Moving Beyond Amyloid in Alzheimer's Therapeutics

Abstract

GEN BiotechnologyVol. 2, No. 2 News FeatureFree AccessMoving Beyond Amyloid in Alzheimer's TherapeuticsAnjali A. SarkarAnjali A. SarkarE-mail Address: sarkaraa@gmail.comAssociate Director, Content Management, IPD Analytics.Search for more papers by this authorPublished Online:18 Apr 2023https://doi.org/10.1089/genbio.2023.29090.aasAboutSectionsPDF/EPUB Permissions & CitationsPermissionsDownload CitationsTrack CitationsAdd to favorites Back To Publication ShareShare onFacebookTwitterLinked InRedditEmail A growing number of start-up companies are pursuing alternative strategies in drug development for patients with Alzheimer's disease that bypass the beta amyloid hypothesis that has dominated the field for decades.Over the past two decades, particularly with the advent of transgenic animal models, drug development for Alzheimer's disease (AD) has been dominated by strategies that target beta-amyloid. As a result, drugs that target other potential pathogenic mechanisms have been systematically sidelined.“I don't blame people for hanging on to the old hypothesis,” says Maria Maccecchini, president and founder of Annovis Bio. “For 20 years, we have been saying that amyloid-beta is the cause of AD. There are amyloid plaques in brains of AD patients. No one disputes that.” But in some cases, plaques arise 20 years before patients develop AD. “There are amyloid plaques, tau tangles, alpha-synuclein Lewy bodies and TDP43 aggregates in AD brains. Removing just one is not enough,” she says.Most of the current treatment options for AD have been around for decades. These include three cholinesterase inhibitors: donepezil (Aricept) approved in 1996, rivastigmine (Exelon) released in 2000, and galantamine (Razadyn) in 2004. Donepezil was approved for the full spectrum of AD, whereas the others are approved for mild-to-moderate AD. In addition, an N-methyl-D-aspartate antagonist memantine (Namenda) was approved for moderate-to-severe dementia in 2003. Thereafter, the development of AD drugs hits an extended dry spell, although a fixed combination of memantine and donepezil (Namzaric) was approved in 2014.Maria Maccecchini, President and Founder of Annovis Bio, developing two drugs for Alzheimer's disease that target two distinct mechanisms: translation and neurotransmission.“In one way or another, all these generic drugs currently in the market address aspects of neurotransmission,” says Hans Moebius, chief medical officer at Athira Pharma, who helped develop memantine. “When we have drugs that address neurotransmission only, we see tachyphylaxis [waning of effect]. These drugs have limited efficacy and may also have limited duration of effects.”The most recently approved drugs are a pair of amyloid-directed monoclonal antibodies: aducanumab (Aduhelm) from Biogen (2021) and lecanemab (Leqembi) from Eisai and Biogen (2023). But these drugs have courted controversy.“Aducanumab and lecanemab are promising approaches to prevent the further accumulation of toxic oligomers of the still soluble species of amyloid-beta,” says Moebius. “On the other hand, a circular argument exists regarding the basics of the amyloid hypothesis. It is known from post-mortem studies that people may have loads of plaque in their brain and no history of dementia during their lifetime. That is a contradiction that has never been properly resolved.”“You often hear in the media that lecanemab improved cognition by 27%. It did not. It worsened cognition by 73%, because from the baseline, cognition goes down 100% with placebo and 73% with lecanemab. So, lecanemab is 27% better than placebo. This is imperceptible to AD patients—27% over 18 months is not something people can feel,” says Maccecchini. “But it is a step in the right direction, because with additive effects of other drugs we may be able to stabilize cognition.”According to Bradford Navia, chief medical officer and executive vice president at Aprinoia, the history of treating AD has had several challenges. “Most approaches have focused on antibodies and recently we have had some success. But the results of the phase III Eisai/Biogen trial for lecanemab show there is a huge unmet need to find new targets and develop novel therapeutic approaches.”Although expectations and controversies continue to rocket and plunge regarding beta-amyloid targeting therapeutic strategies for AD, a handful of renegade biotech companies are exploring lesser known molecular mechanisms implicated in neurodegeneration to uncover treatments for the devastating disease. These strategies are inspiring fresh hope for an aging global population, 55 million of whom are currently suffering from AD—a number that is expected to triple by 20501 and result in well