Clinical evaluation of dried blood spots for the determination of tamoxifen and endoxifen levels

Tamoxifen is a selective estrogen receptor modulator that is widely used for
the treatment and prevention of breast cancer. Endoxifen, formed via
N-desmethyltamoxifen and via 4-hydroxytamoxifen, is considered to be the most
important metabolite. The steady state level of endoxifen is a proposed
predictor of the clinical outcome of tamoxifen treatment. It is suggested that
there is a minimum concentration threshold above which endoxifen is effective
against the recurrence of breast cancer.
Current clinical practice is to measure tamoxifen and metabolite concentrations
in serum, for therapeutic drug monitoring (TDM). The blood samples are drawn
from patients when they visit the Antoni van Leeuwenhoek hospital. In order to
make TDM less invasive for the patient, we set up a method to determine
tamoxifen and endoxifen levels in dried blood spots (DBS). This enables sample
collection by means of one fingerprick, as is previously shown for
antiretroviral drugs.2,3 This technique allows patients to self-sample at home.
No relationship between DBS and serum concentrations of tamoxifen and endoxifen
has been established yet. Since the target concentrations of endoxifen are
determined in serum samples, there is a need to establish the correlation
between DBS and serum concentrations. Therefore, the levels of tamoxifen and
endoxifen have to be determined in simultaneously drawn DBS and serum samples
of patients using tamoxifen.

Clinical evaluation of dried blood spot assay for the determination of
tamoxifen and endoxifen levels

Patients are treated with tamoxifen on a dose according to the prescription of
the physician. No further intervention is needed. A DBS sample will be obtained
simultaneously with the serum sample that is obtained for regular clinical
care.

not applicable