Journal of Personalized Medicine Review Generating a Precision Endoxifen Prediction Algorithm to Advance Personalized Tamoxifen Treatment in Patients with Breast Cancer Thomas Helland 1,2,3,* , Sarah Alsomairy 1, Chenchia Lin 1, Håvard Søiland 3, Gunnar Mellgren 2,3 and Daniel Louis Hertz 1 1 Department of Clinical Pharmacy, University of Michigan College of Pharmacy, Ann Arbor, MI 48109, USA;
[email protected] (S.A.);
[email protected] (C.L.);
[email protected] (D.L.H.) 2 Hormone Laboratory, Department of Medical Biochemistry and Pharmacology, Haukeland University Hospital, 5021 Bergen, Norway;
[email protected] 3 Department of Clinical Science, University of Bergen, 5007 Bergen, Norway;
[email protected] * Correspondence:
[email protected]; Tel.: +47-92847793 Abstract: Tamoxifen is an endocrine treatment for hormone receptor positive breast cancer. The effectiveness of tamoxifen may be compromised in patients with metabolic resistance, who have insufficient metabolic generation of the active metabolites endoxifen and 4-hydroxy-tamoxifen. This has been challenging to validate due to the lack of measured metabolite concentrations in tamoxifen clinical trials. CYP2D6 activity is the primary determinant of endoxifen concentration. Inconclusive results from studies investigating whether CYP2D6 genotype is associated with tamoxifen efficacy may be due to the imprecision in using CYP2D6 genotype as a surrogate of endoxifen concentration Citation: Helland, T.; Alsomairy, S.; without incorporating the influence of other genetic and clinical variables. This review summarizes Lin, C.; Søiland, H.; Mellgren, G.; the evidence that active metabolite concentrations determine tamoxifen efficacy. We then introduce a Hertz, D.L. Generating a Precision novel approach to validate this relationship by generating a precision endoxifen prediction algorithm Endoxifen Prediction Algorithm to and comprehensively review the factors that must be incorporated into the algorithm, including Advance Personalized Tamoxifen genetics of CYP2D6 and other pharmacogenes.