Clinical and laboratory studies with tobramycin

Antibiotic blood concentrations in patients successfully treated with tobramycin

Thirty-nine patients with severe Gram-negative infections were treated with parenteral tobramycin. Thirty-one (79%) were cured of their infection. Tobramycin was most effective in the therapy of patients with urinary tract infections, arthritis and skin and soft tissue infections and relatively less effective in patients with septicaemia, pneumonia, and osteomyelitis. The infection was cured more frequently in patients who achieved a high ratio between the peak serum concentration of tobramycin and the minimal inhibitory concentration of tobramycin against the pathogenic organism (so-called therapeutic ratio). The ratio was greater than 4.0 in 11 of 13 (85%) assays performed in 12 cured patients, whereas this ratio was achieved in only 3 of 10 (30%) instances in 5 patients in whom the therapy failed (P less than 0.05). The latter group also included a greater proportion of patients with an ultimately fatal illness, such as lung cancer and uraemia, compared to the former successfully treated group. Adverse effects of tobramycin on renal function were transitory. No significant effect of tobramycin on the hearing was observed.

Intravenous trimethoprim-sulfamethoxazole alone or combined with tobramycin for infections in cancer patients

A total of 120 episodes of infection in 113 cancer patients were treated with intravenous trimethoprim-sulfamethoxazole (TMP-SMX) alone (92 episodes) or with TMP-SMX plus continuous infusion tobramycin (28 episodes). The overall response rates were 47% and 75%, respectively. The majority of episodes had failed to respond to prior antibiotics. Pneumonia was the most common infection, and Klebsiella pneumoniae was the most common pathogen. TMP-SMX plus tobramycin cured 86% of episodes of septicemia and 76% of episodes of pneumonia, whereas TMP-SMX alone cured 20% and 42%, respectively. The initial neutrophil count did not appear critical in determining the outcome of infection. It was the change in the neutrophil count during the infection that appeared important. The outcome of infection was less favorable where abnormal renal and/or hepatic functions were documented. The sensitivity of the organism in vitro to TMP-SMX and/or tobramycin correlated well with the in vivo response. Intravenous TMP-SMX was well tolerated with a 4% incidence of reversible toxicity. A 15% incidence of renal toxicity was attributable to tobramycin. Intravenous TMP-SMX appears to be useful antimicrobial regimen for the therapy of infections caused by susceptible organisms in cancer patients.

Clinical pharmacology of tobramycin in infants

Tobramycin was used in the treatment of severe infections in infants, mostly newborns and prematures. Serum half-life of the drug was 3.5 h in infants older than one week and above 2,500 g, and 8.0 h in infants below one week and less than 2,500 g. Apparent volume of distribution was similar in both groups, 0.24 l/kg. Intravenous infusions of the drug gave considerably higher plasma levels than intramuscular infections. Particularly in the newborn and premature group intramuscular injections gave fairly unpredictable plasma concentrations.

[Tobramycin Concentration in Human Lung Tissue (author's transl)]

In 27 patients it was experimentally demonstrated that after an i. m. injection of 80 mg tobramycin concentrations are obtained in human lung tissue which for about two hours are higher than the minimal inhibition concentration of most gramnegative organisms. In 14 samples of the latissimus dorsi muscle no detectable tobramycin concentrations were found up two hours after injection of the same dose. The ratio of whole blood concentration and lung tissue concentration is 2 : 1. The measurement of tobramycin concentration in whole blood thus enables the concentration in lung tissue to be estimated. The results show--like all serum and tissue level assays--a high individual variation, which might be dependent on the non-defined factors of binding or metabolism, especially as during the examination period of two hours no concentration balance is accomplished between the two compartments. Administration of 80 mg tobramycin i. m. every six hours is recommended in patients with pulmonary infections due to gramnegative organisms.

Tobramycin: a review of its antibacterial and pharmacokinetic properties and therapeutic use

SYNOPSIS

Tobramycin is a new aminoglycoside antibiotic with a broad antibacterial spectrum in vitro, and pharmacokinetic properties similar to those for gentamicin. Tobramycin is more active than gentamicin against Pseudomonas aeruginosa and active against many gentamicin resistant strains, but is not active against enterobacteriaceae resistant to gentamicin. Theoretically, tobramycin has an advantage over gentamicin against infections caused by P. aeruginosa, but any advantage in clinical practice has yet to be adequately demonstrated. Clinical experience with tobramycin is considerably less than with gentamicin. Whilst tobramycin appears to offer no clear advantages over gentamicin against sensitive organisms it is indicated in infection caused by strains of P. aeruginosa which are resistant to gentamicin, but sensitive to tobramycin. Like gentamicin, tobramycin acts synergistically with corbenicillin and the cephalosporins. The efficacy of the tobramycin-carbenicillin combination has been shown in endocarditis caused by P. aeruginosa which was unresponsive to gentamicin plus carbenicillin. Ototoxicity and nephrotoxicity similar to that seen with other animoglycosides have been encountered in therapeutic trials with tobramycin and wider clinical experience is necessary to determine the relative incidence of these side-effects with gentamicin and tobramycin used under similar conditions. Antimicrobial activity: In comparative studies, in vitro, tobramycin is more active than gentamicin against clinical isolates of Pseudomonas aeruginosa. Similarly, the inhibitory index, which is the ratio between the serum concentration attained at usual therapuetic doses and the minimum inhibitory concentration, for Pseudomonas aeruginosa is higher for tobramycin than for gentamicin. Against Gram-negative bacteria other than Pseudomonas spp. the spectrum of activity of tobramycin is similar to that of gentamicin. For most species the activity of tobramycin is slightly less than that of gentamicin. Gentamicin is consistently more active than tobramycin against Serratia marcescens. Like other aminoglycoside antibiotics, tobramycin is active in vitro in low concentrations against Staphylococcus aureus. Tobramycin is essentially inactive against Streptococcus pyogenes, Streptococcus faecalis and Streptococcus pneumoniae (pneumococci). Maner aminoglycosides and of other antibiotics against various bacteria in vitro, but comparisons between studies cannot always be interpreted literally because the activity of many antibiotics in vitro, including tobramycin, is influenced by the nature of the culture media and the presence of certain salts. The sensitivity of P. aeruginosa to tobramycin is influenced by the magnesium, and calcium content of the culture media whilst that of all species is reduced by sodium ions. Wide variations in the concentration of these ions may result in divergent MIC values and an inappropriate choice of antibacterial agent to treat pseudomonas infection...