Is there a multinational consensus of tobramycin prescribing and monitoring for cystic fibrosis? Survey of current therapeutic drug monitoring practices in USA/Canada, UK/Ireland, and Australia/New Zealand

OBJECTIVES

Sophisticated scientific methods have facilitated dose individualisation with substantial advancements in therapeutic drug monitoring (TDM) practice. It is unclear whether these methods have translated to the clinical setting. This study aimed to determine current TDM practice for tobramycin monitoring in cystic fibrosis (CF) centres in the USA and Canada, UK and Ireland, and Australia and New Zealand due to a high prevalence of CF.

METHODS

A web-based survey was developed and circulated via CF specialist groups within the targeted geographical regions. Themes included centre demographics, tobramycin usage, dosing and infusion practices, TDM practices, and blood sampling methods.

RESULTS

In total 77 responses were received from 75 different CF centres over the 3-month evaluation period (October 2019-January 2020). Respondents were from the USA and Canada (60%), Australia and New Zealand (25%), and the UK and Ireland (15%). Tobramycin was used in 97% of sites, with an international variation in practice across all survey aspects including dosing and infusion practice. TDM-based dose adjustment in the UK and Ireland was most commonly based only on trough sample collection for avoidance of toxicity, where use of computer programs for targeting both efficacy and toxicity endpoints were most common in Australia and New Zealand. The underlying pharmacokinetic basis of that program was not known by 33% of sites who utilised a computer program for tobramycin dose individualisation.

CONCLUSION

There remains substantial heterogeneity in tobramycin management worldwide. Despite two decades of research into TDM of tobramycin, there has been a slow uptake of new technologies and evolution of practice. An improved understanding of TDM processes is required for translation of evidence-based research into clinical practice. International guidelines require updating due to the advances in research to support confidence in the changes in clinical practice.

Necessity of Tobramycin trough Levels in Once Daily Iv-Treatment in Patients with Cystic Fibrosis

BACKGROUND

Once daily intravenous (iv) treatment with tobramycin for Pseudomonas aeruginosa infection in patients with cystic fibrosis (pwCF) is frequently monitored by measuring tobramycin trough levels (TLs). Although the necessity of these TLs is recently questioned in pwCF without renal impairment, no study has evaluated this so far. The aim of this observational study was to evaluate the frequency of increased tobramycin TLs in pwCF treated with a once daily tobramycin dosing protocol.

METHODS

Patient records of all consecutive once daily iv tobramycin courses in 35 pwCF between 07/2009 and 07/2019 were analyzed for tobramycin level, renal function, co-medication and comorbidity.

RESULTS

Eight elevated TLs (2.9% of 278 courses) were recorded in four patients, two with normal renal function. One of these resolved without adjustment of tobramycin dosages suggesting a test timing or laboratory error. In the other patient the elevated tobramycin level decreased after tobramycin dosage adjustment. Six of the elevated levels occurred in two patients with chronic renal failure. In 15 other patients with reduced glomerular filtration rate (GFR) (36 courses) but normal range creatinine no case of elevated tobramycin trough levels was detected. Neither cumulative tobramycin dosages nor concomitant diabetes or nutritional status were risk factors for elevated TLs.

CONCLUSION

Our data show that elevated tobramycin TLs are rare but cannot be excluded, so determination of tobramycin TLs is still recommended for safety.

Review of Tobramycin Dosing in Pediatric Patients With Cystic Fibrosis

OBJECTIVE

An institution's tobramycin pharmacokinetics (PK) database was reviewed to evaluate the efficacy and safety of empiric tobramycin dosing and monitoring strategies used in pediatric patients with cystic fibrosis (CF). The relationship between patient age and tobramycin dosing needed to achieve the area under the curve (AUC) goal was investigated.

METHODS

Retrospective chart review was performed for patients who received tobramycin during a CF exacerbation from 2009 to 2019 who received PK monitoring by pediatric pharmacists. Tobramycin dosing needed to achieve an AUC of 100 mg·hr/L was calculated for each patient. Serum creatinine and concomitant nephrotoxin use were collected as surrogate nephrotoxicity endpoints to evaluate safety.

