WASHINGTON (AP) — Shares of Cell Therapeutics Inc. plummeted Monday after the Food and Drug Administration raised serious questions about the safety and effectiveness of the biotech drugmaker's experimental cancer treatment.

In documents posted online, the FDA said the company's drug pixantrone was associated with potentially fatal cardiac side effects, including heart failure. Additionally, the agency questioned the strength of the company's study, which enrolled significantly fewer patients than originally planned.

Shares of Cell Therapeutics plunged 30 cents, or 27.9 percent, to 76 cents in midday trading. The stock has ranged from 5 cents to $2.23 over the past year.

The FDA will ask its outside panel of cancer experts on Wednesday whether pixantrone's benefits outweigh its risks.

Cell Therapeutics, based in Seattle, has asked the FDA to approve the drug for patients who have advanced non-Hodgkin's lymphoma that has returned or is not responding to other treatments. Non-Hodgkin's lymphoma includes a variety of cancers that originate in the lymph system.

The company found that patients taking pixantrone experienced a complete remission of their cancer for 5.5 months, compared with 3.4 months for patients using an alternative treatment.

But FDA noted that patients taking pixantrone were also more likely to experience complications. Twelve patients in the pixantrone group died of an adverse event, including three from heart failure. That compares with just five patients in the control group.

Last year 19,500 patients died of non-Hodgkin's lymphoma, according to the National Cancer Institute. Life-expectancy varies widely depending on the type and stage of the cancer, though more than half of patients live 10 years or more after being diagnosed.