Currently, there are very few guidelines linking the results of pharmacogenetic tests to specific therapeutic recommendations.
Therefore, the Royal Dutch Association for the Advancement of Pharmacy established the Pharmacogenetics Working Group
with the objective of developing pharmacogenetics-based therapeutic (dose) recommendations. After systematic review
of the literature, recommendations were developed for 53 drugs associated with genes coding for CYP2D6, CYP2C19, CYP2C9,
thiopurine-S-methyltransferase (TPMT), dihydropyrimidine dehydrogenase (DPD), vitamin K epoxide reductase (VKORC1),
uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), HLA-B44, HLA-B*5701, CYP3A5, and factor V Leiden (FVL).
In recent years, there has been substantial progress in the field of pharmacogenetics. The number of publications
on the subject has risen sharply, and the results of the first randomized clinical trial showing that pharmacogenetics
can be used to prevent adverse drug events have been published.
1 Meanwhile, an increasing number of pharmacogenetic tests are becoming available.
2 However, despite US Food
and Drug Administration–approved modifications to more than 30 drug labels to include pharmacogenetic information,
3 guidelines that link the result of a pharmacogenetic test to specific dose recommendations are sparse. Therefore, the
Royal Dutch Association for the Advancement of Pharmacy established the Pharmacogenetics Working Group with the
objectives of developing pharmacogenetics-based therapeutic (dose) recommendations based on systematic review of the
literature and assisting physicians and pharmacists by integrating the recommendations into computerized systems for
drug prescription, dispensing, and automated medication surveillance. The initial results for 85 genotype/phenotype–
drug combinations, comprising 26 drugs, were published in this journal.
4 Here we present recommendations for 27 newlyassessed drugs and updates of the existing monographs.
Results
To date, we have compiled therapeutic (dose) recommendationsfor 163 genotype/phenotype–drug combinations comprising
53 drugs and 11 genes (Table 1; the table’s references are providedin the Supplementary References online). The drugs were
associated with genes coding for CYP2D6 (n = 25), CYP2C19(n = 11), CYP2C9 (n = 7), thiopurine-S-methyltransferase
(TPMT) (n = 3), dihydropyrimidine dehydrogenase (DPD)(n = 3), vitamin K epoxide reductase (VKORC1) (n = 2), uridine
diphosphate glucuronosyltransferase-1A1 (UGT1A1), HLA-B44,HLA-B*5701, CYP3A5, and factor V Leiden (FVL) (all n = 1).
Therapeutic (dose) recommendations were formulated for 39 (73.6%) of the drugs. For clozapine, flupenthixol, and olanzapine,
a gene–drug interaction with CYP2D6 was considered, but no evidence was found in the literature, and hence no recommendations
were required. For 11 of the drugs (20.8%), agene–drug interaction was present, but no therapeutic (dose)
recommendation was deemed necessary. The quality of the retrieved data was scored as category 4 (published
controlled studies of “good” quality; see Supplementary Table S1 online for quality criteria) for 49.1% of the data and
category 3 (published controlled studies of “moderate” quality) for 37.4%. For 59 (36.2%) of the genotype/phenotype–drug
combinations, the clinical relevance of the interaction was classified as category C (long-standing discomfort (48–168 h)
without permanent injury) or higher (see Supplementary TableS2 online for details).

CYP2D6
For CYP2D6 poor metabolizers (PMs), defined as patients carrying two defective alleles, dose reductions are recommended for
clomipramine, flecainide, haloperidol, zuclopenthixol (all 50%); doxepin, nortriptyline (both 60%); imipramine, propafenone
(both 70%); and metoprolol (75%). There were insufficient data to calculate dose adjustments for amitriptyline, oxycodone,
risperidone, and venlafaxine. With respect to tamoxifen, an increased risk for breast cancer relapse is present, and it is
advised that an aromatase inhibitor be considered for treating postmenopausal women with breast cancer. Other recommendations
included the selection of an alternative drug, therapeutic drug monitoring, increased alertness to adverse drug events and
to reduced efficacy, and the recording of an electrocardiogram. For CYP2D6 intermediate metabolizers (IMs), defined as
patients carrying two decreased-activity alleles or one active/ decreased-activity allele and one inactive allele, dose reductions
ranging from 20 to 50% are advised for doxepin, amitriptyline, zuclopenthixol, imipramine, nortriptyline, and metoprolol. There
were insufficient data to calculate dose adjustments for clomipramine, oxycodone, propafenone, risperidone, and venlafaxine.
For tamoxifen, the use of an aromatase inhibitor for treating postmenopausal women with breast cancer and the avoidance of
concomitant use of a CYP2D6 inhibitor are advised. Other recommendations are comparable to the recommendations for PMs.
For CYP2D6 ultrarapid metabolizers (UMs), defined as patients carrying a gene duplication in the absence of inactive
or decreased-activity alleles, dose adjustments ranging from 30 to 150% are recommended for doxepin, imipramine, metoprolol,
nortriptyline, tramadol, and venlafaxine. For eight of the assessed gene–drug combinations, there were insufficient data
to calculate dose adjustments. The metabolic capacity of UMs shows a considerable variability due to the range of gene copy
numbers possible within the definition of UM. Also, the impact of the increased concentrations of drug metabolites to which
UMs are exposed is often unknown. Therefore, the selection of an alternative drug is frequently advised.

