Highly Enantioselective Conjugate Addition of Malonate and β-Ketoester to Nitroalkenes: Asymmetric C−C Bond Formation with New Bifunctional Organic Catalysts Based on Cinchona Alkaloids

The development of readily accessible bifunctional chiral catalysts is a desirable yet challenging goal in catalytic asymmetric synthesis. In this communication, we describe the development of a new class of readily accessible chiral bifunctional organic catalysts that could be derived in one or two steps in high yield from either quinidine or quinine. These catalysts have been shown to catalyze a highly enantioselective conjugate addition of methyl and ethyl malonates and beta-ketoesters to a broad range of beta-substituted nitroalkenes, an synthetically important C-C bond-forming reaction utilizing readily available starting materials. This new catalytic asymmetric reaction proceeds in 91-98% ee and 71-99% yield.

Enantioselective anion exchangers based on cinchona alkaloid-derived carbamates: influence of C8/C9 stereochemistry on chiral recognition

Four diastereomeric chiral stationary phases (CSPs) based on quinine, quinidine, epiquinine, and epiquinidine tert-butyl carbamate selectors were synthesized and evaluated under ion exchange HPLC conditions with a set of racemic N-acylated and N-oxycarbonylated alpha-amino acids as selectands. The enantioseparation potential of quinine- and quinidine-derived CSPs proved to be far superior to that of their C9-epimeric congeners. The absolute configuration of C9 stereogenic center of the cinchonan backbone of these selectors was identified as the structural feature controlling the elution order. Guided by an X-ray structure of a most favorable selector-selectand complex and the observed chromatographic enantioseparation data, a chiral recognition model was advanced. The contributions of ion-pairing, pi-pi donor-acceptor, hydrogen bonding and steric interactions were established as crucial factors.

A common mechanism for blockade of heme polymerization by antimalarial quinolines

The antimalarial quinolines are believed to work by blocking the polymerization of toxic heme released during hemoglobin proteolysis in intraerythrocytic Plasmodium falciparum. In the presence of free heme, chloroquine and quinidine associate with the heme polymer. We have proposed that this association of the quinoline-heme complex with polymer caps the growing heme polymer, preventing further sequestration of additional heme that then accumulates to levels that kill the parasite. In this work results of binding assays demonstrate that the association of quinoline-heme complex with heme polymer is specific, saturable, and high affinity and that diverse quinoline analogs can compete for binding. The relative quinoline binding affinity for heme polymer rather than free heme correlates with disruption of heme polymerization. Mefloquine, another important antimalarial quinoline, associated with polymer in a similar fashion, both in cultured parasites and in the test tube. In parasite culture, blocking heme release with protease inhibitor was antagonistic to mefloquine action, as it is to chloroquine action. These data suggest a common mechanism for quinoline antimalarial action dependent on drug interaction with both heme and heme polymer.

Comparative metabolic capabilities and inhibitory profiles of CYP2D6.1, CYP2D6.10, and CYP2D6.17

Polymorphisms in the cytochrome P450 2D6 (CYP2D6) gene are a major cause of pharmacokinetic variability in human. Although the poor metabolizer phenotype is known to be caused by two null alleles leading to absence of functional CYP2D6 protein, the large variability among individuals with functional alleles remains mostly unexplained. Thus, the goal of this study was to examine the intrinsic enzymatic differences that exist among the several active CYP2D6 allelic variants. The relative catalytic activities (enzyme kinetics) of three functionally active human CYP2D6 allelic variants, CYP2D6.1, CYP2D6.10, and CYP2D6.17, were systematically investigated for their ability to metabolize a structurally diverse set of clinically important CYP2D6-metabolized drugs [atomoxetine, bufuralol, codeine, debrisoquine, dextromethorphan, (S)-fluoxetine, nortriptyline, and tramadol] and the effects of various CYP2D6-inhibitors [cocaine, (S)-fluoxetine, (S)-norfluoxetine, imipramine, quinidine, and thioridazine] on these three variants. The most significant difference observed was a consistent but substrate-dependent decease in the catalytic efficiencies of cDNA-expressed CYP2D6.10 and CYP2D6.17 compared with CYP2D6.1, yielding 1.32 to 27.9 and 7.33 to 80.4% of the efficiency of CYP2D6.1, respectively. The most important finding from this study is that there are mixed effects on the functionally reduced allelic variants in enzyme-substrate affinity or enzyme-inhibitor affinity, which is lower, higher, or comparable to that for CYP2D6.1. Considering the rather high frequencies of CYP2D6*10 and CYP2D6*17 alleles for Asians and African Americans, respectively, these data provide further insight into ethnic differences in CYP2D6-mediated drug metabolism. However, as with all in vitro to in vivo extrapolations, caution should be applied to the clinical consequences.

