Pharmacokinetics of intravenously administered tobramycin in normal volunteers and in renal-impaired and hemodialyzed patients

Tobramycin Inhalation Powder (TIP): An Efficient Treatment Strategy for the Management of Chronic Pseudomonas Aeruginosa Infection in Cystic Fibrosis

Repeated bouts of acute and chronic lung infections are responsible for progressive pulmonary function decline in individuals with cystic fibrosis (CF), ultimately leading to respiratory failure and death. Pseudomonas aeruginosa is the archetypical CF pathogen, causes chronic infection in 70% of individuals, and is associated with an accelerated clinical decline. The management of P. aeruginosa in CF has been revolutionized with the development and widespread use of inhaled antibiotics. Aerosol delivery of antimicrobial compounds in CF enables extremely high concentrations of antibiotics to be reached directly at the site of infection potentially overcoming adaptive resistance and avoiding the potential for cumulative systemic toxicities. Tobramycin inhalation powder (TIP) represents the first dry powder inhaled (DPI) antibiotic available for use in CF. DPIs are notable for a markedly reduced time for administration, ease of portability, and increased compliance. TIP has been developed as a therapeutic alternative to tobramycin inhalation solution (TIS), the standard of care for the past 20 years within CF. Relative to TIS 300 mg nebulized twice daily in on-and-off cycles of 28 days duration, TIP 112 mg twice daily via the T-326 inhaler administered on the same schedule is associated with marked time savings, increased patient satisfaction, and comparable clinical end points. TIP represents an innovative treatment strategy for those individuals with CF and holds the promise of increased patient compliance and thus the potential for improved clinical outcomes.

Pharmacokinetic and Pharmacodynamic Aspects of Antibiotic Use in High-Risk Populations

The study of pharmacokinetics includes the absorption, distribution, metabolism, and elimination of drugs. The pharmacologic effect that a medication has on the body is known as pharmacodynamics. With antimicrobials, pharmacokinetic and pharmacodynamic parameters become especially important because of the association between host drug concentrations, microorganism eradication, and resistance. This article focuses on the pharmacokinetic changes that can occur with antimicrobials when they are used in patients at high risk of infections and how they influence pharmacodynamic effects. The populations described here include patients with obesity and diabetes mellitus, renal or hepatic failure, chronic lung disease, severe burns, and long-term prosthetic devices and the elderly.

Peritoneal Dialysis Alters Tolbutamide Pharmacokinetics in Rats with Experimental Acute Renal Failure

The plasma concentration profile of the antidiabetic agent tolbutamide was investigated in glycerol-induced acute renal failure (ARF) rats receiving or not receiving peritoneal dialysis (PD) to assess the impact of performing dialysis on tolbutamide pharmacokinetics. It was revealed that the plasma concentration of tolbutamide was decreased by 23.4% by performing PD in ARF rats, while it was not changed by PD in normal rats. The decrease in the plasma concentration of tolbutamide was nearly proportional to the increase in its volume of distribution. To clarify the mechanisms responsible for the decreased tolbutamide concentration caused by PD, the plasma protein binding of tolbutamide was examined in normal and ARF rats. The plasma unbound fraction of tolbutamide was higher in ARF rats than in normal rats, and the dissociation constants were 3.5+/-0.7 and 5.5+/-0.2 microg in normal and ARF rats, respectively. These results indicated that the unbound fraction of tolbutamide was increased in ARF rats because of its protein binding being suppressed. It is therefore likely that since a measurable amount of tolbutamide can distribute in peritoneal dialysate in ARF rats, but not in normal rats, the plasma concentration of tolbutamide was decreased by performing PD only in ARF rats. These findings suggest that diabetes medication with tolbutamide should be carefully performed in patients receiving dialysis treatment.

