With anti-amyloid antibodies now consistently hitting their target, tau immunotherapy represents the next frontier. In Alzheimer’s disease, tau tangles correlate far more closely with cognitive decline than plaques do, and tau aggregates are the main pathology in many related disorders. As with amyloid, however, initial trials of anti-tau antibodies have been beset by failures. Already, several antibodies that bind the N- or C-terminus of tau have been scuttled after not doing recipients any good. Meanwhile, preclinical evidence suggests that antibodies that go after the protein’s mid-section, particularly its microtubule-binding region (MTBR), may be better at preventing aggregates from spreading. Several such antibodies have now entered Phase 1 or 2.

  • N-terminal tau antibody semorinemab grabbed its target but did not slow degeneration.
  • Mid-region antibodies PNT001 and bepranemab are in Phase 1.
  • Preclinical antibody PRX005 blocks uptake of tau aggregates by heparan sulfate proteoglycan.

At the 15th International Conference on Alzheimer’s and Parkinson’s Diseases, held virtually March 9–14, researchers discussed a number of these programs. Kristin Wildsmith of Roche offered a first look at biomarker data from the negative Phase 2 trial of the N-terminal-targeting antibody semorinemab. Other speakers touted MTBR-binding antibodies. Wendy Luca of Pinteon Therapeutics showed preliminary Phase 1 findings for PNT001, while Günter Höglinger of the Technical University of Munich presented on UCB’s beprenemab, also in Phase 1. Prothena’s MTBR-binder PRX005 is still preclinical, but the company’s Philip Dolan offered mechanistic data on how it might inhibit the transfer of pathological tau.

Time will tell if this newest crop can perform in the clinic. Wildsmith believes the field is making progress in figuring out how to target the protein. She is encouraged by cerebrospinal fluid data from WashU’s Randall Bateman and colleagues that link CSF MTBR tau with tangles, and specific tau phospho-species with plaques (Mar 2020 news; Dec 2020 news). “That’s really exciting for us as a field. We’re learning so much more about this target,” Wildsmith said in an AD/PD panel discussion.

Tau Strung Out. Researchers are zeroing in on tau’s proline-rich mid-domain and four repeat domains (R1-R4) as the best region for antibodies to target. The microtubule-binding region spans from residues 224–369. [Courtesy of Horie et al., 2021, Brain.]

N-Terminal Strategy: All Duds So Far
Initially, tau’s N-terminus was the favored target, because it produces the highest-affinity antibodies. However, antibodies that bind this region have had no success in Phase 2 trials to date. Biogen’s gosuranemab and AbbVie’s tilavonemab both posted negative findings for progressive supranuclear palsy (PSP) and have been discontinued for that disorder, but remain in trials for Alzheimer’s disease (Jul 2019 news; Dec 2019 news). Biogen presented data at CTAD 2020 showing that gosuranemab hit its target in PSP patients, lowering unbound N-terminal tau in CSF, but had no effect on downstream markers of disease severity.

Similarly, Roche previously announced negative topline results for the Phase 2 TAURIEL trial of semorinemab for AD. Semorinemab targets residues 6–23 in tau’s N-terminus. It did not budge clinical endpoints, and had no effect on tangle accumulation by tau PET (company press release). No safety issues cropped up.

The semorinemab data Wildsmith presented at AD/PD provide another glimpse at the effects of an N-terminal tau antibody on soluble tau and downstream biomarkers. The 16-month TAURIEL trial enrolled 422 people who had prodromal to mild AD and brain amyloid. Participants received either placebo or 1,500, 4,500, or 8,100 mg of semorinemab by infusion once a month. Fewer than a third, or 123 people, participated in the CSF subgroup at baseline. This shrank to about half that number at 49 weeks and about a third at 73 weeks.

First, the good news. As expected, plasma semorinemab rose with dose, with a half-life of 32 days. CSF data showed that about 0.3 percent of the antibody got into the central nervous system, similar to other antibodies. In the two higher-dose groups, N-terminal tau increased in CSF at 49 and 73 weeks, indicating that semorinemab was finding its target. Meanwhile, CSF total tau, as measured by an Elecsys assay that recognizes a mid-domain region, and phosphorylated tau dropped at all doses of drug compared to the placebo groups. This was consistent with mouse data, and demonstrates that the antibody altered tau processing, Wildsmith said. Lower CSF tau is associated with less pathology in AD. Curiously, however, total tau rose in plasma in tandem with semorinemab dose. This trial included tau PET, but Wildsmith did not show that data.

Now, the bad news. The antibody did not budge downstream CSF markers of degeneration and inflammation, such as NfL, neurogranin, S100B, IL-6, and sTREM2. The inflammation marker YKL-40 went the wrong way, increasing in people on semorinemab at 49 and 73 weeks. This astrocytic protein rises as AD progresses, and is linked to brain shrinkage and other deleterious effects (Dec 2020 news). It is unclear why this happened, and Wildsmith did not speculate.