Solution structures of antimalarial drug-heme complexes

Paramagnetic metal centers [such as Fe(III) found within ferriprotoporphyrin IX heme (FPIX)] exert through space effects on the relaxation rate of nearby proton spins that depend critically on the metal-proton distance. We have measured these effects for all protons of several antimalarial drugs that bind to FPIX by systematically varying the drug:heme molar ratio in high field NMR experiments. These measurements allow us to determine precise FPIX Fe-drug H distances for the solution structures of noncovalent complexes formed between FPIX mu-oxo dimers and the antimalarial drugs chloroquine (CQ), quinine (QN), and quinidine (QD). Using these distances, we then performed distance restraint calculations to determine the lowest-energy solution structures of these complexes. Structures were solved for neutral, monoprotic (+1), and diprotic (+2) forms of the drugs. Analysis of these structures allows us to visualize for the first time the stereospecific differences between QN and QD binding to FPIX and the differences in populations of QN and QD solution structures upon changes in digestive vacuolar pH for drug resistant malarial parasites [Dzekunov, S. M., et al. (2000) Mol. Biochem. Parasitol. 110, 107-124]. The data indicate a previously unrecognized key role for the CQ aliphatic chain in stabilizing FPIX-CQ complexes, and suggest how lengthening or shortening the chain might perturb stability. We also define FPIX:drug stoichiometries of 2:1 for the complexes formed at physiological FPIX concentrations, in contrast to the 4:1 and 5:1 stoichiometries previously determined at higher FPIX concentrations [Dorn, A., et al. (1998) Biochem. Pharmacol. 55, 727-736]. These atomic resolution antimalarial drug-heme structures should help elucidate how these drugs inhibit formation of hemozoin during metabolism of heme within the malarial parasite Plasmodium falciparum and assist ongoing development of strategies for circumventing antimalarial drug resistance.

Development of a pharmacophore for inhibition of human liver cytochrome P-450 2D6: molecular modeling and inhibition studies

To gain insight into the specificity of cytochrome P-450 2D6 toward inhibitors, a preliminary pharmacophore model was built up using strong competitive inhibitors. Ajmalicine (1), the strongest inhibitor known (Ki = 3 nM) was selected as template because of its rigid structure. The preliminary pharmacophore model was validated by performing inhibition studies with derivatives of ajmalicine (1) and quinidine (9). Bufuralol (18) was chosen as substrate and the metabolite 1'-hydroxybufuralol (19) was separated by high performance liquid chromatography. All incubations were carried out using human liver microsomes after demonstration that the Ki values obtained with microsomes were in accordance with those obtained with a reconstituted monooxygenase system containing purified cytochrome P-450 2D6. Large differences of Ki values ranging between 0.005 and 100 microM were observed. Low-energy conformers of tested compounds were fit within the preliminary pharmacophore model. The analysis of steric and electronic properties of these compounds led to the definition of a final pharmacophore model. Characteristic properties are a positive charge on a nitrogen atom and a flat hydrophobic region, the plane of which is almost perpendicular to the N-H axis and maximally extends up to a distance of 7.5 A from the nitrogen atom. Compounds with high inhibitory potency had additional functional groups with negative molecular electrostatic potential and hydrogen bond acceptor properties on the opposite side at respective distances of 4.8-5.5 A and 6.6-7.5 A from the nitrogen atom. The superposition of strong and weak inhibitors led to the definition of an excluded volume map. Compounds that required additional space were not inhibitors. This is apparently the first pharmacophore model for inhibitors of a cytochrome P-450 enzyme and offers the opportunity to classify compounds according to their potency of inhibition. Adverse drug interactions which occur when both substrates and inhibitors of cytochrome P-450 2D6 are applied may be predicted.

