Preclinical studies suggest that endogenous opioids and/or opioid medications may contribute to tumor growth. However, endogenous and exogenous opioids have not been modulated over time in cancer patients, who often need opioids for pain control. Most of the analgesic effect of opioids occurs through the activation of the μ-opioid receptor. The most common naturally occurring genetic variation of this receptor in humans is the so-called A118G mutation. Individuals with this mutation have been shown to have a reduced analgesic response to opioid medications. In a recent study, we used this naturally occurring genetic variation to look for evidence that endogenous and/or exogenous opioids influence tumor growth in humans. We hypothesized that if opioids do influence tumor growth, then cancer patients with the A118G mutation, as a group, should have longer survivals than those without it. Using data from the Carolina Breast Cancer Study, we found that, among 2,039 women diagnosed with breast cancer, the presence of A118G was associated with longer breast cancer specific survival. The protective effect of A118G was limited to invasive cases only and appeared to increase with the stage of cancer at diagnosis. This study did not assess whether opioid pain medications have any influence on cancer mortality. Moreover, the study was performed in African Americans and European Americans only, and requires replication.
