Merrimack has no products on the market, but it has six potential cancer treatments in its drug development pipeline. That includes a pancreatic cancer drug in late-stage testing.

SHARE ACTION: Down nearly 19 percent, or 64 cents, to $2.74 in midday trading while broader indexes edged lower. The stock has traded between $2.65 and $7.50 over the past year.