Gentamicin pharmacokinetics and dosing in neonates with hypoxic ischemic encephalopathy receiving hypothermia

STUDY OBJECTIVE

To evaluate the pharmacokinetics of gentamicin in neonates with hypoxic ischemic encephalopathy (HIE) receiving hypothermia and to identify an empiric gentamicin dosing strategy in this population that optimizes achievement of target peak and trough concentrations.

DESIGN

Population pharmacokinetic study using retrospective medical record data.

SETTING

Tertiary neonatal intensive care unit.

PATIENTS

A total of 29 full-term neonates diagnosed with HIE treated with hypothermia who received gentamicin and underwent therapeutic drug monitoring

MEASUREMENT AND MAIN RESULTS

Patient demographics and gentamicin concentration data were retrospectively collected over a 2-year period. A population-based pharmacokinetic model was developed using nonlinear mixed-effects modeling (NONMEM). Using the developed model, Monte Carlo simulations were performed to evaluate the probability of achieving target peak (> 6 mg/L) and trough (< 2 mg/L) gentamicin concentrations for various potential dosing regimens. A one-compartment model best described the available gentamicin concentration data. Birthweight and serum creatinine significantly influenced gentamicin clearance. For the typical study neonate (birthweight 3.3 kg, serum creatinine 0.9 mg/dl), clearance was 0.034 L/hour/kg and volume was 0.52 L/kg. At a 24-hour dosing interval, Monte Carlo simulations predicted target gentamicin peak and trough concentrations could not be reliably achieved at any dose. At a 36-hour dosing interval, a dose of 4-5 mg/kg is predicted to achieve target gentamicin peak and trough concentrations in more than 90% of neonates.

CONCLUSIONS

Gentamicin clearance is decreased in neonates with HIE treated with hypothermia compared with previous reports in nonasphyxiated normothermic full-term neonates. A prolonged 36-hour dosing interval will be needed to achieve target gentamicin trough concentrations in this population. Further prospective evaluation of this dosing recommendation is needed.

Population pharmacokinetics of unbound mycophenolic acid in adult allogeneic haematopoietic cell transplantation: effect of pharmacogenetic factors

AIM

To evaluate pharmacogenetic factors as contributors to the variability of unbound mycophenolic acid (MPA) exposure in adult allogeneic haematopoietic cell transplantation (alloHCT) recipients.

METHODS

A population-based pharmacokinetic (PK) model of unbound MPA was developed using non-linear mixed-effects modelling (nonmem). Previously collected intensive unbound MPA PK data from 132 adult alloHCT recipients after oral and intravenous dosing of the prodrug mycophenolate mofetil (MMF) were used. In addition to clinical covariates, genetic polymorphisms in UGT1A8, UGT1A9, UGT2B7 and MRP2 were evaluated for their impact on unbound MPA PK.

RESULTS

Unbound MPA concentration-time data were well described by a two compartment model with first order absorption and linear elimination. For the typical patient (52 years of age, creatinine clearance 86 ml min(-1)), the median estimated values [coefficient of variation, %, (CV)] of systemic clearance, intercompartmental clearance, central and peripheral volumes of MPA were 1610 l h(-1) (37.4%), 541 l h(-1) (75.6%), 1230 l (37.5%), and 6140 l (120%), respectively. After oral dosing, bioavailability was low (0.56) and highly variable (CV 46%). No genetic polymorphisms tested significantly explained the variability among individuals. Creatinine clearance was a small but significant predictor of unbound MPA CL. No other clinical covariates impacted unbound MPA PK.

CONCLUSIONS

In adult alloHCT recipients, variability in unbound MPA AUC was large and remained largely unexplained even with the inclusion of pharmacogenetic information. Targeting unbound MPA AUC in a patient will require therapeutic drug monitoring.

Drug absorption interactions between oral targeted anticancer agents and PPIs: is pH-dependent solubility the Achilles heel of targeted therapy?

A majority of the novel orally administered, molecularly targeted anticancer therapies are weak bases that exhibit pH-dependent solubility, and suppression of gastric acidity with acid-reducing agents could impair their absorption. In addition, a majority of cancer patients frequently take acid-reducing agents to alleviate symptoms of gastroesophageal reflux disease, thereby raising the potential for a common but underappreciated drug-drug interaction (DDI) that could decrease the exposure of anticancer medication and result in subsequent failure of therapy. This article is a review of the available clinical literature describing the extent of the interaction between 15 orally administered, small-molecule targeted anticancer therapies and acid-reducing agents. The currently available clinical data suggest that the magnitude of this DDI is largest for compounds whose in vitro solubility varies over the pH range 1-4. This range represents the normal physiological gastric acidity (pH ~1) and gastric acidity while on an acid-reducing agent (pH ~4).

Impact of a hospitalwide increase in empiric pediatric vancomycin dosing on initial trough concentrations

STUDY OBJECTIVE

To evaluate the impact of a hospitalwide increase in the recommended vancomycin starting dose from 45 to 60 mg/kg/day on initial vancomycin trough concentrations in children suspected of having an invasive methicillin-resistant Staphylococcus aureus (MRSA) infection.

DESIGN

Retrospective medical record review.

SETTING

Dedicated children's hospital located in a tertiary care, academic medical center.

