IDENTIFICATION OF A NOVEL INHIBITOR (NSC-665564) OF DIHYDROOROTATE DEHYDROGENASE WITH A POTENCY EQUIVALENT TO BREQUINAR

A novel inhibitor of dihydroorotate dehydrogenase (DHO-DH) has been di scovered using data from the National Cancer Institute's in vitro drug screen. Upon analysis of cytotoxicity results from the sixty tumor ce ll lines used in this screen. the COMPARE program predicted that NSC 6 65564 was likely to have the same mechanism of inhibition as brequinar , a known potent inhibitor of DHO-DH. We validated this prediction exp erimentally using MOLT-4 lymphoblast and found the IC50 of brequinar ( 0.5 mu M) and NSC 665564 (0.3 mu M) were comparable and that this indu ced cytotoxicity was reversed by either uridine or cytidine. The enzym e target of NSC 665564 was shown to be identical to that of brequinar when incubation with each drug followed by a 1 h pulse with [C-14] sod ium bicarbonate resulted in cellular accumulation of [C-14]N-carbamyl- L-aspartic acid and [C-14]L-dihydroorotic acid, with concurrent marked depletion of CTP and UTP, The K-i's for NSC 665564 and brequinar were 0.14 and 0.24 mu M, respectively, when partially purified MOLT-4 mito chondria (the site of DHO-DH) were used. These results show that mecha nistic predictions obtained using correlations from the COMPARE algori thm are independent of structure since the structure of NSC 665564 is dissimilar to that of other established DHO-DH inhibitors. (C) 1996 Ac ademic Press, Inc.