Frank A. Sløk

Aarhus University, Aarhus, Central Jutland, Denmark

Are you Frank A. Sløk?

Claim your profile

Publications (15)54.29 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We have previously used homologation of (S)-glutamic acid (Glu) and Glu analogs as an approach to the design of selective ligands for different subtypes of Glu receptors. (RS)-2-Amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), which is an isoxazole homolog of Glu, is a very potent agonist at the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) subgroup of Glu receptors and a moderately potent ligand for the kainic acid (KA) subgroup of Glu receptors. The enantiomers of ACPA were previously obtained by chiral HPLC resolution. Prompted by pharmacological interest in ACPA, we have now prepared the (S)- and (R)-enantiomers of ACPA by stereocontrolled syntheses using (1R,2R,5R)- and (1S,2S,5S)-2-hydroxy-3-pinanone, respectively, as chiral auxiliaries. Furthermore, the 5-ethyl analog of ACPA, Ethyl-ACPA, was synthesized, and (S)- and (R)-Ethyl-ACPA were also prepared using this method. The absolute configurations of (S)- and (R)-ACPA were established by X-ray crystallographic analysis of a protected (1S,2S,5S)-2-hydroxy-3-pinanone imine derivative of (R)-ACPA. The absolute stereochemistry of (S)- and (R)-Ethyl-ACPA was assigned on the basis of a comparison of their properties with those of the enantiomers of ACPA, employing elution order on chiral HPLC columns, as well as circular dichroism (CD) spectroscopy in combination with time-dependent density functional theory. The structural and electronic basis for the Cotton effect observed for such analogs is examined. The lower homolog of ACPA, (RS)-2-amino-2-(3-carboxy-5-methyl-4-isoxazolyl)acetic acid (1), which is a Glu analog, was also synthesized. Affinities and neuroexcitatory effects were determined using rat brain membranes and cortical wedges, respectively, at native AMPA, KA, and N-methyl-D-aspartic acid (NMDA) receptors. The molecular pharmacology of (S)- and (R)-ACPA and (S)- and (R)-Ethyl-ACPA was evaluated at homomeric cloned subtypes of AMPA receptors (iGluR1o,3o,4o) and of KA receptors (iGluR5,6), expressed in Xenopus laevis oocytes. The cloned receptors mGluR1alpha, mGluR2, and mGluR4a, expressed in CHO cell lines, were used to study the effects of the five compounds at metabotropic Glu receptors. In accordance with ligand-receptor complexes known from X-ray crystallography, the conformationally restricted Glu analog 1 was inactive at all Glu receptors studied, and the R-forms of ACPA and Ethyl-ACPA were very weak or inactive at these receptors. At AMPA receptor subtypes, (S)-ACPA and (S)-Ethyl-ACPA showed equally potent agonist effects at iGluR1o and iGluR3o, whereas (S)-Ethyl-ACPA was 6-fold more potent than (S)-ACPA at iGluR4o. (S)-ACPA and (S)-Ethyl-ACPA were approximately an order of magnitude less potent at iGluR5 than at AMPA receptor subtypes, and neither compound showed detectable effects at iGluR6. The binding mode of (S)-Ethyl-ACPA at iGluR2 was examined by docking to the (S)-ACPA-iGluR2 complex.