Drug dosing in chronic kidney disease

Patients with chronic kidney disease (CKD) are at high risk for adverse drug reactions and drug-drug interactions. Drug dosing in these patients often proves to be a difficult task. Renal dysfunction-induced changes in human pathophysiology regularly results may alter medication pharmacodynamics and handling. Several pharmacokinetic parameters are adversely affected by CKD, secondary to a reduced oral absorption and glomerular filtration; altered tubular secretion; and reabsorption and changes in intestinal, hepatic, and renal metabolism. In general, drug dosing can be accomplished by multiple methods; however, the most common recommendations are often to reduce the dose or expand the dosing interval, or use both methods simultaneously. Some medications need to be avoided all together in CKD either because of lack of efficacy or increased risk of toxicity. Nevertheless, specific recommendations are available for dosing of certain medications and are an important resource, because most are based on clinical or pharmacokinetic trials.

Human variability in the renal elimination of foreign compounds and renal excretion-related uncertainty factors for risk assessment

Renal excretion is an important route of elimination for xenobiotics and three processes determine the renal clearance of a compound [glomerular filtration (about 120 ml/min), active renal tubular secretion (>120 ml/min) and passive reabsorption (<120 ml/min)]. Human variability in kinetics has been quantified using a database of 15 compounds excreted extensively by the kidney (>60% of a dose) to develop renal-excretion related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [renal and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was low for both routes of exposure, with coefficients of variation of 21% (oral) and 24% (intravenous) that were largely independent of the renal processes involved. Renal-excretion related uncertainty factors were below the default kinetic uncertainty factor of 3.16 for most subgroups analysed with the exception of the elderly (oral data) and neonates (intravenous data) for whom renal excretion-related factors of 4.2 and 3.2 would be required to cover up to 99% of these subgroups respectively.

Safety of intravenous bolus administration of gentamicin in pediatric patients

OBJECTIVE

To determine the safety of gentamicin administered intravenously as a bolus.

METHODS

All patients (n = 123, ages: up to 18y, 121; 21y, 1; 31y, 1) who received gentamicin intravenously as a bolus over a four-month period were studied retrospectively. Patient demographics, type of infection, dosing regimen, length of therapy, peak and trough serum concentrations, blood urea nitrogen, serum creatinine, and urine output were reviewed. Patients were stratified into four groups and data analyzed statistically.

RESULTS

Mean initial dose (5.32 +/- 2.38 mg/kg/d) was consistent with established guidelines for age and kidney development, with subsequent adjustments based on serum concentrations. Susceptible organisms were eradicated with a mean length of therapy of 6.9 +/- 6.9 days (range 1-35). Patients received a median of nine doses: 42% received doses every eight hours and 33% received doses every 24 hours. No relationship between dosing and abnormal serum creatinine were found (p = 0.69). The estimated cost savings mainly from less nursing time and lower equipment and supply use were $50/patient with bolus administration of gentamicin.

CONCLUSIONS

Intravenous bolus administration was safe in pediatric patients and was associated with lower costs.

Altered tobramycin pharmacokinetics during chemoprophylaxis in bladder surgery

The effect of bladder surgery on the pharmacokinetics of tobramycin in hospitalized patients was studied. Fourteen patients with vesical neoplasia undergoing urinary tract surgery were given tobramycin in a dose of 2 mg/kg of body weight. Each patient received the dose at the induction of anesthesia, about 1 h before surgical incision. For seven patients, the drug was also administered 3 weeks later when nutritional conditions were normal. The pharmacokinetic parameters were determined by a two-compartment open model. Except for renal clearance, no significant difference appeared between pharmacokinetic parameters determined from serum data during peri- and postoperative periods. During this work, tobramycin excretion in urine was studied. Twenty-four hours after drug administration, the mean urine tobramycin levels were 25.5 +/- 9.06 and 41.6 +/- 21.5 micrograms/ml after peri- and postoperative administration, respectively; these values were higher than the MICs for most urinary tract pathogens. Seventy-two hours after perioperative administration, the mean value was still elevated (3.54 micrograms/ml), but 72 h after postoperative administration, the urinary tobramycin concentration was not detectable. The percentages of tobramycin recovered unchanged in urine were 54 and 79% after peri- and postoperative administration, respectively. When tobramycin was administered during surgery, a long terminal log-linear phase, with a mean half-life of 25.6 h, was detected. The ratio of renal clearance to total body clearance was 0.52 and 0.79 after peri- and postoperative administration, respectively.

