Tobramycin Levels Research Articles

Effectiveness of bone cement containing tobramycin. An in vitro susceptibility study of 99 organisms found in infected joint arthroplasty.

We used 99 strains of organisms representative of orthopaedic infections to examine the effectiveness of a bone cement containing tobramycin, employing a modified in vitro Kirby-Bauer susceptibility model. The spectrum was broad, including Gram-positive and Gram-negative aerobic organisms, anaerobes and mycobacteria. Simplex P with added tobramycin was effective against most of the strains, including those which are resistant to typical systemic levels of tobramycin. Although direct correlation between in vitro and in vivo results is difficult, the study showed that tobramycin is stable to the exothermic polymerisation of the cement, and that it is released from the surface of the cement at concentrations high enough to inhibit the growth of most organisms which may be encountered after joint arthroplasty.

Tobramycin penetration into epithelial lining fluid of patients with pneumonia.

To study the penetration of tobramycin in lung tissue evaluated as the concentration in epithelial lining fluid and to characterize the time course of the drug in the treatment of patients with pneumonia. The subjects were 16 patients with pneumonia and taking tobramycin who had clinical indications for bronchoscopy. Bronchoscopy with bronchoalveolar lavage of the pneumonic area was performed once on each patient 1/2%, 2, 4, or 8 hours after the previous tobramycin dose. Urea was used as an endogenous marker for quantification of epithelial lining fluid obtained at bronchoalveolar lavage. Tobramycin concentrations in serum were measured for all patients at the aforementioned 4 time points. Tobramycin concentration was determined by means of fluorescent polarization immunoassay modified for bronchoalveolar samples. Levels of tobramycin in the fluid of the epithelial lining were 2.33+/-0.5 at 1/2 hour, 1.67+/-0.6 at 2 hours, 1.62+/-1.19 at 4 hours, and 0.77+/-0.38 microg/mL at 8 hours. The ratio of epithelial lining fluid to serum concentration of tobramycin was 0.30+/-0.03 at 1/2 hour, 0.42+/-0.16 at 2 hours, 0.64+/-0.37 at 4 hours, and 1.53+/-0.76 at 8 hours. The ratio at peak serum time was similar to that reported for tobramycin and netilmicin. High peak serum concentrations of tobramycin are necessary to obtain microbiologically active concentrations at the alveolar level. The fluid of the epithelial lining constitutes a deep compartment for aminoglycosides. The disappearance of tobramycin was slower than at the serum level.

The use of antibiotic-impregnated cement in infected reconstructions after resection for bone tumours.

We have studied 35 patients with infected reconstructions after segmental resection for bone tumours. Two different regimes of treatment were employed both of which entailed debridement and the use of cement impregnated with antibiotic. In 25 patients gentamicin-PMMA beads were used and in ten an antibiotic-impregnated cement spacer. Better results were achieved with the second procedure in terms of cases healed, the number of operations, time of healing, time of recovery and the functional score. The use of vancomycin in the cement spacer gave better local control.

Application of Neural Networks to Population Pharmacokinetic Data Analysis

This research examined the applicability of using a neural network approach to analyze population pharmacokinetic data. Such data were collected retrospectively from pediatric patients who had received tobramycin for the treatment of bacterial infection. The information collected included patient-related demographic variables (age, weight, gender, and other underlying illness), the individual’s dosing regimens (dose and dosing interval), time of blood drawn, and the resulting tobramycin concentration. Neural networks were trained with this information to capture the relationships between the plasma tobramycin levels and the following factors: patient-related demographic factors, dosing regimens, and time of blood drawn. The data were also analyzed using a standard population pharmacokinetic modeling program, NONMEM. The observed vs predicted concentration relationships obtained from the neural network approach were similar to those from NONMEM. The residuals of the predictions from neural network analyses showed a positive correlation with that from NONMEM. Average absolute errors were 33.9 and 37.3% for neural networks and 39.9% for NONMEM. Average prediction errors were found to be 2.59 and -5.01% for neural networks and 17.7% for NONMEM. We concluded that neural networks were capable of capturing the relationships between plasma drug levels and patient-related prognostic factors from routinely collected sparse within- patient pharmacokinetic data. Neural networks can therefore be considered to have potential to become a useful analytical tool for population pharmacokinetic data analysis.