Practical and highly enantioselective ring opening of cyclic meso-anhydrides mediated by cinchona alkaloids

The cinchona alkaloid-mediated opening of prochiral cyclic anhydrides in the presence of methanol leading to optically active hemiesters is described. Very structurally diverse anhydrides are converted into their corresponding methyl monoesters, and either enantiomer can be obtained with up to 99% ee by using quinine or quinidine as directing additive. After the reaction, the alkaloids can be recovered almost quantitatively and reused without loss of enantioselectivity. Additionally, a catalytic protocol which permits the substoichiometric use of quinidine in the presence of easily accessible pentamethylpiperidine (pempidine) is presented.

High-Throughput Solution-Based Medicinal Library Screening against Human Serum Albumin

High-throughput screening of combinatorial libraries has evolved from studying large diverse libraries to analyzing small, structurally similar, focused libraries. This paradigm shift has generated a need for rapid screening technologies to screen both diverse and focused libraries in a simple, efficient, and inexpensive manner. We have proactively addressed these needs by developing a high-throughput, solution-based method combining size exclusion (SEC), two-dimensional liquid chromatography (2-D LC), and mass spectrometry (MS) for determining the relative binding of drug candidates in small, focused medicinal libraries against human serum albumin (HSA). Two types of libraries were used to evaluate the performance of the system. The first consisted of five diverse ligands with a wide range of hydrophobicities and whose association constants to HSA cover 3 orders of magnitude. A beta-lactam library composed of structurally similar compounds was used to further confirm the validity of the methodology. The ability to distinguish site-specific interactions of drugs competing for individual domains of the HSA receptor is also demonstrated. Comparison of chromatographic profiles of the library components before and after incubation with the receptor using multiple reaction monitoring allowed a ranking of the ligands according to their relative binding affinities. The observed rankings correlate closely with literature values of the association constants between the respective ligands and HSA. This simple, rugged methodology can screen a wide spectrum of chemical entities from combinatorial mixtures in less than 6 min.

Homology modeling of rat and human cytochrome P450 2D (CYP2D) isoforms and computational rationalization of experimental ligand-binding specificities

The ligand-binding characteristics of rat and human CYP2D isoforms, i.e., rat CYP2D1-4 and human CYP2D6, were investigated by measuring IC(50) values of 11 known CYP2D6 ligands using 7-methoxy-4-(aminomethyl)coumarin (MAMC) as substrate. Like CYP2D6, all rat CYP2D isozymes catalyzed the O-demethylation of MAMC with K(m) and V(max) values ranging between 78 and 145 microM and 0.048 and 1.122 min(-1), respectively. To rationalize observed differences in the experimentally determined IC(50) values, homology models of the CYP2D isoforms were constructed. A homology model of CYP2D6 was generated on the basis of crystallized rabbit CYP2C5 and was validated on its ability to reproduce binding orientations corresponding to metabolic profiles of the substrates and to remain stable during unrestrained molecular dynamics simulations at 300 K. Twenty-two active site residues, sharing up to 59% sequence identity, were identified in the CYP2D binding pockets and included CYP2D6 residues Phe120, Glu216, and Asp301. Electrostatic potential calculations displayed large differences in the negative charge of the CYP2D active sites, which was consistent with observed differences in absolute IC(50) values. MD studies on the binding mode of sparteine, quinidine, and quinine in CYP2D2 and CYP2D6 furthermore concurred well with experimentally determined IC(50) values and metabolic profiles. The current study thus provides new insights into differences in the active site topology of the investigated CYP2D isoforms.

