Tamsulosin

Absorption

Oral tamsulosin is 90% absorbed in fasted patients. The area under the curve is 151-199ng/mL*hr for a 0.4mg oral dose and 440-557ng/mL*hr for a 0.8mg oral dose. The maximum plasma concentration is 3.1-5.3ng/mL for a 0.4mg oral dose and 2.5-3.6ng/mL for a 0.8mg oral dose. Taking tamsulosin with food increases the time to maximum concentration from 4-5 hours to 6-7 hours but increases bioavailability by 30% and maximum plasma concentration by 40-70%.

Toxicity

In the event of overdose, patients may experience hypotension and should lie down in a supine position to maintain blood pressure and heart rate. If further measures are required intravenous fluids should be considered. If further progression is required, vasopressors may be used and renal function should be monitored. Dialysis is unlikely to assist in treating overdose because tamsulosin is extensively protein bound.
The oral LD50 in rats is 650mg/kg.
Tamsulosin is not indicated for use in women and no studies have been performed in pregnancy, though animal studies have not shown fetal harm. Tamsulosin is excreted in the milk of rats but there is no available data on what the effect of this tamsulosin exposure may be. Animal studies have shown male and female rat fertility is affected by tamsulosin due to impairment of ejaculation and fertilization. In men, tamsulosin is associated with abnormal ejaculation. Tamsulosin is not mutagenic but may be carcinogenic at levels above the maximum recommended human dose. Female rats experience a slight increase in the rates of mammary gland fibroadenomas and adenocarcinomas.

Description

Tamsulosin hydrochloride is the first in the class of potent and selective α- 1A adrenoceptor antagonists introduced for the treatment of dysuria associated with benign prostatic hypertrophy (BPH). In a clinical study, significant improvement in irritative and obstructive symptoms has been reported for tamsulosin treated patients with BPH. ln vitro studies in human penile erectile tissue and vas deferens indicated that tamsulosin may be of use in treating male sexual dysfunction.

The Uses of Tamsulosin

tamsulosin

Indications

Tamsulosin is indicated for the treatment of signs and symptoms of benign prostatic hyperplasia.
Tamsulosin is also used off label for the treatment of ureteral stones, prostatitis, and female voiding dysfunction.

Background

Tamsulosin is a selective alpha-1A and alpha-1B adrenoceptor antagonist that exerts its greatest effect in the prostate and bladder, where these receptors are most common. It is indicated for the treatment of signs and symptoms of benign prostatic hypertrophy. Antagonism of these receptors leads to relaxation of smooth muscle in the prostate and detrusor muscles in the bladder, allowing for better urinary flow. Other alpha-1 adrenoceptor antagonists developed in the 1980s were less selective and more likely to act on the smooth muscle of blood vessels, resulting in hypotension.
Tamsulosin was first approved by the FDA on April 15, 1997.

Pharmacokinetics

Tamsulosin is an alpha adrenoceptor blocker with specificity for the alpha-1A and alpha-1D subtypes, which are more common in the prostate and submaxillary tissue. The final subtype, alpha-1B, are most common in the aorta and spleen. Tamsulosin binds to alpha-1A receptors 3.9-38 times more selectively than alpha-1B and 3-20 times more selectively than alpha-1D. This selectivity allows for a significant effect on urinary flow with a reduced incidence of adverse reactions like orthostatic hypotension.

Metabolism

Tamsulosin is mostly metabolized in the liver by cytochrome P450 (CYP) 3A4 and 2D6, with some metabolism by other CYPs. CYP3A4 is responsible for the deethylation of tamsulosin to the M-1 metabolite and the oxidative deamination to the AM-1 metabolite, while CYP2D6 is responsible for the hydroxylation of tamsulosin to the M-3 metabolite and the demethylation of tamsulosin to the M-4 metabolite. In addition, tamsulosin can be hydroxylated at a different position by an unknown enzyme to form the M-2 metabolite. The M-1, M-2, M-3, and M-4 metabolites can be glucuronidated, and the M-1 and M-3 metabolites can undergo sulfate conjugation to form other metabolites before excretion.