Modern Drug Discovery
November/December 1999
Modern Drug Discovery, 1999, 2(6) 39-40, 43, 45.
© 1999 American Chemical Society.

Modeling cannabinoid receptors is leading toward therapeutic synthetics.

BY CAROL HART

While the debate over medical use of marijuana has alternately smoldered and blazed for the past two decades, researchers have quietly made discoveries that may ultimately render that debate moot. Synthetic analogs and other drugs acting on the same receptor system may be able to provide therapeutic benefits attributed to marijuana, without the smoke, the contaminants, and the variable potency. Ongoing research into marijuana's pharmacology has the potential for developing new approaches for other important indications, including cognitive disorders, autoimmune disease, and appetite suppression.

Marijuana and Medicine, the comprehensive report released by the Institute of Medicine (IOM) earlier this year, recognized the medical utility of cannabinoids, the active compounds in the marijuana plant, for several important indications while rejecting claims that patients using marijuana to manage symptoms were at significant risk of addiction. However, the IOM report only cautiously endorsed short-term interim studies of smoked marijuana for a few limited indications, stating instead that “the future of medical marijuana lies in classical pharmacological drug development” (1).

That future looks bright. Major scientific discoveries of the past decade include the identification and cloning of two endogenous cannabinoid receptors, CB¹ (found primarily in the brain) and CB² (associated with the immune system); the discovery of endogenous ligands of these receptors; the synthesis of more potent and selective agonists and antagonists; and, most recently, the development of CB¹ and CB² knockout mice.

The history of medicinal marijuana
One of the oldest cultivated plants, marijuana was known in China nearly 5000 years ago, where it was grown for its fiber and the oil in its seed (2). Spreading westward across the Indian subcontinent, it was used in Hindu temple rituals 4000 years ago and valued as a medicine capable of restoring “vital energy”. Its recreational use was noted 2500 years ago by Herodotus, who reported that the Scythians intoxicated themselves with the smoke of burning hemp. Marijuana became known throughout North Africa following the Arab invasions of the early Middle Ages. Cannabis was introduced to Anglo-American medicine in the 19th century by a British physician who had observed its use in India to treat rabies, rheumatism, epilepsy, and tetanus (2). The 1854 U.S. dispensary mentioned epilepsy, rheumatism, menstrual cramps, convulsions, chorea, hysteria, depression, tetanus, gout, and neuralgia, among other indications for “Tilden's extract of Cannabis sativa indica”. Variable potency was a much-noted problem, as it is today. Medical use of marijuana declined in the United States and England as more specific medications, such as aspirin and barbiturates, became available (2).

Smoked marijuana's most obvious and well-known effects include euphoria (but at higher doses dysphoria), sensory heightening (but also potentially analgesia), impaired short-term memory, motor retardation, increased heart rate (tachycardia), and stimulation. Anecdotal reports of therapeutic effects observed with recreational use of marijuana began appearing in the 1960s. Isolation of ⁹-tetrahydrocannabinol (THC), the major psychoactive compound in marijuana, in the mid-1960s had much to do with inspiring interest in the pharmacology and potential medical applications of THC and the 60-odd related cannabinoids found in the plant. (See Structure of THC and two derivatives.)

In the 1970s, Pfizer launched a cannabinoid research program that resulted in the development of a cannabinoid analog, levonantradol, that was 1000 times more potent than THC (3). Clinical trials showed efficacy for postoperative pain and chemotherapy-associated nausea and vomiting; however, side effects (sleepiness, dysphoria, dizziness, thought disturbance, and hypotension, among others) were judged to be excessive, and the project was discontinued. Other candidate drugs were not pursued (3).

Eli Lilly and Company also made a research commitment in the early 1970s that led to the development of the cannabinoid analog nabilone and its approval for the treatment of chemotherapy-associated nausea and vomiting (4). Despite low abuse potential, it was classified as a restricted schedule II narcotic in the United States and never became a viable product. When the FDA requested additional bioavailability studies, Eli Lilly elected to withdraw it from the U.S. market. It has been prescribed in the United Kingdom for more than two decades with no evidence of addiction or abuse (4).

