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The aim of this study was to characterize and validate the population pharmacokinetics of gentamicin in infants and to determine the influences of clinically relevant covariates to explain the inter- and intraindividual variabilities associated with this drug. Infants receiving intravenous gentamicin and with routine therapeutic drug monitoring were consecutively enrolled in the study. Plasma concentration and time data were retrospectively collected from 208 infants (1 to 24 months old) of the Hospital Universitario Severo Ochoa (Spain), of whom 44% were males (mean age [± standard deviation], 5.8 ± 4.8 months; mean body weight, 6.4 ± 2.2 kg). Data analysis was performed with NONMEM 7.2. One- and two-compartment open models were analyzed to estimate the gentamicin population parameters and the influences of several covariates. External validation was carried out in another population of 55 infants. The behavior of gentamicin in infants exhibits two-compartment pharmacokinetics, with total body weight being the covariate that mainly influences central volume (Vc) and clearance (CL); this parameter was also related to creatinine clearance. Both parameters are age related and different from those reported for neonatal populations. On the basis of clinical presentation and diagnosis, a once-daily dosage regimen of 7 mg/kg of body weight every 24 h is proposed for intravenous gentamicin, followed by therapeutic drug monitoring in order to avoid toxicity and ensure efficacy with minimal blood sampling. Gentamicin pharmacokinetics and disposition were accurately characterized in this pediatric population (infants), with the parameters obtained being different from those reported for neonates and children. These differences should be considered in the dosing and therapeutic monitoring of this antibiotic., (Copyright © 2015, American Society for Microbiology. All Rights Reserved.)

This study evaluated the pharmacokinetics of topical creams containing 15% paromomycin ("paromomycin alone") and 15% paromomycin plus 0.5% gentamicin (WR 279,396) in patients with cutaneous leishmaniasis. The investigational creams were applied topically to all lesions once daily for 20 days. Plasma samples were analyzed for simultaneous quantitation of paromomycin and gentamicin isomers and total gentamicin. Pharmacokinetic parameters for gentamicin could not be calculated because detectable levels were rarely evident. After one application, the paromomycin area under the concentration-time curve from 0 to 24 h (AUC0-24) was 2,180 ± 2,621 ng · h/ml (mean ± standard deviation [SD]) for the paromomycin-alone group and 975.6 ± 1,078 ng · h/ml for the WR 279,396 group. After 20 days of application, the paromomycin AUC0-24 and maximum concentration of drug (Cmax) were 5 to 6 times greater than those on day 1 for both treatment groups. For the paromomycin-alone group, the AUC0-24 was 8,575 ± 7,268 ng · h/ml and the Cmax was 1,000 ± 750 ng/ml, compared with 6,037 ± 3,956 ng · h/ml and 660 ± 486 ng/ml for the WR 279,396 group, respectively. Possibly due to large intersubject variability, no differences (P ≥ 0.05) in the AUC0-24 or Cmax were noted between treatment or between sites on day 1 or 20. The percentage of dose absorbed on day 20 was 12.0% ± 6.26% and 9.68% ± 6.05% for paromomycin alone and WR 279,396, respectively. Paromomycin concentrations in plasma after 20 days of application were 5 to 9% of those after intramuscular administration of 15 mg/kg of body weight/day to adults for the systemic treatment of visceral leishmaniasis. Effective topical treatment of cutaneous leishmaniasis appears to be possible with limited paromomycin and gentamicin systemic absorption, thus avoiding drug accumulation and toxicity. (The work described here has been registered at ClinicalTrials.gov under registration no. NCT01032382 and NCT01083576.).

We evaluated the efficacy of tigecycline in a rabbit model of experimental endocarditis caused by a linezolid-resistant clinical strain of Enterococcus faecium. Tigecycline-treated animals had a 2.8-log10-CFU/g reduction in microbial counts in excised vegetations compared with controls. Addition of gentamicin caused a further arithmetical reduction in colony counts. The therapeutic effect was sustained 5 days after completion of treatment, as shown by relapse studies performed in treatment groups.

Obese patients may be at a greater risk for acute kidney injury (AKI) with the use of certain antimicrobial agents that are dosed by weight. Current preclinical models of AKI utilize the male rat within a narrow weight range that limits extrapolation of the generated results. We evaluated the pharmacokinetics and AKI potential of gentamicin in 14-week-old diet-induced obesity-prone (n = 40) and obesity-resistant (n = 40) rats of both sexes. Single daily doses of gentamicin (12.5, 18.75, or 25 mg/kg of body weight) or saline (control) were administered intraperitoneally for 14 doses. Blood samples were collected after doses 1, 7, and 14, assayed by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and analyzed using a nonparametric population pharmacokinetic approach for gentamicin. Urine was collected after doses 1, 3, and 5 and assayed for kidney injury molecule 1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) and normalized to creatinine (Cr) values. Histology was performed on all animals, and the degree of proximal tubular injury was graded. The mean (minimum, maximum) weight of the rats was 330 (136, 580) g. NGAL/Cr predicted AKI better than did KIM-1/Cr and was detectable in male rats after dose 1 and in obesity-prone female rats after dose 5. Proximal tubular injury by histology was significantly higher in male than in female rats. A significant relationship between the gentamicin area under the curve from zero to 24 hours (AUC(0-24)) estimates and the maximum NGAL/Cr ratio was observed. This preclinical model has the potential to aid with dose extrapolation for body size and improve assessment of the toxicology potential of antimicrobials in development.

Gentamicin is a widely used antibiotic in the intensive care unit (ICU). Its dosage is difficult to adapt to hemodialyzed ICU patients. The FDA-approved regimen consists of the administration of 1 to 1.7 mg/kg of gentamicin at the end of each dialysis session. Better pharmacokinetic management could be obtained if gentamicin were administered just before the dialysis session. We performed Monte Carlo simulations (MCS) to determine the best gentamicin pharmacokinetic profile (high peak and low trough concentrations). Then, 6 mg/kg of gentamicin was infused into 10 ICU patients over a period of 30 min. A 4-h-long hemodialysis session was started 30 min after the end of the infusion. Pharmacokinetic samples were regularly collected over 24 h. A one-compartment model with zero-order input and first-order elimination was developed in Nonmem version VI to analyze patients' measured gentamicin concentration-versus-time profiles. Finally, additional MCS were performed to compare the regimen chosen with the FDA-approved gentamicin regimen. High peak concentrations (C(max), 31.8 ± 16.8 mg/liter) were achieved. The estimated C(24) and C(48) values (concentrations 24 and 48 h, respectively, after the beginning of the infusion) were 4.1 ± 2.3 and 1.8 ± 1.2 mg/liter, respectively. The volume of distribution was 0.21 ± 0.06 liter/kg. MCS confirmed that the dosing regimen chosen achieved the target C(max) whereas the FDA-approved regimen did not (31.0 ± 10.9 versus 8.8 ± 3.1 mg · liter(-1)). Moreover, the C(24) values were similar while the AUC(0-24) values were moderately increased (190.8 ± 65.0 versus 135 ± 42.2 mg · h · liter(-1)). Therefore, administration of 6 mg/kg of gentamicin before hemodialysis to critically ill patients achieves a high C(max) and an acceptable AUC, maximizing pharmacokinetic/pharmacodynamic endpoints.