over $2 trillion in direct health care costs.2It would be ideal to have safe and efficacious treatment options for the core and accessory symptoms of AD, with every new agent layering on additional benefits over the standard of care. Drug developers are also eager for economic viability—identifying an agent that will gain uptake with insurers, health care providers, and other payors.Cocktail Therapies for AcetylcholineThe pharmacokinetic and pharmacodynamic profiles of donepezil, the mainstay of anti-AD therapy for a quarter of a century, make it a safe and easy option for AD treatment. But its efficacy is limited, spurring investigations into drugs that can work in tandem to further boost synaptic levels of acetylcholine to bolster global cognition and function.“Cholinesterase inhibitors such as donepezil enhance cerebral cholinergic activity and have the best evidence for symptomatic benefit in AD at the various stages of dementia for periods of at least one year,” says Serge Gauthier, professor emeritus in neurology and psychiatry at McGill University.3 A 6-month study in >2000 patients with mild-to-moderate dementia from Europe, Canada, and the United States treated with donepezil showed improvement of cognition over placebo.4,5“Interestingly, if you stopped the drug without the patient and family knowing, you see the loss of clinical effect of donepezil on cognition,” said Gauthier, as further evidence of the drug's specific benefit. Moreover, Kaplan–Meier survival estimates showed donepezil-treated patients were 38% less likely to decline over 12 months.6To complement the effects of donepezil, AmyriAD Therapeutics, a late-stage biopharmaceutical company, is developing a synthetic small molecule, AD101 (previously ST101)—a first-in-class drug for AD. Although donepezil inhibits cholinesterase, AD101 stimulates the release of the neurotransmitter into the synaptic cleft by acting through neuroselective voltage-gated T-type calcium channels (Fig. 1).FIG. 1. AD101, a phase III ready synthetic small molecule being developed by AmyriAD Therapeutics increases acetylcholine release. It is designed to work together with previously approved donepezil (Aricept) that inhibits the breakdown of acetylcholine for additive improvements in neurotransmission.When administered together with donepezil, AD101 showed additive improvement in animal models and in a randomized phase II clinical trial in AD patients.7 The proof-of-concept trial showed evidence of additive efficacy when doses of up to 180 mg/day AD101 were added to a stable (10 mg/day) regimen of donepezil in 356 AD patients for 6 months. Once approved, the currently phase III-ready AD101 tablet can be administered orally once a day.Although the major manifestations of AD are global cognitive decline and loss of independent function, accessory symptoms include aberrant accumulation of intra- and extracellular proteins, neuroinflammation, mitochondrial dysfunction, and compromised neurotransmission. Sharon L. Rogers, CEO, at AmyriAD Therapeutics, laments that the AD drug development field has focused on disease-modifying therapies at the expense of individual symptom-modifying therapies.“There are 74 disease-modifying therapies in the developmental pipeline versus only 46 symptom-modifying therapies,” says Rogers. “In pharma, we are reconciled to the idea that we don't cure much, we manage things and usually we manage disease with a variety of agents. It is time for us to look at AD as a chronic disease that needs to have that kind of disease management. Disease management combined with disease modification is the future of AD treatment.”And Then There Was NoneOwing to massive neuronal death, few acetylcholine-synthesizing neurons remain in the brain during late stages of AD. Therefore, strategies that block acetylcholinesterase or enhance the release of the neurotransmitter will have limited effect on cognition at advanced stages. However, astrocytes in the brain continue to survive in patients with late-stage AD and produce butyrylcholine, an acetylcholine-like ester that can function as a neurotransmitter and bolster cognition.