Bioanalysis and clinical pharmacology of tamoxifen in breast cancer

Abstract

Tamoxifen, a selective estrogen receptor (ER) modulator, is widely used in the treatment of ER-positive breast cancer. It has been used for over 40 years and has dramatically reduced recurrence and mortality rates of ER-positive breast cancer patients. However, not all patients benefit from tamoxifen treatment as in about 25-30% of the patients the disease recurs. There is an urgent need to gain insight into factors that are predictive for treatment outcome, however, until today the mechanisms of this variable response remain unclear. Studies on the pharmacology of tamoxifen revealed extensive biotransformation and showed that some of its metabolites, especially 4-hydroxytamoxifen and N-desmethyl-4-hydroxytamoxifen (endoxifen), have a much higher affinity for the ER than tamoxifen itself. Endoxifen is suggested to be the most important metabolite, considering it is present at a much higher steady-state serum concentration in patients using tamoxifen than 4-hydroxytamoxifen. There is wide inter-patient variability in tamoxifen pharmacokinetics, however, in general all patients use a standard dose of 20 mg tamoxifen per day. These findings provide a new possibility for treatment optimization; individual dosing based on active metabolites levels. The results of a recent large clinical trial encouraged this optimization option; a significant correlation between endoxifen serum levels and breast cancer recurrence rate was shown and a minimum therapeutic threshold endoxifen serum level was suggested. To enable dose individualization, a sensitive and selective method for the quantitative analysis of tamoxifen and its metabolites in patient serum samples was developed. Subsequently, this method was implemented in clinical practice and used to support clinical studies. Considerable inter-patient variability, but low intra-patient variability, in systemic exposure of tamoxifen and its metabolites was observed. Additionally, it was shown that the prescribed tamoxifen dose was related to endoxifen exposure and that increasing the tamoxifen dose leads to a significantly higher serum concentration of tamoxifen and its metabolite. To ease sampling logistics and as a patient-friendly alternative to venous sampling, a dried blood spot (DBS) method for quantification of tamoxifen and endoxifen was developed. DBS sampling consists of the collection of a whole blood sample, obtained by a finger prick, on a paper DBS card. DBS samples can be transported by regular mail service, since no special conditions for transport and storage of the samples are required. To enable the use of DBS in clinical practice, a bridging study was performed to establish the relationship between DBS and serum concentrations of tamoxifen and endoxifen. After correction for haematocrit and analyte-specific blood cell-to-serum ratio, DBS concentrations were equal to serum concentrations. Additionally, patients were shown to be capable of collecting DBS by themselves at home and they stated to prefer this sampling method over venous sampling. These findings enable DBS sampling at home for the determination of tamoxifen and endoxifen in patient samples, for clinical studies and therapeutic drug monitoring purposes. The insights generated in this thesis could be used to further individualize and optimize tamoxifen treatment, and could be used as a starting point for further research on the association between active metabolite levels and treatment outcome.