General Clinical 
Pharmacology 
Considerations for 
Neonatal Studies for 
Drugs and Biological 
Products 
Guidance for Industry 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
Center for Biologics Evaluation and Research (CBER) 
 
July 2022 
Clinical Pharmacology 
 
    
General Clinical Pharmacology 
Considerations for Neonatal 
Studies for Drugs and 
Biological Products 
 
Guidance for Industry 
 
 
 
Additional copies are available from: 
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Center for Drug Evaluation and Research 
Food and Drug Administration 
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https://www.fda.gov/vaccines-blood-biologics/guidance-compliance-regulatory-information-biologics/biologics-guidances     
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
Center for Biologics Evaluation and Research (CBER) 
 
July 2022 
Clinical Pharmacology 
 
  Contains Nonbinding Recommendations 
 
TABLE OF CONTENTS 
   
 
 
I.  INTRODUCTION............................................................................................................. 1 
II.  BACKGROUND ............................................................................................................... 2 
III.  DEFINITIONS AND SUBGROUP CLASSIFICATIONS ........................................... 3 
IV.  GENERAL CONSIDERATIONS ................................................................................... 5 
A.  PK .................................................................................................................................................... 6 
B.  PD .................................................................................................................................................... 9 
C.  Pharmacogenomics ...................................................................................................................... 10 
D.  Immunogenicity ........................................................................................................................... 10 
V.  NEONATAL STUDY PLANNING ............................................................................... 10 
A.  Approaches to Neonatal Studies ................................................................................................. 10 
B.  Ethics ............................................................................................................................................. 11 
VI.  STUDY DESIGN CONSIDERATIONS ....................................................................... 11 
A.  Study Population .......................................................................................................................... 12 
B.  Dose Selection ............................................................................................................................... 12 
C.  Formulation .................................................................................................................................. 13 
D.  Sample Size ................................................................................................................................... 13 
E.  Sampling ....................................................................................................................................... 14 
F.  Bioanalytical Methods ................................................................................................................. 17 
G.  Data Analysis ................................................................................................................................ 17 
H.  Clinical Study Report .................................................................................................................. 20 
 
 
 
 
   
 
i  Contains Nonbinding Recommendations 
 
General Clinical Pharmacology Considerations for Neonatal Studies 
for Drugs and Biological Products 
Guidance for Industry1 
 
 
This guidance represents the current thinking of the Food and Drug Administration (FDA or Agency) on 
this topic.  It does not establish any rights for any person and is not binding on FDA or the public.  You 
can use an alternative approach if it satisfies the requirements of the applicable statutes and regulations.  
To discuss an alternative approach, contact the FDA office responsible for this guidance as listed on the 
title page.   
 
 
 
 
I.  INTRODUCTION  
 
This guidance is intended to assist sponsors of investigational new drug applications (INDs) and 
applicants of new drug applications (NDAs), biologics license applications (BLAs),2 and 
supplements to such applications who are planning to conduct clinical studies in neonatal 
populations.3,4,5  
  
This guidance provides recommendations for neonatal clinical pharmacology studies, whether 
the studies are conducted pursuant to section 505A of the Federal Food, Drug, and Cosmetic Act 
 
1 This guidance has been prepared by the Neonatal Clinical Pharmacology Working Group in the Office of Clinical 
Pharmacology, Office of Translational Sciences, in the Center for Drug Evaluation and Research (CDER) in 
collaboration with the Division of Pediatric and Maternal Health in the Office of New Drugs, CDER, the Office of 
Pediatric Therapeutics in the Office of the Commissioner, and the Center for Biologics Evaluation and Research at 
the Food and Drug Administration. 
 
2 This guidance is applicable to BLAs submitted under section 351(a) of the Public Health Service Act (PHS Act).  
For the Agencys thinking regarding considerations for BLAs submitted under section 351(k) of the PHS Act, see 
the FDA guidance entitled Clinical Pharmacology Data to Support a Demonstration of Biosimilarity to a Reference 
Product (December 2016).  We update guidances periodically.  For the most recent version of a guidance, check the 
FDA guidance web page at https://www.fda.gov/RegulatoryInformation/Guidances/default.htm.  For additional 
information on biosimilar applications, see the FDA guidance entitled Questions and Answers on Biosimilar 
Development and the BPCI Act (Revision 2) (September 2021). 
 
3 Hereafter in this guidance, the term sponsor refers to sponsors and applicants. 
 
4 For purposes of this guidance, references to drugs includes drugs approved under section 505 of the Federal Food, 
Drug, and Cosmetic Act (FD&C Act or Act) (21 U.S.C. 355) and biological products licensed under 351(a) of the 
Public Health Service Act (PHS Act) (42 U.S.C. 262(a)) that are regulated as drugs.  Hereafter, the term drug will be 
used to refer to all such products. 
 
5 For more information, see the FDA draft guidance entitled General Clinical Pharmacology Considerations for 
Pediatric Studies for Drugs and Biological Products (December 2014), which addresses general clinical 
pharmacology considerations in all pediatric subpopulations, including neonates.  When final, this guidance will 
represent the FDAs current thinking on this topic.   
  1  Contains Nonbinding Recommendations 
 
(FD&C Act),6 section 505B of the FD&C Act,7 or neither.  Effectiveness, safety, or dose-finding 
studies in neonates involve assessing clinical pharmacology information, such as information 
regarding a products pharmacokinetics (PK) and pharmacodynamics (PD) to inform dose 
selection and individualization.  As such, the general considerations described in this guidance 
apply to any neonatal studies which incorporate clinical pharmacology assessments.  This 
guidance does not discuss the timing to initiate neonatal studies.  Questions regarding the 
appropriate timing for the initiation of neonatal studies should be discussed with the relevant 
FDA review division.   
 
The contents of this document do not have the force and effect of law and are not meant to bind 
the public in any way, unless specifically incorporated into a contract.  This document is 
intended only to provide clarity to the public regarding existing requirements under the law.  
FDA guidance documents, including this guidance, should be viewed only as recommendations, 
unless specific regulatory or statutory requirements are cited.  The use of the word should in 
Agency guidance means that something is suggested or recommended, but not required. 
  
 
II.  BACKGROUND 
 
In 2012, the Best Pharmaceuticals for Children Act (BPCA) and the Pediatric Research Equity 
Act (PREA) were made permanent under Title V of the Food and Drug Administration Safety 
and Innovation Act (FDASIA).8  FDASIA requires that if a written request issued under BPCA 
for pediatric studies does not request studies in neonates, the written request must include the 
rationale for not requesting such studies.9 
 
Given that most drugs used in neonatal intensive care units (NICUs) are used off-label, it is 
important that drug information be obtained in neonates to address gaps in neonatal labeling.  In 
addition, therapies should be developed for conditions unique to neonates.  New approaches to 
the study of drugs in neonates should consider the diversity of the patient population and 
underlying conditions that are cared for in NICUs.   
 
