The Journal of Steroid Biochemistry and Molecular Biology
7α-methyl-19-nortestosterone, a synthetic androgen with high potency: structure-activity comparisons with other androgens
Introduction
Steroid hormone receptors have pronounced structural requirements for their ligands as shown by specificity of the response and correlation between receptor binding affinity and bioactivity [1], [2], [3]. This appreciation led to the development and clinical use of a series of synthetic corticosteroids, progestins and estrogens with higher potency than the natural hormones. However, with respect to androgens, no significant progress has been made in developing high potency compounds. An improvement in therapeutic utilization has been brought about mainly by esterification of testosterone (T) molecule and by depot preparations [4], [5]. As early as 1960, marked increase in the androgenic potency was demonstrated for the 7α-methyl derivatives of 19-nortestosterone [6], [7], [8]. However, due to weak activity when given orally, no serious attempts were made to utilize these androgens. Avery et al. [9], reported the synthesis of 7α-methyl-17aβ-propionyloxy-d-homoestra-4,16, dien-3-one, which showed high androgenic activity parenterally and also exhibited an improvement in oral activity. Our studies have shown that 7α-methyl-19-nortestosterone (MENT), a potent androgen could be a better androgen for hormone replacement therapy and male contraception when given parenterally as sustained release formulations [10], [11], [12].
In general, the high potency of 7α-methylated androgens was shown to correlate with their higher binding affinity to androgen receptors [8], [13]. However, other reports showed no correlation between the receptor binding affinity and bioactivity of some 7α-methylated androgens [9], [14], [15]. Thus it was suggested that structural modifications of a compound can lead to changes in its biological activity based on binding affinity, differences in absorption, binding to plasma proteins and/or susceptibility to the action of metabolizing enzymes. The slower metabolism of a synthetic steroid may lead to an increase in its half life in circulation, thus affecting its uptake by target tissues and bioactivity. The activity of a steroid can also be modulated at the level of a target organ by tissue-specific metabolism as is the case with T and 19-nortestosterone (19-NT) [16]. For example, androgenic activity of T is amplified whereas that of 19-NT is reduced upon 5α-reduction in the sex accessory glands. In contrast, their anabolic potency on muscle, where they are not 5α-reduced, is not affected [17].
The present study was undertaken to understand the mechanisms involved in the differences in the biopotency of some of the natural and synthetic androgens. Dose-response curves of their androgenic and anabolic activity in vivo were compared with their relative binding affinities to androgen receptor, sex hormone binding globulin and their in vitro activity in transient transfection assays.
Section snippets
Reagents
Chemicals and solvents were of reagent grade. 7α-methyl-19-nortestosterone (MENT), 7α-cyano-19-nortestosterone (CNNT) and 7α-acetylthio-19-nortestosterone (ATNT) were custom synthesized by SRI International (Menlo Park, CA, USA). The 7β-methyl-19-nortestosterone was kindly made available by Dr Richard P. Blye, Contraceptive Development Branch, Center for Population Research, NIH, Bethesda, MD, USA. Flutamide was a gift from Dr R. Neri, Schering Plough, Kenilworth, NJ, USA. Testosterone (T),
Binding to AR
The binding affinity of androgens to AR was assessed using [3H]-Mibolerone as a tracer. The displacement curves of different androgens are shown in Fig. 1. A comparison of the relative binding affinities showed that the affinity of MENT (IC50=1.82 nM) for AR was the highest followed by DHT (2.39), 19-NT (3.01), T (6.95), DHMENT (24.98) and DHNT (94.74) (Table 1). Thus 19-NT had a 2-fold and MENT almost a 4-fold higher RBA than T. However, the 5α-reduced derivatives of both 19-NT (DHNT) and MENT
Discussion
The present studies demonstrate that the mechanisms which determine the relative biopotency of T, 19-NT and its derivatives not only involve differences in their binding affinities to AR, but also their metabolism in the target tissues which leads to the formation of compounds with greater or lesser activity than the parent compound.
It is well established that T is converted to DHT in the sex accessory glands, and since the binding affinity of DHT to AR is 3–5 times higher than that of T, its
Acknowledgements
We wish to acknowledge the generous support received from US Agency for International Development (Cooperative agreement CCP-A-00-94-00013-04) and NICHD, NIH for Grants HD-29990, HD-13541 and RO1 DK52960 (JFC). We also wish to thank Ms Irene Gorzelany Mr M. Willman and Mr W. DeJesus for technical assistance.
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