Opioid Analgesic Agents and Cancer Cell Biology

Effects of opioids on tumour growth and metastasis in animal models: a systematic review

BACKGROUND

The International Agency for Cancer Research monographs recently classified opium consumption as carcinogenic to humans, with sufficient evidence for carcinogenicity in the larynx, lung, and urinary bladder and limited evidence in the oesophagus, pancreas, stomach, and pharynx. This causes concerns of a potential cancer-promoting effect in the same organs associated with the use of pharmaceutical opioids.

METHODS

We performed a systematic review (registered in the Open Science Framework: osf.io/xyg9p) of the published in vivo preclinical literature to determine whether the effects of opioids on tumour growth and metastasis are organ specific. We investigated whether the opioid category (agonist, antagonist, or peptide), organ of origin of the cancer cells, site of tumour measurement, immune status of rodents, opioid dose, or duration of opioid exposure was associated with reported cancer outcomes.

RESULTS

A total of 118 studies, representing 168 experiments, were included. Most animal experiments (94/168, 56%) reported an anti-cancer effect of opioids and 31 (18%) reported a pro-cancer effect. Of the assessed parameters, opioid category (P<0.001) and opioid dose (P=0.0056) were the only factors significantly associated with the reported cancer outcome. In studies testing morphine, experiments showing a cancer-promoting effect predominantly administered low doses of morphine (the proportion of studies using low-dose morphine was 65% among those reporting pro-cancer outcomes vs 8% among those reporting anti-cancer outcomes).

CONCLUSIONS

We found no relationship indicative of an organ-specific, cancer-promoting effect of opioids.

Opioid Receptor-Mediated and Non-Opioid Receptor-Mediated Roles of Opioids in Tumour Growth and Metastasis

Opioids are administered to cancer patients in the period surrounding tumour excision, and in the management of cancer-associated pain. The effects of opioids on tumour growth and metastasis, and their consequences on disease outcome, continue to be the object of polarised, discrepant literature. It is becoming clear that opioids contribute a range of direct and indirect effects to the biology of solid tumours, to the anticancer immune response, inflammation, angiogenesis and importantly, to the tumour-promoting effects of pain. A common misconception in the literature is that the effect of opioid agonists equates the effect of the mu-opioid receptor, the major target of the analgesic effect of this class of drugs. We review the evidence on opioid receptor expression in cancer, opioid receptor polymorphisms and cancer outcome, the effect of opioid antagonists, especially the peripheral antagonist methylnaltrexone, and lastly, the evidence available of a role for opioids through non-opioid receptor mediated actions.

The TLR4-Active Morphine Metabolite Morphine-3-Glucuronide Does Not Elicit Macrophage Classical Activation In Vitro

Macrophages are abundant in the tumor microenvironment where they adopt a pro-tumor phenotype following alternative polarization induced by paracrine factors from cancer and stromal cells. In contrast, classically activated macrophages have tumoricidal activities, such that the polarization of tumor-associated macrophages has become a novel therapeutic target. Toll-like receptor 4 engagement promotes classical activation of macrophages, and recent literature suggests TLR4 agonism to prevent metastasis and promote survival in experimental metastasis models. A growing number of studies indicate that TLR4 can respond to opioids, including the opioid receptor-inactive morphine metabolite morphine-3-glucuronide (M3G). We measured the activation of TLR4 in a reporter cell line exogenously expressing TLR4 and TLR4 co-receptors, and confirmed that M3G weakly but significantly activates TLR4. We hypothesized that M3G would promote the expression of classical activation signature genes in macrophages in vitro. We exposed mouse and human macrophage cell lines to M3G or the TLR4 activator lipopolysaccharide (LPS), alone or in combination with interferon gamma (IFN-γ). The classical macrophage activation markers tested were iNOS, CD86, IL-6, or TNF-α in RAW 264.7 cells and IL-6, IL-12, IL-23, TNF-α, CXCL10, and CXCL11 in THP1 cells. Our results show that despite exhibiting TLR4-activation ability, M3G does not elicit the expression of classical activation markers in LPS-responsive macrophages.