Interpretation of results

For androgen replacement therapy and male hormonal contraception there is a need for orally active androgens which can substitute the total spectrum of physiological effects of testosterone, preferably at a lower dose. In the search for new orally active androgens, a combination of in vitro and in vivo assays has been used with the aim to select androgens with higher androgen receptor affinity and less metabolic instability than the reference androgen, testosterone. Additional prerequisites of new androgens are some aromatization towards estrogenic metabolites and the absence of androgen receptor activation upon 5a-reduction (like testosterone) to obtain a prostate-sparing effect.

We exploited the fact that nandrolone and 7a-methyl nandrolone (MENT) have 3- and 10-fold increased androgen receptor binding affinity as compared to testosterone, respectively (Bergink etal. 1985; Kumar etal. 1999). However, these andro-gens have not been developed further for oral application due to their limited oral efficacy. The oral efficacy of MENT was expected since both high androgen receptor activity and high metabolic stability have been reported using rat liver microsomes (Agarwal and Monder 1988). Our metabolic stability assay, using intact human hep-atocytes, indicates that there is no difference in the metabolism between nandrolone, MENT and the metabolically unstable reference androgen, testosterone. A plausible explanation for this difference is that the cofactor (NAD) for the most important androgen inactivating enzyme, the microsomal enzyme 17^-hydroxysteroid dehydrogenase type 2, is not added to the microsomal preparation (Puranen etal. 1999). In addition, the low metabolic stability of MENT is also more in line with the pharmacokinetic data in different species and the relative low oral efficacy of MENT in the LH-suppression test using castrated male rats. Our hypothesis was that 7a-alkyl elongation/unsaturation of nandrolone would improve metabolic stability while maintaining androgen receptor affinity. Indeed, 7a-ethylnandrolone is a metabolically relative stable androgen having a good androgen receptor binding affinity. This androgen resulted in good LH suppression in the rat, 4-fold more potent than MENT. Since the pharmacokinetic data in rat, rabbit and monkey indicated low exposure after oral application, this compound was not evaluated further.

Of all nandrolone derivatives tested, OrgXis the most metabolically stable androgen. The excellent in vitro data of OrgX (high receptor activity and metabolic stability) was confirmed by several in vivo tests. After a single oral dose of Org X pharmacokinetic evaluation revealed that both in the rabbit and monkey good kinetics were found. In general, the more or less similar pharmacokinetic behaviour of steroids after oral application in the rabbit and monkey differs from that in the rat. Efficacy studies also indicated that OrgX is a potent oral androgen. In the LH-suppression test using male castrated rats, the oral activity of Org X is 0.3 mg/kg, which is 200, 100 and 45-fold better than the oral efficacy of testosterone, testosterone-undecanoate and MENT, respectively. In the rat osteoporosis test, Org X is around 40-fold more active than testosterone in maintaining trabecular BMD at intact levels. A similar increase in efficacy of Org X for LH/FSH suppression was observed. At this BMD/LH/FSH replacement dose of Org X a clear prostate-sparing effect was observed. The high potency of Org X was confirmed in intact male monkeys. A once-daily dose of only 8 |xg/kg was needed for 60% suppression of endogenous testosterone. Even at this low dose, trough levels (24 hours after application) of 1.8 nmol/l of Org X were found. The relatively low efficacy of 17a-methyltestosterone in the rat LH suppression test was confirmed by the relative high dose (666 |xg/kg) required for testosterone suppression in the male monkey (Ubink et al. 2003). On the basis of ventral prostate weight stimulation in rats, a 50-fold higher potency of 17a-methyltestosterone compared with testosterone was expected (Segaloff 1963). These data indicate that LH suppression in rats is far more indicative of the required profile of a new androgen than ventral prostate stimulation.

22.6 Key messages

• 7a-alkylation of nandrolone increases the AR agonistic activity.

• Org X is a novel 7a-alkylated nandrolone derivative.

• 7a-ethylnandrolone and Org X are metabolically stable compounds as determined after incubation with human hepatocytes.

• 7a-substituted nandrolone derivatives and Org X can be converted in vitro by recombinant human aromatase to active estrogens, just like the natural androgen testosterone.

• Org X is around 40 times more potent than testosterone undecanoate in the castrated male rat in maintaining trabecular mineral bone density, LH and FSH suppression and anabolic effects. At the bone-protecting dose, a prostate-sparing effect of Org X was observed.

• With a once daily oral dose for seven days of 8 ^g/kg of Org X suppression of endogenous plasma testosterone in intact male monkeys was observed.

• Pharmacokinetic data obtained after one single oral dose revealed no differences in the rat between the different nandrolone derivatives.

• Pharmacokinetic parameters after single oral application of the different nandrolone derivatives and reference compounds revealed that the rabbit is more predictive for the monkey than the rat.

• LH suppression in the rat is more indicative of the required profile of a new androgen than ventral prostate stimulation.


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