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Antimicrobial Agents and Chemotherapy, May 2002, p. 1529-1534, Vol. 46, No. 5
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.5.1529-1534.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Crystal Structure of (-)-Mefloquine Hydrochloride Reveals Consistency of Configuration with Biological Activity

Jean M. Karle^1* and Isabella L. Karle²

Department of Medicinal Chemistry, Division of Experimental Therapeutics, Walter Reed Army Institute of Research, Silver Spring, Maryland 20910-7500,¹ Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D.C. 20735-5341²

Received 26 November 2001/ Returned for modification 11 January 2002/ Accepted 4 February 2002

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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 ₆N₂O⁺Cl^- · 0.25 H₂O; M_r, 419.3; symmetry of unit cell, orthorhombic; space group, P2₁2₁2₁; parameters of unit cell, a = 12.6890 ± 0.0006 Å (1 Å = 0.1 nm), b = 18.9720 ± 0.0009 Å, c = 32.189 ± 0.017 Å; volume of unit cell, 7,749 ± 4 ų; number of molecules per unit cell, 16; calculated density, 1.44 g cm^-3; source of radiation, Cu K ( = 1.54178 Å); µ (absorption coefficient), 2.373 mm^-1; room temperature was used; final R₁ (residual index), 0.0874 for 3,692 reflections with intensities greater than 2. 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.

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Quinine and quinidine (Fig. 1), antimalarial agents isolated from the bar k of the Cinchona tree, are diastereomers, but they mirror each other at carbons C-8 and C-9. In vitro quinidine is more potent than quinine against many strains of Plasmodium falciparum. Specifically, quinidine is 2.3 and 2.8 times more potent than quinine against the chloroquine-sensitive Sierra Leone D-6 clone and the chloroquine-resistant Indochina W-2 clone, respectively (15). Quinidine is 2.5 times more potent than quinine and cinchonine (the demethoxyanalog of quinidine) is 2.8 times more potent than cinchonidine (the demethoxy analog of quinine) in vitro against Papua New Guinea FCQ-27/PNG P. falciparum (25). Similarly, quinidine was 2.2 and 3.2 times more potent than quinine against Cameroon chloroquine-resistant FCM 29 and Ivory Coast chloroquine-sensitive L-3 strains, respectively (1). Against clinical isolates from Liberia, quinidine was on average 3 times more potent than quinine (4). Clinical differences in the antimalarial activities of quinine and quinidine have also been reported. Compared to quinine, quinidine was twice as effective against induced McClendon P. falciparum infections (23) and more potent clinically against Thai P. falciparum (18, 26).

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FIG. 1. Chemical structures and numbering schemes (in italics) of (-)mefloquine hydrochloride, quinine salt, and quinidine salt. The asterisks indicate the chiral atoms.

Mefloquine, a synthetic analog of quinine and quinidine, is marketed in racemic form under the trade name Lariam. Mefloquine is basically a structurally simpler form of quinine and quinidine. Mefloquine differs from the cinchona alkaloids in that it has a different substitution on the quinoline ring, a piperidine ring rather than the bicyclo quinuclidine ring, and no vinyl group (see Fig. 1). However, the C-8 and C-9 chiral centers of the cinchona alkaloids are preserved in the mefloquine molecule (numbered C-12 and C-11, respectively). Thus, one enantiomer of mefloquine will share the same stereochemistry as quinine and the other enantiomer of mefloquine will share the same stereochemistry as quinidine at the equivalent chiral centers.

Since quinine and quinidine possess different antimalarial activities, the (+) and (-) isomers of mefloquine were tested against P. falciparum in vitro to determine if they also had different antimalarial activities. Although the difference was not as great as that observed for quinine and quinidine, (+)-mefloquine was 1.81 ± 0.17 and 1.69 ± 0.16 (standard deviations, n = 5) times more active than (-)-mefloquine against the chloroquine-sensitive Sierra Leone D-6 clone and the chloroquine-resistant Indochina W-2 clone, respectively (16).

