AFFiRis: PD01A and PD03A

Mandler and colleagues [85] were the first to test active immunization paradigms on PD animal models; these studies served as the foundation for developing the AFFITOPE (AFF) vaccines. AFFiRis completed phase I clinical trials for the two peptide vaccines PD01A and PD03A, or AFF. AFFiRis’ AFF vaccines implement short peptide fragments that mimic the C-terminus residues of α-Syn (110–130) [46, 85]. AFF peptide produced high titers of antibodies against aggregated forms of α-Syn that were detectable in CSF, plasma, and brain parenchyma using two different tg mouse models overexpressing α-Syn under the PDGF promoter and the thymus cell antigen 1 (Thy1) promoter [85].

AFF^® PD01A on murine Thy1 (mThy1)-α-Syn tg mice reduces α-Syn accumulation and protects against tyrosine hydroxylase (TH) fiber loss and motor impairments (Table 1) [85]. AFF^® PD01A immunization reversed motor dysfunction, impaired spatial memory, and reduced striatal degeneration and α-Syn deposition in PDGF-α-Syn tg mice (Table 1) [85]. AFF^® PD01A on the MSA mouse model demonstrated comparable protection against α-Syn aggregation and TH fiber loss, reduced demyelination, and motor protection [44]. These animal models demonstrated that these peptides improved motor impairment and cognitive function without affecting physiologic α-Syn levels [44, 85]. Although AFFiRis is the only active peptidic vaccine that has reached the clinical development stage, multiple other peptidic vaccines have demonstrated preclinical efficacy in vivo.

The initial vaccination study targeting recombinant α-Syn was conducted in PDGF, which demonstrated significant preservation of synapses attributed to the affinity of the antibodies for membrane-bound aggre species [91]. DNA-based vaccinations offer the potential for a more robust impact on overall proteinopathy by targeting naive protein substrates before aggre recruitment. However, it is important to consider the lack of selectivity for pathologic α-Syn conformers and the potential risk of excessive reduction of functional monomeric α-Syn when developing nucleic acid-based active immunization strategies [85, 90, 92]. In another study, the therapeutic efficacy of PD03, a novel AFF immunotherapy strategy, alone or in combination with Anle138b, was investigated using a proteolipid protein (PLP) α-Syn mice model (Table 1) [46]. The combined therapy of AFF^® PD03 and Anle138b showed improvements in gait deficits and preservation of nigral dopaminergic neurons, accompanied by reduced levels of α-Syn oligos, decreased microglial activation, and lower immunoglobulin G (IgG) binding, resulting in attenuation of neurodegeneration (Table 1) [46]. Both approaches hold promise as potential disease-modifying therapies for α-synucleinopathies [44, 46, 62, 85, 91, 92].

Various phase I clinical trials of AFF^® PD01A/PD03A in patients with PD and MSA have been conducted based on preclinical observations (Table 1). The results of clinical trials were obtained from the clinicaltrials.gov database (Table 2). Two phase I trials evaluated the safety and tolerability of the vaccine in patients with MSA or PD [93]. In the initial clinical trials of AFF^®, a randomized parallel-group phase I study enrolled 32 early PD patients [94]. The dosing regimens tested included a single dose or four doses of 15 μg or 75 μg administered every 4 weeks (Table 2) [62]. Both doses were well tolerated, and no major adverse events related to the drug were observed [62, 93, 94]. Subsequently, the participants continued in the AFF^® 12-month phase Ib follow-up extension study, which included a booster immunization 36 weeks after the initial injection with six doses of AFF^® PD01A (Table 2) [93, 94]. All dosing regimens led to a significant increase in antibodies over time [62, 94]. AFF^® PD01A was well tolerated locally and systemically, with no severe adverse events or significant adverse responses reported. An immunological response was observed in 19 out of 22 vaccinated individuals (86%), with specific serum antibodies produced in 12 of the 19 responders (63%). Compared to the placebo, AFF^® PD01A reduced oligomeric α-Syn in the CSF by 51% and fibrillar α-Syn compared to its monomers [62]. Clinical PD ratings remained stable throughout the trial period, although additional data were not provided due to the lack of a double-blind design. In an open-label evaluation, PD01A patients demonstrated stable clinical ratings compared to the placebo group, leading to the design and initiation of phase II clinical studies (NCT01568099, NCT02618941) [63, 93, 94].

