MECHANISM OF TOXICITY OF FLUOROCYTOSINE AND FLUOROURACIL

The long term objective of this proposal is to improve the clinical
effectiveness of fluoropyrimidine (FP) drugs through a better
understanding of the biochemical and pharmacologic factors that
determine toxicity with a major focus on FP catabolism in
humans. Our knowledge of FP catabolism is still quite limited,
despite the obvious importance in determining availability of
active drug. Initially, the three enzymes in FP catabolism: uracil
reductase (UR), dihydropyrimidinase and ureidopropionase will be
purified from human liver and characterized. FP metabolism will
also be examined in hepatocytes and non-parenchymal cells
isolated from human liver. A study will be made of partial and
complete deficiencies of UR, initially in affected families (to
clarify the genetics of this syndrome) and later in FP-treated
patients to determine whether partial deficiency of enzyme is a
factor in some cases of FP-induced toxicity. The second major
area of this proposal will be to identify and characterize novel
FUra metabolite(s) recently isolated from human bile.
Preliminary studies suggested these to be conjugates of fluoro-
beta-alanine (FBAL) and bile acids (e.g. cholic acid). These
metabolites may have a role in the pathogenesis of FP-induced
cholestasis, seen frequently with hepatic arterial infusion (HAI) of
fluorodeoxyuridine (FdUrd). An attempt will be made to
synthesize FBAL-bile acid conjugates for use in in vivo studies.
The enzyme cholyl-CoA-amino acid N-acyltransferase will be
purified from human liver to determine affinity of FBAL
compared with beta-alanine, taurine and glycine. In human
hepatocytes, FP catabolism will be assessed in absence or
presence of biochemical modulators known to inhibit FP
catabolism in an attempt to decrease the formation of these
conjugates. Utilizing the isolated perfused rat liver, bile flow and
bile acid content will be assessed after HAI of FdUrd or FBAL-
conjugate. Further metabolism of FBAL-conjugates by intestinal
microflora will be evaluated in a semicontinuous culture system.
In clinical studies (3H)-FdUrd will be administered by HAI with
bile analysis to determine presence of metabolites. Late the
presence of the conjugates will be quantitated in serum from
patients treated with HAI FdUrd (non-labelled), with simultaneous
assessment of liver dysfunction. It is hoped by understanding FP
catabolism and the mechanism of FP-induced cholestasis
improved FP regimens can be designed with decreased host
toxicity + increased antitumor effect.