Tobramycin Concentrations Research Articles

Dose-Dependent Effects of Topical Tobramycin in an Animal Model of Pseudomonas Sinusitis

Topical antimicrobials are commonly used to treat Pseudomonas sinusitis after endoscopic sinus surgery. Despite their popularity, there is limited data in the literature to support its use. This study is the first to examine the effects of topical tobramycin in an animal model of Pseudomonas sinusitis. Pseudomonas aeruginosa sinusitis was induced in the maxillary sinus of New Zealand white rabbits. Study groups received various concentrations of topical tobramycin instilled through a single lumen catheter embedded within the maxillary sinus for 7 consecutive days. A control group received normal saline irrigation. Bacterial counts were measured from the nasal lavage after irrigation. The sinonasal complex was analyzed and a histological grading system was used to score the degree of infection at study end. Nasal lavage bacterial counts of the saline control group remained persistently elevated throughout the study. Introduction of increasing concentrations of topical tobramycin caused the bacterial counts to fall to zero by study end. There was a dose-dependent histological response in which increasing concentrations of tobramycin led to a marked reduction but not total elimination of inflammation within the mucosa and underlying bone. As opposed to normal saline irrigations, topical tobramycin led to a significant improvement in the degree of infection within an animal model of P. aeruginosa sinusitis. Despite the elimination of bacteria within the nasal lavage, there was a suggestion of a residual indolent infection as evidenced by persistent inflammation within the mucosa and underlying bone.

Pharmacokinetic studies on tobramycin in horses

The objective of the study was to evaluate the pharmacokinetics of tobramycin in plasma and urine in the horse (n = 7) after intravenous administration of a dose of 4 mg/kg b.w. Plasma tobramycin concentrations were assayed microbiologically and by means of HPLC analyses. Pharmacokinetic parameters, calculated on the basis of concentrations determined with the microbiological assay were not statistically different from those obtained when data from HPLC analysis were used, but the microbiological assay was more sensitive in the detection of low plasma and urine values. The values of the total body clearance (Cl(B)) were 101.4 +/- 30.1 and 130.0 +/- 49.9 mL/kg/h, respectively. The overall extraction ratio was 2.9%. The determined capacity of elimination of tobramycin in horses was similar to those for other aminoglycosides. Within 24 h after treatment, 57.6 +/- 12.2% of injected antibiotic was excreted in the urine.

Reliability of mini-bronchoalveolar lavage for the measurement of epithelial lining fluid concentrations of tobramycin in critically ill patients

To evaluate the reliability of mini-bronchoalveolar lavage (mini-BAL) for the measurement of tobramycin concentrations in epithelial lining fluid (ELF) in comparison with conventional bronchoscopic bronchoalveolar lavage (BAL). Prospective, open-label study. An intensive care unit and research ward in a university hospital. Twelve critically ill adult patients with ventilator-associated pneumonia (VAP). All subjects received intravenous infusions of tobramycin 7-10 mg/kg once daily. After 2 days of therapy, the steady-state serum and ELF concentrations (obtained from BAL and mini-BAL) of tobramycin were determined by means of high-performance liquid chromatography. We observed poor penetration of tobramycin in ELF of approximately approximately 12% with ELF peak concentrations of approximately approximately 3 mg/l with both methods. Good agreement in Bland-Altman analysis (mean +/- SD bias = 0.04 +/- 0.38 mg/l) was observed between the two methods of sampling. Our results suggest that tobramycin 7-10 mg/kg once daily in critically ill patients with VAP might provide insufficient lung concentrations in the case of difficult-to-treat pathogens. Besides, mini-BAL, which is simple, non-invasive and easily repeatable at the bedside, appears to be a reliable method for the measurement of antibiotic concentrations in ELF in comparison with bronchoscopic BAL in critically ill patients with VAP.

