Endocrine Abstracts

Mifepristone (RU38486) is a progesterone and glucocorticoid receptor antagonist which is used clinically to induce medical abortion, and as a research tool, e.g., in metabolic syndrome and depression. We aimed to develop a sensitive method to detect and quantify mifepristone in human plasma.

Analyses were developed using a Surveyor HPLC system, with a mobile phase of methanol:ammonium acetate (5 mM) (70:30; 0.4 ml/min) and a C18 column (5 μm; 2.1×50 mm). The eluate was delivered into a TQ14 Quantum Discovery triple quadrupole mass spectrometer in positive electrospray ionisation mode. Mass spectra were gathered by selective reaction monitoring using argon (collision) and nitrogen (sheath, auxiliary) gases.

The major ion formed from mifepristone was the protonated molecular ion, with m/z 430. Under collisional activation (30V), the major fragment ion was m/z 372, corresponding to loss of the hydroxyl and propionyl group from C17. Alfaxalone was used as an internal standard and yielded a protonated molecular ion at m/z 333 and a major breakdown ion, m/z 297.

Mifepristone was extracted in diethyl ether from plasma (containing 100 ng alfaxalone) of healthy male subjects (18-31y; n=5, BMI 23.1 (S.D. 2.4) kg/m², WHR 0.88 (S.D. 0.08)) 12 h post-dose (200-800 mg po). Recovery of mifepristone and alfaxalone were 30±1% and 70±2%, respectively. The standard curve was linear over the range 0.5–500 ng (r>0.98). Inter-injection reproducibility was 7.1%CV (n=3) and intra-assay reproducibility was 8.3%CV (n=3). The limit of quantitation was 1 ng injected in 10 μl, allowing mifepristone to be detected in plasma (100 μl) at concentrations ranging between 2–20 μM.

LC/MS/MS with electrospray ionisation is a robust method for quantitative analysis of mifepristone in small volumes of plasma.