Nomegestrol Acetate: A Progestin that Deserves Recognition

Re: Seeger H, Wallwiener D, Mueck AO. The effect of progesterone and synthetic progestins on serum- and estradiol-stimulated proliferation of human breast cancer cells. Horm Metab Res 2003; 35 : 76 - 80

The recent article by Seeger et al. [1] is a welcome addition to a controversial area. The results from the Women's Health Initiative study [2] and more recently from the Million Women Study [3] on enhanced breast cancer risk in postmenopausal women using an estrogen/progestin combination clearly indicated the need for a comparison of different progestins with regard to cancer risk.

Seeger et al. [1] studied the effects of progesterone (P) and various synthetic C19- and C21-progestins on the proliferation of MCF-7 breast cancer cells in vitro. The authors investigated P, chlormadinone acetate (CMA), dienogest (DNG), gestodene (GSD), 3-ketodesogestrel (KDG), levonorgestrel (LNG), medroxyprogesterone acetate (MPA), and norethisterone (NET). They tested the progestins with or without E₂ in ”estrogen-free” conditions. Tested alone, CMA, DNG, GSD, KDG, MPA and NET significantly stimulated cell proliferation, but only at high dosages. Sequentially combined with E₂, only CMA inhibited cell proliferation over the whole concentration range. Slight effects were found for DNG, GSD and KDG, whereas P and MPA only showed any effect at the highest concentration. NET had no significant effect on cell proliferation. Continuously combined with E₂, all progestins exhibited an inhibitory effect over the whole concentration range. So, while expending the types of progestins investigated, Seeger et al. studied the effects of progestins derived from P- and from 19-nor-testosterone (19-nor-T). It would have been interesting to include some 19-nor-P derivatives in this excellent study as well.

We would like to broaden the circle further with our knowledge on a progestin belonging to this subgroup, nomegestrol acetate (NOMAC). Actually, progestins can be classified into 3 different subgroups according to their chemical structure as pregnane derivatives such as MPA, CMA, megestrol acetate (MEG), dydrogesterone, 19-nor-T derivatives such as NET and its acetate (NETA), norethynodrel (NEL), LNG (and norgestrel), GSD, DNG, KDG, and 19-nor-P derivatives such as ORG2058, promegestone (often referred to as R5020), trimegestone and NOMAC. 19-nor-P derivatives are structurally different from 19-nor-T derivatives since they all have the C21 carbon of P in the 17β position.

In growth assays on MCF-7 cells, Catherino and Jordan showed that E₂, norgestrel and LNG were able to stimulate proliferation, an effect that was not triggered by NOMAC, R5020, MEG or MPA [4]. In an other study on MCF-7 cells, we found that NOMAC alone or with E₂ had no effect on MCF-7 cell growth [5]. In a different study performed in weak estrogenic conditions, in the presence of phenol red (EST), we compared the effect of NOMAC and MPA with two plating densities and we showed NOMAC and MPA to be equipotent in inhibiting cellular proliferation in both cases.

Schoonen and co-workers tested 8 progestins on 3 different sublines of MCF-7 cells and showed that NET, GES, LNG and KDG were always promoting cell growth while P, MPA, R5020 and ORG2058 were not [6].

Since they contain high levels of P receptor, the T47-D human breast cancer cells are often used in proliferation studies on progestins. In T47-D cells, E₂ and norgestrel were found to stimulate proliferation, while NOMAC, R5020, MPA and MEG had only a meager growth stimulatory capability at only 1 µM. However, the antiestrogen ICI-182,780, which was able to inhibit E₂-induced proliferation, had no effect on the induction elicited by either progestin types [4].

Using EST-T47-D cells, we showed that NOM, ORG2058, R5020, MPA and NETA were able to induce a dose-dependent inhibition proliferation [8]. This effect by progestins was obtained at very low concentrations (1 nM) and remained relatively weak (50 % inhibition) while addition of E₂ did not modify the growth any further.

