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Pretenol-C

 

DESCRIPTION

PRETENOL-C 100 TABLET : A white, 9mm round tablet scored on one side and with marking 'd' on another side.

PRETENOL-C 50 TABLET : A white, 'D'-shaped tablet with marking 'duo862'.

COMPOSITION

PRETENOL-C 100 TABLET : Each tablet contains Atenolol 100 mg, Chlorthalidone 25 mg
PRETENOL-C 50 TABLET : Each tablet contains Atenolol 50 mg, Chlorthalidone 12.5 mg

PHARMACODYNAMICS:

Atenolol and Chlorthalidone have been used singly and concomitantly for the treatment of hypertension. The antihypertensive effects of these agents are additive and studies have shown that there is no significant interference with bioavailability when these agents are given together in the single combination tablet In patients with more severe hypertension Pretenol-C may be administered with other antihypertensive such as vasodilators.

 

Atenolol: A beta-adrenoceptor blocking drug which acts preferentially on beta-receptors in the heat Selectivity decreases with increasing dose. I has little intrinsic sympathomimetic activity and no membrane stabilising activity, Atenolol is a racemic mixture and its activity resides in the S(-) enantiomer. It reduces raised blood pressure by an unknown mechanism and also inhibits exercise induced tachycardia and decreases plasma renin concentration. It causes sight airways obstruction but less than that seen with non-selective beta-blockers. The inhibition of exercise induced tachycardia is correlated with blood levels, but there is no correlation between plasma concentrations and antihypertensive effect Atenolol is effective and well-tolerated in most ethnic populations although the response may be less Afro-Caribbean black patents.


The possible mechanism of the anti-anginal activity of Atenolol appears to be due to a reduction in left ventricular work and oxygen utilisation resulting (mainly) from the decrease in heart rate and contractility. The anti-arrhythmic effect of Atenolol is apparently due to its anti-sympathetic effect There is no evidence that membrane stabilising activity or intrinsic sympathomimetic activity are necessary for anti-arrhythmic efficacy. By its anti-sympathetic effect, Atenolol depresses sinus node function, atrioventricular node function and prolongs atrial refractory periods. It has no direct effect on electrophysiological properties of the HIS-Purkinje system.


Because of their negative inotropic effects, beta-adrenoreceptor blocking agents should be avoided in uncontrolled heart failure.


Chlorthalidone: A monosulfanomyl diuretic which differs chemically from thiazide diuretics in that a double ring system is incorporated in its structure. It is an oral diuretic with prolonged action and low toxicity. The diuretic effect of the drug occurs within 2 hours of an oral dose and continues for up to 72 hours. it produces copius diuresis with greatly increased excretion of sodium and chloride. At maximal therapeutic dosage chlorthalidone is approximately equal in its diuretic effect to comparable maximal therapeutic doses of benzothiazine diuretics. The site of action appears to be the cortical diluting segment of the ascending limb of Henle's loop nephron.


PHARMACOKINETICS
Atenolol: Although absorption of Atenolol is variable and incomplete (40-60%) the virtual lack of liver metabolism results in a relatively systemic bioavailability compared to other beta-blockers. Blood levels in man peak two to four hours after a single 100 mg oral dose and are of the order of 0.4 to 0.9 microgram I ml. Blood levels are consistent and the levels after chronic oral administration are in good agreement with those predicted from single dose results The drug is distributed throughout the body tissues and about 10% of the drug is metabolised. The minor urinary metabolite identified is a hydroxylated derivative. It is reported to be one tenth as active atenolol. The main route of elimination is renal excretion. The plasma half-life, measured by blood level decay or urinary build-up is approximately 7 to 9 hours. In patients with impaired renal function, there is a progressive prolongation of the half life, in patients with normal renal function the therapeutic effect that is, control of raised blood pressure, lasts for at least 24 hours following a 50 mg oral dose.


