The observed improvements in ADAS-Cog and CIBIC+ following treatment with xanomeline provide the first evidence, from a large-scale, placebo-controlled clinical trial, that a direct-acting muscarinic receptor agonist can improve cognitive function in patients with AD. Furthermore, the dramatic and favorable effects on disturbing behaviors in AD suggest a novel approach for treatment of noncognitive symptoms.

Members of the muscarinic acetylcholine receptor family (M1-M5) are known to be involved in a great number of important central and peripheral physiological and pathophysiological processes. Because of the overlapping expression patterns of the M1-M5 muscarinic receptor subtypes and the lack of ligands endowed with sufficient subtype selectivity, the precise physiological functions of the individual receptor subtypes remain to be elucidated. To explore the physiological roles of the M2 muscarinic receptor, we have generated mice lacking functional M2 receptors by using targeted mutagenesis in mouse embryonic stem cells. The resulting mutant mice were analyzed in several behavioral and pharmacologic tests. These studies showed that the M2 muscarinic receptor subtype, besides its well documented involvement in the regulation of heart rate, plays a key role in mediating muscarinic receptor-dependent movement and temperature control as well as antinociceptive responses, three of the most prominent central muscarinic effects. These results offer a rational basis for the development of novel muscarinic drugs.Muscarinic acetylcholine receptors are known to regulate numerous fundamental physiological processes, including the muscarinic actions of acetylcholine on peripheral effector tissues and a multitude of central sensory, vegetative, and motor functions (1-4). In addition, disturbances in central muscarinic neurotransmission have been implicated in a variety of pathophysiological conditions, including Alzheimer's and Parkinson's diseases (1-4).Molecular cloning studies have revealed the existence of five molecularly distinct muscarinic receptor subtypes referred to as M1-M5 (5-7). The M1-M5 receptors are prototypical members of the superfamily of G protein-coupled receptors. Although the odd-numbered muscarinic receptor subtypes (M1, M3, and M5) are selectively linked to G q͞11 proteins, the even-numbered receptors (M2 and M4) are preferentially coupled to G proteins of the G i͞o family (5-7).The M1-M4 receptors are widely expressed throughout the central nervous system and the body periphery (6,(8)(9)(10). Studies with subtype-selective antibodies and in situ mRNA hybridization experiments have shown that most brain regions express several different muscarinic receptor subtypes (8-10). Based on this observation, it has been extremely difficult to assign specific central functions to individual muscarinic receptor subtypes.In addition, the lack of muscarinic agonists and antagonists with pronounced subtype selectivity also has represented a major limitation in studying the physiological roles of the M1-M5 receptors (5-7). This problem is accentuated further in the case of in vivo studies in which the actual concentrations of drugs at their sites of action are difficult to determine because of pharmacokinetic factors.In the body periphery, muscarinic receptors mediate the well known functions of acetylcholine at parasympathetically innervated effector organs, including contraction of smooth muscle, stimulatio...

Members of the muscarinic acetylcholine receptor family (M 1 -M 5 ) are widely expressed in the central nervous system and in the body periphery (1-7). Central muscarinic receptors are known to play key roles in memory and learning as well as in the regulation of many sensory, motor, and autonomic processes (1-3). In the body periphery, muscarinic receptors mediate the well known activities of acetylcholine released from parasympathetic nerves (1-3).Based on the overlapping expression patterns of the different muscarinic receptor subtypes (4-7) and the lack of ligands that display a high degree of receptor subtype selectivity (8, 9), the precise functional roles of the individual muscarinic receptor species remain to be determined. To address this issue, we therefore decided to use gene-targeting technology to generate mouse lines deficient in individual muscarinic receptor subtypes. In this study, we present an initial pharmacologic analysis of a mouse line that lacks functional M 4 muscarinic receptors.At the molecular level, the M 4 receptor subtype, like the structurally closely related M 2 receptor, couples to G proteins of the G i ͞G o family (8, 9). M 4 receptors are expressed abundantly in the striatum (caudate-putamen) and are also present, though at lower levels, in several other brain regions including cerebral cortex and hippocampus (4-7, 10, 11). In the striatum, a region known to be critically involved in extrapyramidal motor control, the M 4 as well as other muscarinic receptor subtypes are coexpressed with D1 and D2 dopamine receptors on striatal projection neurons (12-16). Considerable evidence suggests that complex interactions between these two neurotransmitter receptor systems are critical for the proper regulation of motor control (2,17,18). Consistent with this notion, the severe motor deficits observed in patients suffering from Parkinson's disease and other extrapyramidal motor disorders are thought to reflect an imbalance between muscarinic cholinergic and dopaminergic tone in the striatum (2, 17). To shed light on the potential interactions of M 4 muscarinic receptors with the different dopamine receptor subtypes, we have investigated systematically the locomotor effects induced by administration of D1-and D2-selective ligands in wild-type and M 4 receptor knockout (KO) mice.Interestingly, recent pharmacologic studies suggested that the M 4 receptor subtype may be responsible for mediating muscarinic receptor-dependent antinociception (19,20). Thus, another main focus of the present study was to study muscarinic agonistinduced analgesic effects in the M 4 receptor KO mice. In addition, we also examined the potential involvement of the M 4 receptor subtype in muscarinic receptor-mediated tremor, salivation, and hypothermia, three of the most striking effects caused by administration of centrally acting muscarinic agonists (21).We demonstrate that M 4 receptor-deficient mice show an increase in basal locomotor activity and are hypersensitive to the stimulatory locomotor effects of...

Effective treatments for primary brain tumors and brain metastases represent a major unmet medical need. Targeting the CDK4/CDK6-cyclin D1-Rb-p16/ink4a pathway using a potent CDK4 and CDK6 kinase inhibitor has potential for treating primary central nervous system tumors such as glioblastoma and some peripheral tumors with high incidence of brain metastases. We compared central nervous system exposures of two orally bioavailable CDK4 and CDK6 inhibitors: abemaciclib, which is currently in advanced clinical development, and palbociclib (IBRANCE; Pfizer), which was recently approved by the U.S. Food and Drug Administration. Abemaciclib antitumor activity was assessed in subcutaneous and orthotopic glioma models alone and in combination with standard of care temozolomide (TMZ). Both inhibitors were substrates for xenobiotic efflux transporters P-glycoprotein and breast cancer resistant protein expressed at the blood-brain barrier. Brain K p,uu values were less than 0.2 after an equimolar intravenous dose indicative of active efflux but were approximately 10-fold greater for abemaciclib than palbociclib. K p,uu increased 2.8-and 21-fold, respectively, when similarly dosed in P-gp-deficient mice. Abemaciclib had brain area under the curve (0-24 hours) K p,uu values of 0.03 in mice and 0.11 in rats after a 30 mg/kg p.o. dose. Orally dosed abemaciclib significantly increased survival in a rat orthotopic U87MG xenograft model compared with vehicle-treated animals, and efficacy coincided with a dose-dependent increase in unbound plasma and brain exposures in excess of the CDK4 and CDK6 K i values. Abemaciclib increased survival time of intracranial U87MG tumor-bearing rats similar to TMZ, and the combination of abemaciclib and TMZ was additive or greater than additive. These data show that abemaciclib crosses the blood-brain barrier and confirm that both CDK4 and CDK6 inhibitors reach unbound brain levels in rodents that are expected to produce enzyme inhibition; however, abemaciclib brain levels are reached more efficiently at presumably lower doses than palbociclib and are potentially on target for a longer period of time.