RESULTS

Goal AUC (100 ± 15 mg·hr/L) was achieved based on initial or repeat PK calculations in 43.5% (95% CI, 37.7-49.3) of 85 unique patients across 326 encounters. Patients with calculated recommended doses of 9.5 to 11.9 mg/kg every 24 hours empirically achieved goal AUC in 77% (78/101) of encounters. The odds of achieving goal AUC were 56% higher for children aged 10 vs 5 years (OR = 1.56; 95% CI, 1.04-2.34; p = 0.033) and 32% higher for children aged 15 vs 10 years (OR = 1.32; 95% CI, 1.07-1.61; p = 0.008). Overall rates of acute kidney injury and concomitant nephrotoxin use were 10.8% (95% CI, 6.2-15.5) and 80.7% (95% CI, 74.3-87.1), respectively.

CONCLUSIONS

Desired AUC was achieved by 43.5% of pediatric patients with CF using tobramycin 10 mg/kg every 24 hours. Older patient age was associated with higher initial AUC attainment and fewer dose modifications. Younger children may require higher weight-based dosing to meet AUC goals.

Impact of Patient-Specific Aminoglycoside Monitoring for Treatment of Pediatric Cystic Fibrosis Pulmonary Exacerbations

OBJECTIVE

Aminoglycosides are frequently used for empiric and definitive treatment of cystic fibrosis (CF) pulmonary exacerbations. Various methods have been described for aminoglycoside therapeutic drug monitoring. The objective of this study is to evaluate the effect of patient-specific pharmacokinetic calculations for aminoglycosides used to treat CF pulmonary exacerbations.

METHODS

Ambidirectional cohort study of patients admitted to a children's hospital from June 1, 2018, through February 28, 2019, and June 1, 2019, through February 8, 2021. The primary outcome was the occurrence of dosing changes after analysis of initial serum concentrations in either group. Secondary outcomes included occurrence of nephrotoxicity, duration of antibiotics, and length of stay.

RESULTS

Twenty-four patients (75%) in the intervention group versus zero in the control group required dosing adjustments after initial analysis of serum concentrations were completed (p < 0.001). There was not a statistically significant between-group difference for duration of antibiotics in days (median, 14 vs 13.5; Z, 1.07; p = 0.29) or length of stay (median, 11 vs 11; Z, −0.31; p = 0.76). There was also not a statistically significant between-group difference in forced expiratory volume in one second (FEV1) change from admission to discharge (11.4% vs 13.9%; t, 0.61; Degrees of Freedom, 39; p = 0.55). Two patients (6.25%) in the intervention group experienced nephrotoxicity compared with zero patients in the control group (risk difference, 6.25%; 95% CI, −2.14 to 14.64; number needed to harm, 16).

CONCLUSIONS

Patient-specific pharmacokinetic monitoring led to significantly more dosing changes and was associated with similar patient outcomes as trough-only monitoring. Further studies are needed to identify methods to optimize aminoglycoside dosing and monitoring for these patients with the goal of reducing toxicities while maximizing efficacy.

The indispensable role of pharmacy services and medication therapy management in cystic fibrosis

Care for people with cystic fibrosis (PWCF) is highly complex and requires a multidisciplinary approach where the pharmacist plays a vital role. The purpose of this manuscript is to serve as a guideline for pharmacists and pharmacy technicians who provide care for PWCF by providing background and current recommendations for the use of cystic fibrosis (CF)-specific medications in both the acute and ambulatory care settings. The article explores current literature surrounding the role of pharmacists and pharmacy technicians, proven pharmacy models to emulate, and pharmacokinetic idiosyncrasies unique to the CF population while also identifying areas of future research. Clinical recommendations for the use of CF-specific medications are broken down by organ system including mechanism of action, adverse events, dosages, and monitoring parameters. The article also includes quick reference tables essential to the acute and chronic medication therapy management of PWCF.