In Vitro Kinetics of Drug Release and Pulmonary Retention of Microencapsulated Antibiotic in Liposomal Formulations in Relation to the Lipid Composition

In previous in-vivo studies, we demonstrated that liposomal entrapment of tobramycin resulted in an increased availability of the antibiotic in the lungs without increasing bactericidal efficacy (Omri et al. 1994). With the aim of developing liposomal formulations allowing more efficient liposome-bacteria interactions, we studied the influence of lipid composition on both drug release and pulmonary retention of encapsulated tobramycin. The phase transition temperatures of nine liposome-tobramycin formulations consisting of two synthetic phospholipids (distearoyl phosphatidylcholine (DPSC) or dipalmitoyl phosphatidylcholine (DPPC) with dimyristoyl phosphatidyl-glycerol (DMPG) or dimyristoyl phosphatidylcholine (DMPC)) were determined by differential scanning calorimetry. Liposomes, varying in terms of membrane fluidity and charge were submitted to in-vitro and in-vivo kinetic studies while retention and release of tobramycin were measured by high-performance liquid chromato-graphy (HPLC). Five less fluid liposome formulations showed absence or very low tobramycin release in in-vitro tests and long term pulmonary retention of tobramycin. Four fluid liposome formulations showed in in vitro tests modulated tobramycin release while pulmonary retention of tobramycin was dependent of the presence of charged phospholipids. Administration of charged fluid liposomes in mice showed a low level of tobramycin in the kidneys; non-charged fluid liposomes exhibited a relatively high level of tobramycin retention in the kidneys.

Eradication of mucoid Pseudomonas aeruginosa with fluid liposome-encapsulated tobramycin in an animal model of chronic pulmonary infection.

Despite controversies associated with forms and value of antibiotic therapy for cystic fibrosis patients, antibiotherapy remains a cornerstone in the management of those patients. Locally administered liposome-encapsulated antibiotics may offer advantages over free antibiotics, including sustained concentration of the antibiotic, minimal systemic absorption, reduced toxicity, and increased efficacy. We evaluated the efficacy of free and encapsulated tobramycin in fluid and rigid liposomal formulations administered to rats chronically infected with Pseudomonas aeruginosa. Chronic infection in lungs was established by intratracheal administration of 10(5) CFU of a mucoid variant of P. aeruginosa PA 508 prepared in agar beads. Antibiotic treatments were given intratracheally at time intervals of 16 h. After the last treatment, lung bacterial counts were determined and tobramycin levels in the lungs and kidneys were evaluated by high-performance liquid chromatographic analysis and microbiological assay. Two independent experiments showed that animals treated with encapsulated tobramycin in fluid liposomes had a number of CFU less than the minimal CFU number required to be statistically acceptable compared with > or = 10(6) CFU per pair of lungs for animals treated with encapsulated tobramycin in rigid liposomes, free antibiotic, or liposomes without tobramycin. Tobramycin measured in the lungs at 16 h after the last treatment following the administration of encapsulated antibiotic was still active, and its concentration was > or = 27 micrograms/mg of tissue. Low levels of tobramycin were detected in the kidneys (0.59 to 0.87 micrograms/mg of tissue) after the administration of encapsulated antibiotic, while 5.31 micrograms/mg of tissue was detected in the kidneys following the administration of free antibiotic. These results suggest that the local administration of fluid liposomes with encapsulated tobramycin could greatly improve the management of chronic pulmonary infection in cystic fibrosis patients.

Bone toxicity of locally applied aminoglycosides.

The effect of tobramycin on osteoblasts was studied. Osteoblast-like cells from the MG-63 human osteosarcoma cell line were grown in antibiotic-free media and exposed to concentrations of tobramycin: first at order of magnitude intervals between 0 and 10,000 micrograms/ml, and then at closer intervals between 100 and 1,000 micrograms/ml to more accurately define toxic levels. Cell number and 3H-thymidine incorporation at 0, 24 and 72 h were determined. Similarly, primary cultures of rat osteoblasts were exposed to the same concentrations of tobramycin to confirm the validity of the MG-63 cell line as a model for nontransformed cells. The results of this study demonstrate that local levels of tobramycin of < or = 200 micrograms/ml have little or no effect on osteoblast replication. Concentrations of 400 micrograms/ml significantly decreased cell replication, and concentrations of 10,000 micrograms/ml caused cell death. Results obtained with primary rat osteoblast cultures were similar to those obtained from the MG-63 cultures at the tested tobramycin concentrations.

