Human development is a continuous and dynamic process but developmental changes are more important in certain periods, i.e. in neonates, young infants and near adolescence. The maturation process itself varies between
Fortbildung / Formation continue
28
Vol. 16 No. 2 2005
Intra- and inter-individual variation in drug
metabolism and disposition are central to dif-
ferences in therapeutic response to a stand-
ard dosage regimen. In adults, variability is the
result of complex interactions between genetic
determinants and the environment. In children,
the variability of drug disposition is more com-
plex as the expression of genetic and envi-
ronmental factors is modified by the impact
of the physiological development / matura-
tion
1)–3). Indeed, human development is a con-
tinuous and dynamic process. Developmentalchanges are rapid and important in certain pe-
riods, i.e in neonates, young infants and near
adolescence. In addition, the maturation pro-
cess itself varies between individuals and may
be influenced by exogenous factors such as
maternal factors during pregnancy, nursing,
disease and drug administration.
Pharmacogenetics has been defined as
«… a monogenic trait caused by the pre-
sence in the same population of more than
one allele at the same gene locus and morethan one phenotype regarding drug interac-
tion with the organism. The genotype of low-
est frequency usually represents more than
1% individuals». Such polymorphisms gene-
rally remain undetected in the absence of
drug intake. They may be characterized by the
genotype, referring to an individual’s genetic
constitution and/or the phenotype referring
to the individual expression of genotype.
1. The major pharma-
cogenetic polymorphisms
1.1. Polymorphisms
of drug metabolizing enzymes
Most drugs are lipophilic and metabolized
into hydrophilic compounds which are eli-
minated through the kidneys. Biotransfor-
mation reactions are usually classified into
phase 1 and phase 2 reactions, associated
in various combinations depending on the
drug concerned. Many enzymes operating in
the metabolism of drugs are subject to ge-
netic polymorphisms. For most of them, the
molecular basis responsible for the allelic va-
riants with compromised function is known,
so that it is possible to determine individual
genotypes. By measuring the in vivometa-
bolism of drug substrates specific to the
polymorphic enzyme activity, it is possible to
identify different phenotypes, e.g. fast and
slow metabolizers.
Phase 1 reactions.
CYP2D6
polymorphism was discovered fol-
lowing therapeutic accidents occurring with
the use of debrisoquine for hypertensive
adults
4). More than 50 mutations and 70 dif-
ferent «poor metaboliser» alleles have been
described, with large ethnic differences in
frequency: 7 to 10% of Caucasians and 1 to
2% of Asians are poor metabolisers, while 2
to 3 % are ultrarapid metabolisers, due to
gene duplications or gene multiplications.
This polymorphism is inherited through an
autosomal recessive gene. Homozygous in-
dividuals are characterized by negligible or
no metabolism of a variety of drugs. Several
β-adrenoceptor-blocking agents, antide-
pressants, antipsychotics and antiarrythmic
drugs belong to this group. Numerous ad-
verse drug reactions were associated with
CYP2D6 poor metaboliser phenotype such
as perhexiline hepatotoxicity. CYP2D6 acti-
vity was not detectable in the fœtus, incre-
ased rapidly during the postnatal period, in-
dependently of gestational age, but remained
low during the first month of life (about 20%
Developmental Pharmacogenetics
May Fakhoury, Evelyne Jacqz-Aigrain, Paris
Abstract
Human development is a continuous
and dynamic process but developmen-
tal changes are more important in cer-
tain periods, i.e. in neonates, young in-
fants and near adolescence. The matu-
ration process itself varies between
individuals.
Immaturity in drug metabolism and dis-
position is associated with variability in
pharmacokinetics and drug response.
Most cytochrome P450 activities are not
detectable during fetal life, low at birth
and increase after birth. CYP3A7 is the
major fetal form, with a shift between
CYP3A7 and CYP3A4 occurring after
birth. Most conjugation reactions are im-
mature at birth but increase in the first
months after birth. The pattern of ma-
turation of various metabolic activities
is depending on the enzyme.
The physiological development and ma-
turation of the child may interact with
the expression of genotypic variation in
a way that is different from the expres-
sion in adults. In children, interactions
between pharmacogenetics and deve-
lopment have major impacts on the
pharmacokinetics and response to stan-
dard dosage regimen and are central for
cancer therapy, neonatal care and risks
of adverse drug reactions.
Résumé
Le développement humain est un pro-
cessus dynamique continu. Cependant,
les modifications sont plus importantes
à certaines périodes de la vie: chez le
nouveau-né, le jeune enfant ou l’adoles-
cent. De plus, la maturation est variable
en fonction des enfants.
L’immaturité du métabolisme des médi-
caments est responsable de grandes va-
riabilités dans la pharmacocinétique des
médicaments et la réponse aux traite-
ments.
