At this year’s AD/PD meeting, held March 5-9 in Lisbon, Portugal, no splashy Phase 3, or even Phase 2, data on tau-targeted therapies wowed attendees. Still, Phase 1 and preclinical data showcased a variety of therapeutic approaches the field has latched onto to find treatments for tauopathies. Biogen reported that BIIB113, a small-molecule O-GlcNAcase inhibitor designed to prevent tau from transforming into a pathological form bound to its target in the brain and was safe in healthy volunteers. Similarly, Eli Lilly reported promising Phase 1 findings from its OGA inhibitor, which is being evaluated in a fully enrolled Phase 2 trial in people with AD. On the vaccine front, AC Immune’s phospho-tau vaccine seems to spawn antibodies that thwart tau seeding. Meanwhile, preclinical data on nanobodies and anti-oligomer antibodies encased in slippery micelles hinted that engagement of tau inside of cells might be possible.

  • Two OGA inhibitors safely engage their targets in early phase trials.
  • Antibodies raised by a phospho-tau vaccine stop tau seeding in cells.
  • In mice, nano- and micelle-coated antibodies vanquish intracellular tau.

This new crop of tau-targeted therapies speaks to a shift away from the N-terminally trained antibodies that failed in previous trials, toward approaches that seek to ferret out other, possibly more pathogenic, reservoirs of tau. This includes tau traveling between cells in extracellular vesicles or within tunneling nanotubes, as well as tau pathology brewing within the cells, noted Luc Buée of the University of Lille in France.

In a forum on tau targeted therapies in Lisbon, Bradley Hyman of Massachusetts General Hospital in Boston said that when it comes to figuring out which species of tau to take down and how to do it, the field is still in an exploratory phase. The kinetics of tau release, uptake, and processing are still largely uncertain, he said. “Each of these kinetic steps could be explored, but we have to make a best guess. Sometimes that means just doing the experiment.”

Sugarcoating It
A glycoside hydrolase, O-GlcNAcase, aka OGA, removes N-acetylglucosamine moieties post-translationally bound to the hydroxyl groups of serine and threonine residues. Stripped of these sugars, tau is more likely to form filaments, hence some believe that by bolstering tau glycosylation, OGA inhibitors will prevent neurofibrillary tangles (Liu et al., 2004; Mar 2012 news).

Sugar-Coating Tau. OGT adds N-acetylglucosamine sugars (green) to unbound tau, i.e., free in solution and not attached to microtubules. OGA, on the other hand, removes said sugars. Without them, tau is likelier to become phosphorylated and misfold, heading down the path toward tangles. [Courtesy of Dustin Mergott, Eli Lilly.]

Flavia Nery from Biogen presented the first in-human data on that company’s OGA inhibitor, BIIB113. The compound has been tested for safety, and for target occupancy using a PET tracer for OGA activity that Biogen developed in-house (Cook et al., 2023). A drop in PET signal indicates that the OGA active site is occupied.

In a single-ascending-dose trial, 35 healthy volunteers aged 18-64 who took between 0.5 mg and 50 mg BIIB113 once orally, were monitored. Subsequently, 27 participants, up to age 75, took placebo, 15 mg, or 50 mg BIIB113 daily for 14 days. Results of both the single- and multiple-ascending-dose regimens indicated only mild to moderate adverse events, mostly deemed unrelated to the drug. Headache was the most common complaint among people in the treatment arms, and one person who received the 50 mg dose during the multidose regimen withdrew due to tremor. There were no other serious adverse events.

For target analysis, 10 healthy adults underwent OGA PET scans before and after receiving the drug. Nery reported that 48 hours after receiving 3 mg BIIB113, the PET signal in the brain had dropped by 90 percent, suggesting the drug had broadly engaged the enzyme. In a multidose study, 0.5 mg daily maintained this target occupancy over the two-week interval, Nery reported. Based on these Phase 1 findings, Biogen is planning a Phase 2 trial.

