An Observational Study to Assess Response to Tamoxifen in Breast Cancer Patients

This multi-centre open label single arm non randomized observational study will compare the efficacy in terms of overall response rate and progression free survival of tamoxifen as first line therapy in 3 groups of postmenopausal women with measurable hormone dependent large, locally advanced or metastatic breast cancer. The 3 groups are women with a normal and those with a low 'tamoxifen activity score' based on genetic polymorphisms for CYP2D6 and other genes that are important in the metabolism of tamoxifen using SEQUENOM's MassARRAY technology. The study is subject to Ethical Commission approval and patient consent. The study will necessitate collection of blood for genetic analyses. We will investigate the relation between the studied genotype, the use of drugs that interfere with tamoxifen and tamoxifen-related endpoints like regression of metastatic or locally advanced or large oestrogen receptor positive breast cancer in tamoxifen users. The 'tamoxifen activity score' has been used by a group in the US showing a link with tamoxifen compliance. The score will be adapted to the Belgian situation based on the prevalence of polymorphism in a Belgian population. The efficacy of tamoxifen will be correlated with a predefined 'tamoxifen activity score' which is based on the presence of single nucleotide polymorphisms (SNP) in relevant genes combined with the effect of well known drugs that interfere with the metabolism of tamoxifen. The study will be conducted in several clinical sites in Belgium. All patients will receive tamoxifen 20mg daily. Patients with a large operable or inoperable non-metastatic breast cancer will be considered for surgery no more than 4 months on tamoxifen. If operable, they will postoperatively receive the most appropriate adjuvant therapy and for hormone therapy either continue tamoxifen or receive an oral aromatase inhibitor as decided by the clinician. If women with a locally advanced inoperable breast cancer are not operable after the 4 months of neo-adjuvant therapy, another appropriate salvage therapy will be proposed. Women with metastatic breast cancer will continue treatment until clinical or imaging progression or unacceptable toxicity development. Patients that experience progression of their disease as defined by RECIST criteria will receive salvage therapy by an oral aromatase inhibitor if tamoxifen is given in first line but some patients may require another therapy like chemotherapy. The study will require approximately 14 months to recruit and another 7 months to events/data analyses as the estimated time to progression in this setting is 9-12 months if tamoxifen is given as first line endocrine therapy for those in the metastatic setting