Thus remained the question, does the more active mefloquine enantiomer have the same stereochemistry as quinidine, the more active alkaloid, or does it have the same stereochemistry as quinine, the less active alkaloid? To answer this question, the absolute configuration of (-)-mefloquine hydrochloride was determined by X-ray crystallography.

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(-)-Mefloquine hydrochloride [11 R,12 S-2-piperidyl-2,8-bis(trifluoromethyl)-4-quinolinemethanol hydrochloride] was synthesized under contract by Aerojet Chemical Company (Sacramento, Calif.), resolved by Research Triangle Institute (Research Triangle Park, N.C.) (5), and crystallized from a mixture of ethanol and water acidified to pH 2.3 with HCl. Diffraction data were collected from a clear rectangular needle (0.12 by 0.18 by 2.0 mm) in the -2 scan mode (21) to a maximum 2 value of 112° on a four-circle diffractometer (R3m/V Nicolet; Siemens, Madison, Wis.) with a graphite monochromator. The X-ray source was Cu K radiation (50 kV, 40 mA). The indices ranged from 0 to 13 for h, 0 to 20 for k, and 0 to 34 for l. The total number of independent reflections was 5,804. The standard reflections 451, 085, and 0,0,10 were monitored after every 97 intensity measurements. The standards remained constant within 2.3%. The lattice parameters were based on 25 centered reflections, with 2 values ranging between 35° and 45°. The data were corrected for Lorentz and polarization effects, but no correction for absorption or extinction was used.

The structure was solved routinely by direct phase determination (11). All of the nonhydrogen atoms except the disordered fluorine group and the water molecules were found in the first electron density map. The hydrogen atom attached to O-1D was found in the difference maps. The difference map is an electron density map of the differences between the observed magnitudes of the reflections and the magnitudes calculated from the atoms already positioned in the structure. The resulting electron densities represent the locations of atoms which were not yet identified. Least-squares refinement was performed by using 3,692 reflections with intensity values greater than 2. Coordinates for all atoms except the hydrogen atoms attached to the carbon and N-13 atoms were refined (on F², where F is the structure factor) by using a blocked cascade program in the SHELXTL system (19). The hydrogen atoms bonded to the carbon and N-13 atoms were placed in idealized positions with a fixed bond length of 0.96 Å and were allowed to ride with the carbon atoms. Anisotropic thermal parameters for the C, N, O, Cl, and F atoms and isotropic thermal parameters for the F' atoms, the O atoms of the water molecules, and the H atoms bonded to O-1C and O-1D were refined for a total of 995 parameters. The final values for R₁ (residual index based on the observed structure factor F_O) and wR₂ (a weighted residual index based on F_O²) were 0.0874 and 0.2314, respectively. The absolute configuration Flack parameter was 0.022 ± 0.058 (6, 7). The final differences in electron density were _max equal to 0.85 and _min equal to -0.74 eÅ^-3. The goodness-of-fit value S was 1.05. The value calculated by dividing the maximum change in a parameter by the estimated standard deviation of a parameter (/)_max was 0.085. Atomic scattering factors were those incorporated in SHELXTL (19). Crystallographic parameters were further defined elsewhere (21).

The stereographic images of superimposed structures were created by using a Silicon Graphics Octane workstation (Mountain View, Calif.) using SYBYL software (Tripos, St. Louis, Mo.).

