Development and evaluation of gonadotropin releasing hormone antagonists as SPECT radiotracers

Abstract

The overall aim of the thesis was to evaluate the potential of gonadotropin releasing hormone receptor (GnRH-R) antagonists as radiotracers for the early detection and understanding of Alzheimer`s disease (AD). The molecular imaging field is a symbiosis between organic chemistry, radiochemistry, in vitro biology, as well as preclinical- and clinical trials, therefore an evaluation should involve all respective subfields. In light of this, the development of a new radiotracer is often a time- consuming and laborious process. Consequently, this thesis represents a highly collaborative project involving various subfields and their interplay in the field of molecular imaging. Initially a series of GnRH-R antagonists were synthesized and evaluated for affinity for the GnRH-receptor. A competitive binding study was performed and the synthesized GnRH-R antagonists were compared to control compounds. By the use of a copper mediated late-stage iodination, radiolabeling was further investigated. In addition, an acylation Finkelstein approach was examined by a thorough mechanistic analysis using both experimental and computational methods. The in vitro analyses of two promising GnRH-R antagonists were performed, where human and rat serum stability, a competitive inhibition assay, a metabolic profile (phase 1) and autoradiography evaluated the potential these compounds exhibit as radiotracers. Furthermore, preclinical evaluation of a α-halogenated compound was performed on the basis of literature findings and serum stability analysis. In summary, the project has resulted in several potential GnRH-R antagonist radiotracers. Although more analyses need to be performed, one compound has already been evaluated in preclinical trials and excluded from further investigation. Two more compounds have been radiolabeled and assessed thoroughly in vitro. The compounds showed good affinity for the GnRH-R. Serum stability analysis and the metabolic profiling showed stable compounds for imaging. In addition, several metabolites were identified and the metabolic rate was quantified. Furthermore, autoradiography in rat brain showed binding, which was in compliance with binding affinities for the compound. These new GnRH-R antagonists possess high potential as GnRH- antagonistic radiotracers for the pathophysiologic examination of AD and warrant further preclinical evaluation.