During in utero development, there are significant physiological changes in the fetus involving 
the maturation of organs and tissues, including enzyme systems, receptors, transporters, and 
neurotransmitters.  The normal developmental trajectory of these systems is altered by preterm 
delivery.  Postnatal development can also be adversely affected by concurrent illnesses, resulting 
in altered maturation of organs and tissues and affecting the systems responsible for product 
 
 
6 Section 505A of the FD&C Act is often referred to by the acronym of the Act that created it, the Best 
Pharmaceuticals for Children Act (BPCA). 
 
7 Section 505B of the FD&C Act is often referred to by the acronym of the Act that created it, the Pediatric Research 
Equity Act (PREA). 
 
8 Title V Sec 501(a) of FDASIA can be found at https://www.congress.gov/112/plaws/publ144/PLAW-
112publ144.pdf. 
 
9 See section 505A(d)(1)(A) of the FD&C Act (21 U.S.C. 355a(d)(1)(A)).     
  2  Contains Nonbinding Recommendations 
 
absorption (A), distribution (D), metabolism (M), and excretion (E), known collectively as 
ADME.   
 
Gestational age (GA) at birth, postnatal age (PNA), and other factors (e.g., concurrent illness, 
underlying disease) can independently alter the pharmacokinetic (exposure) and 
pharmacodynamic (response) characteristics of a drug, which are essential components of the 
clinical pharmacology assessment.  For example, a neonate born at 24 weeks gestation who is 4 
weeks PNA is physiologically different compared to a 28-week gestation neonate who has just 
been born.  Therefore, the clinical pharmacology assessment should include a range of 
gestational ages, postnatal ages, and body weights, if feasible, unless the drug is intended to treat 
only a specific neonatal subpopulation. 
 
Leveraging prior knowledge and data obtained from adult, preclinical,10 and other pediatric 
studies coupled with innovative quantitative approaches can help predict neonatal doses and 
optimize clinical trial design.  Using quantitative approaches such as population 
pharmacokinetics (population PK) and physiologically based pharmacokinetic (PBPK) modeling 
is critical to inform neonatal drug development.   
 
Detailed planning of neonatal studies should include input from a multidisciplinary team 
involved in neonatal care, including parents and NICU nurses, in the early stages of study design 
to provide their perspectives of study feasibility and the potential impact of study participation 
on neonates and their families.   
 
For studies conducted to fulfill PREA requirements, the submission of the initial pediatric study 
plan (iPSP)11 is intended to encourage sponsors to consider pediatric studies early in product 
development, and when appropriate, begin planning for these studies.  The iPSP should include 
plans for neonatal studies unless a waiver of the PREA requirements for the neonatal population 
is sought.  If the iPSP contains neonatal studies, the plan should include:  (1) an outline of the 
neonatal study or studies that the sponsor plans to conduct (including, to the extent practicable, 
study objectives and design, age groups (including neonatal subpopulations if relevant), relevant 
endpoints, and the statistical approach); (2) any request for a deferral or partial waiver if 
applicable, along with any supporting information; and (3) other information recommended in 
relevant FDA guidance. 
 
 
III.  DEFINITIONS AND SUBGROUP CLASSIFICATIONS     
 
Historically, the neonatal period was defined as 28 days from delivery.  However, the survival of 
preterm infants as premature as 22 to 23 weeks gestation at birth has necessitated a re-evaluation 
 
10 We support the principles of the 3Rs, to reduce, refine, and replace animal use in testing when feasible.  We 
encourage sponsors to consult with us if it they wish to use a non-animal testing method they believe is suitable, 
adequate, validated, and feasible.  We will consider if such an alternative method could be assessed for equivalency 
to an animal test method.   
 
11 See the FDA guidance entitled Pediatric Study Plans:  Content of and Process for Submitting Initial Pediatric 
Study Plans and Amended Initial Pediatric Study Plans (July 2020).   
 
  3  Contains Nonbinding Recommendations 
 
of this historical definition, due to the weeks or months of NICU care required for maturation 
and growth to term equivalency. 
 
In this guidance, as in the FDA guidance entitled E11(R1) Addendum: Clinical Investigation of 
Medicinal Products in the Pediatric Population (April 2018), the neonatal period is defined for 
the term and post-term newborn as the day of birth plus 27 days, and for the preterm newborn, as 
the day of birth and ending at the expected date of delivery plus 27 days.  This definition is 
consistent with consideration of the neonatal age group to include individuals up to 44 completed 
weeks post-menstrual age (PMA).  PMA has been used to date a gestation from the first day of 
the mothers known or reported last menstrual period and can be used either to define the GA at 
birth or the GA at birth plus the PNA.  On the day of birth, PMA is equal to the GA.  
 
Furthermore, the neonatal population can be categorized into subgroups based on a variety of 
factors.  The following are examples of classifications: 
 
Classification based on GA at birth: 
 
  Preterm neonates at the border of viability:  22 to <24 weeks GA12 
  Extremely preterm neonate:  24 to <28 weeks GA13 
  Very preterm neonate:  28 to <32 weeks GA14 
  Moderate-to-late preterm neonate:  32 to <37 weeks GA15 
  Term neonate:  37 to <42 weeks GA16 
  Post-term neonate:  42 weeks GA at birth17 
 
Classification based on weight at birth: 
 
  Preterm neonates at the border of viability:  <600 grams 
  Extremely low birth weight neonates (ELBW):  <1000 grams 
  Very low birth weight neonates (VLBW):  <1500 grams 
  Low birth weight neonates (LBW):  <2500 grams 
 
 
12 Cummings, J and Committee on Fetus and Newborn, 2015, Antenatal Counseling Regarding Resuscitation and 
Intensive Care Before 25 Weeks of Gestation, Pediatrics, 136(3):588-595. 
 
13 Howson CP, M Kinney, and JE Lawn, editors, March of Dimes, The Partnership for Maternal, Newborn and 
Child Health, Save the Children, World Health Organization. Born Too Soon: The Global Action Report on Preterm 
Birth. Geneva: World Health Organization, 2012, 
https://www.who.int/pmnch/media/news/2012/201204_borntoosoon-report.pdf, accessed June 14, 2021. 
 
14 Ibid. 
 
15 Ibid. 
 
16 The American College of Obstetricians and Gynecologists, 2013, ACOG Committee Opinion No 579: Definition 
of Term Pregnancy, Obstet Gynecol, 122(5):1139-1140. 
 
17 Ibid. 
  4  Contains Nonbinding Recommendations 
 
Other classifications:  
 
  Small for gestational age (SGA) neonates:  Birth weight less than the 10th percentile  
  Large for gestational age (LGA) neonates:  Birth weight greater than the 90th percentile 
  Intrauterine growth restriction (IUGR):  Fetal weight (by ultrasound) less than the 10th 
percentile  
 
Depending on the needs of an individual drug development program, the classifications 
described above could be used to define more homogeneous groups of neonates for inclusion in a 
trial or for stratifying neonatal subgroups enrolled in a trial.  Other classifications may be 
appropriate if supported by rationale and references.  It should be noted that growth charts can 
vary depending upon the geographical region and should be accounted for in classification. 
 