Aminoglycoside antibiotics in clinical use

Aminoglycosides are among the most used antibiotics despite competitive pressure from newer beta-lactam agents. The activity profile, pharmacology, toxicity potential, and methods of toxicity prevention of aminoglycosides are well appreciated after three decades. Nephrotoxicity, ototoxicity, and the added costs of drug level monitoring limit wider usage, but great activity against highly antibiotic resistant gram negative bacteria often outweigh these disadvantages and will likely keep aminoglycosides available for the foreseeable future.

Hemodialysis elimination rates and clearance of gentamicin and tobramycin

The apparent hemodialyzer elimination rate constant and clearance for gentamicin and tobramycin were determined during 100 routine hemodialysis treatments in 49 patients. Three different dialyzers (CDAK 3500, CF 1211, and CF 1511), which vary in membrane composition, surface area, and thickness, were evaluated. The elimination rate constant in each patient was calculated from the slope of the log serum concentration-time curves. Two different elimination rate constants for each patient were derived, one during hemodialysis (KT) and one off hemodialysis (K). The hemodialyzer elimination rate constant (KD) for each dialyzer was calculated as the difference between these two values. The hemodialyzer clearance (Cd) was calculated by multiplying the hemodialyzer elimination rate constant by the volume of distribution of the patient. The KDS of gentamicin and tobramycin by the three dialyzers were significantly different. The gentamicin KD of the CDAK 3500 was lower than the values of the CF 1211 and CF 1511 (0.086 versus 0.123 versus 0.131 h-1, respectively). The Cd of the CDAK 3500 for gentamicin was also significantly lower than that of the CF 1511. Although the CdS of tobramycin for the CF 1211 and CF 1511 were 24 and 43% greater than that for the CDAK 3500, these differences were not statistically significant. The KD and Cd of tobramycin were greater than those of gentamicin for all three dialyzers. These data demonstrate that commonly used hemodialyzers vary markedly with respect to their elimination and clearance characteristics of gentamicin and tobramycin. Clinically, these observations may be helpful in designing the correct dose of gentamicin and tobramycin to achieve maximum drug safety and efficacy in hemodialysis patients.

Tobramycin sulfate elimination in premature infants

The elimination pharmacokinetics of tobramycin sulfate was studied in 25 newborn infants of birth weight 0.7 to 4.7 kg during 31 treatment episodes. The peak serum concentrations after a 2.5 mg/kg dose were usually within the therapeutic range of 5 to 10 micrograms/ml; however, the serum predose trough values were elevated above the theoretical safe limit of 2 micrograms/ml. Because of the prolonged serum elimination half-lives, a calculated extended dosage interval, sometimes greater than 24 hours, was necessary to obtain a predose trough of less than or equal to 2 micrograms/ml. The serum elimination half-lives inversely correlated with gestational age, extrauterine age, birth weight, and creatinine clearance. The very low ratio of tobramycin renal clearance to creatinine renal clearance was virtually constant and indicated a probable tubular reabsorption of tobramycin. A general dosage schedule based on birth weight was derived from the data. An alternative formula was derived to enable prediction of the tobramycin elimination half-life based on a combination of birth weight, gestational age, and extrauterine age for an infant younger than 7 days of age.