    Chirality 09/2004; 16(7):452-66. DOI:10.1002/chir.20060 · 1.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have previously described (RS)-2-amino-3-(3-hydroxy-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid (4-AHCP) as a highly effective agonist at non-N-methyl-d-aspartate (non-NMDA) glutamate (Glu) receptors in vivo, which is more potent than (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) but inactive at NMDA receptors. However, 4-AHCP was found to be much weaker than AMPA as an inhibitor of [(3)H]AMPA binding and to have limited effect in a [(3)H]kainic acid binding assay using rat cortical membranes. To shed light on the mechanism(s) underlying this quite enigmatic pharmacological profile of 4-AHCP, we have now developed a synthesis of (S)-4-AHCP (6) and (R)-4-AHCP (7). At cloned metabotropic Glu receptors mGluR1alpha (group I), mGluR2 (group II), and mGluR4a (group III), neither 6 nor 7 showed significant agonist or antagonist effects. The stereoisomer 6, but not 7, activated cloned AMPA receptor subunits GluR1o, GluR3o, and GluR4o with EC(50) values in the range 4.5-15 microM and the coexpressed kainate-preferring subunits GluR6 + KA2 (EC(50) = 6.4 microM). Compound 6, but not 7, proved to be a very potent agonist (EC(50) = 0.13 microM) at the kainate-preferring GluR5 subunit, equipotent with (S)-2-amino-3-(5-tert-butyl-3-hydroxyisothiazol-4-yl)propionic acid [(S)-Thio-ATPA, 4] and almost 4 times more potent than (S)-2-amino-3-(5-tert-butyl-3-hydroxyisoxazol-4-yl)propionic acid [(S)-ATPA, 3]. Compound 6 thus represents a new structural class of GluR5 agonists. Molecular modeling and docking to a crystal structure of the extracellular binding domain of the AMPA subunit GluR2 has enabled identification of the probable active conformation and binding mode of 6. We are able to rationalize the observed selectivities by comparing the docking of 4 and 6 to subtype constructs, i.e., a crystal structure of the extracellular binding domain of GluR2 and a homology model of GluR5.
    Journal of Medicinal Chemistry 05/2003; 46(8):1350-8. DOI:10.1021/jm0204441 · 5.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Homologation of (S)-glutamic acid (Glu, 1) and Glu analogues has previously provided ligands with activity at metabotropic Glu receptors (mGluRs). The homologue of ibotenic acid (7), 2-amino-3-(3-hydroxy-5-isoxazolyl)propionic acid (HIBO, 8), and the 4-phenyl derivative of 8, compound 9a, are both antagonists at group I mGluRs. Here we report the synthesis and molecular pharmacology of HIBO analogues 9b-h containing different 4-aryl substituents. All of these compounds possess antagonist activity at group I mGluRs but are inactive at group II and III mGluRs.
    Journal of Medicinal Chemistry 03/2002; 45(4):988-91. DOI:10.1021/jm010443t · 5.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The coupling reactions were performed using 3-ethoxy-4-iodo-5-methylisoxazole (4) as the key intermediate. Coupling of 4 under Suzuki–Miyaura or Stille conditions using Pd(PPh3)2Cl2 and arylboronic acids or aryltin analogues, respectively, gave 4-aryl substituted isoxazoles in yields ranging from 49% for the 3-pyridyl analogue 14, to 96% for the 4-pyridyl analogue 12. Under Heck reaction conditions using Pd(PPh3)2Cl2 and 4, analogues of 3-ethoxy-5-methylisoxazole containing vinylic or acetylenic groups in the 4-position were synthesized in yields ranging from 58 to 98%. 3-Ethoxy-5-methylisoxazol-4-ylmagnesium bromide (19), prepared from 4 and isopropylmagnesium bromide, reacted smoothly with benzaldehyde or benzoyl chloride to give the desired 4-[hydroxy(phenyl)methyl] analogue 21 and 4-benzoyl-3-ethoxy-5-methylisoxazole (22), respectively. Transmetallation of 19 with ZnCl2 and subsequent treatment with Pd(PPh3)2Cl2 and 4-iodotoluene gave 3-ethoxy-5-methyl-4-(4-methylphenyl)isoxazole (23) in 80% yield.