Dielectric Relaxation and Crystallization of Ultraviscous Melt and Glassy States of Aspirin, Ibuprofen, Progesterone, and Quinidine

Molecular relaxation in ultraviscous melt and glassy states of aspirin, ibuprofen, progesterone, and quinidine has been studied by dielectric spectroscopy. The asymmetric relaxation spectra is characterized by the Kohlrausch distribution parameter of 0.46 +/- 0.02 for aspirin to 0.67 +/- 0.02 for progesterone. The dielectric relaxation time varies with the temperature, T, according to the Vogel-Fulcher-Tammann Equation, log(10)(tau(0)) = A(VFT) + [B(VFT)/(T - T(0))], where A(VFT), B(VFT), and T(0) are empirical constants. The extrapolated tau(0) at calorimetric glass-softening temperature is close to the value expected. The equilibrium permittivity, epsilon(0), is lowest for ibuprofen which indicates an antiparallel orientation of dipoles in its liquid's hydrogen-bonded structure. A decrease in epsilon(0) with time shows that ultraviscous aspirin, progesterone, and quinidine begin to cold-crystallize at a relatively lower temperature than ibuprofen. epsilon(0) of the cold-crystallized phases are, 4.7 for aspirin at 290 K, 2.55 for ibuprofen at 287 K, 2.6 for progesterone at 320 K, and 3.2 for quinidine at 375 K. It is argued that hydrogen-bonding, the Kohlrausch parameter, extent of localized motions and the long-range diffusion times all determine the physical and chemical stability of an amorphous pharmaceutical during storage.

Characterisation of synthetic beta-haematin and effects of the antimalarial drugs quinidine, halofantrine, desbutylhalofantrine and mefloquine on its formation

Infrared spectroscopy, elemental analysis and X-ray powder diffraction show that the product of 30 min of reaction of haematin in 4.5 M acetate, pH 4.5 at 60 degrees C is identical to beta-haematin prepared in 4.5 M acetic acid at 70 degrees C overnight (pH 2.6). There is no evidence for formation of haem-acetate complex, which could not be isolated, even from 11.4 M acetate solution. The antimalarial drugs quinidine, halofantrine, desbutylhalofantrine and mefloquine were found to inhibit formation of beta-haematin, while 5-, 6- and 8-aminoquinoline and quinoline were found to have no effect. Quinidine was shown to form a complex with ferriprotoporphyrin IX in 40% DMSO with log K = 5.02 +/- 0.03. Log K values for halofantrine and desbutylhalofantrine are 5.29 +/- 0.02 and 5.15 +/- 0.02 respectively (solutions containing 30% acetonitrile in addition to DMSO to solubilise these drugs), which are both stronger than chloroquine under the same conditions (log K = 4.56 +/- 0.02).

Natural product fragment combination to performance-diverse pseudo-natural products

Natural product structure and fragment-based compound development inspire pseudo-natural product design through different combinations of a given natural product fragment set to compound classes expected to be chemically and biologically diverse. We describe the synthetic combination of the fragment-sized natural products quinine, quinidine, sinomenine, and griseofulvin with chromanone or indole-containing fragments to provide a 244-member pseudo-natural product collection. Cheminformatic analyses reveal that the resulting eight pseudo-natural product classes are chemically diverse and share both drug- and natural product-like properties. Unbiased biological evaluation by cell painting demonstrates that bioactivity of pseudo-natural products, guiding natural products, and fragments differ and that combination of different fragments dominates establishment of unique bioactivity. Identification of phenotypic fragment dominance enables design of compound classes with correctly predicted bioactivity. The results demonstrate that fusion of natural product fragments in different combinations and arrangements can provide chemically and biologically diverse pseudo-natural product classes for wider exploration of biologically relevant chemical space.

Why Is Quinidine an Inhibitor of Cytochrome P450 2D6?

