All currently
marketed combination OCs are
composed of a synthetic estrogen plus a progestin. The progestin component provides
most of the contraceptive
protection while the estrogen provides cycle control
and boosts the contraceptive effectiveness of the progestin.
All but one
of the synthetic progestins currently marketed in the United States in hormonal
contraceptives are derivatives of
either 19 nor testosterone or 17 a acetoxyprogesterne . The derivatives of
19 nor testosterone are either estranes or gonanes. The original
OC containing northynodrel an estrane derivative of 19 nor
testosterone is no longer marketed
but the estrane nor ethindrone and its
derivatives along with levonorgestrel and other gonane derivatives
are used in currently marketed
formulations .
The goname
derivatives have greater progestational activity per unit weight than estranes
Modification in the chemical structure
of goname derivatives resulted
in compounds that have altered
biological activity. The magnitude of difference
in androgenic and progestational
effects produced by each
progestin is called
selectivity . the so called third
generation progestins Norgestimate
desogestrel and Gestodene derived from
the gonane norgestrel have high selectivity and demonstrated high progestational activity
and low androgenic activity
when compared with the other gonanes. The
OC formulations containing
desogestrel nor gestimate and Gestodene
are called third generation OCs. The implant
contain etonogestrel a metabolic
of desogestrel.
The newer
selective progestins including DRSP do not counter the effects
of the estrogen component as strongly as the older progestins and are associated with higher
sex hormone binding globulin and
other liver globulins selective
progestin. Adjustment of the new
progestin products with a lower
dose of estrogen is being studied
and may result in a better safety profile.
Only two
estrogen are used in OCPs in the United
States. The so called first generation OCs contain
50 ug of either EE or menstranol . The second generation
OCs contain 20-35 ug of EE OCs
containing one of the three
newer gonane progestins and are called third generation OCs .
All the
synthetic estrogens and progestins in
OCs have an Ethinyl group at position
C17. The presence of this Ethinyl
group enhances the oral activity
of these agents because they are
not as rapidly metabolized as they pass through the intestinal mucosa and the liver through the portal
system EE has about 100
times the potency of an equivalent weight
of conjugated equine estrogen
or estrone sulfate for stimulating synthesis
of hepatic proteins.
The two
estrogens used in OCs , EE and
its 3 methyl either menstranol have different biological potency . Mestranol must be demethylated to EE
to bind to the estrogen cytosol
receptor and become biologically
active. The degree of conversion
of menstranol to EE varies among
individuals although overall EE is
about 1.7 times as potent as the same
weight of mestranol
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