Michael
08-16-2008, 03:41 PM
Currently, most of the drugs approved for the treatment of Alzheimer's disease (http://www.alz.org/alzheimers_disease_standard_prescriptions.asp) are cholinesterase inhibitors (donepezil, galantamine, and rivastigmine), which may improve scores on standardized cognitive tests but have dubious benefits in real-world functionality, (10) and which in any case do nothing to slow the underlying disease process (are symptomatic, rather than "disease-modifying," agents). Memantine, another approved drug, was once thought to be disease-modifying by protecting against excitotoxic neuron death, but this no longer seems to be credible and the drug's central mechanism of action is in doubt.(11-13) A recent " evidence review for a clinical practice guideline" concluded that
Treatment of dementia with cholinesterase inhibitors and memantine can result in statistically significant but clinically marginal improvement in measures of cognition and global assessment of dementia.(14)
... which is in line with the conclusions of the UK National Institute for Health and Clinical Excellence, which sets the formulary for their national healthcare plan.
Clinicians, regulators, patients, and their caregivers would clearly prefer a genuinely disease-modifying therapy, and the pharmaceutical industry has for some time been attempting -- through both scientific and marketing efforts -- to fulfil this unmet need. The main target for such intervention has been beta-amyloid plaques and other aggregates, whose central role in the disease has been the focus of most investigation.
This year's Alzheimer's Association (http://www.alz.org) International Conference on Alzheimer's Disease (ICAD) 2008 in Chicago (http://www.alz.org/icad/) saw a surprisingly large and promising body of work on neurofibrillary tangles (NFTs), the malformed aggregates formed of microtubule-associated tau (http://en.wikipedia.org/wiki/Tau_protein) that constitute the main intracellular pathology in the brain with aging and Alzheimer's disease (vs the extracellular pathology of the beta-Amyloid plaque); in particular, the clinical results of two previously-unknown drugs targeting tau in Alzheimer's patients, along with animal data on one of them.
AL-108
This is Allon Therapeutics' (http://www.allontherapeutics.com/) placeholder name for NAPVSIPQ, or NAP (not to be confused with Neprilysin (NEP) (http://en.wikipedia.org/wiki/Neprilysin), the Abeta-cleaving metalloprotease that is also advancing through the clinical pipeline for AD), an eight-residue fragment of glial-derived activity-dependent neuroprotective protein (ADNP). This agent has previously been shown to be neuroprotective against a variety of insults in animal models, (20) including La Ferla's 'triple-transgenic' mice that produce both NFT and Abeta plaques (21,21a) (in which it lowers both plaque and tangle pathology) and another TG mouse also featuring neurodegeneration induced by tau pathology (22). In vitro studies suggested that part of the protective effect against tau pathology might be mediated by binding to and preventing disruption of microtubules.
In a Phase II trial in 144 patients with amnestic mild cognitive impairment (aMCI) (http://memory.ucsf.edu/Education/Disease/mci.html) presented at ICAD, AL-108 failed to meet its primary outcome target (a composite cognitive memory score (CCMS) composed of tests of memory and executive function), but there were positive-looking findings in a select few subtests of the CCMS (digit span and delayed match-to-sample). The meeting abstract (23) rather disingenuously reports the "A statistically significant, dose-dependent and durable improvement in memory ... in delayed-match-to-sample (DMTS) and digit span" and says that "Data will be presented relating to additional tests of memory and executive function," summarizes this as "Improvement In Cognitive Function" in the title, but fails to mention the lack of effect on the primary outcome.
Allon's press release (http://www.marketwire.com/press-release/Allon-Therapeutics-Inc-TSX-NPC-884162.html) says that there was a trend toward a positive result on the CCMS, and argues that
This data suggests that patients with aMCI have significant memory impairment but no significant impairment of executive function. Thus, AL-108 appears to have an impact on regions of the brain that are impaired in aMCI such as those that are known to control memory (medial temporal lobe, hippocampus and prefrontal cortex). However, no measurable effect of AL-108 on executive function was seen in this population since they were not impaired on tests that involve executive function such as Paired Associates Learning, Spatial Working Memory and One-Touch Stockings of Cambridge.
And, it's also possible that the drug does indeed do just what it is reported to do, but that the clinical benefits are mediated through a moderate but disease-modifying effect with little to no immediate symptomatic boost, so that it takes some time for the benefits relative to untreated patients to appear. This possibility is being recognized as a possible flaw in current Phase II study design for disease-modifying agents in AD (http://www.alzforum.org/new/detail.asp?id=1890), which both the FDA and its European counterpart are struggling to address (more on this below).
While plausible, however, it's equally plausible -- and more parsimonious -- to read the positive results as a Type I error: throw a large enough number of variables onto the clinical wall, and something may coincidentally find a sticky spot.
Fortunately, according to MedPage Today (http://www.medpagetoday.com/MeetingCoverage/ICAD/tb/10304), an anonymous company representative and the lead investigator of the trial seem confident that Allon will continue with research on this drug to clarify these issues:
in the world of Alzheimer's disease research, a near miss is often as good as a hit, so the drug will definitely go forward, according to a spokesperson for Allon Therapeutics, which is developing the drug.
Dr. Schmechel said the most likely next step would be an enhanced phase II trial that will attempt to replicate the positive findings on secondary endpoints, but in patients with mild-to-moderate Alzheimer's disease.
He said that the amnestic mild cognitive impairment patients, while a good population to use when testing safety, were not a good choice for proof of efficacy, because at baseline their cognitive scores were so high.
Future studies will also investigate escalating doses with the intent of identifying minimal and maximal effective dose.
"And we need to test the drug for longer periods," he said.