    Tetrahedron 03/2001; 57(11):2195-2201. DOI:10.1016/S0040-4020(01)00052-7 · 2.82 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have previously described (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) as a potent agonist at the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor subtype of (S)-glutamic acid (Glu) receptors. We now report the chromatographic resolution of ACPA and (RS)-2-amino-3-(3-carboxy-4-isoxazolyl)propionic acid (demethyl-ACPA) using a Sumichiral OA-5000 column. The configuration of the enantiomers of both compounds have been assigned based on X-ray crystallographic analyses, supported by circular dichroism spectra and elution orders on chiral HPLC columns. Furthermore, the enantiopharmacology of ACPA and demethyl-ACPA was investigated using radioligand binding and cortical wedge electrophysiological assay systems and cloned metabotropic Glu receptors. (S)-ACPA showed high affinity in AMPA binding (IC(50) = 0.025 microM), low affinity in kainic acid binding (IC(50) = 3.6 microM), and potent AMPA receptor agonist activity on cortical neurons (EC(50) = 0.25 microM), whereas (R)-ACPA was essentially inactive. Like (S)-ACPA, (S)-demethyl-ACPA displayed high AMPA receptor affinity (IC(50) = 0.039 microM), but was found to be a relatively weak AMPA receptor agonist (EC(50) = 12 microM). The stereoselectivity observed for demethyl-ACPA was high when based on AMPA receptor affinity (eudismic ratio = 250), but low when based on electrophysiological activity (eudismic ratio = 10). (R)-Demethyl-ACPA also possessed a weak NMDA receptor antagonist activity (IC(50) = 220 microM). Among the enantiomers tested, only (S)-demethyl-ACPA showed activity at metabotropic receptors, being a weak antagonist at the mGlu(2) receptor subtype (K(B) = 148 microM).
    Chirality 02/2001; 13(9):523-32. DOI:10.1002/chir.1172 · 1.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have previously described the potent and selective (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor agonist, (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), and the AMPA receptor antagonist (RS)-2-amino-3-[3-(carboxymethoxy)-5-methyl-4-isoxazolyl]propionic acid (AMOA). Using these AMPA receptor ligands as leads, a series of compounds have been developed as tools for further elucidation of the structural requirements for activation and blockade of AMPA receptors. The synthesized compounds have been tested for activity at ionotropic excitatory amino acid (EAA) receptors using receptor binding and electrophysiological techniques, and for activity at metabotropic EAA receptors using second messenger assays. Compounds 1 and 4 were essentially inactive. (RS)-2-Amino-3-[3-(2-carboxyethyl)-5-methyl-4-isoxazolyl]propionic acid (ACMP, 2), on the other hand, was shown to be a selective AMPA receptor antagonist (IC(50) = 73 microM), more potent in electrophysiological experiments than AMOA (IC(50) = 320 microM). The isomeric analogue of 2, compound 5, did not show AMPA antagonist effects, but was a weak NMDA receptor antagonist (IC(50) = 540 microM). Finally, compound 3, which is an isomer of ACPA, turned out to be a very weak NMDA antagonist, and an AMPA receptor agonist approximately 1000 times weaker than ACPA. None of the compounds showed agonist or antagonist effects at metabotropic EAA receptors.
    European Journal of Medicinal Chemistry 02/2000; 35(1):69-76. DOI:10.1016/S0223-5234(00)00104-5 · 3.43 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Philanthotoxin-433 (PhTX-433), a natural polyamine wasp toxin, is a noncompetitive antagonist of certain ionotropic receptors. Six analogues of PhTX-343 (a synthetic analogue of the natural product), in which the secondary amino groups are systematically replaced by oxygen or methylene groups, have been synthesized by coupling of N-(1-oxobutyl)tyrosine with 1,12-dodecanediamine, 4,9-dioxa-1,12-dodecanediamine, or appropriately protected di- and triamines, the latter being obtained by multistep syntheses. The resulting PhTX-343 analogues were purified and characterized, and their protolytic properties (stepwise macroscopic pKa values) were determined by 13C NMR titrations. All analogues are fully protonated at physiological pH. The effects of these compounds on acetylcholine-induced currents in TE671 cells clamped at various holding potentials were determined. All of the analogues noncompetitively antagonized the nicotinic acetylcholine receptor (nAChR) in a concentration-, time-, and voltage-dependent manner. The amplitudes of acetylcholine-induced currents were compared at their peaks and at the end of a 1 s application in the presence or absence of the analogues. Most of the analogues were equipotent with or more potent than PhTX-343. The dideaza analogue PhTX-12 [IC50 of 0.3 μM (final current value)] was the most potent, representing the highest potency improvement (about 50-fold) yet achieved by modification of the parent compound (PhTX-343). Thus, the presence of multiple positive charges in the PhTX-343 molecule is not necessary for antagonism of nAChR. In contrast, the compounds were much less potent than PhTX-343 at locust muscle ionotropic glutamate receptors sensitive to quisqualate (qGluR). The results demonstrate that the selectivity for different types of ionotropic receptors can be achieved by manipulating the polyamine moiety of PhTX-343.