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(-)-Mefloquine hydrochloride crystallized as a secondary amine salt with four different conformations in the crystal, which were arbitrarily labeled A, B, C, and D. Coordinates and thermal parameter U_eq values for the nonhydrogen atoms and coordinates for the refined hydrogen atoms are listed in Table 1. The numbering scheme is shown in Fig. 1. The coordinates listed in Table 1 represent the correct absolute configuration, since the mirror image of this structure gave a higher final R₁ value of 0.0907 and the Flack parameter was 0.751 ± 0.060. The probability that the lower R₁ value correctly determines the absolute configuration is at least 99.5% (10). Also, a Flack parameter value less than twice the estimated standard deviation represents the correct configuration for an enantiopure substance, whereas a Flack parameter value near 1 represents the inverted structure (7). The percentage of (-)-mefloquine in the bulk sample from which the crystal was grown is >99.5%, as determined by high performance liquid chromatography (16).

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TABLE 1. Fractional coordinate values and thermal parameter values for Ueqa

The four conformations of (-)-mefloquine have an absolute configuration of 11R,12S. Molecules A to D possess nearly identical conformations, the main difference being in the rotation of the piperidine ring about the C-11-C-12 bond (Fig. 2). The hydrogen atoms from the hydrochloride salt reside on the piperidine nitrogen atoms, making these nitrogen atoms tetrahedral (Fig. 3). The piperidine ring assumes a chair conformation with atoms N-13, C-14, C-16, and C-17 coplanar such that the root mean square distances of atoms N-13, C-14, C-16, and C-17 from the average plane through these four atoms are 0.026, 0.004, 0.013, and 0.016 Å for molecules A, B, C, and D, respectively. The aryl group lies equatorial to the piperidine ring, with the angle between the average plane of the quinoline ring and the average plane of the piperidine ring ranging from 66.5 to 78.2°. The -54.4 to -71.6° range of torsion angles for the atoms comprising O-1-C-11-C-12-N-13 demonstrates that mefloquine is gauche about the C-11-C-12 bond. The C-4-C-11-C-12-N-13 torsion angles of 168.5°, 171.4°, -176.8°, and -179.4° for the four conformers place the amine group nearly as far as possible from the quinoline ring.

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FIG. 2. Superposition of the four conformations of (-)-mefloquine found in the crystal of (-)-mefloquine hydrochloride, illustrating the similarity of the conformations, which differ mainly in a small rotation of the piperidine ring.

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FIG. 3. Space-filling diagram of (-)-mefloquine salt molecule D, with carbon colored black, oxygen colored red, nitrogen colored deep blue, fluorine colored green, and hydrogen colored cyan. The two labeled hydrogen atoms are the amine and hydroxyl protons, which superimpose with the amine and hydroxyl protons of quinine salt (see Fig. 4A).

In molecules A to D, the fluorine atoms of the trifluoromethyl group attached to C-8 always straddle the quinoline nitrogen atom. However, the trifluoromethyl group attached to C-2A is present in two different conformations (Table 1). This is common behavior for trifluoromethyl groups attached to the 2 and 8 positions of quinoline rings (12).

Since the potencies of mefloquine and the cinchona alkaloids are dependent on the configuration of the aliphatic amine and hydroxyl groups, these groups are likely important features of the pharmacophore. The intramolecular distance from N-13 to O-1 ranges from 2.74 to 2.94 Å in the four conformers. Each hydrogen atom of the N-13 amine groups of (-)-mefloquine forms intermolecular hydrogen bonds to chloride ions (Table 2). All of the hydroxyl groups of (-)-mefloquine and all of the water molecules also form hydrogen bonds. Each chloride ion participates in at least three hydrogen bonds.

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TABLE 2. Hydrogen bond distances and angles

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The absolute configurations of (+)- and (-)-mefloquine have now been established through X-ray crystallography with at least 99.5% certainty (10). Carroll and Blackwell (5) tried to predict the absolute configuration of the optical isomers of mefloquine by comparing their circular dichroism spectra to the circular dichroism spectra of quinine and quinidine. Even though they state in their discussion that their assignment is tentative and that additional work will be necessary before the absolute configuration can be considered rigorously established, the incorrect predictions of the absolute configuration have been used in the literature as established absolute configurations.