“In advanced AD, you don't have to kill acetylcholinesterase because there is no acetylcholine, you have to kill butyrylcholinesterase. That is what ANVS301 does,” says Annovis Bio's president and founder Maria Maccecchini. ANVS301 is anticipated to increase cognition in patients with advanced AD, when Aricept and other acetylcholine-boosting drugs lose efficacy. Annovis has received funding from the National Institutes of Health for the drug's development. In the meantime, however, Maccecchini is conserving her company's resources and focusing on its lead candidate, buntanetap, which targets translation of proteins that could form neurotoxic aggregates if overexpressed (Box 1).Box 1. Nipping Neurotoxic Aggregates in the BudAlong with the aggregation of neurotoxic proteins in the brain, elevated iron levels are a hallmark in several neurodegenerative diseases, including Alzheimer's disease (AD). Iron triggers the overexpression and subsequent aggregation of certain proteins that in turn triggers neurotoxicity and neuronal death in a range of neurodegenerative diseases, including AD. Therefore, a logical strategy in treating AD would be to specifically inhibit translation of proteins with the potential of expressing excessively and aggregating under the influence of iron.The lead candidate from Annovis Bio, buntanetap,16 currently in clinical development for several neurodegenerative indications including AD, does just that. An oral daily pill, the drug inhibits translation of several neurotoxic-aggregating proteins associated with impaired axonal transport, inflammation, and cell death. The drug's half-life is 14 h in spinal fluid and in all animal models tested so far.“We target all four neurotoxic proteins—beta-amyloid, tau, alpha-synuclein, and TDP43,” says Maccecchini. “Therefore, you don't have the toxic cascade and you don't get nerve cell death. What amyloid does is kill neurons, but so does tau, alpha-synuclein, and TDP43. The broader approach, where you lower more than one neurotoxic protein, is a better approach.”Adverse effects of buntanetap observed in rodents at high doses include tremors (doses 10–20 times higher than the effective dose). In dogs and humans, side effects include nausea. “We see 36% healthy volunteers experience nausea and vomiting at 160mg [doses]. The doses we are using for phase III are 10 and 20mg,” clarified Maccecchini. Annovis is currently enrolling participants for phase III trials.“All mRNAs of neurotoxic aggregating proteins that we've looked at have a homologous stem loop that binds to a protein called iron regulatory protein 1 (IRP1). When mRNA is bound to IRP1, it is not translated. IRP1 is an iron responsive protein and opens when iron flows into the cell,” says Maccecchini.Cellular damage increases iron influx, causing IRP1 to open and let go of its captive mRNA, which makes its way to the ribosome to be translated. Continuous influx of iron into cells can cause these proteins to be overexpressed, leading to precipitation, aggregation, and toxic consequences. (When expressed at normal level, these proteins are not toxic and serve necessary physiological functions.)“Our drug sees the interface of IRP1 with the [mRNA] stem loop and inhibits it. IRP1 keeps the mRNA bound,” says Maccecchini. Buntanetap only targets transcripts of potentially neurotoxic proteins that bear this stem loop configuration. “It took us only seven years to prove that!” says Maccecchini.In humans, Annovis has tested buntanetap at very early and early stages of AD in small studies, with positive results. Next, they will test the drug in patients with moderate AD. Theoretically, the stage of disease progression should not affect buntanetap's efficacy, as these neurotoxic aggregates accumulate throughout the course of the disease. “What we do not know yet, is how late you can go and still have an effect,” says Maccecchini.Iron chelators could potentially be used in conjunction with buntanetap for greater efficacy. However, such combinatorial approaches may pose challenges. The iron chelators studied to date are not that good, says Maccecchini—they tend to make patients anemic and are not specific. They also bind to other metals and can be toxic. “But I do believe, chelating iron specifically in the brain—and I am not sure how to do that yet—is a viable option,” she insists.Buntanetap, currently in phase III trials, has not been tested with iron chelators yet. “I am waiting for some company to come up with an iron chelator that works in the brain,” says MaccecchiniPromising TherapiesSeeking alternative AD therapeutic approaches beyond casting a wide net on neurotoxic proteins, Maccecchini would bet on targeting metabolism and inflammation—increasing the energy of the sick cell. “There are several sugar approaches—GLP1, metformin, insulin, keto bodies—that try to change mitochondrial activity,” says Maccecchini. “At this point, it's too early to say which one will work.”Recent studies show injecting gold nanoparticles (AuNP) into the ventricles of the brain in animal models can have anti-inflammatory and antioxidative effects that reverse brain damage.8 Prolonged AuNP treatments prevented neuroinflammation as measured by levels of interlukins-1β and 4 in the hippocampus and cerebral cortex, and oxidative stress as measured by sulfhydryl, nitrite, superoxide dismutase, catalase, and glutathione levels in brain