Tobramycin Serum Concentrations After Aerosol and Oral Administration in Cystic Fibrosis.

We sought to describe tobramycin absorption after aerosol administration to cystic fibrosis (CF) subjects. Serum tobramycin concentrations were determined by modification of the radioimmuno-assay (RIA) technique, lowering the limit of detection from 1.0 &mgr;g ml(minus sign1) to 0.05 &mgr;g ml(minus sign1). In 37 studies, after aerosol delivery of 666 plus minus 195 mg to the airway of 24 patients, in which 222 samples were assayed, only 1 serum sample contained tobramycin at a concentration greater than 1.0 &mgr;g ml(minus sign1). Twenty-six of the 37 studies permitted estimation of pharmacokinetic parameters of tobramycin. The serum clearance of tobramycin following aerosol adminstration is 39.13 plus minus 0.393 L h(minus sign1) (mean plus minus standard error of the mean), with an elimination half-life of 3.072 plus minus 0.194 h. The half-life was significantly longer than that found after intravenous adminstration. The elimination rate constant (K(e)) was calculated to be 0.234 plus minus 0.002 h(minus sign1). Estimated total-body clearance in which systemic absorption was determined from sputum and urinary recovery of tobramycin was 0.094 plus minus 0.002 1 hr(minus sign1) kg(minus sign1). We also studied tobramycin absorption in six CF subjects after ingestion of a 80-mg m(minus sign2) dose, to gain insight into the tobramycin levels observed after swallowing an aerosol. Four out of the six subjects had measurable serum tobramycin concentration after ingestion. The serum concentration-time curve mirrored what was seen after aerosol administration. We concluded that tobramycin has poor systemic absorption in CF subjects after aerosol administration. Tobramycin in serum after aerosol administration is in part due to the gastrointestinal absorption of swallowed drug, as well as absorption from lower respiratory tract.

Transferable amikacin resistance in gram-negative bacterial isolates.

Seven amikacin-resistant strains of Enterobacteriaceae isolated in Slovakia and Germany were included in this study. The strains were also resistant in vitro to high levels of gentamicin, tobramycin, netilmicin and isepamicin. Phosphocellulose paper binding assays indicated that resistance to aminoglycosides was due to synthesis of aminoglycoside acetyltransferase AAC(6')-I a mechanism until now only identified in staphylococci and streptococci. This mechanism of aminoglycoside resistance has also been found in two isolates of Klebsiella pneumoniae from Germany. The substrate profile suggested that in addition to AAC(6')-I and APH(2"), several strains also produced AAC(3)-II. Aminoglycoside resistance was found to be transferable to Escherichia coli 3110 rifr in all isolates, and R plasmids of 36-45 MD were detected in donor and transconjugant strains. All isolated plasmids from transconjugants encoded resistance to aminoglycosides by genes encoding the enzymes AAC(6')-I and APH(2").

Pulmonary retention of free and liposome-encapsulated tobramycin after intratracheal administration in uninfected rats and rats infected with Pseudomonas aeruginosa.

The pulmonary residence time of free and liposome-encapsulated tobramycin was studied with uninfected rats and rats infected with Pseudomonas aeruginosa. Chronic infection in lungs was established by intratracheal administration of 10(8) CFU of P. aeruginosa PA 508 prepared in agar beads. After 3 days, a single dose (300 micrograms) of free or liposome-encapsulated tobramycin was given intratracheally to both infected and uninfected rats. At various time intervals (0.25 to 16 h) after drug instillations, the remaining tobramycin was evaluated in blood, lungs, and kidneys by a microbiological assay. Intratracheal instillation of liposome-encapsulated tobramycin resulted in high and sustained levels of tobramycin in lungs of uninfected and infected rats over the 16-h period studied; however, the tobramycin levels were two times higher in uninfected rats. There was no tobramycin detected in the blood or kidneys from these animals. In contrast, the intratracheally instilled free tobramycin was cleared within 3 and 1 h from the lungs of uninfected and infected animals, respectively. These data suggest that the encapsulation of tobramycin in liposomes can result in a significant increase of its residence time within lungs. This study also shows that pulmonary infection was associated with a lowering of tobramycin levels in lungs.