Chlorthalidone: Absorption and plasma concentration: The bioavailability of an oral dose of 50mg Hygroton is approximately 64%, peak blood concentrations being attained after 8 to 12 hours. For doses of 25 and 50 mg, Cmax values average 1.5 mcg/mL(4.4 micromol/L) and 3.2 mcg/mL (9.4 micromol/L) respectively. For doses up to 100 mg there is a proportional increase in AUC. On repeated daily doses of 50mg, steady-state blood concentrations, measured at the end of the 24-hour-dosage interval, averaging 7.2 mcg/mL (21.2 micromol/L) are reached after 1 to 2 weeks. Distribution: In blood, only a small fraction of chlorthalidone is tree, due to extensive accumulation in erythrocytes and binding to plasma proteins. Owing to the large degree of high-affinity binding to the carbonic anhydrase of erythrocytes, only some 1.4% of the total amount of chlorthalidone in whole blood was found in plasma at steady state during treatment with 50 mg doses. In vitro, plasma protein binding of chlorthalidone is about 76%, and the major binding protein is albumin. Chlorthalidone crosses the placental barrier and passes into breast milk. In mothers treated with 50 mg chlorthalidone daily before and after delivery chlorthalidone levels in foetal whole blood are about 15% of those found in maternal blood. Chlorthalidone concentrations in amniotic fluid and in the maternal milk are approximately 4% of the corresponding maternal blood level. Metabolism and elimination: Chlorthalidone is eliminated from whole blood and plasma with an elimination half-life averaging 50 hours. The elimination hag-life is unaltered after chronic administration. The major part of an absorbed dose of chlorthalidone is excreted by the kidneys, with a mean renal plasma clearance of 60 mL/min. Metabolism and hepatic excretion into bile constitute a minor way of elimination. Within 120 hours, about 70% of the dose is excreted in the urine and in the faeces mainly in unchanged form. Special patient groups: Renal dysfunction does not seem to alter the pharmacokinetics of chlorthalidone, the rate-limiting factor in the elimination of the drug from blood or plasma being most probably the affinity of the drug to the carbonic anhydrase of erythrocytes. In elderly patents, the elimination of chlorthalidone is slower than in healthy young adults, although absorption is the same. Therefore, close medical observation is indicated when treating patents of advanced age with chlorthalidone.

INDICATIONS

Treatment of hypertension in patients for whom the effect of treatment with either beta-blockers or diuretic alone is inadequate. The patient should be stabilised on the two drugs individually before transfer to the fixed combination if the dosage is appropriate.

RECOMMENDED DOSAGE

Dosage of individual drugs to be titrated and patients to be stabilized on the two drugs individually before transfer to the fixed combination if dosage is appropriate. One (50mg) I hat (100 mg) tablet daily of Pretenol C should prove effective in most cases. If the response is considered inadequate then two (50 mg tablets / one (100 mg) tablet daily should be tried. However, higher doses offer a significant risk of hypokalaemia and periodic monitoring of potassium levels should always be undertaken in these patients. Above this dose level there is unlikely to be any further fall in blood pressure and another anti-hypertensive drug such as a vasodilator should be added.

 

Children: There is no paediatric experience with Pretenol-C therefore this preparation is not recommended for children.


A daily dosage of one (100 mg) / two (50 mg) Pretenol C tablets may be inappropriate in the older patient. For these patents, one (50mg) / half (100mg) tablet daily is recommended. The addition of a small dose of a third agent, eg. a vasodilator, may be appropriate should hypertensive control not be achieved on one (50 mg) / half (100mg) Pretenol C tablet daily alone. In patents with severe renal impairment a reduction in daily dose or in frequency of administration may be necessary.

CONTRAINDICATIONS
1. Bronchospasm: Beta adrenergic blockade of the smooth muscle of bronchi and bronchioles may result in an increased airways resistance. These drugs also reduce the effectiveness of asthma treatment This may be dangerous in susceptible patients. Therefore, beta blockers are contraindicated in any patents with a history of airways obstruction or tendency to bronchospasm. Use of cardioselective beta-blockers can also result in severe bronchospasm. If such therapy must be used, great caution should be exercised. Alternative therapy should be considered.
2. Congestive heart failure.
3. Allergic disorders (including allergic rhinitis) which may suggest a predisposition to bronchospasm.
4. Right ventricular failure secondary to pulmonary hypertension.
5. Significant right ventricular hypertrophy.
6. Sick sinus syndromes.
7. Sinus bradycardia (less than 45-50 beats/minute)
8. Second and third degree AV block
9. Shock (including cardiogenic and hypovolaemic shock)
10. Anaesthesia with agents that produce myocardial depression leg. ether, chloroform, cyclopropane)
11. Hypersensitivity to the drug or other sulphonamide-derived drugs
12. Hypotension
13. Metabolic acidosis
14. Severe peripheral arterial circulatory disturbances
15. Untreated phaeochromocytoma
16. Pregnancy and lactation (see Warning: Use in pregnancy and lactation)
17. Hypertension during pregnancy
18. Creatinine clearance lower than 30mL/min
19. Conditions involving enhanced potassium loss, salt-losing nephropathies and prerenal (cardiogenic) impairment of kidney function.


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