Fragile X syndrome (FXS) is an X-linked dominant disorder caused by the expansion of a trinucleotide repeat (CGG)n within the first exon of the fragile X mental retardation 1 (FMR1) gene, which silences the expression of the fragile X mental retardation protein (FMRP).¹ The absence of FMRP, an important regulator of translation of many messenger RNAs involved in synaptic plasticity,² leads to substantial intellectual deficits. The condition has been estimated to affect ∼1:4000 males and 1:7000 females.^3–5 Although both sexes are susceptible, males with FXS typically exhibit more severe symptoms compared with females because the single X chromosome in males is usually fully methylated and not producing FMRP⁶; females typically have some FMRP expression from the FMR1 gene on their second unaffected X chromosome.⁷ Males with FXS are far more likely than females to exhibit significant intellectual and developmental disabilities (85% vs. 25%).⁸

Among individuals with the full mutation (i.e., >200 CGG repeats), symptoms of FXS vary by age and sex, but often include anxiety (80%), social avoidance (80%), stereotyped behaviors (80%), attention-deficit/hyperactivity disorder (70%), autism spectrum disorders (60%), intellectual disability (85% of males and 25% of females), aggression (40%), disrupted sleep patterns (40%), and epilepsy (16%), as well as macroorchidism (90% of adults), prominent ears (60%), long faces (60%), soft skin (50%), and hyperextensible joints (60% of children).^9–11 Traditional allopathic treatment of these patients may involve medications to address issues with sleep (melatonin and clonidine), anxiety (selective serotonin reuptake inhibitors and benzodiazepines), hyperactivity and deficits in attention (psychostimulants), and seizures (carbamazepine, valproic acid, and lamotrigine).¹² Yet, for many patients with FXS, the aforementioned pharmacotherapies, when used alone or in combination, have suboptimal efficacy and tolerability,^11–13 suggesting a persistent unmet medical need for novel, interventional treatment approaches for patients with FXS. Because the clinical abnormalities in these patients (and most recently in children with autism spectrum disorder¹⁴) have been linked, at least in part, with dysregulation of the endocannabinoid system, we briefly review recent research characterizing the involvement of the endocannabinoid system in FXS and present the cases of three individuals (one child and two adults) with FXS who experienced functional benefit after treatment with cannabidiol (CBD)-enriched (CBD+) preparations.

The endocannabinoid system consists of receptors in the brain and peripheral tissues that are involved in numerous physiological processes as well as the endocannabinoids, N-arachidonoylethanolamine (anandamide; AEA) and 2-arachidonoylglycerol (2-AG). The endocannabinoids bind to the G-protein-coupled receptors, cannabinoid 1 and 2 (CB₁ and CB₂),^15,16 and modulate synaptic transmission throughout the central nervous system.^17,18 Receptors are distributed throughout the body, with CB₁ receptors abundantly expressed in the brain and present at lower concentrations in a variety of peripheral tissues and cells and CB₂ receptors expressed primarily in the immune and hematopoietic systems, as well as in the brain, pancreas, and bone.¹⁶

CBD, the primary, noneuphoric exogenous phytocannabinoid in cannabis, may attenuate the loss of endogenous cannabinoid (i.e., endocannabinoid) signaling observed in preclinical models of FXS, allowing a component of the FMRP deficiency inherent in FXS to be bypassed. Specifically, many abnormalities seen in FXS appear to be rooted in dysregulation of the endocannabinoid pathways in the central nervous system, with a reduction of endogenous stimulation of endocannabinoid receptors.^19–21 CBD has the capacity to interact with an FXS-compromised endocannabinoid system. Indeed, deletion of FMRP within a mouse model of FXS led to reduced production of 2-AG, decreasing activation of CB₁ receptors in the central nervous system.²⁰ CBD has been shown to increase 2-AG availability,²² potentially attenuating or reversing one of the biological mechanisms of abnormal cellular function in FXS.²⁰ Importantly, CB₁ protein expression appears unaffected in FMR1 knockout (KO) mice, suggesting that the downstream elements of endocannabinoid signaling can be engaged, even in the absence of FMRP.¹⁹

In addition to the role of 2-AG, recent work has begun to highlight the potential importance of AEA in addressing social impairment as well as deficits in learning and memory among those with FXS. In an FMR1 KO mouse model of FXS, Qin et al. demonstrated that increased levels of AEA were associated with greater cognitive performance.²³ Similarly, Wei et al. utilized mouse models of FXS to show that AEA-mediated signaling at CB₁ receptors, driven by oxytocin, controls social reward²⁴ and that increasing AEA activity resulted in reductions in social impairment.²⁵ Much like its impact on 2-AG, CBD has been shown to increase levels of AEA by binding to fatty acid-binding proteins, which transport AEA to the catabolic enzyme fatty acid amide hydrolase, an enzyme that breaks down AEA.^26–28 Binding to fatty acid-binding proteins is thought to increase AEA availability and CB₁ activation.

The mechanisms underlying the potential benefits of CBD for FXS span far beyond the endocannabinoid system. Beyond providing benefit to patients with FXS through increases in 2-AG and AEA availability, CBD may positively affect synaptic plasticity. Studies of FMR1 KO mice have identified discrete alterations in synaptic plasticity in specific brain regions, including an increase in long-term depression (LTD) in the hippocampus,^23,29 and preclinical data suggest that reducing LTD in FMR1 KO mice requires activation of endocannabinoid receptors.¹⁹ Therefore, it is hypothesized that CBD may increase synaptic plasticity in FXS, facilitating one of the basic cellular mechanisms thought to be associated with learning and improvements in cognition.²⁹

More recent work has also begun to identify deficits in GABA receptor expression among those with FXS. As FMRP has been shown in animal models to enhance expression of GABA receptors, the lack of FMRP among those with FXS has been associated with fewer GABA receptors.³⁰ Indeed, preclinical studies in FMR1 KO mice have consistently shown downregulation of the GABA system.¹¹ As CBD acts as a positive allosteric modulator of GABA-A receptors,³¹ CBD may also act to enhance the binding affinity for GABA.