Cannabinoid research programs at Abbott Laboratories and Sterling Research Foundation also contributed cannabinoid analogs that proved to be of great value in studying the pharmacology of the cannabinoids.

Dronabinol (Marinol), a synthetic oral preparation of THC encapsulated with sesame oil, was approved by the FDA in 1985 for the treatment of chemotherapy-associated nausea and vomiting and later approved for management of AIDS-associated wasting and anorexia. It is being studied for possible benefit in alleviating mood and behavioral changes associated with Alzheimer's disease. Marinol's usefulness is limited by the fact that it has poor bioavailability and delayed onset of action in addition to all the central nervous system (CNS) effects associated with marijuana. Peak plasma levels are not reached until 2–4 h after dosing, compared with 1–2 min for smoked marijuana. As a result, patients complain that they cannot adjust doses of Marinol to minimize CNS side effects, as they can with marijuana (1). Unimed Pharmaceuticals, which manufactures Marinol, and Roxane Laboratories, which jointly markets Marinol, are currently studying new aerosol, nasal spray, and sublingual formulations that may achieve a more rapid onset of action (1).

In July, Marinol was reclassified as a schedule III rather than a schedule II drug, in recognition of its low abuse potential. Prescribing of schedule III drugs is subject to far fewer restrictions than schedule II drugs: Providers may phone in prescriptions, and up to five renewals are permitted within six months, whereas prescriptions for schedule II drugs must be written and signed by the provider, with no renewals permitted. As Billy R. Martin, a panelist in the IOM report and a researcher at the Medical College of Virginia, comments, this rescheduling might be taken as a “good omen” for the future of cannabinoid-related drugs.

The endogenous cannabinoid system
By using powerful new tools such as selective antagonists and knockout mice, the functions of the endogenous cannabinoid system are now being described. These functions potentially include the modulation of mood, memory, cognition, perception, movement, coordination, sleep, thermoregulation, cardiovascular tone, appetite, and immune response (1, 5). The cannabinoid receptors CB¹ and CB² belong to the G-protein coupled superfamily of receptors (see box in “Molecular modeling of opioid analgesics” article). Receptor stimulation activates intracellular G proteins in the first step in the signal transduction pathway, leading to potassium channel activation, calcium channel inhibition, or cyclic AMP inhibition, thereby modulating cyclic AMP–mediated release of peptide and amine messengers. The end result of cannabinoid receptor stimulation depends on other events in the milieu as well as the presence of other molecules competing for receptor binding sites (1).

The two major endogenous ligands identified to date are arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol (2-AG). As their chemical names suggest, both are derived from arachidonic acid, which is also the precursor for the prostaglandins. 2-AG is capable of binding to and activating both receptor types, whereas anandamide only activates the CB¹ receptor (5). As a neurotransmitter, anandamide has some unusual characteristics. According to Cecilia J. Hillard, a neuropharmacologist at the Medical College of Wisconsin, anandamide appears to be released from the postsynaptic nerve terminal and to “swim back” across the synapse to the receptor on the presynaptic terminal in a two-cell feedback system.

The CB¹ receptor is presumed to mediate all the CNS effects of the cannabinoids. “The distribution of these receptors and the signal transduction systems they activate make a lot of sense in terms of what we know about the physiology of these compounds,” said Steven R. Childers, a contributor to the IOM report and a professor of physiology and pharmacology at Bowman Gray School of Medicine. For example, there are very high levels of receptors in the hippocampus involved in memory, and in the cerebellum and basal ganglia mediating the motor effects of cannabinoids.

Signal transduction studies have revealed that THC is actually a fairly weak partial agonist, according to Childers. The number of CB¹ receptors in the brain is very large, comparable to the numbers of the monoamine receptors, such as those for serotonin and dopamine. This abundance of receptors, a phenomenon known as receptor reserve, explains how a partial agonist such as THC is able to produce a response. Basically, the more receptors in the system, the greater the likelihood of an agonist–receptor interaction. The presence of “spare receptors” (receptor reserve) increases sensitivity to an agonist, even a weak one such as THC.

