Diazoxide

Diazoxide is a peripheral vasodilator and has qualitatively the same effect on blood vessels as benzothiazine compounds, but the effect is more rapid and profound. Unlike benzothiazines, diazoxide is non-diuretic and causes retention of sodium and water. It causes a prompt increase in blood glucose by a direct inhibitory action on the secretion of insulin by the beta cells in the Islets of Langerhans.

Diazoxide administered orally produces a prompt dose-related increase in blood glucose level, due primarily to an inhibition of insulin release from the pancreas, and also to an extrapancreatic effect.

The hyperglycemic effect begins within an hour and generally lasts no more than eight hours in the presence of normal renal function.

Diazoxide decreases the excretion of sodium and water, resulting in fluid retention which may be clinically significant.

The hypotensive effect of diazoxide on blood pressure is usually not marked with the oral preparation. This contrasts with the intravenous preparation of diazoxide.

Other pharmacologic actions of diazoxide include increased pulse rate; increased serum uric acid levels due to decreased excretion; increased serum levels of free fatty acids' decreased chloride excretion; decreased para-aminohippuric acid; (PAH) clearance with no appreciable effect on glomerular filtration rate.

The concomitant administration of a benzothiazide diuretic may intensify the hyperglycemic and hyperuricemic effects of

In the presence of hypokalemia, hyperglycemic effects are also potentiated.

Diazoxide-induced hyperglycemia is reversed by the administration of insulin or tolbutamide. The inhibition of insulin release by diazoxide is antagonized by alpha-adrenergic blocking agents.

Oral administration

Diazoxide is extensively bound (more than 90%) to serum proteins, and is excreted in the kidneys. Limited data on oral administration revealed a half-life of 24 and 36 hours in two adults. In four children aged four months to six years, the plasma half-life varied from 9.5 to 24 hours on long-term oral administration. The half-life may be prolonged following overdosage, and in patients with impaired renal function.

IV administration

Diazoxide injection is administered to adults as a rapid intravenous injection at doses of 150mg to 300mg initially for the treatment of severe hypertension or hypertensive crises. Approximately 90% of diazoxide is bound to plasma proteins. Diazoxide crosses the placenta and can cause hyperbilirubinaemia and altered carbohydrate metabolism in the foetus and new-born. Diazoxide is eliminated from the body primarily through glomerular filtration. Its long serum half-life (20-30 hours) reflects the fact that 90% of the drug in serum is bound to albumin and protected from filtration. In patients with impaired renal function, the serum half-life increases with decreasing creatinine clearance. The serum half-life is at least 3 times longer than its hypotensive action, and, when dosage is repeated at intervals of 4 to 12 hours, there is extensive accumulation in the body.

Oral diazoxide in the mouse, rat, rabbit, dog, pig, and monkey produces a rapid and transient rise in blood glucose levels. In dogs, increased blood glucose is accompanied by increased free fatty acids, lactate, and pyruvate in the serum. In mice, a marked decrease in liver glycogen and an increase in the blood urea nitrogen level occur.

In acute toxicity studies the LD₅₀ for oral diazoxide suspension is >5000 mg/kg in the rat, >522 mg/kg in the neonatal rat, between 1900 and 2572 mg/kg in the mouse, and 219 mg/kg in the guinea pig. Although the oral LD₅₀ was not determined in the dog, a dosage of up to 500 mg/kg was well tolerated.

In subacute oral toxicity studies, diazoxide at 400 mg/kg in the rat produced growth retardation, edema, increases in liver and kidney weights, and adrenal hypertrophy. Daily dosages up to 1080 mg/kg for three months produced hyperglycemia, an increase in liver weight and an increase in mortality. In dogs given oral diazoxide at approximately 40 mg/kg/day for one month, no biologically significant gross or microscopic abnormalities were observed. Cataracts, attributed to markedly disturbed carbohydrate metabolism, have been observed in a few dogs given repeated daily doses of oral or intravenous diazoxide. The lenticular changes resembled those which occur experimentally in animals with increased blood glucose levels. In chronic toxicity studies, rats given a daily dose of 200 mg/kg diazoxide for 52 weeks had a decrease in weight gain and an increase in heart, liver, adrenal and thyroid weights. Mortality in drug-treated and control groups was not different. Dogs treated with diazoxide at dosages of 50, l00, and 200 mg/kg/day for 82 weeks had higher blood glucose levels than controls. Mild bone marrow stimulation and increased pancreas weights were evident in the drugtreated dogs; several developed inguinal hernias, one had a testicular seminoma, and another had a mass near the penis. Two females had inguinal mammary swellings. The etiology of these changes was not established. There was no difference in mortality between drug-treated and control groups. In a second chronic oral toxicity study, dogs given milled diazoxide at 50, l00, and 200 mg/kg/day had anorexia and sever weight loss, causing death in a few. Hematologic, biochemical, and histologic examination did not indicate any cause of death other than inanition. After one year of treatment, there is no evidence of herniation or tissue swelling in any of the dogs. When diazoxide was administered at high dosages concomitantly with either chlorothiazide to rats or trichlormethiazide to dogs, increased toxicity was observed. In rats, the combination was nephrotoxic; epithelial hyperplasia was observed in the collecting tubules. In dogs, a diabetic syndrome was produced which resulted in ketosis and death. Neither of the drugs given alone produced these effects.

Although the data are inconclusive, reproduction and teratology studies in several species of animals indicate that diazoxide, when administered during the critical period of embryo formation, may interfere with normal fetal development, possibly through altered glucose metabolism. Parturition was occasionally prolonged in animals treated at term. Intravenous administration of diazoxide to pregnant sheep, goats, and swine produced in the fetus an appreciable increase in blood glucose level and degeneration of the beta cells of the Islets of Langerhans. The reversibility of these effects was not studied.