DETERMINING THE PHARMACOKINETICS AND PHARMACODYNAMICS OF ANTIBIOTICS IN ...

211 Pages·2012·4.6 MB·English

DETERMINING THE PHARMACOKINETICS AND PHARMACODYNAMICS OF
ANTIBIOTICS IN PREMATURE INFANTS

Michael Cohen-Wolkowiez, M.D.

A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in
partial fulfillment of the requirements for the degree of Doctor of Philosophy in
Pharmaceutical Sciences in the Eshelman School of Pharmacy (Division of Pharmacotherapy
and Experimental Therapeutics).

Chapel Hill
2012

Approved by:
Angela D. M. Kashuba, PharmD
Daniel K. Benjamin Jr., MD, MPH, PhD
Dhiren Thakker, PhD
Daniele Ouellet, PhD
Ralph Raasch, PharmD
©2012
Michael Cohen-Wolkowiez, M.D.
ALL RIGHTS RESERVED

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ABSTRACT
MICHAEL COHEN-WOLKOWIEZ: Determining the Pharmacokinetics and
Pharmacodynamics of Antibiotics in Premature Infants
(Under the direction of Angela D. M. Kashuba, Daniel K. Benjamin Jr., Dhiren Thakker,
Daniele Ouellet, and Ralph Raasch)

In the United States, approximately 13% of the annual birth cohort is born preterm
(<37 weeks gestational age). These infants are at high risk of systemic infections, and most
are treated with antimicrobial agents. In spite of their common use, antimicrobials lack
pharmacokinetic (PK) data in preterm infants. Extrapolation of dosing information from
older children and adults to preterm infants often results in therapeutic failures or unwanted
toxicities. Therefore, PK studies in this population are needed. Unique challenges posed by
preterm infants, however, limit the ability to evaluate drug disposition in this population.
Novel, minimal-risk methods can provide the platform to overcome these challenges and
optimize antimicrobial dosing. In this proposal, several of these methods were employed to
deliver antimicrobial dosing recommendations specifically designed for preterm infants. Two
commonly used antimicrobials, piperacillin and metronidazole, were used to evaluate the
utility of these methods. A multiplex liquid chromatography-tandem mass spectrometry
assay for the simultaneous quantification of ampicillin, piperacillin, tazobactam, meropenem,
acyclovir, and metronidazole in ultra-low (<50 uL) human plasma was developed and
validated. This method was used to analyze drug concentration data from sparse, scavenged,
and dried blood spot PK samples collected from preterm infants <32 weeks gestational age at
birth. The scavenged concentration data were analyzed by population PK methodologies and
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used to derive PK parameters and identify covariates (serum creatinine and postmenstrual
age) that explain inter-individual variability in piperacillin and metronidazole clearance.
When compared to historical data from small studies using traditional PK sampling,
piperacillin concentrations were 2–10-fold lower in the scavenged sampling approach, which
prevented the determination of optimal dosing recommendations for piperacillin. In contrast,
scavenged sampling proved successful for metronidazole and allowed for the development of
a simplified, postmenstrual age-based dosing regimen. Piperacillin and tazobactam dried
blood spot sampling proved successful as a proof-of-concept application. Dried blood spot
piperacillin and tazobactam concentrations can be used as surrogate for plasma
measurements. Novel, minimal-risk methods are powerful tools to evaluate the
pharmacokinetics/pharmacodynamics of antimicrobials in preterm infants. The success of
these methods is drug-dependent, and they should be systematically studied prior to universal
implementation.

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ACKNOWLEDGMENTS

I wish to acknowledge and thank all of my committee members (Drs. Angela
Kashuba, Danny Benjamin, Daniele Ouellet, Dhiren Thakker, and Ralph Raasch) for
providing me with guidance and support during my Ph.D. degree. I especially want to thank
Dr. Angela Kashuba for welcoming me into her lab with an open mind and Dr. Daniele
Ouellet for her availability to provide technical guidance.
I want to offer my sincerest gratitude to mentor and committee member, Dr. Danny
Benjamin, who has offered full-fledged support of my career development and overall career
path over the five years at Duke University.
I wish to thank the NICHD at the National Institutes of Health for my K23 Career
Development Award.
I would like to thank my family, who define unconditional support.