While less than convincing, on their own, these results may possibly add further to the case for attacking tau and NFTs as a disease-modifying therapeutic target when considered in the light of the results of research on another tau-targeting drug presented at the conference.
rember™
As many press reports have noted, this previously-"unknown"drug, introduced in a flurry of reports by a team headed by long-time tau researcher Dr. Claude Wischik (http://www.abdn.ac.uk/mediareleases/uploads/files/tau/TauRx_CMW_biog.pdf) (he was part of the team that identified tau as the basis for the paired helical fragment (PHF) of neurofibrillary tangles (NFTs)) and his colleagues at TauRx, is in fact a formulation of the common dye and imaging agent methylthioninium chloride (MTC)/methylene blue (http://en.wikipedia.org/wiki/Methylene_blue). In addition to data from a Phase II human trial of rember with cognitive function as the primary outcome (1), Wischik and colleagues also (a) supported those results with neuroimaging that might suggest a disease-modifying effect;(2,3) (b) reported 2 new animal models of tau pathology that much better recapitulate the NFT neuropathology of human AD than previous models (and support some very interesting aetiological hypotheses) (6,7); and (c) reported some promising-looking results using rember™ in those models (3-5a) .
I. Clinical Results
... rember™ dissolves Tau polymers (Paired Helical Filaments) isolated from AD brain [ in vitro; you can see photos of this (decidedly artificial) process here (http://www.wischik.com/claude/alzheimer.html)-MR], and prevents Tau aggregation in cell models at the nanomolar range (0.15 - 0.58 µM†). MTC has efficacy in Tau transgenic animal models [3-5], reversing cognitive and other behavioural defects, and reversing Tau pathology in the brain.
Methods: An exploratory, dose-range finding, parallel design, double-blind, randomised, placebo-controlled trial of rember™ monotherapy [according to a report from Alzforum (http://www.alzforum.org/new/detail.asp?id=1892), an AD specialist website, "People taking AD drugs, i.e., acetylcholinesterase inhibitors or memantine, were excluded"] was conducted in 332 subjects meeting DSM-IV and NINCDS-ADRDA for probable AD in UK and Singapore [where TauRx is headquartered -MR]. [According to MedPage Today (http://www.printthis.clickability.com/pt/cpt?action=cpt&title=ICAD%3A+Tau-Targeted+Therapy+Slows+Alzheimer%26apos%3Bs+Progre ssion+for+19+Months&expire=&urlID=30054492&fb=Y&url=http%3A%2F%2Fwww.medpagetoday.com%2FMeetingCov erage%2FICAD%2Ftb%2F10320&partnerID=259706), "The dropout rate in each arm of the trial was about 25%, which Dr. Wischik said was about the same as the dropout rate for trials of cholesterase inhibitors." However, as the previously-cited Alzforum story (http://www.alzforum.org/new/detail.asp?id=1892) notes, "the talks excluded certain data that have become standard for pharmaceutical company presentations. For example, it is unclear how many people completed each arm of the trial, and what were the reasons for discontinuation. Other scientists in the field later wondered about how intent-to-treat analysis was handled, and how dropouts may have affected the power of the final data."]
The primary objective was to investigate the effects of oral MTC at 30, 60 and 100 mg three times per day, compared with placebo, on cognitive function (ADAS-cog) in patients with mild or moderate AD stratified by CDR [Clinical Dementia Rating].
ADAS-cog has long been the 'gold standard' for cognitive function in AD trials, although its utility and relationship to real-world cognitive function have been brought into question in recent years. ADAS-cog "measures several cognitive domains, including memory, language and praxis. Total scores range from 0–70, with higher scores (≥ 18) indicating greater cognitive impairment" (8)]. They also included a range of well-validated cognitive and neurological functioning tests, including the Mini-Mental State Exam (MMSE), the Clinical Dementia Rating (http://alzheimer.wustl.edu/cdr/PDFs/CDR_OverviewTranscript-Revised.pdf) scale sum of boxes (CDR-sb -- ie, the sum of all six domains (https://www.cebp.nl/vault_public/filesystem/?ID=1283) of functionality from the CDR, considered a more sensitive measure of global function), the Clinical Global Impression of Change (CGIC), the Alzheimer's Disease Functional Assessment and Change Scale (ADFACS), and the Neuropsychiatric Inventory (NPI)), as secondary outcomes.
Results: ... There was no placebo decline in CDR-mild AD over the first 24 weeks preventing initial efficacy analysis, although efficacy was demonstrated in mild AD by the SPECT-scan outcomes (see (2) below).
In the prespecified analysis at 24 weeks, rember™ produced a significant improvement relative to placebo of -5.4 ADAS-cog units in CDR-moderate subjects at the 60mg [thrice daily] dose.
This result appears quite impressive: while the bar has subsequently been raised, when the cholinesterase inhibitors were approved it was considered adequate to demonstrate any statistically significant effect on ADAS-Cog, and they were approved on 2-4 point effect sizes. In fact, MedPage Today (http://www.printthis.clickability.com/pt/cpt?action=cpt&title=ICAD%3A+Tau-Targeted+Therapy+Slows+Alzheimer%26apos%3Bs+Progre ssion+for+19+Months&expire=&urlID=30054492&fb=Y&url=http%3A%2F%2Fwww.medpagetoday.com%2FMeetingCov erage%2FICAD%2Ftb%2F10320&partnerID=259706) quotes Wischik as saying that this "was twice the effect seen with donepezil (Aricept)".
The abstract is mum about the 30 mg TID dose arm, but according to the Alzforum report, "The low [30 mg TID] dose slope showed a 3.5-point decline," and Dr. Wischik confirmed this in personal communication.
