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New Ways with Digoxin
455
THE LANCET Ways with Digoxin A RECENT survey of hospital inpatients in Belfast showed that 19-8% of patients treated with digoxin New
suffered an adverse reaction to it and that these reactions accounted for one-third of all drug reactions monitored.1 There are many difficulties in using cardiac glycosides. Not only is it difficult to judge their therapeutic effect and therefore dosage in heart-failure with regular rhythm, particularly when they are combined with diuretic therapy, but also many symptoms and most arrhythmias suggestive of digoxin toxicity can also be caused by the intrinsic heart-disease. Two centuries of bedside experience have failed to solve these problems. Perhaps the time is ripe to apply new techniques. During stable maintenance digoxin therapy, myocardial digoxin concentrations bear a relatively constant ratio to serum concentrations as measured with radioactive digoxin.2 Measurement of serum or plasma digoxin concentrations should reflect myocardial digoxin levels and be useful both in guiding treatment and in detecting toxicity. Several methods have now been proposed for this measurement, and two have been clinically assessed. LOWENSTEIN and CORRILL3 demonstrated the feasibility of plasmadigoxin assay by a method based upon the ability of cardiac glycosides to inhibit ion flux across red-cell GRAHAME-SMITH and EVEREST6 membranes.4,5 developed this method further and they have applied it mainly to the diagnosis of digoxin toxicity. The assay requires extraction of digoxin from plasma and with digoxin standards of the ability of the extract to inhibit the uptake of the rubidium ion, 86Rb, by red cells. The lower level of sensitivity of this method was a plasma-digoxin concentration of 0-5 ng. per ml. Therapeutic levels ranged from 0-8 to 4-5 ng. per ml. with a mean of 2-36. Of 9 patients strongly suspected of digoxin toxicity only 1 had a plasma-digoxin concentration of less than 4-5 ng. per ml., the rest ranging from 4-6 to greater than 8 ng. per ml. Another approach is radioimmunoassay. BUTLER and CHEN7 immunised rabbits with a digoxin/ bovine-serum-albumin conjugate, synthetically prepared, and thus raised antibodies specific for the
comparison
1. 2. 3. 4. 5. 6. 7.
Hurwitz, N., Wade,
O. L. Br. med. J. 1969, i, 531. Doherty, J. E., Perkins, W. H., Flanigan, W. J. Ann. intern. Med. 1967, 66, 116. Lowenstein, J. M., Corrill, E. M. J. Lab. clin. Med. 1966, 67, 1048. Schatzmann, H. J. Helv. physiol. pharmec. Acta, 1953, 11, 364. Glynn, I. M. J. Physiol, Lond. 1956, 134, 278. Grahame-Smith, D. G., Everest, M. S. Br. med. J. 1969, i, 286. Butler, V. P., Jr., Chen, J. P. Proc. natn. Acad. Sci. U.S.A. 1967, 57, 71.
