Psychedelics as Novel Therapeutics in Alzheimer’s Disease: Rationale and Potential Mechanisms

The extracted text presents a narrative, mechanistic review rather than a systematic review or meta‑analysis; it does not clearly report a formal search strategy, databases searched, inclusion/exclusion criteria, or dates of literature coverage. Instead, the paper synthesises experimental, preclinical, neuroimaging, and clinical trial data to build a translational rationale. Content areas integrated into the synthesis include: AD pathophysiology (amyloid, tau, synaptic loss, network dysfunction), serotonergic deficits in AD, synaptic and molecular signalling pathways (BDNF, TrkB, mTOR, Arc), neuroinflammation, PET and fMRI markers of metabolism and connectivity, preclinical behavioural and cellular studies of 5-HT2A receptor agonists and antagonists, and human clinical trials and pilot studies examining mood, anxiety, memory, and functional connectivity after psychedelic administration. When available, the authors draw on specific experimental paradigms (e.g. [11C]UCB-J PET for synaptic density in humans, animal reversal‑learning and fear‑extinction paradigms, single‑dose psychoplastogenic assays measuring dendritic spine changes) to connect mechanism to possible clinical endpoints. Where clinical trial data are discussed, the paper summarises key trial designs and outcomes from contemporary placebo‑controlled, wait‑list controlled, and active‑comparator studies of psilocybin and LSD in depression, cancer‑related anxiety, and treatment‑resistant depression, and it notes an ongoing pilot trial (ClinicalTrials.gov NCT04123314) testing moderate and high‑dose psilocybin in patients with early‑stage AD or MCI and depressed mood. The authors do not present new primary data or a quantitative synthesis but instead propose mechanistic hypotheses and suggest trial designs for future translational work.

The review assembles evidence across basic, translational, and clinical domains to support several converging claims. First, AD and normal ageing are associated with reductions in serotonergic neurotransmission; studies report decreased 5-HT2A receptor density and reduced serotonin transporter binding in prodromal AD and AD, changes that are linked to neuropsychiatric symptoms such as depression, agitation, and psychosis. PET imaging with tracers such as [11C]UCB-J indicates early reductions in synaptic density (for example, hippocampal SV2A decreases) that precede overt neuronal loss. Second, key signalling pathways implicated in synaptic maintenance and plasticity—most notably BDNF/TrkB and mTOR—are disrupted in AD. The authors describe evidence that Aβ accumulation reduces BDNF and alters TrkB expression, impairing long‑term potentiation and other plasticity processes, while dysregulated mTOR signalling can both impair autophagy and yet promote neuritogenesis depending on context. Third, inflammation is highlighted as a contributor to AD progression: post‑mortem studies show activated microglia and astrocytes with elevated proinflammatory cytokines (IL‑1β, IL‑6, TNF‑α), and although NSAID trials have not proven effective, targeted anti‑inflammatory strategies are still under consideration. Fourth, neuroimaging studies document functional connectivity alterations in AD, particularly in default mode, salience, and dorsal attention networks; amyloid accumulation and DMN hypoconnectivity are linked to cognitive decline. The paper summarises human neuroimaging findings that single doses of psilocybin can produce lasting changes in network connectivity (for example, decreased amygdala reactivity and increased global functional connectivity up to a month after dosing), suggesting a plausible mechanism for modifying network dysfunction in AD. Preclinical findings reviewed include extensive evidence for 5-HT2A receptor involvement in learning and memory: 5-HT2A agonists and antagonists modulate reversal learning, fear‑extinction, working memory and hippocampal‑dependent consolidation in rodents and primates. Psychoplastogenic effects are a central focus: single doses of psychedelics (psilocybin, DOI, LSD, 5‑MeO‑DMT) have been reported to increase dendritic spine formation, dendritic branching, synapse number, and immediate early gene expression in prefrontal cortex and hippocampus within 24–72 hours, with some structural changes persisting up to a month. In pigs, single‑dose psilocybin increased hippocampal SV2A density and decreased cortical 5‑HT2A density at 24 hours and sustained SV2A increases at 1 week. A 5‑HT2A inverse agonist was reported to reduce brain Aβ levels and improve cognition in an AD mouse model, an effect absent in 5‑HT2A knockout mice. Preclinical anti‑inflammatory data include suppression of TNF‑α induced inflammation by (R)-DOI and LSD in vascular and in vivo mouse models, effects that were 5‑HT2A receptor‑dependent; DMT and 5‑MeO‑DMT may also engage Sigma‑1 receptor pathways to inhibit IL‑1β, IL‑6 and TNF‑α. A clinical study of ayahuasca reported significant reductions in C‑reactive protein 48 hours post‑dose that correlated with mood improvement, but human anti‑inflammatory effects of psychedelics remain preliminary. Human behavioural and clinical data summarised emphasise mood and anxiety benefits after moderate-to-high psychedelic doses. Trials in patients with life‑threatening cancer (n≈51) reported clinically significant reductions in depression and anxiety with effects persisting to 6 months. A wait‑list controlled psilocybin trial (n=27, 24 completers) reported significantly greater reductions in GRID‑HAMD scores and an overall remission rate of 54% at 4 weeks post‑treatment. A double‑blind trial comparing two 25 mg psilocybin doses with 6 weeks of daily escitalopram (n=59) found both treatments reduced depressive symptoms; psilocybin produced numerically greater improvement, with remission at 6 weeks in 57% of the psilocybin group versus 28% on escitalopram and secondary outcomes favouring psilocybin. Acute cognitive effects during intoxication are typically dose‑dependent impairments in working memory and recall, although psychedelics can also enhance vivid autobiographical recall during the acute state. Post‑acute cognitive effects in clinical populations have not been rigorously characterised. The authors also note that neuropsychiatric symptoms in AD are common (over 40% with significant depression) and that high‑dose, psychologically supported psychedelic interventions target depression, anxiety and quality of life, whereas psychosis or hallucinations are generally considered contraindications. Finally, the review highlights gaps: microdosing trials to date have not shown consistent benefits in controlled designs, and definitive clinical evidence for disease‑modifying effects in humans is lacking. The authors reference an ongoing pilot study (ClinicalTrials.gov NCT04123314) testing moderate (15 mg/70 kg) and high (25 mg/70 kg) psilocybin in early‑stage AD/MCI with depressed mood.