Extended Interval Aminoglycoside Treatment for Klebsiella Pneumoniae Endocarditis in an Extremely Low Birth Weight Neonate

Infective endocarditis (IE) in neonates is associated with high mortality and incidence has been increasing over the past two decades. The majority of very low birth weight infants will be treated with at least one nephrotoxic medication during their hospital course. Over one-quarter of very low birth weight neonates exposed to gentamicin may develop acute kidney injury (AKI); this is particularly worrisome as AKI is an independent factor associated with increased neonatal mortality and increased length of stay. AKI during periods of neonatal nephrogenesis, which continues until 34-36 weeks postmenstrual age, may also have serious effects on the long-term nephron development which subsequently puts infants at risk of chronic kidney disease. Extended interval (EI) aminoglycoside (AMG) dosing has been used for decades in adult populations and has proven to reduce AKI while being at least as effective as traditional dosing, although there is limited published research for using an EI AMG in endocarditis in adults or pediatric patients. We describe an extremely low birth weight neonate, born preterm at 24 weeks gestation treated for Klebsiella pneumoniae IE that required AMG therapy who also had concurrent AKI. We utilized EI AMG combination therapy for treatment of Klebsiella pneumoniae endocarditis with good outcome and encourage others to report their experiences to improve our knowledge of EI AMG in this population.

Aminoglycoside dosing and monitoring for Pseudomonas aeruginosa during acute pulmonary exacerbations in cystic fibrosis

Aminoglycosides are commonly used for the treatment of Pseudomonas aeruginosa (PsA) in the setting of acute pulmonary exacerbations (PEx) in pediatric patients with cystic fibrosis (CF). There are controversies and practice differences between institutions related to aminoglycoside dosing and monitoring strategies. The purpose of this review article is to summarize the currently available literature and identify gaps in the literature related to pharmacokinetic parameter goals, aminoglycoside dosing strategies, and methods for monitoring serum aminoglycoside concentrations for treatment of PsA in CF PEx, and throughout will discuss anticipated changes with the increasing availability of highly effective CF transmembrane conductance regulator modulators. This review focuses on tobramycin, as it is the most commonly used aminoglycoside in CF PEx, and will briefly discuss special circumstances surrounding use of amikacin and gentamicin.

Comparison of the predictive value of area under the curve versus maximum serum concentration of intravenous tobramycin in cystic fibrosis patients treated for an acute pulmonary exacerbation

OBJECTIVES

The primary objective of this study was to compare the therapeutic predictive value of area under the curve (AUC₂₄ ) versus maximum concentration (C_max ) in cystic fibrosis (CF) patients receiving intravenous (IV) tobramycin for a Pseudomonas aeruginosa (PsA) acute pulmonary exacerbation (APE). Acute kidney injury (AKI) incidence and the relationship between time undetectable and efficacy were also assessed.

METHODS

A retrospective review was conducted in patients aged at least 1 month with a diagnosis of CF receiving IV tobramycin for treatment of a PsA APE and admitted to the University of Kentucky between August 2015 and August 2019. Patients were excluded if they had no growth of PsA on sputum culture or if two postdose tobramycin levels were not obtained following a dose adjustment of ≥20%.

RESULTS

A total of 44 pediatric and 107 adult patient encounters met inclusion criteria. In patients with therapeutic success (n = 91), 75.8% had an AUC₂₄  ≥80% and 80.3% had a C_max ≥8 times the highest PsA minimal inhibitory concentration. There was a significant correlation between AUC₂₄ and C_max (r[149] = 0.727; p < 0.001). AKI incidence was significantly higher in patients receiving IV tobramycin dosed multiple times daily versus at least every 24 h (χ² [1, 151] = 3.9; p =  0.047).

CONCLUSIONS

The results of this study indicate that both AUC₂₄ and C_max serve as relatively accurate predictors of tobramycin efficacy. Additionally, given the significant increase in incidence of AKI, multidaily dosing of IV tobramycin should be avoided in pediatric and adult patients with CF.