Now, at this point I should clarify some things about the trial's design and analysis, regarding which there has been a lot of confusion in press accounts (even by professional science journalists), for patient clarification of which I am grateful to Dr. Wischik.
The abstract does say that "The 100mg dose was found to have a formulation defect limiting release of the therapeutic form of MTC;" MedPage Today adds that:
Wischik said [rember] was effective when it dissolved in the stomach, but was not effective when the drug was absorbed through the intestines. This was an issue for the 100-mg dose, which had "absolutely no activity because it didn't dissolve in the stomach." Moreover, because the 100-mg dose dissolved in the intestines, it was more likely to cause diarrhea, which was reported by about 30% of patients and was the most common adverse event.
(The Alzforum reporter present at ICAD expands, citing Wischik as indicating that
the major side effects were diarrhea, urinary urgency, and painful urination; there were also some dizziness and falls. Wischik said the side effect profile overall is similar to the three acetylcholinesterase inhibitors that are in wide use, but that diarrhea was more common.
As far as this goes, it is true, and in personal communication Dr. Wischik adds that this was discovered by the end of the 24 week period, but they had thought that since the drug was absorbed and passed into systemic circulation (as demonstrated by the passing of the telltale blue urine), it would be of no import. Instead, as was widely reported, this group was in fact indistinguishable clinically from placebo -- except that there was also some haematological side-effects, which only appeared at this dose (which fact he indicates he also discussed at ICAD), which have previously been observed in the use of this agent for other indications and in industrial exposure.
Dr. Wischik personally communicated a convincing-sounding reason for why a lack of gastric dissolution would modify the pharmacodynamics of the drug, nullifying its pharmacological effects despite its still being absorbed and also increasing the risk of such side-effects -- although he does not have hard pharmacokinetic data from his trial to back it up. I will for present purposes omit discussion of this from this report, but see (16,17) for its basis.
Now, the press either tended to omit, or to actively (though doubtless unintentionally) misrepresent what happened at this point: even the normally-reliable Alzforum, eg, misreports that "rather than analyzing this group for what it was, warts and all, or leaving this group of patients out of the analysis, the investigators instead decided to combine the 100 mg group with the placebo group and compared this pooled set to the 30 mg and 60 groups."
This is not accurate, Dr. Wischik firmly assures me. Instead, as was somewhat ambiguously reported in the Alzheimer's Association's own press release (http://www.alz.org/icad/_release_icad_072908_130pm_trials.asp),"The control group received placebo for the initial 24 weeks and then a minimal efficacy dose subsequently." What happened, in fact, is that for the first 24 weeks, the data for 30, 60, and placebo TID groups are as reported above, and 100 mg TID group was equivalent to placebo in its response. But it was already part of the trial design that at this point the placebo group would be put on 100 mg BID as an active comparator, because of the obvious convenience of twice vs thrice-daily dosing.
This is in fact what they did -- but, again, it appears that the 100 mg formulation was pharmacologically inactive, despite being absorbed, for pharmacodynamic reasons. So instead, what they wound up with (in this analysis) was an ongoing placebo-controlled trial, with an improved placebo because the urine of subjects taking 100 mg BID would turn blue, just like the subjects on the pharmacologically-active 60 mg TID. (Alzforum's misunderstanding of the protocol and analysis extends to this point, too: "methylene blue colors the urine green, raising the question of how a study with this substance can stay blinded at all").
Again, Dr. Wischik has what sounds to my ignorant ears to be a quite convincing explanation for this. If valid, it would actually strengthen the study's conclusions by improving the placebo; moreover, if a small but nontrivial amount were actually absorbed and exerted some longer-term effect, it might even mean that the true drug effect was marginally larger than that reported.
rember ™ [60 mg TID] stabilised the progression of AD over 50 weeks in both mild and moderate AD. The overall effect size was -6.8 ADAS-cog units vs. decline if 7.8 units in the placebo/comparator arm , with significant efficacy demonstrated separately in mild and moderate subgroups. [I]rember™ efficacy was confirmed on all secondary outcomes.(1)
In fact, Dr. Wischik says, there is a minor error in reporting above: the "placebo/comparator arm" actually worsened by 7.2 points, vs a decline of just 0.4 point in 60 mg TID, indicating the same treatment effect but based on what (superstitiously) looks to be an even more convincing observed lack of decline in verum. Moreover, although not reported in the abstract, various (http://www.medpagetoday.com/MeetingCoverage/ICAD/tb/10320) press (http://www.abdn.ac.uk/mediareleases/release.php?id=1444) accounts (http://www.alz.org/icad/_release_icad_072908_130pm_trials.asp) and Dr. Wischik in personal communication confirm that the 60 mg TID group remained stable to the 19 mo mark at the end of the trial.
This result is especially uplifting to the broader AD community, coming as it does in the face of a flurry of recent trials with various agents for AD which have failed to achieve their primary outcomes in Phase II. There is some controversy about what the underlying reason for this might be, but if there is a flaw in current Phase II study design for disease-modifying agents in AD (http://www.alzforum.org/new/detail.asp?id=1890), it is all the more remarkable that rember succeeded so unambiguously (even assuming that we reject the analysis of the 100 mg dose -- see "IV. rember: A Disease-Modifying Agent?" below).
As quoted from the Associated Press story (http://abcnews.go.com/print?id=5473666):
''These are the first very positive results I've seen'' for stopping mental decline, said Marcelle Morrison-Bogorad, director of Alzheimer's research at the National Institute on Aging. ''It's just fantastic.''
