Digoxin is a drug often prescribed to treat heart ailments. The purpose of a study by Parker et al. (A-4)

was to examine the interactions of digoxin with common grapefruit juice. In one experiment, subjects took digoxin with water for 2 weeks, followed by a 2-week period during which digoxin was withheld. During the next 2 weeks subjects took digoxin with grapefruit juice. For seven subjects, the average peak plasma digoxin concentration (Cmax) when taking water is given in the first column of the following table. The second column contains the percent change in Cmax concentration when subjects were taking the digoxin with grapefruit juice [GFJ (%) change]. Use the Cmax level when taking digoxin with water to predict the percent change in Cmax concentration when taking digoxin with grapefruit juice.

Cmax (ngl/ml) with Water Change in Cmax with GFJ (%)

2.34 29.5 2.46 40.7 1.87 5.3 3.09 23.3 5.59 45:1 4.05 35:3 6.21 44:6 2.34 29.5 Source: Data provided courtesy of Robert B. Parker, Pharm.D.