Another AFF^® antibody, AFF^® PD03A, was evaluated in a similar manner as PD01A vaccination in patients with early PD and MSA in a separate clinical trial (NCT02267434), with a 36-week duration and injection doses ranging from 15 μg to 75 μg [63, 94]. AFF^® PD03A induced a sustained IgG antibody response against the PD03 peptide and demonstrated a favorable safety and tolerability profile in PD patients [63]. Both PD03A dosages were tolerated locally and systemically and induced prolonged anti-αSyn titers, with a 58% responder rate [63]. Over 98 patients were treated with either AFF^® PD01A or PD03A and followed for up to 48 months (AFF PD01A) or 12 months (AFF PD03A) [62, 63, 94]. Both treatments elicited an antibody response, with PD01A showing a more significant response than PD03A. AFF PD01A demonstrated safety and tolerability in early-stage PD patients and appeared to reduce α-Syn levels in the blood. However, the larger phase II trial did not show a significant difference in disease progression between the vaccine and placebo groups [63, 93].

United neuroscience: UB-312

UB‐312 is a synthetic peptide vaccine developed to overcome immune tolerance and generate a humoral antibody response to target oligomeric and fibrillary α-Syn. As an active vaccination, UB-312 induces an immune response against the aggregated form of α-Syn [48, 64, 95]. UB-312 targets the 10-AA fragment of the protein’s C-terminus fused to a short peptide that stimulates T-helper cells, which is then mixed with adjuvant for intramuscular injection [42, 49]. The United Biomedical, Inc. and United Neuroscience Ltd. compounds UB-312 in PD were designed based on UBITh technology on the related AD vaccine UB-311 [41, 64, 96]. The so-called UBITh^® approach produced the United Biomedical, Inc. and United Neuroscience Ltd. drugs UB-312 for PD and MSA disorders [43, 97]. Vaccines developed using this technology are made up of heterologous T-helper cell epitopes linked to a short B-cell epitope as an antigenic target site either directly or indirectly via a heterologous spacer [42, 48, 49]. The synthetic T-helper cell determinant stimulates an immunological response that produces many antibodies directed against the target [9, 42, 48, 49]. In the first preclinical study, researchers used guinea pigs and a 12-AA precursor of UB-312-induced antibodies against the toxic form of α-Syn fibrils and oligos but not monomers [42, 49, 50].

Immunohistochemical staining was performed on human cases of PD, DLB, and MSA using guinea pigs immunized with the UB-312 vaccine and IgG fractions from the resulting immune sera (IgG-1, IgG-2, or IgG-3). The immunoreactivity results were compared to a commercially available α-Syn antibody from Novacastra (NOV), commonly used for diagnostic purposes [42, 49]. The vaccine-generated antibodies showed higher detection of α-Syn pathology compared to NOV, with IgG-3 exhibiting the highest levels of α-Syn recognition. These antibodies were able to detect α-Syn inclusions in the SN and basal ganglia of patients with PD, DLB, or MSA. Slot blot analysis confirmed the antibodies’ specificity for native oligos and fibrillar α-Syn [42, 49].

In a study involving young tg (Thy1SNCA/15) and wildtype (wt) mice, a 6-week immunization regimen followed by a 9-week observation period was conducted [48]. Behavioral assessment was performed before immunization and 15 weeks after the first dose. Injection of UB-312 in 10-week-old line 61 tg mice resulted in reduced α-Syn oligos in the brain, hippocampus, and striatum (Table 1) [50]. Vaccinated mice exhibited improved performance in motor tests, such as crossing a beam and hanging from a wire (Table 1) [48, 50]. Therefore, UB-312 was found to bind α-Syn oligos and fibrils in post-mortem brain tissue from PD, MSA, and DLB patients. However, the efficacy of UB-312 in animal models of these disorders was not assessed in preclinical research studies [42, 48–50].

