Although opioid-based painkillers are a standard treatment for relieving cancer pain, they may actually contribute to the progression of the disease, as two recent studies published in the journal Anesthesiology indicate.

One study (2012;116:857-867), led by Patrick A. Singleton, PhD, of The University of Chicago Medicine research institute in Chicago, Illinois, focused on how opioids already present in the body can enhance the malignant tendencies of human lung cancer cells, even without the addition of morphine.

The investigators found that overexpression of µ-opioid receptors in a human non-small cell lung cancer cell line increased tumor cell proliferation, migration, and invasion, all of which are measures of tumor growth and metastasis. When transplanted in mice, human lung cancer cells with additional copies of the opioid receptor grew more than twice as quickly as did tumor cells without the extra receptors. In addition, the cells with the extra opioid receptors were 20 times more likely to metastasize to distant sites.

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Opioid-receptor-blocking agents reduced tumor growth and metastasis, suggesting that the µ-opioid receptor could be a therapeutic target.

In the second research project (2012;116:892-902), Andrey V. Bortsov, MD, PhD, of the anesthesiology department at the University of North Carolina at Chapel Hill, and colleagues evaluated survival rates from an earlier study involving 2,039 women with breast cancer. Women with a genetic variant in the µ-opioid receptor that reduces opioid response were found to be at significantly lower risk for breast cancer–specific mortality. Patients with at least one variant G allele at the µ-opioid receptor A118G gene polymorphism were nearly twice as likely to be alive 10 years after treatment, and those with two variants were four times as likely. Having at least one G allele was also associated with having less advanced disease at diagnosis.

Bortsov’s team concluded that these results supported the hypothesis that endogenous and/or exogenous opioids acting via the µ-opioid receptor may influence cancer outcomes.