Pharmacokinetics of Intraperitoneally Administered Dipyridamole in Cancer Patients

The pharmacokinetics of i.p. administered dipyridamole was studied in six patients to explore the feasibility of using this drug as a modulator of antimetabolite activity in extravascular spaces. Infusions of dipyridamole (50 mg/m2 in 2 liters of normal saline) into the peritoneal cavity resulted in peak drug concentrations 5 to 20 times higher in that cavity than in the plasma. The peritoneal decay data for dipyridamole fitted very well to a single compartment open pharmacokinetic model with one exponential term, while the plasma data are adequately described by a single compartment model with two exponentials (a short absorption phase). The mean peritoneal half-life for total extractable dipyridamole was 3.3 +/- 1.9 (SD) h, and the mean peritoneal clearance was 0.4 +/- 0.3 liters/h/m2. The mean plasma half-life of total dipyridamole in our patients was 2.2 +/- 1.2 h, and the mean clearance value was 5.7 +/- 4.7 liters/h/m2. The area under the concentration versus time curve was calculated to be 626 +/- 312 microM-h for the peritoneal cavity and 45 +/- 20 microM-h for the plasma. Using membrane ultrafiltration, we have measured the concentration of free (non-protein bound) dipyridamole in each patient. While the peritoneal clearance values of free and total drug are comparable, the plasma clearance of free dipyridamole was 47 +/- 39 liters/h/m2. This increased plasma clearance resulted in a plasma area under the concentration versus time curve of 8.3 +/- 5.1 microM-h, which suggests minimal systemic exposure. Our data show that instillation of dipyridamole into the peritoneal cavity resulted in much higher local drug exposure than systemic exposure, confirming the feasibility of using this drug to augment antimetabolite activity within the peritoneal cavity. Since dipyridamole is highly protein bound in the plasma but less so in the peritoneal cavity, these data imply that peritoneal exposure to active (free) dipyridamole is far greater than systemic exposure in our patients.