We have previously shown that Phe(120), Glu(216), and Asp(301) in the active site of cytochrome P450 2D6 (CYP2D6) play a key role in substrate recognition by this important drug-metabolizing enzyme (Paine, M. J., McLaughlin, L. A., Flanagan, J. U., Kemp, C. A., Sutcliffe, M. J., Roberts, G. C., and Wolf, C. R. (2003) J. Biol. Chem. 278, 4021-4027 and Flanagan, J. U., Maréchal, J.-D., Ward, R., Kemp, C. A., McLaughlin, L. A., Sutcliffe, M. J., Roberts, G. C., Paine, M. J., and Wolf, C. R. (2004) Biochem. J. 380, 353-360). We have now examined the effect of mutations of these residues on interactions of the enzyme with the prototypical CYP2D6 inhibitor, quinidine. Abolition of the negative charge at either or both residues 216 and 301 decreased quinidine inhibition of bufuralol 1'-hydroxylation and dextromethorphan O-demethylation by at least 100-fold. The apparent dissociation constants (K(d)) for quinidine binding to the wild-type enzyme and the E216D and D301E mutants were 0.25-0.50 microm. The amide substitution of Glu(216) or Asp(301) resulted in 30-64-fold increases in the K(d) for quinidine. The double mutant E216Q/D301Q showed the largest decrease in quinidine affinity, with a K(d) of 65 microm. Alanine substitution of Phe(120), Phe(481),or Phe(483) had only a minor effect on the inhibition of bufuralol 1'-hydroxylation and dextromethorphan O-demethylation and on binding. In contrast to the wild-type enzyme, a number of the mutants studied were found to be able to metabolize quinidine. E216F produced O-demethylated quinidine, and F120A and E216Q/D301Q produced both O-demethylated quinidine and 3-hydroxyquinidine metabolites. Homology modeling and molecular docking were used to predict the modes of quinidine binding to the wild-type and mutant enzymes; these were able to rationalize the experimental observations.

Sensing metal ions with DNA building blocks: fluorescent pyridobenzimidazole nucleosides

We describe novel fluorescent N-deoxyribosides (1 and 2) having 2-pyrido-2-benzimidazole and 2-quino-2-benzimidazole as aglycones. The compounds were prepared from the previously unknown heterocyclic precursors and Hoffer's chlorosugar, yielding alpha anomers as the chief products. X-ray crystal structures confirmed the geometry and showed that the pyridine and benzimidazole ring systems deviated from coplanarity in the solid state by 154 degrees and 140 degrees , respectively. In methanol compounds 1 and 2 had absorption maxima at 360 and 370 nm, respectively, and emission maxima at 494 and 539 nm. Experiments revealed varied fluorescence responses of the nucleosides to a panel of 17 monovalent, divalent, and trivalent metal ions in methanol. One or both of the nucleosides showed significant changes with 10 of the metal ions. The most pronounced spectral changes for ligand-nucleoside 1 included red shifts in fluorescence (Au(+), Au(3+)), strong quenching (Cu(2+), Ni(2+), Pt(2+)), and substantial enhancements in emission intensity coupled with red shifts (Ag(+), Cd(2+), Zn(2+)). The greatest spectral changes for ligand-nucleoside 2 included a red shift in fluorescence (Ag(+)), a blue shift (Cd(2+)), strong quenching (Pd(2+), Pt(2+)), and substantial enhancements in emission intensity coupled with a blue shift (Zn(2+)). The compounds could be readily incorporated into oligodeoxynucleotides, where an initial study revealed that they retained sensitivity to metal ions in aqueous solution and demonstrated possible cooperative sensing behavior with several ions. The two free nucleosides alone can act as differential sensors for multiple metal ions, and they are potentially useful monomers for contributing metal ion sensing capability to DNAs.

Role of glutamic acid 216 in cytochrome P450 2D6 substrate binding and catalysis

Human cytochrome P450 (P450) 2D6 is an important enzyme involved in the metabolism of drugs, many of which are amines or contain other basic nitrogen atoms. Asp301 has generally been considered to be involved in electrostatic docking with the basic substrates, on the basis of previous modeling studies and site-directed mutagenesis. Substitution of Glu216 with a residue other than Asp strongly attenuated the binding of quinidine, bufuralol, and several other P450 2D6 ligands. Catalytic activity with the substrates bufuralol and 4-methoxyphenethylamine was strongly inhibited by neutral or basic mutations at Glu216 (>95%), to the same extent as the substitution of Asn at Asp301. Unlike the Asp301 mutants, the Gln216 mutant (E216Q) retained 40% enzyme efficiency with the substrate spirosulfonamide, devoid of basic nitrogen, suggesting that the substitutions at Glu216 affect binding of amine substrates more than other catalytic steps. Attempts to induce catalytic specificity toward new substrates by substitutions at Asp301 and Glu216 were unsuccessful. Collectively, the results provide evidence for electrostatic interaction of amine substrates with Glu216, and we propose that both of these acidic residues plus at least another residue(s) is (are) involved in binding the repertoire of P450 2D6 ligands.