Attendees in Lisbon asked how OGA target occupancy might translate into reduction of tau pathology. Nery said that prior studies in mice indicated a target occupancy of 85 percent would be needed, meaning BIIB113 would pass muster at the 3 mg dose. Seiko Ikezu of the Mayo Clinic in Jacksonville, Florida, wondered about potential safety concerns of inhibiting OGA this much, given the enzyme’s role in stripping sugars from proteins across the body. Nery said that Biogen’s initial dosing studies, as well as the broader body of trials that tested other OGA inhibitors, have generally found this class of drugs to be safe. Participants will be closely monitored for any side effects of longer treatment in the upcoming Phase 2, she said.

Of the handful of OGA inhibitors in clinical development, Lilly’s LY3372689 is furthest along. At AD/PD, Lilly’s Dustin Mergott presented preclinical data and findings from Phase 1 studies of the compound in healthy volunteers. He reported that single oral doses ranging from 0.16 to 16 mg of the drug appeared safe, in that adverse events were mild, and did not relate to drug. Pharmacokinetics indicated a plasma half-life of six hours. To analyze target engagement, the scientists used an OGA PET tracer, [18F]LSN3316612, they had developed in collaboration with researchers at the National Institutes of Health (Lu et al., 2020; Shcherbinin et al., 2020). In Lisbon, Mergott reported that in four volunteers, the OGA PET tracer signal plummeted by 97 percent two hours after receiving 1 mg of LY3372689, and the signal was still down by 81 percent 22 hours later. In a subsequent multidose PET study, this target occupancy was sustained over a 14-day period. Finally, in a multiple-ascending-dose study, 40 healthy volunteers took placebo, 1 mg, 3 mg, or 7 mg LY3372689 daily for 14 days. Again, the drug appeared safe and well-tolerated at all doses tested, with only mild adverse events occurring with no relationship to dose. Leveraging this PET data with plasma pharmacokinetics, the investigators settled on 0.75mg LY3372689 as a low dose for Phase 2.

Called Prospect-ALZ, this trial began in 2021 to evaluate two doses of the inhibitor for 76-124 weeks in people with early AD. The trial uses a two-step screening process—plasma p-tau217 followed by tau-PET—to identify participants with tau pathology while minimizing the number of tau-PET scans. The first pass with p-tau217 paid off. Of the 2,177 people recruited for the trial, 1,850 failed screening, most because they had normal levels of plasma p-tau217. Mergott said that more than 1,000 tau-PET scans were avoided by adding this plasma screening step. Ultimately, 327 participants with moderate to high cortical tau pathology based on PET scans were randomized, and the fully enrolled trial expects to read out in late 2024. Change in the integrated Alzheimer’s Disease rating scale among people with moderate cortical tau accumulation serves as the primary endpoint, although secondary endpoints will assess disease progression in the full cohort, which also includes people with high tau tangle burden.

At least one other OGA inhibitor, Asceneuron’s ASN51, is moving toward Phase 2. The company reported favorable safety and target occupancy findings at the CTAD meeting in October 2023 and plans to start a Phase 2 trial this year.

Tau Immunotherapies Branch Out
Small molecule drugs like OGA inhibitors may be relatively cheap and easier to smuggle into the brain than antibodies, but what the latter lack in maneuverability, they gain in specificity. At AD/PD, scientists presented incremental findings on several. These approaches have moved beyond infusing full-size antibodies into the blood. Instead, researchers presented strategies to boost the chances of reaching the target in the brain, such as provoking an enduring immune response with an active vaccine, using small antibody fragments, or encasing full-size antibodies within slippery micelles to help them pass into cells.

