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Gantrisin

Clinical Pharmacology
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Clinical Pharmacology

Gantrisin

Microbiology

The sulfonamides are bacteriostatic agents and the spectrum of activity is similar for all. Sulfonamides inhibit bacterial synthesis of dihydrofolic acid by preventing the condensation of the pteridine with aminobenzoic acid through competitive inhibition of the enzyme dihydropteroate synthetase. Resistant strains have altered dihydropteroate synthetase with reduced affinity for sulfonamides or produce increased quantities of aminobenzoic acid.

Susceptibility Tests

Diffusion Techniques: Quantitative methods that require measurement of zone diameters give the most precise estimate of the susceptibility of bacteria to antimicrobial agents. One such standard procedure3 which has been recommended for use with disks to test susceptibility of organisms to sulfisoxazole uses the 250- or 300- µg sulfisoxazole disk. Interpretation involves the correlation of the diameter obtained in the disk test with the minimum inhibitory concentration (MIC) for sulfisoxazole. Reports from the laboratory giving results of the standard single-disk susceptibility test with a 250- or 300- µg sulfisoxazole disk should be interpreted according to the following criteria:

Zone Diameter (mm)

Interpretation

³ 17

Susceptible

13-16

Moderately susceptible

£ 12

Resistant

A report of ''susceptible'' indicates that the pathogen is likely to be inhibited by generally achievable blood levels. A report of ''moderately susceptible'' suggests that the organism would be susceptible if high dosage is used or if the infection is confined to tissues and fluids in which high antimicrobial levels are attained. A report of ''resistant'' indicates that achievable concentrations are unlikely to be inhibitory, and other therapy should be selected.

Standardized procedures require the use of laboratory control organisms. The 250- or 300- µg sulfisoxazole disk should give the following zone diameters:

Organism

Zone Diameter (mm)

E Coli ATCC 25922

18-26 mm

S. aureus ATCC 25923

24-34 mm

Dilution Techniques: Use a standardized dilution method4 (broth, agar, microdilution) or equivalent with sulfisoxazole powder. The MIC values obtained should be interpreted according to the following criteria:

MIC (mcg/mL)

Interpretation

£ 256

Susceptible
Brand Name: Gantrisin
Generic Name: Acetyl Sulfisoxazole
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