Design and synthesis of 2-(2,2-diarylethyl)-cyclamine derivatives as M3 receptor antagonists and functional evaluation on COPD.

Muscarine acetylcholine receptors (mAChRs) regulate a variety of central and peripheral physiological functions and emerge as important therapeutic targets for a number of diseases including chronic obstructive pulmonary disease (COPD). Inspired by two active natural products, we designed and synthesized a series of 2-(2,2-diarylethyl)-cyclamine derivatives for screening M3 mAChR antagonists. On this skeleton, the structural units including N heterocycle, aryl groups and its substituents on aryl were examined and resulted in a clear structure-activity relationships on the M3 mAChR. In general, these 2-(2,2-diarylethyl)-cyclamine derivatives exhibited good to excellent M3 antagonistic potency and receptor selectivity. The most active 5b-C1 had an IC50 value of 3 nM and the most of compound 6 displayed inactivity against histamine H1 receptor closely related to M3. In in vitro and in vivo evaluations of tracheo-relaxation function, some compounds even showed comparable activity to tiotropium bromide, a known blockbuster drug for COPD. Such excellent properties made these novel compounds potential candidates for COPD drug development.

Synthesis and characterization of oxitropium bromide related substances and novel stability indicating HPLC methods.

The European Pharmacopoeia (Ph. Eur.) described two separate HPLC methods for determination of organic impurities in oxitropium bromide, a synthetic anticholinergic agent used by inhalation in the treatment of asthma and other bronchial disorders, and a potentiometric titration assay method which is not a stability indicating method. During synthetic process development and analytical studies of oxitropium; besides known Ph. Eur.-impurities new process related and degradation impurities were determined, identified by LC-MS, synthesized, characterized, and then used in development and validation studies of oxitropium analytical methods. As a result of these studies, a single HPLC related substances method was developed and validated according to international conference on harmonisation (ICH) guidelines for determination of all oxitropium related substances by using an inertsil ODS-4 (250 mm × 4.6 mm, 5 µm) column at 15 °C with 50 µL injection volume at a wavelength of 210 nm with gradient elution of phosphate-buffer/acetonitrile mixture flowing at a rate of 1.2 mL/min during 60 min. Also, a stability indicating HPLC assay method was developed and validated by using an XBridge C18 (150 mm × 4.6 mm, 3.5 µm) column at 25 °C with 10 µL injection volume at a wavelength of 210 nm and with phosphate-buffer/acetonitrile (85/15) mixture flowing at a rate of 1.0 mL/min during 10 min. Stress-testing and stability studies of oxitropium bromide was carried out and samples were analyzed by using newly developed stability-indicating HPLC assay and related substances methods.

An in vitro study on interaction of anisodine and monocrotaline with organic cation transporters of the SLC22 and SLC47 families.

Current study systematically investigated the interaction of two alkaloids, anisodine and monocrotaline, with organic cation transporter OCT1, 2, 3, MATE1 and MATE2-K by using in vitro stably transfected HEK293 cells. Both anisodine and monocrotaline inhibited the OCTs and MATE transporters. The lowest IC50 was 12.9 µmol·L-1 of anisodine on OCT1 and the highest was 1.8 mmol·L-1 of monocrotaline on OCT2. Anisodine was a substrate of OCT2 (Km = 13.3 ± 2.6 µmol·L-1 and Vmax = 286.8 ± 53.6 pmol/mg protein/min). Monocrotaline was determined to be a substrate of both OCT1 (Km = 109.1 ± 17.8 µmol·L-1, Vmax = 576.5 ± 87.5 pmol/mg protein/min) and OCT2 (Km = 64.7 ± 14.8 µmol·L-1, Vmax = 180.7 ± 22.0 pmol/mg protein/min), other than OCT3 and MATE transporters. The results indicated that OCT2 may be important for renal elimination of anisodine and OCT1 was responsible for monocrotaline uptake into liver. However neither MATE1 nor MATE2-K could facilitate transcellular transport of anisodine and monocrotaline. Accumulation of these drugs in the organs with high OCT1 expression (liver) and OCT2 expression (kidney) may be expected.

