We have recently demonstrated that substitution of two fluorine atoms for hydrogen adjacent to the acetalic linkage in the 2,6-dioxal(1.1.3) heptane system of thromboxane A2 (TXA2) results in a 108 fold decrease in the bimolecular rate constant of hydrolysis. The difluoro derivatives, in contrast to TXA2, which has a half-life of 30 sec at pH 7.4 are stable even at lower pH permitting chemical operations to be performed without hydrolysis. Based on this finding we have undertaken and almost completed a chiral synthesis of 10,10-difluoroTXA2. This material will be tested biologically in several TXA2 sensitive systems. We expect this compound to possess powerful agonistic effects. We have also initiated a more efficient synthesis of this compound, which is suitable for the preparation of receptor antagonists of TXA2 and inhibitors of thromboxane synthase. Such compounds are of potential value for the prevention of platelet aggregation and thrombus formation as well as spastic conditions of the vascular system. We have completed syntheses of 7,7- and 10,10-difluoroarachidonic acids (DFAA) and have begun their evaluation as substrates of enzymes of the eicosanoid cascade. Both are substrates for PGH synthase and form metabolites. A few of the resulting structures have been identified as fluoro-HETE's and diHETE's and it is a major purpose of this application to determine the structure of all the metabolites and, in collaboration with cell biologists and pharmacologists, to explore their biological properties. We will then examine the action of additional enzymes on 7,7- and 10,10-DFAA. Among these will be washed platelets, which supply both PGH synthase and TXA synthase in a single preparation. We shall search for 7,7- and 10,10-difluoro PGH2 and if isolated, examine their conversion by prostacyclin and thromboxane synthase. The DFAA's will also be investigated as potential enzyme inhibitors. Thus, from the known mechanism of leukotriene biosynthesis 7,7-DFAA could be a reversible inhibitor of 5-lipoxygenase while 10,10-DFAA would proceed as far as 5-HPETE, conversion to LTCA being blocked by the fluorines at C-10.
