naphthoquinones and brequinar

The pyrimidine de novo nucleotide synthesis consists of 6 sequential steps. Various inhibitors against these enzymes have been developed and evaluated in the clinic for their potential anticancer activity: acivicin inhibits carbamoyl-phosphate-synthase-II, N-(phosphonacetyl)-L- aspartate (PALA) inhibits aspartate-transcarbamylase, Brequinar sodium and dichloroallyl-lawsone (DCL) inhibit dihydroorotate-dehydrogenase, and pyrazofurin (PF) inhibits orotate-phosphoribosyltransferase. We compared their growth inhibition against 3 cell lines from head-and-neck-cancer (HEP-2, UMSCC-14B and UMSCC-14C) and related the sensitivity to their effects on nucleotide pools. In all cell lines Brequinar and PF were the most active compounds with IC50 (50% growth inhibition) values between 0.06-0.37 µM, Acivicin was as potent (IC50s 0.26-1 µM), but DCL was 20-31-fold less active. PALA was most inactive (24-128 µM). At equitoxic concentrations, all pure antipyrimidine de novo inhibitors depleted UTP and CTP after 24 hr exposure, which was most pronounced for Brequinar (between 6-10% of UTP left, and 12-36% CTP), followed by DCL and PF, which were almost similar (6-16% UTP and 12-27% CTP), while PALA was the least active compound (10-70% UTP and 13-68% CTP). Acivicin is a multi-target inhibitor of more glutamine requiring enzymes (including GMP synthetase) and no decrease of UTP was found, but a pronounced decrease in GTP (31-72% left). In conclusion, these 5 inhibitors of the pyrimidine de novo nucleotide synthesis varied considerably in their efficacy and effect on pyrimidine nucleotide pools. Inhibitors of DHO-DH were most effective suggesting a primary role of this enzyme in controlling pyrimidine nucleotide pools.

Topics: Amides; Antineoplastic Agents; Aspartate Carbamoyltransferase; Aspartic Acid; Biphenyl Compounds; Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing); Carcinoma, Squamous Cell; Cell Line, Tumor; Dihydroorotate Dehydrogenase; Head and Neck Neoplasms; Humans; Isoxazoles; Naphthoquinones; Orotate Phosphoribosyltransferase; Oxidoreductases Acting on CH-CH Group Donors; Phosphonoacetic Acid; Purine Nucleotides; Pyrazoles; Pyrimidine Nucleotides; Ribonucleosides; Ribose

The flavin enzyme dihydroorotate dehydrogenase (DHOD; EC 1.3.99.11) catalyzes the oxidation of dihydroorotate to orotate, the fourth step in the de novo pyrimidine biosynthesis of UMP. The enzyme is a promising target for drug design in different biological and clinical applications for cancer and arthritis. The first crystal structure of the class 2 dihydroorotate dehydrogenase from rat has been determined in complex with its two inhibitors brequinar and atovaquone. These inhibitors have shown promising results as anti-proliferative, immunosuppressive, and antiparasitic agents. A unique feature of the class 2 DHODs is their N-terminal extension, which folds into a separate domain comprising two alpha-helices. This domain serves as the binding site for the two inhibitors and the respiratory quinones acting as the second substrate for the class 2 DHODs. The orientation of the first N-terminal helix is very different in the two complexes of rat DHOD (DHODR). Binding of atovaquone causes a 12 A movement of the first residue in the first alpha-helix. Based on the information from the two structures of DHODR, a model for binding of the quinone and the residues important for the interactions could be defined. His 56 and Arg 136, which are fully conserved in all class 2 DHODs, seem to play a key role in the interaction with the electron acceptor. The differences between the membrane-bound rat DHOD and membrane-associated class 2 DHODs exemplified by the Escherichia coli DHOD has been investigated by GRID computations of the hydrophobic probes predicted to interact with the membrane.

Topics: Amino Acid Sequence; Aniline Compounds; Animals; Atovaquone; Biphenyl Compounds; Catalysis; Crotonates; Crystallography, X-Ray; Dihydroorotate Dehydrogenase; Drug Design; Enzyme Inhibitors; Hydrogen Bonding; Hydroxybutyrates; Immunosuppressive Agents; Models, Molecular; Molecular Sequence Data; Molecular Structure; Naphthoquinones; Nitriles; Orotic Acid; Oxidoreductases Acting on CH-CH Group Donors; Protein Binding; Protein Structure, Secondary; Protein Structure, Tertiary; Rats; Sequence Alignment; Substrate Specificity; Toluidines

