The Endocannabinoid System as a Potential Therapeutic Target for Amyotrophic Lateral Sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by a selective loss of motor neurons from the spinal cord, brainstem and motor cortex. With a prevalence of about 5.5–9.9 per 100,000 persons, ALS is the most common form of motor neuron disease (MND). Although the mechanisms underlying the pathophysiology of this condition are not yet fully understood, it is believed that excitotoxicity, inflammation and oxidative stress play an important role in selective motor neuron death. Despite intensive research, up to this point no cure for ALS has been identified. There is increasing evidence that cannabinoids, due to their anti-glutamatergic and anti-inflammatory actions, may show neuroprotective effects in ALS patients and slow the progression of the disease. Furthermore, cannabis-based medicine may be useful in managing symptoms like pain, spasticity or weight loss. The aim of this chapter is to summarize the current state of research regarding the efficacy and safety of medical cannabis in the treatment of ALS.

1. Introduction

Amyotrophic lateral sclerosis (ALS) is a multisystem neurodegenerative disease leading to the progressive degradation of both upper (UMN) and lower motor neurons (LMN). With its prevalence of about 5.5–9.9 per 100,000 persons, ALS is the most common form of motor neuron disease (MND) [1]. The condition affects primarily the pyramidal motor system, including the motor cortex, cranial nerve motor nuclei and spinal cord motor neurons. The mean age of onset of sporadic ALS is about 60 years with a higher incidence among males [2]. The majority of ALS patients present with limb onset of the disease, resulting in focal muscle weakness and atrophy. Over time, due to the damage of the upper motor neurons, spasticity develops. On the other hand, patients with bulbar onset of ALS initially report dysarthria and dysphagia. The limb symptoms may occur almost simultaneously with bulbar symptoms, and in most cases develop within 1–2 years [3]. Subsequently, the disease involves various body regions, in particular respiratory muscles, causing death within 2–3 years for bulbar onset cases and 3–5 years for limb onset cases [4]. The neurodegeneration progresses in the neighboring cortical regions, including the prefrontal cortex, ventral and medial frontal cortical areas, and eventually involves portions of the parietal and temporal lobes and the deep gray structures. This results in non-motor symptoms of the disease such as impairment of executive functions, behavioral changes and language disorders in up to 50% of cases [5]. The cognitive decline in 10–15% of ALS patients is consistent with the diagnosis of frontotemporal dementia (FTD) [6]. To date, the only therapy widely approved for ALS is an anti-glutamate agent riluzole. It has been proven that riluzole slows the progression of ALS, extends survival and delays the time to tracheostomy. The benefit is, however, very limited and riluzole can extend the average survival time by only 3 months [7, 8]. Another drug, edaravone, was first approved in Japan, followed by South Korea, U.S., Canada, Switzerland, and China. Edaravone is a free radical scavenger that can reduce oxidative stress. Its beneficial clinical effects have been initially proven in Japan [9]. The subsequent study in the USA has showed that an intravenous treatment with edaravone prolonged the survival for 6 months [10]. These findings, however, have not been confirmed in the European cohorts [11, 12]. The third drug, AMX0035 has been approved for the treatment of ALS in the USA and Canada. AMX0035 is a combination of two compounds –tauroursodeoxycholic acid (TUDCA) and sodium phenylbutyrate (PB). It is thought to increase the threshold for cell death by blocking key cell death pathways and reducing the stress on the endoplasmic reticulum (ER) simultaneously [13]. The first randomized, placebo-controlled, phase 2 trial of AMX0035 in ALS (CENTAUR) has shown both functional and survival benefits in ALS patients [14]. Moreover, there are many experimental therapies in development, most of them showing anti-inflammatory and anti- excitotoxic mechanisms of action [15].

2. Possible role of the endocannabinoid system in the motor neuron disease

In recent decades, the endocannabinoid system has been gaining increasing attention as a potential therapeutic target in a number of neurological disorders [16, 17]. Thus far, two cannabinoid receptors (CB) have been identified: the CB receptor type 1 (CB1) and CB receptor type 2 (CB2), both belonging to the G protein-coupled receptors (GPCRs) family. The CB1 is expressed primarily in the central nervous system (CNS), especially in the neocortex, hippocampus, basal ganglia, cerebellum, and brainstem [18]. Apart from the CNS, CB1 has also been identified in numerous peripheral tissues and cell types [19]. On the other hand, the CB2 receptor is abundantly present in the immune system and in smaller quantities in the CNS, especially in human microglia [20].