The main goal of phase I clinical trial was to assess the safety, tolerability, and immunogenicity of UB-312 vaccination in healthy participants [64]. Fifty healthy participants were enrolled in a 44-week, randomized, placebo-controlled, double-blind study (NCT04075318). Participants in seven cohorts were assigned to three intramuscular UB312 or placebo injections (40–2,000 μg) [64]. Safety and tolerability were assessed by adverse events, clinical laboratory, vital signs, electrocardiograms (ECGs), and neurological and physical examinations [64]. Immunogenicity was assessed by measuring serum and CSF anti-α-Syn antibody concentrations [64]. This phase I study in healthy volunteers revealed that UB-312 was generally safe, well tolerated, with no concerning safety findings, and immunogenic, with a high responder rate, antibodies pass the BBB, and induced anti-β-Syn antibodies in serum and CSF of all participants receiving the 300/300/300 μg dose regimen (Table 2) [64]. The company has reported that all patients have completed the vaccination series, and full results are expected by mid-2023. Part A of the UB-312 investigation was a dosage escalation period in which the optimum dose was determined, followed by a safety and tolerability review period in part B. Based on clinicaltrials.gov, in December 2022, researchers at NYU Langone Health registered a phase IB experiment in which four participants with MSA and four with PD will get three priming and five booster injections of 300 g UB-312 over two years [95]. Approximately 6 months following the previous boost, serum and CSF levels of anti-α-Syn antibodies are measured. The project’s estimated start date is April 2023, and the estimated completion date is scheduled for April 2025 [95].

There was no discernible difference between the treatment and placebo groups while using AFF PD01A, and the reason behind this is still unresolved. The organization has not indicated whether it would continue with a more modified therapy. On the other hand, in the phase IA study, which examined safety and tolerability, the UB-312 demonstrated good results. The company plans to begin this year with phase IB, which will treat four people with MSA and four patients with PD. Although the precise future of these active therapies is unknown, it is crucial to be aware that a therapy candidate’s success or failure in clinical trials can be impacted by a number of variables, including effectiveness, safety, tolerability, and the particular endpoints being evaluated. The decision to discontinue or change the development of these candidates can be influenced by a variety of factors, including insufficient effectiveness, unforeseen safety concerns, or strategic decisions made by the sponsor companies.

Prothena-roche: PRX002

Masliah and colleagues [45, 51, 52] published the first study on passive α-Syn immunization in 2011. Several passive antibody therapies are being studied in clinical trials. Studies on prasinezumab (PRX002/RG7935), Lu AF82422, MEDI1341, ABBV-0805, and BIIB054/cinpanemab are now being studied in phase I–III clinical studies for passive immunization against PD and MSA [51, 86]. In the preclinical study, Masliah et al. [51] used a tg mouse model mThy1 (line 61) of PD overexpressing human wt α-Syn was injected with the mAb 9E4 (Table 1) [51]. The antibody binds to the C-terminus and prevents morphological changes that might lead to α-Syn oligomerization [51]. Over 6 months, 10 mg/kg of 9E4 was administered intravenously once a week (Table 1) [51]. There was a reduction in the buildup of calpain-cleaved α-Syn aggres in cortical and hippocampal neurons’ axons and synapses. Additionally, treated mice showed motor improvement on the pole and rotarod and improved cognitive impairment in the water maze when compared with IgG1-treated control animals [51]. In this study, 9E4 antibody was designed to target the C-terminus region of α-Syn since there is scientific evidence showing that α-Syn cleavage at the C-terminus is involved in the generation of toxic α-Syn oligos (Table 1) [51].

Prasinezumab (PRX002, RO7046015, RG7935, NEOD002), a humanized IgG1 mAb, targets aggregated α-Syn in the pathogenesis of PD and DLB. Following the promising result of the PRX002 mouse model with 9E4 antibody. Prothena began a single-center study in 40 healthy adults to compare the safety and pharmacokinetics (PK) of a single infusion of 0.3, 1, 3, 10, or 30 mg/kg to placebo [51, 52]. In March 2014, a phase I single ascending-dose (SAD) study of prasinezumab demonstrated that the antibody was safe, well-tolerated, and dose-dependently reduced the concentration of free α-Syn in the blood down to 4 percent with a maximal effect at a dose of 30 mg/kg (Table 2; NCT02157714) [65, 98].

