Disposition of oral dipyridamole in patients undergoing thallium 201 myocardial imaging

Oral dipyridamole has been used in conjunction with thallium 201 imaging to diagnose coronary artery disease in patients unable to exercise. However, the pharmacokinetic disposition of high-dose dipyridamole, particularly the time to maximum drug concentration, has not been described in patients undergoing oral dipyridamole-thallium 201 scanning. Therefore, we measured serial concentrations in 20 outpatients undergoing thallium 201 testing after a single 400-mg oral dose of dipyridamole. In patients with positive thallium scans (group A), concentrations were slightly higher than in those with negative scans (group B). The variability in concentrations for both groups ranged from 3-118%. The area under the dipyridamole concentration-time curve from 0-4 hours ranged from 2.9-2.3 microgram.hr/ml. The time to peak plasma level for dipyridamole ranged from 0.5-2.5 hours, with two patients having undetectable levels at the time of thallium 201 injection. Dipyridamole concentrations did not correlate with changes in diastolic or systolic blood pressure, or heart rate. Although it is unknown what concentration is required to produce changes in coronary flow velocity, use of oral dipyridamole in this setting resulted in highly variable concentrations. In addition, these data suggest that a small percentage of patients will not have detectable concentrations at the time of the first thallium 201 scan, possibly limiting the usefulness of this agent in conjunction with thallium scintigraphy in some patients.