Prof Clive Ballard, director of research [of the Alzheimer's Society (http://www.alzheimers.org.uk/site/scripts/news_article.php?newsID=319)], says:
This is a major new development in the fight against dementia. It is the first realistic evidence that a new drug can slow cognition decline in people with Alzheimer's, by targeting the protein tangles that cause brain cell death.
This first modestly sized trial in humans is potentially exciting. It suggests the drug could be over twice as effective as any treatment that is currently available.
However we are not there yet. Larger scale trials are now needed ...
II. Neuroimaging
(2) was a nested regional cerebral blood flow (rCBF)-SPECT study of 138 subjects out of the original 332 (again, remember that there was a 25% dropout rate).
Of 135 suitable for analysis, 52, 15, 32 and 36 received placebo, 30mg, 60mg and 100mg tid respectively. Both ROI (region of interest) and SPM (statistical parametric mapping) image analyses ... showed, as expected, that those receiving placebo had significant reduction in rCBF over a 24 weeks [suggesting reduced regional brain activity -MR]. Although subjects who were CDR-mild at baseline showed no clinical decline over 24 weeks [see again (1)], SPECT scan confirmed significant decline in rCBF in this group, suggesting that a masking of decline by cognitive reserve (http://en.wikipedia.org/wiki/Cognitive_reserve) in mild AD may confound clinical outcome analysis. Those receiving rember showed no significant reduction in rCBF regardless of baseline clinical severity. Comparing the groups indicated that remberTM significantly (p<.05 FDR corrected) reduced the rate of rCBF decline particularly in the hippocampal, medial temporal, and temporal regions The 60 mg dose produced the [I]largest effect.(2)
Note that this last sentence suggests some level of pharmacological activity of both the 30 and 100 mg/d dose. Note also that there was a detected decline in regional cerebral blood flow in the placebo group, even in patients with mild disease (who, as (1) indicates, had no detectable clinical deterioration over the initial 24 weeks of this imaging study), while "Those receiving rember showed no significant reduction in rCBF regardless of baseline clinical severity, consistent with the lack of clinical decline of the active group and (especially when considering the regional specificity) reinforcing the suggestion of an underlying disease-modifying effect through a reduction in tau inclusions.
(3) was a cerebral PET glucose-metabolism study in a small (n=18) subset of the 332 original patients: "7, 8 and 4 subjects received placebo, 60mg and 100mg tid respectively". Comparing baseline to 24 weeks, there was
no significant medial temporal FDG (http://en.wikipedia.org/wiki/FDG) decline in the placebo group. Those receiving rember showed significantly[I] increased FDG uptake bilaterally in the medial temporal lobes. Analysis also showed a significant increase in medial temporal lobe uptake when the rember was compared to the placebo group. The 60 mg dose produced the largest effect.(3)
That last sentence seems to again indicate that they observed an effect from the 100 mg dose. Either way, this study reinforces the rCBF data (2) in suggesting an underlying neurodegenerative process that was ongoing in the placebo-treated patients and arrested by rember therapy. Indeed, it would seem to imply a physiological improvement in brain activity in the treated patients, especially (but but not exclusively?) in the 60 mg TID dose.
A couple of caveats. First, the subgroups used in the glucose-metabolism study were quite small, and in neither imaging study was the basis for selection (and therefore for potential confounding bias) made clear. Indeed, on this latter point, "In both imaging modalities, the data on the posters indicated that the placebo groups were two to five years older than the MTC groups" according to Alzforum.
III. Animal and Cellular Models
(a) Models and Mechanisms
Rodents do not naturally form NFTs, and previous transgenic models have typically used mutant tau species responsible for frontotemporal dementia (http://en.wikipedia.org/wiki/Frontotemporal_dementia) and other tauopathies. By contrast, human tau inclusions are based on the aggregated paired helical fragment (PHF) of wild-type tau. The formation of PHFs is usually attributed to the hyperphosphorylation of the protein; however, Dr. Wischik (19) notes that the methods from which this conclusion have been drawn fail to "permit total PHFtau to be measured irrespective of its state of phosphorylation". By using separate antibodies to detect phosphorylated and unphosphorylated tau species, Wischik reports that (depending on sample preparation method) phosphorylated tau accounts for <5% to ~14% of total PHF-bound tau at any stage in pathology. The remainder of the tau in the PHF appears to be unphosphorylated but truncated tau species, which make up the core of the NFT, with the hyperphosphorylated species accumulating at the periphery.
According to Dr. Wischik's research (personal communication, personal website (http://www.wischik.com/claude/alzheimer.html), TauRx website (http://www.taurx.com/science_aggregation_cascade.aspx), and a patent (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=7,335,505.PN.&OS=PN/7,335,505&RS=PN/7,335,505) for "Materials and methods relating to protein aggregation in neurodegenerative disease "), the NFT originates with one of a range of imperfectly-degraded lipofuscin-type materials -- most often ATP synthase subunit 9, but also core protein II of mitochondrial cytochrome bc1 complex (CP2), presenillin, or porin. These materials act as nucleating agents, sequestering tau and leading to an autocatalytic process of cleavage and polymerization:
Early Tau aggregates first accumulate in the cell as particulate Tau complexes (oligomers). Digestion of the aggregated Tau by proteases leaves intact a core of Tau proteins truncated in the vicinity of His-297 and at Glu-391 at the C-terminus. This forms the proteolytically stable footprint of the Tau-Tau binding domain which is able to capture normal full-length Tau protein with high affinity. Further digestion of the newly bound Tau molecule simply reproduces the proteolytically stable unit, which is able in turn to capture a further Tau molecule, and so on. Thus the process is self-propagating and self sustaining.