Daptomycin may attenuate experimental tobramycin nephrotoxicity by electrostatic complexation to tobramycin.

The lipopeptidic antibiotic daptomycin is reported to reduce experimental tobramycin nephrotoxicity (D. Beauchamp, M. Pellerin, P. Gourde, M. Pettigrew and M. G. Bergeron, Antimicrob. Agents Chemother. 34:139-147, 1990; C. A. Wood, H. C. Finkbeiner, S. J. Kohlhepp, P. W. Kohnen, and D. C. Gilbert, Antimicrob. Agents Chemother. 33:1280-1285, 1989). In an attempt to explain these results, the in vivo and in vitro interactions between daptomycin and tobramycin were studied. Tobramycin alone and preincubated with negatively charged phospholipid bilayers (liposomes) was dialyzed against increasing concentrations of daptomycin in buffer at pH 5.4. A significant drop in the concentration of tobramycin was observed when daptomycin was added to the opposite half cells. Furthermore, daptomycin induced a concentration-dependent release of lipid-bound tobramycin. Gold labeling experiments showed that daptomycin could be incorporated into phospholipid layers. Female Sprague-Dawley rats were treated with daptomycin alone, with tobramycin alone, or with the combination over 2 to 10 days. Levels of daptomycin and tobramycin in serum were similar in all groups. The levels of tobramycin in the renal cortex increased significantly with time and, on day 10, reached values of 654 +/- 122 and 844 +/- 298 micrograms/g of tissue (mean +/- standard deviation; not significant) in animals treated with tobramycin and the combination of daptomycin-tobramycin, respectively. No significant difference was observed in the levels of tobramycin in the kidneys between animals treated with tobramycin or the daptomycin-tobramycin combination at any time. By contrast, daptomycin levels were significantly higher in the renal cortexes of animals treated with daptomycin-tobramycin in comparison with those in the renal cortexes of animals treated with daptomycin alone on days 6,8, and 10 (P < 0.01). For immunogold labeling studies, animals were killed 4 h after a single injection of daptomycin alone or daptomycin in combination with tobramycin. Daptomycin was found throughout the matrixes of the lysosomes of proximal tubular cells of animals treated with daptomycin alone. In animals treated with the combination of daptomycin and tobramycin, daptomycin was associated with intralysosomal myeloid bodies. Our results suggest that daptomycin might attenuate experimental aminoglycoside nephrotoxicity by interacting with the aminoglycoside, perhaps electrostatically, and thereby protecting intracellular targets of toxicity.

Antibiotic Impregnated Bone Cement in Total Hip Arthroplasty

A prospective in vivo quantification was performed to measure the elution of tobramycin and vancomycin antibiotics from two commonly used bone cements. Forty patients were divided into four groups: Group I, tobramycin-Simplex; Group II, tobramycin-Palacos-R; Group III, vancomycin-Simplex; and Group IV, vancomycin-Palacos-R. Antibiotic levels were measured from hemovac wound drainage, urine, and serum and compared with control groups who received either intravenous tobramycin or vancomycin. There were no significant differences in daily mean tobramycin levels in hemovac samples between Groups I and II. Tobramycin hemovac levels from Groups I and II were significantly higher than the tobramycin control group. Similarly, no differences were seen in daily mean vancomycin levels of the hemovac samples between Group III and IV; however, the intravenous vancomycin control group had significantly higher levels in the hemovac fluid than Groups III or IV. Tobramycin in the hemovac fluid from Groups I and II was highly bioactive against the control organism. Vancomycin in the hemovac fluid from Groups III and IV had variable bioactivity against the control organism. In 30% of the cases, no vancomycin was detected in the hemovac fluid, and in these cases, the hemovac fluid had no effect on the control organism. Tobramycin elutes to give adequate local tissue levels and releases antibiotic effects when used in an antibiotic bone cement combination. Vancomycin has variable elution properties and is not a predictable additive for the bone cements tested.