The molecular basis of oligomeric organization of the human M3 muscarinic acetylcholine receptor.

G protein-coupled receptors, including the M3 muscarinic acetylcholine receptor, can form homo-oligomers. However, the basis of these interactions and the overall organizational structure of such oligomers are poorly understood. Combinations of site-directed mutagenesis and homogenous time-resolved fluorescence resonance energy transfer studies that assessed interactions between receptor protomers at the surface of transfected cells indicated important contributions of regions of transmembrane domains I, IV, V, VI, and VII as well as intracellular helix VIII to the overall organization. Molecular modeling studies based on both these results and an X-ray structure of the inactive state of the M3 receptor bound by the antagonist/inverse agonist tiotropium were then employed. The results could be accommodated fully by models in which a proportion of the cell surface M3 receptor population is a tetramer with rhombic, but not linear, orientation. This is consistent with previous studies based on spectrally resolved, multiphoton fluorescence resonance energy transfer. Modeling studies furthermore suggest an important role for molecules of cholesterol at the dimer + dimer interface of the tetramer, which is consistent with the presence of cholesterol at key locations in many G protein-coupled receptor crystal structures. Mutants that displayed disrupted quaternary organization were often poorly expressed and showed immature N-glycosylation. Sustained treatment of cells expressing such mutants with the muscarinic receptor inverse agonist atropine increased cellular levels and restored both cell surface delivery and quaternary organization to many of the mutants. These observations suggest that organization as a tetramer may occur before plasma membrane delivery and may be a key step in cellular quality control assessment.

Scopine as a novel brain-targeting moiety enhances the brain uptake of chlorambucil.

The blood brain barrier (BBB) represents the biggest challenge for therapeutic drugs to enter the brain. In our study, we selected chlorambucil (CHL), an alkylating agent, as the model therapeutic agent, and used scopine as a novel brain-targeting moiety. Here, we synthesized Chlorambucil-Scopine (CHLS) prodrug and evaluated its brain-targeting efficacy. The tissue distribution study after i.v. injection revealed that the AUC0-t and Cmax of CHLS in the brain were 14.25- and 12.20-fold of CHL, respectively. Specifically, CHLS accumulated in bEnd.3 and C6 cells in an energy-dependent manner. In C6 cells, superior anti-glioma activity with a significantly decreased IC50 of 65.42 nM/mL was observed for CHLS compared to CHL (IC50 > 400 nM/mL). The safety evaluation, including acute toxicity, pathology, and hematology study, showed minimal toxicity toward nontargeting tissues, and also reached a lower systemic toxicity at 5 mg/kg (i.v.). Our results suggested that scopine is a potential brain-targeting moiety for enhancing the brain uptake efficiency of CHL.

Dualsteric muscarinic antagonists--orthosteric binding pose controls allosteric subtype selectivity.

Bivalent ligands of G protein-coupled receptors have been shown to simultaneously either bind to two adjacent receptors or to bridge different parts of one receptor protein. Recently, we found that bivalent agonists of muscarinic receptors can simultaneously occupy both the orthosteric transmitter binding site and the allosteric vestibule of the receptor protein. Such dualsteric agonists display a certain extent of subtype selectivity, generate pathway-specific signaling, and in addition may allow for designed partial agonism. Here, we want to extend the concept to bivalent antagonism. Using the phthal- and naphthalimide moieties, which bind to the allosteric, extracellular site, and atropine or scopolamine as orthosteric building blocks, both connected by a hexamethonium linker, we were able to prove a bitopic binding mode of antagonist hybrids for the first time. This is demonstrated by structure-activity relationships, site-directed mutagenesis, molecular docking studies, and molecular dynamics simulations. Findings revealed that a difference in spatial orientation of the orthosteric tropane moiety translates into a divergent M2/M5 subtype selectivity of the corresponding bitopic hybrids.

Molecular basis for the long duration of action and kinetic selectivity of tiotropium for the muscarinic M3 receptor.