Following the success of PRX002 in phase II clinical trials, Roche launched PASADENA. A two-year phase II trial evaluating the effectiveness of 1,500 mg or 4,500 mg prasinezumab infusions versus placebo was done in early PD patients (~316) [66]. The primary outcomes of this study included a decrease in the active arm’s part III motor scale Movement Disorder Society-Unified PD Rating Scale (MDS-UPDRS) on the second endpoint [66]. However, the most recent report on the 52-week phase II study found that therapy with PRX002/prasinezumab had no meaningful influence on key end measures compared to placebo expected to be completed in 2026 (NCT03100149) [66]. In May 2021, scientists from Roche began a phase IIB trial (PADOVA) in patients with more advanced PD symptoms than in PASADA (NCT04777331; Table 2). In the PADOVA study, 575 participants taking stable DA agonists received monthly injections of prasinezumab or a placebo for 18 months [67]. The primary outcome was significant advancement on the part III (MDS-UPDRS), defined as a drop of more than 5 points. The study, initiated in late 2022, underwent modifications to align with a confirmed motor advancement event [67]. It encompasses various secondary outcomes, including assessments of motor function, clinical changes, adverse events, PK, and anti-drug antibodies (ADAs). Spanning across 120 locations in North America and Europe, the trial is projected to extend until 2024 [67].

Biogen: BIIB054

Biogen’s mAb, BIIB054 (cinpanemab), binds to N-terminus (residues 1–10) of α-Syn without cross-reactivity to β- or γ-Syn [53, 54]. It is a recombinant human IgG1 antibody derived from the memory B cells of older adults without neurological disorders [54]. Previously, the same technological platform was utilized to create aducanumab, an antibody targeting aggres studied in AD therapy [54]. BIIB054 has an 800-fold greater affinity for fibrillar α-Syn than monomers [54]. Rats and cynomolgus monkeys were given a dose of 10 mg/kg to assess the PK properties (Table 1) [53]. It was shown that antibodies reached the CNS according to dose, peaking between 24 h and 72 h after injection [53]. In the study, wt, tg, and bacterial artificial chromosome (BAC) A53T mice were given injections of PFFs (2 L) [54]. BIIB054 (30 mg/kg) resulted in a 30% reduction of α-Syn spreading, a 20% reduction in the loss of DA transporter (DAT) density in dopaminergic terminals, and a reduced behavioral impairment in the wire hanging test by 50% (Table 1) [54]. Treatment was found to postpone the development of paralysis by 7 days (Table 1) [54]. Preclinical findings provided a strong argument for the clinical development of BIIB054 for the treatment and prevention of PD [54]. Targeting α-Syn by passive vaccination seems to be a promising disease-modifying treatment in animal PD models (Table 1). As a result, this method has been applied in human clinical studies.

A randomized, double-blind, placebo-controlled study tested the safety and PK of BIIB054/cinpanemab (NCT02459886) [68]. Initial cohort of BIIB054 was tested in healthy and mild to moderate PD participants using a single dosage of BIIB054 (15 mg/kg or 45 mg/kg) or a placebo [68]. For 16 weeks, participants underwent clinical, neuroimaging, ECG, and laboratory evaluations [68]. Serum and CSF ratios of BIIB054 concentrations were measured. It was similar to the result obtained in the preclinical studies (0.4%), with a favorable safety profile and the ability to pass the BBB [68]. According to Biogen, cinpanemab is more selective for aggregated than soluble α-Syn but binds soluble protein at high dosages. Overall, BIIB054 was found to be well tolerated, with no significant adverse effects reported (Table 2) [68]. Biogen started SPARK (NCT03318523) to examine the safety and PK of BIIB054/cinpanemab, a two-year phase II multicenter, randomized, double-blind, placebo-controlled trial [69]. Over two years, PD patients were given monthly doses of BIIB054 ranging from 250 mg to 3,500 mg and placebo [99]. The primary objectives of the SPARK research were changes and improvements in the MDS-UPDRS total score and DaT-SPECT changes from baseline to week 52 [69]. However, neither the primary nor secondary endpoints were met, and the study was therefore discontinued from further development [69]. Since BIIB054/cinpanemab binds to the N-terminus and prasinezumab binds to the C-terminus of α-Syn, cinpanemanb results possess less favorable outcomes than prasinezumab.