Although a wide range of dysfunctional cellular processes related to neurodegeneration in ALS has been described, the exact mechanisms underlying its pathogenesis remain largely unknown. Mutations in more than 30 different genes associated with diverse molecular functions has been linked to ALS, which explains approximately 20% cases of the disease [21]. A process of excessive activation of glutamate receptors, resulting in neuronal dysfunction and death called “excitotoxicity” is believed to play an important role in the pathogenesis of many neurological disorders, including ALS [22]. Elevated extracellular glutamate levels activate postsynaptic glutamate receptors, causing an increased influx of calcium into the postsynaptic neurons. Subsequently, excessive postsynaptic calcium activates neurotoxic cascades, resulting in neuronal death. The activation of N-methyl-D-aspartate (NMDA) receptors may lead to cellular death more rapidly than the activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) and kainite receptors [23]. Studies in vitro and in vivo have shown that activation of cannabinoid receptors may inhibit neuronal hyperexcitability associated with activation of both NMDA and non-NMDA subtypes of glutamate receptors [24, 25, 26, 27]. Similarly, the potent anti-inflammatory properties of cannabinoids may be used to reduce chronic neuroinflammation and thus prevent motor neuron toxicity. Both CB1 and CB2 receptors were found to modulate release of endogenous interleukin-1 receptor antagonist (IL-1ra) from primary cultured glial cells, exerting neuroprotective effects [28]. Moreover, recent studies proved the anti-oxidative properties of cannabis, in particular inhibition of oxidative and nitrosative stress as well as reduced production of reactive oxygen species (ROS) [29]. CBD has been also found to regulate mitochondrial calcium concentrations as well as mitochondria-mediated intrinsic apoptosis [30, 31]. Additionally, CBD treatment may exert beneficial effects on skeletal muscle by reducing inflammation and improving muscle recovery [32].

3. Neuroprotective effects of cannabinoids in animal models of ALS

The most commonly used animal model to explore ALS is the SOD (G93A) transgenic mouse. The mouse is genetically engineered to express a mutation in the superoxide dismutase-1 gene (SOD1-G93A) and thus shows a phenotype similar to ALS in humans [33]. Raman et al. have demonstrated, that treatment with delta-9-tetrahydrocannabinol (Δ⁹-THC) in ALS SOD1 mice improved motor deficits and increased survival by 5%, most probably due to its anti-glutamatergic and anti-oxidant mechanisms of action [34]. Weydt et al. have shown, that cannabinol (CBN), a nonpsychotropic cannabinoid, delayed symptom onset in SOD1 transgenic mice without improving the survival [35]. In the subsequent study, Bilsland et al. have investigated the postsymptomatic treatment with an exogenous synthetic cannabinoid as well as genetic augmentation of endocannabinoids. The group has proven, that a potent CB1 and CB2 receptor agonist, WIN 55, 212-2, delayed the disease progression without affecting survival [36]. On the contrary, Shoemaker et al. have demonstrated, that a selective CB2 agonist, AM-1241, extended the lifespan of SOD1 mice by 4%, while WIN 55, 212-2 extended it by even 11% [37]. Zhao et al. have strengthened the evidence for AM-1241, showing its beneficial effect on disease progression [38]. Consequently, Moreno et al. evaluated the efficacy of nabiximols (trade name Sativex®) in SOD1 mice for the first time. Sativex® is a combination of 2.5 mg of CBD and 2.7 mg of Δ9-THC in the form of an oromucosal spray. In this study, Sativex® proved to be effective both in slowing the disease progression and improving survival [39]. Pasquarelli et al. have examined the neuroprotective and anti-inflammatory properties of 2-arachidonoylglycerol (2-AG). 2-AG is an endogenous CB1 and CB2 receptor agonist, which is present at relatively high levels in the central nervous system and is degraded by monoacylglycerol lipase (MAGL). In this study, the MAGL inhibitor KML29 was applied in order to increase the concentration of the 2-AG in the CNS of the SOD1 transgenic mice. This led to a reduction of proinflammatory cytokines and an increase in brain-derived neurotrophic factor (BDNF) expression levels in the spinal cord, the major site of neurodegeneration in ALS. Moreover, the oral KML29 treatment delayed the disease onset and extended life span in SOD1 mice up to 24 days [40]. Espejo- Porras et al. used another animal model, TDP-43 transgenic mice. The mis-metabolism of the RNA/DNA-binding protein TDP-43 (ALS-TDP), in particular, the presence of cytosolic aggregates of the protein is found in the spinal cords of more than 95% of ALS patients. Both the non-selective agonist WIN55,212-2 alone as well as in combination with the selective CB2 agonist HU-308 were shown to delay disease progression in TDP-43 mice [41]. Rodriguez-Cueto explored the anti-inflammatory, anti-oxidative and neuroprotective properties of VCE-003.2. VCE-003.2 is a novel derivative of the non-psychotrophic phytocannabinoid cannabigerol (CBG), which activates the peroxisome proliferator-activated receptor-γ (PPARγ). The treatment with VCE-003.2 was associated with a strong preservation of spinal motor neurons which may be attributed to normalizing the activation and cell function of the astrocytes [42]. An overview of studies investigating the endocannabinoid system in animal models of ALS is shown in Table 1.