AC Immune’s active vaccine, ACI-035.030, is the only one of these approaches in clinical trials. ACI-035.030 comprises liposomes with an antigenic phospho-tau peptide anchored to their lipid bilayer. Two adjuvants, as well as an antigen to rally T-helper cells, are also embedded. The package provokes a robust antibody response against phosphorylated tau (Dec 2022 conference news). The vaccine cleared safety hurdles and demonstrated immunogenicity against phospho-tau in a recently completed a Phase 1b/2a study. At AD/PD, AC Immune’s Marija Vukicevic said that a Phase 2b trial, led by partner Johnson & Johnson, is underway.

She presented no trial data, but detailed the seed-stopping capacity of antibodies raised by the vaccine in nonhuman primates. Using a cell culture model in which filamentous tau extracted from the human brain instigates the aggregation of endogenous tau in primary rat neurons, Vukicevic reported that sera from monkeys injected intramuscularly with ACI-035.030 effectively stopped tau propagation. The more times animals had been treated, the more antibodies they produced. Through a process called affinity maturation, whereby B cells produce antibodies with greater and greater affinity as an immune response matures, these antibodies also became more specific for p-tau antigens, and more adept at stopping seeds in their tracks with each vaccination, suggesting the specificity and functionality of the anti-p-tau antibodies had improved. ACI-035.030 was designed to target extracellular, seed competent tau.

Targeting Intracellular Tau
Until recently, Rakez Kayed of the University of Texas in Galveston had focused his efforts on going after extracellular tau as well. Specifically, he sought to dispatch soluble tau oligomers, which he sees as the primary agents of tau propagation and toxicity. As such, his group developed a suite of tau-oligomer-specific antibodies, aka, TOMAs. Previously, Kayed reported that while TOMAs block tau seeding and propagation in cell culture and animal models, the antibodies were not so good at removing established, intracellular tau pathology (Bittar et al., 2022; Castillo-Carranza et al., 2014). Notably, like most full-size antibodies, TOMAs do not efficiently get into cells.

In Lisbon, Kayed explained what happened when he shifted efforts toward targeting oligomers and other forms of tau inside cells. To do this would require both a different antibody and a new method of delivery. First, he went back to his antibody library and selected a likely candidate that they had previously dismissed. Called tau toxic conformation specific monoclonal-2 (TTCM2), this antibody was not 100 percent oligomer-specific like TOMAs. Instead, it latched onto oligomers, misfolded monomers, and small fibrils of tau. To get TTCM2 into cells, postdoc Sagar Gaikwad worked with scientists at InnoSense, Torrance, California, to package the antibodies within micelles. Made of a mix of polymers, these 100-nanometer-wide particles can slip into cells because their polymer coat melds with the cell membrane. Finally, to sneak past the blood-brain barrier, Gaikwad gave fluorescently labeled “TTCM2-ms” to mice intranasally. Three hours later, the fluorescent micelles had spread widely throughout the brain, inhabiting the olfactory bulb, hippocampus, cortex, cerebellum, and thalamus.

In 15-month-old hTau mice, a single sniff of these TTCM2-ms dramatically lowered existing levels of tau pathology, including aggregates of insoluble, hyperphosphorylated tau as measured by several different antibodies. The treatment also boosted flagging levels of synaptic proteins PSD95 and synaptophysin, and even revitalized memory. Relative to mice that sniffed micelles loaded with control antibodies, those that received TTCM-ms were able to better recognize novel objects, and remember which arms of a maze they’d explored before. Looking closer at synapses in the mouse brain with immunofluorescence, the researchers found that TTCM2-ms treatment reduced the amount of tau aggregates crowding synpases by a third, while doubling total synapse numbers. This is critical, Kayed said, because recent studies have implicated synaptic tau oligomers as a culprit in the spread of tau pathology (May 2023 news; Oct 2023 news).

Targeting Tau for Destruction. TTCM2-ms slip into cells, where the antibody latches onto various forms of pathological tau. TRIM21 binds the Fc fragment of the antibody, and ubiquitinates it, targeting the entire complex to the proteasome. [ Courtesy of Rakez Kayed, UTMB.]

