DRUGS USED IN THE TREATMENT OF IMPOTENCE
Till
recently, there was no really satisfactory method or drug available to
effectively treat male impotence or erectile dysfunc-tion. The following were
tried with varying degrees of success: intracavernosal injection of vasoactive
agents, transurethral delivery of prostaglandin E1, implantation of penile
prosthesis, and venous or arterial surgery. Oral therapy was almost
non-existent (except for tentative trials with apomorphine and phen-tolamine),
until the arrival of sildenafil.
Today, this drug has become one of the largest selling drugs in the world,
since it is claimed to be very effective.
Sildenafil
was developed by Pfizer initially as an anti-anginal and antihypertensive
agent, but as its reputation increased with respect to enhanced sexual
performance among male subjects, the manufacturers decided to switch tracks and
began to adver-tise it as an anti-impotence pill. Since then there has been no
looking back, and sales of sildenafil have soared all over the world, so much
so that it has become the biggest ever grosser among all pharmaceutical
preparations.
·
Male
erectile dysfunction: Sildenafil is effective only
whenthere is at least partial erection. It is not effective in the presence of
severe arteriogenic or venogenic impotence, corporal smooth muscle fibrosis,
and anatomical deformi-ties of the penis. The drug has to be taken an hour
before anticipated sexual activity.
·
Female
impotence: Trials are on to test the efficacy of sildenafil in the
treatment of female impotence. Initial reports are said to be encouraging.
Sildenafil
is rapidly absorbed after oral administration, the absolute bioavailability
being about 40%. Maximum plasma concentrations are achieved in about one hour,
with mean terminal half-life of 3 to 5 hours. It is metabolised by hepatic
microsomal enzyme systems—the CYP3A4 route mainly, and to a lesser extent by
the CYP2C9 route. Major excretion occurs in the faeces (80%) and urine (13%).
Sildenafil
is a selective inhibitor of cyclic GMP specific phos-phodiesterase (PDE) type
5, the predominant enzyme which metabolises cyclic GMP in the corpus
cavernosum. When sexual excitement causes local release of nitric oxide,
inhibi-tion of PDE5 by sildenafil results in increased levels of cyclic GMP,
which leads to smooth muscle relaxation and inflow of blood into the corpus
cavernosum.
Sildenafil
also inhibits PDE6, which is found in retina and is believed to be involved in
phototransduction. This explains the colour vision abnormalities encountered at
high doses of the drug.
·
General:
headache, facial flushing, dyspepsia, and diar-rhoea.
·
CVS:
sudden drop in blood pressure (especially if the patientis on nitrate therapy).
·
Visual:
light sensitivity, bluish tinged vision.
·
Chest pain, stroke, cardiac
arrhythmias, heart failure.
·
Deaths have been reported in elderly
males who participated in sexual activity after ingesting sildenafil. In most
of these
Since there are indications of sildenafil misuse
among the youth (who do not really require the drug), in the form of a
recreational drug, there are concerns about acute toxicity, especially in the
presence of concomitant intake of amyl nitrite. The latter is not uncommonly
abused for its “high” during the course of rave
parties.
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