When designing studies, it is important to consider stratifying the neonatal population to address 
heterogeneity.  While neonates can be grouped by GA and/or weight at birth, PNA is another 
important variable to consider for stratification, as there are significant ADME changes related to 
PNA.  For example, ADME characteristics can be very different for an extremely preterm infant 
in the first few hours of life compared to the first few days after birth and compared to more than 
a week after birth.  These characteristics also are different for an extremely preterm infant 
compared to a moderate-to-late preterm infant.  All of these factors are important in the context 
of the specific drug being studied, as they could directly affect organ and tissue responsiveness 
and drug disposition.  In addition, disparities at both ends of the growth spectrum (e.g., SGA, 
LGA) can impact developmental physiology and pharmacology.  If stratification is based on 
birth weight, LGA infants might be assumed to be more mature than they actually are based 
solely on weight criteria. 
 
 
IV.  GENERAL CONSIDERATIONS 
 
Before initiating neonatal clinical pharmacology studies, the sponsor should assess the available 
scientific information regarding the mechanism of action of the drug, the PK of the drug, and the 
ontogeny of any organs and tissues that are involved in the predicted response to the drug or its 
metabolites.  This scientific information can be derived from several sources, including 
applicable animal models, in vitro studies, and other potentially relevant clinical studies.  This 
information can be used to develop models and perform simulations to inform neonatal clinical 
pharmacology studies.   
 
Model development requires an in-depth knowledge of the ontogeny of the target organs and 
tissues as well as the ontogeny of the organs and tissues involved in the ADME of the drug and 
its metabolites.  The current gaps in this scientific information in neonates can limit the full 
potential of the application of modeling and simulation in this context.  However, as this 
scientific information becomes available, these data can be incorporated into models to inform 
and update the design of neonatal clinical pharmacology studies. 
 
Neonates can be uniquely susceptible to drugs that cross the blood-brain barrier and drugs that 
alter general physiologic parameters.  Because of developmental and growth considerations, for a 
  5  Contains Nonbinding Recommendations 
 
specific drug it could be recommended to follow neonates for potential safety issues longer than 
for older children and adults.  While long-term endpoints could be recommended to assess the 
safety and efficacy of drugs administered in the neonatal period, it is also important to develop 
short-term endpoints where feasible.  Sponsors are encouraged to consult with FDA on this issue 
early in the development process. 
 
A.  PK 
 
Adequate characterization of the PK of a drug can help to optimize dose selection for neonatal 
studies.  In the neonatal population, inter- and intra-individual variability in measures are 
affected by multiple factors, for example, body size, abnormalities in fetal growth, maturation 
(PMA and PNA), underlying illnesses, and concomitant medications.  Factors that can contribute 
to variability should be documented as part of the trial for later analysis.  To account for this 
variability, it could be important to evaluate the product across a wide spectrum of PMA and 
PNA of neonates if the indication to be studied is relevant in those populations.  Sponsors are 
encouraged to consult with FDA on this issue early in the development process. 
 
1.  Absorption 
 
There are multiple developmental changes in neonates that can affect absorption.  Many of these 
factors have unique ontogenic differences in the neonatal population which should be taken into 
consideration in any neonatal trial (e.g., gastric acidity, rates of gastric emptying and 
gastrointestinal motility, surface area of the absorption site, gastrointestinal metabolizing enzyme 
systems, gastrointestinal permeability, biliary function, transporter expression, mode of 
administration, type of enteral feeding, and cutaneous maturation).   
 
Developmental changes in skin, muscle, and fat, including changes in water content and degree 
of vascularization, can affect absorption patterns of medicinal products delivered by 
intramuscular, subcutaneous, or percutaneous routes.   
 
In general, when designing pharmacokinetic studies in neonates, sponsors should consider that 
the absorption for products administered non-intravenously could be different in the neonatal 
population compared to older children.  
 
2.  Distribution 
 
Distribution of a drug can be affected by changes in body composition, such as changes in total 
body water and adipose tissue, which are not necessarily proportional to changes in total body 
weight.  At birth, neonates have a higher total body water content, which is primarily 
extracellular.  The proportion of total body water, as a percentage of body weight, increases with 
decreasing PMA.  After birth, term neonates generally lose up to 10 to 15 percent of their total 
body water in the first postnatal week followed by a return to birth weight by 10 to 14 days PNA.  
For preterm neonates, the total body water loss could be greater than in term neonates, and the 
recovery of birth weight usually takes longer.  Blood flow to an organ or tissue (e.g., brain, liver) 
can differ between term and preterm infants.   Blood flow in neonates can also differ from blood 
  6  Contains Nonbinding Recommendations 
 
flow in older pediatric and adult populations.  These differences could result in altered tissue 
distribution of the drug. 
 
Plasma protein-binding and tissue-binding changes arising from changes in body composition 
with postnatal growth and development can also influence drug distribution.  The concentrations 
of circulating proteins and the degree of protein binding of a drug can be lower in preterm and 
term infants compared to older children and adults.  In addition, serum protein concentrations 
can remain low for weeks in the critically ill preterm infant.  For drugs that are protein bound, 
preterm infants can have an increased exposure to free, unbound concentrations of the drug 
which can impact its efficacy and safety.   
 
When designing pharmacokinetic studies in neonates, sponsors should consider the following 
when feasible: 
 
  Characterize protein binding, particularly for drugs with high protein binding.  For drugs 
that are highly protein bound, collect serum protein levels in neonates to evaluate the 
potential impact on PK (see section G.1. Application of Quantitative Approaches). 
 
  Given the risk of hyperbilirubinemia in neonates, it is important to assess the 
displacement of bilirubin from the albumin binding site if the drug is likely to bind to 
albumin. 
 
3.  Metabolism 
 
Drug metabolism commonly occurs in the liver, but can also occur in the blood, gastrointestinal 
tract, kidney, lung, and skin.  Information on the metabolism of specific drugs in neonates is 
generally limited.  Each metabolic pathway has unique ontogenic characteristics that should be 
considered when designing clinical pharmacology studies in neonates.  In addition, some 
metabolizing enzymes can have different expression and activity in neonates compared to older 
populations (e.g., CYP3A7 is higher and CYP3A4 is lower in neonates compared to adults).18,19  
 
Before conducting a clinical pharmacology study in neonates, sponsors should consider the 
following:  
 
  To plan neonatal studies, a thorough review of scientific literature and the available data 
from adults or older pediatric subjects should be conducted to obtain information about 
the metabolic pathways relevant to the specific drug. 
  
 
18 Lacroix D, M Sonnier, A Moncion, G Cheron, and T Cresteil, 1997, Expression of CYP3A in the Human Liver - 
Evidence that the Shift Between CYP3A7 and CYP3A4 Occurs Immediately After Birth, Eur J Biochem, 
247(2):625-34. 
 