In enterococci, intrinsic low-level resistance to gentamicin does not abolish synergism with a cell wall-active antibiotic while high-level resistance due to acquired aminoglycoside-modifying enzymes does. To study the impact of intermediate levels of resistance to gentamicin (64 < MIC < 500 microg/ml), we selected in vitro three consecutive generations of mutants of Enterococcus faecalis JH2-2 with MICs of gentamicin at 128 microg/ml for G1-1477, 256 microg/ml for G2-1573, and 512 microg/ml for G3-1688. E. faecalis 102, which is highly resistant to gentamicin by enzymatic inactivation was used as control. In in vitro killing curves experiments, gentamicin concentrations allowing bactericidal activity and synergism in combination with amoxicillin increased from 4 microg/ml (1/16th the MIC), 16 microg/ml (one-eighth the MIC), 64 microg/ml (one-quarter the MIC), and 256 microg/ml (one-half the MIC) for strains JH2-2, G1-1477, G2-1573 and G3-1688, respectively. As expected, no bactericidal effect of the combination or synergism could be obtained with strain 102. In rabbits with aortic endocarditis caused by strain G1-1477 or G2-1573, combination therapy with amoxicillin and gentamicin was significantly more active than amoxicillin alone (P < 0.05) but not in those infected with the strains G3-1688 and 102. Thus, intermediate levels of resistance to gentamicin was not associated with a loss of a beneficial effect of the gentamicin-amoxicillin combination in vivo even though higher concentrations of gentamicin were necessary to achieve in vitro synergism. Therefore, the use of an MIC of 500 microg/ml as a clinical cutoff limit to predict in vivo benefit of the combination remains a simple and effective tool.

The objective of this study was to characterize the relationship between gentamicin concentrations during surgery and the development of wound infection following colorectal operations. Despite decades of research in surgical prophylaxis, the relationship between intraoperative antibiotic concentrations and postoperative infection and the concentrations required for effective prophylaxis have not been established. A pharmacodynamic analysis was conducted using data from a previous prospective, randomized, double-blind clinical study which compared two dosage regimens of gentamicin plus metronidazole for prophylaxis in connection with elective colorectal surgery. Univariate and multivariate analyses of risk factors for postoperative wound infection were conducted, and the relationship between intraoperative gentamicin concentrations and surgical outcome was characterized. The gentamicin concentration at the time of surgical closure was one of the strongest independent risk factors for infection (P = 0.02), along with the presence of diabetes mellitus (P = 0.02), stoma (P = 0.04), and advanced age (P = 0.05). Gentamicin concentrations at closure of less than 0.5 mg/liter were associated with an infection rate of 80% (representing 8 of 10 patients with concentrations below that level) (P = 0.003). Receiver operating characteristic curve analysis identified a critical closure concentration of 1.6 mg/liter for effective surgical prophylaxis (P = 0.002; sensitivity, 70.8%; specificity, 65.9%). This study provides new and important information on antibiotic pharmacodynamics in surgical prophylaxis. It demonstrates the critical effect of the antibiotic concentration at closure on wound infection and suggests a significant association between the concentration and other well-established risk factors, like the timing of preoperative antibiotic administration and surgery duration.

The combination of quinupristin-dalfopristin (Q-D) and gentamicin was tested against two strains of gentamicin- and dalfopristin-susceptible methicillin-resistant Staphylococcus aureus (MRSA). One strain was susceptible to macrolides, lincosamides, and streptogramin B type antibiotics (MLS(B)), and the other was constitutively resistant to these antibiotics by virtue of the ermA gene. The checkerboard method and time-kill curves showed that the combination of Q-D and gentamicin was indifferent. A rabbit endocarditis model simulated the pharmacokinetics achieved in humans receiving intravenous injections of Q-D (7.5 mg/kg of body weight three times a day) and gentamicin (3 mg/kg once daily). For the MLS(B)-susceptible strain, a 4-day regimen reduced mean bacterial titers (MBT) in vegetations from 8.5 +/- 0.8 log CFU/g (control group) to 4.1 +/- 2.6 (gentamicin), 3.0 +/- 0.9 (Q-D), and 2.6 +/- 0.5 log CFU/g (Q-D plus gentamicin). For the strain constitutively resistant to MLS(B), a 4-day regimen reduced MBT in vegetations from 8.7 +/- 0.9 log CFU/g (control group) to 5.0 +/- 2.2 (gentamicin), 5.2 +/- 2.2 (Q-D), and 5.1 +/- 2.4 log CFU/g (Q-D plus gentamicin). The differences between control and treatment groups were significant for both strains (P < 0.0001), although there was no significant difference between treatment groups. No resistant variant was isolated from vegetations, and no significant difference in MBT in vegetations of treatment groups after 1-day regimens was observed. This experimental study found no additive benefit in combining Q-D and gentamicin against dalfopristin- and gentamicin-susceptible MRSA.

The consequences on glycopeptide activity of low-level resistance to vancomycin due to VanE-type resistance were evaluated in vitro and in experimental endocarditis caused by Enterococcus faecalis BM4405 (MICs of vancomycin and teicoplanin: 16 and 0.5 microg/ml, respectively), its susceptible derivative BM4405-1, and susceptible E. faecalis JH2-2. After 24 h of incubation, vancomycin at 8 microg/ml was not active against E. faecalis BM4405 whereas it was bacteriostatic against strains BM4405-1 and JH2-2. Against all three strains, vancomycin at 30 microg/ml and teicoplanin at 8 or 30 microg/ml were bacteriostatic but bactericidal when combined with gentamicin. In rabbits with aortic endocarditis due to VanE-type resistant strain BM4405, treatment with a standard dose of vancomycin generated subinhibitory plasma concentrations (i.e., peak of 36.3 +/- 2.1 microg/ml and trough of 6.0 +/- 2.2 microg/ml) and led to no significant reduction in mean aortic valve vegetation counts compared to no treatment of control animals. In contrast, a higher dosing regimen of vancomycin (i.e., resulting in a peak of 38.3 +/- 5.2 microg/ml and a trough of 15.0 +/- 8.3 microg/ml), providing plasma concentrations above the MIC for the entire dosing interval, led to significant and similar activities against all three strains, which were enhanced by combination with gentamicin. Treatment with teicoplanin led to results similar to those obtained with vancomycin at a high dose. No subpopulations with increased resistance to glycopeptides were selected in vitro or in vivo. In conclusion, the use of a high dose of vancomycin was necessary for the treatment of experimental enterococcal endocarditis due to VanE-type strains.