digoxigenin moiety of digoxin and capable of binding tritiated digoxin. Antibodies to digitoxin have also beenraised.8 SMITH et awl.9 utilised digoxin antibodies The patient’s serum is to assay serum-digoxin. incubated with tritiated digoxin and digoxin antibodies ; and the amount of tritiated digoxin binding to the antibody depends upon the amount of nonradioactive digoxin present in the serum. Unbound tritiated digoxin is removed by absorption to charcoal, and the antibody-bound tritiated digoxin is measured by liquid scintillation counting. By this method the therapeutic range of serum-digoxin concentrations is 0-8-2-4 ng. per ml., with a mean of 1-1for patients taking 0-25 mg. of digoxin per day and 1-4 for those taking 0-5 mg. per day. 18 patients with presumed digoxin toxicity had levels ranging from 2-1to 8-7 ng. per ml. The concentrations of digoxin determined by radioimmunoassay seem to be lower than those determined by the 86Rb-transport inhibition. Direct comparison of these two methods is needed to determine whether the 86Rb assay is measuring some biologically active derivative of digoxin which is undetected by radioimmunoassay. These two methods do agree quantitatively in demonstrating the narrow margin between the therapeutic and toxic plasma-digoxin levels. Whereas both of these assays are adequate for clinical use, the radioimmunoassay is probably more precise. Alternatives to these assays, not yet fully assessed, are based upon the inhibition of Na-K activated adenosine-triphosphate (A.T.p.)-ase activity by cardiac glycosides 10 or upon the competition of non-radioactive digoxin with isotopically labelled digoxin for the binding sites on an Na-K activated A.T.P.-ase.11,12 In principle all the methods could be adapted to measure any of the cardiac glycosides in clinical use, but, ingenious though these methods may be, developments in mass spectrometry during the next few years will probably render them all obsolete. Whatever method is used, it is plain that knowledge of the plasma-digoxin concentration will be a great help in managing problems of digoxin therapy and toxicity. Another new development is the potential use of digoxin antibodies in the treatment of digoxin toxicity. Digoxin antibodies bind digoxin and prevent it inhibiting potassium flux into red cells.13 The antibodies are also capable of removing digoxin previously taken up by renal slices and red cells in vitro.13 Though digoxin can apparently be removed from red cells it takes some hours for ion flux to return to normal.13 This finding suggests that either digoxin inhibition of ion flux involves an intermediate step or some crucial biologically active 8.
9. 10. 11. 12. 13.
Oliver, G. C., Parker, B. M., Brasfield, D. L., Parker, C. W. J. clin. Invest. 1968, 47, 1035. Smith, T. W., Butler, V. P., Jr., Haber, E. New Engl. J. Med. 1969, 281, 1212. Burnett, G. H., Conklin, R. L. J. Lab. clin. Med. 1968, 71, 1040. Brooker, G., Jelliffe, R. W. Fedn Proc. 1969, 28, 608. Brooker, G., Appelman, M. M. Biochemistry, 1968, 7, 4182. Watson, J. F., Butler, V. P. Clin. Res. 1968, 16, 252.
456
molecules are not removed by antibody. Whatever the reason, immunisation with digoxin/ bovine-serum-albumin conjugates protects rabbits against lethal doses of digoxin, presumably by binding circulating digoxin. 14 More important from the clinical standpoint are the experiments of SCHMIDT and BUTLER 15 on the reversal of digoxin toxicity in dogs. Digoxin was given to dogs and toxic arrhythmias were induced. Digoxin antibodies reversed the toxic arrhythmias and stopped the gastrointestinal manifestations of toxicity; and in dogs treated with control sera or antibodies unrelated to digoxin the toxic arrhythmias persisted and the dogs died. It remains to be seen how much of this effect is due to removal of digoxin already bound and how much to the binding and inactivation of circulating digoxin. The prospect of being able to reverse serious digitalis toxicity is an exciting one, particularly if the principle can be extended to the treatment of more common drug overdoses.
digoxin
Visible Proof ONE of the most interesting chapters in molecular and metabolic medicine is the one which is not yet quite complete on sickle-cell disease. The clinical and haematological signs-the anaemia, the painful crises, the tendency to thrombolic phenomena-had been recognised for many years 16,17 before INGRAM 188 showed in 1957 that the vital abnormality lay in the substitution of one aminoacid, valine, for the normal one, glutamic acid, in the (3 chains of the haemoglobin molecule. This so altered the physical properties of the haemoglobin that everything else appeared to follow from it. The disease is almost confined to Negroes and is inherited according to mendelian principles so that if two heterozygotes marry and produce four children they tend to have one homozygote sickler, two heterozygotes, and one normal offspring. The homozygote sicklers seldom reach an age to produce a large family, but the heterozygotes often do; and, furthermore, the presence of sickling cells seems to confer some protection against falciparum malaria so that the heterozygotes have a pull on normal individuals in a highly malarious area, perhaps because some of their haemoglobin is not so palatable to the parasites, or because parasitised cells sickle more quickly and, when sickled, are more readily eliminated by phagocytosis.19Another abnormal haemoglobin, HbC, and particularly two variants of it, hxmoglobin C (Georgetown) and haemoglobin C (Harlem), resemble haemoglobin S in their tendency D. H., Butler, V. P., Jr. Circulation, p. 174. 15. Schmidt, D. H., Butler, V. P., Jr. J. clin. Invest. 16. Josephs, H. Bull. Johns Hopkins Hosp. 1928, 43, 17. Henderson, A. B. Am. J. Med. 1950, 9, 757. 18. Ingram, V. M. Nature, Lond. 1957, 180, 326. 19. Luzzatto, L., Nwachuku-Jarrett, E. S., Reddy, 1970, p. 320. 14.