structures. AuNP also restored cognition in rat models as measured by spatial memory tasks.The problem in targeting neuroinflammation, which everyone agrees is supremely deleterious for cognition, is that insights on the specific functions of the numerous inflammatory factors uncovered to date are lacking.Down Less Taken Trophic RoadsA large body of literature vouches for the important role that the hepatocyte growth factor (HGF)/tyrosine kinase receptor (MET) pathway plays in regeneration, protection, and homeostasis, in various cell types, including neurons (Fig. 2).9,10 Although discovered in the liver in the 1980s, subsequent studies have established the system's ubiquity. In AD patients, the expression of the MET receptor is reduced in the hippocampus—important in memory—that is struck by neuronal loss during AD progression.11FIG. 2. Positive modulation of the HGF/MET system affects the multiple cell types in the brain that includes neurons and glia. While triggering the HGF/MET system in neurons activates neurotrophic and neuroprotective pathways, triggering the HGF/MET system in glia inhibits neuroinflammation. HGF, hepatocyte growth factor; MET.In a recent review in the Journal of Alzheimer's Disease, Hans Moebius, chief medical officer, and Kevin Church, chief scientific officer at Athira Pharma, discuss the potential for enhancing the HGF/MET system to address the unmet need in AD therapeutic development.12Disparities in the development of diagnostic tools have hampered the development of new AD drugs, but recent advances in proteomics have identified proteotoxic clusters that connect perfusion, oxygenation, neuroinflammation, and microglial activation with neuronal death. “Even before you get to the slippery slope of losing neurons in the hippocampus or in the prefrontal cortex—the AD predilection areas—these neurons are suffering because they do not have enough trophic support. Their homeostasis is in question,” says Moebius.Translational research on neurotrophic factors such as brain-derived neurotrophic factor and nerve growth factor has tried administering these directly to raise their levels in the brain, without tangible therapeutic benefits.Such developments led Athira Pharma to turn its attention to the HGF/MET system, upstream of these other neurotrophic factors. Athira's lead candidate, fosgonimeton (ATH-1017) for mild-to-moderate AD, is a small molecule in phase II/III trials. The drug offers potential neuroprotective, neurotrophic, and anti-inflammatory effects by enhancing the HGF/MET neurotrophic system. Preclinical studies on fosgonimeton support its efficacy as a neuroprotective and anti-inflammatory agent in animal models of AD. Two clinical trials aim to corroborate these findings: the completed 26-week exploratory phase II trial ACT-AD (NCT04491006) and the ongoing phase II/III LIFT-AD (NCT04488419).“We have a highly specific, small molecule that positively modulates to this brain receptor system and has a multimodal downstream effect on many of the key pathways that are known to be compromised in AD,” says Moebius.Fosgonimeton is a prodrug that is injected daily subcutaneously. It is quickly converted into the active metabolite, fosgo-AM (active metabolite of fosgonimeton), which travels into the brain and enhances the HGF/MET system through an array of secondary messenger systems. “Our preclinical research shows increased number of functional synapses. Neurons are making new connections. We are also seeing an increase of axonal sprouting and an increase in NMDA-mediated currents,” says Church.Athira is currently developing fosgonimeton for patients with mild-to-moderate AD. “We feel this is where the largest present medical need is,” says Moebius, “and also this requires less extensive trials. More extensive trials are more taxing for the patients.”Whereas trials in the predementia phase require treatment periods of 72 weeks or longer, at mild-to-moderate phases of the disease those periods are only 26 weeks, as per FDA guidelines, which makes a big difference for sponsoring companies.At least two other companies are targeting the HGF/MET pathway for other peripheral disease indications. Mitsubishi Tanabe has developed an intramuscularly injectable plasmid, approved in Japan for complications of diabetes mellitus—peripheral ulcerations due to reduced perfusion. Helixmith is developing a plasmid that targets the system for peripheral polyneuropathy. However, plasmids will not cross the blood–brain barrier. “That is only possible with smaller molecules like fosgonimeton,” says Moebius.Complexities in CompositesNovel therapeutics can sometimes interact with existing agents to the detriment or benefit of the patient, which makes drug–drug interactions a concern for pharmaceutical companies.