Cystic fibrosis (CF) patients receive repeated courses of aminoglycoside therapy. These patients would consequently be expected to be more susceptible to cochleotoxicity, a recognized side effect with single courses of aminoglycoside therapy. The primary aim of this retrospective study was to establish the incidence and severity of auditory deficit in CF patients. Standard (0.25- to 8-kHz) and high-frequency (10- to 16-kHz) pure-tone audiometry was carried out in 70 CF patients, and the results were compared with the results from 91 control subjects. These subjects were further divided into pediatric and adult groups. Of 70 CF patients, 12 (1 pediatric) displayed hearing loss considered to be caused by repeated exposure to aminoglycosides. There was a nonlinear relationship between the courses of therapy received and the incidence of hearing loss. The severity of the loss did not appear to be related to the number of courses received. Assuming the risk of loss to be independent for each course, preliminary estimates of per course risk of hearing loss were less than 2%. Upon comparison with previous clinical studies and experimental work, these findings suggest that the incidence of cochleotoxicity in CF patients is considerably lower than would be expected, suggesting that the CF condition may confer protection against aminoglycoside cochleotoxicity.

Listeria monocytogenes-infected phagocytes are present in the bloodstream of experimentally infected mice, but whether they play a role in central nervous system (CNS) invasion is unclear. To test whether bacteria within infected leukocytes could establish CNS infection, experimentally infected mice were treated with gentamicin delivered by surgically implanted osmotic pumps. Bacterial inhibitory titers in serum and plasma ranged from 1:16 to 1:256, and essentially all viable bacteria in the bloodstream of treated mice were leukocyte associated. Nevertheless, CNS infection developed in gentamicin-treated animals infected intraperitoneally or by gastric lavage, suggesting that intracellular bacteria could be responsible for neuroinvasion. This was supported by data showing that 43.5% of bacteria found with blood leukocytes were intracellular and some colocalized with F-actin, indicating productive intracellular parasitism. Experiments using an L. monocytogenes strain containing a chromosomal actA-gfpuv-plcB transcriptional fusion showed that blood leukocytes were associated with intracellular and extracellularly bound green fluorescent protein-expressing (GFP+) bacteria. Treatment with gentamicin decreased the numbers of extracellularly bound GFP+ bacteria significantly but did not affect the numbers of intracellular GFP+ bacteria, suggesting that the latter were the result of intercellular spread of GFP+ bacteria to leukocytes. These data demonstrate that infected leukocytes and the intracellular L. monocytogenes harbored within them play key roles in neuroinvasion. Moreover, they suggest that phagocytes recruited to infected organs such as the liver or spleen are themselves parasitized by intercellular spread of L. monocytogenes and then reenter the bloodstream and contribute to the systemic dissemination of bacteria.

The activity of gentamicin at various concentrations against two strains of Enterococcus faecalis was investigated in vitro and in a rabbit model of aortic endocarditis. In vitro, gentamicin at 0.5 to 4 times the MIC failed to reduce the number of bacteria at 24 h. Rabbit or human serum dramatically increased gentamicin activity, leading to a >/=3-log(10) CFU/ml decrease in bacterial counts when the drug concentration exceeded the MIC. Susceptibility testing in the presence of serum was predictive of in vivo activity, since gentamicin alone significantly reduced the number of surviving bacteria in the vegetations if the peak-to-MIC ratio was greater than 1. However, gentamicin selected resistant mutants in rabbits. The intrinsic activity of gentamicin should be taken into account in evaluation of combinations of gentamicin and cell wall-active agents against enterococci.

The pharmacokinetics of isepamicin following administration of a 1-g dose were evaluated for 18 healthy male volunteers between the ages of 26 and 38. In a randomized crossover fashion, each volunteer received doses of isepamicin by a 30-min intravenous infusion and as an intramuscular injection. Blood samples were collected at specified times after dosing and assayed for isepamicin by a validated radioimmunoassay method. The individual plasma concentration-time curves were analyzed by noncompartmental methods. In general, the pharmacokinetics after intravenous infusion and intramuscular injection were similar. As expected, the maximum concentration of isepamicin in serum following intramuscular injection (37.2 microg/ml) was lower than the observed concentration at the end of infusion (66.7 microg/ml). The areas under the concentration-time curves from 0 h to infinity following intramuscular and intravenous administration were 164.8 and 154.5 microg x hr/ml, respectively, indicating complete absorption following intramuscular administration. The respective mean terminal-phase half-life (t1/2) values were 2.6 and 3.6 h. Although t1/2 was slightly longer following intravenous infusion, the small difference in the observed t1/2 values was not considered to be clinically significant. Total body clearances following intramuscular injection and intravenous infusion were 1.3 and 1.4 ml/min/kg, respectively, which were similar to renal serum creatinine clearances in healthy volunteers (> 1.14 ml/min/kg). The drug was safe and well tolerated. The results of the present study clearly show complete absorption of isepamicin following intramuscular administration. The similarity in the pharmacokinetics after intravenous infusion and intramuscular dosing would permit interchangeable administration of isepamicin by either route without compromising clinical efficacy.

Two doses of gentamicin (2 and 7 mg/kg of body weight) were administered to 11 healthy volunteers in a randomized, crossover single-dose study to compare their pharmacokinetics. Doses were infused over 1 h with a syringe infusion pump, and 14 concentrations in sera were obtained over an 8-h period. Concentration in serum versus time data were fitted to a two-compartment pharmacokinetic model. In addition, to mimic the clinical setting, subjects' data were fitted by the Sawchuk-Zaske method. Distributional and postdistributional peak concentrations, along with the last obtained concentration in serum, were utilized to compare the following pharmacokinetic variables: volume of distribution at steady state (Vss), half-life, clearance (CL), and maximum concentration in serum (Cmax). With two-compartment pharmacokinetic fitting, significant differences in distribution half-life (average, 21.8 and 41.6 min [P < or = 0.05]) and gentamicin CL (76.6 +/- 6.6 and 67.2 +/- 4.2 ml/min/1.73 m2 [P < or = 0.001]) were found between traditional-dose and high-dose groups, respectively. When the data for concentrations in sera were fitted to a one-compartment pharmacokinetic model by using either the distributional or the postdistributional Cmax, statistically significant differences (P < or = 0.001) were found between Vss, half-life, CL, and Cmax values for both dosage groups. The results show that the pharmacokinetics of gentamicin at a large dose differ significantly from those at the traditional dose. This information has direct implications for once-daily aminoglycoside (ODA) literature when the Cmax values reported are distributional and therefore show falsely high Cmax/MIC ratio estimates. In addition, ODA nomogram dosing tools developed with distributional Cmax values are probably inaccurate.