Schmidt,
1968, 38, suppl. 6, 1969, 48, 74a. 397.
S.
Lancet, Feb. 14,
sickle and in that the corpuscles containing them are abnormally vulnerable to trauma. 20 From an early age the homozygote sicklers are unable to produce a concentrated urine,16,17,21,22 although in most other respects their renal function is within normal limits, and such patients must therefore be excluded from any study of the concentrating power of the kidney. 23 The heterozygotes are affected too, but only later in life, and, since the ability of the kidney to concentrate the urine is now acknowledged to reside in the counter-current exchange-and-multiplier systems of the renal medulla and the very specialised vasa recta which supply the medulla with blood, it was natural to suppose that the long crescent-shaped masses of haemoglobin in the sickled cells did not circulate properly through them. This view was supported by observations that hypertonicity, which is the normal state of affairs in the medulla, promoted sickling and, in consequence, an increased viscosity of the blood.24 It was made almost certain by the discovery that the abnormality could be corrected, but only early in life, by multiple transfusions into sicklers of normal blood which slowly replaced the almost stagnant blood in the vasa recta.25,26 Visible proof of the abnormality in and permanent damage to the vasa recta has now been forthcoming and is reported on p. 450 this week. Patients with haemoglobin C even in its pure form have so far been found to concentrate their urines normally, but no report seems to have emerged as yet on patients with haemoglobin C
to
(Georgetown)
or
(Harlem).
Another renal abnormality of sicklers is a failure to acidify the urine to the normal extent 21-29; and, with the evidence now available, it is natural to suggest that the final addition of H ions to the tubular urine in exchange for sodium must take place in the medulla and depend upon the blood-supply in the vasa recta. It is not clear at the moment how, if the blood-supply to the medulla of the kidney is so abnormal in sicklers, the cells of the collecting ducts and Henle’s loops obtain the oxygen they requirenot only for their existence but for their activities, particularly the reabsorption of sodium to which so much of the oxygen supply to the kidney seems to be devoted.3O This indeed may be the explanation of 20. 21.
22. 23. 24. 25. 26.
Wintrobe, M. W. Clinical Hæmatology. London, 1967. Kunz, H. W., Pratt, E. L., Mellin, G. W., Cheung, M. W. Pediatrics, Springfield, 1954, 13, 352. Berlyne, G. M. in Renal Disease (edited by D. A. K. Black); pp. 517-545. Oxford, 1967. McCance, R. A., Crowne, R. S., Hall, T. S. Clin. Sci. 1969, 37, 471 Perillie, P. E., Epstein, F. H. J. clin. Invest. 1963, 42, 570. Keitel, H. G., Thomson, D., Itano, H. A. ibid. 1956, 35, 998. Statius, V., Eps, L. W., Romeny-Wachter, C., Ch. ter, H., la PorteWijsman, L. W., Schouten, H., Boudier, A. M. S. Abstracts of 3rd International Congress of Nephrology, Washington, D.C.,
1966. 27. Ho Ping Wong, H., Alleyne, G. A. O. Lancet, 1968, ii, 954. 28. Goossens, J., Statius van Eps, L. W., Shouten, H., Giterson, A. L. Abstracts of 4th International Congress of Nephrology; p. 455. Stockholm, 1969. 29. Ho Ping Wong, H., Alleyne, G. A. O. Br. med. J. 1969, iii, 271. 30. Knox, F. G., Fleming, J. S., Rennie, D. W. Am. J. Physiol. 1966, 210, 751.
