NEW YORK (AP) — Shares of Ariad Pharmaceuticals had their biggest one-day drop Wednesday after the company reported serious side effects of its cancer pill Iclusig, prompting regulators to halt enrollment in new tests for the drug.

THE SPARK: Ariad said after two years of research, 11.8 percent of patients treated with Iclusig in a clinical study had suffered serious blood clots in an artery. That’s more than the company had previously reported: after collecting 11 months’ worth of data from the study it said 8 percent of patients had those side effects.

Ariad said about 20 percent of patients treated with the drug experienced a serious or non-serious blood vessel event.

As a result the company the Food and Drug Administration has halted enrollment of new patients in all clinical trials of Iclusig, and patients who are being treated with the drug in those trials will have their dosages reduced. Ariad said it is consulting with the FDA and other health regulators about changing the label on Iclusig to reflect the new information.

THE BIG PICTURE: The Food and Drug Administration approved Iclusig in December to treat two rare types of leukemia. Ariad reported $13.9 million in Iclusig sales during the second quarter. European Union regulators approved the drug in July, and the Cambridge, Mass., company is running other clinical trials in the hopes of broadening Iclusig’s marketing approval and increasing sales.

THE ANALYSIS: Jefferies & Co. analyst Eun Yang slashed her sales estimates for Iclusig on the news. She now expects sales will peak at about $420 million a year rather than $1.1 billion.

“This highlights underlying safety concerns that the Street has had for Iclusig and its likely limited commercial potential,” she said. Yang kept a “Buy” rating on the stock but lowered her price target to $7 per share from $23.

SHARE ACTION: Shares of Ariad Pharmaceuticals Inc. dropped $11.69, or 68 percent, to $5.45 in afternoon trading. Earlier the stock fell to $4, its lowest price since November 2010.