La majorité des activités cytochromes
P450 (CYP) présents chez l’adulte ne
sont pas détectables pendant la vie foe-
tale et apparaissent après la naissance.
A l’inverse CYP3A7 est une forme stric-
tement fœtale qui disparaît après la
naissance et est remplacée par CYP3A4.
Les réactions de conjugaison sont, elles
aussi, immatures à la naissance mais
augmentent dans les premiers mois de
vie. Le profil de maturation des diffé-
rentes voies métaboliques varie en
fonction des enzymes.
Chez l’enfant, les interactions entre
pharmacogénétique et développement
peuvent avoir un impact important sur
la pharmacocinétique et la réponse thé-
rapeutique. Ceci explique que pour la
plupart des médicaments la posologie
soit adaptée à l’âge de l’enfant. Ceci est
particulièrement important pour l’ad-
ministration des médicaments chez le
nouveau-né ainsi que dans le risquè d’ef-
fets indésirables.
29
Vol. 16 No. 2 2005 Fortbildung / Formation continue
of adult’s levels. Phenotypic studies con-
ducted in small populations of children, have
shown that the adult phenotypic distribution
pattern was attained at 3 years of age. In chil-
dren, the number of clinically used drugs de-
pendent on CYP2D6 is limited. However, co-
deine and tramadol are metabolized to acti-
ve compounds morphine and O-desmethyl
tramadol, respectively and the analgesic ef-
fectiveness of these drugs is low in patients
of the slow metabolizer (developmental or
pharmacogenetic) phenotype.
Additional polymorphisms were described:
CYP2C1
9deficient patients 5% of Caucasians
and more than 20 % of Orientals are cha-
racterized by negligible or no metabolism of
a variety of drugs, such as S-mephenytoin,
citalopram, diazepam, omeprazol, lanzo-
prazol, etc. Although the polymorphism
may be of clinical importance in certain si-
tuations in pediatric pharmacotherapy, very
little is known about the clinical implications
in children. CYP2C9 is polymorphic and res-
ponsible for the oxidative metabolism of
widely used drugs such as anticoagulants,
non-steroidal antiinflammatory drugs, phe-
nytoin, etc. with clinical implications predo-
minantly in the treatment of cardiovascular
disease and epilepsy.
T
he CYP3A subfamily 5)is the predominant cy-
tochrome P450 subfamily, and comprises
three major isoforms: CYP3A4, CYP3A5,
CYP3A7, very closely related as they share
at least 85 % amino acid sequence. The
CYP3A4 enzyme is the predominant form
in adults important for the metabolism of a
large number of commonly used drugs wit-
hin the groups of antiepileptics, immuno-
suppressants, cytostatics, antibiotics etc.
CYP3A5 is predominantly an extrahepatic
form, under genetic control. CYP3A7 is the
major CYP isoform detected in embryonic,
fetal and newborn liver with a shift between
the CYP3A7 and CYP3A4 occurring after
birth. CYP3A7 is the enzyme responsible for
the drug metabolising activity initially des-
cribed in the human fetal liver.
Phase 2 reactions.
N
-acetyltransferases (NAT1 and NAT2) are
polymorphically expressed in human popu-
lations. NAT2 activity is transmitted as an
autosomal recessive trait. In Caucasian po-
pulations, 50–70 % of individuals are slow
acetylators, whereas the percentage is only
5% in Eskimo populations, and more than90% in Egyptians [During fœtal life, low NAT2
activity becomes perceivable already in
mid trimester After birth, all children are slow
metabolizers up to about the age of 2
months, after which the proportion of fast
metabolizer phenotype is successively in-
creased. At about 3 years of age, phenoty-
pic distribution is similar to that found in
adults.
T
hiopurine methyltransferase (TPM T)6)–7) .
TPMT is a cytosolic enzyme under pharma-
cogenetic control, metabolizing thiopurine
drugs, 6-mercaptopurine, azathioprine and
thioguanine, widely used in pediatric pa-
tients. 6-MP is prescribed during mainte-
nance therapy for childhood acute lympho-
blastic leukemia, azathioprine is used in the
management of inflammatory bowel disease.
The genetic polymorphism of TPMT activity
is of major clinical importance and TPMT ge-
notype should be determined in all patients
prior to treatment with thiopurines as pa-
tients deficient in TPMT activity are at very
high risk of developing myelosuppression
when treated with standard doses of 6-mer-
captopurine. In addition, in children with
acute lymphoblastic leukemia having a de-
tectable TPMT activity, 6-TGN concentra-
tions were negatively correlated with TPMT
activity and were predictive of better out-
come.