THE LANCET Ways with Digoxin A RECENT survey of hospital inpatients in Belfast showed that 19-8% of patients treated with digoxin New
suffered an adverse reaction to it and that these reactions accounted for one-third of all drug reactions monitored.1 There are many difficulties in using cardiac glycosides. Not only is it difficult to judge their therapeutic effect and therefore dosage in heart-failure with regular rhythm, particularly when they are combined with diuretic therapy, but also many symptoms and most arrhythmias suggestive of digoxin toxicity can also be caused by the intrinsic heart-disease. Two centuries of bedside experience have failed to solve these problems. Perhaps the time is ripe to apply new techniques. During stable maintenance digoxin therapy, myocardial digoxin concentrations bear a relatively constant ratio to serum concentrations as measured with radioactive digoxin.2 Measurement of serum or plasma digoxin concentrations should reflect myocardial digoxin levels and be useful both in guiding treatment and in detecting toxicity. Several methods have now been proposed for this measurement, and two have been clinically assessed. LOWENSTEIN and CORRILL3 demonstrated the feasibility of plasmadigoxin assay by a method based upon the ability of cardiac glycosides to inhibit ion flux across red-cell GRAHAME-SMITH and EVEREST6 membranes.4,5 developed this method further and they have applied it mainly to the diagnosis of digoxin toxicity. The assay requires extraction of digoxin from plasma and with digoxin standards of the ability of the extract to inhibit the uptake of the rubidium ion, 86Rb, by red cells. The lower level of sensitivity of this method was a plasma-digoxin concentration of 0-5 ng. per ml. Therapeutic levels ranged from 0-8 to 4-5 ng. per ml. with a mean of 2-36. Of 9 patients strongly suspected of digoxin toxicity only 1 had a plasma-digoxin concentration of less than 4-5 ng. per ml., the rest ranging from 4-6 to greater than 8 ng. per ml. Another approach is radioimmunoassay. BUTLER and CHEN7 immunised rabbits with a digoxin/ bovine-serum-albumin conjugate, synthetically prepared, and thus raised antibodies specific for the
comparison
1. 2. 3. 4. 5. 6. 7.
Hurwitz, N., Wade,
O. L. Br. med. J. 1969, i, 531. Doherty, J. E., Perkins, W. H., Flanigan, W. J. Ann. intern. Med. 1967, 66, 116. Lowenstein, J. M., Corrill, E. M. J. Lab. clin. Med. 1966, 67, 1048. Schatzmann, H. J. Helv. physiol. pharmec. Acta, 1953, 11, 364. Glynn, I. M. J. Physiol, Lond. 1956, 134, 278. Grahame-Smith, D. G., Everest, M. S. Br. med. J. 1969, i, 286. Butler, V. P., Jr., Chen, J. P. Proc. natn. Acad. Sci. U.S.A. 1967, 57, 71.
