Other antibiotics

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Chapter: Medicinal Chemistry : Antibiotics

Examples of other antibiotics are chloramphenicol, rifampicin and mupirocin.


Other antibiotics

Examples of other antibiotics are chloramphenicol, rifampicin and mupirocin.

 

i. Chloramphenicol or chloromycetin

Chloramphenicol has a spectrum of activity resembling that of the tetracyclines except that it exhibits a bit less activity against some gram-positive bacteria. It is isolated from Salmonella venezuelae by Ehrlich et al in 1947. It contains chlorine and is obtained from an actinomycete, and thus, named as chloromycetin. It is specifically recommended for the treatment of serious infections caused by H. influnzae, S. typhi (typhoid), S. pneumoniae, and N. meningitides. Its ability to penetrate into the CNS presents an alternative therapy for meningitis and exhibits antirickettsial activity.

Structure


Properties and uses: Chlorampenicol is a white or greyish-white or yellowish-white crystalline powder or fine crystals, slightly soluble in water, soluble in alcohol and propylene glycol. It was the first, and still is the only therapeutically important antibiotic to be produced in competition with microbiological processes. It contains a nitrobenzene moiety and is a derivative of dichloroacetic acid. Since it has two chiral centres, four isomers are possible. The D-(-) threo is the biologically active form. It is used in the treatment of typhoid fever caused by S. typhi. The most serious adverse effect of chloramphenicol is bone marrow depression and fatal blood dyscrasias.

Assay: Dissolve the sample in water, dilute with the same solvent, and measure the absorbance at the maximum of 278 nm using ultraviolet spectrophotometer.

Dose: Usual adult dose is 500 mg every 6 h.

Dosage forms: Chloramphenicol capsules I.P., B.P., Chloramphenicol ear drops I.P., B.P., Chloramphenicol eye ointment I.P., B.P., Chloramphenicol eye drops B.P.

Synthesis


 

SAR of Chloramphenicol


a.            Modification of p-nitrophenyl group.

b.           Modification of dichloroacetamide side chain.

c.            Modification of 1, 3-prepanediol.

Modification of p-nitrophenyl group: The p-nitrophenyl group may be modified through the following ways:

a.            Replacement of the nitro group by other substituents leads to a reduction in activity.

b.           Shifting of the nitro group from the para position also reduces the antibacterial activity.

c.            Replacement of phenyl group by the alicyclic moieties results in less potent compounds.

Modification of dichloroacetamido side chain: Other dihalo derivatives of the side chain are less potent although major activities are retained.

Modification of 1,3-propanediol: If the primary alcoholic group on C-1 atom is modified, it results in a decrease in activity; hence, the alcoholic group seems to be essential for activity.

 

ii. Rifampicin


Properties and uses: Rifampicin is a reddish-brown or brownish-red crystalline powder, slightly soluble in water, acetone, and alcohol and soluble in methanol. It is a broad-spectrum bactericidal antibiotic, structurally similar to complex macrocyclic antibiotic obtained from S. mediterrani. They belong to a new class of antibiotics called as ansamycins. Five types, that is, rifampicin A, B, C, D, and E are present. It penetrates well into cerebrospinal fluid and is, therefore, used in the treatment of tuberculous meningitis.

Assay: Dissolve the sample in methanol and dilute it with the same solvent. Dilute the solution with phosphate buffer solution pH 7.4 and measure the absorbance at the maximum at 475 nm, using phosphate buffer solution pH 7.4 as blank.

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