Michael J. Callahan

In vitro and in vivo evaluation of the subtype-selective muscarinic agonist PD 151832

PD 151832 is a potent partial muscarinic agonist that displays a high level of functional selectivity for the muscarinic m1 receptor subtype, as evidenced by its selective stimulation of PI turnover and cellular metabolic activity in transfected Hm1-CHO cells at concentrations that produce minimal stimulation of other cloned human muscarinic receptors. PD 151832 enhanced the amplification of Hm1-transfected NIH-3T3 cells at concentrations lower than those required to produce similar effects in Hm2 or Hm3-transfected cells. The functional m1 selectivity of PD 151832 is consistent with its improvement of mouse water maze performance at doses far lower than those required to produce peripheral parasympathetic side effects.

Chapter 54: Subtype selective muscarinic agonists: potential therapeutic agents for Alzheimer's disease

Publisher Summary Senile dementia of Alzheimer type (AD) is a progressive neurodegenerative disease of unknown etiology. AD and related dementias are characterized by significant neuronal pathology in discrete cortical and subcortical brain regions. The basal forebrain cholinergic system, among other neurotransmitter systems, is severely affected in Alzheimers disease. Significant loss of forebrain cholinergic neurons accompanied by decreased neocortical choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, the major anabolic and catabolic enzymes for acetylcholine, is consistently seen in the brains of demented subjects. Moreover, reduction in ChAT and AChE activity are correlated with the degree of dementia and severity of neuropathological hallmarks of AD. A close association, therefore, exists between cholinergic biochemical abnormalities and this disease. Replacement of lost cholinergic function should provide palliative relief from the cognitive symptoms accompanying AD. Several approaches to cholinergic replacement have been tried in AD. Clinical studies with cholinesterase inhibitors and acetylcholine releasing agents have shown significant, albeit weak activity. The activity of AChE inhibitors and ACh releasing agents is dependent on the state of forebrain cholinergic neurons. In contrast, postsynaptic muscarinic receptors on cholinoceptive neurons in the neocortex and hippocampus are relatively spared in AD. Agents acting directly at these sites and mimicking the actions of acetylcholine should restore lost cholinergic function and retain efficacy throughout AD. Unfortunately, reducing theory to practice in this area has been difficult and the clinical efficacy of muscarinic agonists in AD is equivocal. The limited clinical utility of muscarinic agonists can be attributed to their propensity to induce unwanted cholinergic-mediated parasympathetic effects— that is, (nausea, hypersalivation, sweating), poor oral bio-availability, short durations of action and perhaps subtype selectivity for receptors mediating unwanted effects. This limited clinical utility should not be surprising since these are old agents that were not specifically designed for the use in treating the cognitive decline associated with aging and dementia. Newer muscarinic agonists with improved efficacy to side effect ratios and optimized durations of action are needed.

Combining tacrine with milameline reverses a scopolamine-induced impairment of continuous performance in rhesus monkeys.

Abstractu2002n Rationale: Cholinomimetic therapy in Alzheimer’s disease (AD) has been hampered by narrow efficacious dose ranges and dose-limiting side effects. These limitations highlight the need for an alternative therapeutic approach for the symptomatic treatment of AD. Objectives: To determine in rhesus monkeys if combined treatment with the acetylcholinesterase inhibitor tacrine (Cognex) and the muscarinic agonist milameline improve behavioral efficacy in a scopolamine-reversal task without potentiating adverse side effects. Methods: Behavioral performance of rhesus monkeys was measured using a continuous performance task. The effects of tacrine and milameline, separately or in combination, were determined following administration of an impairing dose of the anticholinergic scopolamine. In addition, tacrine and milameline were given similarly in the absence of scopolamine to determine the presence of adverse side effects. Results: Tacrine and milameline, separately or in combination, reversed the scopolamine-induced decrease in responses on a continuous performance task. Administered in combination, tacrine and milameline significantly improved performance on this task at lower doses and across a broader dose range than when given separately. In the absence of scopolamine, combined treatment did not potentiate the appearance of side effects or produce adverse events significantly different from those observed with either compound alone. Conclusions: Tacrine and milameline given in combination broadened the range of doses significantly reversing a scopolamine-induced impairment without potentiating adverse side effects.

CI-1017, a functionally M1-selective muscarinic agonist: design, synthesis, and preclinical pharmacology.

The five muscarinic receptor subtypes (M1-M5) are characterized by seven helices that define a transmembrane cavity which serves as the binding pocket for agonists and antagonists. The five cavities appear to be topographically different enough to permit subtype selectivity among antagonists but not among classical agonists which tend to be smaller in size than antagonists. It was reasoned that synthesis of muscarinic agonists longer/larger than their classical counterparts might result in subtype selectivity. M1 subtype selectivity was found in a class of 1-azabicyclo[2.2.1]heptan-3-one, O-(3-aryl-2-propynyl) oximes. One of these, CI-1017, improved spatial memory of hippocampally deficient mice and nbM-lesioned rats at doses of 1.0-3.2 and 0.1-0.3 mg/kg, respectively, while producing parasympathetic side effects only at very high doses (100-178 mg/kg). Additionally, CI-1017 inhibited production of amyloidogenic A beta and increased secretion of soluble APP. Thus, CI-1017, besides treating AD symptomatically, may also retard its progression. CI-1017 has recently completed phase I clinical trials.