Some fundamental and technical aspects of the quantitative analysis of pharmaceutical drugs by matrix-assisted laser desorption/ionization mass spectrometry

The purpose of the present paper was to study some of the underlying physical and technical aspects of high-throughput quantitative matrix-assisted laser desorption/ionization (MALDI) of small drug molecules. A prototype MALDI-triple quadrupole instrument equipped with a high repetition rate laser was employed. Initially, the detection limits and dynamic ranges for the quantitation of four drugs (quinidine, danofloxacin, ramipril and nadolol) were determined. Internal standards were carefully chosen for each of these analytes in terms of structure similarity and fragmentation pathways. Three organic matrices were tested for these assays, resulting in different crystallization behaviors and measurement reproducibilities. alpha-Cyano-4-hydroxycinnamic acid yielded the best results and was subsequently employed for the quantitative determination of all four analytes. Further experiments considered the role of laser energy and pulse rate on the ablated areas as well as ion signals. Light microscope and scanning electron microscope images allowed the examination of the ablated area of the MALDI spots. The images showed convincing evidence that the ablated area was virtually void of crystals after analysis, with no preferential removal of material in the center of the laser's path. Average values for the amount of material ablated were determined to be 3.9+/-0.5% of the total spot size, and as low as 19.5 attomoles of analyte were detectable for our most sensitive analyte, ramipril. It was calculated that, under these assay conditions, it was possible to accurately quantify less than 1 femtomole of all analytes with the use of appropriately pure internal standards. These studies showed very promising results for the quantitative nature of MALDI for small molecules with molecular weights less than 500 Da.

Crystal structure of (-)-mefloquine hydrochloride reveals consistency of configuration with biological activity

The absolute configuration of (-)-mefloquine has been established as 11R,12S by X-ray crystallography of the hydrochloride salt, thus allowing comparison of the configuration of mefloquine's optical isomers to those of quinine and quinidine. (-)-Mefloquine has the same stereochemistry as quinine, and (+)-mefloquine has the same stereochemistry as quinidine. Since (+)-mefloquine is more potent than (-)-mefloquine in vitro against the D6 and W2 strains of Plasmodium falciparum and quinidine is more potent than quinine, a common stereochemical component for antimalarial activity is implicated. The crystal of (-)-mefloquine hydrochloride contained four different conformations which mainly differ in a small rotation of the piperidine ring. These conformations are essentially the same as the crystalline conformations of racemic mefloquine methylsulfonate monohydrate, mefloquine hydrochloride, and mefloquine free base. The crystallographic parameters for (-)-mefloquine hydrochloride hydrate were as follows: C17H17F (6)N(2)O(+)Cl(-) .0.25 H2O; M(r), 419.3; symmetry of unit cell, orthorhombic; space group, P2(1)2(1)2(1); parameters of unit cell, a = 12.6890 +/- 0.0006 A (1 A = 0.1 nm), b = 18.9720 +/- 0.0009 A, c = 32.189 +/- 0.017 A; volume of unit cell, 7,749 +/- 4 A(3); number of molecules per unit cell, 16; calculated density, 1.44 g cm(-3); source of radiation, Cu Kalpha (lambda = 1.54178 A); mu (absorption coefficient), 2.373 mm(-1); room temperature was used; final R(1) (residual index), 0.0874 for 3,692 reflections with intensities greater than 2sigma. All of the hydroxyl and amine hydrogen atoms participate in intermolecular hydrogen bonds with chloride ions. The orientation of the amine and hydroxyl groups in (+)-mefloquine may define the optimal geometry for hydrogen bonding with cellular constituents.

