Pharmaceutica acta Helvetiae最新文献

Kappa opioid receptors derive their name from the prototype benzomorphan, ketocyclazocine (1a) which was found to produce behavioral effects that were distinct from the behavioral effects of morphine but that were antagonized by the opioid antagonist, naltrexone. Recent evidence suggests that agonists and antagonists at kappa opioid receptors may modulate the activity of dopaminergic neurons and alter the neurochemical and behavioral effects of cocaine. Kappa agonists blocked the effects of cocaine in squirrel monkeys in studies of cocaine discrimination and scheduled-controlled responding. Studies in rhesus monkeys suggested that kappa opioids may antagonize the reinforcing effects of cocaine. These studies prompted the synthesis and evaluation of a series of kappa agonists related to the morphinan, l-cyclorphan (3a) and the benzomorphan, l-cyclazocine (2). We describe the synthesis and preliminary evaluation of a series of morphinans, structural analogs of cyclorphan 3a–c, the 10-keto morphinans 4a and b, and the 8-keto benzomorphan 1b, structurally related to ketocyclazocine (1a). In binding experiments l-cyclorphan (3a), the cyclobutyl (3b), the tetrahydrofurfuryl 3c and the 10-keto 4b analogs had high affinity for mu (μ), delta (δ) and kappa (κ) opioid receptors. Both 3a and 3b were more selective for the κ receptor than the μ receptor. However, 3b was 18-fold more selective for the κ receptor in comparison to the δ receptor, while cyclorphan (3a) had only a 4-fold greater affinity for the κ receptor in comparison to the δ receptor. The cyclobutyl compound 3b was found to have significant μ agonist properties, while 3a was a μ antagonist. All compounds were also examined in the mouse tail flick and writhing assay. Compounds 3a and 3b were κ agonists. Correlating with the binding results, compound 3a had some δ agonist properties, while 3b was devoid of any activity at the δ receptor. In addition, compounds 3a and 3b had opposing properties at the μ opioid receptor. The cyclobutyl compound 3b was found to have significant μ agonist properties, while 3a was a μ antagonist.

Multiple populations of pentameric nicotinic acetylcholinergic (nACh) receptors exist and several may be classified as central or neuronal. Neuronal nACh receptors, however, are primarily of the α₄β₂ and α₇ types, and these have been the focus of most recent investigations aimed at the development of novel agents and identification of pharmacophores. Selectivity data are limited. Furthermore, because several populations of nACh receptors might indirectly influence a given functional effect, it is difficult to discuss structure–activity relationships (SAR) in terms of differential SAR, or to formulate SAR on the basis of functional studies. For the most part, studies are limited to the formulation of structure–affinity relationships (SAFIR) for the binding of agents at nACh receptors, and for these the α₄β₂ population has been the most extensively investigated. SAFIR and newer agents are reviewed here with reference to earlier studies. Novel agents now have been identified that bind with up to 30 times higher affinity than nicotine and these are providing new insight into the understanding of nACh receptors.

Cholinergic neurons degenerate in Alzheimer's disease, resulting in cognitive impairments and memory deficits, and drug development efforts have focused on selective M₁ muscarinic agonists. 5-(3-Ethyl-1,2,4-oxadiazol-5-yl)-1,4,5,6-tetrahydropyrimidine trifluoroacetic acid (CDD-0102) stimulates M₁ muscarinic receptors in rat brain [Messer, W.S., Jr., Abuh, Y.F., Liu, Y., Periyasamy, S., Ngur, D.O., Edgar, M.A., El-Assadi, A.A., Sbeih, S., Dunbar, P.G., Roknich, S., Rho, T., Fang, Z., Ojo, B., Zhang, H., Huzl, J.J., III, Nagy, P.I., 1997a. J. Med. Chem. 40, 1230–1246.] and improves memory function in rats with lesions of the basal forebrain cholinergic system. Moreover, CDD-0102 exhibits oral bioavailability, few side effects and low toxicity, and thus represents a viable candidate for clinical studies. Despite the development of functionally selective agonists such as xanomeline and CDD-0102, there is room for improvements in ligand affinity and selectivity. The high degree of amino acid homology within transmembrane domains has hindered the development of truly selective agonists. Site-directed mutagenesis, biochemical and molecular modeling studies have identified key amino acid residues such as Thr192 and Asn382 in the binding of agonists to M₁ receptors [Huang, X.P., Nagy, P.I., Williams, F.E., Peseckis, S.M., Messer, W.S., Jr., 1999. Br. J. Pharmacol. 126, 735–745.]. Recent work has implicated residues at the top of transmembrane domain VI in the binding of muscarinic agonists and activation of M₁ receptors [Huang, X.P., Williams, F.E., Peseckis, S.M., Messer, W.S., Jr., 1998. J. Pharmacol. Exp. Ther. 286, 1129–1139.]. Thus, residues such as Ser388 represent molecular targets for the further development of agonists with improved M₁ receptor affinity, selectivity and activity.

