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Selective androgen receptor modulators The discovery and development of selective androgen receptor modulators (SARMs) provides the opportunity to design molecules that are not only orally active, but that target androgen receptors in different tissues to elicit the specific desired activity [223]. The desired activity profile of novel SARMs is described in Table 18.5. An ideal SARM for the treatment of primary or secondary male hypogonadism would have the following profile: orally active, with a pharmacokinetic profile consistent with once a day administration; capable or incapable of stimulating the prostate, seminal vesicles, and other sex accessory tissues at doses equipotent to those needed to provide increases in muscle mass and strength along with fat-free mass; support bone growth; and maintain/restore erectile function, libido, virilization, and male habitus. Unlike testosterone, these SARMs are not substrates for 5a-reductase activity. Other activities that are considered undesirable should be diminished or eliminated, such as potential liver toxicity, blood pressure effects, and fluid retention, induction of gynecomastia, infertility, and overstimulation of erythropoiesis. On the other hand, use of SARMs for selected indications provides the rationale for developing molecules with distinct tissue specificity. For example, if the target is bone growth in elderly men with osteopenia or osteoporosis, but with no overt signs of hypogonadism, a more anabolic SARM with clear effects on bone and muscle, but lesser activity on prostate or other sex accessory tissues, would be more desirable.

The compound 7a-methyl-19-nortestosterone (MENT) has a biopotency per molecule 10 times greater than T [224] and it does not undergo 5a-reduction, which may or may not be an advantage for its effects on the prostate. It retains, however, its capacity to be aromatized to estradiol [225]. It cannot be regarded as a SARM since there is no tissue selectivity based on receptor mechanisms, but it is potentially prostate-sparing. The effects of MENT on the prostate are two to three times as potent as those of T, but studies with MENT given in a dose sufficient to sustain gonadotropin suppression and anabolism (1/10 of the amount of T) showed a 50% loss in

Hormones, Metabolism, Aging, and Men's Health Table 18.5 Desired profile of activity of new selective androgen receptor modulators (SARMs): male applications.

Indications

Tissue/parameter

Hypogonadism

Selected indications*

Prostate/sex accessory tissues

Stimulatory, but less than DHT

Weak or neutral

Libido

Stimulatory

Stimulatory/neutral

Inhibition of gonadotropins

Present

Absent/reduced

Hair growth

Stimulatory

Neutral

Bone growth

Stimulatory

Stimulatory

Muscle mass/strength

Stimulatory

Stimulatory

Fat-free mass

Increase

Increase

Lipids/cardiovascular risk factors

Neutral

Neutral/beneficial

Blood pressure/fluid retention

Neutral

Neutral

Erythropoiesis

Weakly stimulatory

Stimulatory

Liver function (enzyme elevation)

Neutral

Neutral

Breast (gynecomastia)

Neutral

Neutral

* Selected indications may include glucocorticoid-induced osteoporosis, androgen replacement in eldery men, HIV-wasting, cancer cachexia, certain anemias, muscular dystrophies, and male contraception. DHT, dihydrotestosterone. With permission of Negro-Villar A. J Clin Endocrin Metab 1999;84:3459-3462 [224].

* Selected indications may include glucocorticoid-induced osteoporosis, androgen replacement in eldery men, HIV-wasting, cancer cachexia, certain anemias, muscular dystrophies, and male contraception. DHT, dihydrotestosterone. With permission of Negro-Villar A. J Clin Endocrin Metab 1999;84:3459-3462 [224].

prostate volume. MENT is capable of restoring and maintaining sexual behavior in the human male with plasma levels in the order of 15-20% of normal T levels [226]. The pharmacokinetics of MENT administered as MENT acetate subdermal implants has shown that serum MENT levels remain at a steady state level during the four weeks of implant use and are clearly dose-dependent [227].

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