For example, 43 percent of patients taking the highly effective leukemia drug Gleevec were also on another medicine that could diminish its efficacy, while 68 percent were taking something that could potentially raise the toxicity level, the study found.

Not surprisingly, the study found that the vast majority of the cancer drugs were prescribed by an oncologist, while the other medicines were typically prescribed by a primary care physician.

"More communication needs to take place across all doctors that are prescribing for the patient," said Medco's Steve Bowlin, who is presenting the study findings on Friday at the American Society for Clinical Pharmacology and Therapeutics meeting in Washington.

The Medco drug interaction study looked at pharmacy claims of about 11,600 patients who had been prescribed any of nine oral drugs known as kinase inhibitors, used to treat a variety of cancers. They include Gleevec and Tasigna from Novartis; Pfizer Inc's Sutent; Nexavar from Onyx Pharmaceuticals Inc and Bayer AG; Tarceva, sold by Roche Holding and Japan's Astellas Pharma Inc; Sprycel from Bristol-Myers Squibb Co; and GlaxoSmithKline Plc's Tykerb.

The medications that could potentially cause such adverse "drug-drug interactions" (DDI) include commonly used heartburn drugs known as proton pump inhibitors (PPIs), such as Nexium and Prevacid; steroids; a class of blood pressure and heart medications known as calcium channel blockers; and some antibiotics and antifungal treatments.

At the high end of the range, 74 percent of patients who were taking Glaxo's kidney cancer drug Votrient were also taking a drug that could increase toxicity, researchers said.

Fifty-seven percent of Tarceva patients were also taking a drug that could weaken effectiveness, more than for any of the other cancer treatments.

Sutent had the lowest incidence of potentially troubling drug interactions, with 23 percent of patients taking a medicine that could decrease its effectiveness and 24 percent on a drug that could increase Sutent's toxicity.

"The fact that one-quarter to 75 percent of patients on these oral drugs may not be getting the full benefit of their treatment or may in fact be putting their health at further risk because of another medication they are taking is concerning," Dr. Milayna Subar of Medco's Oncology Therapeutic Resource Center said in a statement.

The PPI drugs for heartburn and acid reflux were the most likely to alter the effectiveness and toxicity of Gleevec, researchers said. For the lung cancer drug Tarceva, steroids and PPIs were most likely to decrease efficacy, while certain antibiotics were most likely to increase toxicity.

Even though the Tarceva label cautions against using the antibiotic ciprofloxacin when taking that cancer drug, the analysis found that the two were co-prescribed for a significant number of patients.

"There are other therapeutic alternatives that could be prescribed that would not have the same potential for interaction," said study co-author Eric Stanek, who called for increased surveillance of patient prescribing.

The analysis also found that the overlap for patients taking one of the oral cancer drugs and a medicine that could cause problems was rarely for just a day or two.

"In many instances, 30 or 40 percent of the time people were on a cancer drug they were also on a DDI drug," Bowlin said. "On average, it was substantial amount of time."

Medco, a pharmacy benefit manager that is in the process of being acquired by rival Express Scripts Inc, offers a Drug Utilization Review program under which its oncology pharmacists can alert doctors about potentially troubling medication interactions.

(Reporting By Bill Berkrot; Editing by Gerald E. McCormick)

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