Therapeutic Drug Monitoring of Inhaled Tobramycin in a Post-Hematopoietic Cell Transplant Patient With Bronchiolitis Obliterans and End-Stage Renal Disease

There are no widely accepted dose alterations for inhaled tobramycin in the setting of renal dysfunction, and serum concentrations are not typically monitored. Herein we describe a case report of a 16-year-old female with a history of 2 hematopoietic cell transplants and a kidney transplant who received inhaled tobramycin for chronic Pseudomonas aeruginosa management. The patient developed chronic kidney disease, and tobramycin concentrations were monitored. Initially she received a reduced dose of inhaled tobramycin, with repeated doses based on serum concentrations. The dose was increased, but serum concentrations obtained the following day remained higher than desired, leading to a suspicion of delayed systemic absorption. Tobramycin administration was changed from immediately prior to dialysis to the evening prior to the next day's dialysis session, and serum concentrations were consistently <1 mg/L postdialysis. In conclusion, systemic absorption of inhaled tobramycin in non-cystic fibrosis (CF) patients may differ compared to that observed in CF patients. Renal dysfunction may lead to systemic accumulation of inhaled tobramycin, and the timing of inhaled tobramycin administration with respect to dialysis has a potentially significant influence on drug clearance. Thus, monitoring may be required. Further cases are required to verify these observations.

Individualizing piperacillin/tazobactam dosing in adult patients with cystic fibrosis: can tobramycin measurements help?

Background

Empirical models to predict β-lactam pharmacokinetics (PK) using information from routine aminoglycoside therapeutic drug monitoring (TDM) have been proposed for critically ill patients; however, no such models exist for patients with cystic fibrosis (CF).

Objectives

To investigate whether PK parameters of tobramycin could be used to predict those of piperacillin.

Methods

A non-interventional, open-label PK study was conducted in hospitalized adults treated with piperacillin/tazobactam and tobramycin for acute pulmonary exacerbations of CF. Six serum samples per patient were collected and analysed. One- and two-compartment population PK models with linear, Michaelis-Menten or mixed elimination were evaluated for both drugs within the PmetricsTM package for R. Models were developed and compared iteratively using the log-likelihood and Akaike information criterion (AIC) objective functions.

Results

Nine primarily female (n = 8) and Caucasian (100%) adult CF patients were enrolled. The median (IQR) age, height, weight and serum creatinine of included patients was 31 (27-32) years, 51.4 (49.9-55.8) kg, 162.6 (160.0-165.1)  cm and 0.6 (0.5-0.6) mg/dL, respectively. The final model with the lowest objective function values consisted of one compartment with first-order elimination for tobramycin and two compartments with mixed-order elimination for piperacillin with the elimination rate constant of piperacillin modelled as a linear function of the elimination rate constant of tobramycin.

Conclusions

A relationship was identified between the elimination rate constants of tobramycin and piperacillin. Validation of this relationship in larger studies of adult patients with CF is needed before application to the precision dosing of piperacillin/tazobactam in this patient population.

Extended Interval Tobramycin Pharmacokinetics in a Pediatric Patient With Primary Ciliary Dyskinesia Presenting With an Acute Respiratory Exacerbation

The pharmacokinetics of tobramycin in patients with ciliary dyskinesia have not been previously reported. A 10-year-old female patient with primary ciliary dyskinesia was admitted to the general pediatrics floor with an acute respiratory exacerbation after several months of worsening lung function that was unresponsive to oral antibiotics. Extrapolating from cystic fibrosis dosing regimens, she was given intravenous tobramycin 320 mg (10.3 mg/kg/day) on admission as a result of concern for a Pseudomonas aeruginosa infection. Two-point pharmacokinetic monitoring revealed a maximum serum concentration (Cmax) of 18.9 mg/L and a 24-hour area under the curve (AUC_0-24hr) of 58.8 (mg × hr)/L, as well as a volume of distribution (Vd) of 0.5 L/kg and an elimination rate (Ke) of 0.34 hr^-1. After a dosage increase to tobramycin 400 mg (12.8 mg/kg/day), pharmacokinetic parameters on 2 assessments were as follows: Vd 0.37 to 0.39 L/kg, Ke 0.33 to 0.39 hr^-1, Cmax 27.8 to 28.7 mg/L, and AUC_0-24h 78.4 to 89.4 (mg × hr)/L. This was the first case report of aminoglycoside pharmacokinetics in a patient with ciliary dyskinesia. The administration of larger doses (up to 12.8 mg/kg/day) of extended-interval tobramycin, similar to the treatment recommendation of at least 10 mg/kg/day for cystic fibrosis patients, was necessary in this patient to achieve serum concentrations that were appropriate for treatment.