We have developed a chromatographic assay with high sensitivity and specificity to quantify G418 sulfate (Geneticin), an antibiotic used routinely in molecular genetics experiments for selecting eukaryotic transformants. With this method, G418 in tissues and plasma samples can be quantitated without the confounding factors often associated with biological assays. After removal of proteins in homogenized tissue or plasma samples with methanol (2:1, vol/vol), the amino group of G418 was derivatized with 1-fluoro-2,4-dinitrobenzene (DNFB) to form the UV-visible G418-DNFB product. The DNFB-derivatized G418 was separated on a reversed-phase C18 column with an acetonitrile and water gradient as the mobile phase. Under these assay conditions, the detection limit for G418 sulfate in buffer, plasma, and tissues was recorded at 78 ng/ml and the linearity was recorded for concentrations up to 100 micrograms/ml. The data obtained from this analysis indicate that this assay can be used for the quantitative determination of G418 sulfate in plasma and tissue samples.

Current data indicate that once-daily aminoglycoside therapy is as efficacious as traditional multiple daily dosing and equally or less toxic. Our experience with once-daily gentamicin, 6 mg/kg of body weight led to a 10% (3 of 33 patients) occurrence of documented ototoxicity after prolonged aminoglycoside exposure.

Wound infections frequently originate from the subcutaneous tissue. The effect of gentamicin in subcutaneous tissue has, however, normally been evaluated from concentrations in blood or wound fluid. The aim of the present study was to investigate the pharmacokinetic properties of gentamicin in human subcutaneous adipose tissue by a microdialysis technique. Seven healthy young volunteers each had four microdialysis probes placed in the fat (subcutaneous) layer of the abdominal skin. After the administration of a 240-mg gentamicin intravenous bolus, consecutive measurements of the drug concentrations in serum and subcutaneous interstitial fluid were obtained simultaneously for 6 h. The tissue gentamicin concentration peaked after 10 to 30 min. The peak concentration in the tissue was 6.7 +/- 2.0 mg.liter-1 (standard deviation), equivalent to 39.1% of the peak concentration in serum. The area under the concentration-versus-time curve for the first 6 h in the tissue was 1,281 +/- 390.0) mg.min liter-1, equivalent to 59.7% of the area under the concentration-versus-time curve in serum. It is concluded that the microdialysis technique can be used to make dynamic and quantitative measurements of the gentamicin concentration in human subcutaneous tissue. In this adipose tissue, the peak concentrations of gentamicin were approximately seven times the MIC for Pseudomonas aeruginosa and 33 times the MIC for Staphylococcus aureus after the administration of an intravenous bolus of 240 mg, indicating the presence of sufficient concentrations in the adipose tissue to be effective against common bacteria.

The dispositions of vancomycin (VCM) and gentamicin (GM) in plasma and perilymph after single and multiple administrations and the effects of multiple administrations of VCM or GM alone and the combination of both drugs on auditory function were studied in male guinea pigs. The pharmacokinetic parameters of VCM and GM obtained from plasma drug concentration-time data after single and multiple (22 days) intramuscular administrations of VCM (200 mg/kg of body weight) alone and GM (50 mg/kg) alone were not significantly different from those after combined administration of VCM (200 mg/kg) and GM (50 mg/kg). There was no change in the penetration ratio of VCM and GM into perilymph between administration of VCM or GM alone and the combination of both drugs. Furthermore, the hearing threshold of guinea pigs was not affected by VCM or GM alone or the combination of both drugs within the range of therapeutic VCM and GM levels in plasma in humans.

We adapted an in vitro pharmacodynamic model of infection to incorporate simulated endocardial vegetations. The bactericidal activities of teicoplanin, vancomycin, gentamicin, and various combinations of these drugs were studied against a strain of methicillin-susceptible Staphylococcus aureus obtained from a patient being treated for endocarditis at Detroit Receiving Hospital. Bacteria were grown overnight, concentrated, and added to a mixture of cryoprecipitate (80%) and thrombin (10%) to achieve approximately 5 x 10(9) CFU/g. Fibrin clots (8 to 10) were suspended into the model, removed at 24, 48, and 72 h in duplicate, weighed, and homogenized in 1.25% trypsin. Control experiments were conducted to characterize the growth kinetics. The following antibiotics were administered to simulate the pharmacokinetics of the drugs in humans: teicoplanin at 3 and 15 mg/kg of body weight, vancomycin at 15 mg/kg, and gentamicin at 1 mg/kg. Fibrin clot samples used to detect resistance were plated on antibiotic-containing tryptic soy agar plates. For the teicoplanin and vancomycin regimens, protein binding to cryoprecipitate, thrombin, and fibrin clot was determined to be 32, 43, and 50% and 26, 28, and 29%, respectively. In comparison with no treatment, vancomycin or teicoplanin at 15 mg/kg or either of these regimens combined with gentamicin significantly reduced bacterial counts (P < 0.0001). Monotherapy with teicoplanin at 3 mg/kg or gentamicin resulted in no killing activity. Combination treatment with teicoplanin at 3 mg/kg and gentamicin resulted in the killing of approximately 2 log10 CFU/g by 72 h and the development of resistance to gentamicin. The results obtained with the in vitro model of endocarditis are similar to the results reported by several investigators with the rabbit model of infective endocarditis. This unique infection model is useful for designing initial drug dosage regimens and may be predictive of drug efficacy against infective endocarditis.

This report describes the effects of endotoxin and a thromboxane receptor antagonist, L-655,240, on kidney function and the intrarenal pharmacokinetics of aminoglycosides. The rationale for these studies was that thromboxane antagonists may eventually be used in combination with aminoglycosides in patients with gram-negative sepsis and endotoxemia. As aminoglycosides are nephrotoxic and endotoxin has already been shown to increase the renal uptake of gentamicin, we investigated the possibility that thromboxane antagonists might interfere with the nephrotoxic potential of both substances. A decrease in the volume of distribution and an increase in the intracortical concentration of gentamicin were observed in animals given endotoxin. Compared with animals given endotoxin alone, those which received endotoxin plus L-655,240 had significant accumulation of gentamicin in the renal cortex and medulla, as determined by the area under the concentration-time curve, and a significant reduction in the total clearance of the antibiotic (P < 0.05). This difference in uptake could not be attributed to hypotension or changes in the glomerular filtration rate or renal plasma flow. L-655,240 alone did not modify gentamicin pharmacokinetics but did decrease p-aminohippuric acid secretion. Thromboxane antagonists in the context of endotoxemia increase intrarenal uptake of aminoglycosides. If these compounds are to be used as therapeutic agents when endotoxin is present, their influence on renal handling of nephrotoxic drugs needs to be considered. Multiple-dosing regimens deserve investigation.