Reference	Model	Substance	Outcome
Raman et al. [34]	SOD1 mice	Δ^9-THC	Δ^9-THC delayed disease progression in SOD1 mice and expanded their lifespan by 5%.
Weydt et al. [35]	SOD1 mice	cannabinol	Cannabinol delayed disease onset in SOD1 mice.
Bilsland et al. [36]	SOD1mice	WIN 55, 212-2; CB1 and Faah ablation	Delayed disease progression in SOD1 mice.; Expanded lifespan in SOD1 mice by 13%.
Shoemaker et al. [37]	SOD1 mice	AM-1241; WIN-55,212-2	AM-1241 extended lifespan of SOD1 mice by 4%; WIN-55,212-2 extended lifespan of SOD1 mice by 11%
Zhao et al. [38]	SOD1 mice	AM-1241	Delayed disease progression in SOD1 mice.
Moreno et al. [39]	SOD1 mice	combination of cannabidiol and; Δ^9-THC; (Sativex®)	Delayed disease progression in SOD1 mice and improved survival.
Pasquarelli et al. [40]	SOD1 mice	2-AG	Delayed disease progression in SOD1 mice and improved survival.
Espejo- Porras et al. [41]	TDP-43 (A315T) mice	WIN55,212-2; HU-308	Delayed disease progression in TDP-43 mice.
Rodriguez-Cueto [42]	SOD1 mice	VCE-003.2	Improved survival of spinal motor neurons. Delayed disease progression in SOD-1 mice.

Table 1.

Summary of studies exploring the endocannabinoid system in animal models of ALS. Studies are presented in chronological order.

SOD 1, superoxide dismutase-1; Δ9-THC, Δ9-tetrahydrocannabinol; FAAH, fatty acid amide hydrolase; CB1, Cannabinoid receptor type 1; 2-AG, 2-arachidonoylglycerol.

4. Clinical research with cannabis-based medicine in ALS

In 2004 Amtmann et al. conducted the first survey on marijuana use among 131 patients with ALS. In this group of patients, 13 males reported using cannabis in the previous 12 months. Cannabis smokers reported reduction of depression, appetite loss, pain, spasticity, drooling and weakness [43]. To date, only a very limited number of studies regarding the potential use of CBM in ALS has been conducted. A small pilot study conducted by Gelinas et al. in a group of 20 ALS patients found THC to be effective against muscle cramps, fasciculations, insomnia and lack of appetite [44]. Subsequently, Weber et al. investigated the efficacy of oral THC in the treatment of ALS-related cramps in a group of 27 patients. The participants were randomly assigned to receive 5 mg THC twice daily followed by placebo or vice versa for 2 weeks. The intensity of cramps was assessed using the visual analogue scale (VAS). Unfortunately, no statistically significant improvement of cramp intensity, number of cramps, or fasciculation intensity was observed. No serious adverse events were noted, one of the participants complained about dizziness [45]. Joerger et al. examined the pharmacokinetics (PK) and tolerability of oral THC in ALS patients. The group showed, that adverse events (drowsiness, euphoria, orthostasis, sleepiness, vertigo and weakness) occurred more frequently in patients receiving 10 mg compared to 5 mg THC. No serious adverse events were reported. A marked interindividual variability regarding the absorption and elimination of THC was also observed [46]. Meyer et al. explored the efficacy of nabiximols in the treatment of ALS – related spasticity in a retrospective, mono-centric cohort study with 32 patients. The participants were treated with an oromucosal spray, the mean dose was 5.5 actuations per day. Spasticity was rated using the Numering Rating Scale (NRS) and the patient’s experience was evaluated with the net promoter score (NPS) and treatment satisfaction questionnaire (TSMQ-9). Moderate to severe spasticity was associated with an elevated number of daily THC:CBD actuations and stronger recommendation rate (NPS) [47]. A larger, multicentre, double-blind, randomized, placebo-controlled study (CANALS) was conducted by Riva et al. The group investigated the efficacy of nabiximols (trade name Sativex®) in the treatment of ALS-related spasticity in a group of 59 patients (29 in the nabiximols group and 30 in the placebo group). The participants self-titrated the oromucosal spray during the first 2 weeks of treatment, receiving up to 12 puffs per day, then maintained the dose for 4 weeks. After 6 weeks of treatment a statistically significant reduction of spasticity evaluated with the Modified Asworth Scale (MAS) was observed in the nabiximols group. No serious adverse events occurred in either group. The most common adverse effects in the nabiximols group were asthenia, somnolence, vertigo, and nausea [48]. Only recently, the EMERALD trial, a randomized, double-blind, placebo-controlled study was initiated to investigate the efficacy of cannabis-based medicine (CBME) on slowing down the progression of ALS. The investigational product contains 25 mg of CBD and less than 2 mg of THC (approximately 10%) formulated as a capsule. A total number of 30 patients with probable or definite ALS diagnosis based on the El Escorial criteria, with the symptom duration of <2 years, has been included in the study. The primary objective of the study is to evaluate the efficacy of CMBE on the disease progression measured with ALS Functional Rating Scale-Revised and forced vital capacity (FVC) score after 6 months of treatment [49].