Several recent advances are leading to a better understanding of sigma receptors. Here we focus on our recent findings regarding cellular functions of sigma-2 receptors and discuss their possible clinical implications. Agonists at sigma-2 receptors induced changes in cell morphology and apoptosis in various cell types. Sigma-2 receptor activation produced both transient and sustained increases in [Ca⁺⁺]i, derived from different intracellular stores. These changes in [Ca⁺⁺]i and cytotoxic effects are mediated by intracellular sigma-2 receptors. Sigma-2 agonists induced apoptosis in drug-resistant cancer cells, enhanced the potency of DNA damaging agents, and down-regulated expression of p-glycoprotein mRNA. Thus, sigma-2 receptor agonists may be useful in treatment of drug-resistant cancers. Sigma radioligands have been used in tumor imaging. We also discuss how sigma-2 antagonists might prevent the irreversible motor side effects of typical neuroleptics. Sigma-2 receptors may subserve a novel signalling pathway to apoptosis, involved in regulation of cell proliferation and/or viability.

Advances in the understanding of the structure, function, and distribution of central nervous system (CNS) nicotinic receptors has provided the impetus for new studies examining the role(s) that these receptors and associated processes may play in CNS functions. Further motivation has come from the realization that such receptors are changed in degenerative neurologic diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Ongoing investigations of the molecular substructure of CNS nicotinic receptors and their pharmacology have begun to open up new possibilities for novel CNS therapeutics with nicotinic agents. Exploiting these possibilities will require understanding of the role(s) that these receptor systems play in human cognitive, behavioral, motor, and sensory functioning. Clues from careful studies of human cognition and behavior are beginning to emerge and will provide direction for studies of potentially therapeutic novel nicotinic agents. Modulation of these receptors with the ultimate goal of producing therapeutic benefits is the goal of these investigations and drug development. This paper will review studies from our laboratory and others that point to the importance of CNS nicotinic mechanisms in normal human cognitive and behavioral functioning as well as their role in disease states. In addition, this paper will examine potential clinical applications of nicotine and/or nicotinic agonists in a variety of CNS disorders with particular emphasis on structural brain disease including: movement disorders such as Parkinson's disease and Tourette's syndrome, cognitive/behavioral disorders such as Alzheimer's disease, attention deficit/hyperactivity disorder, and schizophrenia, and other more speculative applications. Important results from early therapeutic studies of nicotine and/or nicotinic agonists in these disease states are presented. For example, recent studies with nicotine and novel nicotinic agonists such as ABT-418 by our group in AD patients suggest that nicotinic stimulation can improve the acquisition and retention of verbal information and decrease errors. Preliminary results from a series of studies examining the acute and subchronic quantitative effects of nicotine on cognitive and motor functioning in Parkinson's disease suggest that acute nicotine administration and stimulation improves some aspects of cognitive and motor performance and may improve the processing speed of more complex tasks. The most likely near-term applications of novel nicotinic agonists in CNS disorders are likely to be in those disorders that are degenerative in nature, e.g. Parkinson's disease and Alzheimer's disease, or other movement disorders such as Tourette's syndrome. The most likely direct therapeutic role for nicotinic agonists is as augmentation therapy in combination with other agents rather than as monotherapy, except earl