Safety of Extended Interval Tobramycin in Cystic Fibrosis Patients Less an 6 Years Old

OBJECTIVES

The primary objective of this study was to assess the nephrotoxicity and ototoxicity risks of extended interval tobramycin in cystic fibrosis (CF) patients who are <6 years old. A secondary objective included analyzing pharmacokinetic parameters in this age group.

METHODS

A retrospective chart review was conducted of patients with CF who were <6 years old, admitted for an acute pulmonary exacerbation from January 1, 2003, to January 1, 2014, and treated with intravenous tobramycin.

RESULTS

The median baseline serum creatinine (SCr) was 0.26 mg/dL among the 31 patients included in the study. Of the 20 patients who experienced increases in SCr, the absolute median increase was 0.065 mg/dL (0.033-0.1 mg/dL). Abnormal audiograms seen in 4 patients are attributable to middle ear effusion present on exam. The median dose of tobramycin was 11.7 mg/kg (11.3-12 mg/kg), the elimination constant was 0.4 hr^-1(0.32-0.47 hr^-1), half-life was 1.7 hr (1.5-2.1 hr), volume of distribution was 0.37 L/kg (0.31-0.47 L/kg), and median peaks and troughs fell within ranges of 20 to 30 mg/L (20.9-32.7 mg/L) and <0.01 mg/L, respectively.

CONCLUSIONS

Extended interval dosing tobramycin is safe in CF patients who are <6 years old. There was no drug-related ototoxicity, and some nephrotoxicity was observed. When dosed at 12 mg/kg, similar pharmacokinetics were seen among all age groups, and concentrations were within the desired range.

Bayesian Estimation of Tobramycin Exposure in Patients with Cystic Fibrosis

Fixed tobramycin (mg/kg) dosing is often inappropriate in patients with cystic fibrosis (CF), as pharmacokinetics are highly variable. The area under the concentration-time curve (AUC) is an exposure metric suited to monitoring in this population. Bayesian strategies to estimate AUC have been available for over 20 years but are not standard practice in the clinical setting. To assess their suitability for use in clinical practice, three AUC estimation methods using limited sampling were compared to measured true exposure by using intensive sampling tobramycin data. Adults prescribed once daily intravenous tobramycin had eight concentrations taken over 24 h. An estimate of true exposure within one dosing interval was calculated using the trapezoidal method and compared to three alternate estimates determined using (i) a two-sample log-linear regression (LLR) method (local hospital practice); (ii) a Bayesian estimate using one concentration (AUC₁); and (iii) a Bayesian estimate using two concentrations (AUC₂). Each method was evaluated against the true measured exposure by a Bland-Altman analysis. Twelve patients with a median (range) age and weight of 25 (18 to 36) years and 66.5 (51 to 76) kg, respectively, were recruited. There was good agreement between the true exposure and the three alternate estimates of AUC, with a mean AUC bias of <10 mg/liter · h in each case, i.e., -8.2 (LLR), 3.8 (AUC₁), and 1.0 (AUC₂). Bayesian analysis-based and LLR estimation methods of tobramycin AUC are equivalent to true exposure estimation. All three methods may be suitable for use in the clinical setting; however, a one-sample Bayesian method may be most useful in ambulatory patients for which coordinating blood samples is difficult. Suitably powered, randomized clinical trials are required to assess patient outcomes.

Evaluation of a Once/Day Tobramycin Regimen to Achieve Target Concentrations in Adult Patients with Cystic Fibrosis

STUDY OBJECTIVE

To evaluate the success of an initial tobramycin dosing regimen to achieve target peak and trough concentrations in adult patients with pulmonary exacerbations of cystic fibrosis (CF).

DESIGN

Retrospective single-center medical record review.

SETTING

Large tertiary care academic medical center.

PATIENTS

A total of 186 patient encounters where 112 patients with CF were treated for acute pulmonary exacerbations with 10 mg/kg/day of tobramycin between January 1, 2009, and December 5, 2014.