CYP2C9
Seven CYP2C9 substrates were assessed. For phenytoin, dose reductions of 25% (*1/*2, *1/*3) and 50% (*2/*2, *2/*3, *3/*3)
are recommended. For acenocoumarol and phenprocoumon, although clinically relevant gene–drug interactions are present,
no dose adjustment is recommended because of strict international normalized ratio monitoring by the Dutch Thrombosis
Service.5 The need for adjustment of the initial dose is currently under investigation.6 In addition to the CYP2C9 genotype, the
VKORC1 genotype is an important determinant of coumarin response. Therefore, the status of both CYP2C9 and VKORC1
should be considered when identifying candidates for intensified international normalized ratio monitoring. Despite a clear
pharmacokinetic effect of the gene–drug interaction, no recommendations were formulated for any of the sulfonylureas; the
absolute risk for hypoglycemia is low, and the dose is titrated in response to plasma levels of glucose/glycosylated hemoglobin.

CYP2C19
The number of CYP2C19 substrates assessed increased from 1 to 12, and the CYP2C19*17 allele (resulting in UMs) was added.
Recommendations have been made with respect to all drugs except moclobemide and rabeprazole. Several articles have
reported that the use of proton pump inhibitors results in better clinical efficacy in PMs and IMs as compared to extensive
metabolizers. These results were scored as clinical relevance category AA# (AA: no statistically significant kinetic or clinical
effect; “#” indicates a positive effect). Because of the risk of undertreatment, dose increases ranging from 50 to 400% are
advised for UMs who are receiving treatment with proton pump inhibitors. In the case of voriconazole, because of its nonlinear
pharmacokinetics, no dose adjustment is recommended.

UGT1A1
The UGT1A1*28 allele is associated with irinotecan toxicity. Although results are not consistent, there is sufficient evidence
that a reduction in the initial dose by 30% is required for regimens containing >250 mg/m2 of irinotecan prescribed to
homozygous carriers of the UGT1A1*28 allele. This is in agreement with the Food and Drug Administration–mandated label
change. No dose reduction is recommended for heterozygous carriers of the UGT1A1*28 allele because dose reduction might
result in undertreatment.

TPMT
TPMT catalyzes the S-methylation of the thiopurine drugs 6-mercaptopurine, azathioprine, and thioguanine. Selection of
an alternative drug is advised for IMs and PMs. If this is not possible, the dose should be reduced by 50 and 90%, respectively.
The data for thioguanine were insufficient for calculating dose adjustments.

HLA-B44
There was some evidence that HLA-B44-negative patients show less response to treatment with ribavirine. However, given that
~90% of the population is HLA-B44-negative and that no alternative treatment is available, no action is advised.HLA-B*5701
To date, the association between HLA-B*5701 genotype and the hypersensitivity reaction to abacavir remains the only example
of a randomized clinical trial of pharmacogenetics. The advice regarding selection of an alternative drug for treating HLAB*
5701-positive patients is in agreement with the recommendations of the Food and Drug Administration and the European
Medicines Agency.

CYP3A5
Because of the large number of publications, studies limited to healthy volunteers, pharmacokinetic end points, or liver transplantations
were excluded. Although an interaction between CYP3A5 genotype and tacrolimus metabolism exists, no action is
advised because in Dutch transplantation hospitals the tacrolimus dose is titrated in response to therapeutic drug monitoring.

VKORC1
The VKORC1 genotype appears to contribute more to the variability in coumarin dose requirements than the CYP2C9 genotype
does. The presence of the VKORC1 C1173T polymorphism results in a decrease in dose requirements of acenocoumarol
and phenprocoumon. However, for reasons identical to those related to the coumarin–CYP2C9 interaction, it was decided
not to advise a dose reduction.

FVL
Patients with a positive (family) history of thrombotic events, and who are also carriers of the FVL allele, are advised to avoid
the use of estrogen-containing oral contraceptives.

DPYD
Three DPD substrates were evaluated: 5-fluorouracil, its oral prodrug capecitabine, and tegafur. Selection of an alternative
drug is advised for PMs, defined as homozygous carriers of a nonfunctional allele. For IMs, defined as heterozygous carriers
of a nonfunctional allele, a dose reduction of 50% is advised for 5-fluorouracil and capecitabine.

