The difficulty in developing drugs for specific subtypes of some neurotransmitter receptors is that the transmitter binding site is normally highly conserved. One way around this is to design molecules that modulate a putative allosteric site(s) in the receptor. This is the take-home message of the paper of Jones et al., 2008, that describes some interesting effects in vitro and in vivo of TBPB, a novel allosteric activator of the M1 muscarinic receptor (mAChR). The authors show a high selectivity of TBPB for the rat M1 mAChR as compared to the human M2-M5 mAChR subtypes using one readout in vitro, namely agonist-evoked increases of intracellular calcium ions. Furthermore, TBPB, like other selective and non-selective M1 agonists, elevates α-APPs in cell-based assays that contain mainly the M1 mAChR (in this study, PC12 cells co-transfected with human M1 mAChR and APP Swedish mutation). In another interesting aspect of this paper, the authors describe the effects of TBPB in animal models that may have some predictive value for the treatment of symptoms associated with...  Read more

The difficulty in developing drugs for specific subtypes of some neurotransmitter receptors is that the transmitter binding site is normally highly conserved. One way around this is to design molecules that modulate a putative allosteric site(s) in the receptor. This is the take-home message of the paper of Jones et al., 2008, that describes some interesting effects in vitro and in vivo of TBPB, a novel allosteric activator of the M1 muscarinic receptor (mAChR). The authors show a high selectivity of TBPB for the rat M1 mAChR as compared to the human M2-M5 mAChR subtypes using one readout in vitro, namely agonist-evoked increases of intracellular calcium ions. Furthermore, TBPB, like other selective and non-selective M1 agonists, elevates α-APPs in cell-based assays that contain mainly the M1 mAChR (in this study, PC12 cells co-transfected with human M1 mAChR and APP Swedish mutation). In another interesting aspect of this paper, the authors describe the effects of TBPB in animal models that may have some predictive value for the treatment of symptoms associated with schizophrenia. While clearly this paper is an important step forward in our understanding of M1 mAChR activation, its allosteric site(s), and the important role of this receptor in affective disorders such as schizophrenia, several issues need to be addressed regarding this paper and development of M1 agonists for treatment of CNS diseases such as Alzheimer’s (AD) or schizophrenia, in general:

1. We have succeeded over the years in developing functionally selective M1 agonists of the AF series (e.g., AF102B, AF150(S), AF267B, and more) with high specificity and ability to cross the blood-brain barrier (see attached papers). AF267B, for example, is orally available, penetrates the blood-brain barrier, and has an excellent pharmacokinetic profile both in preclinical and clinical studies, a high bioavailability, and a wide safety margin. AF267B (originally from our lab, >99.9 percent chemical and enantiomeric purity) was shown to reduce both Aβ and tau pathologies in the hippocampus and cortex, and to reverse cognitive deficits in 3xTg-AD mice (Caccamo et al., 2006). These effects of AF267B were observed without side effects at 1 and 3 mg/kg (ip, daily administration for two months), respectively—doses 45 or 15 times lower than those in which overt effects such as salivation were seen. This showed clearly that the compound has a wide safety margin and that the beneficial effects of AF267B can be attributed to M1 mAChR activation. Furthermore, in the same study, reduced M1 mAChR activation (by dicyclomine, a relatively selective M1 antagonist) increased Aβ production, tau hyperphosphorylation, and cognitive deficits in the same model.

2. Different receptor reserves of mAChR subtypes in cell cultures and different experimental design can provide results that may differ from one lab to another. Given this caveat and the difficulty in predicting whether a putative M1 agonist, be it orthosteric or allosteric, will be solely selective for the M1 mAChR both in vitro and in vivo, one has to use several tests in order to obtain a valid estimate, and of course the final test will be in clinical studies. In this context, AF267B (originally from our lab, >99.9 percent chemical and enantiomeric purities) emerged in our studies as a highly selective functional M1 agonist as evidenced, inter alia, in cell cultures stably transfected with the human M1-M5 mAChR subtypes by mobilization of intracellular calcium ions (EC50 = 1.7 μM [almost full agonist vs. carbachol] with marginal effects [20-10 percent] on M3 and M5 mAChR and no effects on M2 and M4 mAChR subtypes when tested at 100 μM). AF267B emerged also as a selective M1 partial agonist vs. its effect on M3 and M5 mAChR as assayed by an increase in phosphoinositides turnover in the same cell cultures. Additionally, in cell cultures such as PC12 cells stably transfected with rat M1 mAChR, AF267B was a partial agonist (readout: phosphoinositides elevation or arachidonic acid release, but inactive on cAMP release mediated by the M1 mAChR). Moreover, AF267B was inactive as an agonist on cell cultures that lack a significant population of the M1 mAChR, but still have intrinsic M3 mAChR (e.g., PC12 or SH-SY5Y cells). For in vivo selectivity of AF267B toward M1 mAChR-mediated functions, see point 1 above.