As discussed in the introduction, a stereospecific component of the antimalarial activities of the cinchona alkaloids and mefloquine has been observed. (-)-Mefloquine has the same absolute conformation as quinine (Fig. 4A), and therefore, (+)-mefloquine has the same absolute conformation as quinidine (Fig. 4B). This relationship is mirrored in their relative potencies, with quinidine possessing more antimalarial potency than quinine both in vitro and in vivo (1, 4, 15, 18, 23, 25, 26), and in certain strains in vitro, (+)-mefloquine is more potent than (-)-mefloquine (16).

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FIG. 4. (A) Stereodiagram of the superposition of (-)-mefloquine salt with quinine salt showing the identical positionings of the quinoline rings and the hydroxyl and amine groups. Colors identifying atoms are as described in the Fig. 3 legend. The diagram can be viewed in three dimensions with the aid of a stereoviewer (Hubbard Scientific Co., Northbrook, Ill.) or by holding the drawing steady approximately 45 cm from your eyes and allowing your eye muscles to relax until the center image comes into focus. (B) Stereodiagram of the superposition of (+)-mefloquine salt with quinidine salt showing the identical positionings of the quinoline rings and the hydroxyl and amine groups. The structure of (+)-mefloquine was obtained by inverting the crystal structure of (-)-mefloquine. Magenta dots were drawn at the van der Waals radius of the common hydroxyl and amine protons to depict the regions of hydrogen bonding with a common receptor.

Stereoselectivity plays a role in human mefloquine pharmacokinetics. (-)-Mefloquine has consistently shown significantly higher peak concentrations and areas under the curve as well as longer plasma half-lives than (+)-mefloquine (2, 3, 8, 9, 24). Taggart et al. (23) reported that quinine produces higher plasma concentrations than quinidine.

Human central nervous system responses to the enantiomers of mefloquine may also differ. The (-)-enantiomer of mefloquine is 100 to 400 times more potent than the (+)-enantiomer as an adenosine receptor agonist; thus, the (+)-enantiomer may have reduced neuropsychiatric side effects compared to racemic mefloquine (20).

The crystalline conformations of (-)-mefloquine hydrochloride are virtually identical to the crystalline conformations of racemic mefloquine with regard to their hydrochloride salts (13), methylsulfonate salts (14), and free bases (17). Table 3 shows that the distances between the aliphatic nitrogen atom and hydroxyl oxygen atom, the positions of the hydroxyl and amine groups, and the positions of the quinoline and piperidine rings are essentially identical for all of the mefloquine molecules. Thus, although mefloquine contains rotatable bonds, the conformation of mefloquine is independent of crystalline environments and is not dependent on whether mefloquine is a salt or a free base or whether it is racemic or optically pure. The solution nuclear magnetic resonance spectra of the racemate for either the free base or the HCl salt are also consistent with this conformation (7, 23).

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TABLE 3. Comparison of geometries of different forms of mefloquine

Structure-activity studies suggest that the amine and hydroxyl groups of mefloquine need to be available for hydrogen bonding with cellular constituents. Mefloquine loses its antimalarial activity if the amine and hydroxyl groups are acetylated (22). Formation of an O-methyl or O-ethyl derivative or conversion of the saturated 2-piperidyl group to an unsaturated 2-pyridyl group also eliminates activity (22).

The implication from studies of the stereochemistry and stereospecificity of antimalarial potency of mefloquine and the cinchona alkaloids is that these compounds have a common malaria receptor (14). In Fig. 4B, dots have been drawn around the hydroxyl and amine groups at the van der Waals radius of the hydrogen atoms which are common to the salts of both (+)-mefloquine and quinidine. The complementary surface to the dot surfaces shown in Fig. 4B outlines the approximate geometry of the hydrogen bonding regions of the proposed receptor. Hydrogen bonds to the receptor would form with donor-hydrogen-acceptor angles generally in the range of 150° to 180°. Thus, if a common receptor for antimalarial activity exists, then the geometry of the pharmacophore is defined by (+)-mefloquine and quinidine as illustrated in Fig. 4B.

