Question

Digoxin is most often used to enhance the force of contraction of cardiac myocytes in individuals diagnosed with congestive heart failure. Although digoxin plasma drug concentrations demonstrate first‐order kinetics, it is considered a “narrow therapeutic” drug because of the central nervous system and cardiac toxicities that are observed when drug levels exceed 2 ng/ml.

General Digoxin Drug Information: (Population Estimates)

Typical Dosing Interval: Bioavailability:
Plasma Protein Binding: Elimination:

Normal Clearance: Renal Clearance: Normal Half‐Life: Therapeutic Range: Sample Collection:

Digoxin Dosage Forms and Strengths:

Daily or Every 24 hours Tablets 75%; Elixir 80% 25% (Albumin) Primarily Kidneys

143 ml/min or 8.58 L/hr
70‐80% of the dose excreted in the urine
36 hours with normal renal function
0.5 – 1.2 ng/ml in heart failure
Collect specimen 8‐12 hours after administration of oral dose.

Digoxin Immediate‐Release Tablets: 0.0625 mg, 0.125 mg, 0.25 mg; Elixir 0.05 mg/ml; Injection 0.1 mg/ml, 0.25 mg/ml

PART 1: GENERAL ASSESSMENT QUESTIONS

1. Based on the General Digoxin Information provided in the case, which of the following daily maintenance dosage regimens would most likely achieve a steady‐state concentration of 1.0 ng/ml in an adult patient with normal renal function and digoxin clearance?

2. Based on the General Digoxin Information provided in the case, which of the following drug elimination mechanisms has a significant impact on digoxin’s clearance?

3. Which transporter is associated with impacting digoxin's absorption from the gastrointestinal tract and its clearance by the kidney?

Answer 1

1) Studies have shown that the time taken to reach a steady state of digoxin as 1.0ng/ml is approximately one week. Hence one should wait for atleast 7 days before serum digoxin concentration is measured in case when the treatment has just started or the dosage is just being changed..
2) The metabolic clearence of digoxin averages approximately 0.8mL/kg/minute. The elimination of digoxin is proportional to the total dose following first order kinetics. After intravenous administration to healthy patient 50 to 70% of the dose is excreted in unchanged form through urine.Renal clearence of digoxin is equivalent to the creatinine clearance. In heart failure patient's both metabolic and renal components of digoxin clearence is greatly decreased and the metabolic component decrease more dramatically.Nearly 25 th 28% 0f digoxin is excreated outside the kidney.

3) 70% to 80% of oral digoxin is absorbed in the proximal part of the small intestine. The degree of binding to serum albumin is 20 % - 30%. Nearly all of the digoxin excreted in the urine is unchanged. The main route of eliminating digoxin is by urinary excretion which is closely related to the glomerular filteration rate.