3. Regarding the allosteric agonist TBPB, the authors emphasize its value in the non-amyloidogenic processing of APP as mediated by M1 mAChR. This is an important finding that confirms studies with other muscarinic agonists, including the functionally selective M1 agonists from the AF series. However, effects of TBPB in cell culture or brain slices that contain several mAChR subtypes were not reported in the paper. Notably, in such preparations the M1 agonists of the AF series appeared even more potent than the non-selective agonist carbachol most probably due to their selective effects on the M1 mAChR.

4. By binding to allosteric sites on the receptor, allosteric modulators can sometimes be more selective than orthosteric agonists for the M1 vs. M2-M5 mAChR. However, paradoxically one cannot disregard the scenario that such an allosteric agonist will bind or modulate other G protein-coupled receptors (GPCR) in spite of its excellent selectivity for the M1 mAChR. While the orthosteric site is the target that dictates the selectivity of a given neurotransmitter for a receptor including its subtypes, the exact orthosteric site is not found in other GPCRs. However, we lack enough compelling information for whether the allosteric site(s) for one receptor (e.g., the M1 mAChR) is found exclusively in this particular receptor and cannot be found in some other GPCR. Thus, while the orthosteric agonist AF267B was shown to bind specifically only to the mAChR subtypes when tested on a plethora of GPCRs and other receptors and enzymes, it is not clear from the paper whether TBPB is solely specific for the M1 mAChR. In fact, in the range of its full effects on the M1 mAChR, TBPB also binds to the D2 receptors in the μM range.

5. Finally, the clinical value of a selective M1 agonist, be it orthosteric or allosteric, is dictated by its pharmacokinetic profile and bioavailability. In this context, AF267B has an excellent pharmacokinetic profile producing the metabolite, N-des-methyl analog, a highly selective partial M1 agonist, in vitro and in vivo. The pharmacokinetic profile of TBPB was not mentioned, and no effects of low doses were reported in in vivo studies. Notably, based on the lesson learned with xanomeline (highly selective in vitro, yet bad pharmacokinetics and selectivity in vivo) selectivity for a mAChR subtype in vitro is not a guarantee for a good selectivity in vivo or a promising bioavailability/pharmacokinetic profile. Definitively, more studies are required to substantiate the practical value of TBPB and to show whether it has a preclinical profile that can match a good M1 candidate for AD treatment.

References:
Fisher A. M1 muscarinic agonists target major hallmarks of Alzheimer's disease--the pivotal role of brain M1 receptors. Neurodegener Dis. 2008;5(3-4):237-40. 2008. Review. Abstract

Fisher A. Cholinergic treatments with emphasis on M1 muscarinic agonists as potential disease-modifying agents for Alzheimer's disease. Neurotherapeutics, 2008. Abstract

Caccamo A, Oddo S, Billings LM, Martinez-Coria H, Fisher A, LaFerla FM. M1 receptors play a central role in modulating AD-like pathology in transgenic mice. Neuron 2006;49:671-682. Abstract

