MORPHINE, which is given as a painkiller to many people with cancer, might stimulate the growth of tumours, say researchers in the US. Their worrying findings have been questioned by others in the field, but all agree that further studies are urgently needed to settle the issue.
In test-tube experiments and in mice, Kalpna Gupta and her colleagues at the University of Minnesota Medical School found that morphine encourages the growth of blood vessels, known as angiogenesis. The increased blood supply accelerated the growth of breast tumours in mice (Cancer Research, vol 62, p 4491).
Although the researchers have not yet looked for this effect in people, Gupta warns that morphine could be harmful for patients with any form of solid tumour that depends on a healthy blood supply. She stresses that nobody should yet consider altering their use of morphine because of her findings. 鈥淏ut clinical studies must be done,鈥� she says.
Advertisement
The findings have provoked a mixed response. Roy Bicknell, an angiogenesis expert at the Weatherall Institute of Molecular Medicine in Oxford, thinks the study deserves serious attention. He says it shows the drug mimics the effect of a blood vessel growth factor known as VEGF. It is also the first time the molecular pathway of morphine鈥檚 action has been worked out for cells outside the central nervous system.
But many other researchers are sceptical, especially as several previous studies have suggested that morphine slows cancer development by relieving pain and the associated stress. Stress can suppress the immune response and encourage the growth of tumours, says Ikuo Saiki of the Toyama Medical and Pharmaceutical University in Japan.
His research suggests that morphine actually helps prevent the spread of some cancer cells to other areas in the body. His group will publish these results in a forthcoming issue of the journal Cancer Letters. But different cancers may react very differently to the drug, he adds.
George Stefano, director of the Neuroscience Research Institute at the State University of New York, who has studied the effect of morphine on tumour growth, is also sceptical. He believes the morphine concentrations used were unrealistically high, though Gupta says her group matched the doses to the concentrations found in blood samples from several thousand patients.
More studies are now needed to confirm Gupta鈥檚 results and, more importantly, to see if they apply to people. 鈥淭he researchers have shown that morphine makes mouse endothelial cells proliferate,鈥� says Bicknell. 鈥淏ut we don鈥檛 know whether it will do the same in man.鈥