We compared ciprofloxacin, rifampin, and gentamicin treatments, alone and in combination, for 5 days in the therapy of experimental aortic valve endocarditis in rats caused by a clinical isolate of vancomycin-resistant Enterococcus faecium. The MICs and MBCs of vancomycin, ciprofloxacin, rifampin, and gentamicin were 250 and > 1,000, 3.1 and 6.3, 0.098 and 1.6, and 12.5 and > 50 micrograms/ml, respectively. Infected rats were sacrificed after completing 5 days of therapy. Additional rats within each treatment group were followed for 5 days beyond the last dose of antibiotic therapy. Although survivals in the different groups were not significantly different after 5 days of therapy, survival was significantly better 5 days beyond the last dose of antibiotic therapy in rats treated with rifampin-containing regimens. The combination of ciprofloxacin and gentamicin was bactericidal in vitro and in vegetations from rats with enterococcal endocarditis. Rifampin alone was similarly bactericidal in vivo, but it was not significantly better than rifampin in combination with other antibiotics. Subpopulations resistant to rifampin, but not ciprofloxacin, were detected in the inoculum and in most vegetations during therapy. However, the combination of ciprofloxacin plus both gentamicin and rifampin reduced both the rifampin-susceptible and -resistant population in vegetations of 9 of 10 animals below the level of detection after 5 days of therapy. Nevertheless, a residual enterococcal population apparently remained in numbers of < 2 log10 CFU/g after 5 days of therapy, which resulted in relapse. Perhaps a longer course of therapy would have eliminated this residual population and improved efficacy.

Infections with enterococci that are resistant to multiple antibiotics are an emerging clinical problem. We evaluated the antibiotic treatment of experimental enterococcal endocarditis caused by two strains with different mechanisms of penicillin resistance. Enterococcus faecalis HH-22 is resistant to aminoglycosides and penicillin on the basis of plasmid-mediated modifying enzymes; Enterococcus raffinosus SF-195 is susceptible to aminoglycosides but is resistant to penicillin on the basis of low-affinity penicillin-binding proteins. Animals infected with strain HH-22 received 5 days of treatment with the following: no treatment; daptomycin (20 mg/kg of body weight twice daily [b.i.d.], intramuscularly [i.m.]), vancomycin (20 mg/kg b.i.d., intravenously), or ampicillin (100 mg/kg three times daily, i.m.) plus gentamicin (2.5 mg/kg b.i.d. i.m.). Although vancomycin was superior to ampicillin-gentamicin (P less than 0.01), daptomycin was significantly better than all other treatment regimens (P less than 0.01) in reducing intravegetation enterococcal densities, although no vegetations were rendered culture negative by this agent. Animals infected with strain SF-195 received 5 days of no therapy, ampicillin, ampicillin-gentamicin, vancomycin, or daptomycin (all at the dosage regimens described above). Daptomycin, vancomycin, and ampicillin-gentamicin each lowered intravegetation enterococcal densities significantly better than did ampicillin monotherapy or no treatment (P less than 0.01); moreover, these three treatment regimens rendered significantly more vegetations culture negative than did ampicillin monotherapy or no treatment (P less than 0.05). Serum daptomycin levels remained above the MICs and MBCs for both enterococcal strains throughout the 12-h dosing interval used in the study. Daptomycin and vancomycin were both active in vivo in these models of experimental enterococcal endocarditis caused by penicillin-resistant strains, irrespective of the mechanism of resistance. This activity correlated with the unique cell wall sites of action of these agents (binding to lipoteichoic acid and pentapeptide precursor, respectively) compared with the sites of action of beta-lactams (penicillin-binding proteins). Beta-Lactamase production by strain HH-22 precluded in vivo efficacy with ampicillin-gentamicin combinations. In contrast, this combination was active in vivo against strain SF-195, which exhibited intermediate-level penicillin resistance (MIC, 32 micrograms/ml), likely reflecting the ability of high-dose ampicillin to achieve enough binding to low-affinity penicillin-binding proteins to cause augmented aminoglycoside uptake.

Aminoglycoside bactericidal activity during the first 24 h of treatment probably is a determining parameter in the prognosis of severe gram-negative infections in immunocompromised patients. To identify the predictive factors involved in the definition of the best therapeutic regimen for Enterobacter cloacae and Serratia marcescens infections, we studied different gentamicin, tobramycin, and amikacin regimens by using an experimental model of rabbit endocarditis. Two factors appear to play an important role in predicting in vivo efficacy: (i) the level of in vivo bactericidal activity, which can differ widely from one aminoglycoside to another for the same bacterial strain and from one strain to another of the same species, and (ii) the critical serum drug concentration (CSC, in milligrams per liter), defined as the lowest serum antibiotic concentration capable of producing a significant CFU reduction (P less than 0.05) in endocarditis vegetations 24 h after the beginning of a continuous infusion. Stepwise regression analysis showed that for gentamicin and S. marcescens, the area under the concentration-time curve above the CSC and then the time above the CSC are the determining parameters for efficacy (R = 0.69; F = 13.5; P = 0.001), whereas for amikacin and S. marcescens, the time above the CSC and then the area under the concentration-time curve above the CSC predict efficacy (R = 0.74; F = 24.0; P = 0.0001). The lowest CSC is that of amikacin (about 8 mg/liter); those of gentamicin and tobramycin are about 15 mg/liter. In severe S. marcescens infections, intermittent amikacin dosing offers excellent bactericidal activity within the first 24 h.

The pharmacokinetic parameters of gentamicin and tobramycin were evaluated and compared for 260 patients with pleural effusions and 1,049 patients without pleural effusions by chest radiograph. Pharmacokinetic data were collected prospectively and analyzed by using our aminoglycoside data base. Univariate analysis revealed that the patients with pleural effusions demonstrated significantly lower serum albumin concentrations, greater aminoglycoside volumes of distribution, longer elimination half-lives, and lower peak and higher trough concentrations in serum than the patients without pleural effusions. Patients with pleural effusions were significantly older and had lower total body weight. Stepwise multiple linear regression analysis revealed that lower total body weight and serum albumin concentration, presence of pleural effusion, and greater age were associated with significantly greater volumes of distribution. Calculated creatinine clearance, age, total body weight, and shock were associated with reduced aminoglycoside clearance in these patients.

