BIOCHEMICAL PHARMACOLOGY OF PENCLOMEDINE (NSC-338720)

Penclomedine (PEN) is a synthetic pyridine derivative that has been se lected for clinical development based on its activity against human an d mouse breast tumors implanted in mice. Its mechanism of action was u nclear, and we were interested in determining its mechanism of cytotox icity in vitro and in vivo. We found chromosome breaks, gaps, and exch anges in P388 ascites cells from BD2F1 mice treated with 200 mg/kg PEN . Maximal observed damage occurred 24 hr after drug administration. Al kaline elution indicated only limited DNA strand breaks and interstran d cross-linking; in vitro, PEN (75 mu g/mL) inhibited RNA and DNA synt heses almost completely. In addition, incubation of [C-14]PEN with rat liver S-9 fraction in the presence of calf thymus DNA resulted in the stable transfer of radioactivity to DNA. Addition of butylated hydrox ytoluene, a free radical scavenger, to the incubation mixture inhibite d the binding of drug to DNA, implicating free radicals as the ultimat e reactive species. These data suggest that PEN can be metabolized to free radical, DNA-reactive products, and that its cytotoxicity is due to chromosomal damage produced by monofunctional alkylation. As an alt ernate mechanism, the ability of PEN to inhibit cellular dihydroorotat e dehydrogenase was explored. Although PEN is an inhibitor of this enz yme in cells in vivo, in vitro, and in isolated cell sonicates, HPLC a nalyses of ribonucleotide triphosphate pools in P388 cells showed that all triphosphates had increased, especially UTP. Addition of uridine to the cell culture failed to prevent PEN-mediated cytotoxicity, sugge sting that inhibition of de novo pyrimidine biosynthesis was not likel y to be an important mechanism of action of this drug. These data sugg est that PEN is activated in cells to a free radical that binds DNA.