    Journal of Medicinal Chemistry 11/1999; 42(25). DOI:10.1021/jm9903747 · 5.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: (RS)-2-Amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), which is a potent and selective agonist at (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptors, has previously been shown to desensitize AMPA receptors to a much lower degree than AMPA itself. We now report the synthesis of [3H]ACPA (32.5 Ci/mmol), the neurochemical and pharmacological characterization of [3H]ACPA binding, and a comparison of the distribution of [3H]ACPA, [3H]AMPA, and [3H](S)-5-fluorowillardiine binding sites in rat brain. Under equilibrium conditions, [3H]ACPA was shown to bind to a single population of receptor sites on rat brain membranes. [3H]ACPA was shown to bind with single and similar affinities (15-45 nM) to cloned AMPA receptor subunits (GluR1-4), expressed in insect cells, whereas a K(D) value of 330 nM was determined for the binding of [3H]ACPA to cloned kainic acid preferring GluR5 subunits. Whereas Bmax and K(D) values for [3H]ACPA binding, determined using filtration techniques, were different from such obtained in centrifugation assays, Bmax and K(D) values as well as association and dissociation constants were not significantly affected by the addition of the chaotropic agent KSCN. K(D) values, determined under equilibrium conditions, were, however, markedly different from K(D) values derived from kinetic data. Furthermore, the results of analyses of these kinetic data were consistent with the existence of two different populations of [3H]ACPA binding sites. The pharmacology of [3H]ACPA binding sites was characterized using a series of AMPA receptor agonists and antagonists. Whereas addition of KSCN had little effect on the affinities of AMPA receptor agonists for [3H]ACPA binding, this chaotropic agent reduced the affinities of AMPA receptor antagonists structurally related to AMPA. Based on these and previously reported data, the AMPA receptor agonists, ACPA, AMPA and (S)-5-fluorowillardiine, seem to bind to and activate AMPA receptors in a nonidentical fashion, and these three agonists together may be useful tools for studies of AMPA receptor mechanisms.