Development of M1 subtype selective muscarinic agonists for Alzheimer's disease: Translation of in vitro selectivity into in vivo efficacy

Clinical trials in Alzheimers disease (AD) with first generation muscarinic agonists (e.g., arecoline, oxotremorine, pilocarpine) produced inconsistent results due to poor pharmacokinetic properties and a lack of separation between central and peripheral activities observed with these compounds. Second generation agonists have sought to optimize physicochemical properties, and in most cases, target specific subtypes of muscarinic receptor to overcome these limitations. Based upon receptor distribution in both central and peripheral nervous systems, agonists of the m1/M1 subtype seem to possess the desired profile for AD treatment. For the discovery and characterization of these selective agents, the use of clonal cell lines expressing the five subtypes of muscarinic receptors (m1–m5) has become pivotal. However, their use is not without limitations. Results from functional assays (e.g., activation of second messengers) reliably measures subtype selectivity, whereas receptor binding underestimates selectivity. From a novel series of azabicyclic oximes, PD151832 has been chosen for further development and data obtained with this M1 muscarinic agonist is used to exemplify central and peripheral animal models of cholinergic activity and the ability to translate in vitro subtype selectivity into in vivo efficacy. Drug Dev. Res. 40:133–143, 1997.

PD 142676 (CI 1002), a novel anticholinesterase and muscarinic antagonist

Inhibition of brain acetylcholinesterase (AChE) can provide relief from the cognitive loss associated with Alzheimers disease (AD). However, unwanted peripheral side effects often limit the usefulness of the available anticholinesterases. Recently, we identified a dihydroquinazoline compound, PD 142676 (CI 1002) that is a potent anticholinesterase and a functional muscarinic antagonist at higher concentrations. Peripherally, PD 14276, unlike other anticholinesterases, inhibits gastrointestinal motility in rats, an effect consistent with its muscarinic antagonist properties. Centrally, the compound acts as a cholinomimetic. In rats, PD 142676, decreases core body temperature. It also increases neocortical arousal, as measured by quantitative electroencephalography, and cortical acetylcholine levels, measured by in vivo microdialysis. The compound improves the performance of C57/B10j mice in a water maze task and of aged rhesus monkeys in a delayed match-to-sample task involving short-term memory. The combined effect of AChE inhibition and muscarinic antagonism distinguishes PD 142676 from other anticholinesterases and may be useful in treating the cognitive dysfunction of AD and produce fewer peripheral side effects.

Selective Muscarinic Agonists for Alzheimer Disease Treatment

Replacement therapy in Alzheimer Disease (AD) with muscarinic agonists may be of therapeutic benefit in alleviating certain cognitive deficits associated with the disorder. Based upon this “cholinergic hypothesis” (Bartus et al., 1982), a number of clinical trials were conducted over the last decade with a variety of agonists (Table I). The overall results with these compounds were equivocal with some patients reporting positive responses, some showing no responses, and others unable to complete the trials due to troublesome cholinergic side effects. Two conclusions from these studies were that either the design (e.g. outcome measures) of the trials were insufficient to determine efficacy or that the compounds themselves had major deficiencies which prevented the cholinergic hypothesis from being adequately tested.

In Vitro and In Vivo Effects of a Dual Inhibitor of Acetylcholinesterase and Muscarinic Receptors, CI-1002

The observed loss of cholinergic innervation in the frontal cortex and hippocampus of Alzheimer’s Disease (AD) brains prompted the testing of centrally acting anticholinesterases to restore cholinergic tone and to produce cognitive improvement. The success of the anticholinesterase tacrine (THA, Cognex®) in benefitting the cognitive function of many individuals with AD (Knapp et al., 1994) validates this approach and encourages us to find even better compounds with which to treat the disease. Currently, a number of anticholinesterases, both reversible and irreversible inhibitors of acetylcholinesterase (AChE), are being studied for the treatment of AD (Jaen and Davis, 1993). However, many appear to be limited in their therapeutic usefulness by the production of peripheral cholinergic side-effects at the same drug concentrations that also improve cognitive performance (Kumar and Calache, 1991)). In an effort to overcome this limitation, we synthesized and characterized a series of substituted dihydroquinazolines that show anticholinesterase activity. The most potent of this series, CI-1002 (1,3-dichloro-6,7,8,9,10,12-hexahydroazepino[2,1-b]quinazoline),hasa potency similar to tacrine in inhibiting AChE, but unlike tacrine acts like an antagonist at muscarinic receptors at higher concentrations. The novelty of this dual action prompted us to characterize the in vitro and in vivo effects of CI-1002 to determine if it could be useful in the treatment of AD. Our data suggest that CI-1002 enhances performance of animals on cognitive tasks while producing few, if any, overt peripheral cholinergic side-effects. Thus, the combined effects of CI-1002, as an anticholinesterase and as a muscarinic antagonist, may improve the therapeutic potential of the compound.

Biochemistry, Pharmacodynamics and Pharmacokinetics of CI-1002, A Combined Anticholinesterase and Muscarinic Antagonist

The inhibition of brain acetylcholinesterase (AChE) has proven itself beneficial in treating the dementia associated with Alzheimer’s Disease (AD). A six-month, multicenter clinical trial showed that statistically significant improvements in measures of cognitive function (ADAS-Cog), global assessments by clinicians and caregivers, and activities of daily living occur in AD individuals receiving tacrine (Cognex®), an AChE inhibitor (Knapp et al., 1994). The cognitive changes produced by tacrine accompany a delay in the time to nursing home placement (Knopman et al., 1996). This translates into increasing the time AD patients spend in their homes and delays entrance into costly institutionalized care. The greatest benefit occurs at the 160 mg/day dose of tacrine. However, a number of patients (25%) fail to receive the highest dose because of associated cholinergically mediated side effects (Knapp et al., 1994). Thus, eliminating cholinergic side effects should increase the comfort of those receiving the AChE inhibitor while enlarging the number of AD patients who otherwise would not attain the benefits of higher doses.