Br. J. clin. Pharmac. (1992), 34, 102-105

Therapeutic use of atrial natriuretic factor JOHN M. C. CONNELL, ALAN G. JARDINE & DAVID B. NORTHRIDGE' MRC Blood Pressure Unit and 'Department of Cardiology, Western Infirmary, Glasgow Gil 6NT

and so enhance its action (Maack et al., 1987). Use of such ligands has been shown to enhance the efficacy of exogenous ANF, with amplification of natriuresis and diuresis in experimental animals (Almeida et al., 1989), and this might offer one way to exploit the potential of the hormone. However, orally active C-receptor specific blockers are not yet available and this interesting approach remains of theoretical value only. Another way of utilising ANF is to block its metabolic degradation. The hormone is initially metabolised by a zinc-containing metallo-endopeptidase (EC 3.4.24.11, neutral endopeptidase (NEP)), an enzyme which is widely distributed, being present in kidney, lung, gut, brain and placenta (Erdos & Skidgell, 1989). The enzyme metabolises, in addition to ANF, a range of

Discovery of a novel hormone involved in cardiovascular homeostasis, and which is abnormal in pathophysiological states, raises prospects of therapeutic exploitation which may lead to exciting new options for disease management-witness the exploitation of the renin-angiotensin system in this regard. Atrial natriuretic factor (ANF), a 28-amino acid polypeptide secreted from cardiac atria with actions which are generally antagonistic to angiotensin II, was discovered in 1981 (deBold et al., 1981). Since that time a decade of intensive research into the physiology and pathophysiology of this hormone have followed (for review see Kenyon & Jardine, 1989). How have therapeutic applications for exploitation of this novel peptide fared? The physiological actions of ANF include natriuresis, diuresis, venodilatation with a consequent fall in cardiac filling pressures and a modest fall in blood pressure (Kenyon & Jardine, 1989; Richards et al., 1985, 1988a; Tonolo et al., 1986; Weidmann et al., 1986): these suggest that activation of the receptors for ANF ought to be of benefit in sodium retaining states, cardiac failure and hypertension. In human subjects with hypertension short term infusion of ANF causes exaggerated natriuresis and a small fall in blood pressure (Tonolo et al., 1986): administration of the hormone for a longer period of time (5 days) causes a sustained fall in blood pressure (Janssen et al., 1989). In heart failure acute infusions result in variable diuretic effects but consistent and beneficial haemodynamic changes. These are encouraging data and have led to efforts to exploit the therapeutic potential of ANF in disease states. However, because of its physical nature, the hormone is not orally bioavailable and alternative ways of utilising it have been sought. Most of the actions of ANF are accounted for by activation of membrane bound guanylate cyclase, leading to generation of cyclic GMP (cGMP) (Chinkers et al., 1989). One form of the ANF receptor (GC) incorporates guanylate cyclase activity within its structure; this receptor modulates the known physiological actions of the hormone in a wide range of tissues (Chinkers, 1990). Unfortunately, effective and specific ligands for this receptor are not available. Another ANF binding site ('C-ANF receptor') exists but is not associated with any obvious second messenger system and has no known physiological function (Chinkers, 1990). It has been proposed that this binding site acts as a clearance mechanism for ANF, and may regulate access of the peptide to the GC receptor. Thus, truncated forms of ANF bind specifically to the C-receptor and increase the availability of the native hormone to the GC receptor

endogenous peptides including bradykinin, angiotensin II and endothelin (Erdos & Skidgell, 1989; Vane, 1990). NEP cleaves ANF at the cys 105-phe 106 bond resulting in an inactive, ring-deleted structure (Kenny & Stephenson, 1988). Inhibition of NEP prevents this inactivation of ANF and so prolongs its biological availability (Olins et al., 1989). What evidence is there that this might be a useful therapeutic option? Various specific inhibitors of NEP have been described in the last few years including phosphoramidon (Lafferty et al., 1989), thiorphan (Olins et al., 1989), SQ 29,072 (Seymour et al., 1989), SCH 34,826 (Sybertz et al., 1989), UK 69,578 (Danilewicz et al., 1989), [(±) candoxatrilat] and related compounds. All have been shown in vitro to delay the breakdown of ANF. In vivo, these compounds prolong the elimination half-life of exogenous ANF and raise basal levels of the hormone. These effects in experimental animals coincide with natriuresis and diuresis which can be blocked by ANF antisera, suggesting that the actions are specific consequences of inhibition of ANF breakdown (Shepperson et al., 1991). Furthermore, these changes are associated with an increase in urinary excretion of cGMP (Sybertz et al., 1989, 1990) which is consistent with increased activation of guanylate cyclase. Following acute administration in normotensive animals no major changes in blood pressure or heart rate are seen, although blood pressure has been reported to fall following 5 days administration of SCH 34,826 to spontaneously hypertensive rats (Sybertz et al., 1990). In other studies, blood pressure has been shown to fall following chronic administration of this class of drug to animals with volume-dependent hypertension (e.g. DOCA salt) (Shepperson et al., 1991; Sybertz et al., 1989). In animal models of heart failure, beneficial effects, including diuresis and improvement in central

Correspondence: Dr John M. C. Connell, MRC Blood Pressure Unit, Western Infirmary, Glasgow Gll 6NT

