Structural analogs of alkylacetylglycerophosphocholine inhibitory behavior on platelet activation

Several analogs of alkylacetylglycerophosphocholine (AGEPC; platelet-activating factor) were investigated as potential selective inhibitors of AGEPC-induced activation of washed rabbit platelets. Two particular compounds, CV-3988 (rac-3-(N-n-octadecylcarbamoyloxy)-2-methoxypropyl-2-thiazolioethyl++ + phosphate) and U66985 (1-O-octadecyl-2-acetyl-sn-glycero-3-phosphoric acid-6'-trimethylammonium-hexyl ester) emerged as particularly active and effective inhibitors. Aggregation and secretion profiles, as well as the degradation of inositol phospholipids and production of phosphatidic acid, were used as monitors of their inhibitory capabilities. U66985 was the most effective inhibitor, giving an IC50 value of 4.1 +/- 1.5 X 10(-8) M against a challenge of 1 X 10(-10) M AGEPC in the secretion assay. Phospholipid turnover was blocked completely at this inhibitor concentration. On the other hand, while CV-3988 was an effective inhibitor, a higher concentration was required and a more restricted range of activity was noted with an IC50 value of 5.9 +/- 1.3 X 10(-7) M against a challenge of 1 X 10(-10) M AGEPC in the secretion assay. While CV-3988 did indeed completely block the turnover of inositol phospholipids and phosphatidic acid formation, these effects were noted at a higher concentration than with U66985. On the basis of data obtained in desensitization experiments with AGEPC and U66985, it appears that each inhibitor occupies the same receptor site as the agonist, AGEPC. These results illustrate the usefulness of these AGEPC analogs in exploring the biochemical characteristics of the interaction of AGEPC with a cell.