Analytical Interference of Exemestane With Androstenedione Immunoassays

Exemestane, an aromatase inhibitor commonly used for breast cancer treatment, shares structural similarities with sex steroids analyzed in clinical laboratories. We aimed to investigate the influence of exemestane cross-reactivity in the measurement of sex steroids across various immunoassays.

We conducted a multicenter study involving measurements of androstenedione, testosterone, estradiol, progesterone, and 17-hydroxyprogesterone in serum samples from women undergoing exemestane therapy (N=15; 25 mg/day). Measurements were performed using liquid chromatography-mass spectrometry (LC-MS) and various commercially available chemiluminescence immunoassays, ELISA, and radioimmunoassay. In-vitro cross-reactivity was assessed by adding exemestane and 17-hydroexemestane to serum samples.

Patients undergoing exemestane therapy had markedly falsely elevated androstenedione results in all immunoassays evaluated (N=4), which correlated with serum exemestane levels. In-vitro experiments confirmed this interference to be caused by cross-reactivity with exemestane. Additionally, one immunoassay yielded falsely elevated estradiol results in 20% of patients. However, in-vitro experiments did not confirm this to be caused by cross-reactivity with exemestane or 17-hydroexemestane.

Exemestane cross-reacts with androstenedione immunoassays, causing falsely elevated results in treated patients. This analytical interference may raise unnecessary concerns, leading to expensive diagnostic workups.