Psilocybin (a classic serotonergic psychedelic drug) has received appraisal for use in psychedelic-assisted therapy of several psychiatric disorders. A less explored topic concerns the use of repeated low doses of psychedelics, at a dose that is well below the psychedelic dose used in psychedelic-assisted therapy and often referred to as microdosing. Psilocybin microdose users frequently report increases in mental health, yet such reports are often highly biased and vulnerable to placebo effects. Here we establish and validate a psilocybin microdose-like regimen in rats with repeated low doses of psilocybin administration at a dose derived from occupancy at rat brain 5-HT_2A receptors in vivo. The rats tolerated the repeated low doses of psilocybin well and did not manifest signs of anhedonia, anxiety, or altered locomotor activity. There were no deficits in pre-pulse inhibition of the startle reflex, nor did the treatment downregulate or desensitize the 5-HT_2A receptors. However, the repeated low doses of psilocybin imparted resilience against the stress of multiple subcutaneous injections, and reduced the frequency of self-grooming, a proxy for human compulsive actions, while also increasing 5-HT₇ receptor expression and synaptic density in the paraventricular nucleus of the thalamus. These results establish a well-validated regimen for further experiments probing the effects of repeated low doses of psilocybin. Results further substantiate anecdotal reports of the benefits of psilocybin microdosing as a therapeutic intervention, while pointing to a possible physiological mechanism.

The protein alpha-synuclein (αSYN) plays a central role in synucleinopathies such as Parkinsons's disease (PD) and multiple system atrophy (MSA). Presently, there are no selective αSYN positron emission tomography (PET) radioligands that do not also show affinity to amyloid-beta (Aβ). We have previously shown that radiolabeled antibodies, engineered to enter the brain via the transferrin receptor (TfR), is a promising approach for PET imaging of intrabrain targets. In this study, we used this strategy to visualize αSYN in the living mouse brain. Five bispecific antibodies, binding to both the murine TfR and αSYN were generated and radiolabeled with iodine-125 or iodine-124. All bispecific antibodies bound to αSYN and mTfR before and after radiolabelling in an ELISA assay, and bound to brain sections prepared from αSYN overexpressing mice as well as human PD- and MSA subjects, but not control tissues in autoradiography. Brain concentrations of the bispecific antibodies were between 26 and 63 times higher than the unmodified IgG format 2 h post-injection, corresponding to about 1.5% of the injected dose per gram brain tissue. Additionally, intrastriatal αSYN fibrils were visualized with PET in an αSYN deposition mouse model with one of the bispecific antibodies, [¹²⁴I]RmAbSynO2-scFv8D3. However, PET images acquired in αSYN transgenic mice with verified brain pathology injected with [¹²⁴I]RmAbSynO2-scFv8D3 and [¹²⁴I]RmAb48-scFv8D3 showed no increase in antibody retention compared to WT mice. Despite successful imaging of deposited extracellular αSYN using a brain-penetrating antibody-based radioligand with no cross-specificity towards Aβ, this proof-of-concept study demonstrates challenges in imaging intracellular αSYN inclusions present in synucleinopathies.

Immunotherapy targeting aggregated alpha-synuclein (αSYN) is a promising approach for the treatment of Parkinson’s disease. However, brain penetration of antibodies is hampered by their large size. Here, RmAbSynO2-scFv8D3, a modified bispecific antibody that targets aggregated αSYN and binds to the transferrin receptor for facilitated brain uptake, was investigated to treat αSYN pathology in transgenic mice. Ex vivo analyses of the blood and brain distribution of RmAbSynO2-scFv8D3 and the unmodified variant RmAbSynO2, as well as in vivo analyses with microdialysis and PET, confirmed fast and efficient brain uptake of the bispecific format. In addition, intravenous administration was shown to be superior to intraperitoneal injections in terms of brain uptake and distribution. Next, aged female αSYN transgenic mice (L61) were administered either RmAbSynO2-scFv8D3, RmAbSynO2, or PBS intravenously three times over five days. Levels of TBS-T soluble aggregated αSYN in the brain following treatment with RmAbSynO2-scFv8D3 were decreased in the cortex and midbrain compared to RmAbSynO2 or PBS controls. Taken together, our results indicate that facilitated brain uptake of αSYN antibodies can improve treatment of αSYN pathology.