The disposition of isepamicin, an investigational aminoglycoside antibiotic, was evaluated in 30 subjects with various degrees of renal function. The subjects were divided into five groups: those with normal renal function (creatinine clearance [CLCR], greater than 80 ml/min/1.73 m2), those with mild renal insufficiency (CLCR, 50 to 80 ml/min/1.73 m2), those with moderate renal insufficiency (CLCR, 30 to 49 ml/min/1.73 m2), those with severe renal insufficiency (CLCR, 5 to 29 ml/min/1.73 m2), and those maintained on hemodialysis (CLCR, less than 5 ml/min/1.73 m2). Subjects on hemodialysis were studied both during hemodialysis and during an interdialytic period. The volumes of distribution of isepamicin were not significantly different among the five groups of patients. The total body clearance (CLP) and renal clearance (CLR) of isepamicin significantly decreased as CLCR decreased. The CLP of isepamicin and CLCR were significantly related [(COP = 0.391.[CLCR] + 1.83; r2 = 0.878)]. Nonrenal clearance of isepamicin did not differ between groups. Hemodialysis augmented the CLP of isepamicin by approximately 25-fold. The amount of isepamicin recovered in the dialysate was 60.6 +/- 15.8% of the dose administered. The maximal rebound of the isepamicin concentration in plasma after cessation of hemodialysis was observed at 0.78 +/- 0.7 h. Concentrations in plasma increased 32.7 +/- 22.9% over that measured at the end of hemodialysis. These data indicate that dosage adjustments are necessary in subjects with decreased renal function.

This study was undertaken to compare the susceptibility to inactivation of isepamicin with amikacin and gentamicin when exposed to different beta-lactams, beta-lactamase inhibitors, and heparin. The aminoglycosides (5, 10, 20, and 50 micrograms/ml) were incubated in human serum with ampicillin, azlocillin, aztreonam, carbenicillin, ceftazidime, piperacillin, and ticarcillin (100 and 600 micrograms/ml) and with clavulanate, cilastatin, 1:1 imipenemcilastatin, oxacillin, and sulbactam (20 and 120 micrograms/ml) for 48 h at 37 degrees C. Aminoglycoside concentrations were measured by fluorescence polarization immunoassay (FPI) after 0, 8, and 48 h of incubation and by radial diffusion bioassay after 48 h of incubation. Each of the three aminoglycosides was also added to whole blood containing either heparin (100 U/ml) or 0.5% EDTA as a control and assayed after 6 h by FPI. The degree of inactivation of isepamicin by the beta-lactams was significantly less than that by amikacin (P less than 0.003) and gentamicin (P less than 0.0002) when determined by bioassay. Piperacillin, carbenicillin, and azlocillin produced the greatest amount of inactivation, and cilastatin and oxacillin produced the least. A similar pattern was observed when the degree of inactivation was measured by FPI. A significant difference in the degree of inactivation was noted between isepamicin and gentamicin (P less than 0.003 at 8 h and P less than 0.006 at 48 h) but not between isepamicin and amikacin (P greater than 0.7 at 8 h and P greater than 0.08 at 48 h). Aminoglycoside determinations by FPI were not influenced by the presence of heparin. In summary, isepamicin was found to be at least as stable as amikacin against inactivation by beta-lactam compounds and beta-lactamase inhibitors. Heparin (100 U/ml) did not influence aminoglycoside determinations by FPI.

Aminoglycosides are usually considered to be concentration-dependent antibiotics and to have similar pharmacodynamic and pharmacokinetic properties. To verify the equivalent activity of the aminoglycosides on a susceptible strain, we tested the killing rate of three aminoglycosides (gentamicin, tobramycin, and amikacin) on one strain of Serratia marcescens both in vitro and in vivo by using a rabbit model of left-ventricle endocarditis. Despite, similar MICs, the time-kill curve of gentamicin was consistently better than those of amikacin and tobramycin, whatever the concentration of each antibiotic used (1, 2, 4, 8, 16, or 32 mg/liter), after a 5-h incubation. The in vivo bacterial reduction in the vegetations was measured 24 h after administration of an intravenous 48-mg/kg bolus of each antibiotic or at the end of a 24-h continuous intravenous infusion of the same dose. Gentamicin was significantly more effective when administered as a bolus than when administered as a continuous infusion (2.8 +/- 0.2 versus 6.4 +/- 1.5 log10 CFU/g of vegetation, respectively; P less than 0.01), whereas amikacin was more effective as a continuous infusion than as a bolus injection (3.6 +/- 2.0 versus 7.5 +/- 1.3 log10 CFU/g of vegetation, respectively; P less than 0.01). Tobramycin was not very effective, whatever the dosage tested (approximately 6.5 to 7 log10 CFU/g). These results suggest that concentration-dependent bactericidal activities, both in vitro and in vivo, may vary greatly among aminoglycosides despite similar MICs.

The mechanism by which endotoxin (lipopolysaccharide [LPS]) modifies the intrarenal distribution and the nephrotoxic potential of gentamicin is unknown. We studied the influence of LPS on the intracortical accumulation kinetics of gentamicin in rats infused intravenously for 6 h, during which time steady-state levels of the antibiotic in serum were achieved. We compared gentamicin accumulation rates (V) in normal rats and in rats receiving LPS (0.5 and 5 mg/kg) as levels in serum (S) varied from 0.5 to 130 micrograms/ml. The pharmacokinetic parameters of gentamicin were previously measured in the three groups of rats that were studied in order to reach and maintain in each rat the desired levels of antibiotic in serum during the 6 h of infusion. Two hours before the infusion of gentamicin, LPS was injected intravenously over a period of 15 min. In normal rats, the increase in S was associated with a nonlinear increase in V. The Michaelis-Menten kinetics, which was the best-fitting function, gave an apparent Vmax (maximal capacity of uptake) of 195.03 +/- 9.75 micrograms/g per h and an apparent Km (concentration in serum at Vmax/2, an index of affinity) of 34.91 +/- 4.45 micrograms/ml (linear transformation of the experimental data by the Hanes-Woolf plot: r = 0.93, n = 85). In the rats that received LPS, the increase in S was associated with a linear increase of V: for LPS at 0.5 mg/kg, V = 27.00 + 1.50 S (r = 0.94, n = 80); for LPS at 5 mg/kg, V = 22.72 + 1.48 S (r = 0.94, n = 75). We conclude that endotoxin modifies the accumulation kinetics of gentamicin in the kidney cortices of rats.

Three antimicrobial combinations, ticarcillin plus cephalothin (T+C), ticarcillin plus gentamicin (T+G), and cephalothin plus gentamicin (C+G), were administered to 105 febrile granulocytopenic cancer patients at the Baltimore Cancer Research Center as part of a multi-institutional prospective randomized antibiotic trial. The sera from 32 of these patients (T+C-10 patients, T+G-10 patients, and C+G-12 patients) obtained 1 h post-antibiotic administration were examined for bactericidal activity against 11 strains each of the most common pathogens infecting the granulocytopenic host: Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, and Staphylococcus aureus. Each of the three antibiotic regimens produced a high degree of bactericidal activity in these sera against S. aureus and E. coli. P. aeruginosa was equally, although poorly, killed by sera containing ticarcillin (T+G, T+C), whereas C+G produced no measurable serocidal activity (P < 0.05). Sera with C+G killed K. pneumoniae more effectively than T+G; T+C produced the least killing effect of the three regimens against this organism (P < 0.05). The bactericidal activity of the serum from these 32 patients supplements the overall clinical results of the multi-institutional antibiotic trial and suggests that T+G is a useful initial regimen for empiric therapy of febrile episodes in granulocytopenic cancer patients.

