Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with motor condition that causes the paralysis of the patient and his/her subsequent death. With ALS being a disease without a cure or effective treatment, cannabinoids are presented as molecules able to delay the symptomatology to improve patients' quality of life.
The main cause of this pathology is the progressive death of neurons responsible for transmitting the nerve impulse to the muscles, also known as motor neurones. Found mainly in the grey matter of the spinal cord (the internal part of the same), as they die the weakness of the limbs increases (this usually first affects one type and then spreads to the other). This entails a muscle atrophy that will evolve in the late stages towards total paralysis, which typically causes a respiratory failure that leads to the death of the patient. In addition, during the development of the disease other secondary symptoms related to pain, lack of appetite, or lack of sleep arise.
ALS was described for the first time in 1869 by the French doctor J.M. Charcot. Today it is considered a rare disease, since its prevalence reflects that there are about 2 people who suffer per every 100,000 inhabitants. The condition tends to be more common in men than in women at a ratio 3 to 1. Most cases are considered sporadic, around 90%, while the remaining 10% are cases of family origin, that is, there is genetic inheritance. From sporadic cases studies have been conducted on environmental factors as possible causes of the disease. Pesticides, heavy metals and even excessive physical activity have been the subject of studies without generating a direct relationship due to not having found significant differences in the populations studied. However, in both cases multiple proteins and genes responsible for triggering the ALS have been identified. In the 1990s the first mutation identified was in the gene of the SOD1 protein, which has led to it being the most studied. It was not until 2006 that TDP43 was also identified as another protein the mutation of which is associated with ALS. In recent years other genes have been added to the list of mutations involved, such as FUS, or more prevalent the C9orf72 gene. It should be noted that depending on which protein is involved, in addition to producing a characteristic symptoms of ALS, it can also reflect symptoms of dementia. In any case, most mutations in these genes generally produce an incorrect protein folding and as a result, functional defects that trigger the death of motor neurons.
Among these events can be found: protein aggregation as a trigger of cell death (protein malformations eventually collapse the neuronal systems causing their death); failures in cellular respiration of motor neurons that trigger oxidative stress (they are large-sized neurons with very long axons to connect with the muscles and therefore their energy expenditure is greater); excess glutamate in the synapses that promote an excitotoxicity (glutamate is a neurotransmitter that activates motor neurons, but excessive activation can lead to their death); marked glial activation associated with an increase of inflammation (by astrocytes, which are the main neuronal trophic support and microglia to emulate functions of the immune system but within the central nervous system), etc.
However, the fact that for so many years there was only evidence of the SOD1 mutated protein as responsible for the disease, has held back the study of the pathology itself and of potential drugs to treat it. Many molecules and drugs have been tested with some success in preclinical trials, but when moving on to clinical trials and trials with patients they did not achieve the expected results. That is why for years the only available drug was riluzol, a molecule focused on preventing excitotoxicity by glutamate, but with a very narrow therapeutic window. In 2017 a second molecule was approved for the treatment of ALS, called edaravone. This compound focused on reducing oxidative stress has better results in the cases of early diagnosis. Finally is masitinib, a third compound awaiting approval focused on mitigating glial inflammation which has had good results in preclinical and clinical trials. This situation reflects a need to find new molecules that can serve as a better therapeutic response.
Why then should cannabinoid molecules be considered useful for treating ALS? In the first place because cannabinoids have already been tested as good therapeutic agents in other neurodegenerative pathologies, for example, Alzheimer's disease or Parkinson's disease. Secondly, cannabinoids might have a relevant advantage over other molecules or drugs, as there are some molecules with a broad spectrum of action. Unlike the drugs already mentioned that focus on a single pathological aspect of the disease, cannabinoids may address more than one at a time. Their neuroprotective, antioxidant, anti-inflammatory, etc. qualities are well-known.
However, this idea contrasts with the scarce existence of pharmacological experiments in animal models, and even less clinically. In fact, the most relevant trials in patients have been conducted from 2010 onwards. They focused on the application of Δ9-THC only for treatment of secondary symptoms of the disease. Actually, the expected results were not obtained most probably due to the high concentration used, as effects arising from the psychoactivity of the cannabinoid were recorded.
What is known exactly from the preclinical models and experimental models with animals? Use of the experimental models has allowed studying the endocannabinoid system in the disease, and as in other diseases, it has been seen that certain elements of the same are altered and that we can modulate these changes for the sake of adjusting a specific treatment.
By way of an explanation, the endocannabinoid system, like other neurotransmitter systems such as the glutamate or the dopamine system, is formed by various receptors in different cells. The main ones are the CB1 receptor (of neurons and which the activation of which produces the psychoactive effects of Δ9-THC) and CB2 (characteristic of glial cells and mediator of inflammatory effects). These, in a natural manner, are activated by endogenous molecules in the body (in this case, the so-called endocannabinoids), the most well-known being anandamide and 2AG (2 arachidonoyl glycerol), which are regulated by synthetic enzymes (NAPE-PLD and DAGL) and degradation enzymes (FAAH and MAGL). Cannabinoids that come from the Cannabis sativa plant like the aforementioned Δ9-THC or the CBD (cannabidiol) and act on the different elements of the endocannabinoid system, are called Phyto cannabinoids.
Now we know the constitution of the endocannabinoid system, most preclinical studies on this system in ALS have been carried out with models of mutated SOD1. The first pharmacological trials were performed with Δ9-THC with the idea of increasing the survival of motor neurons through activation of CB1 receptors that these cells possess. Despite obtaining a certain positive effect, it has been found that the best therapeutic effects found have been mediated by the action of the CB2 receptor. This is due to the fact that over the years the role of glial activation has become increasingly relevant (astrocytes and microglia) not so much as a consequence of the ALS, but as a cause of the same. In addition, in parallel, studies on the endocannabinoid system in this model of mutated SOD1 reflected that there was an increase in levels of CB2. With these two premises, pharmacological trials were carried out with the synthetic cannabinoid AM-1241, selective for the CB2 receptor exclusively. This treatment managed to significantly delay the evolution of the disease correlated with a lower glial activation, i.e. less inflammation. Another way studied with these same positive effects was to block the degradation enzyme MAGL. When this enzyme is blocked, the levels of the endocannabinoid 2AG (2-arachidonoyl glycerol) increase more than normal, which leads to a greater activation mainly of the CB2 receptor, and somewhat lower than the CB1.
In addition, this same CB2 receptor overexpression has been found in a new model of ALS with the mutated protein TDP43. As was the case in the SOD1 model, we obtained the same toxic events resulting from a glial activation with a parallel activation of CB2 receptors in these cells. Pharmacological treatment using another selective synthetic cannabinoid, in this case HU-308, once again obtained a delay of the symptoms of the disease and greater survival of motor neurons. This results in mice that develop the disease later with a greater survival.
In conclusion, on the basis of that cannabinoids have already been demonstrated as therapeutic compounds for a variety of neurodegenerative diseases, and ALS a disease of this type in which there are a multitude of toxic events, it is clear that the strategy of using a direct pharmacology versus a single target is ineffective for treating it. It is precisely this reason that makes cannabinoids potentially beneficial molecules for treatment of ALS. Whether of plant origin or synthesised in the laboratory, the information obtained from the different pharmacological trials with cannabinoids has reflected that modulation of the endocannabinoid system is a reality with a positive effect. In ALS, in particular, there is a strong base for thinking that the cannabinoid receptors that mediate the activation of CB2 receptor (whose overexpression could be a marker of the disease) can help patients by delaying the symptomatology of the disease, improving quality of life and prolonging it. The only thing left is to put it into practice.