Repurposing potential of β-adrenergic agonists and antagonists: A PharmLines Initiative study for cognitive status and Alzheimer’s disease (A retrospective cohort study)

Inherent problem
Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by debilitating cognitive ability including memory loss, lack of judgement in decision-making, and poor perception [1]. Currently, approximately 280,000 persons in the Netherlands are living with AD; this number is expected to increase to 420,000 by the year 2030, owing to the increasing proportion of elderly persons in the country. AD is the most common cause of dementia cases in the Netherlands [2].

Neuropathology
AD is neuro-pathologically characterized by extracellular accumulation of plaques consisting of insoluble amyloid-β (Aβ) peptides and intracellular neurofibrillary tangles (NFTs) composed of hyper-phosphorylated tau protein in characteristic brain regions [3,4]. These AD hallmarks are known to cause cognitive decline via loss of cholinergic neurons [5,6,7].

Current treatments
The main medication class used in the treatment of AD is cholinesterase inhibitors (ChE-Is); however, use of ChE-Is in the treatment of AD remains controversial. Notably, ChE-Is available today are not intended to cure AD, but rather slow cognitive decline and alleviate symptoms of AD [8,9]. Studies have revealed such limited efficacy of current AD medications that the effects of such may not be clinically significant [9]. Lack of efficacy coupled with the risk of gastrointestinal side effects from current AD medications elucidates the need for novel treatments.

Disease mechanism
‘Cholinergic hypothesis’
The most widely accepted disease mechanism outlines AD primarily as a disease of the cholinergic system [9]. This so-called ‘cholinergic hypothesis’ is supported on consensus that deficiency in cholinergic transmission is involved in the pathophysiology of learning and memory impairment associated with AD. Similarly, a decrease in cholinergic tone in cognition-related brain areas in patients with AD supports this [9]. Currently available anti-AD medications have been developed based on the cholinergic hypothesis and thus act as acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibitors; these ChE-Is prevent the degradation of the neurotransmitter acetylcholine (ACh) by inhibiting AChE and/or BuChE, two cholinesterase enzymes which degrade ACh, thus increasing cholinergic tone in characteristic brain areas [9]. Further, studies have shown possible benefits of ChE-Is in the treatment of dementia associated with PD [10]. However, the cholinergic hypothesis assumes that only ACh contributes to the pathology of neurodegeneration, while research indicates ACh may not be the only neurotransmitter or biochemical pathway suitable as a target for AD therapies [3-7,9,11,12].

Neurotransmission hypothesis
Some researchers propose deficits or surplus of several different neurotransmitters may be responsible for the pathology of AD in addition to ACh, with potentially over twenty distinct biochemical pathways implicated in AD pathology [5,6,12]. Specifically, Fitzgerald et al. (2019) hypothesizes some neurotransmitters (glutamate, GABA, and noradrenaline) may be biased towards excitatory responses while others (5-HT, ACh, Mt, and DA) may be biased towards inhibitory responses in patients with AD; with this, it is suggested that most AD patients have an excess in noradrenergic tone and deficiency in the other neurotransmitters [6]. Agreeably, Cipres-Flores et al. (2019) suggests testosterone and adrenergic pathways are implicated in AD etiology as β-adrenergic receptors (β-AR), specifically the β2-AR, participate in modulation of memory [5]. These findings indicate that the mechanism(s) by which AD manifests and progresses is complex and requires therapies which target more than one pathway implicated in neurodegenerative disorders, including AD [5,6].

Lysosomal hypothesis
Studies support the hypothesis that the mechanism involved in AD progression is dysregulation or impairment of lysosomal pathways [1,3,4,7]. Autophagy is an intracellular pathway responsible for organized elimination of waste products via lysosomes and mediates the secretion of Aβ [3]. Abnormalities in the lysosomal system have been reported in the brain of AD patients and it is hypothesized that these abnormalities lead to an increase in γ-secretase, an enzyme which participates in Aβ production via endoproteolysis. Importantly, Ni et al. (2006) demonstrated in vitro and in vivo that β2-AR activation enhances γ-secretase activity and this agonist-induced activity is mediated by endocytosis and late endosomes and lysosomes [4,13]. This enhanced γ-secretase activity is linked to production of Aβ fragments formed from endoproteolysis of amyloid precursor protein (APP). APP synthesis and secretion is regulated by cAMP; interestingly, cAMP levels are reportedly elevated in cerebral micro-vessels and cerebral spinal fluid (CSF) in the brain of AD patients, implicating the cAMP pathway in neurodegeneration [1,3,4,7,14,15].