An overview of clinical studies investigating the efficacy and safety of the cannabis – based medicine in ALS patients is shown in Table 2.

Reference	Number of patients	Substance	Results	Safety
Gelinas et al. [44]	20	oral Δ 9-THC	Improvement of effective against muscle cramps, fasciculations, insomnia and lack of appetite.	No data.
Weber et al. [45]	22	oral Δ 9-THC	No significant improvement of cramp intensity, number of cramps, or fasciculation intensity.	No study-related SAEs.; AEs: dizziness.
Joerger et al. [46]	9	oral Δ 9-THC	AEs more frequent in patients receiving 10 mg compared to 5 mg THC. A marked interindividual variability regarding the absorption and elimination of THC.	No study-related SAEs.; AEs: drowsiness, euphoria, orthostasis, sleepiness, vertigo, and weakness
Meyer et al. [47]	32	nabiximols (Sativex®)	Higher efficacy in subgroups of ALS patients with moderate to severe spasticity.; High treatment satisfaction (TSQM-9).	Not data.
Riva et al. [48]	59	nabiximols (Sativex®)	Significant reduction of ALS-related; spasticity	No SAEs.; AEs: asthenia, somnolence, vertigo, and nausea.

Table 2.

Summary of clinical studies investigating the efficacy and safety of the cannabis – Based medicine in ALS patients. Studies are presented in chronological order.

Δ9-THC, Δ9-tetrahydrocannabinol; AEs, adverse events; SAEs, Serious Adverse Events.

5. Safety profile of cannabis-based medicine in patients with ALS

Thus far, very little is known about the safety and tolerability of CBM in patients with ALS. The only study focusing specifically on the tolerability of oral THC showed, that adverse events (drowsiness, euphoria, orthostasis, sleepiness, vertigo, and weakness) were more frequent in ALS patients receiving 10 mg compared to 5 mg THC [46]. This contrasts with findings in patients with multiple sclerosis and healthy subjects [50], who tolerated single doses even up to 15 mg without significant adverse events [51]. However, the abovementioned study included only 10 participants [46]. The other available preliminary results suggested that the safety profile of CBM in ALS is similar to that in other groups of patients and did not report any serious adverse events [45, 47, 48]. A meta-analysis conducted by Whiting, including diverse populations of patients treated with CBM, showed that the treatment with cannabinoids can be associated with a greater risk of adverse events (AE), including serious adverse events (SAE). The most common short-term AEs encompass dizziness, dry mouth, nausea or vomiting, fatigue, somnolence, euphoria, disorientation, drowsiness, confusion, loss of balance, and hallucinations. Moreover, up to this date no study evaluating the long-term AEs of cannabinoids has been conducted [52].

6. Conclusions

Taking into consideration the above-mentioned reports, there is a valid rationale for the use of cannabis-based medicine in the symptomatic treatment of patients with ALS. The efficacy of cannabis in the management of spasticity, drooling and anorexia has been proven not only in the small RCTs with ALS patients, but also in large RTCs including other groups of patients suffering from similar symptoms in the course of other neurological diseases, most commonly in multiple sclerosis (MS) patients. Moreover, there is increasing evidence that cannabinoid compounds may exert neuroprotective effects and prolong the survival in the animal models of ALS. Therefore, larger RTCs are urgently needed to confirm the therapeutic potential of cannabis in slowing down the disease progression.