View all comments by Abraham Fisher

The paper by Jones et al. describes TBPB, a novel compound that represents a new generation of highly specific drugs. The study is important for the AD field because this drug will allow the role of the M1 muscarinic receptor in AD to be more clearly defined, including its potential for AD therapeutics. There is a long and well-known history of research on the cholinergic system in AD that led to the development of cholinesterase inhibitors as approved therapies. However, the therapeutic utility of cholinesterase inhibitors is modest, and because these drugs lead to non-specific activation of many different subtypes of muscarinic and nicotinic receptors, side effects are frequent and tolerability suboptimal. For these reasons, development of selective agonists has been a long sought goal for AD treatment, dating back to the discovery of the cholinergic deficiency in AD. Indeed, a wealth of preclinical and clinical data have supported the prediction that highly specific M1 agonists would be more efficacious for cognitive and behavioral symptoms of AD. Such drugs would also be...  Read more

The paper by Jones et al. describes TBPB, a novel compound that represents a new generation of highly specific drugs. The study is important for the AD field because this drug will allow the role of the M1 muscarinic receptor in AD to be more clearly defined, including its potential for AD therapeutics. There is a long and well-known history of research on the cholinergic system in AD that led to the development of cholinesterase inhibitors as approved therapies. However, the therapeutic utility of cholinesterase inhibitors is modest, and because these drugs lead to non-specific activation of many different subtypes of muscarinic and nicotinic receptors, side effects are frequent and tolerability suboptimal. For these reasons, development of selective agonists has been a long sought goal for AD treatment, dating back to the discovery of the cholinergic deficiency in AD. Indeed, a wealth of preclinical and clinical data have supported the prediction that highly specific M1 agonists would be more efficacious for cognitive and behavioral symptoms of AD. Such drugs would also be expected to be better tolerated with fewer cholinergic side effects than cholinesterase inhibitors. However, the development of selective drugs has been an elusive goal despite decades of intensive efforts by big pharma.

There has been a resurgence of interest in M1 receptor-based therapeutics for AD with the findings of LaFerla's and Fisher's groups in 2006 that AF267B (Caccamo et al., 2006), a purported M1 agonist, reduced amyloid and tau pathologies in the triple transgenic AD mouse model. The current study is important as it represents a true breakthrough in the pharmacology of the muscarinic cholinergic system, with the development of a truly selective M1 agonist termed TBPB. As the authors show, TBPB has anti-amyloidogenic effects on cells, and activates central M1 receptors to produce expected behavioral, cognitive, and neurophysiological benefits. Moreover, the study demonstrates that AF267B is not as selective for M1 as previously believed. Conn's group carefully assesses the specificity of AF267B and shows that this compound also activates M3 receptors—perhaps even better than M1. While this might be a potential beneficial property for anti-amyloid effects, it makes it impossible to tease out the role of M1 vs. M3 from previous studies, and its potential as a drug could be limited by M3 side effects (which are likely a major source of the peripheral cholinergic side effects plaguing current drugs, particularly GI, and can limit dosing). Thus, TBPB and other M1 selective drugs may have improved efficacy and tolerability.

The pharmacological breakthrough comes from targeting the receptor to a different binding site than has been conventionally targeted for cholinergic drugs. TBPB binds to an allosteric binding site, which is more highly divergent among the five closely related muscarinic receptor family members. Other drugs acting at this site have been recently described by Acadia Pharmaceuticals. With this knowledge in hand, one might expect newer generations of highly selective drugs for M1 and the other muscarinic receptor subtypes. Additional studies of these drugs will be necessary to determine if they will be advantageous for clinical applications, including neuroprotection for AD.

View all comments by Allan Levey

M1 receptors play a central role in modulating AD-like pathology in transgenic mice
The activation of M1 and M3 muscarinic receptors have long been regarded as a promising approach for AD therapy, because it was shown that they activate the non-amyloidogenic α-secretase pathway. However, the clinical trials did not support their application in clinical practice. The development of new M1 agonists with higher specificity, which can cross the blood-brain barrier, has been the main aim of the research group of Abraham Fisher for several years. The results with such a newly developed M1 agonist obtained in a triple-transgenic mouse model now provide new hope that these compounds will be more successful in clinical trials.