19 Fanni D, R Ambu, C Gerosa, S Nemolato, M Castagnola, P Van Eyken, G Faa, and V Fanos, 2014, Cytochrome 
P450 Genetic Polymorphism in Neonatal Drug Metabolism:  Role and Practical Consequences Towards a New 
Culture in Neonatology, Int J Immunopathol Pharmacol, 27(1):5-13. 
 
  7  Contains Nonbinding Recommendations 
 
  As the postnatal ontogeny of many of these metabolic pathways has not been fully 
elucidated, it could be recommended to perform additional in vitro or preclinical studies.  
 
  When appropriate, microdosing studies in neonates can be considered to provide 
preliminary pharmacokinetic information and assess for potential ontogenic differences 
in the metabolic pathway compared to older populations.20,21,22 
 
  Modeling and simulation approaches should be considered early to inform dosing and 
account for available information (e.g., ontogeny, PK in older pediatric age groups). 
 
4.  Excretion 
 
Drug excretion by the kidneys is the net result of glomerular filtration, tubular secretion, and 
tubular reabsorption.  The glomerular filtration rate (GFR) is low in neonates, particularly in 
those born before 32 weeks PMA and increases rapidly after birth.23,24,25   For drugs that are 
primarily renally excreted, systemic exposure of a drug is related to both PMA and PNA.26  
Pulmonary and gastrointestinal/biliary routes of excretion can also be important for certain drugs 
and could be affected by the ontogeny of those organ systems.   
 
Before conducting a clinical pharmacology study in neonates, sponsors should consider the 
following: 
 
 
20 Roth-Cline M and RM Nelson, 2015, Microdosing Studies in Children:  A US Regulatory Perspective, Clin 
Pharmacol Ther, 98(3):232-3.  
21 Mooij MG, E van Duijn, CAJ Knibbe, K Allegaert, AD Windhorst, J van Rosmalen, NH Hendrikse, D Tibboel, 
WHJ Vaes, and SN de Wildt, 2017, Successful Use of [14C]Paracetamol Microdosing to Elucidate Developmental 
Changes in Drug Metabolism, Clin Pharmacokinet, 56(10):1185-1195.  
 
22 Ethical considerations for conducting a microdosing study are similar to those for a single-dose pharmacokinetic 
study.  See the FDA draft guidance General Clinical Pharmacology Considerations for Pediatric Studies for Drugs 
and Biological Products (December 2014) for more information.  When final, this guidance will represent the 
Agencys current thinking on this topic. 
 
23 Anderson BJ and NHG Holford, 2008, Mechanism-Based Concepts of Size and Maturity in Pharmacokinetics, 
Annu Rev Pharmacol Toxicol, 48:303-32. 
 
24 Rhodin, MM, BJ Anderson, AM Peters, MG Coulthard, B Wilkins, M Cole, E Chatelut, A Grubb, GJ Veal, MJ 
Keir, and NHG Holford, 2009, Human Renal Function Maturation:  A Quantitative Description Using Weight and 
Postmenstrual Age, Pediatr Nephrol, 24:67-76. 
 
25 Mahmood I and MA Tegenge, 2019, A Comparative Study Between Allometric Scaling and Physiologically 
Based Pharmacokinetic Modeling for the Prediction of Drug Clearance From Neonates to Adolescents, Pediatr 
Pharmacol, 59(2):189-197. 
 
26 Wang J, SS Kumar, CM Sherwin, R Ward, G Baer, GJ Burckart, Y Wang, and LP Yao, 2019, Renal Clearance in 
Newborns and Infants: Predictive Performance of Population-Based Modeling for Drug Development, Clin 
Pharmacol Ther, 105(6):1462-1470. 
  8  Contains Nonbinding Recommendations 
 
  The ontogeny of transport systems, particularly those involved in active transport, have 
not been well elucidated in neonates.  When relevant to the specific drug, ontogenic 
differences in a transport system could have an impact on the neonatal PK of the drug.  
 
  For drugs that are known substrates of transporters, information gained from the conduct 
of pharmacokinetic studies in neonates can help elucidate the ontogenic trajectory of the 
transporter of interest.  
 
5.  Clearance 
 
Plasma clearance is defined as the volume of plasma which is completely cleared of a drug in a 
given time period.  Clearance as a function of age (PMA and PNA) in addition to weight are 
generally valuable parameters for determining the dose for each neonatal subgroup and can 
change rapidly based on the PNA.   
 
  Clearance from target organs and tissues (e.g., brain) can also differ between neonates 
and older children and adults; therefore, compartment sampling (e.g., cerebrospinal fluid) 
to characterize the tissue exposure, when feasible, can be useful to determine the optimal 
dosing. 
 
  As the clearance of a drug can be substantially different in various neonatal subgroups 
based on both PMA and PNA, it may be recommended to assess the clearance of a drug 
in each subgroup being studied. 
 
6.  Additional Factors 
 
As increasing scientific data are garnered related to the prenatal and postnatal ontogeny of organs 
and tissues for ADME parameters in each of the neonatal subgroups, this information could be 
used to generate quantitative models such as PBPK to help design subsequent dosing strategies 
in those subgroups.27 
 
B.  PD 
 
Sponsors should collect and analyze both pharmacokinetic and, whenever possible, 
pharmacodynamic data in neonatal studies to determine how the two are linked with respect to 
exposure-response (E-R).  PD could include the effect of the drug on biomarkers or clinical 
endpoints for both safety and efficacy.  These measurements can help to determine if the E-R 
relationship of the drug in neonates is similar to that observed in older children and adults.  If the 
relevant clinical endpoints cannot be measured directly, then an appropriate biomarker, if 
available, can be considered.  As drugs given to neonates can affect multiple organ systems, it 
could be recommended to evaluate several biomarkers.  In neonates, the ontogeny of the tissues 
and organs that are targeted by the drug can be critically important in predicting the potential 
 
27 See the FDA guidance entitled Physiologically Based Pharmacokinetic Analyses  Format and Content (August 
2018). 
  9  Contains Nonbinding Recommendations 
 
degree of response, thus altering the E-R relationships.  These data are integral to any 
consideration of extrapolating efficacy data from studies in adults or other pediatric age groups. 
 
Before conducting a clinical pharmacology study in neonates, all prior information should be 
considered.  E-R relationships of the drug in adults and other pediatric age groups can help 
inform the neonatal studies.  A poor or incomplete understanding of the natural history and 
pathophysiologic mechanisms of many neonatal conditions hinders the identification of clinically 
relevant pharmacodynamic biomarkers.  Therefore, sponsors should initiate discussions early 
with the FDA when considering the use of novel biomarkers of response in neonatal studies. 
 
C.  Pharmacogenomics 
 
Genetic differences that affect both the exposure of and response to a drug are increasingly 
documented, but the relationship between genomic profiles and developmentally regulated gene 
expression has not been extensively studied in the neonatal population.  Therefore, consider the 
following, if feasible: 
 
  If there are pharmacogenetic differences that affect the PK, efficacy, or safety of a drug 
in older children and adults, pharmacogenetic analysis is recommended in neonates.   
 