Evaluation of the in vivo efficacy of topical tobramycin against Pseudomonas sinonasal biofilms

There is increasing evidence that bacterial biofilms are present within the sinonasal mucosa of patients with medically recalcitrant chronic rhinosinusitis. The antibiotic concentrations needed to eradicate biofilms are much higher than those commonly used and pose an increased risk for systemic side effects. Topical medications are an alternative approach to deliver high concentrations to the mucosa while limiting systemic side effects. This is the first study to evaluate the effects of a topical antibiotic on Pseudomonas biofilms in an animal model of sinusitis. Pseudomonas sinusitis was established in nine rabbits. An irrigation catheter was placed within the lumen of the maxillary sinus, and the rabbits were irrigated with 7 days of normal saline or varying concentrations of tobramycin diluted in normal saline. Bacterial cfu were determined for nasal lavage fluid as well as physically disrupted mucosa, and scanning electron microscopy (SEM) of the mucosal surface was performed at the end of the study. Increasing concentrations of topical tobramycin resulted in the eradication of viable bacteria within the lumen of the sinus but did not eradicate Pseudomonas attached to the mucosa. SEM detected biofilms within the mucosa even in those rabbits treated with high concentrations of topical tobramycin. Bacterial biofilms attached to the sinonasal mucosa are resistant to topical saline and tobramycin irrigation. SEM was useful in the identification of biofilms, but did not provide a quantitative evaluation for efficacy of treatment.

Novel tobramycin inhalation powder in cystic fibrosis subjects: Pharmacokinetics and safety

Aerosolized antibiotics are associated with a high treatment burden that can result in non-adherence to chronic therapy. We evaluated the pharmacokinetics (PK) and safety of tobramycin inhalation powder (TIP), a novel dry-powder formulation designed to deliver a high payload of tobramycin topically to the lungs for management of chronic Pseudomonas aeruginosa infections. This was a multi-center, open-label, sequential-cohort, single-dose, dose-escalation study using the standard 300 mg dose of tobramycin solution for inhalation (TSI) as an active control. Subjects were randomized to TIP or TSI in a 3:1 ratio in each of five cohorts. Measurements included serum and sputum tobramycin concentrations, administration time, serum chemistries, acute change in lung function, and adverse events (AEs). Out of 90 randomized subjects, 86 had data for safety analysis; and 84 had data for PK analysis. Serum tobramycin PK profiles were similar for TIP and TSI. Four capsules of 28 mg TIP (total tobramycin dose 112 mg) produced comparable systemic exposure to 300 mg TSI, in less than one-third the administration time. The most common AEs associated with TIP were cough (20%) and dysgeusia (17%). TIP allows for faster and more efficient pulmonary delivery of tobramycin than TSI and has a safety profile that supports continued clinical investigation. The increased rate of local respiratory tract irritation noted with TIP is not unexpected with a high-payload powder formulation. The development of dry powder inhaled antibiotics may represent an important advance in the treatment of chronic lung infections.

Inhaled tobramycin solution-associated recurrent eosinophilia and severe persistent bronchospasm in a patient with cystic fibrosis: a case report

BackgroundDelivery of tobramycin by inhalation to the lungs of patients with cystic fibrosis (CF) who are infected with Pseudomonas aeruginosa has been proven to be effective and safe. The aerosol administration allows high concentrations of tobramycin to be delivered to the site of infection with limited systemic absorption. In rare patients, systemic absorption of inhaled tobramycin may be significant enough to produce toxic effects, such as renal and vestibular toxicities.Case presentationWe report a patient with CF who developed recurrent eosinophilia and severe persistent bronchospasm following repeated administration of preservative-free tobramycin by inhalation, beginning at 16 months of age. Also, he developed similar signs and symptoms when he was administered tobramycin intravenously on one occasion at 5 1/2 years. The patient had a history of environmental allergies. Temporal sequence of his signs and symptoms after each administration of tobramycin (similar to re-challenge testing), and his improvement after discontinuation of the drug strongly suggest an adverse drug reaction.ConclusionHypersensitivity reaction should be considered in patients who develop recurrent eosinophilia and deterioration of pulmonary function following the use of tobramycin by inhalation or by intravenous administration.