http://www.taurx.com/images/kineticModel.jpg
TauRx Website: Science - Tau Aggregation Cascade (http://www.taurx.com/science_aggregation_cascade.aspx)
The apparent requirement for incompletely-degraded mitochondrial components and other typical constituents of lysosomal failure as nucleating agents led to the hypothesis that the formation of PHF and NFT is ultimately due to endosomal-lysosomal failure. On this basis, "An inducible model of Tau aggregation in cell culture and two distinct transgenic mouse lines were generated. In one mouse, a repeat domain Tau fragment was directed to the endoplasmic reticulum to seed an artificial nucleation event. In a second line, mice expressed mutant Tau." (5a) More specifically, the two lines were generated using inducible human tau cDNA constructs under the control of a murine Thy1-promoter and with membrane localisation signal sequences "to nucleate aggregation": one using truncated tau "(amino acids 295-390 of the 441-residue isoform)" and the other using full-length but mutant human tau ("double mutation (P301S/G335D)").(6) P301S is a common frontotemporal dementia FTDP-17 mutation; I am unclear as to the significance of G335D.
There was age-dependent increase in tau-immunoreactivity in the brains from ... mice expressing the repeat domain fragment . The tau aggregates was [I]not in the form of classical filaments seen in AD, as they are neither argyrophilic nor thioflavin-S positive. They are similar to the early stage amorphous aggregates described in AD. The anatomical pattern and progression of the tau-immunoreactivity was similar to that seen in AD, described by Braak staging. ["Regions with abundant pathology in mice expressing mutant tau include visual cortex, hippocampus, entorhinal cortex and subiculum, whereas less abundant pathology in “low count regions” was observed in amygdala, auditory cortex and retrosplenial cortex. "(5a)]
This transgenic mouse line was characterised by subtle behavioural changes observed as early as two months of age, leading to a robust cognitive endophenotype by 6 months. ["Heterozygous and homozygous animals of two lines ... [that] different in transgene expression levels were ... tested in a battery of cognitive and motor tasks at 6, 10 and 15 month of age in both cross-sectional and longitudinal designs. ... [A]nxiety levels were normal. Non-associative memory tested in the open field was impaired paralleled by hyperactivity. Spatial memory was impaired in a gene-dose related manner ... In heterozygous mice, although muscle tone was normal, motor learning/coordination and sensory-motor abilities were compromised. Deficits were less severe in a longitudinal relative to cross-sectional study design, but independent of gene-expression levels." (7)]. In contrast, two lines of mice expressing full-length tau with the double mutation showed tau aggregates that were both argyrophilic and thioflavin-S positive, as well as being strongly labelled with mAb 7/51. Expression of the mutant tau was associated with prominent motoric impairment in these mice.(6)
The mutant tau animals are consistent with previous models using untargeted mutant tau species. However, it's the truncated WT tau that is more interesting. The replication of progression through the brain observed in human aging and AD, and of cognitive rather than motor-function impairments, is much more consistent with human aging and disease; it's unfortunate that they don't form mature NFTs, but the fact that they still exhibit such symptoms from more amorphous neuropathology may suggest (as other research has implied) that the NFT per se is only a minor component of the problem, which may be more due to oligomeric tau species, and/or the sheer sequestering of unmodified WT tau, and/or a toxic fain-of-function of accumulating misprocessed tau species not yet stably incorporated into the NFT, and/or the early impairment of the endosomal-lysosomal system. Alternatively/additionally, the lack of longitudinal progression may indicate that the time-dependent formation of the mature NFT (and, presumably, the associated neuronal death) drives the further clinical progression of the disease in humans.
(b)Effects of rember Treatment
The brains were examined, following 2-3 weeks treatment with oral MTC, for Tau pathology using mAb 7/51.
Results: The pathological processing of Tau in the human brain has been reproduced in a cell model in vitro. MTC inhibits this process with an EC50 of 0.59 mM ... MTC also dissolves PHFs isolated from the brain with an EC50 of 0.13 µM.† Postmortem examination of brain tissue from [treated] mice expressing truncated Tau showed significant reduction in counts of Tau-positive neurons in hippocampus and entorhinal cortex, regions believed to be important in spatial learning tasks. ... Histopathological analysis showed that MTC (2 or 5 mg/kg/day) given intravenously for 17 days reduced pathology in both models. In the mutant Tau mice, the lower dose produced reduction in the low count regions, whereas those areas having greater pathology required a higher dose (5 mg/kg/day).(5a)
This is, obviously, very tantalizing, especially in light of the clinical and neuroimaging results in humans, suggesting as it does that the human effects are caused by the dissociation of PHF by the drug (1-3). It's also a good sign that the release of repeat-domain tau monomers from the PHF core apparently does not itself cause neurotoxic effects, as might otherwise have been feared.
The descriptions of the effects of the IV and oral dosing regimens are not totally clear, but my ab silentio reading is that the oral dosing was sufficient to slow and apparently reverse the accumulation of amorphous tau pathology in the truncated tau mice, but that IV administration (leading, presumably, to higher local concentrations of active drug species) was required to clear out the tangled species in the mutant tau mice. This might be taken to suggest that clearance of mature NFTs will not be attainable using rember (more on this below): it seems unlikely that the investigators would have proceeded to IV administration with 2 doses without trying a higher oral dose first, so that the lack of discussion again seems to suggest that this just couldn't be made to work.
Speculation on human tranlatability, however, is complicated by the fact that the fibrillized NFTs in the mutant mice do not have the same structure as those in 'normal' human aging and AD, whereas the truncated-tau model simply does not form NFT. Even were it to turn out to be as suggested above, however, it seems possible that a regimen of relatively long periods of oral rember to remove amorphous species, combined with less frequent periods of IV administration to remove the slower-forming fibrillized forms, could keep all age-related tau accumulations beneath the threshold of pathology.
