The study suggests MS patients who use mitoxantrone are 10 times more likely to develop colorectal cancer.
Furthermore, the study - published in the journal Neurology - supports previous research that suggests the drug raises the risk of acute myeloid leukemia (AML).
Mitoxantrone - brand name Novantrone - is a drug approved for the reduction of disease relapses among patients with aggressive multiple sclerosis (MS) who do not respond to other therapies.
The drug works by suppressing the immune system, reducing the activity of T cells, B cells, and macrophages that are believed to promote the attack on myelin - the protective coating that surrounds nerves.
However, Dr. Mathias Buttmann, of the University of Würzburg, Germany, and colleagues note that the use of mitoxantrone for MS is limited, primarily because previous research has suggested the drug raises the risk of heart problems and AML.
For this latest study, the team further investigated the association between mitoxantrone and cancer.
Colorectal cancer rate three times greater with mitoxantrone use
Using medical records of patients treated at a German hospital-based MS center between 1994-2007, the researchers identified 676 individuals with MS who were treated with mitoxantrone.
On comparing cancer incidence among these patients with the general population of Germany, the researchers found that patients with MS treated with mitoxantrone were at 10 times greater risk of AML.
The rate of colorectal cancer was found to be three times higher among mitoxantrone-treated patients than the general population - an association that had not been identified previously.
The rate of breast cancer and other cancer types was no higher for patients treated with mitoxantrone than the general population, the researchers report.
Cancer risk 'acceptably low to justify mitoxantrone use' in severe cases
Of four patients treated with mitoxantrone who developed AML, three were treated successfully and were alive at the end of follow-up.
Of the seven mitoxantrone-treated patients who developed colorectal cancer, three died during follow-up. The researchers say this indicates that colorectal cancer is more life-threatening for MS patients treated with mitoxantrone than AML.
Older age at first use of mitoxantrone was found to be a risk factor for cancer development. However, cumulative mitoxantrone dose and use of other drugs that suppress the immune system did not appear to affect cancer risk.
While the findings clearly indicate an increased risk of AML and colorectal cancer with mitoxantrone use, the team says the overall cancer incidence in this cohort "appears acceptably low to justify mitoxantrone treatment in severely affected patients with MS if no better therapeutic alternative is available."
"Mitoxantrone is the only approved treatment for people with secondary progressive MS without relapses and should be considered in people where the disease is evolving quickly.
Also, many of the new and highly effective MS drugs are not available to people in a number of countries for economic reasons, so mitoxantrone is being used for people with very active relapsing forms of the disease."
Dr. Mathias Buttmann
Dr. Buttmann notes that their findings need to be confirmed in a larger study. If they are confirmed, he suggests that MS patients undergo colonoscopies following mitoxantrone use to screen for colorectal cancer.