digoxigenin moiety of digoxin and capable of binding tritiated digoxin. Antibodies to digitoxin have also beenraised.8 SMITH et awl.9 utilised digoxin antibodies The patient’s serum is to assay serum-digoxin. incubated with tritiated digoxin and digoxin antibodies ; and the amount of tritiated digoxin binding to the antibody depends upon the amount of nonradioactive digoxin present in the serum. Unbound tritiated digoxin is removed by absorption to charcoal, and the antibody-bound tritiated digoxin is measured by liquid scintillation counting. By this method the therapeutic range of serum-digoxin concentrations is 0-8-2-4 ng. per ml., with a mean of 1-1for patients taking 0-25 mg. of digoxin per day and 1-4 for those taking 0-5 mg. per day. 18 patients with presumed digoxin toxicity had levels ranging from 2-1to 8-7 ng. per ml. The concentrations of digoxin determined by radioimmunoassay seem to be lower than those determined by the 86Rb-transport inhibition. Direct comparison of these two methods is needed to determine whether the 86Rb assay is measuring some biologically active derivative of digoxin which is undetected by radioimmunoassay. These two methods do agree quantitatively in demonstrating the narrow margin between the therapeutic and toxic plasma-digoxin levels. Whereas both of these assays are adequate for clinical use, the radioimmunoassay is probably more precise. Alternatives to these assays, not yet fully assessed, are based upon the inhibition of Na-K activated adenosine-triphosphate (A.T.p.)-ase activity by cardiac glycosides 10 or upon the competition of non-radioactive digoxin with isotopically labelled digoxin for the binding sites on an Na-K activated A.T.P.-ase.11,12 In principle all the methods could be adapted to measure any of the cardiac glycosides in clinical use, but, ingenious though these methods may be, developments in mass spectrometry during the next few years will probably render them all obsolete. Whatever method is used, it is plain that knowledge of the plasma-digoxin concentration will be a great help in managing problems of digoxin therapy and toxicity. Another new development is the potential use of digoxin antibodies in the treatment of digoxin toxicity. Digoxin antibodies bind digoxin and prevent it inhibiting potassium flux into red cells.13 The antibodies are also capable of removing digoxin previously taken up by renal slices and red cells in vitro.13 Though digoxin can apparently be removed from red cells it takes some hours for ion flux to return to normal.13 This finding suggests that either digoxin inhibition of ion flux involves an intermediate step or some crucial biologically active 8.
9. 10. 11. 12. 13.
Oliver, G. C., Parker, B. M., Brasfield, D. L., Parker, C. W. J. clin. Invest. 1968, 47, 1035. Smith, T. W., Butler, V. P., Jr., Haber, E. New Engl. J. Med. 1969, 281, 1212. Burnett, G. H., Conklin, R. L. J. Lab. clin. Med. 1968, 71, 1040. Brooker, G., Jelliffe, R. W. Fedn Proc. 1969, 28, 608. Brooker, G., Appelman, M. M. Biochemistry, 1968, 7, 4182. Watson, J. F., Butler, V. P. Clin. Res. 1968, 16, 252.
456
molecules are not removed by antibody. Whatever the reason, immunisation with digoxin/ bovine-serum-albumin conjugates protects rabbits against lethal doses of digoxin, presumably by binding circulating digoxin. 14 More important from the clinical standpoint are the experiments of SCHMIDT and BUTLER 15 on the reversal of digoxin toxicity in dogs. Digoxin was given to dogs and toxic arrhythmias were induced. Digoxin antibodies reversed the toxic arrhythmias and stopped the gastrointestinal manifestations of toxicity; and in dogs treated with control sera or antibodies unrelated to digoxin the toxic arrhythmias persisted and the dogs died. It remains to be seen how much of this effect is due to removal of digoxin already bound and how much to the binding and inactivation of circulating digoxin. The prospect of being able to reverse serious digitalis toxicity is an exciting one, particularly if the principle can be extended to the treatment of more common drug overdoses.
digoxin
Visible Proof ONE of the most interesting chapters in molecular and metabolic medicine is the one which is not yet quite complete on sickle-cell disease. The clinical and haematological signs-the anaemia, the painful crises, the tendency to thrombolic phenomena-had been recognised for many years 16,17 before INGRAM 188 showed in 1957 that the vital abnormality lay in the substitution of one aminoacid, valine, for the normal one, glutamic acid, in the (3 chains of the haemoglobin molecule. This so altered the physical properties of the haemoglobin that everything else appeared to follow from it. The disease is almost confined to Negroes and is inherited according to mendelian principles so that if two heterozygotes marry and produce four children they tend to have one homozygote sickler, two heterozygotes, and one normal offspring. The homozygote sicklers seldom reach an age to produce a large family, but the heterozygotes often do; and, furthermore, the presence of sickling cells seems to confer some protection against falciparum malaria so that the heterozygotes have a pull on normal individuals in a highly malarious area, perhaps because some of their haemoglobin is not so palatable to the parasites, or because parasitised cells sickle more quickly and, when sickled, are more readily eliminated by phagocytosis.19Another abnormal haemoglobin, HbC, and particularly two variants of it, hxmoglobin C (Georgetown) and haemoglobin C (Harlem), resemble haemoglobin S in their tendency D. H., Butler, V. P., Jr. Circulation, p. 174. 15. Schmidt, D. H., Butler, V. P., Jr. J. clin. Invest. 16. Josephs, H. Bull. Johns Hopkins Hosp. 1928, 43, 17. Henderson, A. B. Am. J. Med. 1950, 9, 757. 18. Ingram, V. M. Nature, Lond. 1957, 180, 326. 19. Luzzatto, L., Nwachuku-Jarrett, E. S., Reddy, 1970, p. 320. 14.