The Lon protease of Pseudomonas aeruginosa is induced by aminoglycosides and is involved in biofilm formation and motility

Pseudomonas aeruginosa is an important nosocomial opportunistic human pathogen and a major cause of chronic lung infections in individuals with cystic fibrosis. Serious infections by this organism are often treated with a combination of aminoglycosides and semi-synthetic penicillins. Subinhibitory concentrations of antibiotics are now being recognized for their role in microbial persistence and the development of antimicrobial resistance, two very important clinical phenomena. An extensive screen of a P. aeruginosa PAO1 luciferase gene fusion library was performed to identify genes that were differentially regulated during exposure to subinhibitory gentamicin. It was demonstrated that subinhibitory concentrations of gentamicin and tobramycin induced a set of genes that are likely to affect the interaction of P. aeruginosa with host cells, including the gene encoding Lon protease, which is known to play a major role in protein quality control. Studies with a lon mutant compared to its parent and a complemented strain indicated that this protein was essential for biofilm formation and motility in P. aeruginosa.

The peculiarities of Pseudomonas aeruginosa resistance to antibiotics and prevalence of serogroups

Pseudomonas aeruginosa is one of the most common nonfermenting aerobic gram-negative microorganisms identified in clinical specimens of hospitalized patients. The emergence of multidrug-resistant (MDR) Pseudomonas aeruginosa strains is a growing concern in hospital-acquired infections. Typing of strains is important for identifying the sources of infection as well as prevention of cross-infections and monitoring of the efficacy of antimicrobial therapy. The aim of this study was to evaluate the antimicrobial resistance and prevalence of Pseudomonas aeruginosa serogroups isolated at Kaunas University of Medicine Hospital, Lithuania. Minimum inhibitory concentrations of piperacillin, cefoperazone, ceftazidime, cefotaxime, cefepime, imipenem, meropenem, gentamicin, amikacin, tobramycin, and ciprofloxacin for 609 Pseudomonas aeruginosa strains isolated from various clinical specimens between November 2001 and November 2002 were determined by the microdilution method in Mueller-Hinton agar using interpretative guidelines of National Committee for Clinical Laboratory Standards. Serogroups of Pseudomonas aeruginosa strains were identified using serums of Seiken Co. Ltd (Tokyo, Japan), containing antibodies against antigens of Pseudomonas aeruginosa O-group. Pseudomonas aeruginosa strains were the most sensitive to ceftazidime (78.9%), imipenem (73.6%), meropenem (70.9%) and the most resistant to gentamicin (54.1%) and ciprofloxacin (52.5%). Multidrug-resistant strains made up 9.85% of all Pseudomonas aeruginosa strains investigated. Multidrug-resistant Pseudomonas aeruginosa strains were 1.5-3.5 times more resistant to antibiotics compared to non-multidrug-resistant strains, except to amikacin: multidrug-resistant strains were more sensitive (81.7%) than non-multidrug-resistant Pseudomonas aeruginosa strains (61.0%). Pseudomonas aeruginosa serogroups O:E and O:B were the most common serogroups (34.7% and 29.0%, respectively) followed by serogroups O:I (11.4%) and O:A (10.1%). Pseudomonas aeruginosa serogroup O:E strains were the most prevalent among multidrug-resistant strains (48.3%). The results of our study show that serogroup O:E was the most prevalent serogroup of Pseudomonas aeruginosa in our hospital, and its resistance to antibiotics was the highest.

Efficacy, Safety, and Local Pharmacokinetics of Highly Concentrated Nebulized Tobramycin in Patients with Cystic Fibrosis Colonized with Pseudomonas aeruginosa