Incretin mimetics, especially dual/triple agonists, are effective for type 2 diabetes and obesity, though mechanisms remain unclear. This study applied PET using [⁶⁸Ga]Ga-DO3A-Exendin-4 and [⁶⁸Ga]S02-GIP-T4 to assess GLP-1R and GIPR occupancy of SAR441255 (a GLP-1R, GIPR, and GCGR agonist) and tirzepatide in pig pancreas and CNS. In vitro binding assays on frozen HEK293 cell sections overexpressing human GLP-1R, GIPR, or GCGR assessed [⁶⁸Ga]Ga-DO3A-Exendin-4 specificity and competition with tirzepatide and SAR441255, and [⁶⁸Ga]S02-GIP-T4 with SAR441255. In vivo, the GLP-1R occupancy by SAR441255 and tirzepatide, and GIPR occupancy by SAR441255, was evaluated in healthy pigs using PET/CT initiated at tracer injection. A subcutaneous dose of the study drug was then administered, and a second scan was performed 2.5 h later. Occupancy was determined by comparing pancreatic and CNS tracer SUV before and after dosing. Two animals were used to compare the tracers directly. [⁶⁸Ga]Ga-DO3A-Exendin-4 and [⁶⁸Ga]S02-GIP-T4 showed high specificity for GLP-1R and GIPR, respectively, and competed with the study drugs in vitro. In vivo, SAR441255 induced dose-dependent GLP-1R occupancy (>70%) in pancreas and pituitary and (>60%) in CNS, while tirzepatide showed lower occupancy. SAR441255 also reduced pancreatic [⁶⁸Ga]S02-GIP-T4 uptake by 23 ± 8.5%, indicating GIPR engagement. PET imaging in pigs demonstrated in vivo GLP-1R engagement by SAR441255 and tirzepatide, and GIPR engagement by SAR441255 in the pancreas. SAR441255 exhibited dose-dependent GLP-1R occupancy in the pancreas and brain regions linked to appetite regulation. The study was funded by Uppsala Diabetes Center, Diabetesfonden, ExoDiab, Diabetes Wellness Sweden, Barndiabetesfonden, Science for Life Laboratory, and the Swedish Research Council.

Background: In preclinical studies, the positron emission tomography (PET) imaging with [¹¹C]UCB-A provided promising results for imaging synaptic vesicle protein 2A (SV2A) as a proxy for synaptic density. This paper reports the first-in-human [¹¹C]UCB-A PET study to characterise its kinetics in healthy subjects and further evaluate SV2A-specific binding. Results: Twelve healthy subjects underwent 90-min baseline [¹¹C]UCB-A scans with PET/MRI, with two subjects participating in an additional blocking scan with the same scanning procedure after a single dose of levetiracetam (1500 mg). Our results indicated abundant [¹¹C]UCB-A brain uptake across all cortical regions, with slow elimination. Kinetic modelling of [¹¹C]UCB-A PET using various compartment models suggested that the irreversible two-tissue compartment model best describes the kinetics of the radioactive tracer. Accordingly, the Patlak graphical analysis was used to simplify the analysis. The estimated SV2A occupancy determined by the Lassen plot was around 66%. Significant specific binding at baseline and comparable binding reduction as grey matter precludes the use of centrum semiovale as reference tissue. Conclusions: [¹¹C]UCB-A PET imaging enables quantifying SV2A in vivo. However, its slow kinetics require a long scan duration, which is impractical with the short half-life of carbon-11. Consequently, the slow kinetics and complicated quantification methods may restrict its use in humans.