How did TTCM2-ms find and destroy its intracellular targets? Gaikwad found that this depended on TRIM21, a cytosolic Fc receptor that also serves as an E3 ubiquitin ligase. It whisks antibodies that end up in the cell, and their cargo, to the proteasome for destruction. Broadly expressed in neurons and other cell types, this atypical Fc receptor may have evolved to deal with viruses that bust into the cytoplasm with antibodies clinging to their capsids (McEwan, 2016). Gaikwad found that in seeding assays in tau biosensor cell lines, knocking down TRIM21 thwarted the seed-stopping effects of TTCM-ms. In hTau mice treated intranasally with TTCM-ms, Gaikwad found TTCM2, TRIM21, and tau aggregates comingling within neurons. The findings jibe with a recent report that TRIM21 is required for the effectiveness of tau immunotherapies (Mukadam et al., 2023).

Kayed told Alzforum that his lab continues to investigate the mechanisms involved in the coordinated takedown of intracellular tau by TTCM2 and TRIM21. With an eye toward clinical development, TTCM2 has been fully humanized.

Other scientists with their sights on intracellular tau are taking a leaf out of the TRIM21 book. In Lisbon, Bengt Winblad of the Karolinska Institute, Stockholm, described how he developed proteolysis targeting chimeras, aka, PROTACs, for tau. First described more than 20 years ago, these engineered molecules comprise three connected parts: a ligand that binds to a target of interest, e.g., tau; a linker; and a ligand that binds an E3 ubiquitin ligase (Sakamoto et al., 2001). Once the PROTAC binds the target and the ligase, the latter adds ubiquitin, diverting the target to the proteasome for disposal. Winblad has generated a library of these molecules, pairing small molecules that latch onto paired helical filaments of tau, with others that ensnare an E3. He is currently testing out the top contenders in neuronal cell culture studies and mouse models of tauopathy.

Attack of the PROTACs. A PROTAC links a protein-binding domain with a ligand for E3 ubiquitin ligase. Once both the protein target and E3 are bound, E3 adds ubiquitin residues to the target protein, relegating the whole complex to proteasomal degradation (right). [Courtesy of Bengt Winblad, Karolinska Institute.]

Others are using the PROTAC method to promote the proteasomal degradation of tau-targeted single-domain antibodies, aka nanobodies. Produced naturally by camelids such as camels and llamas, these pared-down antibodies contain only a single variable heavy domain (VHH). Like full-size antibodies, they are exquisitely specific for their targets, however, they are small enough to easily slip across the blood-brain barrier and even enter cells via bulk endocytosis. Buée has generated a library of such nanobodies against tau. Previously, he reported that one of them, Z70,  recognizes the filament-driving, microtubule-binding region of tau, and that it thwarted tau aggregation intracellularly and vanquished tauopathy in a mouse model (May 2023 conference news). In Lisbon, Buée said that preliminary findings from his lab suggest that the efficiency of nanobodies can be bolstered significantly by rigging them up with PROTACs, in which the anti-tau VHH serves as the tau-nabbing portion of the PROTAC. Unlike the full-size antibodies that Kayed smuggles into cells with micelles, nanobodies lack the Fc domain that binds TRIM21, making the PROTAC approach critical to rev proteasomal degradation of the nanobody and its cargo.

Although he maintains that intracellular targeting is critical to stop the progression of tau pathology, Buée has also explored whether nanobodies might squelch tau inside of cells by preventing its uptake from the outside. While he found that Z70 blocked uptake, another nanobody, H3-2, did so even more efficiently. Upon binding to tau’s C-terminus, H3-2 forms a dimer, effectively preventing tau from being taken up into cells. He did not explain how the single-chain nanobody dimerizes, but he said other nanobodies do not do this. The findings raise the possibility of using combinations of nanobodies to interfere with different stages of tau propagation.