While there have been several N-terminal tau antibodies, far fewer target tau’s C-terminus. One that did, Roche’s RG7345, was dropped after Phase 1. Lundbeck’s LuAF87908, which binds the C-terminus, is completing Phase 1 and should read out soon.

Hot Zone. Prothena’s antibody screen found that those binding the shaded yellow region, spanning tau’s repeat domains R1 and R2, best blocked tau uptake into cells. [Courtesy of Prothena.]

Mid-Region: Up and Coming
The new hot tickets in the field are antibodies targeting tau’s middle, especially the MTBR. This region drives tau aggregation, and in cell culture, antibodies that bind here best prevented the spread of misfolded forms (von Bergen et al., 2005; Apr 2018 conference news). Earlier this month, the DIAN-TU trial chose Eisai’s E2814, which recognizes a motif in the MTBR, for its anti-tau arm (Mar 2021 news). 

Several more such antibodies are in trials. Janssen has an MTBR-targeting antibody, JNJ-63733657, that has just begun Phase 2. Biogen’s mid-domain BIIB076 completed Phase 1 in AD in 2020, but no results or future plans have been announced. The antibody remains listed on Biogen’s pipeline. Lilly’s zagotenemab will read out for Phase 2 in AD later this year. It was derived from the research antibody MC1, which is reputed to bind amino acids 312–322 in tau’s mid-region. However, Lilly previously reported that zagotenemab recognizes a conformational epitope involving the N-terminus and is selective for aggregated tau, leaving it unclear in what category it belongs (Alam et al., 2017). 

AD/PD showcased three more mid-tau antibodies. Pinteon Therapeutics’ PNT001 is the furthest along, having completed Phase 1. It recognizes phosphoThr231 in the MTBR. Luca noted that it blocked tau aggregate seeding in lysates from AD frontal cortex and hippocampus by 88 percent. In the Tg4510 mouse model of tauopathy, five months of dosing lowered insoluble tau and serum NfL, a marker of degeneration, while improving performance in the Morris water maze.

The Phase 1 single-ascending-dose study enrolled 49 healthy volunteers at three U.S. study sites, who received either 33, 100, 300, 900, 2,700, or 4,000 mg. In each dose group, six people got PNT001 and two placebo, each as a 30-minute infusion. Participants were followed for four months, donating CSF at days three and 28. The antibody was well-tolerated, Luca reported.

Serum levels of PTN001 increased with dose as expected. In CSF, too, the antibody concentration reflected the dose given, and stayed constant through day 28. For doses of 900 mg and above, the CSF concentration topped 45 ng/ml. This is the minimum concentration necessary for this antibody to bind tau well, Luca noted. Pinteon will take these top three doses into Phase 2. The company is still analyzing serum and CSF tau, as well as CSF markers of inflammation. Meanwhile, Pinteon has started a multiple-ascending-dose trial for traumatic brain injury, and is planning a similar trial in AD.

UCB’s bepranemab is in Phase 1 for PSP. This antibody, now being developed in partnership with Roche, recognizes residues 235–250 in the MTBR. This stretch overlaps the end of the proline-rich region and the beginning of the first repeat. At AD/PD, Höglinger showed cell culture data indicating that as little as 0.3 nM of bepranemab prevented uptake of tau aggregates. In the same assay, antibodies against the N-terminus required more than 10 times that concentration to have an effect.

The Phase 1b study enrolls 24 people with PSP at sites in Belgium, Germany, Spain, and the U.K. Eighteen participants take bepranemab, six placebo, for one year. Researchers will collect CSF to assess whether the antibody hit its target and how it was metabolized. The trial is expected to complete this year, after which participants will have the option to go on to an open-label extension, or a 16-week follow-up to collect safety data. No safety concerns have arisen so far, Höglinger noted.

UCB initially planned to run a Phase 3 PSP trial, dubbed auTAUnomy, but put it on hold and is now prioritizing Phase 2b for AD, Höglinger said. More money and more patients are available for AD research, and such a trial will still allow UCB and Roche to evaluate safety and efficacy in a tauopathy. The sidelined PSP trial would have run for two years and included mortality as an outcome measure, as amyotrophic lateral sclerosis trials do. It would have been the first PSP trial to use a combined assessment of function and survival, known as CAFS (Berry et al., 2013). Höglinger believes such a design makes sense. “It would be a good step forward in getting meaningful outcomes in clinical trials, because it focuses on a longer observation period and incorporates time to the most meaningful milestone to patients, which is survival,” he said.

Werner Poewe of Innsbruck Medical University, Austria, agrees that one-year trials are too short for many neurodegenerative diseases, and longer trials would serve the field better. The effects of a drug may accrue over time, and the magnitude of change may be too small to see in a short timeframe, he suggested. “I am worried that we are throwing away valid drugs and targets,” he said at AD/PD.

Why Is Mid-Tau Better? Preclinical Data for PRX005 Offers Clues

Pass the Tau, Please. Tau (red) needs to bind sulfated proteoglycans (brown twigs) to exit and enter cells.