Salt and cosolvent effects on ionic drug loading into microspheres using an O/W method

Salt effects on aqueous solubility and microsphere entrapment efficiency of a model ionic drug (quinidine sulfate) were studied. Poly-D,L-lactic acid (PLA) microspheres were prepared using an O/W solvent evaporation method with various electrolytes added in different concentrations to the aqueous phase. Salts affect microsphere drug loading by changing the aqueous solubility of both the drug and the organic solvent (dichloromethane, DCM). Quinidine sulfate solubility was depressed by either a common ion effect (Na(2)SO(4)) or by formation of new, less soluble drug salts (e.g., bromide, perchlorate, thiocyanate) for which solubility products (K(sp)) were estimated. Inorganic salts depress DCM aqueous solubility to different extents as described by the Hofmeister series. NaClO(4) and NaSCN depressed drug solubility to the highest extent, resulting in microspheres with high drug loading (e.g., >90%). Other salts such as Na(2)SO(4) did not depress quinidine sulfate solubility to the same extent and did not improve loading. The use of a cosolvent (ethanol) in the organic phase improved microsphere drug loading and resulted in a uniform microsphere drug distribution with smooth release profiles.

Inhibition of cytochrome P450 2D6: structure-activity studies using a series of quinidine and quinine analogues

Several pharmacophore models have suggested that substrates and inhibitors of cytochrome P450 2D6 (P450 2D6) possess a nitrogen with a positive charge that participates in a charge-pair interaction with the aspartate 301 residue. In an effort to investigate this paradigm for P450 2D6 binding, an analogue series of the stereoisomers quinidine and quinine were synthesized and screened for binding affinity as measured by inhibition. Results revealed that bulky substituents added to the quinuclidine nitrogen (quaternary salts) did not affect the inhibitory potency of quinidine (IC(50) = 0.02 microM), suggesting minimal contribution to binding affinity of this inhibitor by the purported ionic-binding interaction. Meanwhile, substantial decreases in inhibitory potency were observed for the N-methyl, N-ethyl, and N-benzyl quininium salts, suggesting that the quaternary nitrogen of this antipode interacts with a distinct region of the P450 2D6 active site as compared to the corresponding nitrogen of quinidine. Interestingly, esterification of quinidine resulted in a substantial loss of inhibitory potency, likely due to disruption of a hydrogen-bonding interaction of the hydroxyl group. This suggests that hydrogen bonding contributes more to the tight binding of quinidine than does the charge-pair interaction of the positively charged nitrogen. Moreover, benzoyl ester formation of quinine caused the binding orientation to switch from type II to type I, with concomitant restoration of P450 2D6 inhibitory potency. Thus, it appears that both hydrogen bonding and the ionic interaction of the basic nitrogen of quinine contribute to inhibitory potency, while the hydroxyl group also apparently contributes to directing type II binding. Overall, results suggest that when analyzing a series of compounds that include stereoisomers for development of predictive pharmacophore/protein models describing P450 2D6 binding, it may be inappropriate to assume that the ionic interaction of the basic nitrogen with aspartate 301 represents the primary binding interaction.

Catalytic enantioselective sulfinyl transfer using cinchona alkaloid catalysts

[reaction: see text] Practical reaction conditions for the catalytic enantioselective synthesis of sulfinate esters are reported. Commercially available cinchona alkaloids were found to be superior catalysts for the sulfinyl transfer reaction of tert-butanesulfinyl chloride and a variety of benzyl alcohols. Sulfinyl transfer with 2,4,6-trichlorobenzyl alcohol and 10 mol % of the commercially available, inexpensive catalyst quinidine provided the pure sulfinate ester product in 92% isolated yield and with 90% ee.