Netilmicin (Netromycin, Schering-Plough)

Netilmicin sulfate, the 1-N-ethyl derivative of sisomicin, is a new aminoglycoside recently released for use in Canada and not yet released in the U.S. Its place in therapeutics, compared with gentamicin (G), tobramycin (T), and amikacin (A), is not yet established. Preliminary work in animals has suggested a lower incidence of nephrotoxicity and ototoxicity than with other aminoglycosides, and in vitro work has suggested some activity against G/T-resistant organisms. However, netilmicin appears to be virtually identical to G,T, and A in antimicrobial spectrum (except for its poorer activity against P. aeruginosa), human toxicity, and clinical use. For G/T-resistant organisms, amikacin is still the aminoglycoside of choice. In summary, netilmicin has not been demonstrated to have significant advantages over other aminoglycosides (G,T,A), and it is more expensive; thus, its potential value is limited.

Reappraisal of guidelines for pharmacokinetic monitoring of aminoglycosides

The rationale for pharmacokinetic monitoring of aminoglycosides is critically reviewed. Retrospective studies suggest that for optimal antibacterial effect, peak serum gentamicin concentrations should exceed 5 micrograms/ml, despite the fact that these concentrations are indirect measures of the concentration of drug at the site of infection. When quantitative results of antimicrobial susceptibility are known (e.g. MIC or MBC), limited data suggest that for most infections, the peak serum aminoglycoside concentration should exceed the minimum inhibitory concentration by four-fold. However, the optimal duration for which the serum concentration should exceed the MIC or MBC during each dosing interval and the detrimental effect of prolonged subinhibitory drug concentrations have not been evaluated. Furthermore, the immunological competence of the host and the pathogenicity of the infecting organism are important factors in achieving antibacterial response. Using serum creatinine as an indirect and relatively late indicator of nephrotoxicity, nadir gentamicin concentrations greater than 2 micrograms/ml may predispose patients to develop nephrotoxicity. In addition, recent information indicates that patients who accumulate excessive amounts of aminoglycosides in their tissues may be at higher risk for developing nephrotoxicity; these patients may be identified based on the extent of accumulation in their nadir concentrations with continuous dosing. The aminoglycosides diffuse into the inner ear fluids slowly and diffuse out with a half-life of decline in inner ear fluid concentrations slower than that in serum. High transient peak serum concentrations probably do not contribute significantly to the risk of ototoxicity. However, there is evidence from early clinical trials, studies using continuous infusions of aminoglycosides, and animal studies indicating that elevated nadir serum concentrations relate to the development of ototoxicity. There is considerable interpatient variability in the peak serum concentration, even when identical dosages based on body weight or surface area are administered. Similarly, the half-life for decline in serum concentrations is highly variable from patient to patient, even in patients with stable normal renal function. Absorption after intramuscular administration is reliable in most patients, although critically ill patients may experience erratic absorption. The distribution of aminoglycosides is altered in obese patients because of differences in extracellular fluid content between fat and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics of intravenous amikacin after rapid and slow infusion with special reference to hemodialysis

The pharmacokinetics of amikacin, a recently introduced aminoglycoside structurally related to kanamycin, were determined in healthy volunteers after rapid and slow constant-rate intravenous administration of a 7.5 mg/kg dose. The elimination profile of amikacin can be described by two compartment open model kinetics. Peripheral distribution of the drug is extremely rapid, and beta-phase concentrations decay with a half-life averaging about 2 hours, while inter-compartmental equilibrium is achieved in a little over 30 minutes. The volume of distribution averages about 25% of body weight. During hemodialysis, amikacin extraction from the blood reaches 97% +/- 17% (mean +/- 95% confidence interval) that of creatinine and 89% +/- 20% that of blood urea nitrogen. A method of administration adapted to the kinetic properties of the antibiotic is proposed.

The pharmacology of newer aminoglycosides, with a consideration of the application to clinical situations

This has been an overview of the pharmacology of tobramycin, a new potent aminoglycoside, particularly active against Pseudomonas aeruginosa. Close attention must be given to the administration of this compound, and to the administration of all aminoglycosides, if one is to achieve adequate serum and tissue concentrations without provoking a toxic renal or otic reaction. Differences in methods and dosage of administration depending upon age, status of renal function, and route of administration are discussed. The basic principles reviewed can be applied to all the agents in this class.