LONDON ¿ Among the active ingredients of cannabis could lie the key to new drugs for the treatment of the brain tumor known as malignant glioma, which frequently proves fatal within a year of diagnosis. Researchers in Spain have shown that a proportion of rats with experimental gliomas apparently can be cured of their tumors when injected with cannabinoid compounds.
The team, led by Manuel Guzman, lecturer in biochemistry at Complutense University in Madrid, is carrying out experiments to optimize the dose, type and delivery of the cannabinoids in the rat model. Guzman told BioWorld International: ¿We are still quite a long way from the clinical application of this finding but we hope that maybe in a year¿s time we will be in a good position to start human clinical trials.¿
Guzman and his colleagues are seeking partners to help them develop their work. Their findings are published in the March 2000 edition of Nature Medicine, in a paper titled ¿Anti-tumoral action of cannabinoids: Involvement of sustained ceramide accumulation and extracellular signal regulated kinase activation.¿
Commenting on the paper, Daniele Piomelli, of the Department of Pharmacology at the University of Calif ornia, Irvine, said the ¿bleak scenario¿ facing patients with a diagnosis of glioma ¿ aggressive treatment consisting of surgery, radiation therapy and chemotherapy, with a median survival rate of 40 to 50 weeks ¿ alone should provide sufficient motivation to continue the studies begun by the Spanish group.
In a News & Views article titled ¿Pot of gold for glioma therapy¿ in the same issue of Nature Medicine, Piomelli concludes that ¿the risk of typical cannabinoid side effects ¿ euphoria, amnesia, decreased psychomotor performance and hypotension ¿ may be outweighed by the therapeutic advantages."
Four years ago, Guzman and his colleagues were studying the signal transduction mechanisms involved in the modulation by cannabinoids of glucose uptake. He told BioWorld International, ¿Just by chance, we started to use cell lines derived from brain tumor cells and we observed that when we added the cannabinoids, these cells died. At the beginning, we thought something was wrong with the cultures, but since it kept happening we started to think that a specific mechanism might be involved.¿
Further research followed, including the experiments reported in the Nature Medicine paper. Guzman and his team aimed to find out whether cannabinoids such as delta9-tetrahydrocannabinol (THC), which is the main active ingredient of marijuana, could inhibit the growth of malignant cells in animals and, if so, by what mechanism.
The group had already shown that THC could induce apoptosis of glioma cells in vitro. For their in vivo experiments, they injected glioma cells into the brains of rats to induce malignant tumors. Rats subjected to this procedure normally die 12 to 18 days later.
The researchers decided to evaluate the effects of both THC and a synthetic agonist of cannabinoid receptors called WIN-55,212-2. Because the latter is highly potent, it could be used at lower doses, with possibly fewer side effects. Twelve days after the tumor cells had been introduced, these cannabinoids were pumped into the same site. This infusion continued for another seven days.
Results of these experiments were encouraging. The rats treated with either of the two cannabinoids survived significantly longer than control rats (p<0.01). Of the 15 rats treated with THC, the treatment was ineffective in three, which died by days 16 to 18. But nine outlived the untreated rats and survived from 19 to 35 days. In a further three THC-treated rats, the tumor was completely eradicated.
Out of the 15 rats treated with WIN-55,212-2, treatment was ineffective in six of the animals. Another four rats survived for an extended period lasting between 19 and 43 days. In five, the tumor was completely eradicated.
The researchers reported, ¿An MRI scan of one of the THC-treated rats showed that after cannabinoid administration there was a total absence of tumoral mass, and in its place, a residual . . . image interpreted as a fibrous scar appeared at the site of injection. There has been no recurrence in any of the eight surviving rats, which have been monitored periodically by MRI.¿
Two cannabinoid receptors have so far been discovered in mammalian tissues. These are known as CB1, which is present in the central nervous system, and CB2, which is mainly present in cells of the immune system. It is the CB1 receptor which is thought to mediate all the psychotropic effects of cannabis.
Guzman¿s group set out to establish which of the receptors mediated the actions of the cannabinoids seen in the experiments. He and his colleagues found that, whereas brain cells normally express only the CB1 receptor, the transformed glial cells used for the injections also express the CB2 receptor.
"We observed,¿ Guzman told BioWorld International, "that cannabinoids could induce cell death through activation of either of the CB receptors. Both receptors could trigger a complete set of reactions within the cell leading to cell death.¿
The observation that CB2 appears in the glial cells once they become malignant is very important, he added. ¿If we were able to activate only the CB2 receptors, we should be able to kill tumor cells without exerting any psychotropic effect, because these effects are supposed to be mediated by CB1.¿
Further experiments showed that activation of the CB receptors led to the generation within the cell of a lipid called ceramide, which is known to be a mediator of apoptosis. Ceramide, in turn, activates a protein kinase called raf1, and this activates a further protein kinase called Mek. Mek in turn activates a kinase called extracellular signal-regulated kinase (ERK). Guzman said, ¿ERK is supposed to get into the nucleus of the cell and modulate the expression of genes encoding proteins involved in the induction of cell death, but we don¿t know exactly how it does this. We are studying this at the moment.¿
Future studies will attempt to elucidate how activation of the cannabinoid receptors leads to the generation of ceramide.
Other plans include experiments aimed at optimizing the delivery of cannabinoids. ¿These are lipophilic compounds and so they are not easy to get into the tumor,¿ Guzman pointed out. ¿We would also like to test other cannabinoids, including the most potent synthetic cannabinoid, HU210.¿