The clinical trials followed substantial preclinical research in rodent models employing cinpanemab and prasinezumab, which revealed significant reductions in PD-like symptoms. Both studies enrolled patients in the early stages of PD, recognizing the limited potential for reversing downstream pathological processes in late-stage PD, even with the stoppage of basic pathogenic mechanisms [67–69]. In conclusion, while the studies involving α-Syn mAbs (cinpanemab and prasinezumab) did not show efficacy in early PD, the findings highlight the need to reconsider the potential limitations of preclinical studies and clinical trials, particularly the translation from experimental models to endpoint evaluations [67–69]. Incorporating developing technology may aid in overcoming the problems of α-Syn-based immunotherapy. Despite the detrimental results, it would be premature to terminate the investigation of these therapeutic techniques.

AstraZeneca: MEDI1341

MEDI1341 is an α-Syn mAb distinguished by its high affinity and specificity [55]. MEDI1341 possesses therapeutic properties by binding to the C-terminus of monomeric and aggregated α-Syn, most likely in the epitope at AA 103–129 [55]. In in vitro model (SH-SY5Y cells), the antibody inhibited cell-to-cell transmission and spreading of human α-Syn PFF [55]. Preclinical testing of MEDI1341 demonstrated rapid infiltration in the brain and reduced free α-Syn unblocked α-Syn propagation [55]. Intravenous administration of MEDI1341 to rats at 100 mg/kg showed a decrease in CSF α-Syn levels by 81.5% at peak, with the drop remaining at 45% after 4 days, and decreased brain interstitial fluid (ISF) levels by 75% after intravenous administration (Table 1) [55]. In a series of target engagement tests, MEDI1341 was examined for its ability to enter and capture extracellular α-Syn in the CNS [55]. MEDI1341 was also administered intravenously to cynomolgus monkeys and resulted in measurable amounts in the CNS, although at 0.2% of serum levels, up to 88% less free α-Syn was present in the CSF (Table 1) [55]. The combination of characteristics and properties of MEDI1341 mentioned above are novel and different from other α-Syn antibodies currently in human clinical trials. AstraZeneca’s MEDI1341/TAK-341 is currently in early clinical development. Recently, two phase I clinical research is conducted to examine the safety, tolerability, and PK properties of MEDI1341 and its capacity to sequester α-Syn in the CNS [55].

In the study, SAD was administrated in healthy volunteers and multiple ascending-dose in patients with PD (NCT04449484, NCT03272165; Table 2) [55]. The aim of phase I study was to evaluate safety and tolerability, PK, and pharmacodynamics of SAD of MEDI1341. In October 2017, AstraZeneca initiated phase I with 36–48 healthy volunteers (Table 2). In August 2019, AstraZeneca changed qualifying criteria to exclude participants with severe eye disease and added vision and eye health assessment to the critical objectives. From August 2020 to January 2022, AstraZeneca conducted a second phase I study in patients with PD, evaluating several ascending dosages. Takeda took over development in November 2022, with a phase II study in 138 patients with MSA. After one year, the study outcome is a change in the Unified MSA Rating Scale part 1. Secondary results include PK, other clinical indicators, survival time, adverse events, and ADAs. No data disclosed has been released.

At the time of this review, no data from the study had been released. These data imply that MEDI1341 offers promise as a potential therapeutic option for α-Syn-related illnesses, and further clinical research will provide more information about its safety, effectiveness, and disease-modifying effects.