MEASUREMENTS AND MAIN RESULTS

Baseline demographics, clinical characteristics, tobramycin data, and pharmacokinetic variables were collected. The primary outcome evaluated the success of the initial tobramycin dosing regimen in attaining the target peak concentration. Secondary end points were achievement of the target trough concentration, achievement of combined peak and trough targets, and incidence of nephrotoxicity. Bivariate and multivariate analyses were performed to evaluate factors associated with achieving target concentrations. Of the 186 patient encounters, 41% achieved the target peak with the first dosing regimen, 62% achieved a target trough, and 23% achieved the target peak and trough. Nephrotoxicity occurred in 10% of patient encounters. A body mass index (BMI) of 18.5-24.9 kg/m(2) was associated with higher odds of meeting the target peak compared with a BMI lower than 18.5 kg/m(2) (odds ratio [OR] 24.5; 95% confidence interval [CI] 5.2-117.2). Conversely, a BMI of 18.5-24.9 kg/m(2) was associated with lower odds of attaining the target trough compared with a BMI lower than 18.5 kg/m(2) (OR 0.16; 95% CI 0.05-0.56). Higher volume of distribution and elimination rate constants (Kel ) were associated with significantly lower odds of achieving the target peak. In addition, higher Kel values were associated with significantly higher odds of achieving the target trough.

CONCLUSIONS

The current initial tobramycin regimen did not achieve target serum tobramycin concentrations reliably. Optimization of the initial CF tobramycin dosing regimen is warranted.

Extended-Interval Aminoglycoside Use in Cystic Fibrosis Exacerbation in Children and Young Adults: A Prospective Quality Improvement Project

This is a prospective quality improvement project for patients with cystic fibrosis who are 5 years of age and older who were admitted for intravenous antibiotic administration as part of treatment of cystic fibrosis exacerbation. The goal of this project was to compare the pharmacokinetics of once-daily versus thrice-daily aminoglycoside use when treating cystic fibrosis exacerbation in different age groups. Of the total of 119 patient encounters, 82.4% were started on once-daily dosing, and the remainder were started on thrice-daily dosing. Patients with pharmacokinetics allowing the continuation of once-daily dosing differed from patients who required a switch to thrice-daily dosing in terms of baseline forced expiratory volume in 1 second, forced expiratory flow from 25% to 75% of vital capacity, age, and body mass index (BMI) but were similar in BMI percentiles. The once-daily dosing group had higher mean 18-hour level, higher mean half-life, higher mean area under the curve, and lower mean elimination constant. This study showed that aminoglycoside clearance is higher in younger children.

Infectious Diseases Pharmacotherapy for Children With Cystic Fibrosis

Cystic fibrosis (CF) affects several organs, most notably the lungs, which become predisposed to infections with potentially severe consequences. Because of physiologic changes and infection characteristics, unique approaches to antimicrobial agent selection, dosing, and administration are needed. To provide optimal acute and long-term care, pediatric health care providers must be aware of these patient features and common approaches to antimicrobial therapy in CF, which can differ significantly from those of other infectious diseases. The purpose of this article is to review common respiratory pathogens, pharmacology of commonly used antimicrobial agents, and unique pharmacokinetics and dosing strategies often used when treating children with CF.

Spanish consensus on the prevention and treatment of Pseudomonas aeruginosa bronchial infections in cystic fibrosis patients

Pseudomonas aeruginosa is the main pathogen in bronchopulmonary infections in cystic fibrosis (CF) patients. It can only be eradicated at early infection stages while reduction of its bacterial load is the therapeutic goal during chronic infection or exacerbations. Neonatal screening and pharmacokinetic/pharmacodynamic knowledge has modified the management of CF-patients. A culture based microbiological follow-up should be performed in patients with no infection with P.aeruginosa. At initial infection, inhaled colistin (0,5-2MU/tid), tobramycin (300mg/bid) or aztreonam (75mg/tid) with or without oral ciprofloxacin (15-20mg/kg/bid, 2-3weeks) are recommended. In chronic infections, treatment is based on continuous administration of colistin or with a 28-day on-off regimen with tobramycin or aztreonam. During mild-moderate exacerbations oral ciprofloxacin (2-3weeks) can be administered while serious exacerbations must be treated with intravenous combination therapy (beta-lactam with an aminoglycoside or a fluoroquinolone). Future studies will support antibiotic rotation and/or new combination therapies. Epidemiological measures are also recommended to avoid new P.aeruginosa infections and "patient-to-patient transmission" of this pathogen.