cAMP hypothesis
Some researchers argue impairment within the cAMP pathway is the main contributor to AD-like neurodegeneration. Evidence has shown co-immunolocalization of cAMP and Aβ in AD brains, while elevation of cAMP levels are concomitant with increased apoE aggregation [15,16]. With this, it is hypothesized that cAMP up-regulation promotes neurodegeneration via cAMP-dependent protein kinase (PKA) phosphorylation of tau [15,17]. This implicates β-ARs as some studies propose that Aβ stimulates cAMP and apoE levels via initial interaction with β-ARs, which supports hypotheses that β-AR agonists or antagonists are potential therapeutics in preventing cognitive decline [16]. Additionally, studies have shown activation of β2-ARs increase the release of neurotropic factors via the cAMP/PKA pathway, presumably providing protection against neurodegeneration [18]. 

Inflammation hypothesis
Chronic inflammation is implicated in the etiology of neurodegenerative disorders as inflammatory processes cause injurious local changes. Researchers hypothesize that dysregulation of the lysosomal pathways leads to this inflammation. Elevation of intracellular cAMP has been shown to be responsible for pro-inflammatory cytokine induction, presumably contributing to neuronal dysfunction and apoptosis [1,8,19]. Interestingly, studies have shown depletion of norepinephrine (NE)- the endogenous β2-AR agonist- increases microglial-induced neuro-inflammation, with administration of NE thereafter providing protection to neurons from apoptosis [18].

Cerebral blood flow hypothesis
Cerebral blood flow regulation impairment has also been implicated in AD etiology. Vascular factors are known to be associated with dementia, heart disease and diabetes, with smoking being a risk factor for developing AD. The Kungsholmen Project study determined that a baseline systolic blood pressure greater than 180 mmHg was associated with an adjusted relative risk for incident AD of 1.5 [20]. Zhang et al. (2020) provide an alternative mechanism whereby AD progression is caused by disrupted regulation of vascular tone, which in turn leads to Aβ aggregation due to poor clearance by the glymphatic system [11]. Cerebral blood flow regulation impairment is considered a hallmark in AD pathology and studies have shown that the glymphatic system removes Aβ from characteristic brain regions. With this, disruption of vascular tone may influence the efficiency of the glymphatic system making impaired glymphatic function a useful target for anti-AD therapies [11]. 

β-AR agonists/antagonists
Many studies have been completed with the aim of determining the therapeutic potential of β-AR agonists and antagonists alike in improving cognitive functioning and attenuating the progression of neuropathological hallmarks of neurodegeneration. Interestingly, these studies have provided inconsistent results. [21,22] Some studies, such as that of Branca (2014), Desai (2020), B. Zhang (2020), and Hutten (2022) demonstrate the therapeutic potential of β-AR agonists for AD. Conversely, studies from that of Dobarro (2013), L. Zhang (2017), N-N. Yu (2010), and J-T. Yu (2010) support the therapeutic potential of β-AR antagonists [23]. Additionally, some researchers have hypothesized that both β-AR excitement and inhibition may attribute to attenuated cognitive decline depending on the individual’s neuro-modulatory balance [5,6]. Notably, the aforementioned studies constitute both animal models as well as epidemiological research, where conflicting results have manifested from both research types.

Branca and colleagues describe administration of a β2-AR antagonist exacerbating cognitive deficits in mouse models of AD [22]. Similarly, an epidemiological study by Hutten et al. (2022) showed a decreased risk of developing AD with exposure to β-AR agonists and increased risk with exposure to β-AR antagonists [7]. Conversely, Dobarro et al. (2013) describe improved cognitive functioning and attenuation of AD pathological hallmarks in mouse models upon administration of propranolol [23]. Likewise, a review on the roles of β-ARs in AD pathogenesis by Yu et al. (2010) resolved blocking β-ARs assuage the pathological process of AD [4]. Additionally, several criticisms of epidemiological studies, namely by Mittal et al., have surfaced; specifically, Hopfner et al. argue that the protective benefits of salbutamol diminish when adjusting for further confounders [24,25].

Further studies needed
Few pharmacoepidemiological studies have been completed with the aim of determining the potential therapeutic value of β-AR agonists and antagonists on the market today; notably, these reported studies have encompassed fairly small cohorts [7]. Only one study has been reported on the comparative therapeutic value of salbutamol and propranolol in human populations, despite numerous studies proposing salbutamol and propranolol as potential therapeutics for neurodegenerative diseases [1,3,5,6,7,11,12]. Fitzgerald et al. (2021) directly calls for retrospective epidemiological analysis of medical records in the context of AD in order to advance the understanding of disorders of neurodegeneration in humans [6].