Progressive respiratory failure due to Pseudomonas aeruginosa colonization is the most significant morbidity in patients with cystic fibrosis (CF). This trial was designed to investigate the efficacy and safety of a highly concentrated (300mg/4mL) tobramycin solution for inhalation (TSI) [Bramitob] in patients with CF and P. aeruginosa infection. Fifty-nine patients were randomized to receive a 4-week treatment with tobramycin or placebo administered twice daily via the Pari LC Plus nebulizer and Pari TurboBoy compressor, followed by a 4-week run-out phase. Pulmonary function (forced expiratory volume in 1 second [FEV(1)], forced vital capacity [FVC], and forced expiratory flow at the midportion of vital capacity [FEF(25-75%)]), P. aeruginosa susceptibility, microbiologic results, and in vitro minimum inhibitory concentration for 90% of strains (MIC(90)) were the efficacy outcome measures, while safety was monitored by the recording of adverse events, audiometry (bone conduction at 250-8,000Hz frequency), laboratory tests, physical examination and general health condition. The concentration of tobramycin attained in sputum was measured in a cohort of 21 patients. FEV(1) significantly increased from baseline in the tobramycin group compared with no change in the placebo group: the absolute difference between groups (intent-to-treat population) of predicted normal was 13.2% at week 2 (p = 0.002) and 13.3% at week 4 (p = 0.003). Significant differences in favor of the tobramycin group were also observed for FVC and FEF(25-75%). The microbiologic results at the end of the treatment period (P. aeruginosa-negative culture, persistence, superinfection) showed a significantly better outcome in the tobramycin group compared with placebo (p = 0.033). The effects of tobramycin on pulmonary function and microbiology were not maintained at the end of the run-out phase. Mean sputum concentrations of tobramycin after the first dose (695.6 +/- 817.0 microg/mL) were similar to those measured after the last dose (716.9 +/- 799 microg/mL) and were superior to the detected specific MIC(90). The proportion of patients with drug-related adverse events was lower in the tobramycin group and no signs of renal or auditory toxicity were observed. The 4-week administration of a highly concentrated TSI significantly improved pulmonary function and microbiologic outcome compared with placebo and was well tolerated. The results of this study should be confirmed in further long-term trials in larger populations.

Clinical Pharmacology Study of Bramitob??, a Tobramycin Solution for Nebulization, in Comparison with Tobi??

To compare in vitro characteristics and pharmacokinetics of Bramitob, a preservative-free tobramycin solution for nebulization, and Tobi in patients with cystic fibrosis (CF) and Pseudomonas aeruginosa infection. In vitro characteristics of Bramitob and Tobi were evaluated using Pari TurboBoy/LC Plus and the Systam 290 LS nebulizers. In the randomized, double-blind, two-way crossover pharmacokinetic study, 11 patients with CF received a single nebulized dose (300mg) of Bramitob or Tobi, separated by a 7-day washout period. Plasma and sputum tobramycin concentrations were measured immediately before and over 24 hours after administration. Bramitob and Tobi performed alike during nebulization. The fine particle fraction was 33-37% and the mass median aerodynamic diameter was <5microm. Nine patients completed the pharmacokinetic study. Tobramycin plasma profiles after administration of Bramitob or Tobi were similar, with a peak at 90 and 72 minutes after inhalation of Bramitob and Tobi, respectively. The elimination half-life was ~5 hours for both products. The relative bioavailability of Bramitob to Tobi was 1.01, indicating comparable systemic exposure. Peak sputum concentration of tobramycin was 816 +/- 681 microg/g for Tobi and 1289 +/- 851 microg/g for Bramitob and was >400 microg/g (threshold sufficient for an antibacterial effect against P. aeruginosa) in 5 out of 9 patients receiving Tobi and 8 out of 9 patients receiving Bramitob. All adverse events were considered mild and judged not related to the study drugs. In vitro performance of Bramitob((R)) was similar when nebulized with Pari TurboBoy k/LC Plus and Systam 290 LS nebulizers and comparable to that of TobiThe systemic bioavailability of tobramycin was similar after administration of either Bramitob or Tobi; however, in sputum samples the tobramycin peak concentration was slightly greater after administration of Bramitob than after Tobi.

Acute Renal Failure Associated with Vancomycin- and Tobramycin-Laden Cement in Total Hip Arthroplasty

