Analysis of estrogens in serum and plasma from postmenopausal women: past present, and future

Abstract

Previous studies have shown that the selection of women who are at high breast cancer risk for treatment with chemoprevention agents leads to an enhanced benefit/risk ratio. However, further efforts to implement this strategy will require the development of new models to predict the breast cancer risk of particular individuals. Postmenopausal women with elevated plasma or serum estrogens are at increased risk for breast cancer. Therefore, the roles of various enzymes involved in the biosynthesis of estrogens in postmenopausal women have been reviewed in detail. In addition, the potential genotoxic and/or proliferative effects of the different estrogen metabolites as risk factors in the etiology of breast cancer have been examined. Unfortunately, much of the current bioanalytical methodology employed for the analysis of plasma and serum estrogens has proved to be problematic. Major advances in risk assessment would be possible if reliable methodology were available to quantify estradiol and its major metabolites in the plasma or serum of postmenopausal women. High performance liquid chromatography (HPLC) coupled with radioimmunoassay (RIA) currently provides the most sensitive and best validated immunoassay method for the analysis of estrone and estradiol in serum samples from postmenopausal women. However, inter-individual differences in specificity observed with many other immunoassays have caused significant problems when interpreting epidemiologic studies of breast cancer. It is almost impossible to overcome the inherent assay problems involved in using RIA-based methodology, particularly for multiple estrogens. For reliable measurements of multiple estrogens in plasma or serum, it will be necessary to employ stable isotope dilution methodology in combination with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Extremely high sensitivity can be obtained with pre-ionized estrogen derivatives when employed in combination with a modern triple quadrupole mass spectrometer and nanoflow LC. Using [(13)C(6)]-estrone as the internal standard it has proved possible to analyze estrone as its pre-ionized Girard T (GT) derivative in sub-fg (low amol) amounts on column. This suggests that in the future it will be possible to routinely conduct LC-MS assays of multiple estrogen metabolites in serum and plasma at even lower concentrations than the current lower limit of quantitation of 0.4pg/mL (1.6pmol/L). The ease with which the pre-ionization derivatization strategy can be implemented will make it possible to readily introduce high sensitivity stable isotope dilution methodology in laboratories that are currently employing LC-MS/MS methodology. This will help conserve important plasma and serum samples as it will be possible to conduct high sensitivity analyses using low sample volumes.

Figure 1

Biosynthesis of estrogens from circulating C-19 androgens arising from cholesterol metabolism in the adrenal cortex of menopausal women.

Figure 2

Enzymes involved in estradiol metabolism.

Figure 3

Enzymes involved in estrone metabolism

Figure 4

Derivatives used to enhance the ionization efficiency of estrogens in order to improve sensitivity for GC-MS/MS and LC-MS/MS analysis.

Figure 5

Analysis of estrone in the range 0.625 fg to 10.00 fg (2.3 amol to 37.0 amol) on columns as the GT derivative using LC-multiple reaction monitoring (MRM)/MS. Chromatography was performed using a Halo C18 column (150 × 0.1 mm × id, 2.7 μm, 90 Å; Advanced Materials Technology, Wilmington, DE) using a linear gradient of water/acetonitrile at a flow rate of 1,000 nL/min. MS was conducted using a Thermo Analytical Vantage triple stage quadruple mass spectrometer. MRM/MS was conducted on the following ions m/z 384 (M⁺, estrone) → m/z 157 and m/z 390 (M⁺, [¹³C₆]-estrone)→ m/z 157. Data points represent the means ± SEM (n=3).