In the paper by Caccamo et al., the M1 agonist AF267B is shown to reduce both the Aβ and tau pathology in the hippocampus and cortex, and to reverse cognitive deficits. Recently my group, in collaboration with the group of Fred van Leuven (Belgium) has shown that overexpression of the α-secretase ADAM-10 prevents amyloid plaque...  Read more

M1 receptors play a central role in modulating AD-like pathology in transgenic mice
The activation of M1 and M3 muscarinic receptors have long been regarded as a promising approach for AD therapy, because it was shown that they activate the non-amyloidogenic α-secretase pathway. However, the clinical trials did not support their application in clinical practice. The development of new M1 agonists with higher specificity, which can cross the blood-brain barrier, has been the main aim of the research group of Abraham Fisher for several years. The results with such a newly developed M1 agonist obtained in a triple-transgenic mouse model now provide new hope that these compounds will be more successful in clinical trials.

In the paper by Caccamo et al., the M1 agonist AF267B is shown to reduce both the Aβ and tau pathology in the hippocampus and cortex, and to reverse cognitive deficits. Recently my group, in collaboration with the group of Fred van Leuven (Belgium) has shown that overexpression of the α-secretase ADAM-10 prevents amyloid plaque formation and hippocampal defects in an Alzheimer disease mouse model, and in this way a proof of concept has been provided that up-regulation of the α-secretase activity in vivo could preclude the formation of Aβ and alleviate cognitive deficits (Postina et al., 2004). The paper by the LaFerla group and Fisher now moves an important step further by showing convincingly that small compounds, like the newly developed M1 agonist, can reverse AD-like pathology in transgenic mice.

Concerning the up-regulation of the ADAM-17, I would like to respectfully question the results the authors provided. From our own experiments, we know that in brain homogenates, due to the low abundance of ADAMs, it is very difficult to quantitate their amount without enrichment of these glycoproteins by lectin chromatography. As the authors had not enriched these enzymes and do not show the molecular weights of the ADAM enzymes and also of BACE, these experiments concerning the expression of α- and β-secretase are not definitive just yet. One also has to take into account that activation of the α-secretase may be achieved without increasing its protein level. This has just been described in a paper by Kojro et al., 2006. From my point of view, it would also have been interesting to see whether the muscarinic agonist also increased the amount of secreted APP (sAPPα).

In spite of some open questions concerning the mechanism, the authors now have made an important step forward to causal therapy of AD and have provided evidence that compounds activating the non-amyloidogenic α-secretase pathway might have clinical relevance. This work opens a way to new clinical trials with a new generation of M1 agonists.

References:
Kojro E, Postina R, Buro C, Meiringer C, Gehrig-Burger K, Fahrenholz F. The neuropeptide PACAP promotes the {alpha}-secretase pathway for processing the Alzheimer amyloid precursor protein. FASEB J. 2006 Mar;20(3):512-4. Epub 2006 Jan 9. Abstract

Postina R, Schroeder A, Dewachter I, Bohl J, Schmitt U, Kojro E, Prinzen C, Endres K, Hiemke C, Blessing M, Flamez P, Dequenne A, Godaux E, van Leuven F, Fahrenholz F. A disintegrin-metalloproteinase prevents amyloid plaque formation and hippocampal defects in an Alzheimer disease mouse model. J Clin Invest. 2004 May;113(10):1456-64. Erratum in: J Clin Invest. 2004 Aug;114(4):598. 36919

View all comments by Falk Fahrenholz

Caccamo and colleagues did a great job in demonstrating the potentially important role of M1 receptors in treatment of AD.

One question I have based on my limited knowledge is: Does the receptor agonist have to be specific to the M1 receptor? Would agonist(s) with broader specificity have any side effect(s)? Would M1-specific agonist(s) be more beneficial than donepezil?"

In this respect, I would have used a control group treated with donepezil to demonstrate that M1 agonist(s) would be better medications than aceetylcholine esterase inhibitors. Such a control group would also help to clarify the pathophysiological relevance of the 3XTg mice as an AD model.

View all comments by Takaomi Saido

The news that a specific M1 muscarinic receptor agonist can reverse both cognitive deficits as well as the amyloid and tau pathology in this mouse model is most interesting.

I wonder whether the study by Ganzinelli and colleagues (1) explains why you don't see AD in people with schizophrenia. They report that "circulating antibodies from schizophrenic patients interacting with cerebral M1 muscarinic acetylcholine receptors can act as an inducer of m(1) mAChR-mRNA, and neuronal nitric oxide synthase (nNOS) mRNA gene expression of rat frontal cortex." Might these antibodies be expected to reverse the amyloid pathology in AD?

