Choice of Antimicrobial Agent Part 3 | Drug Factor affecting selection of Antibiotic | MCQ GPAT NEET

Drug factors

When any one of a number o f AMAs could be used to treat an infection, choice among them is
based upon specific properties of these AMAs:
1. Spectrum of activity: For definitive therapy, a narrow-spectrum drug which selectively affects the concerned organism is preferred, because it
is generally more effective than a broad-spectrum AMA, and is less likely to disturb the normal microbial flora. However. for empirical therapy,
often a broad-spectrum drug has to be used to cover all likely pathogens.
2. Type of activity: Many infections in patients with normal host defence respond equally well to bacteriostatic and bactericidal AMAs. But several acute infections resolve faster with a cidal than a static drug. because
the cidal drug directly reduces the number of bacteria al the site o f infection. while the static drug only prevents increae in their number.
Many bactericidal drugs exert prolonged postantibiotic effect (PAE) so that maintenance of drug level continuously above the MIC is not
essential. With bacteriostatic AMAs the bacteria start multiplying quickly when drug level falls below the MIC, resulting in relapse of infection.
A bactericidal antibiotic is clearly superior to bacteriostatic one in treating patients with impaired host defence, life-threatening infections,
infections at less accessible sites (SABE) or when carrier state is possible (e.g. typhoid).

3. Sensitivity of the organism: Assessed on the basis of MIC values (if available) and consideration of PAE.

4. Relative toxicity: Obviously, a less toxic antibiotic is preferred, e.g. a P-lactam over an aminoglycoside or erythromycin over clindamycin.

5. Pharmacokinetic profile: For optimum action the antibiotic has to be present at the site of infection in sufficient concentration for an adequate length of time. This depends on their pharmacokinetic characteristics.
Most antibiotics are given at 2 to 4 half-life intervals- thus anaining therapeutic concentrations
only intermittently. For many organisms, aminoglycosides, fluoroquinolones and metronidazole produce 'concentration-dependent killing', i.e. inhibitory effect depends on the ratio of peak
concentration to the MIC. The same daily dose of gentamicin produces better action when given as a single dose than if it is divided into are inflamed. Ampicillin, cephalosporins and erythromycin attain high biliary concentration.

6. Route of administration: Many AMAs can be given orally as welI as parenterally, but aminoglycosides, penicillin G, ticarcillin, many cephalosporins, vancomycin, etc. have to be given by injection only. For less severe infections, an oral antibiotic is preferable; but for serious infections, e.g. meningitis, spreading cellulitis, septicaemias, a parenteral antibiotic would be more reliable.

7. Evidence of clinical efficacy: Relative value of different AM As in treating an infection is decided on the basis of comparative clinical trials. Optimum dosage regimens and duration of treatment are also determined on the basis of such trials. Reliable clinical trial data, if available, is the final guide for choice of the antibiotic. '

8. Cost: Less expensive drugs are to be pre 2- 3 portions. On the other hand, beta-lactams, glycopeptides and macrolides produce 'timedependent killing'. i.e. antimicrobial action depends on the length or time the concentration remains above the MIC; division of daily dose improves the effect. However, the doses should be so spaced that the surviving organisms again start multiplying and a cidal action is exerted.
Penetration to the site of infection also depends on the pharmacokinetic properties of the drug. A drug which penetrates better and attains higher concentration at the site of infection is likely to be more effective . The
fluoroquinolones have excellent tissue penetration- attain high concentrations in soft tissues, lungs, prostate, joints, etc. Ciprofloxacin and rifampin have very good intracellular penetration. Cefuroxime. ceftriaxone, chloramphenicol, ciprofloxacin attain high CSF concentration. On the other hand, penicillins and aminoglycosides
penetrate poorly into CSF unless meninges

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