“THC is not really a very effective drug,” Childers said. “Historically, that's why we've needed to wait for the pharmaceutical development of more potent and specific compounds to study the actions of cannabinoids. With only THC itself, we wouldn't be able to understand very much of what is going on.”

The functions of the CB² receptor, found primarily in the immune system, are less clear. Most of the early studies seemed to suggest that the cannabinoids were primarily immunosuppressive, commented Hillard. But the later literature, using lower concentrations of more selective compounds, “would almost suggest that the cannabinoid receptor is part of an immune-stimulating system.” Depending on dosage, some immune cells are suppressed while others are stimulated. “The acute effects of cannabinoids on immune function are not obvious,” Childers said. “They are fairly complex and appear to be more modulatory in nature.” CB² knockout mice do not appear to have any immune system deficiencies, though more subtle changes in the immune system may be present.

The CB¹ and CB² receptors have only 44% similarity in their amino acid sequence, making the design of selective agonists and antagonists more feasible (5). Some are already available for research purposes, and at least one is now in clinical trials, as detailed in the section “Cannabinoid antagonists”.

Synthetic cannabinoid agonists
As a partial and nonselective agonist at both cannabinoid receptors, THC is far from being the ideal agent in this class. In addition to appetite stimulation and suppression of nausea and vomiting, possible therapeutic uses of cannabinoid receptor agonists would include (6):

• postoperative pain, cancer pain, and neuropathic pain;
• multiple sclerosis and spinal injury symptoms, such as muscle spasms, pain, and tremor;
• bronchial asthma; and
• inflammatory disorders.

Of these potential indications, analgesia has received the most attention from academic and industry researchers. While there is yet no compound with a selective pharmacologic profile, Martin points out that there are now several distinct chemical classes of compounds known to interact with the CB¹ receptor, such as the aminoalkylindoles and the anandamides. Some of these compounds have reduced psychoactivity while retaining their analgesic effects. “We spent a lifetime making THC derivatives, and almost all had a 1:1 ratio for analgesia to sedation. Within these new classes, some of those ratios are up 10- or 20-fold,” Martin said.

One technical barrier, as Childers points out, is the fact that all these compounds are highly lipophilic. Research efforts are under way to develop water-soluble derivatives, but there are risks inherent in this approach. “A more water-soluble compound may produce a more rapid onset of action and in fact be a more abusable form of cannabinoid than THC itself,” Childers commented. “You have to be careful what you wish for.”

	Definitions
Ligand	A compound that activates a receptor and triggers its characteristic response
Agonist	A compound that interacts with a receptor to mimic the effects of the endogenous ligand.
Antagonist	A compound that interacts with a receptor to inhibit its activation by an agonist or ligand.
Partial agonist	An agonist that is unable to induce the maximum activation of a receptor.
Nonselective agonist	A compound that activates a variety of receptors.

Early studies of cannabinoid-induced analgesia produced conflicting results, depending on animal model and dosage. More contemporary studies using objective measures, such as recordings from individual sensory neurons, are now allowing researchers to specifically measure the effects of cannabinoids on different pain pathways. “There is more and more evidence coming out of these studies that the cannabinoid receptor system is an analgesic system,” said Childers.

Even more important, it appears to be distinct from the endogenous opioid system. “That suggests that certain types of pain that do not respond to opioid drugs might be treatable with cannabinoids. A drug that could treat other kinds of pain that opiates don't touch, now that's exciting,” said Hillard.

Cannabinoid antagonists
Although there may be other companies quietly pursuing cannabinoid research, the researchers interviewed all said that the French pharmaceutical concern Sanofi Recherche, now the Sanofi-Synthélabo Group, appears to have the most extensive drug development program in this field. Its specific CB¹ receptor antagonist, SR141716A, is now in clinical trials, reportedly to study its potential as an appetite suppressant. Sanofi has developed the first selective CB² receptor antagonist, designated SR144528, but it is not clear whether Sanofi intends to explore clinical applications for this compound as well.