Martin et al. (2) report lower protein levels of nNOS in the Tg2576 transgenic mouse model. Has anyone used nitroglycerine as a nitric oxide donor in AD and, if so, what have been the results?

References:
1. Ganzinelli S, Borda T, Sterin-Borda L. Regulation of m(1) muscarinic receptors and nNOS mRNA levels by autoantibodies from schizophrenic patients. Neuropharmacology. 2006 Mar;50(3):362-371. Epub 2005 Nov 11. Abstract

2. Martin BL, Tokheim AM, McCarthy PT, Doms BS, Davis AA, Armitage IM. Metallothionein-3 and neuronal nitric oxide synthase levels in brains from the Tg2576 mouse model of Alzheimer's disease. Mol Cell Biochem. 2006 Feb;283(1-2):129-37. Abstract

View all comments by Mary Reid

Reply by Abraham Fisher to Takaomi Saido
In reply to Takaomi Saido’s questions: The progression of Alzheimer disease (AD), associated with loss of the cholinergic neurons and decreases in acetylcholine (ACh), limit the therapeutic potential of the FDA-approved acetylcholinesterase inhibitors (AChEIs) such as donepezil, galantamine, rivastigmine, or tacrine. Postsynaptic M1 muscarinic receptors (M1 mAChR) are predominant in cerebral cortex and hippocampus and have a major role in hippocampal-based learning and memory, particularly for short-term memory, which is impaired in AD. As M1 mAChR are relatively preserved in AD, use of M1 muscarinic agonists in AD treatment is rational.

Unlike AChEIs, M1 muscarinic agonists in theory are independent of ACh levels in the brain, and thus less affected by the extent of degeneration of presynaptic cholinergic terminals. Whilst activation of M1 mAChR is advantageous, stimulation of the other mAChR subtypes leads to side effects. Therefore, the ideal M1 muscarinic agonist should be devoid of M2, M3, and M5 agonistic effects....  Read more

Reply by Abraham Fisher to Takaomi Saido
In reply to Takaomi Saido’s questions: The progression of Alzheimer disease (AD), associated with loss of the cholinergic neurons and decreases in acetylcholine (ACh), limit the therapeutic potential of the FDA-approved acetylcholinesterase inhibitors (AChEIs) such as donepezil, galantamine, rivastigmine, or tacrine. Postsynaptic M1 muscarinic receptors (M1 mAChR) are predominant in cerebral cortex and hippocampus and have a major role in hippocampal-based learning and memory, particularly for short-term memory, which is impaired in AD. As M1 mAChR are relatively preserved in AD, use of M1 muscarinic agonists in AD treatment is rational.

Unlike AChEIs, M1 muscarinic agonists in theory are independent of ACh levels in the brain, and thus less affected by the extent of degeneration of presynaptic cholinergic terminals. Whilst activation of M1 mAChR is advantageous, stimulation of the other mAChR subtypes leads to side effects. Therefore, the ideal M1 muscarinic agonist should be devoid of M2, M3, and M5 agonistic effects. Some muscarinic agonists improved cognition and reduced psychotic episodes in AD patients. However, a scarcity of selective M1 muscarinic agonists has limited the clinical use of muscarinic agonists in AD due to side effects observed at higher doses. The failure of most of the tested agonists was due to the compounds’ inadequate M1 selectivity and poor pharmacokinetics, narrow safety margin, and side effects. AF267B is an M1 selective agonist, is orally available, penetrates the blood-brain barrier, and has a high bioavailability and wide safety margin.

M1 mAChR-mediated activation of α-secretase can increase α-APPs, preventing the formation of Aβ. Activation of M3 mAChR also elevates α-APPs, yet selective M1 agonists are preferable to prevent peripheral M3 mediated-side effects (e.g., gastrointestinal). M2 mAChR and M4 mAChR are ineffective in activating α-secretase as tested by α-APPs release, and the M2 mAChR may even have an inhibitory effect.

