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Therapeutic Function

Antihypertensive

Biological Functions

Diazoxide (Hyperstat) is chemically similar to the thiazide diuretics. It is devoid of diuretic activity and causes Na+ and water retention. Diazoxide is a very potent vasodilator and is available only for intravenous use in the treatment of hypertensive emergencies. The mechanism by which diazoxide relaxes vascular smooth muscle is related to its ability to activate potassium channels and produce a hyperpolarization of the cell membrane.

Biological Activity

Antihypertensive, activates ATP-dependent K + channels. Induces activation of PKC ε , an intermediate in the opening of mitoK ATP channels, results in cardioprotection against hypoxia-induced death. Blocks desensitization of AMPA receptors.

Biochem/physiol Actions

Selective ATP-sensitive K+ channel activator in both vascular smooth muscle and pancreatic β-cells; antihypertensive.

Mechanism of action

Diazoxide reduces peripheral vascular resistance and blood pressure by a direct vasodilating effect on the VSM with a mechanism similar to that described for minoxidil by activating (opening) the ATP-modulated potassium channel. Thus, diazoxide prolongs the opening of the potassium channel, sustaining greater vasodilation on arterioles than on veins. The greatest hypotensive effect is observed in patients with malignant hypertension. Although oral or slow IV administration of diazoxide can produce a sustained fall in blood pressure, rapid IV administration is required for maximum hypotensive effects, especially in patients with malignant hypertension. Diazoxide-induced decreases in blood pressure and peripheral vascular resistance are accompanied by a reflex response, resulting in an increased heart rate, cardiac output, and left ventricular ejection rate. In contrast to the thiazide diuretics, diazoxide causes sodium and water retention and decreased urinary output, which can result in expansion of plasma and extracellular fluid volume, edema, and congestive heart failure, especially during prolonged administration. Diazoxide increases blood glucose concentration (diazoxide-induced hyperglycemia) by several different mechanisms: by inhibiting pancreatic insulin secretion, by stimulating release of catecholamines, or by increasing hepatic release of glucose. The precise mechanism of inhibition of insulin release has not been elucidated but, possibly, may result from an effect of diazoxide on cell-membrane potassium channels and calcium flux.

Pharmacology

The hemodynamic effects of diazoxide are similar to those of hydralazine and minoxidil. It produces direct relaxation of arteriolar smooth muscle with little effect on capacitance beds. Since it does not impair cardiovascular reflexes, orthostasis is not a problem. Its administration is, however, associated with a reflex increase in cardiac output that partially counters its antihypertensive effects. Propranolol and other -blockers potentiate the vasodilating properties of the drug. Diazoxide has no direct action on the heart. Although renal blood flow and glomerular filtration may fall transiently, they generally return to predrug levels within an hour.

Pharmacokinetics

Following rapid IV administration, diazoxide produces a prompt reduction in blood pressure, with maximum hypotensive effects occurring within 5 minutes. The duration of its hypotensive effect varies from 3 to 12 hours, but ranges from 30 minutes to 72 hours have been observed. The elimination half-life of diazoxide following a single oral or IV dose has been reported to range from 21 to 45 hours in adults with normal renal function. In patients with renal impairment, the half-life is prolonged. Approximately 90% of the diazoxide in the blood is bound to plasma proteins. Approximately 20 to 50% of diazoxide is eliminated unchanged in the urine, along with its major metabolites, resulting from the oxidation of the 3-methyl group to its 3-hydroxymethyl- and 3-carboxyl-metabolites.

Side effects

Since diazoxide is not often used for long-term treatment, toxicities associated with chronic use are rare.The chief concern is the side effects associated with the increased workload on the heart, which may precipitate myocardial ischemia and Na+ and water retention. These undesirable effects can be controlled by concurrent therapy with a β-blocker and a diuretic. Diazoxide may cause hyperglycemia, especially in diabetics, so if the drug is used for several days, blood glucose levels should be measured. When used in the treatment of toxemia, diazoxide may stop labor, because it relaxes uterine smooth muscle.

Synthesis

7-chloro-3-methyl-2-H-1,2,4-benzothiadiazin-1,1-dioxide (21.3.14), is synthesized by condensating 2-aminosulfonyl-4-chloroaniline with triethyl orthoacetate.

Veterinary Drugs and Treatments

Oral diazoxide is used in canine and ferret medicine for the treatment of hypoglycemia secondary to hyperinsulin secretion (e.g., insulinoma). Insulinomas are apparently very rare in the cat; there is little experience with this drug in that species. In human medicine, intravenous diazoxide is sometimes used for treating severe hypertension.

Drug interactions

Potentially hazardous interactions with other drugsAntihypertensives and vasodilators: enhanced hypotensive effect.MAOIs: withdraw at least 14 days before starting diazoxidePhenytoin: may reduce phenytoin levels.

Environmental Fate

Diazoxide is a potassium channel activator, which causes local relaxation in smooth muscles by increasing membrane permeability to potassium ions. Consequently, voltage-gated calcium ion channels are ineffective, inhibiting the generation of an action potential. The primary mechanism by which diazoxide lowers blood pressure is by direct relaxation of medium sized blood vessels. The cardiac output and renin secretion increases, resulting in elevated angiotensin II levels and retention of salt and water. When used to treat low blood sugar, diazoxide decreases insulin release from the pancreas.

Metabolism

Diazoxide lowers blood pressure within 3 to 5 minutes after rapid intravenous injection, and its duration of action may be 4 to 12 hours. Interestingly, if diazoxide is either injected slowly or infused its hypotensive action is quite modest.This is believed to be due to a rapid and extensive binding of the drug to plasma proteins. Both the liver and kidney contribute to its metabolism and excretion.The plasma half-life is therefore prolonged in patients with chronic renal failure.

Definition

ChEBI: A benzothiadiazine that is the S,S-dioxide of 2H-1,2,4-benzothiadiazine which is substituted at position 3 by a methyl group and at position 7 by chlorine. A peripheral vasodilator, it increases the oncentration of glucose in the plasma and inhibits the secretion of insulin by the beta- cells of the pancreas. It is used orally in the management of intractable hypoglycaemia and intravenously in the management of hypertensive emergencies.

Brand name

Hyperstat (Schering); Proglycem (Baker Norton).

General Description

Diazoxide is 7-chloro-3-methyl-4H-benzo[e][1,2,4]thiadiazine-1,1-dioxide , and is currentlyavailable in the United States only as a 50-mg/mLoral suspension (Proglycem); discontinued formulations includedcapsules for oral administration, and injectable formsthat typically found use for indications other than hypoglycemicconditions. Diazoxide is a cyclic benzenesulfonamide,although the free acid in solution can exist in threetautomeric forms, and the 4H tautomer most likely predominatesto a very high proportion. Partly because of theadditional nitrogen in the quinazoline ring structure, themolecule is somewhat more acidic (pKa～8.4, 8.6)than benzenesulfonamide (pKa～10).