Antagonizing the human M3 muscarinic receptor (hM3R) over a long time is a key feature of modern bronchodilating COPD drugs aiming at symptom relief. The long duration of action of the antimuscarinic drug tiotropium and its kinetic subtype selectivity over hM2R are investigated by kinetic mapping of the binding site and the exit channel of hM3R. Hence, dissociation experiments have been performed with a set of molecular matched pairs of tiotropium on a large variety of mutated variants of hM3R. The exceedingly long half-life of tiotropium (of more than 24 h) is attributed to interactions in the binding site; particularly a highly directed interaction of the ligands' hydroxy group with an asparagine (N508(6.52)) prevents rapid dissociation via a snap-lock mechanism. The kinetic selectivity over hM2R, however, is caused by differences in the electrostatics and in the flexibility of the extracellular vestibule. Extensive molecular dynamics simulations (several microseconds) support experimental results.

Probing biochemical mechanisms of action of muscarinic M3 receptor antagonists with label-free whole cell assays.

Binding kinetics of drugs is increasingly recognized to be important for their in vivo efficacy and safety profiles. However, little is known about the effect of drug binding kinetics on receptor signaling in native cells. Here we used label-free whole cell dynamic mass redistribution (DMR) assays under persistent and duration-controlled stimulation conditions to investigate the influence of the binding kinetics of four antagonists on the signaling of endogenous muscarinic M3 receptor in native HT-29 cells. Results showed that DMR assays under different conditions differentiated the biochemical mechanisms of action of distinct M3 antagonists. When co-stimulated with acetylcholine, tiotropium, a relatively slow binding antagonist, was found to selectively block the late signaling of the receptor, suggesting that acetylcholine attains its binding equilibrium faster than tiotropium does, thereby still being able to initiate its rapid response until the antagonist draws up and fully blocks the signaling. Furthermore, DMR assays under microfluidics allowed estimation of the residence times of these antagonists acting at the receptor in native cells, which were found to be the determining factor for the blockage efficiency of M3 receptor signaling under duration-controlled conditions. This study demonstrates that DMR assays can be used to elucidate the functional consequence of kinetics-driven antagonist occupancy in native cells.

Structure and dynamics of the M3 muscarinic acetylcholine receptor.

Acetylcholine, the first neurotransmitter to be identified, exerts many of its physiological actions via activation of a family of G-protein-coupled receptors (GPCRs) known as muscarinic acetylcholine receptors (mAChRs). Although the five mAChR subtypes (M1-M5) share a high degree of sequence homology, they show pronounced differences in G-protein coupling preference and the physiological responses they mediate. Unfortunately, despite decades of effort, no therapeutic agents endowed with clear mAChR subtype selectivity have been developed to exploit these differences. We describe here the structure of the G(q/11)-coupled M3 mAChR ('M3 receptor', from rat) bound to the bronchodilator drug tiotropium and identify the binding mode for this clinically important drug. This structure, together with that of the G(i/o)-coupled M2 receptor, offers possibilities for the design of mAChR subtype-selective ligands. Importantly, the M3 receptor structure allows a structural comparison between two members of a mammalian GPCR subfamily displaying different G-protein coupling selectivities. Furthermore, molecular dynamics simulations suggest that tiotropium binds transiently to an allosteric site en route to the binding pocket of both receptors. These simulations offer a structural view of an allosteric binding mode for an orthosteric GPCR ligand and provide additional opportunities for the design of ligands with different affinities or binding kinetics for different mAChR subtypes. Our findings not only offer insights into the structure and function of one of the most important GPCR families, but may also facilitate the design of improved therapeutics targeting these critical receptors.

The thermosalient phenomenon. "Jumping crystals" and crystal chemistry of the anticholinergic agent oxitropium bromide.

