The activities of tobramycin derivatives acetylated and ethylated on the
6
'-
N, 2'-
N and 3-
N positions were examined. The MICs of these derivatives against tobramycin sensitive strains indicated that 2'-
N-ethylated and
6
'-
N-ethylated derivatives have a fairly good activity, and confirmed that the 3-
N position is the most important one for antibiotic activity since 3-
N derivatives were less active. The MICs of these derivatives against tobramycin resistant strains, and their inactivation by tobramycin modifying enzymes were examined. These results showed that 2'-
N or
6
'-
N ethylation protects the drug against inactivation by AAC(2') or AAC(
6
'), respectively, and 2'-
N-ethyltobramycin and
6
'-
N-ethyltobramycin were active against strains containing these modifying enzymes. On the other hand, 3-
N ethylation protects the drug against inactivation by AAC(3) but 3-
N-ethyl tobramycin does not inhibit strains containing this enzyme.
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