Radioimmunological Analysis of Cyproterone Acetate in Human Serum: Comparison with a gas chromatographic/mass spectrometric method and influence of each method on the outcome of a bioequivalence trial

Abstract

Cyproterone acetate (CAS 427-51-0, CPA) is a steroid hormone with antiandrogenic an progestogenic properties, which has been used in the therapy of prostate carcinoma in men, and for the treatment of severe acne and hirsutism in women. The aim of the present study was to compare two equally sensitive analytical methods, a radioimmunoassay (RIA) and a gas chromatographic/mass spectrometric method (GC/MS), for the quantitative determination of CPA in human serum samples and to assess their suitability for bioequivalence trials. To this end, serum samples which had been collected during a study on the bioequivalence of two CPA-containing formulations (Androcur® 100 and Androcur® 50) were analysed using both methods. Basic pharmacokinetic parameters, like Cmax, tmax, t1/2, and AUC were calculated from each data set and corresponding parameters were compared by statistical methods. A comparison of the drug concentration-time curves obtained with both analytical methods revealed that in particular at later sampling times (48 to 120 h post dose) concentration values generated by RIA were slightly higher by about 20-40% than those measured with GC/MS. This indicates the presence of cross-reacting metabolite(s), most likely the 15β-hydroxy-cyproterone acetate. Accordingly, the values of Cmax, AUC(0-120 h) and AUC were overestimated by the RIA method by about 10-20%, 5% and 10%, respectively. However, there was a high correlation between corresponding parameters derived from RIA and GC/MS analysis. Although AUC values were slightly overestimated when the RIA method was used, this had no influence on the decision about bioequivalence because the mean ratio of the target variables Cmax and AUC was not affected. The variance of the pharmacokinetic parameters which is relevant for the bioequivalence test were even lower for RIA based values than those calculated after GC/MS analysis. In conclusion, it was found that although the GC/MS method is superior to RIA in terms of specificity, both methods were equally suited to demonstrate the bioequivalence of the two CPA-containing formulations. Thus, in future studies of this kind, the more simple RIA may be used instead of GC/MS.