Cross Reactivity between the Roche Elecsys® Progesterone Assay and Exemestane.
Authors: Dieudonné A. , Billen J. , Geerts I. , Leunen K. , Paridaens R. , Vandenberghe J. , Van Calster B. , Wildiers H. , Neven P. 
Published: December 2010

Abstract
Background:Oral aromatase inhibitors (AI) and to a lesser extend tamoxifen can promote recovery of ovarian function in postmenopausal breast cancer patients. Biochemical monitoring of ovarian function is important as folliculogenesis may lead to pregnancy, vaginal bleeding and loss of efficacy of the oral AI. Biochemical monitoring of oestradiol in exemestane users is problematic because exemestane metabolites cross react in most immunoassays for oestradiol. Following our finding of luteal phase progesterone levels in serum of some postmenopausal women with vaginal bleeding on exemestane, we assessed potential cross reactivity between exemestane and its 4-OH metabolite in the Roche Elecsys® progesterone assay. We also studied the frequency of such progesterone activity in serum from consecutive exemestane users and correlated values with FSH.Methods:Progesterone concentrations were measured with the Elecsys® assay in blanco serum, blanco serum with pure exemestane and blanco serum with the pure 17-OH-exemestane metabolite. Several concentrations of the purifided 17-OH-exemestane metabolite in DSMO solvent were then prepared using the Elecsys Diluent MultiAssay and progesterone concentration was measured in these samples using the Elecsys® assay. Cross sectional blood samples from 94 exemestane users were measured for progesterone activity with this assay and also for FSH using the Roche Modular E170® (F. Hoffmann-La Roche Ltd, Basel, Switserland).Results:Using this progesterone assay, 41% of exemestane users had luteal phase progesterone levels (&gt;1.7 ng/mL). There is cross reactivity with exemestane but more importantly with the 17-OH metabolite (table). The level of progesterone and cross reactivity for 2 known concentrations of the 17-OH-metabolite are also shown in this table. There is a correlation between progesterone picked up by the Elecsys® assay and FSH which increased with progesterone activity.Elecsys® assay RocheProgesterone (ng/mL)Blanco serum0.030Blanco serum + exemestane0.313Blanco serum + 17-OH-exemestane0.781Blanco serum + 2500 ng-ML 17-OH-exemestane16.20.65 %*Blanco serum + 5000 ng ng/mL 17-OH-exemestane32.50.65%/*(*): percentage of cross reactivityConclusion:Progesterone levels in exemestane users measured with the Elecsys® assay from Roche were often overestimated due to cross reactivity with exemestane and its main metabolite. This progesterone assay can not be used to define menopause or distinguish postmenopausal from menstrual bleeding in exemestane users. Higher FSH levels with higher progesterone activity in this assay suggest a link between circulating exemestane/metabolites and the quantitative downregulation of postmenopausal estrogens by this AI, but serial rather than cross sectional measurements are required to confirm this.We acknowledge Pfizer for providing us with pure exemestane and 17-OH-metabolite.
Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5147.