To describe 2 cases of acute renal failure (ARF) associated with the use of antibiotic-laden cement incorporated in total hip arthroplasties (THA). An 82-year-old female received a right THA with antibiotic-laden cement spacers. She developed ARF 5 months following implantation, concurrent with an elevated serum tobramycin concentration of 5.5 microg/mL. After explantation of the prosthesis and spacers, serum creatinine and antibiotic concentrations decreased to within normal limits. A 79-year-old male received antibiotic-laden cement spacers in a revision of his right THA due to infection. ARF developed 1 1/2 months after the revision; a serum tobramycin concentration was 2.9 microg/mL. Serum creatinine and antibiotic serum concentrations decreased to within normal limits with explantation. More than 250 000 joint replacements are performed yearly in the US. A common complication is infection, which occurs in 1-2% of primary replacements and 3-4% of revisions of previously infected prostheses. Antibiotic-laden cement is used for prosthesis placement to prevent or treat infection, while minimizing systemic drug exposure. Both patients described here received antibiotic-laden spacers during THA and subsequently developed ARF in conjunction with elevated serum tobramycin concentrations. Use of the Naranjo probability scale and consideration of possible contributing factors suggest a probable association of the antibiotic-laden cement and the development of ARF in these patients. Antibiotic-laden cement with aminoglycosides and/or vancomycin has the potential for systemic toxicity and should be used according to guidelines and with increased vigilance and prudent monitoring in patients at increased risk for nephrotoxicity.

Nebulisers comparison with inhaled tobramycin in young children with cystic fibrosis

Background This randomised cross-over pilot study was undertaken in 10 cystic fibrosis children aged 10 to 63 months to describe lung absorption of tobramycin delivered by the PariLC+/PariTurboboyN (Pari GmbH) and the disposable NL9M/AtomisorBoxPlus (Diffusion Technique Française) nebulising systems. Methods Each child inhaled 300 mg tobramycin delivered with one or the other apparatus via a facemask in two separate and standardised sessions. Urine was collected for 6 h. Tobramycin concentrations determined by immunoprecipitation were expressed in mg per g of creatinine and compared by a Wilcoxon test for matched pairs. The influences of age, weight and Brasfield score on this parameter were evaluated by correlation tests, and those of sex, previous nebulisation treatment, and crying or coughing were evaluated by Student's t-test. Results The amount of tobramycin measured in urines was low and variable. Median values for urinary tobramycin concentration were 47.6 mg/g (14.9–79.6) with the PariLC+ and 42.6 mg/g (6.3–112.8) with the NL9M ( p = 0.6). PariLC+ delivered tobramycin in 22 min and NL9M in 12 min ( p = 0.005). Crying or coughing dramatically reduced the amount of tobramycin collected. Conclusion This pilot study shows that evaluation of nebulisers based on tobramycin renal excretion is feasible in young children with cystic fibrosis.

Therapeutic drug monitoring of once daily tobramycin in cystic fibrosis—caution with trough concentrations

Background: Once daily intravenous aminoglycoside dosing (ODD) is widely used to treat acute Pseudomonas aeruginosa exacerbations in patients with cystic fibrosis. Controversy exists as to what is the most appropriate method of therapeutic drug monitoring (TDM) of such therapy with recommendations including trough plasma concentrations of < 1 mg/L or < 2 mg/L, area under curve (AUC) and various nomograms. This study aimed to compare the exposures to ODD of tobramycin in adults and children with cystic fibrosis using the AUC and trough TDM approaches. Methods: Using a mono-exponential software program to calculate AUC from 2 plasma concentrations, AUCs were determined in 22 adults with pre-dose tobramycin concentrations < 1 mg/L. The exposure of 5 children with reduced tobramycin clearances was simulated at the usual recommended dose of 10 mg/kg/daily but retaining a trough < 1 mg/L. Results: A tobramycin dose of 10 mg/kg of tobramycin in these patients with normal serum creatinine and a trough concentration < 1 mg/L resulted in exposures in excess of those associated with conventional 8-hourly dosing. Conclusions: The TDM approach of a trough < 1 mg/L, as used with conventional 8-hourly tobramycin dosing, is not relevant to ODD.

