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Quinolones: SAR of Quinolones

  • Quinolones are a class of synthetic antibacterial agents that are effective in treating various bacterial infections, including urinary tract infections (UTIs).

  • They are derived from the parent compound nalidixic acid, which was discovered in the 1960s.

  • Later generations of quinolones are referred to as fluoroquinolones due to the addition of a fluorine atom at position 6 of the quinolone core structure.

SAR of Quinolones

  • Structure-activity relationships (SAR) of quinolones are essential for understanding their potency, spectrum of activity, and pharmacokinetic properties.

  • Key structural features of quinolones and their relationships with antimicrobial activity include:

1. Central ring:

  • The quinolone core structure consists of a bicyclic system containing a pyridone ring fused to a quinoline ring. The presence of a carbonyl group at position 4 and a carboxylic acid at position 3 are essential for antibacterial activity.

2. Fluorine at position 6:

  • The addition of a fluorine atom at position 6 on the quinolone core structure greatly enhances the antimicrobial potency and spectrum of activity. This modification results in the fluoroquinolone class of compounds.

3. Substituents at position 7:

  • The nature of the substituent at position 7 of the quinolone core structure affects the potency and spectrum of activity. Generally, a piperazinyl or similar amine-containing group at position 7 enhances activity against Gram-negative bacteria and atypical pathogens. For example, ciprofloxacin and levofloxacin have a piperazinyl group at this position.

4. Substituents at position 1:

  • The presence of an alkyl or cycloalkyl group at position 1 of the quinolone core structure is essential for antimicrobial activity. The nature of the substituent at position 1 can affect pharmacokinetic properties, such as oral bioavailability and tissue distribution. For instance, moxifloxacin has a cyclopropyl group at position 1, which contributes to its enhanced activity against Gram-positive bacteria and anaerobes.

5. Substituents at position 8:

  • The presence of a halogen atom (typically chlorine or fluorine) at position 8 of the quinolone core structure can increase the potency and spectrum of activity against certain bacterial species. However, it can also increase the potential for phototoxicity. Lomefloxacin, for example, has a fluorine atom at position 8, which contributes to its increased activity against Gram-negative bacteria, but it also has a higher risk of phototoxicity compared to other fluoroquinolones.

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