“There is little overlap because all the current immunological approaches are targeting very early stages of the disease, while we are currently targeting mild to moderate stage,” says Moebius. “From a pharmacological perspective, these therapeutic approaches should be orthogonal. There is no theoretical pharmacological reason why a monoclonal antibody and a small molecule like fosgonimeton should interact. I could imagine, in the future, a combination therapy of these.”Topline results released in June 2022 from Athira's ACT-AD study, however, showed no statistical change in the primary endpoint among all participants in the study—latency to form P300 brainwaves (which only occur when an individual actively detects a target stimulus among a train of standard stimuli). This endpoint is a predictive measure of cognitive performance. Further subgroup analysis revealed a drug–drug interaction between fosgonimeton and acetylcholinesterase inhibitors (e.g., Aricept), which diminished the effect of fosgonimeton. Patients taking fosgonimeton alone showed improvements in measures of cognition and function, as well as a reduction in plasma levels of neurofilament light chain (NfL), a biomarker of neurodegeneration.This led Athira to modify its protocol for its phase II/III LIFT-AD trial to exclude AD patients on acetylcholinesterase inhibitors. Athira hopes to complete enrollment of up to an additional 150 patients for the LIFT-AD trial by mid-2023 and release results in 2024.Hans Moebius, Chief Medical Officer at Athira Pharma, who was directly involved in the development of memantine for Alzheimer's disease, is now developing fosgonimeton that targets the HGF/MET system.Lili Zhang, Head of Preclinical Development at APRINOIA.Beyond Memory Toward Composite EndpointsAmong other cell types, MET is expressed in neurons and glia such as oligodendroglia, astroglia, and microglia. Therefore, it is reasonable to speculate that fosgonimeton will act on multiple cell types to have broad functional consequences.“We must not look at neurons in isolation. Neurons can only properly function in concert with the supportive matrix of glial cells around them. We are addressing this complex multi cell-type situation,” says Moebius. This is also true for peripheral nerves where Schwann cells, instead of oligodendroglia, isolate axons for the saltatory prolongation of signals. “There isn't a brain-neuron specificity to our basic approach. It could have broad application to treat neurodegenerative diseases and also provide peripheral neuroprotection.”In addition to memory loss, AD is marked by loss of independent functional capacity, mood disturbances, aggression, and other behavioral and psychiatric symptoms. Moebius hopes to detect fosgonimeton's as-yet-unknown benefits on such symptoms by remaining open to research outcomes in the clinic.In the ongoing LIFT-AD trial, he says, “we're not only employing measures of cognitive change but also of instrumental activities of daily living and e.g., the neuropsychiatric inventory. We're also assessing pharmacoeconomic endpoints because the moment you can improve daily-life activities, you could expect people will maintain their independence longer. That is a core problem in AD, which none of the current approaches address sufficiently.”Positron emission tomography (PET) scans can reliably confirm (or rule out) protein aggregates in the brain, but determining comprehensive quantitative endpoints for clinical trials that combine multiple molecular and behavioral biomarkers is a greater challenge. How should investigators assess such effects? Moebius's team employs a composite primary outcome called the global statistical test.13 “This is an unbiased and unweighted composite score. We're not determining up front what is more important—cognition or function,” he says.For novel therapeutic approaches like the positive modulation of HGF/MET with fosgonimeton, using such composite endpoints can enable the assessment of the full spectrum of the intervention's benefits.Tearing Down TauTargeted protein degradation using proteolysis targeting chimera (PROTAC) is a new therapeutic modality that harnesses a cell's own protein clearance systems.14 Aprinoia, a clinical-stage biotech company, is the first to develop a PROTAC for neurological disease. It works by degrading tau aggregates. A wealth of basic and clinical data shows tau aggregates are toxic to brain cells and pivotal in neuronal degeneration and dementia, associated with AD and rarer tauopathies. Many experts believe tau may be a better surrogate for cognitive decline than amyloid.