Source: Health Products Regulatory Authority (ZA) Revision Year: 2022 Publisher: Pfizer Laboratories (Pty) Ltd, 85 Bute Lane, Sandton 2196, South Africa Tel: +27(0)11 320 6000 / 0860 734 937 (Toll-free South Africa)
Category and class: A 2.6 Tranquillisers
Prazepam is a benzodiazepine derivative with anxiolytic and central muscle relaxant activity.
Benzodiazepines exert their effects through enhancement of the gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex. GABA is an inhibitory neurotransmitter that exerts its effects at specific receptor subtypes designated GABA-A and GABA-B. GABA-A is the primary receptor subtype in the CNS and is thought to be involved in most of the inhibitory neurotransmission in the CNS.
Benzodiazepines enhance the effects of GABA by increasing GABA affinity for the GABA receptor. Binding of GABA to the site opens the chloride channel resulting in a hyperpolarised cell membrane that prevents further excitation of the cell.
Benzodiazepines reportedly act as agonists at the benzodiazepine receptors, which have been shown to form a component of a functional supramolecular unit known as the benzodiazepine-GABA receptor-chloride ionophore complex.
Prazepam is readily and completely absorbed from the gastro-intestinal tract independently from the gastric pH.
The bioavailability of desalkylprazepam due to prazepam is 51 ± 5%. Prazepam pharmacokinetics is characterized by constant blood levels and absence of plasma peaks. After single doses of 20 mg prazepam tablets the highest blood level of the active metabolite is reached after 5-6 hours; then blood levels slowly decrease. The distribution volume is 14,4 ± 5,1 l/kg. Prazepam is extensively bound (97,5%) to plasma proteins. Following repeated doses, blood concentrations increase in several days reaching the steady-state at day 9. After discontinuation of the drug, blood levels decrease gradually.
Because of extensive first-pass metabolism in the liver, prazepam is not present – or present in very small amounts – in peripheral blood. The principal active metabolite found in peripheral blood is desalkylprazepam. The other major metabolites include 3-hydroxyprazepam and oxazepam; they are partially conjugated with glucuronic acid, ready for excretion in the urine, and have no clinical activity.
The elimination half-life of the active metabolite is 60 hours and it is more prolonged in the elderly, in obese patients and in patients with liver disease and with hepatic cirrhosis.
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