Schmidt,
1968, 38, suppl. 6, 1969, 48, 74a. 397.
S.
Lancet, Feb. 14,
sickle and in that the corpuscles containing them are abnormally vulnerable to trauma. 20 From an early age the homozygote sicklers are unable to produce a concentrated urine,16,17,21,22 although in most other respects their renal function is within normal limits, and such patients must therefore be excluded from any study of the concentrating power of the kidney. 23 The heterozygotes are affected too, but only later in life, and, since the ability of the kidney to concentrate the urine is now acknowledged to reside in the counter-current exchange-and-multiplier systems of the renal medulla and the very specialised vasa recta which supply the medulla with blood, it was natural to suppose that the long crescent-shaped masses of haemoglobin in the sickled cells did not circulate properly through them. This view was supported by observations that hypertonicity, which is the normal state of affairs in the medulla, promoted sickling and, in consequence, an increased viscosity of the blood.24 It was made almost certain by the discovery that the abnormality could be corrected, but only early in life, by multiple transfusions into sicklers of normal blood which slowly replaced the almost stagnant blood in the vasa recta.25,26 Visible proof of the abnormality in and permanent damage to the vasa recta has now been forthcoming and is reported on p. 450 this week. Patients with haemoglobin C even in its pure form have so far been found to concentrate their urines normally, but no report seems to have emerged as yet on patients with haemoglobin C
to
(Georgetown)
or
(Harlem).
Another renal abnormality of sicklers is a failure to acidify the urine to the normal extent 21-29; and, with the evidence now available, it is natural to suggest that the final addition of H ions to the tubular urine in exchange for sodium must take place in the medulla and depend upon the blood-supply in the vasa recta. It is not clear at the moment how, if the blood-supply to the medulla of the kidney is so abnormal in sicklers, the cells of the collecting ducts and Henle’s loops obtain the oxygen they requirenot only for their existence but for their activities, particularly the reabsorption of sodium to which so much of the oxygen supply to the kidney seems to be devoted.3O This indeed may be the explanation of 20. 21.
22. 23. 24. 25. 26.
Wintrobe, M. W. Clinical Hæmatology. London, 1967. Kunz, H. W., Pratt, E. L., Mellin, G. W., Cheung, M. W. Pediatrics, Springfield, 1954, 13, 352. Berlyne, G. M. in Renal Disease (edited by D. A. K. Black); pp. 517-545. Oxford, 1967. McCance, R. A., Crowne, R. S., Hall, T. S. Clin. Sci. 1969, 37, 471 Perillie, P. E., Epstein, F. H. J. clin. Invest. 1963, 42, 570. Keitel, H. G., Thomson, D., Itano, H. A. ibid. 1956, 35, 998. Statius, V., Eps, L. W., Romeny-Wachter, C., Ch. ter, H., la PorteWijsman, L. W., Schouten, H., Boudier, A. M. S. Abstracts of 3rd International Congress of Nephrology, Washington, D.C.,
1966. 27. Ho Ping Wong, H., Alleyne, G. A. O. Lancet, 1968, ii, 954. 28. Goossens, J., Statius van Eps, L. W., Shouten, H., Giterson, A. L. Abstracts of 4th International Congress of Nephrology; p. 455. Stockholm, 1969. 29. Ho Ping Wong, H., Alleyne, G. A. O. Br. med. J. 1969, iii, 271. 30. Knox, F. G., Fleming, J. S., Rennie, D. W. Am. J. Physiol. 1966, 210, 751.