Pergamon Free Radical Biology & Medicine,Vol. 18, No. 2, pp. 239 247, 1995 Copyright ~q) 1995 ElsevierScience Ltd Printed in the USA. All rights reserved 0891-5849/95 $9.50 + .00 0891-5849(94)E0123-5 Original Contribution A POTENT CHAIN-BREAKING ANTIOXIDANT ACTIVITY OF THE CARDIOVASCULAR DRUG DIPYRIDAMOLE LUIGI IULIANO,* JENS Z. PEDERSEN,* GIUSEPPE ROTILIO,* DOMENICO FERRO,* and FRANCESCO VIOLI* *Institute of I Clinical Medicine, University La Sapienza, Rome, Italy; and *Department of Biology, University Tor Vergata, Rome, Italy (Received 5 January 1994; Revised 29 April 1994; Accepted 25 May 1994) Abstract--The antioxidant properties of the antithrombotic drug dipyridamole have been studied using lipid oxidation assays based on the generation of peroxy radicals by azo compounds. Dipyridamole was observed to prevent both peroxidation of arachidonic acid micelles in aqueous solution and peroxidation of methyl linoleate in organic solvents; in contrast to vitamin E, dipyridamole was found to scavenge both hydrophilic and hydrophobic radicals. The rate constant for the reaction of dipyridamole with methyl linoleate peroxyl radicals at 37°C was calculated as 2 × 106 M's ~, in comparison to 1 × 106 M Ls ~ of vitamin E under the same conditions. The antioxidant efficiency of the drug was confirmed in experiments with radiolysis-induced oxidation and through measurements of malondialdehyde production and diene formation. As a result of radical scavenging, a relatively stable dipyridamole radical was formed that could be detected by electron spin resonance spectroscopy. The particular antioxidant properties of dipyridamole may explain the vasodilating and antiplatelet effects of this cardiovascular drug. Keywords--Dipyridamole, Antioxidants, Lipid peroxidation, Free radicals, Atherosclerosis, Cardiovascular drugs, Platelets, INTRODUCTION Dipyridamole is a drug used in clinical practice for the treatment of cardiovascular diseases because of its vasodilating and antiplatelet activity) Although this compound has been used for many years, its mecha- nism of action is still not known. It has been suggested to inhibit platelet phosphodiesterase, to stimulate the release of prostacyclin, or to inhibit the transport of adenosine; however, there is no consensus as to the mechanism that actually causes the observed effects in vivo) Among the different effects described, dipyri- damole has also been reported to possess antioxidant activity. Studies from this and other laboratories have demonstrated that the drug inhibits lipid peroxidation 2'4 and scavenges superoxide 4 and hydroxyl radicals. 5 These reactive species are involved in the reactions that lead to initiation of the lipid peroxidation process, and it has been proposed that the superoxide and hy- droxyl radical scavenging activity could be linked to Address correspondence to: Luigi Iuliano, Institute of I Clinical Medicine, University La Sapienza, 00185 Rome, Italy. the capacity of dipyridamole to inhibit lipid peroxida- tion. 5 Because lipid peroxidation proceeds through propa- gation reactions depending on the generation of per- oxyl radicals, we undertook this study to investigate if dipyridamole can react directly with peroxyl radicals. The scavenging of peroxyl radicals is an important point in the characterization of an antioxidant mole- cule 6 because lipid peroxidation is not only induced by superoxide and HO'-mediated reactions, but also by peroxyl radicals. Furthermore, peroxyl radicals have been described to react with biological targets different from iipids, 7-~° which eventually causes a va- riety of tissue damage and pathologic events. To study the reactivity of dipyridamole, we used a system based on peroxyl-generating azoinitiators, ff'j2 The oxidation of lipids induced by azoinitiators can be represented by the following scheme: k~ R--C--N=N--C--R --, 2R--C" + N2 (1) R--C" + 02 ~ R--CO0" (2) R--COO" + LH ~ R--COOH + L" (3) 239