 
* Corresponding author. Mailing address: Walter Reed Army Institute of Research, Division of Experimental Therapeutics, 503 Robert Grant Ave., Silver Spring, MD 20910-7500. Phone: (301) 319-9633. Fax: (301) 319-9449. E-mail: jean.karle{at}na.amedd.army.mil.

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1
1. Basco, L. K., C. Gillotin, F. Gimenez, R. Farinotti, and J. Le Bras. 1992. In vitro activity of the enantiomers of mefloquine, halofantrine, and enpiroline against Plasmodium falciparum. Br. J. Clin. Pharmacol. 33:517-520.[Medline]
2
2. Bergqvist, Y., and J. Al Kabbam. 1993. Enantioselective high-performance liquid chromatographic determination of (SR)- and (RS)-mefloquine in plasma using N-benzyloxycarbonyl-glycyl-L-proline as chiral counter ion. J. Chromatogr. Biomed. Appl. 620:217-224.[CrossRef]
3
3. Bergqvist, Y., M. Doverskog, and J. Al Kabbam. 1994. High-performance liquid chromatographic determination of (SR)- and (RS)-enantiomers of mefloquine in plasma and capillary blood sampled on paper after derivatization with (-)-1-(9-fluorenyl)ethyl chloroformate. J. Chromatogr. B Biomed. Appl. 652:73-81.[CrossRef]
4
4. Bjorkman, A., M.Willcox, N. Marbiah, and D. Payne. 1991. Susceptibility of Plasmodium falciparum to different doses of quinine in vivo and to quinine and quinidine in vitro in relation to chloroquine in Liberia. Bull. W. H. O. 69:459-465.[Medline]
5
5. Carroll, F. I., and J. T. Blackwell. 1974. Optical isomers of aryl-2-piperidylmethanol antimalarial agents. Preparation, optical purity, and absolute stereochemistry. J. Med. Chem. 17:210-219.[CrossRef][Medline]
6
6. Flack, H. D., and G. Bernardinelli. 1999. Absolute structure and absolute configuration. Acta Crystallogr. Sect. A 55:908-915.[CrossRef][Medline]
7
7. Flack, H. D., and G. Bernardinelli. 1999. Reporting and evaluating absolute structure and absolute-configuration determinations. J. Appl. Crystallogr. 33:1143-1148.[CrossRef]
8
8. Gimenez, F., R. Farinotti, A. Thuller, G. Hazebroucq, and I. W. Waimer. 1990. Determination of the enantiomers of mefloquine in plasma and whole blood using a coupled achiral-chiral high-performance liquid chromatographic system. J. Chromatogr. Biomed. Appl. 529:339-346.[CrossRef]
9
9. Gimenez, F., R. A. Pennie, G. Koren, C. Crevoisier, I. W. Waimer, and R. Farinotti. 1994. Stereoselective pharmacokinetics of mefloquine in healthy Caucasians after multiple doses. J. Pharm. Sci. 83:824-827.[CrossRef][Medline]
10
10. Hamilton, A. C. 1965. Significance tests on the crystallographic R factor. Acta Crystallogr. 18:502-510.[CrossRef]
11
11. Karle, J., and I. L. Karle. 1966. The symbolic addition procedure for phase determination for centrosymmetric and noncentrosymmetric crystals. Acta Crystallogr. 21:849-859.[CrossRef]
12
12. Karle, J. M., and A. K. Bhattacharjee. 1996. Trifluoromethyl group disorder in crystalline 2,8-bis(trifluoromethyl)-4-hydroxymethylquinoline. J. Chem. Crystallogr. 26:615-619.[CrossRef]
13
13. Karle, J. M., and I. L. Karle. 1991. Structure of the antimalarial (±)-mefloquine hydrochloride. Acta Crystallogr. Sect. C Cryst. Struct. Commun. 47:2391-2395.[CrossRef]
14