The multitude of animal models available complicates comparison among studies from various labs. In this context, the 3xTg-AD model offers the possibility to answer fundamental questions in AD pathology and therapeutic strategies. How validated are the animal models? One validation should include studies of AChEIs, even if such compounds are not expected to cause a disease modification (e.g., reduction in Aβ levels and/or tau hyperphosphorylation). Whether AF267B is more beneficial than donepezil cannot be addressed properly since donepezil was not yet tested in the 3xTg-AD mice. Yet, our study and previous findings indicate that AF267B, unlike donepezil, in addition to treatment of cognitive deficits, can be also regarded as a disease modifier. While few studies reported some beneficial effects of donepezil on behavioral deficits (memory and learning) in Tg mice such as Tg2576, APP23, and AD11, none of these studies reported a decrease in Aβ levels and/or tau hyperphosphorylation.

References:
Dong H, Csernansky CA, Martin MV, Bertchume A, Vallera D, Csernansky JG. Acetylcholinesterase inhibitors ameliorate behavioral deficits in the Tg2576 mouse model of Alzheimer's disease. Psychopharmacology (Berl). 2005 Aug;181(1):145-52. Epub 2005 Oct 15. Abstract

Van Dam D, Abramowski D, Staufenbiel M, De Deyn PP. Symptomatic effect of donepezil, rivastigmine, galantamine and memantine on cognitive deficits in the APP23 model. Psychopharmacology (Berl). 2005 Jun;180(1):177-90. Epub 2005 Jan 15. Abstract

Capsoni S, Giannotta S, Stebel M, Garcia AA, De Rosa R, Villetti G, Imbimbo BP, Pietra C, Cattaneo A. Ganstigmine and donepezil improve neurodegeneration in AD11 antinerve growth factor transgenic mice. Am J Alzheimers Dis Other Demen. 2004 May-Jun;19(3):153-60. Abstract

View all comments by Abraham Fisher

The paper by LaFerla and colleagues adds to considerable evidence suggesting that M1 receptor activation leads to decreased amyloidogenic processing of APP while reduced M1 receptor activation, by means of either pharmacological agents or cholinergic lesion, results in increased Aβ production.

We have done considerable work in this field that readers may find interesting (see attached citations for examples). In particular, we have evaluated three of Abraham Fisher’s M1-selective compounds, including AF267B, and found that all three reduced CSF and cortical levels of Aβ in rabbits.

The present conventional wisdom that cholinergic agents are only palliative and do not affect disease progression is challenged by the collective contrary data that has accumulated over the last 10 years. It is likely that cholinergic therapy would have its greatest potential effects on Aβ deposition and disease progression if it were given as primary prevention, since Aβ deposition has already reached a plateau by the time the clinical diagnosis of Alzheimer disease is...  Read more

The paper by LaFerla and colleagues adds to considerable evidence suggesting that M1 receptor activation leads to decreased amyloidogenic processing of APP while reduced M1 receptor activation, by means of either pharmacological agents or cholinergic lesion, results in increased Aβ production.

We have done considerable work in this field that readers may find interesting (see attached citations for examples). In particular, we have evaluated three of Abraham Fisher’s M1-selective compounds, including AF267B, and found that all three reduced CSF and cortical levels of Aβ in rabbits.

The present conventional wisdom that cholinergic agents are only palliative and do not affect disease progression is challenged by the collective contrary data that has accumulated over the last 10 years. It is likely that cholinergic therapy would have its greatest potential effects on Aβ deposition and disease progression if it were given as primary prevention, since Aβ deposition has already reached a plateau by the time the clinical diagnosis of Alzheimer disease is made. (1,2)

References:
1. Beach TG, Walker DG, Roher AE, and Potter PE. Anti-Amyloidogenic Activity of Cholinergic Agents. Drug Dev. Res. 56 (2002) 242-247. 2. Beach TG, Walker DG, Potter PE, Sue LI, Fisher A. Reduction of cerebrospinal fluid amyloid beta after systemic administration of M1 muscarinic agonists. Brain Res. 2001 Jun 29;905(1-2):220-3. Abstract

View all comments by Thomas Beach

The results reported by LaFerla and colleagues are very promising. However, before embarking on large-scale clinical studies with selective M1 agonists, certain points, which might have important implications both with respect to the efficacy and safety of these agents, deserve consideration.

