By Pat Anson, Editor

An international team of researchers may have discovered why some pain medications are inneffective: they target receptors on the surface of nerve cells that have moved out of reach.

Their findings, published in the journal Science Translational Medicine, may lead to the development of a new class of pain medication that is more potent and less prone to side effects than opioids and non-steroidal anti-inflammatory drugs (NSAIDs).

"Opioids and NSAIDs do not work for everyone and have unacceptable side effects, particularly when used over a long period of time," said Nigel Bunnett, PhD, a professor of surgery and pharmacology at Columbia University Medical Center.

"However, previous efforts to develop more effective analgesics have been stalled by our limited understanding of the mechanisms that allow nerves to sense and transmit pain signals."

Many pain medications work by targeting protein receptors on the surface of nerve cells that transmit pain signals. One receptor – known as the neurokinin 1 receptor (NK1R) -- causes pain and inflammation when activated.

In a series of laboratory experiments on rodents, Bunnett and his colleagues discovered that NK1R, when stimulated by pain, quickly moves from the cell surface to inside the cell membrane, where it continues to function outside the reach of pain medication. Researchers found that when they added a lipid (fat molecule) to painkillers that can cross the cell membrane, they effectively blocked NK1R and provided potent and durable pain relief to the rodents.

"From these experiments, we have demonstrated that designing NK1R inhibitors that are capable of reaching the endosomal network within nerve cells may provide much longer-lasting pain relief than currently available analgesics," said Bunnett. "We think that modification of many existing compounds, as we did with NK1R inhibitors, may have the potential to enhance the effectiveness of many different classes of medications."

The next step for researchers is to see if the same results can be found in humans. If proven, it could mean that current pain medications could be redesigned to make them more effective.

"This is a proof-of-concept study that shows that we can re-engineer current pain drugs and make them more effective. The challenge is now to translate the technology into human clinical trials. This is a complex and challenging path – but the ultimate benefits to patients with nerve pain are potentially highly significant," said Dr. Meritxell Canals of Monash Institute of Pharmaceutical Sciences at Monash University in Australia.

The study was supported by grants from Australia’s National Health and Medical Research Council, the Australian Research Council, and Takeda Pharmaceuticals.