14. Karle, J. M., and I. L. Karle. 1991. Crystal structure and molecular structure of mefloquine methylsulfonate monohydrate: implications for a malaria receptor. Antimicrob. Agents Chemother. 35:2238-2245.[Abstract/Free Full Text]
15
15. Karle, J. M., I. Karle, L. Gerena, and W. K. Milhous. 1992. Stereochemical evaluation of the relative sensitivity of Plasmodium falciparum to the cinchona alkaloids. Antimicrob. Agents Chemother. 36:1538-1544.[Abstract/Free Full Text]
16
16. Karle, J. M., R. Olmeda, L. Gerena, and W. K. Milhous. 1993. Plasmodium falciparum: role of absolute stereochemistry in the antimalarial activity of synthetic amino alcohol antimalarial agents. Exp. Parasitol. 76:345-351.[CrossRef][Medline]
17
17. Oleksyn, B. J., L. Lebioda, and J. Sliwinski. 1980. Crystal structure investigation of mefloquine, a new potent antimalarial drug, p. 200-204. In Z. Kaluski (ed.), Proceedings of the 3rd Symposium on Organic Crystal Chemistry. University Adam Mickiewicz, Poznan, Poland.
18
18. Phillips, R. E., D. A. Warrell, N. J. White, S. Looareesuwan, and J. Karbwang. 1985. Intravenous quinidine for the treatment of severe falciparum malaria: clinical and pharmacokinetic studies. N. Engl. J. Med. 312:1273-1278.[Abstract]
19
19. Sheldrick, G. M. 1985. Crystallographic algorithms for mini and maxi computers, p. 175-189. In G. M. Sheldrick, C. Krüger, and R. Goddard (ed.), Crystallographic computing, vol. 3. Oxford University Press, Oxford, England.
20
20. Shepard, R. D., and A. Fletcher. 11 September 1998. Use of (+)-mefloquine for the treatment of malaria with reduced side-effects. Int. Patent Appl. WO 9839003.
21
21. Stout, G. H., and L. H. Jensen. 1989. X-ray structure determination. A practical guide, 2nd ed. John Wiley & Sons, New York, N.Y.
22
22. Sweeney, T. R. 1981. The present status of malaria chemotherapy: mefloquine, a novel antimalarial. Med. Res. Rev. 1:281-301.[CrossRef][Medline]
23
23. Taggart, J. V., D. P. Earle, R. W. Berliner, C. G. Zubrod, W. J. Welch, N. B. Wise, E. F. Schroeder, I. M. London, and J. A. Shannon. 1948. Studies on the chemotherapy of the human malarias. III. The physiological disposition and antimalarial activity of the cinchona alkaloids. J. Clin. Investig. 27(Suppl.):80-86.
24
24. Wallen, L., O. Ericsson, I. Wikstrom, and U. Hellgren. 1994. High-performance liquid chromatographic method for the enantioselective analysis of mefloquine in plasma and urine. J. Chromatogr. B Biomed. Appl. 655:153-157.[CrossRef][Medline]
25
25. Wesche, D. L., and J. Black. 1990. A comparison of the antimalarial activity of the cinchona alkaloids against P. falciparum in vitro. J. Trop. Med. Hyg. 93:153-159.[Medline]
26
26. White, N. J., S. Looareesuwan, D. A. Warrell, T. Chongsuphajasiddhi, D. Bunnag, and T. Harinasuta. 1981. Quinidine in falciparum malaria. Lancet ii:1069-1071.[CrossRef]

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Antimicrobial Agents and Chemotherapy, May 2002, p. 1529-1534, Vol. 46, No. 5
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.5.1529-1534.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Karle, J. M. Articles by Karle, I. L. Search for Related Content PubMed PubMed Citation Articles by Karle, J. M. Articles by Karle, I. L.