PROTACs: A Promising Approach for Neurodegenerative Diseases

Proteolysis Targeting Chimeras (PROTACs) are emerging as a promising approach for treating neurodegenerative diseases (NDDs). PROTACs leverage the cell's ubiquitin-proteasome system to precisely target and eliminate neurodegeneration-associated proteins and aggregates, addressing a core challenge in the treatment of neurodegenerative diseases. Their ability to circumvent drug resistance and their versatile design tailored for various targets position PROTACs as a groundbreaking advancement in NDD therapy.

These molecules have shown potential towards the treatment of Alzheimer disease (AD), Huntington’s disease (HD), and Parkinson’s disease (PD), which are characterized by the accumulation of aberrant proteins—such as β-amyloid and hyperphosphorylated tau in AD, α-synuclein in PD, and mutant huntingtin (mHtt) in HD, respectively. Various NDD-associated proteins that serve as targets for PROTACs are depicted in the figure below (Figure 1).

In recent years, several investigations have been conducted, aiming to develop the PROTACs which can effectively degrade neurodegeneration-associated aberrant proteins. Some of these studies are highlighted in the subsequent sections.

Tau

Tau protein plays a crucial role in stabilizing microtubules within neurons, but in neurodegenerative disorders like AD and frontotemporal dementia with parkinsonism-17 (FTDP-17), it undergoes hyperphosphorylation and aggregation. This pathological transformation disrupts neuronal function and contributes to neurodegeneration.

Recent advancements in PROTACs targeting tau include C004019, developed by Wang et al. in 2021. This PROTAC utilizes the Von Hippel–Lindau (VHL) E3 ligase to specifically target and degrade tau. In preclinical studies, C004019 has demonstrated significant efficacy in reducing tau levels in various cellular (HEK293 and SH-SY5Y cells) and improving cognitive function in animal models (htau-transgenic and 3xTg-AD mice). Another notable development is TH006, introduced by Chu et al. in 2016. This PROTAC uses a tau-recognition motif linked to VHL E3 ligase and has shown effectiveness in promoting tau degradation in tau-overexpressing Neuro-2a cells and 3xTg-AD mouse brains while reducing Aβ-induced neurotoxicity.

Further progress was made by Silva et al. in 2019 with QC-01-175, a PROTAC incorporating the tau PET tracer 18F-T807 linked to the cereblon (CRBN) E3 ligase. This PROTAC has demonstrated higher efficacy in degrading A152T and P301L mutant tau compared to wild-type tau, suggesting a more precise treatment strategy. Silva’s team continued their work in 2022 with the development of FMF-06-038 and FMF-06-049, second-generation PROTACs that show enhanced degradation of both total and phosphorylated tau in FTD neurons, with improved specificity and prolonged tau reduction.

In addition, Kargbo’s 2019 work introduced pyridoindole-based PROTACs. These utilize a pyridoindole motif connected to various E3 ligase ligands and have shown success in degrading phosphorylated tau in human tau-P301L and tau-A152T neurons. Importantly, these PROTACs have demonstrated the ability to cross the blood-brain barrier in mouse models. Arvinas Inc. has also advanced tau-targeting PROTACs, which have shown a reduction in insoluble tau aggregates in murine tauopathy models. These advancements in tau-targeting PROTACs represent a significant leap forward towards tackling tau-associated neurodegenerative diseases like Alzheimer disease (AD), frontotemporal dementia with parkinsonism-17 (FTDP-17), Pick disease (PiD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD).

α-Synuclein

α-Synuclein, a presynaptic protein with no secondary structure, is implicated in Parkinson’s disease (PD) and other neurodegenerative disorders due to its tendency to misfold and aggregate. In 2020, Crew’s group patented several α-synuclein degraders, achieving 30-65% degradation in HEK293 TREX α-synuclein A53T cells. Similarly, Kargbo’s patent detailed six small-molecule PROTACs targeting α-synuclein, with several achieving over 65% degradation. Novel PROTACs, including those based on the aggregation inhibitor sery384 have effectively reduced α-synuclein aggregates and associated toxicity.