Additional polymorphisms were described for
5’-Uridine -diphosphate – glucuronosyl trans –
ferases ( UDP GTs), gluthathione S -tranfera –
ses (G ST)… Indeed, the glucuronidation of
acetaminophen (a substrate of UDPGT1A6
and 1A9) is absent in human fetal liver and
replaced by sulphate conjugaison. The glu-
curonidation of morphine (a substrate of
UDPGT2B7), is negligeable in human fetal li-
ver, immature at birth and rapidly increasing
thereafter. In infants, morphine clearance in-
creases with age reaching 80% of adult va-
lues by 6 months
8).
T
herapeutic consequences. The therapeutic
consequences depend on the importance of
the deficient enzyme for the overall meta-
bolism of the drug and/or the influence on
the metabolism pattern. In slow metaboli-
zers, the accumulation of the drug in a poor
metabolizer may be associated with exces-
sive therapeutic or toxic effects. Hovewer,
metabolic deviation leading to metabolites
not formed in extensive metabolizers is also
possible. In fast metabolizers, excessive
Table 1: Major phase 1 and phase 2
enzymes with some substrates
used in paediatric patients.
(The list of substrates is not exhaustive)
Enzyme Substrates
CYP1A2 Caffeine,
Carbamazepine,
Theophylline
CYP2A6 Acetaminophen,
Nicotine
CYP2B6 Cyclophosfamide,
Ifosfamide
CYP2C8 Diazepam,
Diclofenac,
Tricyclic antidepressants
CYP2C9* Losartan
NSAIDs (Celecoxib, Ibuprofen,
Indomethacin, Naproxen)
Phenytoin
CYP2C19* Diazepam,
Citalopram
Lanzoprazole
Omeprazole
S-mephenytoin
CYP2D6*Antiarrhythmic drugs,
Codeine
Dextromethorphan
Ethylmorphine
Odansetron
Perphenazine
Serotonin reuptake inhibitors
S-Mianserin
Tolterodine
Tricyclic antidepressants,
Zuclopenthixol
CYP2E1 Acetaminophen
Caffeine
Ethanol
CYP3A4 Cisapride
Cortisol
Cyclosporine,
Diazepam
Erythromycin
Ethosuximide
Midazolam
Nifedipine
Ritonavir
Tacrolimus
CYP3A7 Dehydroepiandrosterone,
Ethinylestradiol
NAT2 Caffeine
Dapsone
Isoniazid
Sulfametoxazole
TPMT 6-Mercaptopurine
6-Thioguanine
UDPGTs Acetaminophen
Bilirubin
Chloramphenicol
Diclofenac
Ibuprofen
Ketoprofen
Mycophenolic acid
Morphine
Naproxen
Fortbildung / Formation continue
30
Vol. 16 No. 2 2005
metabolism is a result causing risk of un-
dertreatment and insufficient clinical effica-
cy. Disequilibrium between phase I and pha-
se II reactions may account for a high risk of
adverse consequences including induction of
cancer, or immunotoxicity.
1.2. Polymorphism of ABC transporters
Data on the development aspects of trans-
porters or drug targets are still limited, alt-
hough this may have a major impact on the
pharmacological response in children.
T
he A TP binding cassette ( ABC ) family com-
prises at least eight multi-drug resistance-
associated proteins with a central role in
the absorption, distribution and elimination
of many drugs. P-glycoprotein (P-gp) is a trans-
membrane ATP-dependent efflux pump
9). It
is encoded by the MDR1(ABCB1) gene. It is
located in many tissues: in the gastrointe-
stinal tract its location in the brush border of
the apical surface of mature enterocytes in
the small intestine contributes to the che-
mical protection of the organism. Its ex-
pression is genetically controlled and MDR1
function is correlated with a mutation in exon
26 (C3435T) of the MDR1 gene. Substrates
of P-glycoprotein include glucocorticoids,
anticancer drugs, immunossuppressants,
HIV1 protease inhibitors and many other
drugs. As an example, digoxin pharmacoki-
netics is affected by the C3435T polymor-
phism in exon 26.
1.3. Polymorphism of target proteins
Target proteins can be a receptor, an enzy-
me, or another type of protein. Genetic po-
lymorphisms affecting these drug targets can
contribute to the pathogenesis of the disease
and modify the pharmacological response in
children
(10 ).
Polymorphisms of the
β2-adrenergic recep –
tor ( AD RB2) have been implicated in the va-
rying response to β-agonists in patients with
asthma. Polymorphisms of the promoter re-
gion (variable number of tandem repeats) af-
fecting ALOX5 geneexpression has been as-
sociated with the response to inhibitors of
ALOX5. A number of additional polymor-
phisms of potential clinical importance
were described in adults: polymorphisms of
the angiotensin converting enzyme (ACE) af-
fecting the sensitivity to ACE inhibitors, of the
5-hydroxytryptamine receptor affecting the
response to neuroleptics, and of dopamine
D3 receptor associated with drug inducedtardive dyskinesia. However, studies should
be conducted in this field as there is almost
no data on the ontogeny of the different drug
targets.
