Chloramphenicol derivatives

An interesting exception to the absolute validity of the tifth postulate is the considerable activity of chloramphenicol derivatives in cell-free model systems of protein synthesis when these derivatives are substituted with amino acyl residues instead of with dichloroacetyl as is the antibiotic itself (rev. in 2°)). This has been traced to the necessity of the dichloroacetyl grouping in aiding in the permeation of the antibiotic through the bacterial envelope 21 The amino acyl derivatives have very low antibacterial activity 20. Permeation failures of actinomycin D, macrolides and distamycin A with respect to certain families of bacteria occlude the action of these antibiotics on their intracellular drug receptors and target reactions but can be overcome experimentally by measures which render test organisms permeable. 

Modified Hammett substituent constants (100) were used by Garrett et al. to describe the bacterostatic activities of a series of sulfanilamides (101). Hansch also used the homolytic substituent constants (ER) of Yamamoto and Otsu (102) in analyzing the activity of selected chloramphenicol derivatives (103). The resulting correlations led to the hypothesis of a free-radical mechanism of chloramphenicol action. Substituent measures of it electron charge density distributions (07 and ov)... 

Hansch et al. have used the hemolytic substituent constants, Er, of Yamamoto and Otsu for correlations with antibacterial activity against E. ooli for a series of chloramphenicol derivatives. To determine the importance of electronic effects relative to hydrophobic properties, the authors performed regression analyses on the data of Garrett et al. Using Er derived from free-radical reactions and the hydrophobic parameter, it, of Hansch, they obtained eq 1 and eq 2... 

The very first experiences gained with METABOLEXPERT seem to support these ideas. Though some of the metabolic transformations of the chloramphenicol derivatives seemed to be straightforward for experts in metabolism research, the chemists of the cooperating synthetic team were happy to get predictions about expected metabolites within a few hours and thought that a good part of superfluous synthetic efforts was spared. 

Studies on the biosynthesis of chloramphenicol by Streptomyces vene-zuelae. based on a carbon-by-carbon degradation of the antibiotic using labeled glucose, were consistent with the shlkimate pathway of aromatic biosynthesis.15/ The structure-activity relationships of 37 chloramphenicol derivatives were studied. The trifluoro derivative is 1,7 times more active against E. coll than the parent.158... 

The bacterial CAT enzyme catalyzes the transfer of acetyl groups to chloramphenicol from acetyl coenzyme A (acetyl CoA). In a typical assay, this reaction is monitored with relabeled chloramphenicol After separation by thin-layer chromatography (TLC), the acetylated and nonacetylated forms can be distinguished by autoradiography, and quantitation is achieved by isolating the forms and measuring their radioactivity in a scintillation counter. Quantitative CAT assays have been performed on Drosophila tissue culture and dissociated cell extracts (Di Nocera and Dawid 1983 Benyajati and Dray 1984 Thummel et al. 1988 Krasnow et al. 1989 Ye et al. 1997). CAT can also be detected with commercially available antibodies. In addition, a nonradioactive CAT assay exists that utilizes a fluorescent chloramphenicol derivative (Molecular Probes). 

Haughland, R. P Kang, H. C. Young, S. L. Melner, M. H. Fluorescent chloramphenicol derivatives for determination of chloramphenicol acetylbansferase activity. U.S. Patent 5364764,1994. 

Chloramphenicol. Only chloramphenicol and a few closely related analogues fall iato this group. Chloramphenicol, a nitro benzene derivative of dichloroacetic acid, inhibits proteia biosyathesis. 

Plasmid-mediated bacterial inactivation of chloramphenicol and thiamphenicol can potentially lead to three products, the 3- 0-acetyl (3), 1-0-acetyl (4), and 1,3-di-O-acetyl (5) derivatives as shown in Figure 1. 

The preferred substrates of acetyltransferases are amino-groups of antibiotics, like chloramphenicol, strepto-gramin derivatives, and the various aminoglycosides. The modification is believed to block a functional group involved in the drug-target-interaction. All acetyltransferases use acetyl-coenzyme A as cofactor. 

Plasmid- ortransposon-encoded chloramphenicol acetyltransferases (CATs) are responsible for resistance by inactivating the antibiotic. CATs convert chloramphenicol to an acetoxy derivative which fails to bind to the ribosomal target. Several CATs have been characterized and found to differ in properties such as elecfrophorefic mobilify and cafalyfic acfivify. 

An important feature of the antibiotic chloramphenicol (9) is the presence of the dichloroacetamide function. Inclusion of this amide in a simpler molecule, teclozan (15), leads to a compound with antiamebic activity. Whether this is cause and effect or fortuitous is unclear. The synthesis begins with alkylation of the alkoxide derived from ethanolamine (10) with ethyl iodide to give the aminoether (11). Reaction of a,a -dibromo-p-xylene (12) with 2-nitropropane in the presence of base leads to dialdehyde (13). The reaction probably proceeds by O-alkylation on the nitropropyl anion... 

P-Nitro alcohols can be hydrogenated to the corresponding amino alcohols with retention of configuration the stereoselective Henry reaction is a useful tool in the elaboration of pharmacologically important P-amino alcohol derivatives including chloramphenicol, ephedrine, norephedrine, and others. Some important P-amino alcohols are listed in Scheme 3.11.107... 

When the desired reaction is not essential, creative thinking must be employed. Adding a selectable moiety may create derivatives of a natural substrate that allow for selection. Hwang et al. [41] used this approach to derive a screen for enantioselective hydrolases. By linking the antibiotic chloramphenicol to either the R- or, S -enantiomer of 2-phenylbutyric acid, they showed that Exiguobacterium acetylicum could hydrolyse the l -form (as the released chloramphenicol inhibited growth) but not the, S -form. However, one must be aware that activity on a derivative may not always correspond to the activity on the actual desired substrate. 

Fig. 11.7. The two pathways of dechlorination of chloramphenicol (11.39). Cytochrome P450 catalyzed oxidation to yield the oxamic acid derivative (11.40), and hydrolytic dechlorination to yield both oxamic acid (11.40) and primary alcohol (11.41) metabolites [75].

In contrast to oxidative dechlorination, the hydrolytic dechlorination of chloramphenicol replaces a Cl-atom with a OH group to yield a (monochlo-ro)hydroxyacetamido intermediate. The latter, like the dichloro analogue, also eliminates HC1, but the product is an aldehyde that is far less reactive than the oxamoyl chloride intermediate. Chloramphenicol-aldehyde undergoes the usual biotransformation of aldehydes, namely reduction to the primary alcohol 11.41 and dehydrogenation to the oxamic acid derivative 11.40 (Fig. 11.7). 

In conclusion, the oxamic acid derivative is produced by two distinct metabolic pathways, namely by oxidative and hydrolytic dechlorinations. In contrast, the primary alcohol metabolite 11.41 can be produced only by hydrolytic dechlorination and is, thus, an unambiguous marker of this pathway. The alcohol 11.41 is a known urinary metabolite of chloramphenicol in humans. 

Figure 10.5. Selected examples of TAR binders. From left to right Arginine-kanamycin A conjugate, diphenylfuran derivative, poly amine-acridine conjugate, and chloramphenicol-neomycin B conjugate.

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