Pharmacokinetics and pharmacodynamics of aerosolized antibacterial agents in chronically infected cystic fibrosis patients

Bacteria adapt to growth in lungs of patients with cystic fibrosis (CF) by selection of heterogeneously resistant variants that are not detected by conventional susceptibility testing but are selected for rapidly during antibacterial treatment. Therefore, total bacterial counts and antibiotic susceptibilities are misleading indicators of infection and are not helpful as guides for therapy decisions or efficacy endpoints. High drug concentrations delivered by aerosol may maximize efficacy, as decreased drug susceptibilities of the pathogens are compensated for by high target site concentrations. However, reductions of the bacterial load in sputum and improvements in lung function were within the same ranges following aerosolized and conventional therapies. Furthermore, the use of conventional pharmacokinetic/pharmacodynamic (PK/PD) surrogates correlating pharmacokinetics in serum with clinical cure and presumed or proven eradication of the pathogen as a basis for PK/PD investigations in CF patients is irrelevant, as minimization of systemic exposure is one of the main objectives of aerosolized therapy; in addition, bacterial pathogens cannot be eradicated, and chronic infection cannot be cured. Consequently, conventional PK/PD surrogates are not applicable to CF patients. It is nonetheless obvious that systemic exposure of patients, with all its sequelae, is minimized and that the burden of oral treatment for CF patients suffering from chronic infections is reduced.

Clinical impact of laboratory error on therapeutic drug monitoring of once-daily tobramycin in cystic fibrosis: Case series

Once-daily dosing intravenous tobramycin is commonly used to treat cystic fibrosis pulmonary exacerbations. Clinicians often utilize historical therapeutic drug monitoring data to individualize the dose among patients who have been treated with tobramycin previously. This case series involves three patients with cystic fibrosis who had supra-therapeutic tobramycin levels despite use of a once-daily dosing that produced therapeutic drug levels during a previous hospital admission, raising questions about the validity of these levels. Investigation into several potential sources of error led to the discovery of an analyzer error in the laboratory. Once the laboratory’s tobramycin analyzer was recalibrated, the reported levels were comparable to historical levels. This case series emphasizes the clinical importance of critically analyzing reported levels, and specifically, the importance of utilizing past therapeutic drug monitoring data, if available, for all patients treated with intravenous tobramycin. If a patient was therapeutic on a similar dose of tobramycin during a previous admission, a dose adjustment may not be necessary, and clinicians should consider repeating levels while pursuing alternative explanations for the discrepant serum levels.

Evaluation of serum concentrations achieved with an empiric once-daily tobramycin dosage regimen in children and adults with cystic fibrosis

OBJECTIVES

To assess the ability of an empiric once-daily dosing (ODD) tobramycin regimen to achieve desired serum concentrations in patients with cystic fibrosis (CF); to determine an optimal dosage regimen, using pharmacodynamic parameters; and to evaluate clinical response, adverse effects, and patient/parent satisfaction with ODD.

METHODS

This was a prospective single-center trial in patients with CF who are 5 years of age and older requiring intravenous antibiotics. Tobramycin, 10 mg/kg every 24 hours, was infused over 60 minutes, and two serum concentrations were analyzed using 1-compartment pharmacokinetic modeling. Simulations were performed to identify dosage regimens that maximized desired pharmacodynamic parameters. Other data included demographics, symptoms, spirometry, adverse events, and satisfaction with ODD.

RESULTS

A total of 14 children and 11 adults completed the study. Empiric doses resulted in mean peak tobramycin concentrations of 28.7 ± 5.5 mg/L and undetectable trough concentrations. Only 42% of patients achieved desired peak serum concentrations (20-30 mg/L) with the empiric regimen. A regimen of 12 mg/kg every 24 hours would achieve modified pharmacodynamic goals with an acceptable peak range of 20 to 35 mg/L. Forced expiratory volume in 1 second improved in 15 of 20 (75%) patients with ODD. Two patients experienced reversible vestibular adverse effects attributed to tobramycin. All patients were satisfied or very satisfied with ODD because of convenience and ease of use.