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TABLE OF CONTENTS

LIST OF TABLES .............................................................................................................. viii
LIST OF FIGURES ................................................................................................................x
LIST OF ABBREVIATIONS .............................................................................................. xii
INTRODUCTION ..................................................................................................................1
Chapter
I. DEVELOPMENT OF A LIQUID CHROMATOGRAPHY-
TANDEM MASS SPECTROMETRY ASSAY OF SIX
ANTIMICROBIALS IN PLASMA FOR PHARMACOKINETIC
STUDIES IN PREMATURE INFANTS ..............................................................9
Introduction ...........................................................................................................9
Methods...............................................................................................................10
Results .................................................................................................................17
Discussion ...........................................................................................................19
II. POPULATION PHARMACOKINETICS OF PIPERACILLIN
USING SCAVENGED SAMPLES FROM PRETERM INFANTS ..................38
Introduction .........................................................................................................38
Methods...............................................................................................................39
Results .................................................................................................................44
Discussion ...........................................................................................................48
III. POPULATION PHARMACOKINETICS OF METRONIDAZOLE
USING SCAVENGED SAMPLES FROM PRETERM INFANTS ..................64
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Introduction .........................................................................................................64
Methods...............................................................................................................66
Results .................................................................................................................71
Discussion ...........................................................................................................74
IV. PIPERACILLIN AND TAZOBACTAM IN DRIED BLOOD
SPOTS: ASSAY DEVELOPMENT AND CLINICAL
APPLICATION IN PRETERM INFANTS ........................................................97
Introduction .........................................................................................................97
Methods...............................................................................................................98
Results ...............................................................................................................104
Discussion .........................................................................................................106
SUMMARY AND FUTURE DIRECTIONS .....................................................................124
APPENDICES ....................................................................................................................130
Appendix 1: Metronidazole Final Model Control File and Output ..................130
Appendix 2: Metronidazole PK Dataset ...........................................................137
Appendix 3: Piperacillin Final Model Control File and Output .......................155
Appendix 4: Piperacillin PK Dataset ................................................................163
REFERENCES FOR SUMMARY .....................................................................................198
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LIST OF TABLES

Table
1.1. Individual stock solutions .............................................................................24
1.2. Optimal precursor and product ions and instrument parameters
by compound .................................................................................................25
1.3. Summary of standard curve, retention times, and extraction
efficiency.......................................................................................................26
1.4. Matrix effects ................................................................................................27
1.5. Summary of accuracy and precision .............................................................29
1.6. Drug concentrations in patient samples ........................................................30
2.1 Clinical data by gestational age group ..........................................................54
2.2 Model-building process ................................................................................55
2.3 Final population pharmacokinetic model parameter estimates .....................56
2.4 Individual empirical Bayesian pharmacokinetic parameter
estimates by gestational age group ...............................................................57

3.1. Dosing schemes used to assess pharmacodynamic target
achievement ..................................................................................................82
3.2. Clinical data by gestational age group ..........................................................83
3.3. Model-building process ................................................................................84
3.4. Final population pharmacokinetic model parameter estimates .....................85
3.5. Individual empirical Bayesian pharmacokinetic parameter
estimates by gestational age group ...............................................................86
4.1. Optimal precursor and product ions and instrument parameters
by compound ...............................................................................................111

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4.2. Summary of standard curve, retention times, and extraction
efficiency.....................................................................................................112
4.3. Summary of accuracy and precision ...........................................................113
4.4. Validation of piperacillin partial dilutions ..................................................114
4.5. DBS and plasma concentrations for piperacillin and
tazobactam ..................................................................................................115

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LIST OF FIGURES

Figure
1.1. Blank plasma sample run in a) negative (tazobactam) and
b) positive (top to bottom: metronidazole, acyclovir, ampicillin,
meropenem, piperacillin) ionization mode ...................................................32
1.2. Lower limit of quantification in a) negative (top to bottom:
tazobactam and dicloxacillin [IS]) and b) positive (top to
bottom: dicloxacillin [IS], metronidazole, acyclovir, ampicillin,
meropenem, piperacillin) ionization mode. Upper limit of
quantification in c) negative (top to bottom: tazobactam and
dicloxacillin [IS]) and d) positive (top to bottom: dicloxacillin [IS],
metronidazole, acyclovir, ampicillin, meropenem, piperacillin)
ionization mode .............................................................................................34
2.1. Base model scatter plots of CL ETA1 estimates and the following:
BGA (A), PNA (B), PMA (C), and SCR (D) ...............................................58
2.2. Final population pharmacokinetic model diagnostic plots:
observed vs. population prediction (A) and individual
prediction (B), weighted residuals vs. population predictions
(C) and time (D) ............................................................................................59
2.3. Visual predictive check of piperacillin dose-normalized
concentrations versus time ............................................................................60
2.4. Weight-normalized piperacillin clearance versus serum creatinine (A)
and body weight (B)......................................................................................61
2.5. Pharmacodynamic target attainment rates by gestational age group ............62
3.1. Base model scatter plots of CL ETA1 estimates and the
following: BGA (A), PNA (B), PMA (C), and SCR (D) ..............................87
3.2. Final population PK model diagnostic plots: observed vs.
population predictions (A) and individual predictions (B);
weighted residuals vs. population predictions (C) and time (D) ..................88
3.3. Visual predictive check of metronidazole concentrations versus time .........89
3.4. Weight-normalized metronidazole clearance versus postmenstrual
age (A) and serum creatinine (B) ..................................................................90
x