    European Journal of Pharmacology 07/1999; 373(2-3):251-62. DOI:10.1016/S0014-2999(99)00269-1 · 2.68 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Our previous publication (J. Med. Chem. 1996, 39, 3188-3194) described (RS)-2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (Homo-AMPA) as a highly selective agonist at the mGlu6 subtype of metabotropic excitatory amino acid (EAA) receptors. Homo-AMPA has already become a standard agonist for the pharmacological characterization of mGlu6 (Trends Pharmacol. Sci. Suppl. 1997, 37-39), and we here report the resolution, configurational assignment, and pharmacology of (S)- (6) and (R)- (7) Homo-AMPA. Using the "Ugi four-component condensation", 3-(3-ethoxy-5-methylisoxazol-4-yl)propanal (10) was converted into the separable diastereomeric derivatives of 6 and 7, compounds 12 and 11, respectively. Deprotection of 12, in one or two steps, gave extensively racemized 6, which was converted in low yield into 6 (99.0% ee) through several crystallizations. 6 (99.7% ee) and 7 (99.9% ee) were finally obtained by preparative chiral HPLC. The configurational assignments of 6 and 7 were based on 1H NMR spectroscopic studies on 12 and 11, respectively, and circular dichroism studies on 6 and 7. Values of optical rotations using different solvents and the chiral HPLC elution order of 6 and 7 supported the results of the spectroscopic configurational assignments. The activities of 6 and 7 at ionotropic EAA (iGlu) receptors and at mGlu1-7 were studied. (S)-Homo-AMPA (6) was shown to be a specific agonist at mGlu6 (EC50 = 58 +/- 11 microM) comparable in potency with the endogenous mGlu agonist (S)-glutamic acid (EC50 = 20 +/- 3 microM). Although Homo-AMPA did not show significant effects at iGlu receptors, (R)-Homo-AMPA (7), which was inactive at mGlu1-7, turned out to be a weak N-methyl-D-aspartic acid (NMDA) receptor antagonist (IC50 = 131 +/- 18 microM).
    Journal of Medicinal Chemistry 11/1997; 40(22):3700-5. DOI:10.1021/jm9703597 · 5.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low-temperature single-crystal structure determinations have been carried out on isoxazol-3-ol, 5-methyl-isoxazol-3-ol, isothiazol-3-ol and 5-methylisothiazol-3-ol, the heterocyclic ring systems used as carboxy group bioisosteres in numerous neuroactive analogues of 4-aminobutyric acid (GABA) and glutamic acid. All compounds form hydrogen-bonded dimers in the solid state. The OH · · · N hydrogen bonds are shorter in isoxazol-3-ols than in isothiazol-3-ols. The excess molecular van der Waals volume of the sulfur-containing ring systems as compared to the corresponding isoxazol-3-ols amounts to about 15%. The sulfur substitution significantly affects the position of the 5-substituents in relation to the heterocyclic ring. Such effects may contribute to the observed differences in pharmacological effects of the structurally related isoxazol-3-ol and isothiazol-3-ol amino acids. The geometries of the compounds have been optimized by ab initio calculations at the HF/6-31G* level, and in some cases also at the MP2/6-311G** level. The gas-phase calculations are in agreement with the experimental data, especially when correction for the effects of hydrogen bonding is made, as estimated using a complex between isoxazol-3-ol and formic acid. Calculated dipole moments of isoxazol-3-ols and isothiazol-3-ols are similar. Isoxazol-3-ol is more acidic than isothiazol-3-ol by 1.7 pKa unit as determined by 13C NMR titration, and the differences in acidity are believed to be one of the major factors causing the differences in the biological actions of isoxazol-3-ol amino acids and the corresponding isothiazol-3-ol analogues.
    Journal of the Chemical Society Perkin Transactions 2 09/1997; DOI:10.1039/A700332C
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have previously proposed the existence of a lipophilic cavity of the 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptor recognition site capable of accommodating alkyl substituents of limited size in the 5-position of the isoxazole ring. In order to indirectly elucidate the approximate extent of this proposed cavity we have synthesized and pharmacologically characterized a number of AMPA analogues. For most of these AMPA analogues, a positive correlation between AMPA receptor affinity and agonist effect was observed. The only exception was demethyl-AMPA (8a), which showed relatively high AMPA receptor affinity (IC50 = 0.27 μM) but remarkably weak agonist potency (EC50 = 900 μM). Whereas the ethyl analogue of AMPA (Et-AMPA) (IC50 = 0.030 μM; EC50 = 2.3 μM) has previously been shown to be slightly more potent than AMPA (IC50 = 0.040 μM; EC50 = 3.5 μM), substitutions of a propyl or a butyl group for the methyl group of AMPA to give 8b (IC50 = 0.090 μM; EC50 = 5.0 μM) or 8f (IC50 = 1.0 μM; EC50 = 32 μM), respectively, result in progressive loss of the AMPA agonist effect. Analogues containing larger groups, such as isopentyl (8e), 1-propylbutyl (8g), 2,2-dimethylpropyl (8h), or benzyl (14) groups, were very weak or totally inactive as AMPA receptor ligands.