Schoonen and co-workers also tested progestins on T47-D cells, showing that NET, GES, LNG and KDG stimulated cellular proliferation at 0.01 µM and reached at 1 µM a level comparable to that of E₂. The pregnanes P, MPA, R5020 and ORG2058 at 1 µM had a slight effect of 15 - 20 % that of E₂, while the MPA effect appeared to be a little stronger [7].

We also made use of T47-D cells and found that at 1 µM, NET, NEL, and LNG were able to induce a very modest but significant increase in cell proliferation when NOMAC, ORG2058, R5020 and MEG were completely devoid of such a growth-promoting potential, where all progestins, inducing proliferation or not, were able to inhibit the E₂-induced growth. The antiprogestin RU486 was completely able to prevent the inhibition of E₂-induced proliferation elicited by NOMAC and R5020, suggesting a PgR-mediated inhibition of cell growth. To guarantee that the above observed effects on MCF-7 or T47-D proliferation were indeed an estrogen receptor-mediated mechanism, transient transfection studies were performed using a luciferase reporter construct containing three tandem copies of the Xenopus vitellogenin A2 estrogen response element. Since the PgR is not able to interact with and stimulate transcription from an estrogen response element, stimulation of luciferase by compounds represents an estrogen receptor mediated event [4]. In MCF-7 and T47-D cells, E₂, norgestrel and RU486 were able to stimulate transcription, while NOMAC, MEG, MPA and R5020 did not acted as estrogens. Moreover, transcriptional stimulation induced by the estrogenic compounds was blocked by ICI 182,780. These results demonstrated that NOMAC was unable to act as an estrogen in MCF-7 or T47-D cells, while norgestrel and RU486 did show such activity on both cell lines.

It has been demonstrated that 19-nor-T derived progestins are biotransformed into several metabolites that exhibit altered hormone properties in target tissues - A-ring reduction of NET, GSD, or LNG produces the corresponding 5α-dihydro and 3β,5α-tetrahydrometabolites that bind to the ER and exert in vivo estrogenic effects [9]. In this regard, only C-19 steroids with a hydroxyl group at C-3 and C-17 have any significant affinity for ER. Computer-based quantitative structure-activity relationship studies of ligand-receptor interactions revealed that ER is sensitive to the addition of steric bulk at the 17-position on the steroid ring [10]. They also showed that the molecular changes elicited by the 19-nor substitution might increase the mobility and electronic density of the A-ring allowing the alignment of the 3ß-hydroxy group and the A-ring hydrogen atoms resembling an estrogen-like environment.

19-nor-progestins such as NOMAC differ from 19-nor-T-derivatives in many respects, beginning with their inability to be metabolized to compounds binding to the ER. They deserve to be considered as a well-identified different group of progestins. Notably, the most widely prescribed HRT regimens in France combine transdermal E₂ with micronized P or progestins such as dydrogesterone, R5020, CMA and NOMAC, medrogestone, while MPA is used in less than 3 % of all cases.

References

• 1 Seeger H, Wallwiener D, Mueck A O. The effect of progesterone and synthetic progestins on serum- and estradiol-stimulated proliferation of human breast cancer cells.  Horm Metab Res.. 2003;  35 76-80
• 2 Writing group for Women’s Health Initiative Investigators . Risks and benefits of estrogen plus progestin in healthy postmenopausal women. Principal results from the Women’s Health Initiative Randomized controlled trial.  JAMA. 2002;  288 321-333
• 3 Million Women Study Collaborators . Breast cancer and hormone-replacement therapy in the Million Women Study.  The Lancet. 2003;  362 419-427
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• 6 Schoonen W GEJ, Joosten J WH, Kloosterboer H J. Effects of two classes of progestagens, pregnane and 19-nortestosterone derivatives, on cell growth of human breast tumor cells: I. MCF-7 cell lines.  J Steroid Biochem Mol Biol. 1995;  55 423-437
• 7 Schoonen W GEJ, Joosten J WH, Kloosterboer H J. Effects of two classes of progestagens, pregnane and 19-nortestosterone derivatives, on cell growth of human breast tumor cells: II. T47D cell lines.  J Steroid Biochem Mol Biol. 1995;  55 439-444
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