Capillary electrophoretic separation, immunochemical recognition and analysis of the diastereomers quinine and quinidine and two quinidine metabolites in body fluids

The capillary electrophoretic separation and immunochemical recognition of the two naturally fluorescing, cationic diastereomers quinine (QN) and quinidine (QD), their hydroderivatives and two major QD metabolites (3-hydroxyquinidine and quinidine-N-oxide) was investigated. Plain aqueous phosphate buffers and an alkaline buffer containing dodecyl sulfate micelles are shown to be incapable of resolving the two diastereomers. However, incorporation of an additional chemical equilibrium (with beta-cyclodextrin) in the case of capillary zone electrophoresis (CZE) and the presence of a small amount of an organic solvent as buffer modifier (2-propanol) in dodecyl sulfate based micellar electrokinetic capillary chromatography (MECC), were found to provide separation media which lead to complete resolution of QN, QD and the other compounds of interest. Furthermore, for MECC- and CZE-based immunoassay formats, a commercially available antibody against QD was found to be a perfect discriminator between QD and QN. It was determined to recognize QD and the two QD metabolites (cross reactivity of 20--30%) but not QN. MECC and CZE with laser induced fluorescence (LIF) detection are shown to be suitable to determine QD and metabolites in urine and plasma (quinidine-N-oxide only) collected after single dose intake of 50 mg QD sulfate and of QN in urine, saliva and serum samples that were collected after self-administration of 0.5 l of quinine water (25 mg of QN). With direct injection of a body fluid, MECC with LIF was found to provide 10 ng/ml detection limits for QD and QN. This ppb sensitivity is comparable to that obtained in HPLC assays that are based upon drug extraction. Furthermore, MECC and CZE assays with UV detection are shown to provide the ppm sensitivity required for therapeutic drug monitoring and clinical toxicology of QD and QN.

Solution phase enantioselective recognition and discrimination by electrospray ionization--mass spectrometry: state-of-the-art, methods, and an eye towards increased throughput measurements

A review is given from the stand-point of applying methods utilizing electrospray ionization - mass spectrometry (ESI-MS) for quantitative binding (affinity and selectivity) determinations in small molecule host-guest (receptor-ligand, selector-selectand, etc.) systems. Advantages over more commonly utilized and traditional solution phase approaches, both in the context of developing new chiral selector molecules for separation and purification of enantiomers and in drug discovery applications, are presented. Although the majority of studies employing methods such as host-guest screening, competitive binding, and titrations have focused on large protein-ligand, DNA-ligand, and RNA-ligand systems, the use of ESI-MS for studying small molecule and chiral recognition systems is growing. For the latter case, greater care must be given to considering the effects of the ESI process on the ionization of the species involved in equilibria of interest. Some basic mechanistic and practical concerns for performing solution-phase-targeting quantitative measurements by ESI-MS are given in this light. Finally, an account of the application of these methods in a high throughput format is given, highlighting the potential of traditional and novel screening and titration approaches which allow scientists to screen the performance of hundreds, if not thousands, of compounds in a single day.

Stereoelectronic features of the cinchona alkaloids determine their differential antimalarial activity

For most potent antimalarial activity, the cinchona alkaloids appear to require certain electronic features, particularly a sufficiently acidic hydroxyl proton and an electric field direction pointing from the aliphatic nitrogen atom towards the quinoline ring. These observations are the result of an analysis of molecular electronic properties of eight cinchona alkaloids and an in vivo metabolite calculated using ab initio 3-21G quantum chemical methods in relation to their in vitro IC50 values against chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum parasites. The purpose is to provide a profile of the electronic characteristics necessary for potent antimalarial activity for use in the design of new antimalarial agents and to gain insight into the mechanistic path for antimalarial activity. Distinguishing features of the weakly active epiquinine and epiquinidine include a higher dipole moment, a different direction of the electric field, a greater intrinsic nucleophilicity, lower acidity of the hydroxyl proton, a lesser electron affinity of the lowest unoccupied molecular orbitals, and a higher proton affinity than the active cinchona alkaloids. A moderately potent quinine metabolite possesses some, but not all, of the same electronic features as the most potent cinchona alkaloids. Both the positioning of the hydroxyl and aliphatic amine groups and their electronic features appear to play a crucial role for antimalarial potency of the cinchona alkaloids, most likely by controlling the ability of these groups to form effective intermolecular hydrogen bonds.