The pharmacokinetics of intratracheally instilled and intravenously injected gentamicin were compared in the rat and analyzed by a one-compartment open model. The maximum concentration of gentamicin in plasma occurred within 10 min after intratracheal instillation. Considerable amounts of gentamicin were absorbed from lungs after intratracheal instillation, as shown by its concentrations in plasma and elimination in urine. The data suggest that the absorption of gentamicin from the pulmonary system would be sufficient to maintain therapeutic levels of this agent in plasma.

Measurements of gentamicin concentrations in serum by the luciferase method are affected by sodium phosphate, glucose, and heparin. The effects of the first two factors can be eliminated by omitting phosphate and increasing the level of glucose in the culture medium. The effects of heparin cannot be easily circumvented, thus precluding use of this agent as an anticoagulant. Bilirubin, which purportedly inhibits measurement of gentamicin in serum by the disk diffusion assay, does not affect the accuracy of the luciferase method.

This study was concerned with the effect of Cetomacrogol (polyethylene glycol 1000 monocetyl ether), a nonionic surfactant, on the absorption of gentamicin and amikacin from the gastrointestinal tract of rats. A 200-mg dose of Cetomacrogol coadministered orally with 10 mg of gentamicin resulted in a mean peak gentamicin blood concentration of 14.1 microgram/ml, compared with 67.8 microgram/mg when the same gentamicin dose was administered intramuscularly. The area under the curve after administration of the oral mixture was 23% of that after the intramuscular dose. The rectal administration of the mixture resulted in a mean peak gentamicin blood level of 8.2 micrograms/ml, compares to 16.5 microgram/ml when the mixture was administered orally. A 50-mg dose of amikacin coadministered orally with 200 mg of Cetomacrogol resulted in a mean peak amikacin blood level of 13.3 microgram/ml, compared to 310 microgram/ml when this amikacin dose was administered intramuscularly. Cetomacrogol augments the intestinal absorption of gentamicin and amikacin in rats. If the toxicity of the combination in humans is limited, the combination may be potentially clinically useful.

Six different antibiotic treatment regimens were compared for efficacy in rabbits with endocarditis induced by inoculation with a nutritionally variant strain of streptococcus. Seven untreated animals, sacrificed at day 11, had vegetations containing 8.89 +/- 1.35 log10 CFU/g, none of which was sterile. The vegetations from the rabbits in all treated groups had bacterial titers significantly lower than those of the controls (P less than 0.001). Vegetations from penicillin-treated animals averaged 5.14 +/- 1.00 log CFU/g, and no vegetations were sterile. Treatment with penicillin plus gentamicin or amikacin was more effective than treatment with penicillin alone, resulting in 3.99 +/- 0.94 log CFU/g of vegetation and sterile lesions in 5 of 12 animals. Treatment with vancomycin alone was as least as efficient as that with penicillin plus an aminoglycoside, resulting in an average of 3.33 +/- 0.96 log CFU/g of vegetation and sterile lesions in five of eight animals. Treatment with vancomycin plus an aminoglycoside was not superior to treatment with vancomycin alone, resulting in an average of 3.68 +/- 1.37 log CFU/g of vegetation and sterile lesions in 8 of 13 animals. These in vivo results correlated poorly with the in vitro susceptibility of the strain to the various antibiotics, as measured by the time-kill method. These results support the current practice of using vancomycin as alternative therapy when a penicillin-aminoglycoside combination is ineffective or contraindicated in patients with endocarditis caused by nutritionally variant streptococci.

To assess the possible role of intravenous (i.v.) infusion rate as a determinant of degree and rate of interstitial fluid penetration, six rabbits, each with four intraperitoneal implanted capsules, were studied by crossover design after a single dose of 1.7 mg of gentamicin per kg by either slow 2.5-min i.v. bolus or 30 min i.v. infusion. The mean serum peak antibiotic level after slow bolus was 17.4 mug/ml. After 30 min of infusion, mean serum peak was 8.3 mug/ml (P < 0.025). Mean capsule fluid antibiotic levels at 30 min, 1, and 2 h were 0.9 mug/ml, 1.6 mug/ml, and 1.8 mug/ml, respectively, after slow bolus and 0.6 mug/ml, 0.9 mug/ml, and 1.3 mug/ml after 30-min infusion (P < 0.05 at 30 min, P < 0.001 at 1 h, and P < 0.05 at 2 h). Comparison of capsule levels beyond 2 h revealed no significant differences, and peak capsular concentrations achieved by the two methods were similar. Slow 2.5-min i.v. bolus administration of gentamicin established higher interstitial fluid levels during the first 2 h of therapy and may be the preferred mode of delivery when rapid extravascular penetration is desired.

The effectiveness of gentamicin administered by the intranasal route in healthy human volunteers was studied. Thirty minutes after gentamicin in a solution of 1% sodium glycocholate was administered at 2 mg/kg of body weight, gentamicin concentrations of 2.48 +/- 0.42 mg/ml were present in the sera. Serum concentrations decreased to 0.94 +/- 0.13 mg/ml at 180 min. No serious side effects were observed. There was slight local irritation after a single dose. When administered intranasally without glycocholate, gentamicin was not present in the sera in detectable amounts.

A procedure is described that uses high-pressure liquid chromatography for assay of gentamicin in serum. The technique involves extraction of gentamicin from serum by using a CM-Sephadex column and analysis by reverse-phase, ion-pair chromatography. Continuous-flow, post-column derivatization with o-phthaladehyde is used to form fluorescent products for detection. The possible scope of this method for analysis of other aminoglycoside antibiotics is illustrated by application to sisomicin and netilmicin.

A radiometric assay for gentamicin was compared with an established radioimmunoassay protocol. The coefficients of variation for within-run, day-to-day, and overall runs were consistently higher for the radiometric method as compared with the radioimmunoassay. The coefficient of correlation for 84 patient sera tested by the two methods for gentamicin levels was 0.82. Though less precise, the radiometric method was felt to be an acceptable means of determining gentamicin levels in laboratories where the radioimmunoassay is unavailable.