    European Journal of Medicinal Chemistry 01/1997; 32(4):329-338. DOI:10.1016/S0223-5234(97)89085-X · 3.43 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The homologous series of acidic amino acids, ranging from aspartic acid (1) to 2-aminosuberic acid (5), and the corresponding series of 3-isoxazolol bioisosteres of these amino acids, ranging from (RS)-2-amino-2-(3-hydroxy-5-methylisoxazol-4-yl)acetic acid (AMAA, 6) to (RS)-2-amino-6-(3-hydroxy-5-methylisoxazol-4-yl)hexanoic acid (10), were tested as ligands for metabotropic excitatory amino acid receptors (mGlu1 alpha, mGlu2, mGlu4a, and mGlu6). Whereas AMAA (6) and (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propinoic acid (AMPA, 7) are potent and highly selective agonists at N-methyl-D-aspartic acid (NMDA) and AMPA receptors, respectively, the higher homologue of AMPA (7), (RS)-2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (homo-AMPA, 8), is inactive at ionotropic excitatory amino acid receptors. Homo-AMPA (8), which is a 3-isoxazolol bioisostere of 2-aminoadipic acid (3), was, however, shown to be a specific and rather potent agonist at mGlu6, approximately 4 times weaker than the nonselective excitatory amino acid receptor agonist (S)-glutamic acid. 2-Aminoadipic acid (3), which shows a complex excitatory amino acid synaptic pharmacology, was an agonist at mGlu6 as well as mGlu2. AMPA (7) and the higher homologue of homo-AMPA (8), (RS)-2-amino-5-(3-hydroxy-5-methylisoxazol-4-yl)pentanoic acid (9), showed relatively weak agonist effects at mGlu6. It is concluded that homo-AMPA (8) is likely to be a useful tool for studies of the pharmacology and physiological role of mGlu6. We describe a new versatile synthesis of this homologue of AMPA and the synthesis of compound 10.
    Journal of Medicinal Chemistry 09/1996; 39(16):3188-94. DOI:10.1021/jm9602569 · 5.48 Impact Factor
  • Neuropharmacology 06/1996; 35(6). DOI:10.1016/0028-3908(96)84664-8 · 4.82 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 1H-NMR spectroscopy is a convenient method for determination of diethylcarbamazine (DEC) in urine, and can be used to monitor medication with the drug. Urine samples were mixed with 10% of deuterium oxide as a spectrometer field frequency lock, which is the only sample pretreatment required. Tailored excitation with the 1331 pulse was used for water peak suppression. The quantification of DEC was carried out with the triplet of the N-ethyl group, for which the T1 relaxation time was 1 s. In aqueous solutions, amounts below 1 microgram ml-1 of DEC could be easily detected. In urine, the detectability depended on the level of chemical noise but was better than 10 micrograms ml-1. The accuracy and precision of the method were better than 15%. Analysis of urine from volunteers receiving a single therapeutic dose of DEC (6 mg kg-1 body weight orally) showed that the drug was eliminated in unchanged form during 2 days, in agreement with earlier results. The concentration of DEC in urine several hours after the intake exceeded 100 micrograms ml-1 making the 1H-NMR assay rapid and easy. No significant amounts of the N-oxide of DEC could be detected.
    Journal of Pharmaceutical and Biomedical Analysis 04/1996; 14(5):543-9. DOI:10.1016/0731-7085(95)01664-3 · 2.83 Impact Factor
  • European Journal of Medicinal Chemistry 01/1996; 31(7):515-537. DOI:10.1016/0223-5234(96)89549-3 · 3.43 Impact Factor