Prothena’s Dolan showed more data on the merits of targeting tau’s middle. His team screened a panel of antibodies, and found that those that bound repeat regions 1 and 2, near the beginning of the MTBR, worked best to block internalization of tau aggregates in cell culture. They selected one such antibody, PRX005, that bound at the start of R1 (see image above). This antibody binds all splice forms of tau, as R2 is the one that gets spliced out in 3-repeat tau. PRX005 recognizes phosphorylated and unphosphorylated forms with equal affinity, Dolan said. It also lit up neurofibrillary tangles and dystrophic neurites in AD brain tissue. 

Dolan noted his company’s choice of antibody was also influenced by Bateman’s CSF data on MTBR tau. “That solidified for us that it was a region worth pursuing. It’s mechanistically distinct and, in our experiments, was a bit of a hot spot,” he said at AD/PD.

Curiously, PRX005 inhibited tau binding to heparan sulfate proteoglycans. HSPGs on the cell surface are believed to both secrete and take up tau (Mar 2013 conference news; Katsinelos et al., 2018; image above). In culture, even picomolar quantities of PRX005 disrupted tau-HSPG binding. This helped cells; PRX005 protected rat cortical neurons from tau toxicity. Dolan did not discuss whether PRX005 might interfere with HSPG binding of other, physiological cargo.

The antibody worked in animals, too. The researchers treated PS19 mice, which express P301S tau, with PRX005 starting at 6 months of age. Four months later, treated mice had less ptau199/202 and ptau212 in the brainstem, and were able to hang longer from a grid, indicating improved grip strength. In a different model, mice injected with AD lysate, weekly antibody injections likewise prevented tau aggregates. Dolan noted that they used both models because some experiments indicate tau internalization may happen differently in each.

Höglinger asked if PRX005 would block tau aggregation. “We think it probably would,” Dolan said. He questioned how much that would help, given that most tau aggregation is intracellular, and most antibodies stay outside cells. This may help with safety, since antibodies that enter cells could interfere with tau’s physiological functions. Dolan believes the risk of intracellular effects is small, and PRX005’s main mode of action will be to stop cell-to-cell propagation. He did not discuss a timeline for clinical trials.—Madolyn Bowman Rogers

Comments

  1. On the choice of epitope for Tau therapeutic antibodies:

    For bepranemab, we have published on the antibody and the choice of epitope (Courade et al., 2018).

    We had raised a broad spectrum of antibodies, run them through our functional assay using human AD tau seeds, and determined epitopes of the best antibodies only afterwards. At the time, we were surprised to find that the mid-region antibody was so superior to previously described reference antibodies, several of which were in clinical development. The focus on human AD tau seeds (transgenic mouse-derived seeds are more forgiving) and a rigorous quantitative assay were critical for selection of the antibody.

    References:

    . Epitope determines efficacy of therapeutic anti-Tau antibodies in a functional assay with human Alzheimer Tau. Acta Neuropathol. 2018 Nov;136(5):729-745. Epub 2018 Sep 20 PubMed.

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References

News Citations

  1. Different CSF Phospho-Taus Match Distinct Changes in Brain Pathology
  2. MTBR-243 Tau: A Fluid Biomarker for Tangles Themselves?
  3. AbbVie’s Tau Antibody Flops in Progressive Supranuclear Palsy
  4. Gosuranemab, Biogen’s Anti-Tau Immunotherapy, Does Not Fly for PSP
  5. Not Just a Biomarker: YKL-40 Links Glial Clock to Amyloidosis
  6. To Block Tau’s Proteopathic Spread, Antibody Must Attack its Mid-Region
  7. Aiming at the Tangle’s Heart? DIAN-TU Trial to Torpedo Tau’s Core
  8. Tau, α-Synuclein Spread: Crazy Stuff—How Might It Work?

Therapeutics Citations

  1. Gosuranemab
  2. Tilavonemab
  3. Semorinemab
  4. RG7345
  5. Lu AF87908
  6. E2814
  7. JNJ-63733657
  8. BIIB076
  9. Zagotenemab
  10. PNT001
  11. Bepranemab

Antibody Citations

  1. Tau (MC1)

Research Models Citations

  1. rTg(tauP301L)4510
  2. Tau P301S (Line PS19)

Paper Citations

  1. . Tau aggregation is driven by a transition from random coil to beta sheet structure. Biochim Biophys Acta. 2005 Jan 3;1739(2-3):158-66. PubMed.
  2. . [O2–14–05]: Preclinical Characterization of an Antibody [LY3303560] Targeting Aggregated Tau. Alzheimer's & Dementia, July 2017
  3. . The Combined Assessment of Function and Survival (CAFS): a new endpoint for ALS clinical trials. Amyotroph Lateral Scler Frontotemporal Degener. 2013 Apr;14(3):162-8. Epub 2013 Jan 17 PubMed.
  4. . Unconventional Secretion Mediates the Trans-cellular Spreading of Tau. Cell Rep. 2018 May 15;23(7):2039-2055. PubMed.

External Citations

  1. company press release

Further Reading