Reinfusion of whole blood following addition of tobramycin powder to the wound during total knee arthroplasty

This study prospectively examined nine human volunteers who underwent unilateral cementless total knee arthroplasty and had 600 mg of tobramycin powder added to their wound just prior to fascial closure. Serum levels of tobramycin were measured at 30 minutes, 4 hours, 8 hours, and 12 hours after surgery. Tobramycin levels in the reinfused whole blood were measured with each reinfusion. Patients were reinfused up to 12 hours after surgery and then the reservoir was left to suction drainage. All patients had significant levels of serum tobramycin 30 minutes after surgery. The average serum level was 5.5 μg/ml (range, 3.0–10.6 μg/ml). This level was achieved prior to any reinfusion and represented systemic absorption of tobramycin from the bleeding surfaces of the muscle and bone. All patients received at least one reinfusion of 400 ml of whole blood in the first 8 hours after surgery. Two thirds of the patients received a second reinfusion within the same time frame. Serum tobramycin levels measured postreinfusion indicated that the risk of attaining potential toxic levels of aminoglycoside was not dependent on reinfusion, but on absorption from the wound. The dose of tobramycin in the drain was measured and found to be excessive and potentially toxic (range, 27–312 mg; average, 132 mg). Despite this fact, reinfusion was not as likely to produce toxic serum levels as was local uptake of the antibiotic. Although no patient in this series had any complaints relating to the high dose of tobramycin (deafness or renal failure), caution should be exercised when using antibiotics in a reinfusion system. Because most of the antibiotic delivered into the wound is absorbed immediately the need for irrigating the wound with plain saline prior to reinfusion is not necessary

Overproduction of 3'-aminoglycoside phosphotransferase type I confers resistance to tobramycin in Escherichia coli

Escherichia coli HM69, isolated from urine, was resistant to high levels of kanamycin (MIC, > 1,000 micrograms/ml) and a low level of tobramycin (MIC, 8 micrograms/ml). Phosphocellulose paper-binding assays and molecular cloning indicated that resistance to both aminoglycosides was due to synthesis of a 3'-aminoglycoside phosphotransferase type I, an enzyme that phosphorylates kanamycin but not tobramycin. The structural gene for the enzyme was borne by an 80-kb conjugative plasmid, pIP1518, and was nearly identical to aphA1 of Tn903. Incubation of extracts of resistant cells with tobramycin or kanamycin led to a decrease (> 80%) of antibiotic activity as determined by a microbiological assay. Heat treatment showed that loss of activity was reversible and dependent upon the native enzyme. In the presence of ATP, only inactivation of kanamycin was reversible. These results suggest that resistance to low levels of tobramycin was due to formation of a complex between the enzyme and the antibiotic.

Optimal aminoglycoside dosing regimen for penicillin-tobramycin synergism in experimental Streptococcus adjacens endocarditis.

The combination of penicillin and aminoglycoside is the recommended therapy for endocarditis caused by nutritionally variant streptococci (NVS). However, the optimal aminoglycoside dosing regimen remains controversial. We compared the efficacies of four regimens of tobramycin alone or combined with procaine penicillin in the therapy of rabbits with endocarditis caused by Streptococcus adjacens, a new species of NVS. Animals were injected intramuscularly for 4 days with procaine penicillin (150,000 U/kg of body weight twice daily) or tobramycin at a low dose (3 mg/kg every 24 h) or a high dose (12 mg/kg every 24 h) either once or three times daily (t.i.d.) alone or in combination with procaine penicillin. Additional groups of animals were treated with the combination regimens for a shorter period of time (2 days) in order to demonstrate a possible difference in the rapidity of efficacy between the regimens. The MICs and MBCs were 0.015 and 1 micrograms/ml and 8 and 16 micrograms/ml for penicillin and tobramycin, respectively. The mean peak tobramycin levels in plasma were 2.4 +/- 1.3 (1 mg/kg t.i.d.), 5.4 +/- 3.7 (4 mg/kg t.i.d.), and 25 +/- 9.3 (12 mg/kg once daily). The mean penicillin levels in serum were always above the MIC. In vitro kill curves plotted at the time that peak concentrations were reached in plasma showed a concentration-dependent killing effect of tobramycin alone but not in combination with penicillin. In vivo, low-dose tobramycin was significantly less effective than the high dose. Results for the combinations of the different dosing regimens of tobramycin with procaine penicillin were not significantly different. Our results suggest that (i) against susceptible strains of streptococci, aminoglycoside alone exhibits a concentration-dependent killing effect both in vitro and in vivo; (ii) against NVS strains, combinations of penicillin and high- or low-dose tobramycin are equally effective; and (iii) aminoglycoside given once daily or at a low dose t.i.d. with penicillin could be a cost-effective alternative with reduced toxic risk for patients with NVS endocarditis when the bacteria are susceptible to the killing activities of both compounds.