Garcia‑Romeu and colleagues interpret the assembled data as providing a plausible, multi‑modal rationale for investigating classic psychedelics in early‑stage AD. They argue that psychoplastogenic properties—rapid induction of structural and functional neuroplasticity mediated by 5-HT2A receptor stimulation with downstream upregulation of BDNF, amplification of AMPA signalling with subsequent mTOR activation, and increased Arc expression—could counteract synaptic deficits and regional atrophy characteristic of AD. Alongside potential anti‑inflammatory effects and lasting modulation of large‑scale brain networks, these mechanisms could plausibly contribute to both symptomatic relief of neuropsychiatric comorbidity (depression, anxiety, apathy) and, speculatively, to slowing or reversing aspects of neurodegeneration. The authors position these ideas relative to prior work on psychedelics in psychiatry, noting robust antidepressant and anxiolytic signals in controlled trials and persistent changes in well‑being and life satisfaction across diverse populations. They recommend prioritising early‑stage AD or MCI populations for initial clinical research because of lower likelihood of psychosis or advanced cognitive impairment that would complicate consent and safety. Both high‑dose, psychologically supported sessions (targeting subjective therapeutic experiences associated with mood benefit) and repeated low‑dose regimens (targeting psychoplastogenic or anti‑inflammatory effects with reduced subjective intensity) are proposed as complementary research strategies. Key limitations and uncertainties are acknowledged. The review notes the preponderance of preclinical data and the limited direct human evidence in AD; translation from animal models to human disease remains unproven. The necessity of subjective, mystical‑type experiences for durable therapeutic benefit is debated: some effects may be driven primarily by biology and occur without strong psychoactive effects, whereas mood and existential outcomes seen in psychiatric trials may depend substantially on acute subjective effects plus psychological support. Safety concerns are raised for advanced AD where psychosis, hallucinations or severe behavioural disturbance may be exacerbated. The authors call for cautious, hypothesis‑driven clinical trials incorporating neuroimaging and biomarker endpoints (e.g., SV2A PET, Aβ/tau measures, cytokines, BDNF) to test both symptomatic and potential disease‑modifying outcomes. Overall, the discussion frames classic psychedelics as a promising but still largely preclinical avenue for AD therapeutics that warrants carefully designed translational and early‑phase clinical research, with treatment modality tailored to the clinical target and stage of disease.

The authors conclude that classic psychedelics with psychoplastogenic properties merit further investigation as potential treatments for early‑stage AD or MCI. They emphasise three converging biological pathways—5‑HT2A receptor‑driven upregulation of neocortical BDNF, amplification of AMPA receptor activity leading to mTOR activation, and increased Arc protein expression—that could underlie rapid and lasting increases in neural plasticity. Such plasticity‑promoting actions might slow or reverse cortical atrophy in cognitive regions deficient in endogenous plasticity factors like BDNF, and thereby represent a novel disease‑modifying approach. In addition, the authors note psychedelics’ antidepressant and anxiolytic effects could address prevalent neuropsychiatric comorbidities in AD and improve quality of life. They recommend pursuing both low‑dose biological paradigms and high‑dose psychologically assisted approaches, tailoring choice to the clinical problem, and conducting rigorous translational and clinical research to resolve remaining questions about mechanisms, safety, and efficacy.