Absence of Immunoreactive Luteinizing Hormone following Gonadotropin-Releasing Hormone Agonist Therapy in Women with Endometriosis

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Abstract
Pituitary desensitization following infusion of gonadotropin-releasing hormone (GnRH) is measurable if bioactivity instead of immunoreactivity is considered. We hypothesized that GnRH agonist therapy induces the same kind of desensitization, but that radioimmunoassays (RIA) for gonadotropins based on polyclonal antibodies cannot show this effect because they recognize inactive fragments of gondadotropins. To test this hypothesis we measured luteinizing hormone (LH) with two different assays: one RIA was based on a polyclonal rabbit anti-hLH, while the other one was an immunoradiometric assay (IRMA) based on 2 different mouse monoclonal anti-hLH. LH measurements were performed on plasma samples obtained from 13 women with laparoscopically proven endometriosis and treated with microcapsules of the GnRH agonist D-Trp6-GnRH (Ferring) once a month. The correlation between LH measurements with both assays in 36 control plasma samples and in another 13 samples obtained before treatment in women with endometriosis was excellent (r = 0.959). In contrast, in women treated with GnRH agonist, the RIA yielded values ranging from undetectable to 12 mIU/ml, whereas 60 out of 66 values were undetectable with the IRMA. We conclude that the monoclonal anti-hLH antibodies in the IRMA either recognize an epitope close to the active site and/or do not recognize the biologically inactive LH fragments which are known to be produced during GnRH agonist therapy. Thus, monoclonal-antibody-based IRMA provide a new and interesting clinical tool to follow the effects of therapies which desensitize the gonadotropic function of the pituitary.