Formulation and Characterization of Lipid-Coated Tobramycin Particles for Dry Powder Inhalation

This study was conducted to develop and evaluate the physicochemical and aerodynamic characteristics of lipid-coated dry powder formulations presenting particularly high lung deposition. Lipid-coated particles were prepared by spray-drying suspensions with different concentrations of tobramycin and lipids. The solid-state properties of the formulations, including particle size and morphology, were assessed by scanning electron microscopy and laser diffraction. Aerosol performance was studied by dispersing the powders into a Multistage Liquid Impinger and determining drug deposition by high-performance liquid chromatography. Particle size distributions of the formulations were unimodal, narrow with more than 90% of the particles having a diameter of less than 2.8 microm. All powder formulations exhibited mass median diameters of less than 1.3 and 3.2 microm, as determined by two different laser diffraction methods, the Malvern's Mastersizer and Spraytec, respectively. The fine particle fraction varied within a range of 50.5 and 68.3%. Lipid coating of tobramycin formulations resulted in a reduced agglomeration tendency and in high fine particle fraction values, thus improving drug deposition. The very low excipients content (about 5% m/m) of these formulations offers the benefit of delivering particularly huge concentrations of antibiotic directly to the site of infection, while minimizing systemic exposure, and may provide a valuable alternative treatment of cystic fibrosis.

Measurement of tobramycin by reversed-phase high-performance liquid chromatography with mass spectrometry detection

Analysis of tobramycin faces challenges owing to its significant basicity, hydrophilicity and lack of a UV absorbing chromophore. Chromatographic methods, coupled with derivatization to introduce chromophores for tobramycin analysis, were extensively studied. A direct reversed-phase HPLC method for tobramycin analysis has not been reported. Here, we would like to report a simple LC/MS method for quantitative analysis of tobramycin in pharmaceutical formulations. Reversed-phase HPLC analysis of tobramycin was achieved using a pH stable C18 column with basic (pH 11) aqueous mobile phase (ammonium hydroxide buffer), while direct detection was carried out employing a single quadruple mass detector in negative mode via electrospray ionization. This unique separation–detection combination provided simple and specific determination of tobramycin. This method was found to be linear at a tobramycin concentration range of 0.2–0.8 mg/mL with a correlation coefficient value of 0.999. The quantitation limit and detection limit were calculated as 0.210 and 0.063 μg/mL, respectively, with 99.994% confidence. This method was successfully applied to measure tobramycin content in matrices containing tobramycin and other pharmaceutical formulation ingredients. Recoveries of 101.8, 97.8 and 106.7% were obtained for tobramycin spiked in the pharmaceutical formulation at concentrations of 1.68, 1.0 and 0.35 mg/mL, respectively. The relative standard deviations for six injections of spiked samples ranged from 0.2 to 3.2%, indicating good method repeatability.

Determination of tobramycin in human serum by capillary electrophoresis with contactless conductivity detection

A study on the determination of the antibiotic tobramycin by CE with capacitively coupled contactless conductivity detection is presented. This method enabled the direct quantification of the non-UV-absorbing species without incurring the disadvantages of the indirect approaches which would be needed for optical detection. The separation of tobramycin from inorganic cations present in serum samples was achieved by optimizing the composition of the acetic acid buffer. Field-amplified sample stacking was employed to enhance the sensitivity of the method and a detection limit of 50 microg/L (S/N = 3) was reached. The RSDs obtained for migration time and peak area using kanamycin B as internal standard were typically 0.12 and 4%, respectively. The newly developed method was validated by measuring the concentration of tobramycin in serum standards containing typical therapeutic concentrations of 2 and 10 mg/L. The recoveries were 96 and 97% for the two concentrations, respectively.

Safeness of Simplex-Tobramycin Bone Cement in Patients with Renal Dysfunction Undergoing Total Hip Replacement

To compare the pharmacokinetic profile of tobramycin in blood, urine, and at the operative site following the use of Simplex-tobramycin bone cement in primary total hip replacement between patients with and without renal dysfunction. Six patients with renal dysfunction underwent cemented primary total hip replacement for osteoarthritis. The elution characteristics of Simplex-tobramycin bone cement in the 6 patients with renal dysfunction were compared with 9 patients who had normal renal function. Blood, urine, and drainage fluid specimens were collected for 72 hours postoperatively. Very high concentrations of tobramycin were seen in the drainage fluid of the 2 groups. Mean serum tobramycin levels peaked at postoperative 3 hours, and declined rapidly to negligible levels at 72 hours in both groups. Mean urinary tobramycin concentrations peaked at postoperative 12 hours and declined rapidly until 48 hours in both groups. Urinary tobramycin was excreted significantly more slowly in renal dysfunction group in the first 12 hours, but not thereafter. Although serum creatinine levels of the renal dysfunction group were higher throughout the study period, the difference was not significant. Both groups achieved excellent local delivery of the antibiotic with minimal systemic concentrations. Simplex-tobramycin bone cement appears to be an effective and safe means to deliver antibiotic for patients with renal dysfunction who undergo total hip replacement.