The anticholinergic agent oxitropium bromide possesses rich crystal chemistry, most remarkably exhibiting a strong thermosalient effect ("jumping crystal" effect), a mechanical property with potential applications in organic-based actuators. The thermosalient effect, manifested in forceful jumps of up to several centimeters, was investigated by a combination of structural, microscopic, spectroscopic, and thermoanalytical techniques, providing data on which to base a proposed mechanism for the phenomenon. Direct observation of the effect in a single crystal and structure determination of both phases revealed that the jumping of the crystals is a macroscopic manifestation of a highly anisotropic change in the cell volume. The cell distortion is accompanied by a conformational change of the oxitropium cation, which triggers increased separation between the ion pairs in the lattice at nearly identical separation between the cation and the anion within each ion pair. At the molecular level, the cation acts as a molecular shuttle composed of two rigid parts (epoxy-aza-tricyclic-nonyl portion and phenyl ring) that are bridged by a flexible ester linkage. The structure of the rigid, inert aza-tricyclic portion remains practically unaffected by the temperature, suggesting a mechanism in which the large, thermally accumulated strain is transferred over the ester bridge to the phenyl ring, which rotates to trigger the phase transition. Mechanistic details of the higher temperature solid-state phenomena are also presented. The high-temperature phase can also be obtained by grinding or UV irradiation of the room-temperature phase. In addition, if it is irradiated with UV light in the presence of KBr, the high-temperature phase undergoes intramolecular photochemical rearrangement. Heating the high-temperature phase to slightly below the melting temperature results in an additional solid-state reaction that results in the conversion of the salt to a mixture of neutral compounds.

Biochemical properties, pharmacokinetics and pharmacological response of tiotropium in chronic obstructive pulmonary disease patients.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is a progressive disease with increasing incidence and mortality. Tiotropium is an inhaled long-acting anti-cholinergic for the maintenance treatment of COPD. OBJECTIVE: To review biochemical and pharmacokinetic data on tiotropium and discuss in the context of tiotropium's efficacy and safety in COPD. METHODS: Review of previously done pharmacokinetic studies performed by the manufacturer of tiotropium. Data obtained through peer-reviewed publications and regulatory websites. RESULTS/CONCLUSIONS: The long duration of action with tiotropium is owing to prolonged, competitive binding to M(3) muscarinic receptors. Tiotropium is poorly absorbed following inhalation, which largely limits side effects. Metabolism of absorbed drug is minimal and excretion is largely through the kidneys. Tiotropium is efficacious and well tolerated by patients with COPD.

Tiotropium fumarate: an interesting pharmaceutical co-crystal.

A new salt-co-crystal of tiotropium fumarate with fumaric acid has been discovered, and found to be the most stable solid form of tiotropium fumarate. This type of structure consists of matched cations and anions (a salt) together with a nonionized free acid moiety as the co-former (co-crystal), and is unique amongst the large number of tiotropium salts that have been prepared and characterized. The stoichiometry cation/anion/co-former of 2:1:1 corresponds to a simple polymorph of the 1:1 salt, and its identity as a co-crystal has been established by single-crystal X-ray diffraction with some corroborating evidence from the Raman and infrared spectra. A detailed investigation of the bonding and geometry of the three crystalline forms of the fumarate indicates that the hydrogen bonding motifs are very similar, and that conformational differences arising from the packing of the two thiophene rings into the crystal structure is probably important in determining their relative stabilities. A comparison with the structures of other tiotropium salts indicates that there is a correlation of the dihedral angle between the two tiotropium thiophene rings with the stability of the crystal forms.

Structural elucidation of in vivo and in vitro metabolites of anisodine by liquid chromatography-tandem mass spectrometry.

Liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESIMSn) was employed to investigate the in vivo and in vitro metabolism of anisodine. Feces, urine and plasma samples were collected after ingestion of 20 mg anisodine to healthy rats. Feces and urine samples were cleaned up by liquid-liquid extraction and solid-phase extraction procedures (C18 cartridges), respectively. Methanol was added to plasma samples to precipitate plasma proteins. Anisodine was incubated with homogenized liver and intestinal flora of rats in vitro, respectively, followed by extraction with ethyl acetate. LC-MSn was used for the separation and identification of the metabolites using C18 column with mobile phase of methanol/0.01% triethylamine solution (2 mM, adjusted to pH 3.5 with formic acid) (60:40, v/v). The results revealed that five metabolites (norscopine, scopine, alpha-hydroxytropic acid, noranisodine and hydroxyanisodine) and the parent drug existed in feces. Three new metabolites (dimethoxyanisodine, tetrahydroxyanisodine and trihydroxy-methoxyanisodine) were identified in urine. Four metabolites (norscopine, scopine, hydroxyanisodine and anisodine N-oxide) and the parent drug were detected in plasma. Two hydrolyzed metabolites (scopine and alpha-hydroxytropic acid) were found in rat intestinal flora incubation mixture, and two metabolites (aponoranisodine and anisodine N-oxide) were identified in homogenized liver incubation mixture.

An efficient approach to the asymmetric total synthesis of (-)-anisodine.

-Anisodine (l-6,7-epoxy-3-tropyl-alpha-hydroxytropate), which was isolated from the medicinal plant Scopolia tanguticus Maxim, was the first efficiently prepared using 6-beta-acetyltropine as the starting material via a key step of the Sharpless asymmetric dihydroxylation (AD). The intermediate compounds 10 and 11 showed promising cholinergic activity.

Quaternary ammonium derivatives as spasmolytics for irritable bowel syndrome.

Quaternary ammonium derivatives such as cimetropium, n-butyl scopolammonium, otilonium and pinaverium bromide have been discovered and developed as potent spasmolytics of the gastrointestinal tract. Their pharmacological activity has been proven in both "in vivo" and "in vitro" studies of hypermotility. "In vitro" experiments showed that they possess antimuscarinic activity at nM level but only pinaverium and otilonium are endowed with calcium channel blocker properties. These latter compounds relaxed the gastrointestinal smooth muscle mainly through a specific inhibition of calcium ion influx through L-type voltage operated calcium channels. Molecular pharmacology trials have indicated that pinaverium and otilonium can bind specific subunits of the calcium channel in the external surface of the plasma membrane and in this way they block the machinery of the contraction. Recent evidence showed that otilonium is able to bind tachykinin NK(2) receptors and not only inhibits one of the major contractile agents but can reduce the activation of afferent nerves devoted to the passage of sensory signals from the periphery to the central nervous system. Thanks to their typical physico-chemical characteristics, they are poorly absorbed by the systemic circulation and generally remain in the gastrointestinal tract where they exert the muscle relaxant activity by a local activity. Some differences exists in the absorption among these compounds: both n-butyl scopolammonium and cimetropium are partially taken up in the bloodstream, pinaverium has a low absorption (8-10 %) but is endowed with an excellent hepato-biliary excretion and otilonium, which has the lowest absorption (3 %), is almost totally excreted by faeces. Quaternary ammonium derivatives are widely used for the treatment of irritable bowel syndrome and recent meta-analyses have supported their efficacy in this disease. Due to its therapeutic index, the use of n-butyl scopolammonium is more indicated to treat acute colics than a chronic disease such as irritable bowel syndrome. Taking into consideration the published trials carried out with validated methodology in irritable bowel syndrome, cimetropium and otilonium are the best demonstrated drugs for the improvement in global assessment, pain and abdominal distension.

Effect of purification followed by solubilization of receptor material on quantitative receptor assays for anticholinergic drugs.

In order to optimize quantitative receptor assays for anticholinergics, the different receptor preparations resulting from the purification and the solubilization of the P2 pellet from the calf striatum were evaluated. The dissociation constants for two chemically different anticholinergics, the tertiary amine scopolamine and the quaternary amine oxyphenonium, were calculated from inhibition studies of 3H-NMS binding in buffer and plasma. The Kd values for both anticholinergics were similar for all the membrane-bound receptor preparations (unpurified and the purified P2 pellet) either in buffer or in plasma. More pronounced differences were observed between the membrane-bound and solubilized receptors. By introducing the solubilized receptor as well, differences between the individual anticholinergics appeared. On the one hand, for scopolamine, a gain in sensitivity of 1.5-2.8 in plasma was observed for the solubilized receptor. On the other hand, in the case of oxyphenonium, a dramatic loss in sensitivity (by a factor of about 24) was observed with the solubilized receptor, as compared to the membrane-bound receptor, in buffer. Very interestingly, however, when the solubilized receptor was used in plasma, a lowering of the Kd value was found for both anticholinergics, i.e. the assays became more sensitive. Such an effect (not observed for the membrane-bound receptor) could be obtained only when the percentage of digitonin present in the assay was at least 0.12% (w/v) or higher.

