Quinolones
The quinolones have a nucleus of two fused six-member
rings that when substituted with fluorine become fluoroquinolones, which are
now the dominant quinolones for treatment of bacterial infections. Among the
fluoroquinolones, ciprofloxacin,
norfloxacin, and ofloxacin, the
addition of a piperazine ring and its methylation alter the activity and
phar-macologic properties of the individual compound. The primary target of all
quinolones is DNA topoisomerase (gyrase), the enzyme responsible for nicking,
supercoiling, and sealing bacterial DNA during replication. Bacterial
topoisomerases have four subunits, one or more of which are inhibited by the
particular quinolone. The enhanced activity and lower fre-quency of resistance
seen with the fluoroquinolones is attributed to binding at multiple sites on
the enzyme. This greatly reduces the chance a single mutation can lead to
resistance, which was a problem with the first quinolone, nalidixic acid, a
single-binding site agent.
The fluoroquinolones are highly active and
bactericidal against a wide range of aer-obes and facultative anaerobes.
However, streptococci and Mycoplasma
are only margin-ally susceptible, and anaerobes are generally resistant.
Ofloxacin has significant activity against Chlamydia,
whereas ciprofloxacin is particularly useful against P. aeruginosa. Fluoroquinolones has several favorable pharmacologic
properties in addition to their broad spectrum. These include oral
administration, low protein binding, good distribution to all body
compartments, penetration of phagocytes, and a prolonged serum half-life that
allows once- or twice-a-day dosing. Norfloxacin and ciprofloxacin are excreted
by hepatic and renal routes, resulting in high drug concentrations in the bile
and urine. Ofloxacin is excreted primarily by the kidney.
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