Researchers led by UCSF scientists are reporting that an experimental pain drug known as a kappa-opioid brings pain relief to female rats but not males, a finding that adds weight to a recent UCSF clinical finding, and highlights, they say, the need to evaluate drugs by gender.
Traditionally, kappa-opioids have been dismissed as ineffective analgesics in humans, though the drugs have shown mixed results in animal studies, depending on how they have been administered.
The finding, published in the March issue of Pain, may help to resolve the controversy about the drug's effectiveness, the researchers say, and underscores a weakness in traditional drug screening: Until the early 1990s, most drugs, including kappa-opioids, were primarily evaluated in men.
"The problem of gender differences, particularly in response to opioid drugs, is extremely important and widely under-appreciated," says the senior author of the study, Howard Fields, MD, PhD, a leading expert on the brain mechanisms of pain and a pain-treatment specialist. Fields is UCSF professor of neurology, a member of the Keck Center for Integrative Neuroscience and director of the UCSF Wheeler Center for the Neurobiology of Addiction.
"There may be classes of drugs that are particularly effective in women that don't have the side effects of currently available potent drugs," says Fields. "Kappas are an example, but it may be true for a lot of drugs and we just don't know it because we haven't looked. Drug companies might be throwing away a perfectly good drug because it doesn't work in males."
The specific finding is important because morphine, a class of opioid and the painkiller most often used for severe pain, has limitations - over time, people can develop tolerance to the drug and/or become dependent on it. As a result, researchers are intent on identifying an alternative class of opioids that lack the drug's limitations.