Tiotropium bromide (Ba 679 BR), a novel long-acting muscarinic antagonist for the treatment of obstructive airways disease.

Tiotropium bromide (Ba 679 BR) is a novel potent and long-lasting muscarinic antagonist that has been developed for the treatment of chronic obstructive airways disease (COPD). Binding studies with [3H]tiotropium bromide in human lung have confirmed that this is a potent muscarinic antagonist with equal affinity for M1-, M2- and M3-receptors and is approximately 10-fold more potent than ipratropium bromide. Tiotropium bromide dissociates very slowly from lung muscarinic receptors compared with ipratropium bromide. In vitro tiotropium bromide has a potent inhibitory effect against cholinergic nerve-induced contraction of guinea-pig and human airways, that has a slower onset than atropine or ipratropium bromide. After washout, however, tiotropium bromide dissociates extremely slowly compared with the dissociation of atropine and ipratropium bromide. Measurement of acetylcholine (ACh) release from guinea-pig trachea shows that tiotropium bromide, ipratropium bromide and atropine all increase ACh release on neural stimulation and that this effect is washed out equally quickly for the three antagonists. This confirms binding studies to transfected human muscarinic receptors which suggested that tiotropium bromide dissociates slowly from M3-receptors (on airway smooth muscle) but rapidly from M2 autoreceptors (on cholinergic nerve terminals). Clinical studies with inhaled tiotropium bromide confirm that it is a potent and long-lasting bronchodilator in COPD and asthma. Furthermore, it protects against cholinergic bronchoconstriction for > 24 h. This suggests that tiotropium bromide will be a useful bronchodilator, particularly in patients with COPD, and may be suitable for daily dosing. The selectivity for M3- over M2-receptors may also confer a clinical advantage.

Soft drugs. 13: Design and evaluation of phenylsuccinic analogs of scopolamine as soft anticholinergics.

Concepts involved in the design of soft drugs (drugs which, after achieving their therapeutic role, are metabolized in a predictable manner and at a controlled rate to non-toxic moieties) have been applied to methscopolamine. Selected aliphatic and cycloaliphatic esters (4a-e) of a hypothetical metabolite (2a) of methscopolamine with a phenylsuccinic acid basic structural moiety were designed and found to have anticholinergic activity and to revert to the original hypothetical metabolite in biological media with fairly short half lives. The pA2 values of the soft drugs were determined and the hypothetical metabolite was found 100 times less potent than the most potent compound (4a) of the series. The anticholinergic activities were found to be dependent on the length of the side chain alcohol.

Soft drugs. 12: Design, syntheses and evaluation of soft anticholinergics.

Concepts involved in the design of soft drugs (drugs which, after achieving their therapeutic role, are metabolized in a predictable manner and at a controlled rate to non-toxic moieties) have been applied to methscopolamine (1). Selected aliphatic and cycloaliphatic esters (3) of a hypothetical carboxylic acid metabolite (2) of methscopolamine were designed and found to have anticholinergic activity. Six soft drug analogs of methscopolamine were tested for mydriatic activity in rabbit eye. At equieffective doses, the AUC24 hrs and the mydriatic recovery time were found to be significantly less with some of the soft drugs compared to methscopolamine and soft drug 3a was found to be shorter acting than tropicamide. At equieffective doses the AUC24 hrs for soft drugs ranged from 23.2% to 187% of that of methscopolamine. Significant dilation of the untreated eye was observed with scopolamine but not with the soft drugs after unilateral administration. Soft drug 3a exhibited only 2.3% of the AUC6 hrs (untreated eye) of that of methscopolamine.