“We're developing some exciting diagnostic tracers to visualize aggregate proteins like tau. That ties into our therapeutic approach,” says CMO Navia, who believes intracellular protein degradation-based approaches will add value to existing extracellular antibody-driven strategies. “With the [Eisai Biogen's] amyloid approach, clinical efficacy was modest at best, despite the dramatic lowering of amyloid plaque.”PROTACs consist of two active domains and a linker. Although the “target binder” domain binds to the misfolded protein, the other is an E3 ligase ligand. The tripartite PROTAC drags the misfolded protein to the proteosome to be degraded and cleared from the system.“Misfolded proteins are very difficult to target with small molecules. PROTACs allow us to drug these historically undruggable targets,” says Lili Zhang, head of preclinical development at Aprinoia. “PROTACs offer a better option than enhancing E3 ligases because these enzymes are non-specific. PROTACs add unprecedented specificity, more so than small molecules, because the molecule must bind not just the target but also the E3 ligase. With PROTACs, you can do pretty much anything if you can find a binder to the target protein.”The specificity of the PROTAC agent derives not from E3s, which may be promiscuous in their substrate selection, but from the “warhead” target binder of the tripartite molecule. “You don't need to know exactly which E3 the cell uses for a particular target. You can engage it to any E3 to drag the target to the proteasome,” clarifies Zhang.Another key advantage of PROTAC involves its pharmacology, which sets it apart from traditional small molecule drugs. Zhang explains, “You want a typical inhibitor to occupy the target's active site continuously. But the PROTAC molecule drags the target to the proteasome and releases it. You can recycle each molecule.” Recycling implies a lower requirement for the drug, which translates into better toxicity profiles.“[Our] platform docks the toxic protein through binders and delivers it to the proteosome, where is gets broken down further. That kind of approach has never been tried before in neurology, as far as I know,” says Paul Tempest, Aprinoia's head of medicinal chemistry discovery.What potentially gives Aprinoia a competitive edge is their proprietary PET tracer “warheads” that can cross the blood–brain barrier to detect tau aggregates and act as target binder domains of the PROTAC molecule.15,16 Aprinoia's PET tracers for tau are now in phase II/III clinical trial (NCT05326009). The tracer APN 1607 effectively visualizes different forms of tau across diverse tauopathies. The binding properties of the tracer were leveraged to create tau binders for the PROTAC platform to target toxic tau aggregates.“Using PROTAC allows small molecules to have an outsized effect,” says Tempest. “With the PROTAC system, you can go after larger targets, like protein aggregates, with a small molecule.”For PROTACs to degrade tau, it must reach within neurons in the brain. “We are designing our small molecule compounds based on our PET tracers. Our compounds target a unique binding site on tau,” says Tempest. “Because we know the SAR [structure–activity relationship] around that binding site thoroughly, we know exactly where we can add or delete functionality to make these compounds very specific and tailor their properties so that they get through the blood brain barrier and into neurons,” says Tempest.PROTACs could be used as a standalone or complementary treatment strategy. Navia believes novel therapeutics create opportunities for cocktail therapies consisting of several antibodies or PROTACs in combination with antibodies, which could have additive benefits. “There is an emerging school of thought that perhaps the right approach to AD, like other neurodegenerative diseases, and oncology, may be combination therapies targeting different proteins using different approaches,” says Navia.Stemming the TideFor the more distant future, Maccecchini hitches her hope wagon to stem cell therapy. “Stem cell therapy for AD is way in the future, because we don't really understand how to differentiate stem cells at the right time, in the right place, and integrate them into the brain to do what a normal neuron would do there and then,” says Maccecchini. “At some point we will.”Although the various AD therapies developed so far and in development might only prevent patients from getting worse by blocking the synthesis of neurotoxic proteins, degrading toxic protein aggregates, increasing cellular energy, or curbing neuroinflammation, precision stem cell therapies promise the recovery of brain cells lost to the disease.Anjali Sarkar, PhD, is a former science and technology editor with Genetic Engineering and Biotechnology News. She is currently associate director, Content Management, with IPD Analytics, E-mail: (sarkaraa@gmail.com).