Pharmacokinetics of tobramycin and gentamicin in abusers of intravenous drugs

The kinetics of aminoglycoside elimination were determined in 18 hospitalized narcotics abusers receiving gentamicin or tobramycin for treatment of severe infection. Rapid aminoglycoside elimination (requiring doses of > 5 mg/kg per day to maintain adequate drug levels) was noted in 12 of the 18 patients (18 of 27 clearance studies). Patients found to have rapid elimination were younger (P <0.01) and had larger drug distribution volumes (P <0.005), greater measured creatinine clearances (P <0.05), and lower creatinine levels in serum (P <0.001) than those with normal elimination. Nevertheless, by regression analysis, age, creatinine levels in serum, creatinine clearances, and drug distribution volumes proved to be unreliable predictors of individual aminoglycoside clearance. Measured drug half-life in serum appeared to be the only reliable predictor of drug clearance (r = 0.93). Patients with rapid drug elimination had aminoglycoside clearances 16 to 43% greater than measured creatinine clearances, suggesting an extraglomerular route of drug elimination. We conclude that in drug abuse patients a significant and clinically unpredictable interpatient variation occurs in aminoglycoside elimination and that accurate serum kinetics are needed to determine therapeutic dosing. In addicts younger than 35 years, with a creatinine level of <1.0 mg/100 ml in serum, the risk of inadequate therapy is high if standard dosing guidelines are followed. For this group, initial dosing of 8 mg/kg per day, with a drug half-life determination on the first dose, is recommended. Pharmacokinetic analysis is critical for all drug abusers treated with aminoglycosides for serious infection.