Traditional Chinese, Indian, and Thai medicine uses the tropical plant Tabernaemontana divaricata (crepe jasmine) to alleviate fever and pain. The basis for that pain-relieving power has been a riddle, with the answer hidden somewhere in the long list of potentially active alkaloids found in the plant’s bark (Kam et al., 2004; Pratchayasakul et al., 2008). Glenn Micalizio and colleagues at The Scripps Research Institute, Jupiter, Florida, recently succeeded in synthesizing an exceedingly rare and chemically complex component of T. divaricata, the indole alkaloid conolidine, and showed that it acts as a potent non-opioid pain killer in animal models. The work, which appeared online May 23 in Nature Chemistry, opens up the study of a new class of natural products as potential analgesics, albeit with a currently mysterious mechanism of action.
Other natural products related to conolidine have been suggested to act as opioids (Pratchayasakul et al., 2008; Ingkaninan et al., 1999), but nothing was known about conolidine itself, because of its low abundance and the lack of a synthetic strategy for members of the C5-nor stemmadenine molecular family to which it belongs. Attracted by the challenge, and the promise of biological activity, first author Michael Tarselli and coworkers devised a nine-step asymmetric synthesis that yielded the (+) or (–) isomers of conolidine, or a racemic mixture. They were able to produce hundreds of milligrams of the compound, enough for Scripps pharmacologist and coauthor Laura Bohn to test in animals.
To their surprise, Bohn and her colleagues found that conolidine was active in mouse models of pain. When given by intraperitoneal injection, either of the two isomers, or the mixture, reduced pain behavior in models of visceral pain (abdominal acetic acid injection) and inflammatory pain (formalin injection to the paw). In the formalin model, conolidine diminished pain-related behaviors in both the acute and the persistent pain phases, with the (–) isomer displaying a similar potency to morphine sulfate. However, the compound’s actions were not identical to opioids: In two acute thermal nociception assays—hot plate and tail immersion tests—conolidine had no effect. Pharmacokinetic experiments showed that conolidine was present in both the plasma and brain for up to four hours after injection.
Despite the proposed opioid activities of conolidine’s molecular cousins, the researchers could not find any evidence that the compound interacts with opioid receptors. To try to identify other targets, Bohn sent the compound to the National Institute of Mental Health’s Psychoactive Drug Screening Program at the University of North Carolina at Chapel Hill, where it was tested for binding to ~50 CNS receptors, channels, and transporters. A few proteins in the collection bound conolidine, but those hits failed to hold up in functional assays. So Bohn and her group are continuing to hunt for targets. In addition to molecular screening, Bohn says, the team is trying to identify the cells that respond to conolidine, and they have some early hints that the compound may be acting in the dorsal root ganglion.
Conolidine’s lack of binding partners, while leaving the researchers scratching their heads about a mechanism for its analgesic effects, could also signal good news about its side effect profile. “It’s not hitting some of the major neurotransmitter receptors that are known to be problematic for psychoactive properties,” Bohn said. In open-field activity tests, the compound had no effect on locomotor activity in mice, which is a classical central nervous system effect of morphine. That offers hope that conolidine may carry out its analgesic activities without opioids’ notorious complications.