And these effects were associated with reversal of tau-induced cognitive impairments:
At 6-8 months, female [mice homozygously TG for the repeat domain fragment of human tau] were assessed in a spatial problem-solving task in the water maze... Spatial training to find a submerged platform proceeded until a criterion was met (3 consecutive trials under 20 sec), upon which animals progressed to the next platform location...
Results: ... At this age, ... Transgenic mice required a considerably longer training regime to acquire criterion for each problem. Two weeks [I]pre-treatment with [oral] MTC (15 and 45 mg/kg) reversed this deficit in the Tau transgenic mice, but had no reliable effect on wild-type controls. ... This problem-solving task has also been shown in PDAPP mice.
That last sentence is ambiguous, but intriguing. PDAPP mice (http://www.alzforum.org/res/com/tra/app/pdapp.asp) are transgenic for mutant human APP under a human platelet derived growth factor-beta chain gene promoter; they produce plaque and some impairments of working and memory, but do not produce NFTs and are not a terribly good phenotypic/behavioral match for human AD. Is the abstract indicating that the effects of rember on "This problem-solving task has also been shown in PDAPP mice"? That would be remarkable in a model that does not exhibit tau pathology but does accumulate Abeta-based lesions and cognitive deficits that presumably bears some connection to human AD . I doubt that this can be the case -- surely, such a result would have been noted in the many press accounts, and explicated and emphasized in the abstract -- but it is difficult to understand why otherwise it would have been noted, except as an offhanded tip of the hat to Wischik's skepticism of any "amyloid hypothesis" for AD.
Moreover, surprisingly, rember was found to reverse deficits in learning and memory induced in WT mice by the anticholinergic drug scopolamine: in fact, it outperformed rivastigmine (Exelon®), whose primary (and likely only meaningful) mechanism of action is precisely as an enhancer of cholinergic neurotransmission through inhibition of acetylcholinesterase:
MTC was administered after scopolamine either at a fully effective dose or at a sub-effective dose co-administered with rivastigmine.
Results: MTC, on a weight basis, was less toxic and more effective than rivastigmine when the drugs were administered singly after scopolamine. Although rivastigmine only reversed the scopolamine-induced deficits in learning, MTC reversed both short- and long-term memory in addition to the learning deficits. Co-administration of sub-effective doses of rivastigmine and MTC elicited a temporal synergy in reversing learning deficits and a full reversal of scopolamine-induced memory impairments at subefficacy doses for each drug individually. There were no non-specific side effects of MTC in this paradigm.(5)
IV. rember™: A Disease-Modifying Agent?
So: we have an agent that dissociates the repeat-domain tau monomers from the PHF core in vitro in cell culture, reduces NFT neuropathology and cognitive and neurological deficits in a convincing-looking transgenic animal model of human age- and AD-related NFT neuropathology, and in a small Phase II trial arrested the progression of clinical AD over 19 mo while the 'placebo-comparator' group declined progressively over the evaluated 50 weeks, leading to a progressive increase in the effect size. In fact, according to the Alzheimer's Association ICAD press release (http://www.alz.org/icad/_release_icad_072908_130pm_trials.asp), a "mixed effects" statistical model of the data showed "an 81 percent reduction of long run rate of progression of decline over 50 weeks (p < 0.0001)." Moreover, this was associated with neuroimaging apparently demonstrating maintainance or even partial restoration of metabolic activity in areas of the brain vulnerable to NFT pathology and dysfunction for the initial 24 weeks (the period for which this was tested).
With the sum of this evidence in hand, Wischik's team quite reasonably takes the results of the trial as "a first approximation to supporting disease modifying efficacy." A drug that only affects symptoms would be expected to yield an initial (and, hopefully (but not necessarily), sustained) improvement of cognitive performance relative to placebo -- but because the underlying neurodegenerative process would be ongoing, the rate of decline would be unaffected, so that absolute cognitive ability would slowly erode at a similar pace in both groups. By contrast, a disease-modifying agent would be expected to slow the underlying rate of decline, resulting a progressive increase in effect size over time.*
http://www.primarypsychiatry.com/userdocs/articleimages/91/1107PP_Kennedy_F_big.gif
Fig 1. Taken from Kennedy, 2007 (18).
In this case, rember appears not merely to have slowed the rate of degeneration (blue line in Kennedy's figure above), but to have nearly arrested it (green line) over the course of one and a half years -- a truly epochal result, if validated in larger Phase III trials with full adherence to the prespecified design and analysis.
There is definitely a faint aroma of cooking going on here, and how we can form a reasonable intention-to-treat analysis is beyond me, but even if we ignore the 'placebo/comparator' group and the effect size, or entertain (with Alzforum) the possibility "that including the highest dose with placebo affected the progression of the true placebo group, inflating the effect size" (which could conceivably happen if 100 mg of the faulty capsules BID was somehow neurotoxic, thereby actually accelerating the rate of their decline and producing an iatrogenic differential in relative cognitive performance) -- even under such an hypothesis, I say, it is frankly hard to credit any other explanation for so protracted an arrest of symptoms (and associated neuroimaging abnormalities) in the 60 mg TID group than a powerful, disease-modifying effect of the drug -- provided that we rule out frank fraud, or a total misdiagnosis and selective randomization of "normally" aging people at baseline.