D.  Immunogenicity 
 
Immunogenicity to certain products (e.g., therapeutic proteins, peptides, oligonucleotides) can 
negatively impact the PK, PD, safety, and/or efficacy of the drug.  The immune response in the 
neonatal period can differ from that of adults or older children; furthermore, how these 
differences affect the immunogenicity risk is not fully understood.  Therefore, the 
immunogenicity of a drug should be assessed in neonatal trials regardless of the knowledge 
gained in adults and older children. 
 
 
V.  NEONATAL STUDY PLANNING 
 
The following sections describe considerations for specific trial design elements when 
developing a neonatal study plan.  Study planning should consider the ethics and the studies or 
approaches that would provide substantial evidence of effectiveness.  
 
A.  Approaches to Neonatal Studies 
 
There are several approaches to providing substantial evidence of effectiveness for drugs for the 
pediatric population.  In some cases, existing data in adults and other pediatric populations can be 
leveraged to provide this substantial evidence of effectiveness.  This concept is often referred to 
as pediatric extrapolation.28   
 
28 For more information, see the FDA draft guidances entitled Demonstrating Substantial Evidence of Effectiveness 
for Human Drug and Biological Products (December 2019) and General Clinical Pharmacology Considerations for 
 
  10  Contains Nonbinding Recommendations 
 
 
Considering the distinct disease processes seen in the neonatal population, it is expected that 
pediatric extrapolation of effectiveness from other populations (e.g., adults, older children) 
would be infrequently possible.  Most drugs developed for an indication specific to neonates will 
require at least one adequate and well-controlled study to establish substantial evidence of 
effectiveness.  Regardless of the approach used to provide evidence of effectiveness, safety data 
should be obtained for all drugs studied in neonates.  The magnitude of the safety database that 
should be obtained is determined by several factors, including for example, experience with 
similar drugs in populations of older children, adults, and neonates, the seriousness of the 
adverse reactions in the adult or pediatric populations, the rarity of the condition, and the 
potential for unique susceptibility to particular adverse events. 
 
B.  Ethics 
 
It is recommended that an IRB reviewing neonatal research have specific expertise in neonatal 
trials.29  Furthermore, it may be appropriate to utilize an independent Data and Safety 
Monitoring Board (DSMB) to oversee the neonatal trials; in such cases, the DSMB should also 
have expertise with neonatal trials.  
 
 
VI.  STUDY DESIGN CONSIDERATIONS 
 
Intensive blood sampling for pharmacokinetic studies can rarely be undertaken in neonates 
because of their limited circulating blood volume.  Furthermore, variability in the study 
population (e.g., a population undergoing rapid and varying rates of maturation) makes 
collection of clinical pharmacology information (e.g., PK, PD) uniquely challenging.  Hence, it 
is important to use all available information and innovative approaches when designing a 
neonatal pharmacokinetic study.  Modeling and simulation and pharmacologic considerations are 
often critical for the successful completion of a study.  Some approaches that can inform the 
design and dose selection of neonatal studies include population PK, PBPK modeling, and/or 
pharmacokinetic/pharmacodynamic modeling approaches.30  However, relevant ontogenic data 
with respect to ADME should be available before robust and accurate models can be developed 
for use in neonatal clinical studies. 
 
The following sections describe considerations for specific trial design elements. 
 
Pediatric Studies for Drugs and Biological Products (December 2014).  When final, these guidances will represent 
the Agencys current thinking on this topic.   
29 Ethical considerations for pediatric studies and the role of the Institutional Review Board (IRB) in clinical trials 
involving pediatric participants are discussed in the FDA draft guidance General Clinical Pharmacology 
Considerations for Pediatric Studies for Drugs and Biological Products (December 2014).  When final, this 
guidance will represent the Agencys current thinking on this topic.  See also 21 CFR part 50, subpart D.   
 
30 See the following FDA guidances:  (1) Physiologically Based Pharmacokinetic Analyses  Format and Content 
(August 2018); (2) Exposure-Response Relationships  Study Design, Data Analysis, and Regulatory Applications 
(April 2003); and (3) Population Pharmacokinetics (February 2022).   
 
  11  Contains Nonbinding Recommendations 
 
 
A.  Study Population 
 
When conducting clinical pharmacology studies in neonates, the population enrolled should 
involve neonates that have the condition of interest or, in some cases, neonates who might be at 
risk for the condition of interest.   
 
To account for variability in age and development, the sponsor should include as wide a 
spectrum of PMA and PNA in neonates as appropriate to cover neonatal subgroups for which the 
indication to be studied is relevant (see section III. Definition and Subgroup Classifications).  
When including a wide spectrum of neonates, the sponsor should plan for subgroup analyses (see 
section VI.G. Data Analysis).31   
 
B.  Dose Selection 
 
Selection of an appropriate dose range to be studied is critical in deriving rational dosing 
recommendations for the neonatal population.  Sponsors should use all existing pharmacokinetic 
and pharmacodynamic data (from adults, older pediatric populations, etc.) to help determine an 
initial dose in neonates.  Clinical trial simulations that integrate PK, PD, biomarkers, and disease 
progression can help make this initial determination.  Dose selection in neonates should also 
consider the PMA and PNA.32 
 
The rapid changes in growth and development occurring in neonates can require dosing 
adjustments over short periods of time (e.g., in certain instances, the initial dosing of anti-
infective agents changes after 24 hours).  Depending on the range of PNAs and PMAs being 
studied and the intended study duration, dosing regimens could become even more complex.  
Often, significant uncertainty about the dose in neonates necessitates alternative approaches that 
could involve titration of the dose, adaptive trial designs, or the use of therapeutic drug 
monitoring (TDM) during the trial.  TDM can be particularly useful when there is known drug 
toxicity, or higher exposures are expected in neonates.   
 
Given the unique ADME characteristics in neonates, different dosing regimens should be studied 
to optimize the exposure in various neonatal subpopulations.  Neonates could require higher drug 
exposures than those in older children and adults to achieve adequate clinical benefit; in such 
cases, additional safety data should be collected to support the use of higher doses in neonates. 
 
Given the uniqueness of some neonatal conditions, it is possible that in certain circumstances 
first-in-human studies should be conducted in the neonatal population (e.g., the drug is 
developed specifically for the neonatal condition, and nonclinical data preclude the possibility of 
studying the drug in healthy adult volunteers).  In a first-in-human scenario (e.g., the target 
 
31 Sponsors should be aware that individual jurisdictions may have differing privacy regulations regarding the 
collection of some of the information to assess PMA and PNA (e.g., precise birth date). 
 
32 For additional information regarding dose selection, see the FDA draft guidance entitled General Clinical 
Pharmacology Considerations for Pediatric Studies for Drugs and Biological Products (December 2014).  When 
final, this guidance will represent the FDAs current thinking on this topic. 
  12  Contains Nonbinding Recommendations 
 
population is the neonatal population only), where sufficient data from adults or older children 
are lacking, sponsors should initiate discussions early with the FDA to determine potential 
approaches to dose selection. 
 