Discussion
We have developed pharmacogenetics-based therapeutic (dose) recommendations for 163 genotype/phenotype–drug combinations
comprising 53 drugs and 11 genes. These recommendations include updates on the 26 existing therapeutic (dose) recommendations as well as recommendations for 27 new gene–drug combinations. The recommendations issued since October 2006 are available through most automated drug prescription, dispensing, and medication surveillance systems in the Netherlands.
The Pharmacogenetics Working Group initiative is not the first to develop guidelines with pharmacogenetics-based dose recommendations. A 2001 paper on CYP2D6 phenotype–based dose recommendations for antidepressants represents an early step.7
A more recent example is the inclusion of pharmacogeneticinformation in coumarin dosing algorithms.6,8 Furthermore,
several groups have developed databases that are devoted to disseminating knowledge in the area of pharmacogenetics, e.g.,
PharmGKB (http://www.pharmgkb.org/). However, our recommendations are the first to be available nationwide during the
process of drug prescribing and dispensing. Our approach has some limitations, though. First, pharmacogenetics
was not the primary objective for most of the studies we assessed; therefore, many of the studies were underpowered,
with insufficient sample size per genotype or phenotype. Second, the end points assessed were often pharmacokinetic ones and
the result of single-dose experiments in healthy volunteers— not representative of the conditions in daily clinical practice.
However, since our previous report, the number of studies with pharmacogenetics as the primary objective has increased
significantly.4 In our opinion, there is currently only limited evidence to justify population-wide prospective pharmacogenetic screening.
A pharmacogenetic test prior to drug prescription is obligatory only for trastuzumab. Yet there are indications that patients with
a non-wild-type genotype may be at increased risk for an aberrant drug response. Therefore, we formulated recommendations
for patients with a previously determined genotype. In current clinical practice, the number of such patients is limited and consists
mainly of subjects who were genotyped after unexplained adverse drug events or lack of response to “normal” drug dose.
However, with the continuous decline in the costs of pharmacogenetic tests and the increasing number of laboratories with
genotyping infrastructure, this number is bound to increase. The recommendations of the Pharmacogenetics Working
Group focus on the combination of a single gene with a single drug. However, the predictive value of a single genetic variant
with regard to drug response is often limited, and combinations of multiple genetic variants may be involved. For example,
only 5–18% and 15–37% of the variation in warfarin dose requirements are explained by CYP2C9 and VKORC1 genotypes,
respectively.9–13 Models that combine information on both genetic and nongenetic factors are able to explain up to 50% of
the variation in warfarin dose requirements.8 The formulation of recommendations that consider combinations of multiple genes
presents a significant challenge for the future, given that very large study populations will be required to gather significant
numbers of patients with combinations of rare genotypes. A second challenge is the integration of gene–drug and drug–drug
interactions. To date, drug–drug interactions have been considered characteristic only of the drugs involved. However, in the
light of current knowledge of pharmacogenetics, this might no longer be valid. For example, the interaction between a CYP2D6
inhibitor and a CYP2D6 substrate requires different management for CYP2D6 IMs than for CYP2D6 PMs. Therefore, the
combination of gene–drug and drug–drug interactions may have major implications for drug prescribing and dispensing.
Research in this field is only starting to evolve.14

In conclusion, we have developed pharmacogenetics-based therapeutic (dose) recommendations for 53 drugs. The recommendations
are available nationwide during the process of drug prescribing and dispensing. We believe that the availability of the therapeutic (dose) recommendations during the process of therapeutic decision making represents an important step in the clinical use of pharmacogenetic information.

Methods
A detailed description of the methods used for data collection, data assessment, and preparation of gene–drug monographs has previously been provided in this journal.4 In brief, a list of genetic polymorphisms affecting pharmacokinetics and pharmacodynamics, including an overview of drug substrates, was compiled. For each drug, a systematic search of the literature was performed. Review articles and studies involving nonhuman subjects and in vitro experiments were excluded. Each gene–drug interaction was scored on two parameters. First, the quality of evidence for the gene–drug interaction was scored on a five-point scale ranging from 0 (lowest evidence) to 4 (highest evidence) (Supplementary Table S1). Population size was not included as a parameter for assessing the quality of evidence, but dose adjustments were calculated as the population size–weighted mean. Second, the clinical relevance of the potential gene–drug interaction was scored on a seven-point scale ranging from AA (lowest impact) to F (highest impact) (Supplementary
Table S2). For each gene–drug interaction, a risk analysis containing a review of the selected articles, their assigned levels of evidence and clinical relevance, and a therapeutic (dose) recommendation were compiled.
Recommendations included those related to dose adjustments as well as advice on therapeutic strategy (e.g., therapeutic drug monitoring, selection of alternative drugs, and warning for adverse drug events).

SUPPLEMENTARY MATERIAL is linked to the online version of the paper at
http://www.nature.com/cpt
Acknowledgments
No additional funding was received. We thank Jean Conemans and
Ingeborg Wilting for their valuable contributions as former members of the
Pharmacogenetics Working Group.
Conflict of Interest
The authors declared no conflict of interest.
© 2011 American Society for Clinical Pharmacology and Therapeutics
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