Metabolism of Synthetic Steroids by Animals and Man

Abstract

The metabolism of naturally occurring oestrogens in humans is outlined and compared with that in other species and with the metabolism of synthetic oestrogens. There is a close similarity in humans between the metabolism of the naturally occurring oestrogens and that of ethynyl \x=req-\ oestradiol with the exception that the latter is less extensively metabolised than the former. With the exception of the baboon, the metabolism of oestrone and oestradiol appears to differ significantly in other species from that of the human. Our knowledge of the metabolism of oestrone and oestradiol and the synthetic oestrogens in the baboon is still meagre and much more information is required before one can ascertain to what extent metabolism in this species can serve as a model for that in humans.Little information is available concerning the metabolism of synthetic progestogens in species other than man and rabbits. There are many similarities between these two species both in the metabolism of progesterone and of synthetic progestogens related both to 19-nortestosterone and to 17\g=a\-hydroxyprogesterone. Insufficient information is available to know how far these similarities extend.Little is known about the in vitro metabolism of either the synthetic oestrogens or synthetic progestogens. Such information as is available does not suggest that in vitro metabolic studies, with their well-known limitations, are likely to be helpful. In vivo metabolism of oestrogensThe metabolism in humans of the naturally occurring oestrogens is briefly outlined to aid comparison with that in other species and also with the meta¬ bolism of synthetic oestrogens.A. Metabolism of oestrone and oestradiol in humans The major pathways for the metabolism of oestrogens are outlined in Fig. 1.Oestrone and oestradiol are interconvertible by 17/3-hydroxysteroid oxidoreductase, the dehydrogenation of oestradiol to oestrone being more rapid than the reduction of oestrone to oestradiol so that the equilibrium between oestrone and oestradiol favours oestrone. Although a 17a-hydroxysteroid oxidoreductase is present in man (Breuer 8c Schott 1966), 17a-oestradiol is not an important metabolite but it is important in some species. One of the main metabolic pathways for oestrone is 16a-hydroxylation leading to 16a-hydroxyoestrone and oestriol. Some of the oestriol produced is oxidized to 16-oxo-oestradiol which can undergo reduction to 16/?-hydroxyoestradiol (16-e/«oestriol). The second major pathway appears to be hydroxylation at C-2 with the formation of 2-hydroxyoestrone which is readily converted to 2-methoxyoestrone by an 0-methyl transferase. Demethylation of 2-methoxyoestrogens occurs to only a slight extent. It seems likely that oestrone sulphate rather than oestrone is the precursor for the hydroxylated metabolites found in urine (Fishman 8c Hellman 1973). Hydroxylation may also occur at C-6. Oestradiol -17 a Ha^/V^" HC 2-Hydroxyoestrone 16a-Hydroxyoestrone lóB-Hydroxyoestrone * OH O Me O HO OH -OH 2-Methoxyoestrone Oestriol Fig. 1.Metabolism of oestrog...

Alterations in prednisolone disposition as a result of time of administration, gender and dose.

“…Thus for the low dose, cortisol is able to displace prednisolone from binding sites but this effect is a weak one, however at the high dose cortisol does not displace prednisolone from plasma binding sites (Figures 5 and 6a). Cortisol and prednisolone differ in structure only by the presence of a double bond in ring A and are thought to undergo a number of common metabolic transformations (Fotherby & James, 1972). Competitive inhibition of one or more of these metabolic transformations may be the mechanism of the circadian reduction in the clearance of unbound prednisolone at low doses.…”

Alterations in prednisolone disposition as a result of oral contraceptive use and dose.

“…In addition oestrogens are known to reduce the clearance of unbound cortisol (Brien, 1980). a compound which has a close structural similarity to and undergoes parallel metabolic transformations with prednisolone (Fotherby & James, 1972). While these studies were in progress Boekenoogen et al (1983) reported a 50% reduction of free prednisolone clearance following 40 mg prednisolone doses in women taking oral contraceptives compared to control women.…”

Plasma concentrations of ethynyl oestradiol and norethindrone after oral administration to women

“…Studies on the metabolism of these compounds in women have demon¬ strated that the ethynyl group in norethindrone remains intact (Kamyab, Fotherby & Littleton, 1967 ; Kamyab, Fotherby & Klopper, 1968 ;Fotherby, 1974), but that the ring A is transformed into several reduced products, i.e. 17rz-ethynyl-17ß-hydroxy-5a-oestran-3-one and 17ct-ethynyl-5a-oestrane-3ß,17ß-diol (Murata, 1968), 17o>ethynyl-5a-oestrane-3a,17ß-diol and 17a-ethynyl-5ß-oestrane3a,17ß-diol (Gerhards, Hecker, Hitze, Nieuweboer & Bellmann, 1971 ; Stillwell, Horning, Horning, Stillwell & Zlatkis, 1972).…”