Einar Sigurdsson of New York University also uses the single-domain antibody approach to target both tau and α-synuclein pathologies (Congdon et al., 2022). At previous meetings, and more recently in preprint articles, he reported that PROTACs significantly enhanced the clearance of pathogenic targets in mouse models of tauopathy and synucleinopathy (May 2023 conference news; Jiang et al., 2024; Sigurdsson et al., 2024). In Lisbon, he reiterated that, and also reported on yet another approach, expressing nanobodies from viral vectors. In the A53T mouse model of synucleinopathy, intravenous injection of an adeno-associated virus vector carrying a gene for an α-synuclein-specific sdAb not only prevented synucleinopathy, but reversed it in older mice that had substantial Lewy body pathology.—Jessica Shugart


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News Citations

  1. Can a Little Sugar Keep Tau From Souring Neurons?
  2. Two New Stabs at Vaccinating People Against Pathologic Tau
  3. Abnormal Tau Slips into Synapses Long Before Tangles Form
  4. In Alzheimer’s, Tau Oligomers in Synapses Act as ‘Eat Me’ Signal
  5. New Arrows Aimed at Tau: Single-Domain Antibody, Peptibody, Vaccine

Therapeutics Citations

  1. BIIB113
  2. LY3372689
  3. ASN51
  4. ACI-35

Research Models Citations

  1. htau

Paper Citations

  1. . O-GlcNAcylation regulates phosphorylation of tau: a mechanism involved in Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Jul 20;101(29):10804-9. PubMed.
  2. . Development of a PET Tracer for OGA with Improved Kinetics in the Living Brain. J Nucl Med. 2023 Oct;64(10):1588-1593. Epub 2023 Jul 6 PubMed.
  3. . PET ligands [18F]LSN3316612 and [11C]LSN3316612 quantify O-linked-β-N-acetyl-glucosamine hydrolase in the brain. Sci Transl Med. 2020 May 13;12(543) PubMed.
  4. . Brain Target Occupancy of LY3372689, an Inhibitor of theO-GlcNAcase (OGA) Enzyme: Translation from rat to human. Alzheimer's & Dementia, 2020
  5. . Passive Immunotherapy Targeting Tau Oligomeric Strains Reverses Tauopathy Phenotypes in Aged Human-Tau Mice in a Mouse Model-Specific Manner. J Alzheimers Dis. 2022;90(3):1103-1122. PubMed.
  6. . Passive immunization with Tau oligomer monoclonal antibody reverses tauopathy phenotypes without affecting hyperphosphorylated neurofibrillary tangles. J Neurosci. 2014 Mar 19;34(12):4260-72. PubMed.
  7. . Surveillance for Intracellular Antibody by Cytosolic Fc Receptor TRIM21. Antibodies (Basel). 2016 Nov 2;5(4) PubMed.
  8. . Cytosolic antibody receptor TRIM21 is required for effective tau immunotherapy in mouse models. Science. 2023 Mar 31;379(6639):1336-1341. Epub 2023 Mar 30 PubMed.
  9. . Protacs: chimeric molecules that target proteins to the Skp1-Cullin-F box complex for ubiquitination and degradation. Proc Natl Acad Sci U S A. 2001 Jul 17;98(15):8554-9. Epub 2001 Jul 3 PubMed.
  10. . Single domain antibodies targeting pathological tau protein: Influence of four IgG subclasses on efficacy and toxicity. EBioMedicine. 2022 Oct;84:104249. Epub 2022 Sep 10 PubMed.
  11. . Single-Domain Antibody-Based Protein Degrader for Synucleinopathies. 2024 Mar 13 10.1101/2024.03.11.584473 (version 1) bioRxiv.
  12. . Anti-tau single domain antibodies clear pathological tau and attenuate its toxicity and related functional defects. Research Square, Feb 26, 2024 Research Square

External Citations

  1. Prospect-ALZ

Further Reading

No Available Further Reading