C.  Formulation 
 
Proposed trials in the neonatal population should use an age-appropriate formulation.  Neonates 
could present unique challenges associated with formulations and dosing.  All aspects of the 
formulation, including the salt forms of the active ingredient, the excipients, and the volume of 
the unit dose, should be considered.  Consideration should be given to the total volume for 
parenteral formulations to be administered, as the total intake volume may be limited and should 
also take into account the volume already received through parenteral nutrition and other 
standard-of-care drugs.  Formulations should be developed to permit accurate dosing, especially 
given the potentially small unit doses.  Studies of drugs in neonates should account for and 
capture information on potential interactions with tubing used for both parenteral and enteral 
administration and any potential interactions with co-administered fluids (including parenteral 
nutrition), enteral nutrition, and other therapeutic products.   
 
The route of administration is important in neonates, given that many neonates are critically ill 
and unable to receive enteral products.  While most products are developed for parenteral 
administration, other routes (e.g., enteral, inhalational, intraocular, transcutaneous, 
intramuscular, subcutaneous, rectal) can be considered when appropriate, depending on the 
condition and the clinical status of the neonate.  The bioavailability of any non-parenteral 
formulation used in neonatal studies should be characterized in relation to the formulation used 
in older children and adults.  Typically, bioavailability studies of age-appropriate formulations 
are conducted in adults; however, the potential for developmental differences in absorption 
between neonates and adults should be considered for dosing. 
 
Considerations for excipients are particularly important in the neonatal population.  The 
accumulation of excipients can be significantly higher in neonates because of immature organ 
function.  In general, the sponsor should minimize the use of excipients in neonatal formulations 
whenever possible.  Excipients with known toxicity in neonates should not be used (e.g., ethanol, 
propylene glycol, benzyl alcohol). 
 
D.  Sample Size 
 
Sponsors should consider the number of neonates in various subpopulations to establish accurate 
dosing.  Justification should be provided for the sample size selected.  The precision of 
pharmacokinetic and pharmacodynamic parameters in the sample size calculation is critical for 
neonatal studies.33  For example, one approach is to prospectively target a 95 percent confidence 
interval within 60 percent and 140 percent of the geometric mean estimates of clearance and 
 
33 For more information, see the FDA draft guidance entitled General Clinical Pharmacology Considerations for 
Pediatric Studies for Drugs and Biological Products (December 2014).  When final, this guidance will represent the 
Agencys current thinking on this topic. 
 
  13  Contains Nonbinding Recommendations 
 
volume of distribution for the drug in each pediatric stratum with at least 80 percent power.34  
Prior knowledge of the disease, drug exposure, and pharmacodynamic response from older 
children and adult data can be used to estimate the sample size for neonatal studies.  The sponsor 
should account for sources of variability, including inter- and intra-subject variability, 
differences between neonatal subgroups, and differences between neonates and older children 
and adults in the final selection of the sample size for each neonatal subgroup.   
 
Given the challenges associated with conducting studies in neonates, alternative and innovative 
approaches to traditional sample size requirements may be appropriate if they improve the 
interpretability of trial results.  Clinical trial simulations that integrate pharmacokinetic and 
pharmacodynamic aspects can aid in the design the clinical pharmacology trials with feasible 
sample sizes.  Practical considerations should be taken into account when determining the 
sample size if it is not possible to recruit adequate numbers of participants to achieve the desired 
precision of parameter estimates.  The study sample sizes for neonatal subgroups should be 
discussed with the Agency prior to conducting the clinical pharmacology study.   
 
E.  Sampling 
 
More often than not, blood samples are the primary samples collected in neonatal studies.  Other 
types of samples, such as cerebrospinal fluid, urine, or saliva can be informative but are not as 
readily collected for the characterization of PK and PD. 
 
1.  Considerations for Blood Sample Volume Limits 
 
Blood sample volumes for research studies should be limited to the least possible volume 
required for testing to minimize risk to the neonate.  The sponsor should account for blood drawn 
for the study in addition to blood drawn for routine clinical assessments.  If possible, blood 
samples for research studies should be timed with clinically indicated blood draws to minimize 
the blood volume and decrease the number of needle sticks or draws from an indwelling 
catheter.35,36  In some situations, it may be possible to improve the feasibility of pharmacokinetic 
studies by using blood from scavenged samples (i.e., samples obtained from surplus blood drawn 
during clinical care). 
 
Greater consideration should be given for infants where illnesses specifically impact the ability 
to replace hemoglobin.  It is important to know how slowly red cells will be replenished in the 
hospitalized neonate (which reflects GA, PNA, and the severity and type of illness) when 
determining the number of samples and sample volumes for the purposes of the study.  In 
 
34 For more information, see the FDA draft guidance entitled General Clinical Pharmacology Considerations for 
Pediatric Studies for Drugs and Biological Products (December 2014).  When final, this guidance will represent the 
Agencys current thinking on this topic. 
 
35 Howie SRC, 2011, Blood Sample Volumes in Child Health Research: Review of Safe Limits, Bulletin of the 
World Health Organization, 89:45-53. 
 
36 Veal GJ, 2014, Blood Volumes in Pediatric Clinical Trials:  A Review of Current Regulations and Guidance for 
Research Studies, Clin. Invest, 4:1005-1011. 
 
  14  Contains Nonbinding Recommendations 
 
general, neonatal blood volumes are approximately 85 mL/kg, increasing to 105 mL/kg by the 
end of the first month.37,38,39  Several studies have looked at the association between blood draws 
and the potential negative impact on neonatal blood volume and hemoglobin levels, including the 
need for transfusion in some instances. 40,41,42   
 
Several academic centers and IRBs have published guidelines for total blood volume (TBV) 
limits for neonatal studies (including blood draws for both research and clinical care purposes).43  
In general, these guidelines describe acceptable ranges between 1 to 5 percent of the TBV for a 
single draw or over a 24-hour period and 3 to 10 percent of the TBV over a month.  The sponsor 
should consider the amount of blood drawn for clinical purposes and the clinical status of the 
neonate.  In addition, a minimum hemoglobin should be set before a research blood draw.  
Literature data on minimum hemoglobin values in neonates are limited.  However, some 
institutions have minimum values, for example 7.0 g/dL for a stable neonate and 9.0 to 10 g/dL 
for a neonate with respiratory or cardiovascular compromise.44  These values would be 
dependent on the PMA and PNA of the neonates in the study. 
 
When planning a neonatal pharmacokinetic and/or pharmacodynamic study, sponsors should 
justify their proposed sampling scheme and the number of samples to be collected per neonate, 
considering PMA and PNA. 
 