CONCLUSIONS

An empiric tobramycin regimen of 10 mg/kg every 24 hours did not achieve desired serum concentrations for most patients, although all patients demonstrated clinical improvement. Desired tobramycin concentrations with modified pharmacodynamic goals could be achieved by using an empiric dosage of 12 mg/kg every 24 hours. Prospective evaluation of this regimen with individualized patient monitoring is needed to ensure safety and efficacy and to monitor the effects on microbial resistance patterns.

National survey of extended-interval aminoglycoside dosing in pediatric cystic fibrosis pulmonary exacerbations

OBJECTIVES

The Cystic Fibrosis Foundation recently deemed the use of extended-interval dosing (EID) of aminoglycosides acceptable for the treatment of cystic fibrosis (CF) pulmonary exacerbations, but current practices across United States (US) pediatric CF accredited care centers and affiliate programs are unknown. The objectives of this research are to characterize the practice trends, dosing strategies, therapeutic drug monitoring practices, and adverse drug reaction monitoring of EID of aminoglycosides in the treatment of pulmonary exacerbations across US pediatric CF programs.

METHODS

A 38-question online survey was distributed on behalf of the author by the CF Foundation to all US pediatric CF accredited care centers and affiliate programs.

RESULTS

Of the 70 participating CF programs (42.2% survey response rate), 94.3% reported using EID of aminoglycosides (as once-daily or twice-daily dosing), whereas 84.3% reported using once-daily EID in their pediatric CF population. The frequency of EID use increased with patient age. Tobramycin dosed 10 mg/kg per day every 24 hours, infused over the course of 30 minutes, in combination with an antipseudomonal beta-lactam, was the most commonly cited regimen. Monitoring of aminoglycoside serum concentrations was reported by 98.5% of programs, with a tobramycin peak of 25 to 30 mg/L and trough of less than 1 mg/L targeted most frequently. Nephrotoxicity was commonly monitored through serum creatinine measurements, whereas ototoxicity was monitored by audiometry in approximately one-half of programs.

CONCLUSIONS

This study indicates that the use of EID of aminoglycosides across US pediatric CF accredited care centers and affiliate programs is common, particularly among adolescents, with tobramycin being the preferred agent.

Simplified estimation of aminoglycoside pharmacokinetics in underweight and obese adult patients

Aminoglycosides are an important class of agents that are used in combination antimicrobial regimens to treat bacterial pathogens. Dosing of aminoglycosides is typically based on total body weight. However, the most appropriate alternative body size descriptor for dosing aminoglycosides at the extremes of weight (underweight and obese) is not known. Also, the predictive performance of newer formulas to assess kidney function, such as the modification of diet in renal disease (MDRD) and chronic kidney disease-epidemiology (CKD-EPI) equations compared to the Cockcroft-Gault equation to predict aminoglycoside clearance, is not known. We sought to examine dosing of aminoglycosides across the extremes of weight using a variety of formulas to assess kidney function. Pharmacokinetic data were obtained from a set of prospectively collected data (1982 to 2003) of 2,073 (53.5% male) adult patients that included 497 tobramycin- and 1,576 gentamicin-treated cases. The median (minimum, maximum) age, weight, and body mass index were 66 (18, 98) years, 70.0 (29.7, 206.7) kg, and 24.4 (11.3, 73.8) kg/m(2), respectively. The percentage of underweight, normal-weight, overweight, and obese cases based on the World Health Organization classification were 8.8%, 45.5%, 26.5%, and 19.2%, respectively. The aminoglycoside volume of distribution was normalized to several alternative body size descriptors. Only lean body weight estimated by the method of S. Janmahasatian et al. (Clin. Pharmacokinet. 44:1051-1065, 2005) normalized the volume of distribution for both tobramycin and gentamicin across all weight strata, with the estimate being approximately 0.45 liter/kg. Aminoglycoside dosing can be simplified across all weight strata with the use of lean body weight. The CKD-EPI equation best predicts aminoglycoside clearance.