Plant constituents: potential pharmacological and therapeutic tools?

DKF Newsletter November 2002 / Research MEM
The traditional and empirical use of plants in folk medicine is a rich ethnopharmacological source for the evaluation of therapeutically interesting biogenic compounds. In case of unknown active principles, in-vitro and in-vivo bioassay guided chromatographic fractionation of plant extracts is performed followed by spectroscopic structure elucidation (1H-, 13C-NMR, MS). Such a procedure was used to identify the constituent(s) of onion (Allium cepa L.) which exhibit(s) bone resorption inhibitory activity (in cooperation with Dr. Roman Mühlbauer, group of bone biology; http://www.cx.unibe.ch/dkf5/bone_biol/group.html). The use of bone metabolism modulating natural products opens a nutritional approach to prevent osteoporosis. In addition, it has recently been shown by Mühlbauer et al., that several monoterpenes, the major compounds of essential oils, inhibit bone resorption in vitro and in vivo. Menthol, for example, showed a strong activity in rats at a dose of 400 mg/kg b.wt. Due to the highly volatile character of this lipophilic monoterpene an appropriate analytical method based on headspace solid-phase microextration (SPME) followed by gas chromatography-mass spectrometry (GC-MS) had to be established for the validation of in-vitro bioassays as well as the acquisition of pharmacokinetic profiles and identification of potentially active metabolites. This procedure allowed the determination of menthol in complex biological matrices (blood, urine etc.) even at the 5 ng/mL level.
Hemp (Cannabis sativa L.) is not only the worldwide most abused illegal drug but also a potent medicinal plant with a broad therapeutic spectrum known already 5000 years ago in the ancient China. In the nineties, two cannabinoid receptors (CB-1, CB-2) and several endogenous ligands (anandamides) were found. This endocannabinoid system, in view of the existence of the endomorphine system postulated for a long time, seems to be a very promising target for new agonistically or antagonistically acting drugs to treat movement disorders, obesity, spasticity, anorexia/cachexia, pain etc. The most relevant phytocannabinoid, delta-9-tetrahydrocannabinol (THC), is an exogenous CB-1/CB-2 agonist and is responsible for most therapeutic but also adverse (e.g. psychotropic) effects of Cannabis. Nowadays, numerous anecdotal reports exist from patients, suffering for example from chronic pain, spasticity and appetite loss, and experiencing positive effects after consumption of Cannabis products (mostly hemp tea or cigarette). The lack of scientific evidence motivated us to conduct several controlled clinical studies on patients as well as on healthy subjects. When administering oral THC in form of MarinolÒ (FDA approved) to spinal-cord injured, paralyzed patients, a reduction of spasticity and pain and an improvement of bladder functions could be observed (project realized in collaboration with The Rehabilitation Center for Paralyzed and Brain-Injured Patients, REHAB, Basel). In an ongoing trial at the Department of Neurology, University Hospital Bern (PD Johannes Mathis), patients suffering from the Restless Legs Syndrome (RLS) get MarinolÒ to study in the sleep laboratory the reliefing effects of THC. The antinociceptive effect of THC, also shown by animal experiments, could not be confirmed in a placebo-controlled pain study (in collaboration with PD Michele Curatolo et al., Department of Anaesthesiology), when giving 20 mg oral THC to healthy subjects and using experimental pain models (pressure, cold, heat, electrical stimulation). The morphine-potentiating effect, claimed by chronic pain patients who practice comedication with Cannabis, could not be confirmed, too. The pharmacokinetic profiling by GC/MS revealed an extensive first pass metabolism, which encouraged us to initiate a second pain study with an inhalable and an injectable THC application form. The pharmaceutical challenge consisted of the formulation of physiologically tolerable aqueous preparations (aerosol, injection solution) of the extremely water insoluble THC. First results show similar pharmacodynamic characteristics of pulmonally and intravenously administered THC. Pharmacokinetic data are not yet available, as the bioanalytical work (plasma profiling of THC and metabolites) is still in progress.

Constipation is one of the most troublesome side-effects of opioid therapy and often adds to the distress of patients being treated for chronic pain. Whilst the analgesic action of opioids is mediated through the central nervous system, constipation is a consequence of their action on the gut wall, reducing intestinal secretion and motility in both small and large bowel, with a lengthening of gut transit time. Naloxone (NX), a non-specific opioid receptor ligand, acts peripherally and centrally as antagonist mainly on m-opioid receptors. In man, NX undergoes an extensive first pass liver effect with its main metabolite naloxone-3-glucuronide (NX3G). In contrast to the easily absorbed NX, the highly polar NX3G should not reach the systemic circulation and antagonize the central opioid analgesia. Therefore, oral NX3G could be used to antagonize the opioid-induced constipation. This hypothesis was the basis for a project, which is realized in collaboration with Prof. Ueli Scheurer et al., Department of Gastroenterology. The aims of this project are: (a) to examine the effect of NX and NX3G on the isolated vascularely perfused rat colon with exclusion of major physiological glucuronidation sites of NX by the liver and other organs with exception of the colon; (b) to acquire pharmacokinetic profiles of NX and NX3G in healthy subjects; (c) to study by radioscintigraphy, whether oral NX or NX3G selectively antagonizes morphine-delayed colonic transit in healthy subjects. The influence of NX3G on central morphine analgesia will be monitored by an experimental pain test (ice water).

For detailled informations about group members, other projects,
instrumentation, papers etc. or questions see
brenneisen@dkf5.unibe.ch
http://www.cx.unibe.ch/dkf5/brenneisen/brenne.html