2.  Sampling Schemes 
 
Given the blood volume considerations for neonates, sparse sampling is a practical approach for 
obtaining pharmacokinetic data in neonatal studies.  To effectively inform sparse sampling in 
neonates, it is essential to leverage what is known about the ontogeny of relevant organ and 
 
37 Guidelines for Blood Volumes in Clinical Trials (Especially in Pediatric Clinical Trials): 
http://onbiostatistics.blogspot.com/2011/02/guidelines-for-blood-volumes-in.html.  
 
38 Howie, S, 2011, Blood Sample Volumes in Child Health Research: Review of Safe Limits, Bull World Health 
Organ, 89(1):46-53. 
 
39 Pearson H, 2003, Blood and Blood Forming Tissues, Rudolphs Pediatrics, 21st ed, New York:McGraw-Hill, 
1521. 
 
40 Madsen LP, MK Rasmussen, LL Bjerregaard, SB Nhr, and F Ebbesen, 2000, Impact of Blood Sampling in Very 
Preterm Infants, Scand J Clin Lab Invest, 60:125-32. 
 
41 Hack KE, CM Khodabux, JS von Lindern, HA Brouwers, SA Scherjon, HJ van Rijn, JA van Hilten, A Brand, and 
GC Page-Christiaens, 2008, Need for Blood Transfusion in Premature Infants in 2 Dutch Perinatology Centres 
Particularly Determined by Blood Sampling for Diagnosis, Ned Tijdschr Geneeskd, 152:1419-25. 
 
42 Testa M, F Birocchi, P Carta, and V Fanos, 2006, Causes of Anaemia in Very Low Birth Weight Infants:  
Phlebotomy Losses are not the First Accused, Minerva Pediatr, 58:263-7. 
 
43 Howie, S, 2011, Blood Sample Volumes in Child Health Research: Review of Safe Limits, Bull World Health 
Organ, 89(1):46-53. 
 
44 Howie, S, 2011, Blood Sample Volumes in Child Health Research: Review of Safe Limits, Bull World Health 
Organ, 89(1):46-53. 
  15  Contains Nonbinding Recommendations 
 
enzyme systems as well as pharmacokinetic information that could be available in adults or older 
children.  The sampling scheme should allow for the characterization of the clinically relevant 
exposure metrics that inform dosing.   
 
Clinical practicality and the least burdensome approaches for neonates should also be taken into 
account when determining a feasible sampling scheme.  In some situations, it may be possible to 
increase PK study feasibility by using opportunistic sampling (i.e., sampling around the time of 
clinically indicated blood draws) and scavenged samples for pharmacokinetic sampling and 
characterization. 
 
When using opportunistic or scavenged samples, storage conditions should ensure sample 
stability, given that these samples are not generally collected with the primary intention of 
characterizing PK, and the approach to their collection and handling can differ from traditional 
pharmacokinetic samples.  Careful, prospective planning is required when using an opportunistic 
approach and scavenged sampling, as there is less control of the sampling time with respect to 
the dosing time of the drug of interest and other concomitant medications.  Lack of planning 
could increase the possibility that pharmacokinetic samples over critical periods of the dosing 
interval will not be collected, potentially making the information obtained unreliable.  
 
The protocol should specify a standardized collection scheme, storage and handling conditions, 
accurate recording of the sampling times, the dose, and the dosing time of the drug of interest as 
well as any concomitant medications.  The sponsor should assess the correlation between 
scavenged sample concentrations and prospectively collected pharmacokinetic sample 
concentrations to understand the extent to which drug concentration measurements are 
affected.45  The acceptability of this approach depends on the quality and quantity of samples, 
the number of subjects, the total number of samples, and the variability of the data.  When 
planning to employ such approaches, sponsors should consult with the FDA. 
 
3.  Sample Collection Methods 
 
Sampling technique is critical including when using an available neonatal indwelling intravenous 
or intra-arterial catheter.  Any sampling plan should also take into consideration the use of 
umbilical catheters and small caliber vascular access devices.  If possible, pharmacokinetic 
samples should be obtained from a separate site other than that used for the administration of the 
drug product.  Blood samples are generally collected and analyzed for drug concentrations via 
direct puncture to a vein from venous blood.  While it is ideal to collect blood samples for 
analysis from the circulating blood volume, capillary or arterial sampling such as heel sticks can 
be used in the neonatal population if acceptable from a bioanalytical perspective.   
 
When possible, opportunistic sampling or scavenging of biological fluids that are already being 
collected as part of routine clinical care such as cerebrospinal fluid or bronchial fluid, can 
provide additional pharmacokinetic information.  For example, cerebrospinal fluid collected for 
clinical purposes can add to the understanding of the PK of the drug.  However, proper collection 
 
45 Autmizguine J, DK Benjamin Jr, PB Smith, M Sampson, P Ovetchkine, M Cohen-Wolkowiez, and KM Watt, 
2014, Pharmacokinetic Studies in Infants Using Minimal-Risk Study Designs, Curr Clin Pharmacol, 9(4):350-358. 
 
  16  Contains Nonbinding Recommendations 
 
and storage of the sample as well as recording the time the sample was collected relative to the 
administration of the drug are critical to obtaining interpretable data.  Urine and saliva collection 
could be considered, but the interpretation of data from such samples is also complicated and 
requires careful consideration before collecting.  Non-invasive sampling using fluids can be 
useful if correlated with outcomes or blood or plasma drug levels.  The volume of these samples 
in neonates can be small, and validation of the analysis in these small volumes should be 
provided. 
 
From a feasibility perspective, dried matrix spots represent a potential methodology for acquiring 
biological samples.46  Dried matrix samples consist of a collection of biological fluid on blotting 
paper and typically require low volumes.  There are several dried matrix spot methods which can 
include dried blood spots (DBS), dried urine spots (DUS), and dried plasma spots (DPS).  The 
most common dried matrix spot used in the neonatal population is DBS.  Its minimally invasive 
sampling technique, the low blood volume required, and the ease of sample storage and handling 
are potential advantages of DBS.  When using such an approach, bioanalytical validation should 
be conducted (see section VI.F. Bioanalytical Methods).  If considering using such an approach, 
sponsors should initiate discussions with the FDA.  
 
F.  Bioanalytical Methods 
 
An accurate, precise, sensitive, specific, and reproducible analytical method to quantify the drug 
and metabolites in the biologic fluids of interest is essential.  Given the small volumes of study 
samples from neonates, sponsors should consider special assay techniques (e.g., ultra-low blood 
volume drug assays).  These techniques should be validated so that these methods can be used 
with confidence in neonatal studies.47  
 
Standardized sample collection, and validated methods can reduce the number of limitations 
associated with the biomatrix.  Before use in a neonatal pharmacokinetic assessment, the sponsor 
should account for any bias in concentration measurements of the microsampling methodology 
compared to those from a traditional sampling approach (e.g., plasma concentrations) that might 
have been used during the drug development in adults.  
 
Sponsors are advised to obtain feedback from the relevant FDA review division early in the 
neonatal drug development process to determine the appropriate bioanalytical methods for the 
drug. 
 