As Wischik notes (quoted by the Alzheimer Association), the combination of stable clinical status and neuroimaging results, in the face of declines on all fronts in the 'placebo/comparator' group
appear to meet the draft EMEA clinical guidelines for disease-modifying therapy, supported by SPECT and PET evidence of efficacy in brain regions heavily affected by tau pathology
The "draft EMEA clinical guidelines" are the July, 2007 draft Guideline on medicinal products for the treatment of alzheimer’s disease and other dementias (http://www.emea.europa.eu/pdfs/human/ewp/055395endraft.pdf) released by the Committee For Medicinal Products For Human Use of the European Medicines Agency (http://www.emea.europa.eu/), which is the EU's drug-regulatory body ("FDA"). Under this prima facie very reasonable-sounding proposal, a drug would be indicated for disease modification if it could show delayed onset of a milestone disability, as well as evidence of a change in the underlying pathogenesis of the disease through validated biomarkers as surrogate endpoints. The given neuroimaging results are not such biomarkers, but they are a close correlate granted their correspondence to areas vulnerable to NFT pathology and the supporting evidence from the TG mice.
Unfortunately, in any case, there are no validated biomarkers for AD (or PD). The Committee acknowledges this fact, but suggests that they are likely to accept a combination of neuroimaging (eg, PIB labeling of Abeta plaque) and CSF markers (various species of Abeta and forms of tau in the case of AD). Unfortunately, while these markers are reasonably well-validated for diagnosis of AD, the proper interpretation of changing CSF Abeta species (and, to a lesser extent, tau species) as indicators of disease progression remains unresolved, and will therefore present a problem for their use in evaluating drug effects.
Alzforum reports,
During the press briefing and again after his talk, Wischik was asked whether the company had included CSF tau/phospho-tau measurements in the trial. (... Research shows that CSF tau and phospho-tau go up before AD diagnosis and during its course.) Wischik replied that TauRx did not, because prior research made it impossible to predict whether CSF tau should go up or down in response to treatment with MTC, and that he would have had to pre-specify this issue prior to the trial.
This is a real quandary: an agent like rember, that "dissolve[s] Tau aggregates into the truncated repeat domain monomers that comprise the PHF core ... does not affect normal Tau-Tubulin interaction which also occurs through the repeat domain" (7) might conceivably increase CSF tau species, which is normally associated with disease progression and generally interpreted as an indicator of neuronal death and ensuing release of interneuronal phosphotau. However, this might not occur, precisely because the route out of the brain and across the BBB is normally the result of neuronal death and the disease-related compromisation of the BBB (respectively); the former would not apply, and the latter would possibly depend on the degree of progression of the individual patient's disease. Conceivably, the brain might metabolize away the liberated truncated tau monomers; indeed, what exactly happens with such would be a very useful question to explore in Wischik's TG mice. One might speculate at least a transient increase when the drug were first administered, possibly followed over long-term treatment by a gradual decline if brain levels can be normalized or at least reach a new, lower equilibrium. But if MTC mobiizes NFT or PHF out of the brain into the CSF, the resulting rise in tau species might be mistaken for an acceleration of neurodegeneration.
While no details are given, and we cannot be sure that it is not merely aspirational, the TauRx website (http://www.taurx.com/pipeline_diagnostics.aspx) offers a potential way out of this quandary, both for rember specifically and for the flaw in the use of CSF tau species as an AD biomarker generally: the in vivo imaging of NFT:
TauRx Therapeutic Ltd is also developing several lead molecules which it hopes to test in brain scans to measure the build up of tangles. This allows Tau pathology to be diagnosed early and treatment initiated before permanent damage is done.
Overall, the bullish-sounding study abstract conclusions do seem to be warranted:
Conclusions: This represents the first evidence that TAI monotherapy with rember™ is a viable disease-modifying treatment for mild and moderate AD which may also have preventative application at preclinical Braak [Neurofibrillary pathology scoring system] stages of AD.(1)
V. Caveats
As MedPage Today notes, "limitations include the fact that the study was conducted and presented by Dr. Wischik, who is co-founder and chair of TauRx Therapeutics." In fact, from the abstracts, all of the investigators are officers, employees, or consultants of TauRx or WisTa Laboratories. According to Dr. Wischik's biography (http://www.abdn.ac.uk/mediareleases/uploads/files/tau/TauRx_CMW_biog.pdf), these are twin arms of a commercial entity he heads: " Wischik is the Chairman of TauRx, which undertakes clinical development and commercialisation, and of WisTa Laboratories which owns and manages the discovery platform."
Again, the post hoc reinterpretation of the 100 mg BID dose as a placebo group must raise eyebrows, and makes a bit of a joke of intention-to-treat. Alzforum quotes Lon Schneider of USC as expressing skepticism:
“If the investigators could have gotten statistical significance or a statistic with a p value <0.05 with the placebo group alone, then they would have done that. The 100 mg dose was associated with adverse events. It looks like the investigators decided that because it was ineffective, none of it was absorbed, and then they came up with the explanation about drug bound to the matrix of the capsule."
On the other hand, "Most commentators felt that if there is a real treatment effect in this data, even just a kernel, this would be exciting as a general signal that targeting tau aggregation can in fact make people better. " And, once again, it is hard to chalk up a 19-month arrest of diagnosed AD to anything other than a robust, disease-modifying effect.
Alzforum also notes that "Another, difficulty ... is that Dr. Wischik has not published any data on the drug. ... Dr. Wischik said he planned to publish three major papers -- preclinical, imaging, and clinical data -- but he would not do so until after his company had secured an okay from the FDA for a phase III trial.[/QUOTE]
In personal communication, Dr. Wischik indicated that this was a matter of time allocation: he would rather pursue advancing the drug into clinical trials as rapidly as possible, and leave the formal process of peer review and the exchange of ideas to the back seat. If the financial backing of the privately-held company is as solid as seems implied by its chief investor (see below), Dr. Wischik may on this front have the freedom of not being dependent on the normal channels of scientific communication as a means to lend prestige and credibility to either his own academic career, or the venture capitalist community to which a fledgling biotech would normally have to pitch its story.