While brain muscarinic (M1) receptor density has generally been found to be preserved across all stages of AD, numerous observations from in-vitro studies indicate a loss of the coupling of cortical M1 receptors to G-proteins, which could limit the efficacy of these agents.

Additionally, the precise origin of the M1 receptor-mediated reductions in CSF Aβ levels, which have been reported in pilot clinical studies with this class of compounds in AD, is not known, and this isolated finding may not necessarily reflect brain M1 receptor activation.

Paradoxically, there is also in-vivo evidence that certain central and peripheral responses to nonspecific M1 agonists may be increased in AD patients (1,2) which could potentially influence the safety of these drugs in this...  Read more

The results reported by LaFerla and colleagues are very promising. However, before embarking on large-scale clinical studies with selective M1 agonists, certain points, which might have important implications both with respect to the efficacy and safety of these agents, deserve consideration.

While brain muscarinic (M1) receptor density has generally been found to be preserved across all stages of AD, numerous observations from in-vitro studies indicate a loss of the coupling of cortical M1 receptors to G-proteins, which could limit the efficacy of these agents.

Additionally, the precise origin of the M1 receptor-mediated reductions in CSF Aβ levels, which have been reported in pilot clinical studies with this class of compounds in AD, is not known, and this isolated finding may not necessarily reflect brain M1 receptor activation.

Paradoxically, there is also in-vivo evidence that certain central and peripheral responses to nonspecific M1 agonists may be increased in AD patients (1,2) which could potentially influence the safety of these drugs in this population.

References:
1. Pomara N, Sitaram N. Science. 1995 Mar 17;267(5204):1579-80; author reply 1580-1. No abstract available. Abstract

2. Pomara N, Stanley M, LeWitt PA, Galloway M, Singh R, Deptula D. Increased CSF HVA response to arecoline challenge in Alzheimer’s disease. J Neural Transm Gen Sect. 90:53-65, 1992. Abstract

View all comments by Nunzio Pomara

The studies reported by Ray and coworkers represent a fundamental advance. They provide further evidence that it will be possible to develop highly selective activators of the M1 muscarinic receptor by targeting allosteric sites. Unlike recently discovered allosteric agonists of M1, i.e., TBPB, AC42, and 77-LH-28-1, BQCA is not an M1 agonist but an allosteric potentiator of responses of M1 to ACh. This is similar to other recently described allosteric potentiators of M1, M4, and M5, and may offer potential advantages as well as disadvantages relative to allosteric agonists. However, BQCA is the first M1 allosteric potentiator that has both exquisite selectivity and pharmacokinetic properties needed for use in studies in animal models that are relevant to Alzheimer disease. Thus, when combined with recently described highly selective systemically active M1 allosteric agonists, this provides the first set of tools to allow systematic studies aimed at determining the optimal properties of a drug that should be advanced as a clinical candidate for treatment of AD and other disorders....  Read more

The studies reported by Ray and coworkers represent a fundamental advance. They provide further evidence that it will be possible to develop highly selective activators of the M1 muscarinic receptor by targeting allosteric sites. Unlike recently discovered allosteric agonists of M1, i.e., TBPB, AC42, and 77-LH-28-1, BQCA is not an M1 agonist but an allosteric potentiator of responses of M1 to ACh. This is similar to other recently described allosteric potentiators of M1, M4, and M5, and may offer potential advantages as well as disadvantages relative to allosteric agonists. However, BQCA is the first M1 allosteric potentiator that has both exquisite selectivity and pharmacokinetic properties needed for use in studies in animal models that are relevant to Alzheimer disease. Thus, when combined with recently described highly selective systemically active M1 allosteric agonists, this provides the first set of tools to allow systematic studies aimed at determining the optimal properties of a drug that should be advanced as a clinical candidate for treatment of AD and other disorders. Also, a highly selective M1 antagonist as well as selective M4 and M5 allosteric potentiators have become available in the last year, which provide an unprecedented set of tools to allow major advances in this important area over the coming years.

View all comments by P. Jeffrey Conn