G.  Data Analysis 
 
There are two basic approaches for performing pharmacokinetic analyses in pediatric 
participants:  (1) a standard non-compartmental pharmacokinetic approach; and (2) a population 
 
46 See the FDA guidance entitled Bioanalytical Method Validation (May 2018). 
 
47 See the FDA guidance entitled Bioanalytical Method Validation (May 2018). 
 
  17  Contains Nonbinding Recommendations 
 
PK approach.48  The population PK approach is more feasible in the neonatal population as it 
minimizes the total number of samples and hence the total volume of blood sampled per 
individual.  Population PK approaches leverage prior information obtained from studies in adults 
and older children in conjunction with data collected from neonatal studies to provide estimates 
of the drugs pharmacokinetic parameters and their associated variability.  However, any models 
that are developed for use in neonates should take into consideration all the ADME factors for 
each PMA and PNA subgroup and be supported by scientific data.49,50 
 
1.  Application of Quantitative Approaches 
 
The application of modeling and simulation (M&S) as a tool for dose selection in neonates 
should take into account the considerable variability in neonatal subgroups driven by differences 
in growth and maturation.  In the absence of conducting a clinical trial in a large, diverse cohort 
of neonates, M&S can provide insights into dosing if such models are well informed, formulated, 
and executed.  First, internal and external evaluations of the model should be performed to 
ensure that estimates of the drugs pharmacokinetic parameters are adequate and precise.  Then, 
the model can be used to simulate dosing scenarios in the population for which the model was 
developed.  Any trial design as a product of M&S should be flexible enough to mitigate the 
uncertainties inherent in the model outcomes.  For example, in the commonly used sequential or 
staged study design, progressively younger cohorts are studied sequentially so that the trajectory 
of the dose-exposure or E-R relationships can be assessed.  This conservative approach is widely 
used but can also significantly delay drug development in neonates, who are the youngest and 
most vulnerable age group.  Alternative study designs (e.g., adaptive study designs) coupled with 
M&S can help streamline neonatal drug development. 
 
2.  Population PK 
 
The population PK approach described in the FDA guidance entitled Population 
Pharmacokinetics (February 2022) has been the most commonly used approach in neonatal drug 
development studies.  Population PK allows for the analyses of sparse (limited number of blood 
samples per individual) and unbalanced data (unequal distribution of blood samples in various 
 
48 For more information, see the FDA draft guidance for industry entitled General Clinical Pharmacology 
Considerations for Pediatric Studies for Drugs and Biological Products (December 2014).  When final, this 
guidance will represent the Agencys current thinking on this topic. 
 
49 Kern, SE, 2009, Challenges in Conducting Clinical Trials in Children:  Approaches for Improving Performance, 
Expert Rev Clin Pharmacol, 2(6):609-17. 
 
50 Rodieux, F, M Wilbaux, JN van den Anker, and M Pfister, 2015, Effect of Kidney Function on Drug Kinetics and 
Dosing in Neonates, Infants, and Children, Clin Pharmacokinet, 4(12):1183-204. 
 
  18  Contains Nonbinding Recommendations 
 
parts of the concentration-time profile in the individuals).  These factors are particularly 
important as both scenarios are typically present in neonatal studies.51,52 
 
3.  PBPK Modeling 
 
Another quantitative approach is PBPK modeling, a mechanistic modeling approach that 
incorporates the understanding of physiology and compound-specific information to predict the 
PK and/or PD of a drug.53,54  While PBPK prediction incorporates a more mechanistic 
understanding, its application in neonates is particularly challenging due to the limited 
understanding of rapid changes in neonatal physiology and the maturation of ADME processes 
in this population.   
 
4.  Covariates and Phenotype Data 
 
In general, the following covariates for each neonate should be considered as part of data 
analysis:  GA, birth weight, birth length, birth head circumference, PMA, PNA, current weight, 
body surface area (BSA), race or ethnicity, sex, diagnoses, concomitant and recent medications 
or intravenous fluids (including blood transfusions), type and amount of enteral feedings, and 
relevant laboratory tests that reflect the function of the organs responsible for drug metabolism 
and drug excretion.  The sponsor should examine the relationships between the covariates and 
the PK of the drug of interest to assess the potential contribution of the covariates to the 
variability of pharmacokinetic parameters.  Having enough subjects with or without the 
covariates of interest is important to determine the impact of these factors on the drugs PK.  
Also, the impact of pharmacogenetic factors could be critical to data analysis in some instances; 
therefore, sponsors are encouraged to collect DNA samples in neonatal pharmacokinetic studies 
when feasible.  
 
A quantitative model can incorporate covariates to reflect the importance of individual 
characteristics (e.g., body size, PNA or PMA) or extrinsic factors (e.g., presence of concomitant 
medication) on pharmacokinetic parameters, resulting in more precise estimates of the PK of the 
drug on the next cohort.55  
 
 
51 Wang J, AN Edginton, D Avant, and GJ Burckart, 2015, Predicting Neonatal Pharmacokinetics from Prior Data 
Using Population Pharmacokinetic Modeling, J Clin Pharmacol, 55(10):1175-83. 
 
52 Ku, LC and PB Smith, 2015, Dosing in Neonates:  Special Considerations in Physiology and Trial Design, Pediatr 
Res, 77(1-1):2-9. 
 
53 See the FDA guidance entitled Physiologically Based Pharmacokinetic Analyses  Format and Content (August 
2018). 
 
54 Maharaj, AR and AN Edginton, 2014, Physiologically Based Pharmacokinetic Modeling and Simulation in 
Pediatric Drug Development, CPT Pharmacometrics Syst Pharmacol, 3(11):1-13. 
 
55 Wang J, AN Edginton, and G Burckart, 2015, Using Modeling and Simulation for Neonatal Drug Development, 
NeoReviews, 16(11):e648-52. 
 
  19  Contains Nonbinding Recommendations 
 
H.  Clinical Study Report 
 
In addition to other clinical pharmacology information, it is important to capture safety data in all 
clinical pharmacology studies of neonates.  Classification of adverse events in neonates can be 
difficult given concomitant illnesses and medications.56  Any potential adverse events related to 
drug administration should be documented in the clinical study report.57 
 
 
 
 
 
56 Salaets, T, MA Turner, M Short, RM Ward, I Hokuto, RL Ariagno, A Klein, S Beauman, K Wade, M Thomson, E 
Roberts, J Harrison, T Quinn, G Baer, J Davis, K Allegaert, International Neonatal Consortium, Development of a 
Neonatal Adverse Event Severity Scale Through a Delphi Consensus Approach, Arch Dis Child, 104(12):1167-
1173. 
 
57 For more information, see the FDA draft guidance entitled General Clinical Pharmacology Considerations for 
Pediatric Studies for Drugs and Biological Products (December 2014), which describes the elements of the Clinical 
Study Report for neonatal studies.  When final, this guidance will represent the Agencys current thinking on this 
topic. 
 
  20