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Therapeutic use of ANF haemodynamic measurements, have been observed (Cavero et al., 1990; Marguiles et al., 1990). Taken together, therefore, these data are encouraging and suggest that the approach might be of benefit in human cardiovascular disease. Initial studies in normal man with an intravenous agent (candoxatrilat) showed that plasma levels of ANF rose within 30 min of administration of the drug and remained high for several hours (Jardine et al., 1990). This was associated with natriuresis, diuresis and suppression of plasma active renin concentration, changes similar to those seen with low dose ANF infusion. Subsequent studies with an orally available agent (UK 79,300, candoxatril) have shown similar effects after single or dual dose administration (O'Connell et al., 1992; Richards etal., 1990). These short term studies have not been associated with important changes in blood pressure or heart rate. However, chronic administration of candoxatril in normal man causes only a small initial natriuresis with no sustained effect on cumulative sodium balance, and has no effect on blood pressure (O'Connell et al., 1990). Lefrancois and colleagues (1990) did report a preliminary uncontrolled study in which sinorphan (a derivative of thiorphan) caused a fall in both systolic and diastolic blood pressure. However in a larger placebo controlled study in essential hypertension administration of candoxatril for 30 days caused a small fall in erect systolic blood pressure without altering supine blood pressure or diastolic pressure (Bevan et al., 1991). Intravenous (±)-candoxatrilat has been given acutely to subjects with mild chronic heart failure. Basal ANF levels in these subjects were elevated, and administration of the drug caused a further 2-3 fold increase in concentrations within 1 h (Northridge et al., 1990). This was associated with natriuresis and diuresis. The principal acute haemodynamic effect was a fall in right atrial and pulmonary artery wedge pressures (cardiac preload), with no change in heart rate, blood pressure or cardiac output. Similar acute responses have been described following administration of sinorphan by mouth in an open and uncontrolled study in subjects with severe heart failure (Khan et al., 1990). In a canine model of heart failure the NEP inhibitor SQ 28,603 caused natriuresis and diuresis; these changes were greater than those which would have been expected for the observed increased in plasma levels of ANF, suggesting that the actions of the drug were due either to intrarenal changes in ANF metabolism or to other effects of the agent (Cavero et al., 1990; Marguiles et al., 1990). In this regard it is of interest that recent evidence suggests that candoxatril administration in man causes an increase in circulating levels of brain natriuretic peptide (BNP) which is structurally similar to ANF and which binds to a guanylate cyclase linked receptor (GC-B) to effect natriuresis (Lang et al., 1991). Data on chronic administration of NEP inhibitors in heart failure are awaited with interest. These current data with NEP inhibitors in man suggest that the initial expectations of benefit in cardiovascular disease may not be fulfilled. Why should this be? Firstly, the effects of these agents may be limited by failure to maintain chronic elevation of plasma ANF. Richards and colleagues (1991) reported that after 5 days' treat-

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ment with candoxatril in normal males basal ANF levels were not changed, although the elimination half-life of exogenous ANF remained prolonged, so that the failure to maintain elevation of circulating levels of the hormone does not reflect escape of NEP from inhibition. Instead, it seems likely that there is a fall in secretion rate of the endogenous hormone. The reason for this is unclear, but may reflect a subtle change in plasma volume with a

consequent drop in atrial filling pressure. Nevertheless, in hypertensive subjects chronic dosing may sustain elevated levels of ANF. Thus, an approximately twofold increase in ANF was noted after 30 days of treatment with candoxatril in subjects with essential hypertension (Bevan et al., 1991), and further data on the effects of chronic inhibition of NEP in pathophysiological states are awaited with interest. It is not certain however, that alterations in circulating ANF levels are important in the mechanism of action of NEP inhibitors: changes in local ANF concentrations may be equally relevant, and these have not been assessed during chronic NEP inhibitor administration. It is noteworthy that despite difficulty in demonstrating elevation of plasma ANF, cGMP excretion remained increased during chronic administration of candoxatril (O'Connell et al., 1990) and other NEP inhibitors (Sybertz et al., 1989, 1990). Other possible mechanisms of action, including inhibition of the breakdown of the related natriuretic compound BNP (see above) have also been suggested. Secondly, chronic administration of candoxatril appears to be associated with a fall in ANF receptor number (at least on platelets (O'Connell et al., 1990)), and this may limit long term effectiveness of the compounds. It is of interest that a similar fall in ANF receptor number has been reported in subjects with congestive cardiac failure, presumably as a consequence of chronically increased circulating levels of the hormone leading to receptor down-regulation (Schiffrin, 1987). Lastly, NEP has a wide range of potential endogenous substrates including angiotensin II and endothelin, and chronic inhibition of the enzyme, by delaying breakdown of pressor peptides, may have actions beyond inhibition of ANF metabolism which could limit the therapeutic value of this class of drugs. In summary, what of the therapeutic potential of ANF? NEP inhibitors at present represent the only practical pharmacological means of activating ANF receptors. The role of these agents in heart failure is of interest and remains to be fully assessed, although may be eclipsed by more encouraging data with angiotensin converting enzyme inhibitors (The SOLVD investigators, 1991). However, there is some current interest in compounds with combined actions to inhibit both angiotensin converting enzyme and NEP, and these may prove to be potent hypotensive and natriuretic agents (Seymour et al., 1991). Reports of the effects of such compounds in experimental animals and man will be awaited with interest. In hypertension the current data are disappointing. It should be recalled, however, that in experimental animal models NEP inhibitors are most effective in volume-dependent hypertension and studies in patient subgroups with hypertension with pathophysiological features which are similar to this (low renin hypertension and hypertension in black subjects) may be worthwhile. At present, therefore, the tantalising therapeutic

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potential of ANF remains unfulfilled, although this should be regarded as an interim report only. Inhibitors of neutral endopeptidase have effects consistent with increased activation of ANF receptors, but may be of

limited value due to lack of specificity. For the future, specific agonists for the ANF guanylate cyclase receptor(s) might be the best therapeutic approach, although these remain currently unavailable.

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(Received 7 November 1991, accepted 3 March 1992)