The EMIT (Enzyme Multiplied Immunoassay Technique) for serum gentamicin determination was evaluated by a standard procedure. The precision, accuracy, and specificity were assessed and proved satisfactory. Comparison with a bioassay was done, and results for patient samples showed a correlation coefficient of 0.95 between the two methods. Advantages of the enzyme immunoassay system were the provision of results within 15 min of receipt of blood in the laboratory, a requirement of minimal technical expertise, and an applicability to both large and small workloads. The EMIT proved during evaluation to be a practical alternative to current bioassays for the determination of serum gentamicin concentrations.

Gentamicin was successfully incorporated into neutral, anionic, and cationic liposomes, and the percentage of gentamicin incorporated was found to be a function of lipid concentration. Gentamicin did not leak out of the liposomes over a 3-week period at 4 degrees C. When liposome-associated gentamicin was administered intravenously to rabbits, its serum half-life was greatly prolonged. Intragastric administration of dipalmitoylphosphatidylcholine liposomes containing gentamicin resulted in the appearance of gentamicin in serum. Liposome-associated gentamicin, when administered intravenously, led to the appearance of gentamicin in the liver and spleen, which was not observed when rabbits were injected with free gentamicin.

Serum concentration determinations of netilmicin obtained with bioassay and the enzyme multiplied immunoassay technique were compared. There was no statistically significant difference between the precision of the two techniques (P greater than 0.05). When clinical serum was used, a systematic error was demonstrated between the enzyme multiplied immunoassay technique and bioassay.

Three procedures (radioimmunoassay, fluorescence immunoassay, and enzyme immunoassay) for the determination of gentamicin and tobramycin levels were compared. These systems were evaluated on the basis of accuracy, reproducibility, specificity, and cost. All three systems showed a high degree of accuracy and precision. The fluorescence immunoassay gave significantly lower values for gentamicin and tobramycin at levels below 5 microgram/ml. Of the three systems, the radioimmunoassay was the least expensive, provided that a minimum of 10 analyses were performed daily; single tests required more technical time and were more costly. Costs of single tests by fluorescence immunoassay and enzyme immunoassay systems were similar. The enzyme immunoassay system performed equally well for both gentamicin and tobramycin in giving rapid and accurate results.

In a crossover study, single doses of netilmicin and gentamicin were administered intramuscularly, each at 1.0 and 2.5 mg/kg. The serum concentrations, analyzed by a two-compartment open model with a first-order absorption, indicated that the pharmacokinetics of the drugs are essentially the same. Both drugs were rapidly absorbed and distributed after administration. No significant differences were observed between the two drugs in disposition half-life, rate of distribution and elimination, area under the serum concentration-time curve, urinary excretion, total body clearance, and renal clearance. After 1.0 mg/kg, the maximum serum concentration of netilmicin (5.18 microgram/ml) was only slightly lower than that for gentamicin (5.76 microgram/ml), but no difference was found after the 2.5-mg/kg dose.

A homogenous enzyme immunoassay for the quantitative determination of netilmicin in serum was developed. The procedure utilizes a commercially available assay for gentamicin (EMIT; Syva Co., Palo Alto, Calif.). The method was adapted to a microcentrifugal analyzer, and log-logit regression analysis was performed with a computer. The results of samples assayed by this method correlate well with microbioassay (r2 = 0.985) and radioimmunoassay (r2 = 0.986). This method is not only precise and accurate, but also very rapid and economical and compares favorably to other available methods of netilmicin assay.

Patients' sera were divided into three portions when the initial gentamicin level was determined and were stored at -20, 4, and 25 degrees C in plastic or glass tubes. Gentamicin levels were repeated after 1 and 2 days of storage at the respective temperatures. There was no significant difference in gentamicin levels among portions, except those from a patient in renal failure with high serum concentrations of carbenicillin.

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.

Hypoxemia causes significant alterations in aminoglycoside serum pharmacokinetics in adult rats and newborn rabbits. The aminoglycoside serum clearances were diminished in hypoxemic animals. Comparisons of intravenous and intramuscular administration imply a delay in uptake of drug from intramuscular injection sites under hypoxemic conditions.

The current substrate-labeled fluorescent immunoassay for gentamicin has been modified by substituting monoclonal antibodies for conventional antiserum to gentamicin. The standard curve generated with gentamicin monoclonal antibody was essentially linear from 0 to 12 micrograms/ml, could detect 0.4 micrograms of gentamicin per ml, and had overall intra- and interassay precision values (coefficients of variation) of 1.9 and 5.0%, respectively. The substrate-labeled fluorescent immunoassay produced with gentamicin monoclonal antibody gave results with clinical specimens comparable (r = 0.991) to those obtained with the commercially available substrate-labeled fluorescent immunoassay and also with an enzyme immunoassay, a radioimmunoassay, and a fluorescent immunoassay. This technologically state-of-the-art assay employs both a nonisotopic label and monoclonal antibodies. It offers excellent precision, sensitivity, lot-to-lot reproducibility, linearity, and reagent stability.

Accuracy, precision, and clinical laboratory utility of the TDX (Abbott Laboratories), Auto-ICS (Beckman Instruments, Inc.), COBAS-Bio (Roche Analytical Instruments, Inc.) with reagent kits (Syva), and EMIT (Syva) for gentamicin and tobramycin serum assay were assessed. TDX, COBAS-Bio, and EMIT analytical systems showed a proportional bias of less than 10% for recovery studies and a coefficient of variation less than 5% for within-run precision. The results of the recovery studies with the Auto-ICS showed a proportional bias of 25% with gentamicin and 16% with tobramycin. The within-run precision expressed as the coefficient of variation for the Auto-ICS was 6.7% for gentamicin and 8.6% for tobramycin. In comparisons involving gentamicin- and tobramycin-containing patient samples, the results with the TDX analytical system showed the best agreement with the COBAS-Bio. For the determination of these two antibiotics, the TDX analytical system provided the best overall accuracy and precision.

A radioimmunoassay and an enzymatic assay for gentamicin have been compared. The correlation coefficient for results of gentamicin assays performed by the two methods with 45 serum specimens was 0.90. A similar standard curve for the radioimmunoassay was obtained with gentamicin complex, with gentamicin Cl, Cla, or C2, or with sisomicin as ligand, but tobramycin did not compete with [(3)H]gentamicin for binding to the antiserum.

The microbiological bioassay, the adenylation method, and the radiometric, enzyme, and fluorescent immunoassay methods for assaying serum gentamicin were compared. The precision, reproducibility, and specificity of each method was assessed and proved satisfactory, with the exception of the radioimmunoassay, which gave artificially high results. Good correlations between the other four methods was obtained in a comparison involving 103 patient sera. The other aspects of performance were also compared, namely, simplicity, speed, cost, ease of automation, and application to large and small workloads. The enzyme immunoassay performed best in this comparison, being accurate, specific, rapid, and very simple to perform. However, other laboratories might find that workload, staffing, and available equipment make other methods more attractive.