GSK-3β

Glycogen synthase kinase 3β (GSK-3β) plays a critical role in neurodegenerative diseases due to its involvement in key cellular processes. In 2021, Sun’s team pioneered the first GSK-3β degrader, which achieved 44.2% degradation at 2.8 μM through the ubiquitin-proteasome system and provided protection against glutamate-induced cell death in HT-22 cells. This was followed by the development of small-molecule GSK-3β PROTACs, such as PG-21 by Jiang et al., which recruits GSK-3β to the CRBN E3 ligase, resulting in over 44% degradation and offering neuroprotection in HT-22 cells. Newer PROTACs combining GSK-3β inhibitors with pomalidomide for E3 ligase recruitment have demonstrated effective GSK-3β degradation and reduced neurotoxicity in SH-SY5Y cells, with minimal cytotoxicity at concentrations below 20 µM.

mHTT

Mutant huntingtin (mHTT) protein aggregation is central to pathology of Huntington's disease (HD). In 2017, Ishikawa’s group created small-molecule PROTACs by linking ubiquitin ligase ligands to mHTT probes. These compounds successfully reduced mHTT levels in mHTT-transfected HeLa cells and in HD patient-derived cells.

Recent PROTAC developments include those by Arvinas and Origami Therapeutics. Arvinas’s PROTACs selectively target mHTT, degrading it without affecting wild-type HTT, while Origami’s ORI-113 aims to eliminate protein aggregates from the body. A newer PROTAC, incorporating ubiquitin E3 ligases, MV1 and mHTT ligands, showed improved efficacy in degrading mHTT in fibroblasts from HD patients. Targeting mHTT with PROTACs presents a promising strategy for addressing Huntington’s disease pathology and may improve treatment outcomes.

LRRK2

Leucine-rich repeat kinase 2 (LRRK2), associated with Parkinson’s disease through mutations like G2019S, has also been targeted with PROTACs. Dömling's group developed PROTACs in 2020 that targeted LRRK2 with good cell permeability but limited efficacy. More recent advancements include XL01126, which achieves over 80% LRRK2 degradation in multiple cell lines, with DC50 values within 15–72 nM, Dmax values ranging from 82 to 90%, is orally bioavailable (F = 15%) and crosses the blood-brain barrier in mice. Arvinas has developed optimized LRRK2 PROTACs that ensure prolonged brain penetration and degradation, achieving up to 50% degradation for 2-3 days after administration. These developments highlight the potential of LRRK2 PROTACs as effective compounds for the treatment of Parkinson’s disease.

TDP-43

TAR DNA-binding protein (TDP-43), which is normally a nuclear protein involved in gene regulation, forms cytoplasmic aggregates in amyotrophic lateral sclerosis (ALS) and sporadic and familial frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions (FTLD-U), contributing to disease pathology. In 2023, Tseng et al. developed JMF4560, a PROTAC targeting C-terminal TDP-43 aggregates. This PROTAC, linking benzothiazole aniline to pomalidomide via a PEG4 linker, effectively reduced C-TDP-43 aggregates, and alleviated related toxicity in Neuro-2a cells and C. elegans, while selectively degrading C-TDP-43 and improving motility in C. elegans.

TRK Receptors

Tropomyosin receptor kinase (TRK) receptors (TRKA, TRKB, TRKC) are crucial for neuronal development and function, activating pathways like PI3K/AKT and RAF/MEK/ERK that affect cell proliferation and survival. In 2020, Chen et al. developed selective TRKA degraders by linking the pan-TRK inhibitor GNF-8625 to thalidomide, achieving potent degradation with DC50 values as low as 0.36 nM in cellular models. In 2019, Zhao et al. introduced the first TRKC PROTACs, including a pomalidomide-based degrader with significant TRKC degradation and new treatment possibilities for TRKC-overexpressing cancers.

Conclusion

In conclusion, PROTACs represent a groundbreaking approach to treating neurodegenerative diseases by selectively targeting and degrading pathological proteins that contribute to disease progression. Their ability to specifically address the plausible causes of NDDs such as Alzheimer, Parkinson’s, and Huntington’s provides a significant advantage over traditional therapies. However, the clinical trials of PROTACs are limited by challenges related to blood-brain barrier (BBB) permeability. To effectively reach their targets within the central nervous system, PROTACs must overcome the BBB, which is influenced by drug-like properties such as molecular weight, lipophilicity, and solubility-the "Rule of 5." Addressing these challenges is crucial for optimizing PROTAC design and function, ultimately enabling these novel therapeutics to revolutionize treatment strategies and offer new hope for patients suffering from neurodegenerative diseases.