VI. Going Forward
MedPage Today says,
Dr. Wischik said that a phase III trial with 1,000 to 1,200 patients is being planned, but details await talks with the FDA. He said "my agents have contacted the FDA, and we hope to meet by the end of the year."
According to the University of Aberdeen's press release, "Following these results from the largest Phase 2 clinical trial of a disease modifying treatment conducted to date, TauRx is now planning a Phase 3 trial which should begin next year. If the Phase 3 trial confirms the Phase 2 findings, the drug could be available by 2012."
While this seems an extremely rapid timeline, the Phase II results do suggest that it might take just a few months to show a clinical benefit at least as great as that of established drugs. Moreover, as the MedPage Today interview notes,
At issue, he said, was whether the FDA would require toxicity trials in primates to be conducted prior to a phase III trial or concurrently with phase III. "Because this is an old drug , that has been around for years [with a surprising range of indications (http://en.wikipedia.org/wiki/Methylene_blue#Medicine), from malaria to UTI -MR], we hope that the toxicity study could be done concurrently," he said.
Dr. Wischik elaborated in personal communication that they have subsequently developed a new, stabilized formulation of rember, which keeps rember/MTC in its reduced form and has double the potency of the study formulation (I presume this refers to their animal models -- embarrassingly, my notes and memory fail me here) and none of the observed haematological toxicity (which was seen at 100 mg TID)). However, they have not yet decided whether to use this formulation, or simply to maintain stricter quality control over the existing one, which worked fine when the capsules dissolved properly.
Fortunately, it seems unlikely that progress will be derailed by financial difficulties. Aside from the enormous attention that Dr. Wischik's study has garnered, which could attract more venture capital for future rounds of investment, the Times of London (http://business.timesonline.co.uk/tol/business/industry_sectors/health/article4453874.ece) indicates that the venture is already backed by one of Singapore's two sovereign wealth funds; Singapore, as is now well-known, is aggressively pursuing its establishment as a global hub for biotech, notably reified in the form of .
And there's more in the pipeline, according to TauRx (http://www.taurx.com/pipeline_second.aspx): "Our lead compound rember™ has demonstrated activity on the synuclein fibres which accumulate in the brains of Parkinson’s disease patients. We are currently evaluating rember™ in preclinical and animal models." In fact, their homepage (http://www.taurx.com/) indicates that "TauRx is also planning a trial in Parkinson’s as rember™ also has activity on the synuclein fibres which accumulate in the brains of these patients in the form of Lewy bodies" -- an assertion repeated on the University of Aberdeen's press release (http://www.abdn.ac.uk/mediareleases/release.php?id=1444).
VII. Implications for SENS
Again, these are extremely promising result in themselves; even if "all" rember does is buy some breathing time and prolong the functional lives of patients with a specific AD diagnosis, it will be a wonder-drug. But the potential appears clearly present for its surprising inclusion in the first wave of SENS as the intervention against tau pathology, before a more biotechnologically-sophisticated and fully 'engineering'-based solution. (ie, lysosomal fortification with xenoenzymes (15) and related strategies aimed at the actual removal of existing neurofibrillary tangles (http://www.mfoundation.org/forums/showpost.php?p=1415&postcount=1) as currently envisioned).
As noted, however, it appears possible (ab silentio) that rember is incapable of disaggregating the mature NFT (5a), and the evidence for endosomal-lysosomal failure may indicate a near-term need for such, whatever rember's intrinsic potential.
It seems certain that such robust interventions will be required at some point -- if not as first-line therapy (as currently envisioned, pending confirmation and fuller characterization of the effects of rember) then either as a supplement to deal with relatively advanced (chronologically or disease-specifically) cases, as an occasional supplement for all to deal with smaller amounts of slower-accumulating NFTs, or as part of a later iteration of the SENS platform for the same purpose. It will presumably be a matter of 'when' rather than 'if.'
Alternatively, if for some reason rember will not fit into SENS as a panel of interventions against 'normal' aging, it still seems likely to relieve an enormous amount of misery and suffering for AD patients and their loved ones, and cost from society. This alone would be quite enough for us to owe Dr. Wischik and colleagues a debt of thanks.
-Michael
*Indeed, it's one of the challenges for a long-term, "gerontological" (http://www.mfoundation.org/index.php?pagename=sens_index) disease-modifying agent that it might prove very difficult to prove its efficacy: an agent that [B]only slows the ongoing accumulation of aging damage, thereby reducing the rate of decline, might offer only a very small initial effect size, which might be difficult to detect in any but extremely well-powered trials. This is yet another reason to favor "engineering" approaches to age-related degenerative diseases, which would reverse the accumulation of aging damage (http://www.mfoundation.org/index.php?pagename=sens_index).
†From Alzforum (http://www.alzforum.org/new/detail.asp?id=1892): "For background, a compound’s half-maximal inhibitory concentration (IC50) is used for in-vitro assays ... the assay simply contains a target and a drug, and the investigator can assume that 100 percent of the drug finds the target. The EC50 is typically used in cell culture and is lower [sic: higher] than the IC50 because the compound has to cross the cell membrane and distribute through the cells. The key parameter for humans is the effective dose (ED50). That is because the [pharmacodynamic] component is large as the drug spreads in the body, and only a small fraction finds the desired target, especially if it is in the brain and intracellular. Between the EC50 and the ED50, there can be two to three orders of magnitude difference. Because of these issues, companies typically prefer compounds that start out with an IC50 in the low nanomolar range.
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