2. Methods in pharmacogenetics
At the gene level, pharmacogenetic altera-
tions leading to allelic variants may include:
1) partial or total gene deletion, insertion or
duplication,
2) SNPs (Single Nucleotide Polymorphisms)
affecting the coding region, the perigenic
region, or the non-coding region. The lar-
ger the gene, the larger the number of ex-
pected SNPs. On average about four
SNPs of functional importance are ex-
pected per gene. Such SNPs are gene-
rally non-synonymous, i.e located in the
coding region and associated with an
amino acid change.
The majority of the SNPs will not have any cli-
nical importance, depending primarily on the
role of the polymorphic trait and the thera-
peutic index of the drug. Indeed, the func-
tional significance of all the genetic variations
identified through rapid-throughput se-
quencing will require demonstration of a
functional correlate. Sometimes, powerful
statistical approaches are required for this
purpose.
3. Implications of developmental
pharmacogenetics in therapeutics
3.1. Impact on pharmacokinetics
Delayed maturation and pharmacogenetic
polymorphisms of drug metabolizing enzy-
mes have an important impact on the phar-
macokinetics of many drugs, resulting in alower clearance and prolonged elimination
half-life. Examples include drugs highly
metabolized such as carbamazepine, mor-
phine, phenytoin, theophyllin, or drugs pri-
marily cleared by renal elimination such as
gentamicin or digoxin. As the age related
changes in pharmacokinetics are non mono-
tonic, age-appropriate doses should be given
in neonates, infants and children in order to
optimize therapy.
Paediatric pharmacokinetic models, are
now developed, based on genetic, physiolo-
gical, demographic and clinical attributes of
the patient population, in order to predict
drug disposition in paediatric patients and to
estimate average dose requirements accor-
ding to age.
3.2. Pharmacogenetics
and adverse drug reactions
The possible implications of pharmacoge-
netics for the risk of ADRs was underlined by
Philips et al
11)who reported recently that 59%
of drugs in the ADR studies were metaboli-
zed by at least one polymorphic enzyme in
comparison to 22% of randomly selected
drugs. The pharmacogenetic mechanisms
may involve accumulation of the parent drug
in slow metabolisers, formation of toxic meta-
bolites with disequilibrium between the bio-
activation and detoxification processes, im-
mune-mediated ADR, most often associated
with the HLA (human leukocyte antigen).
Adverse drug reactions are a significant cau-
se of morbidity and mortality in children
12 ).
This is the case in neonates having immatu-
re glucuronidation, who developed a grey
baby syndrome from treatment with chlo-
ramphenicol in the 1960s. The example of ci-
sapride is more recent: the CYP3A4 meta-
Pharmacogenetics of drug metabolising enzymes CYP450: cytochrome P450; ADH:alcohol dehydrogenase; UGTs:uridine 5’-triphosphate glucuronosyltransferase;
NAT:N-acetyl transferase; GST:glutathion S-transferase; TPMT:thiopurine methyltransferase
Elimination Phase I
FCYP450: 2C 2D 3A
FADH
Festerases
Phase II
FUGTs
FNATs
FGSTs
FTPMT
Phase I
O2 ROH
XOH XOR XORXOH
XX
Phase II Phase III
Phase IV
31
Vol. 16 No. 2 2005 Fortbildung / Formation continue
bolism of cisapride is immature in neonates
resulting in drug accumulation that might ex-
plain QTc prolongation
13 ). In older infants,
aged two to six years, some metabolic acti-
vities appear to exceed the corresponding
adult activities. This may result in an inba-
lance between bio activation and detoxifi-
cation reactions, leading to high concentra-
tions of toxic reactive metabolites.
4. Conclusions
The relationship between phenotype and
genotype may be different from what it is
seen in adults, due to the physiological de-
velopment and maturation of the child. As a
result, variability in pharmacokinetics and
response to standard dosage regimen is
greater in children than in adults. Therefore,
pharmacological studies as a function of age
are very important for the definition of do-
sage regimens suitable for children and for
limiting the risk of side effects and toxicity.
Address for correspondence:
Professeur Evelyne Jacqz-Aigrain
Department of Paediatric Pharmacology
and Pharmacogenetics
Center for Paediatric Clinical Investigations
Hopital Robert Debré, 48 Boulevard Serurier
75019 Paris – France
Tel: +33 (0)1 4003 2150
Fax: +33 (0)1 4003 4759
evelyne.jacqz
-aigrain@rdb.ap -hop -paris.fr
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Informations complémentaires
Auteurs
Prof. Evelyne Jacqz-Aigrain , Department of Paediatric Pharmacology and Pharmacogenetics Center for Paediatric Clinical Investigations Hopital Robert Debré, France