AbbVie: ABBV-0805

AN0805/ABBV-0805, also known as BAN0805, is a humanized mAb targeting α-Syn oligos and protofibrils at the C-terminus [58]. Preclinical investigations studied the effects of ABBV-0805 on two mouse models of PD, one expressing human α-Syn and the other expressing A53T mutant human α-Syn (Table 1) [58]. Mice were treated with ABBV-0805 at a range of 0.1, 1, 10, and 20 mg/kg or control for 2–4 months; treatment was assessed by measuring α-Syn levels, motor function, and survival (Table 1) [58]. ABBV-0805 decreased the level of α-Syn fibrils, prevented motor impairment, and extended longevity; the antibody generally protected against α-Syn toxicity [58]. These findings suggest that ABBV-0805, a novel antibody selective for soluble aggregated α-Syn, has therapeutic potential for PD [58]. Further research is needed to determine the long-term effects and safety of ABBV-0805 in humans [57, 58]. Lindström and colleagues [56] created a novel α-Syn protofibril-specific mAb47 similar to ABBV-0805.

The antibody mAb47 was evaluated in 14-month-old A30P tg mutant mice under the Thy1 promoter, resulting in the formation of α-Syn fibrils (Table 1) [56]. Mice were treated for 14 weeks with a weekly intraperitoneal injection. mAb47 decreased soluble α-Syn protofibril levels in the spinal cord without changing the level of monomeric forms and decreased motor dysfunction compared with untreated mice [56, 57]. Although levels of α-Syn protofibril, monomeric, and fibrillar were decreased, there was no difference in levels across groups in the brain and spinal cord tissue [56, 57]. In a similar study, Thy1 α-Syn mice were weekly immunized with specific antibodies (Syn-O1, Syn-O4, and Syn-F1) that exclusively recognized oligos and fibrils but not monomeric α-Syn to prevent accumulation and synaptic alterations, resulting in reduced astrogliosis and microglia activation in the hippocampus [100]. In 2019 AbbVie, in partnership with BioArtic, began a phase I trial of ABBV-0805 in healthy volunteers (Table 2). Phase I data from AbbVie on PK excellent tolerance and safety supported moving forward to phase II. During placebo-controlled phase IB with 32 idiopathic, mild to moderate PD patients, ABBV-0805 was withdrawn by the firm in July 2020, citing strategic considerations. At the time of this review, no results from the trial have been published yet. It was planned to randomly assign them to receive three increasing doses of the antibody, or a placebo administered intravenously every 28 days, with a fourth dosage added pending preliminary findings (Table 2; NCT04127695). Although ABBV-0805 showed promising preclinical results, further clinical investigations and trials are necessary to fully understand its efficacy, safety, and potential as a therapeutic option for PD.

H. Lundbeck: Lu AF82422

The humanized mAb IgG1 Lu AF82422 targets the C and N-terminus of all forms of α-Syn [59]. Blood cells present α-Syn, yet most human blood cell types do not bind to the antibody [58]. A nonclinical study was completed in rats and cynomolgus monkeys at dosages up to 600 mg/kg delivered intravenously every 10 days to support a first-in-human (FIH) SAD trial with Lu AF82422 (Table 1) [59]. No-observed-adverse-effect met the criteria at the highest dosage tested, and there were no treatment-related side effects. A single dose showed PK parameters in the predicted range for mAb with a soluble target and target engagement in plasma and CSF [59]. A preclinical study showed enough safety margins between documented safe doses in animals and the intended dosage levels in the FIH SAD study [59]. The study concluded that Lu AF82422 represents a promising therapeutic approach for synucleinopathies and further clinical investigation [59]. In July 2018, Lundbeck initiated a phase I SAD trial with Lu AF82422 in healthy non-Japanese and Japanese subjects and patients with PD (NCT03611569; Table 2). Participants were given a single dose of 50 mg/mL or a placebo by infusion, followed by a 12-week monitoring period. Although no findings were provided, the manufacturer classified this antibody as well tolerated. Lundbeck announced the AMULET trial, which will confirm the efficacy of Lu AF82422 as a therapy to slow the rate of disease development in 64 patients with MSA. Lu AF82422 will be administered intravenously once every four weeks for a maximum of 72 weeks (NCT05104476). Finally, the humanized mAb Lu AF82422 targets multiple forms of α-Syn and has demonstrated encouraging preclinical outcomes. Nonclinical trials revealed safety and efficacy, paving the way for additional clinical research. A phase I experiment with Lu AF82422 in healthy persons and PD patients showed satisfactory tolerability. The AMULET trial appears to be the next step in evaluating the efficacy of Lu AF82422 in reducing disease progression in patients with MSA.