The impact of CBD on serotonin represents a mechanism by which CBD may aid in reduction of social anxiety and resulting avoidance experienced by patients with FXS. Indeed, the anxiolytic effects of CBD have been reported in over 30 preclinical studies, using multiple models of anxiety (i.e., generalized anxiety, stress-induced anxiety, panic disorder, and contextual fear conditioning),^32,33 as well as in a growing number of human studies, including within-social anxiety.^32,34 Several preclinical studies have identified the serotonin 1A receptor as one mechanism through which CBD exerts its anxiolytic effects.^35,36

We report the cases of three patients with diagnosed FXS who received oral forms of botanically derived CBD+ solutions and reported clinical symptom improvement.

Based on its morbidity and prevalence, its association with a number of co-occurring problems, such as seizures, anxiety, and sleep disturbances,¹³ and the paucity of efficacious therapeutic options, FXS represents an important public health problem.⁶ The present article provides a brief review of recent research that documents the promise of CBD as a therapeutic agent for patients with FXS. Also described are the cases of one pediatric and two adult patients with FXS for whom CBD+ treatment appears to have contributed to positive changes in anxiety and/or language skills, with no observed adverse events.

Before starting CBD+ treatment, Patient 1 experienced heightened symptoms of anxiety, frequent tantrums, and sleep difficulties. Over the first month of CBD+ monotherapy, and subsequent 3 months of CBD+ treatment combined with speech, language, and occupational therapy, the patient made considerable progress with feeding and weight gain, exhibited better oral–motor coordination, had decreased social avoidance and sensory sensitivities, and showed improvements in attention span/engagement, the frequency and severity of atypical motor movements, and general level of hyperactivity. Upon discontinuation of CBD+ treatment, the patient's prior symptoms reemerged. While a strong therapy program and later addition of other medications likely contributed to this patient's overall developmental progress, the temporally related improvements in anxiety, feeding, tantrums, and sleep—evident when CBD+ treatment was initially started as monotherapy and then reinstated following cessation—are compelling support for the benefits of CBD+ treatment for this patient. By maintaining his adaptive functioning scores from 1 to 3 years of age, the patient demonstrated a significant improvement over the characteristic developmental trajectory of young male children with FXS, where a decrease in adaptive functioning scores is typically observed between 2 and 6 years of age.⁴⁰

Patient 2 showed a similarly encouraging response to CBD+ treatment, experiencing reduced anxiety, improved use of language, and better sleep within 1 week of beginning treatment with a CBD+ solution. It is also remarkable that the patient continued to demonstrate symptom improvement over the initial 6-week treatment period, with longer-term follow-up highlighting continued use of CBD+ solution with sustained benefit.

In Patient 3, the use of CBD+ solution was also associated with a positive effect in a higher functioning female with FXS and long-standing anxiety symptoms. Similar to Patient 1, treatment discontinuation was associated with a recurrence of anxiety symptoms, with reinitiation of CBD+ treatment leading to symptom improvement and resulting long-term use.

The present findings, coupled with the available preclinical data, highlight the potential for CBD as an intervention for individuals with FXS. The existing literature combines to demonstrate that CBD may positively impact individuals with FXS through many mechanisms, including the endocannabinoid system, GABA, and serotonin. While a number of drugs have been developed to target specific systems (e.g., GABA agonists), CBD has the potential to yield a multifaceted benefit to individuals with FXS due to its multiple mechanisms of action. CBD has not only been shown to be generally well tolerated relative to other treatments used in this population,⁴¹ but also numerous studies have documented its benefits in terms of sleep quality,^42,43 anxiety (including social anxiety),^34,44 and cognitive impairment⁴⁵—symptoms experienced by the individuals profiled in the present case series.

These data serve as stepping stones upon which proof-of-concept open-label trials should be based. As with many patients, however, those discussed herein used orally administered botanical CBD+ solution that is not regulated by the Food and Drug Administration and, thus, inconsistencies in availability, quality, purity, and labeling^46,47 make research, interpretations, and clinical recommendations challenging. The present case series is limited by its reliance on manufacturer-reported cannabinoid content as well as the lack of multimethod assessment of patient symptomatology, including clinimetric data. The observed clinical benefit of CBD+ treatment in case studies, particularly with respect to caregiver-reported behavioral outcomes, must also be interpreted with caution given the significant placebo effects that have been documented in clinical trials of CBD.⁴⁸ Only placebo-controlled trials will be able to elucidate the true therapeutic effects of CBD/CBD+ treatment on FXS symptomatology. Until rigorous clinical trials have demonstrated the efficacy of CBD/CBD+ treatment for FXS, current treatments for the many behavioral problems associated with FXS should be utilized before off-label use of CBD+ products.

In an effort to overcome existing limitations, future studies should independently test patient samples to confirm actual constituents of each CBD+ preparation⁴⁷ and utilize well-validated caregiver-reported assessments of anxiety and other FXS symptomatology, in addition to unstructured caregiver- and clinician-based reports, in an attempt to more completely track each patient's course while in clinical care. Due to inconsistencies observed in many oral botanical preparations, rigorous examinations of pharmaceutical-grade preparations of CBD should be explored as potential treatments for children and adolescents with FXS.