PATIENTS

A total of 182 children aged 1 month-12 years with normal renal function who had suspected MRSA infections treated with vancomycin during two different starting dose recommendation periods: 45 mg/kg/day divided every 8 hours during July 2006-June 2007 (low-dose group [88 children]) and 60 mg/kg/day divided every 6 hours during July 2008-June 2009 (high-dose group [94 children]).

MEASUREMENT AND MAIN RESULTS

Data on patient demographics, vancomycin doses, and initial vancomycin trough concentrations were collected. No significant demographic differences were noted between patients in the low-dose and high-dose groups. The mean ± SD initial vancomycin trough level increased from 7 ± 5 μg/ml in the low-dose group to 9 ± 5 μg/ml in the high-dose group (p<0.001). The percentage of patients with an initial trough level less than 5 μg/ml declined from 38% (33/88 children) in the low-dose group to 17% (16/94 children) in the high-dose group (p<0.001), whereas the percentage of patients with an initial trough concentration in the potentially adverse range (> 20 μg/ml) did not change between the two groups (2% vs 2%, p=0.9). Less than 14% (13/94 children) achieved a trough level in the range of 15-20 μg/ml in the high-dose group.

CONCLUSION

An increase in the recommended vancomycin starting dose to 60 mg/kg/day decreased the likelihood of an initial low vancomycin trough level (< 5 μg/ml), with no increase in the proportion of patients with trough levels in a potentially toxic range. The 60-mg/kg/day dose did not consistently achieve a vancomycin trough of 15-20 μg/ml, a goal suggested by some experts for adults. Comparative effectiveness studies are needed to directly evaluate vancomycin dosing regimens and clinical outcomes for children with invasive MRSA infections.

Prediction of vancomycin pharmacodynamics in children with invasive methicillin-resistant Staphylococcus aureus infections: a Monte Carlo simulation

BACKGROUND

Due to the emergence of community-associated strains, the prevalence of invasive methicillin-resistant Staphylococcus aureus (MRSA) infections has increased substantially in pediatric patients. A vancomycin AUC(0-24)/MIC index >400 best predicts treatment outcomes for invasive MRSA infection in adults. Data on whether recommended vancomycin doses in children achieve this break point are lacking.

OBJECTIVE

This study aimed to assess the likelihood that currently recommended vancomycin doses in children achieve AUC(0-24)/MIC >400.

METHODS

Vancomycin AUC(0-24)/MIC predictions were conducted across a range of dosages (40-70 mg/kg/d) using a Monte Carlo simulation (n = 5000). AUC(0-24) was calculated as daily dose divided by vancomycin clearance, and daily dose was fixed for a given simulation. Three literature-reported estimates in children were used to define vancomycin clearance and its variance. For the MIC distribution of MRSA isolates, susceptibility data were obtained from the University of California, San Francisco Children's Hospital, San Francisco, California (n = 180; 40% < or =0.5 mg/L; 59% = 1 mg/L; and 1% = 2 mg/L).

RESULTS

Using the recommended empiric dosage of 40 mg/kg/d, 58% to 66% of children were predicted to achieve AUC(0-24)/MIC >400. Increasing the vancomycin dosage to 60 mg/kg/d substantially increased the likelihood (88%-98%) of achieving this pharmacodynamic target. On sensitivity analysis, a dosage of 40 mg/kg/d was more strongly influenced by small changes in MIC compared with 60 mg/kg/d.

CONCLUSIONS

Recommended empiric vancomycin dosing in children (40 mg/kg/d) was not predicted to consistently achieve the pharmacodynamic target of AUC(0-24)/MIC >400 for invasive MRSA infections. A vancomycin dosage of 60 mg/kg/d was predicted to optimize achievement of this target in children.

Delayed distribution of active vasodilation and altered vascular conductance in aged skin

Reflex vasodilation is attenuated in aged skin during hyperthermia. We used laser-Doppler imaging (LDI) to test the hypothesis that the magnitude of conductance and the spatial distribution of vasodilation are altered with aging. LDI of forearm skin was compared in 12 young (19- to 29-yr-old) and 12 older (64- to 75-yr-old) men during supine passive heating. Additionally, iontophoresis of bretylium tosylate was performed in a subset of subjects to explore the involvement of sympathetic vasoconstriction in limiting skin blood flow. Passive heating with water-perfused suits clamped mean skin temperature at 41.0 +/- 0.5 degrees C, causing a ramp increase in esophageal temperature (T(es)) to </=38.5 degrees C. LDI scans were performed at baseline and at every 0.2 degrees C increase in T(es). LDI at bretylium and control sites was identical, suggesting no influence of noradrenergic vasoconstriction. Forearm vascular conductance (venous occlusion plethysmography) was reduced in the older men (P </= 0.001) at every elevated T(es). Mean cutaneous vascular conductance (CVC) of the scanned area was reduced in the older men at 0.2 degrees C </= DeltaT(es) </= 0.8 degrees C. Early in heating (0.2 degrees C </= DeltaT(es) </= 0.6 degrees C), older men also responded with a reduced vasodilated area (P </= 0.05), implying a slower recruitment or filling of skin microvessels. The results indicate that the area of vasodilation and CVC within the vasodilated area are reduced in aged skin during early passive heating, but only CVC is reduced at DeltaT(es) = 0.8 degrees C.