Numerous preclinical investigations have revealed passive immunization mechanisms with significant results that have yet to be tested in clinical trials. PDGFb, a tg mouse, was used to test the C-terminus antibody (Ab274; Table 1) [47]. The passively immunized treated group improved behavioral impairments and neurodegeneration [47]. After 4 weeks of therapy, Ab274 successfully reduce aggregated α-Syn transmission (Table 1) [47]. Similarly, preincubating recombinant α-Syn with 1H7 and 5C1 reduced C-terminus cleavage in the mThy1 α-Syn tg mice model [45]. mAbs 1H7, 5C1, and 5D12 were also particularly effective in reducing C-terminus-truncated α-Syn and higher-molecular-weight aggre (Table 1) [45, 52, 91]. The effect of passive immunization against aggregated α-Syn was studied in a mouse model of axonal transit and accumulation of α-Syn. Non-tg, α-Syn knockout, and mThy1 α-Syn tg mice were given unilateral intracerebral injections of a lentiviral (LV)-α-Syn vector construct, followed by systemic treatment with the mAb 1H7 (which recognizes AA 91–99) or control IgG, for 3 months (Table 1) [45, 52, 91]. 1H7 antibody decreased axonal accumulation and improved behavioral abnormalities, suggesting immunotherapy may help synucleinopathy models with behavioral impairment [52]. Overall, Games et al. [45], Masliah et al. [51], and Spencer et al. [52] discovered antibody variants, such as 1H7 and 5C1, specifically targeting the C-terminus of α-Syn, blocking calpain-mediated cleavage and hindered the spread of aggre α-Syn species from cell-to-cell [45, 51, 52].

Anti-α-Syn antibodies that are N-terminally directed have also been created and tested in a PD rat model. The model was created by injecting a recombinant adeno-associated viral (rAAV) vector expressing human wt α-Syn under the control of the chicken β-actin (CBA) promoter into the skin (Table 1) [60]. Control IgG or antibodies against the N-terminus (AB1) or central region (AB2) of α-Syn were administrated intraperitoneally to rats every 2 weeks for 3 months. AB1 and AB2 resulted in a substantial but not statistically significant improvement in behavioral measures and strong protection against the death of nigral dopaminergic neurons [60]. Overall, α-Syn expression and microglial activity in the nigral area were reduced (Table 1) [60]. Syn303 and Syn211, two N-terminus antibodies, were designed to target misfolded α-Syn in a mouse model after being injected with PFFs α-Syn (Table 1) [61]. Weekly intraperitoneal treatment for up to 26 weeks prevented the spread of α-Syn aggres and alleviated behavioral impairments and dopaminergic cell loss (Table 1) [61]. These mAbs could bind to aggregated α-Syn at the N-terminus to promote clearance of extracellular α-Syn and mitigate its pathological consequences (Table 1) [60, 61]. Additional research on PI for synucleinopathies is being conducted, and phase I clinical studies assessing safety and acceptability have been completed. Phase I study on people living with PD and healthy males began in December 2020 by UCB Biopharma/Novartis (Table 2). The trial will enlist 64 patients in the UK and evaluate the safety, tolerability, and PK of single- and multiple-ascending doses of UCB7853 infusion. The trial’s completion date was moved from September 2022 to April 2023 after UCB suspended enrollment in June 2021 (NCT04651153). Early clinical trials suggest that immunotherapy is often well tolerated. The CSF contains much fewer mAbs than the plasma, which may limit their efficacy (Table 2). Overall, PI targeting α-Syn, such as the one stated above, have shown promise in preclinical and early clinical research. More research and clinical trials are required to determine their long-term efficacy and safety in the treatment of synucleinopathies including PD and MSA.