Dietary Supplements and Cancer Treatment: A Risky Mixture

Patients undergoing cancer treatment often experience not only pain and discomfort from their disease, but also the potentially debilitating side effects caused by their treatments.

Some patients who do not receive adequate palliative care to lessen these side effects, or those driven to do anything possible to feel better while fighting their disease, may turn to dietary supplements advertised as having anticancer effects or being supportive of general health, frequently without consulting a health care professional.

These supplements are often herbs or other natural products. “A common, false belief is that ‘if it’s natural, it must be safe,’” said Dr. Barrie Cassileth, chief of the Integrative Medicine Service at the Memorial Sloan-Kettering Cancer Center. “But herbs and other dietary supplements are biologically active compounds, and they frequently have negative interactions with prescription pharmaceuticals.”

Furthermore, as a growing number of studies have shown, commonly used herbs and supplements can interact with cancer chemotherapy or radiation therapy, causing potentially life-threatening effects.

Widespread Use

In a systematic review published by Dr. Cassileth and a colleague in 1998, the use of complementary and alternative medicine (CAM) by cancer patients, including herbs and other dietary supplements, ranged from 7 percent to 64 percent in 21 different studies, with the average being about 31 percent.

CAM use has most likely increased during the past 11 years. In a 2004 study from the Mayo Clinic Comprehensive Cancer Center, more than 80 percent of patients enrolled in early phase chemotherapy trials concurrently used supplemental vitamins, herbs, or minerals, which were often explicitly not allowed as part of the trial protocols.

A study from a Midwestern oncology clinic published in 2005 showed that 65 percent of patients receiving chemotherapy had also taken dietary supplements, not including vitamins. Twenty-five percent of these had used one or more herbal therapies that are thought to have negative interactions with chemotherapy drugs. The majority of the patients did not consult a health care professional prior to supplement use.

Dangerous Interactions

“We tell patients that if you are on any chemotherapy or undergoing radiation, or planning for it in the future: no herbs, no antioxidants, no dietary supplements, across the board. Particularly herbal agents, because they can interact with and decrease the level of chemotherapy or any medication that enters your body,” explained Dr. Cassileth.

These effects are due to pharmacokinetic interactions—what happens when biologically active compounds in an herb alter the way a chemotherapy drug is absorbed, distributed in the body, metabolized, or eliminated. These interactions can happen for many reasons, including interference with the enzymes in the liver that normally break down the drugs, or interactions with the transporters that carry drugs across cell membranes. St. John’s Wort, garlic extract, and Echinacea are examples of commonly used herbal products thought to pharmacokinetically interact with chemotherapy drugs.

Pharmacokinetic interactions can have two potentially disastrous consequences. One is that less chemotherapy drug circulates in the bloodstream than is needed, leading to treatment failure. The other is the opposite effect: if the chemotherapy drugs are not broken down and removed from the body as expected, severe side effects can occur as a result of an overdose.

Unwanted Protection for Cancer Cells

Even antioxidant supplements such as vitamin E have the potential to interfere with treatment. Radiation therapy and some types of chemotherapy work by generating free radicals that damage cells’ DNA. Antioxidants can block this therapeutic effect.

“People are taking high-dose antioxidant supplements thinking they’re only going to protect normal cells, but both preclinical and clinical data show that they may protect both normal and tumor cells,” explained Dr. Brian Lawenda, a radiation oncologist from the Naval Medical Center in San Diego.

In 2008, Dr. Lawenda and his colleagues performed a review of results from the published randomized clinical trials testing antioxidants with radiation therapy or chemotherapy, and they concluded that the use of high doses of “supplemental antioxidants during chemotherapy and radiation therapy should be discouraged because of the possibility of tumor protection and reduced survival.”

A Silver Lining?

Even though doctors now caution patients not to mix and match over-the-counter supplements with cancer treatment, the bioactivity of herbs and other natural products are of interest to some cancer researchers looking for ways to enhance chemotherapy. While many herbs have been shown to interfere with chemotherapy, some may actually improve its efficacy.

“The interaction between certain complementary approaches and conventional treatment is an area of special interest to the Office of Cancer Complementary and Alternative Medicine (OCCAM),” said Dr. Jeffrey White, director of OCCAM. “We’re looking for ways to use natural products to improve the therapeutic index of conventional therapy.”

In addition to supporting ongoing clinical trials testing several traditional Chinese medicines in combination with the chemotherapy drugs irinotecan, capecitabine, and gemcitabine, OCCAM recently released two program announcements (PA-09-167 and PA-09-168) that will provide funding for researchers interested in testing synergistic interactions between natural products and traditional therapies

— Sharon Reynolds