Effects of N-trimethylchitosan on transcellular and paracellular transcorneal drug transport

The effects of N-trimethylchitosan (TMC) on the transcorneal transport of dexamethasone, taken as a marker of the transcellular penetration route, and of tobramycin, a marker of the paracellular route, were studied by assessing the TMC effect on the intraocular pharmacokinetics of each marker. The drugs were topically applied via erodible inserts (weight, 20 mg; diameter, 6 mm; drug dose, 0.3 mg) based on poly(ethylene oxide), containing 10% w/w medicated TMC microspheres (diameter <2.5 μm). Before application, drug release and insert erosion kinetics, and release mechanism were studied in vitro. With either drug, introduction of 10% TMC into insert did not substantially alter the release and erosion rates, hence this formulation was apt to isolate the transcorneal penetration enhancing effect of TMC. Ocular pharmacokinetics were determined in the rabbit model. TMC produced significant increases of dexamethasone C max (5.69 ± 0.49 vs. 3.07 ± 0.31 μg/ml) and AUC (619.3 ± 32.5 vs. 380.5 ± 32.0 μg min/ml) in the aqueous with respect to the reference TMC-free insert. On the other hand, TMC was unable to yield tobramycin concentrations in the aqueous exceeding the determination limit (0.5 μg/ml). In conclusion, TMC enhances transcorneal transport via the transcellular route, whereas it is unable to effectively open the tight junctions between corneal cells.

92 Intrapulmonary concentrations of tobramycin and ceftazidime achieved after intravenous administration for the treatment of Cystic Fibrosis lung infections

92 Intrapulmonary concentrations of tobramycin and ceftazidime achieved after intravenous administration for the treatment of Cystic Fibrosis lung infections

Stimulating Biofilm Formation

As illustrated by Fleming's famous observation that a contaminating mold inhibited bacterial growth, most antibiotics derive from substances produced by microorganisms, which may allow them to compete with other microorganisms. Noting that Streptomyces tenebrarius (which produces the aminoglycoside antibiotic tobramycin) and Pseudomonas aeruginosa (a human pathogen that can cause chronic, life-threatening infections) are both found in soil, Hoffman et al. investigated the hypothesis that P. aeruginosa had evolved adaptive responses to tobramycin. Low concentrations of tobramycin, which failed to inhibit the growth rate or alter global protein synthesis, induced the formation of biofilms (adherent communities of bacteria that contribute to antibiotic resistance and persistent infection) in P. aeruginosa and Escherichia coli . Three other aminoglycoside antibiotics also stimulated biofilm formation. By screening for mutants with attenuated biofilm production in response to tobramycin but normal biofilm formation in its absence, the authors identified three strains with mutations in a gene that they named arr (for aminoglycoside response regulator). arr was predicted to encode a membrane protein with two transmembrane domains and an EAL domain [found in cyclic di-guanosine monophosphate (c-di-GMP) phosphodiesterases]. Complementation with wild-type arr , but not a mutant in which a conserved glutamate in the EAL domain was replaced with alanine, restored tobramycin-induced biofilm formation. Membranes from arr -mutant bacteria were less effective at degrading c-di-GMP than were those from wild-type bacteria, and exogenous GTP (which inhibits c-di-GMP phosphodiesterases) inhibited tobamycin-stimulated biofilm formation in wild-type cells. Thus, the authors conclude that biofilm formation is a defensive response to aminoglycoside antibiotics that involves changes in bacterial c-di-GMP content. L. R. Hoffman, D. A. D'Argenio, M. J. MacCoss, Z. Zhang, R. A. Jones, S. I. Miller, Aminoglycoside antibiotics induce bacterial biofilm formation. Nature 436 , 1171-1175 (2005). [Online Journal]