Streptomycin pharmacokinetics

Pharmacokinetics Following IM injection of 1 g of streptomycin as the sulfate, a peak serum leyel of 25 to 50 mcg/mL is reached within 1 hour, diminishing slowly to about 50% after 5 to 6 hours. Appreciable concentrations are found in all organ tissues except the brain. Streptomycin is excreted by glomerular filtration. 

TTie major clinically important aminoglycosides are amikacin (Amikin), gentamicin Garamycin), kanamycin (Kantrex), netilmicin Netromycin), neomycin Myci-fradin), streptomycin, and tobramycin (Nebcin). Their pharmacokinetic characteristics are shown in Table 46.1. 

Results of pharmacokinetic studies of streptomycin are in most cases also applicable to dihydrostreptomycin and vice versa. In animals, the absorption of both streptomycin and dihydrostreptomycin is poor via the oral route but rapid after intramuscular administration. In cattle, peak serum levels were obtained 1 h after intramuscular injection of either streptomycin or dihydrostreptomycin (18), whereas serum concentrations produced in sheep and horses paralleled those obtained in cattle (19). As a result, most of an oral dose is recovered in the feces whereas most of a parenteral dose is recovered in the urine. However, if kidney function is severely impaired, little of an intramuscularly administered dose is excreted in the urine. 

In the purposeful search that followed the demonstration of the clinical efficacy of penicillin, streptomycin was obtained from Streptomyces griseus in 1944, cultured from a heavily manured field, and also from a chicken s throat. Aminoglycosides resemble each other in their mode of action, and their pharmacokinetic, therapeutic and toxic properties. The main differences in usage reflect variation in their range of antibacterial activity crossresistance is variable. 

These agents are all secondary drugs used only for treatment of disease caused by resistant M. tuberculosis or by nontuberculous mycobacteria. They aU are given parenteraUy and have similar pharmacokinetics and toxicity. Since these agents are potentially ototoxic and nephrotoxic, no two drugs from this group should be used simultaneously, and they should not be combined with streptomycin. 

Tobramycin is almost identical to gentamicin in both its pharmacodynamic and pharmacokinetic properties. However, it is much less active than either gentamicin or streptomycin when used in combination with a penicillin in the treatment of enterococcal endocarditis. The answer is (E). 

Describe the pharmacodynamic and pharmacokinetic properties of the first-line drugs used in tuberculosis (isoniazid, ethambutol, pyrazinamide, rifampin, and streptomycin). 

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