The ribostamycin aminoglycosides are pseudotrisaccharides compounds 28-36 that lack the ring IV (neosamine) of neomycins in their structure [45-52]. They show antibacterial activity greater than neamine, but less than neomycins.
(e) 4,6-Disubstituted 2-Deoxystreptamine. These pseudotrisaccharides (compounds 37-61) are effective against a variety of gram-positive and gram-negative bacteria but are highly susceptible to the drug resistance enzymes (see below). Kan-amycins B and C are minor components of the kanamycin A producing organism Streptomyces kanamyceticus . In comparison to kanamycins A and B, kanamycin C (39) has a much weaker antibacterial activity against S. aureus and E. coli. Tobramycin 42 or 3'-deoxykanamycin B is a clinically useful antibiotic because of its ability to avoid phosphorylation at the 3'-position by the resistance enzymes aminoglycoside 3' -phosphotransferases.
Another important class of antibiotics in this group is the gentamicins (compounds 45-60 ), of which about 25 different compounds have been isolated from the same or different culture filtrates [35,57,58,59,60,61]. Commercially available gen-tamycin C, which is a valuable therapeutic agent, is a mixture of gentamicins C1, Cia, and C2.
Gentamicins A3, B, and B1 bear a hydroxyl group instead of an amine at the C2'-position. Sisomicin 61 and related compounds are derivatives of gentamicin C, which have an unsaturated amino sugar moiety in their structures. Sisomicin is more active than the gentamicin C mixture, but it is also twofold more toxic (compounds 61-65 ; see pg. 317).
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