In theory, a cannabinoid antagonist would produce effects opposite to THC and the other cannabinoid agonists. As Martin observes, a CB¹ antagonist might also be beneficial for treating cognitive disorders involving short-term memory loss. There is evidence from a couple of models, he said, that a cannabinoid antagonist could enhance memory. His laboratory has shown that THC disrupts memory specifically through its action on the cannabinoid receptors, demonstrating that the well-known effects of marijuana on short-term memory do not result from some nonselective action of cannabinoids. Hillard points out that antagonists may also have some application in preventing the massive vasodilation and precipitous drop in blood pressure that accompany shock, as recent studies indicate that endogenous cannabinoids released in the immune system may be involved (5).

Because endogenous cannabinoids are also regulators of motor activity and coordination, it is possible that a cannabinoid antagonist could be used in the management of some types of movement disorders. As Childers points out, all these applications are speculative, based on animal studies indicating functions of the endogenous system. “We do not yet have the clinical experience with these compounds in humans to know whether they would have significant effects,” he said. Unlike the agonists, cannabinoid antagonists would presumably have no abuse potential, and the possibility of scheduling would not be a deterrent to development.

Uptake inhibitors
Compounds have also been developed and tested in animal models that selectively block uptake of anandamide, thereby amplifying its signal. As Martin comments, this is basic research, since no one yet knows precisely what functions anandamide serves, with current conjectures pointing to roles in cognition and pain. “There is interest in anandamide possibly working as a peripheral analgesic, and there's research activity ongoing with regard to the role of anandamide-like compounds in inflammation, because arachidonic acid is the precursor to both anandamide and the prostaglandins,” he said.

In the past, Martin said, companies contemplating drug discovery programs in this area have been concerned about the psychoactivity and drug abuse liability of potential therapeutic compounds. As research moves further away from THC, these concerns seem much less pertinent. “Agents that manipulate endogenous anandamide levels are a pretty far remove from the marijuana plant,” Martin said. Although much basic research remains to be done, Martin suggests that this is an ideal time for companies to make a long-term commitment to begin or revive cannabinoid research programs.

References
(1) Marijuana and Medicine: Assessing the Science Base; Joy, J. E., Watson, Jr., S. J., Benson, Jr., J. A., Eds.; National Academy Press: Washington, DC, 1999; p. 288.

(2) Peters, H.; Nahas, G. G. A brief history of four millennia. In Marihuana and Medicine; Nahas, G. G., Sutin, K. M., Harvey, D. J., Agurell, S., Eds.; Humana Press: Totowa, NJ, 1999; pp. 3–7.

(3) Koe, B. K. Levonantradol. In Marihuana and Medicine; Nahas, G. G., Sutin, K. M., Harvey, D. J., Agurell, S., Eds.; Humana Press: Totowa, NJ, 1999; pp. 553–560.

(4) Lemberger, L. Nabilone: A synthetic cannabinoid of medicinal utility. In Marihuana and Medicine; Nahas, G. G., Sutin, K. M., Harvey, D. J., Agurell, S., Eds.; Humana Press: Totowa, NJ, 1999; pp. 561–566.

(5) Wagner J. A.; Varga, K.; Kunos, G. Cardiovascular actions of cannabinoids and their generation during shock. J. Mol. Med. 1998, 76, 824–836.

(6) Pertwee, R. G. Cannabinoid receptors and their ligands in brain and other tissues. In Marihuana and Medicine; Nahas, G. G., Sutin, K. M., Harvey, D. J., Agurell, S., Eds.; Humana Press: Totowa, NJ, 1999; pp. 177–185.

Carol Hart is a science writer based in Narberth, PA. Comments and questions for the author may be e-mailed to mdd@acs.org, faxed to 202-776-8058, or mailed to Modern Drug Discovery, 1155 16th St., NW, Washington, DC 20036.

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