Studies on digitalis. VI. The effect of heparin on serum protein binding of digitoxin and digoxin The aim of the present study was to investigate the nature of the previously reported changes in the serum protein binding of digitoxin and digoxin in uremic patients under treatment with hemodialysis. Kinetic studies on protein binding during hemodialysis showed that the free fraction of digitoxin rose from 2.6% to 6.9% after 5 min of hemodialysis and remained elevated during the dialyzing period. Free digoxin rose from 78.3 % to 87.1 % during the same period. In vitro hemodialysis experiments showed that such changes occurred only in vivo. Injection of heparin (5,000 IU) to control subjects produced similar kinetic changes in the protein binding of digitoxin and digoxin. Free fatty acids changed in the same way. These results indicate that the heparin-induced release of free fatty acids causes displacement of digitoxin and digoxin from their albumin-binding sites. Patients on hemodialysis have lower serum levels of digitoxin and cardioactive metabolites (mean, 8.9 nglml) than control patients (mean, 16.7 nglmi) (p < 0.005) on similar doses (mean, 0.085 mglday). They should be maintained on the same digitoxin doses as uremic and control patients, but serum digitoxin levels should be adjusted to 10 to 15 ng Iml in hemodialysis patients compared to 15 to 25 ng Iml in uremic patients and in patients with normal renal function.

Liv Storstein, M.D., and Helge Janssen, M.D. Oslo, Norway

Medical Department B and Hemodialysis Unit, Surgical Department A, University Clinics, Rikshospitalet

An earlier investigation29 reported normal digitoxin protein binding in uremia. In patients on hemodialysis, marked changes in digitoxin and digoxin protein binding occur during an 8-hr dialyzing period. Predialysis binding is normal and end-dialysis values are significantly decreased.

Supported by grants from The Norwegian Council on Cardiovascular Diseases, The University of Oslo and Norwegian Research Council for Science and the Humanities. Received for pUblication Nov. 18, 1975. Accepted for publication Feb. 12, 1976. Reprint requests to: Dr. Liv Storstein, University Clinic, Rikshospitalet, Pilestredet 32, Oslo, Norway.

The kinetics of protein binding changes during hemodialysis had been studied when U. Ravens, M.D., University ofKiel,* commented on the similarity in the kinetics of protein binding changes and release of free fatty acids following heparin injection. The aim of our study is to elucidate the nature of these differences. In vivo heparin experiments and determination of free fatty acids and the protein binding of digitoxin and digoxin indicate a causal relationship between the two. There may be a clinical significance in the effects of the 'Personal communication.

15

Storstein and Janssen

16

Clinical Pharmacology and Therapeutics

Table I. Kinetic changes in digitoxin and digoxin protein binding during hemodialysis Minutes Patient

0

Digitoxin 1. 2. 3. 4a. 4b. 5. 6. 7. 8. 9. 10. 11.

2.2 2.3 2.8 2.7 3.9 2.2 2.8 2.2 2.8 2.9 2.4 2.0

Mean SD

I

5

I

10

I

15

3.4 6.1

3.3 5.3

3.7 6.0

11.1

11.5

11.6

I

Hours 30

1

3.3 3.4 4.5 8.2 11.5 12.0

2.7 2.8 3.4 7.3 10.3 8.8 2.9 5.3 3.9 8.7 2.9 2.2

I

2

I

3-4

3.3 4.9

2.8 8.6

8.5 2.7 4.3 4.1 8.9 3.1

8.0 2.5

I

6-7

12.2 11.4 5.8 3.2 3.7 3.4 8.5 3.9

6.8

6.7

7.1

7.0 5.1 11.2 4.2 3.8

2.6

6.9

6.7

7.0

6.8

5.5

5.0

5.5

6.5

0.5

3.2

3.5

3.0

3.4

2.9

2.4

3.3

3.7

Digoxin 1. 2. 3. 4. 5.

75.2 78.6 78.4 81.1 78.4

84.1 90.0

85.5 88.9

86.2 86.7

86.6 83.5 85.1 94.1 93.1

84.5 81.2 83.2 90.0 96.1

77.9 87.7 92.6

78.4 92.9 98.2

95.8 93.0

Mean

78.3

87.1

87.2

86.5

88.5

87.0

86.1

90.0

94.4

2.1

4.2

2.4

0.4

4.8

6.0

7.5

10.3

2.0

SD

decreases in protein binding on the overall pharmacokinetics of digitoxin and digoxin. Material and methods

Kinetic studies. Plasma was obtained by plasmapheresis in patients on treatment with hemodialysis. Samples were drawn at zero time and after 5, 10, 15, and 30 min and 1,2,3 to 4 and 6 to 7 hr after the start of hemodialysis. Digitoxin protein binding was examined in 10 patients and digoxin protein binding in 5 patients. Experimental hemodialysis studies. In vitro dialysis experiments were performed by dialyzing blood from the blood bank in a closedcircuit system. Dialysis was carried out with a Drake-Willock machine in a Gambro-Lundia dialyzer. The following fluids were dialyzed against each other: (1) blood-blood, (2) bloodlow potassium dialyzing fluid (1.5 mEq/L), (3) blood-normal potassium dialyzing fluid (4.0 mEq/L), (4) blood-20% Aminess "Astra" in

Ringer's solution. Aminess contains 8 essential amino acids and histidine. In vivo heparin studies. Seven persons with normal renal and hepatic function were given a bolus injection of 5,000 IV heparin, the same dose as given to the hemodialysis patients at the start of hemodialysis. Samples were drawn at zero time and after 5, 10, 15, 30, and 60 min. Digitoxin protein binding was studied in 5 patients, digoxin protein binding in 4, and free fatty acids in 4. Laboratory methods. Protein binding was studied in vitro with radioactive-labeled glycoside and equilibrium dialysis. 26 Serum digitoxin levels in patients on hemodialysis who were on maintenance treatment with the drug were determined with the modified 86Rb method used in our laboratory. 10 Free fatty acids were analyzed with a commercial kit method (Boehringer Gmbh, Mannheim). Serum (0.2 ml) and a copper

Effect of heparin on digitalis binding

Volume 20 Number 1

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DIGITOXIN 14

8

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6

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4

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to

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