The influence of time of administration on tamoxifen pharmacokinetics.

The selective estrogen-receptor modulator tamoxifen is still an important and
frequently used drug for the treatment of estrogen receptor positive breast
cancer. Tamoxifen reduces the risk of recurrence and of mortality, however, not
all women benefit from the tamoxifen therapy. In addition, treatment-related
adverse reactions (i.e. hot flashes) also vary greatly between patients.
Inter-individual variability in metabolism of tamoxifen, which is influenced by
both genetic and environmental factors, may contribute to the differences in
efficacy and toxicity of tamoxifen. However, even after correcting for CYP2D6
genotype and use of CYP2D6 inhibiting drugs, large inter-individual variability
in endoxifen concentrations have been found, suggesting other factors which may
influence tamoxifen pharmacokinetics and endoxifen concentrations.

For further improvement of rational pharmacotherapy, both inter-individual
variability and intra-individual variability should be considered. In addition
to inter-individual differences in pharmacokinetics by polymorphisms in
metabolizing enzymes and inhibiting co-medication, drugs may vary in potency
and/or toxicity associated with the rhythmicity of biochemical and
physiological factors, depending on the administration time.

Chronopharmacology describes the effect of timing of drug administration on the
pharmacokinetics and pharmacodynamics of the drug. The time of drug
administration may affect pharmacokinetics and thereby potentially treatment
response. It has been shown that the response of several drugs differs,
depending on the time of administration. Because of circadian variation in the
activity of gastrointestinal, hepatic and renal processes, the pharmacokinetics
of drugs may change as a function of time of drug administration.

As absorption, distribution, metabolism and elimination have been shown to be
influenced or regulated by the circadian clock, tamoxifen plasma
pharmacokinetics may vary with time and may depend on time of administration
(i.e. morning versus evening administration). Therefore, the influence of
circadian rhythm on the pharmacokinetics of tamoxifen and its metabolites will
be examined.

Diurnal changes in estrogen levels have been observed in premenopausal women.
In case of fluctuations in estrogen levels during the day, this may have
consequences on tamoxifen administration. To investigate diurnal changes in
estrogen levels, plasma concentrations of this hormone will be determined on
three time points during a day.

Amendment:
The influence of circadian rhythms on the pharmacokinetics of tamoxifen were
examined in mice. Results of the preclinical study, in which 6 different dosing
times (8 a.m., 12 p.m., 4 p.m., 8 p.m., 12 a.m. and 4 a.m.) of tamoxifen on the
pharmacokinetics of tamoxifen and its metabolites were investigated, suggest
circadian changes in the pharmacokinetics of tamoxifen. A 34% difference in
tamoxifen pharmacokinetics was observed between 8 a.m. and 8 p.m. dosing in
mice. However, highest levels were reached after dosing at midnight.
Translating these results to humans, levels of tamoxifen and its active
metabolites might be higher after dosing in the afternoon (at 1 p.m.). To
observe whether tamoxifen pharmacokinetics differ between morning (8 a.m.),
afternoon (1 p.m.) and evening (8 p.m.) dosing, possibly reaching higher levels
of tamoxifen and active metabolites after dosing at 1 p.m., a third time-point
(1 p.m.) of tamoxifen administration will be included in the study (to
investigate the effects on pharmacokinetics).

To determine the influence of morning versus evening administration on the
pharmacokinetics of tamoxifen and its metabolites.

Amendment: To determine the influence of time of administration (morning (8
a.m.), afternoon (1 p.m.) and evening (8 p.m.)) on the pharmacokinetics of
tamoxifen.

This study is a pharmacokinetic study intended to investigate the influence of
morning versus evening administration on the pharmacokinetics of tamoxifen.
Patients who are treated with tamoxifen in a dose of 20 or 40 mg will be
included in this trial; nine patients who use tamoxifen in the morning (group
1) and nine patients who use tamoxifen in the evening (group 2). For the
primary study objective, the following study plan will be applied. In group 1,
tamoxifen will be administered in the morning (+/- 8 AM) during at least 4
weeks. Patients will be hospitalized for a 24-hour period and pharmacokinetic
samples will be drawn. Patients will switch to administration of tamoxifen in
the evening. During the second period of the study, tamoxifen will be
administered in the evening (+/- 8 PM) for at least 4 weeks in patients of
group 1 and these patients will undergo a second 24-hour pharmacokinetic
sampling period. Patients in group 2 will undergo the same pharmacokinetic
sampling periods, only in the opposite cycle order; first during administration
of tamoxifen in the evening (+/- 8 PM; for at least 4 weeks) and a second
PK-sampling period after tamoxifen administration in the morning (+/- 8 AM; for
at least 4 weeks).

Amendment:
Patients already enrolled in the study (and completed 2 pharmacokinetic
sampling periods) will be asked to undergo a third period of 24-hour
pharmacokinetic sampling after tamoxifen administration at 1 PM for at least 4
weeks. After morning or evening administration of tamoxifen, depending on
group, patients will switch to administration of tamoxifen in the afternoon
(+/- 1 PM; for at least 4 weeks) and will undergo a third 24-hour
pharmacokinetic sampling period.
However, after completing the study, a number of patients stopped tamoxifen
treatment (finishing 2.5 years of tamoxifen treatment). In addition, some
patients may not wish to participate in the study for a third time of 24-hour
sampling. Therefore, to eventually obtain pharmacokinetic results from 18
patients, who have completed three periods of different times of tamoxifen
administration (morning, afternoon and evening) followed by 24-hour
PK-sampling, several new patients have to be included in the study
(additionally to the 18 enrolled patients). Participation to the third period
of the study (administration of tamoxifen in the afternoon and PK-sampling),
requires an additional informed consent form that has to be signed by the
patient (patient information form (amendment), version 4). Newly included
patients should also sign this patient information form.

On the pharmacokinetic sampling days, plasma samples will also be used for
determination of 4β-hydroxycholesterol to cholesterol ratio*s as an endogenous
marker of CYP3A4/5 activity.

For the secondary study objective, patients will be followed during the study
period. At baseline and on pharmacokinetic sampling days, adverse effects will
be recorded. Adverse effects during the study should be recorded by the patient
in a patient diary. Differences in adverse effects will be determined.

To determine a patient*s chronotype (determination of so called *morning* or
*evening persons*) patients will be asked to complete a short questionnaire
(including 5 questions).

To investigate diurnal changes in estrogen levels, plasma concentrations of
this hormone will be determined on three time points during a day. As its was
shown that estrogen levels peaked in the morning (~ 8 AM) and trough levels
were observed in the later afternoon/evening, concentrations of estrogens in
plasma will be determined in the morning (8.00 AM), afternoon (4.00 PM),
evening (8.00 PM) and night (04.00 AM).

Patients will switch from administration of tamoxifen in the morning to
administration in the evening or from tamoxifen administration in the evening
to administration in the morning.

Amendment: Patients will switch for a second time from tamoxifen administration
in the morning/evening to administration in the afternoon.

Patients are only at risk for complications associated with blood sampling
(venepuncture). However, bloodsamples will be drawn by trained staff. In case
of significant different endoxifen exposure during morning, afternoon or
evening tamoxifen administration, a recommendation on time of administration
for tamoxifen can be given. This may contribute to optimization of tamoxifen
therapy.