Thiazide Diuretics

Thiazide Diuretics Chemische Eigenschaften,Einsatz,Produktion Methoden

• Biologische Funktion

  Thiazide diuretics consist of two distinct groups: those containing a benzothiadiazine ring, such as hydrochlorothiazide and chlorothiazide, referred to as thiazide diuretics, and those that lack this heterocyclic structure but contain an unsubstituted sulfonamide group.The latter are called thiazidelike diuretics; they include metolazone, xipamide, and indapamide. The major thiazide and thiazidelike drugs available in the United States are bendroflumethiazide, benzthiazide, chlorothiazide, hydrochlorothiazide, hydroflumethiazide, methyclothiazide, polythiazide, and trichlormethiazide; and chlorthalidone, indapamide, metolazone, and quinethazone, respectively.
  Despite the structural distinctions, the drugs share the functional attribute of increasing sodium and chloride excretion by inhibiting Na⁺–Cl^- cotransport in distal convoluted tubules.
  Although chlorothiazide and its subsequently developed congeners retain the sulfamyl group SO₂NH₂, which is necessary for carbonic anhydrase inhibition, their primary effect does not rely on carbonic anhydrase inhibition.
  The thiazidelike compounds, including chlorthalidone (Hygroton), quinethazone (Hydromox), and metolazone (Zaroxolyn) have similar mechanisms of action, but they differ substantially from one another in their duration of action, the degree of carbonic anhydrase inhibition, and the dose required for maximum natriuretic activity.

• Mechanism of action

  Thiazide diuretics act in the distal convoluted tubule, where they block Na⁺–Cl^- cotransport. The Na⁺–Cl^- cotransport takes place on the luminal surface of distal convoluted tubules.Thus, to exert their diuretic action, the thiazides must reach the luminal fluid. Since the thiazide diuretics are largely bound to plasma proteins and therefore are not readily filtered across the glomeruli, access to the luminal fluid is accomplished by the proximal tubule organic acid secretory system. The drugs then travel along the nephron, presumably being concentrated as fluid is abstracted, until they reach their site of inhibitory action in the distal convoluted tubule.
  Especially at higher doses, administration of some of the thiazides results in some degree of carbonic anhydrase inhibition. However, at usual doses, only chlorothiazide shows any appreciable carbonic anhydrase inhibitory activity.

• Clinical Use

  Thiazides, especially hydrochlorothiazide (Dyazide, Esidrix, HydroDIURIL, Oretic), are useful adjunctive therapy in controlling the edema associated with congestive heart failure, cirrhosis, premenstrual tension, and hormone therapy.They are widely used in the treatment of hypertension whether or not it is accompanied by edema.They can be used in patients with renal disease; however, their diuretic activity is proportional to the residual tubular functional capacity of the kidney. The thiazides do not prevent toxemia in pregnancy, nor are they useful in the treatment of it.

• Nebenwirkungen

  Thiazides should be used cautiously in the presence of severe renal and hepatic disease, since azotemia and coma may result. The most important toxic effect associated with this class of diuretics is hypokalemia, which may result in muscular and central nervous system symptoms, as well as cardiac sensitization. Periodic examination of serum electrolytes for possible imbalances is strongly recommended. Appropriate dietary and therapeutic measures for controlling hypokalemia are described later in this chapter. The thiazides also possess some diabetogenic potential, and although pancreatitis during thiazide therapy has been reported in a few cases, the major mechanism contributing to the potential for glucose intolerance is not known.

• Stoffwechsel

  Orally administered thiazides are rapidly absorbed from the gastrointestinal tract and begin to produce diuresis in about 1 hour. Approximately 50% of an oral dose is excreted in the urine within 6 hours. These compounds are organic acids and are actively secreted into the proximal tubular fluid by the organic acid secretory mechanism. There also appears to be an extrarenal pathway for their elimination involving the hepatic–biliary acid secretory system that is particularly important for thiazide elimination when renal function is impaired.
  The thiazides have a variable effect on elimination of uric acid, which also is secreted by the renal acid secretory mechanism. Administration of thiazide diuretics, especially at low doses, may elevate serum uric acid levels and cause goutlike symptoms. Following large doses, thiazides may compete with uric acid for active reabsorption and thereby may promote uric acid elimination rather than impair it.