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.

Cranioplasty, vertebral body replacement, and spinal fusion with tobramycin-impregnated methylmethacrylate

A prospective analysis of cranioplasty, vertebral body replacement, and spinal fusion using tobramycin-impregnated methylmethacrylate in 65 patients is presented. Cranioplastic methacrylate (1 packet) was mixed with 1.2 g of powdered tobramycin and placed into the desired location. The polymer was irrigated with bacitracin until it became solid. All patients received prophylactic medication with intravenously administered nafcillin and cephalosporin perioperatively and for 48 hours postoperatively. The follow-up time ranged from 7 to 57 months (mean, 32.2 months). Serum tobramycin levels remained below 0.5 microgram/ml in all patients tested, regardless of the time interval. Blood urea nitrogen and creatinine levels remained within normal limits in all patients, and there has been no nephrotoxicity or ototoxicity. There has been 1 infection among the 65 patients (1%) to date. It occurred in a patient who had a previously treated local infection. Cranioplasty, vertebral body replacement, or spinal fusion with tobramycin-impregnated methylmethacrylate is safe and may reduce the incidence of infection.

Cranioplasty, Vertebral Body Replacement, and Spinal Fusion with Tobramycin-impregnated Methylmethacrylate

Abstract A prospective analysis of cranioplasty, vertebral body replacement, and spinal fusion using tobramycin-impregnated methylmethacrylate in 65 patients is presented. Cranioplastic methacrylate (1 packet) was mixed with 1.2 g of powdered tobramycin and placed into the desired location. The polymer was irrigated with bacitracin until it became solid. All patients received prophylactic medication with intravenously administered nafcillin and cephalosporin perioperatively and for 48 hours postoperatively. The follow-up time ranged from 7 to 57 months (mean, 32.2 months). Serum tobramycin levels remained below 0.5 μg/ml in all patients tested, regardless of the time interval. Blood urea nitrogen and creatinine levels remained within normal limits in all patients, and there has been no nephrotoxicity or ototoxicity. There has been 1 infection among the 65 patients (1%) to date. It occurred in a patient who had a previously treated local infection. Cranioplasty, vertebral body replacement, or spinal fusion with tobramycin-impregnated methylmethacrylate is safe and may reduce the incidence of infection.

Antibiotic levels in bronchial tree and in serum during selective digestive decontamination

Selective digestive decontamination has been found to prevent pulmonary infections in mechanically ventilated patients. The aims of this study were: 1) to determine whether detectable levels of antibiotics could be found in bronchial tree secretions of patients receiving SDD, and 2) to evaluate antibiotic serum levels. In 15 patients receiving mechanical ventilation and SDD for 10 days or more, tobramycin and amphotericin B levels were determined every 3 days in the following specimens: tracheal aspirates, distal bronchial secretions and blood samples. 82% of tracheal aspirates contained detectable (greater than 0.18 mg/l), tobramycin concentrations; the levels varied widely between patients and large day-to-day variations were observed. Every patient had at least 1 tracheal aspirate with tobramycin level higher than 0.5 mg/l during his course. 40% of distal specimens contained detectable tobramycin levels (10 patients). Serum determinations showed detectable concentration of tobramycin in 50% of the specimens (9 patients). Two patients with renal failure had serum tobramycin levels higher than 2 mg/l. In 13 tracheal aspirates cultures were positive and 15 species were isolated; 13 had a MIC higher than the corresponding tobramycin level in tracheal secretions. We conclude that substantial levels of antibiotics can be found frequently in respiratory tract specimens of patients receiving SDD. Therefore, the usual microbiological criteria used to assess respiratory tract infection may be unreliable in this setting and other criteria may be required. Follow-up of antibiotic serum levels is required, especially in patients with renal failure.

Quantitation of Scierai Tobramycin Levels

We have devised a new method to solubilize sclera in order to permit quantitative determination of tobramycin by commercially available and widely utilized means. The sclera, containing tobramycin, was digested with Alcalase (Subtilisn A, NOVO Enzymes) and the tobramycin level was quantitated by TDX autoflourophotometry. This technique is accurate and sensitive. The tissue digestion and antibiotic assay are relatively simple to perform and should be applicable to a variety of antibiotics and possibly other therapeutic agents.