Mitochondrially-bound dihydroorotate dehydrogenase (EC 1.3.99.11) catalyzes the fourth sequential step in the de novo synthesis of uridine monophosphate. The enzyme has been identified as or surmised to be the pharmacological target for isoxazol, triazine, cinchoninic acid and (naphtho)quinone derivatives, which exerted antiproliferative, immunosuppressive, and antiparasitic effects. Despite this broad spectrum of biological and clinical relevance, there have been no comparative studies on drug-dihydroorotate dehydrogenase interactions. Here, we describe a study of the inhibition of the purified recombinant human and rat dihydroorotate dehydrogenase by ten compounds. 1,4-Naphthoquinone, 5,8-hydroxy-naphthoquinone and the natural compounds juglon, plumbagin and polyporic acid (quinone derivative) were found to function as alternative electron acceptors with 10-30% of control enzyme activity. The human and rat enzyme activity was decreased by 50% by the natural compound lawsone ( > 500 and 49 microM, respectively) and by the derivatives dichloroally-lawsone (67 and 10 nM), lapachol (618 and 61 nM) and atovaquone (15 microM and 698 nM). With respect to the quinone co-substrate of the dihydroorotate dehydrogenase, atovaquone (Kic = 2.7 microM) and dichloroally-lawsone (Kic = 9.8 nM) were shown to be competitive inhibitors of human dihydroorotate dehydrogenase. Atovaquone (Kic = 60 nM) was also acompetitive inhibitor of the rat enzyme. Dichloroally]-lawsone was found to be a time-dependent inhibitor of the rat enzyme, with the lowest inhibition constant (Ki* = 0.77 nM) determined so far for mammalian dihydroorotate dehydrogenases. Another inhibitor, brequinar was previously reported to be a slow-binding inhibitor of the human dihydroorotate dehydrogenase [W. Knecht, M. Loffler, Species-related inhibition of human and rat dihyroorotate dehydrogenase by immunosuppressive isoxazol and cinchoninic acid derivatives, Biochem. Pharmacol. 56 (1998) 1259-1264]. The slow binding features of this potent inhibitor (Ki* = 1.8 nM) with the human enzyme, were verified and seen to be one of the reasons for the narrow therapeutic window (efficacy versus toxicity) reported from clinical trials on its antiproliferative and immunosuppressive action. With respect to the substrate dihydroorotate, atovaquone was an uncompetitive inhibitor of human dihydroorotate dehydrogenase (Kiu = 11.6 microM) and a non-competitive inhibitor of the rat enzyme (Kiu = 905/ Kic = 1,012 nM). 1.5 mM po

Topics: Animals; Atovaquone; Benzoquinones; Binding, Competitive; Biphenyl Compounds; Dihydroorotate Dehydrogenase; Enzyme Inhibitors; Humans; Inhibitory Concentration 50; Kinetics; Naphthoquinones; Oxidoreductases; Oxidoreductases Acting on CH-CH Group Donors; Quinones; Rats; Substrate Specificity

The computer algorithm COMPARE provides information regarding the biological mechanism of action of a compound. In this study, excellent correlations were obtained for 2,2'-[3,3'-dimethoxy[1,1'-biphenyl]-4,4'-diyl)diimino]bis- benzoic acid (redoxal) and 1-(p-bromophenyl)-2-methyl-1H- naphth[2,3-d]imidazole-4,9-dione (BNID) and two well-studied dihydroorotate dehydrogenase (DHOD) inhibitors, dichloroallyl lawsone and brequinar, in terms of antiproliferative activity against tumor cell lines in vitro. When redoxal and BNID were incubated with MOLT-4 cells for 72 hr, 50% growth inhibition was achieved at 0.7 and 3.5 microM, respectively. After 24 hr of incubation, pyrimidine triphosphate pools were shown to be decreased by 50% by redoxal (1 microM) and BNID (0.25 microM). Addition of either uridine (50 microM) or cytidine (100 microM) antagonized the cellular cytotoxicity caused by either drug; uridine corrected the UTP and CTP deficit, whereas cytidine corrected only the CTP deficit. Exposure of MOLT-4 cells to a 1 microM concentration of either drug for 18 hr followed by a 1-hr exposure to [14C]bicarbonate showed a 97% decrease of incorporation of [14C] into pyrimidine triphosphates accompanied by a 91- and 82-fold increase in radioactive incorporation into L-dihydroorotate and N-carbamyl-L-aspartate, respectively. By direct exposure of DHOD prepared from MOLT-4 cell mitochondria to a range of concentrations of the two drugs, apparent Ki values of 0.33 microM (redoxal) and 0.53 microM (BNID) were determined. These data provide direct evidence for inhibition of DHOD by redoxal and BNID in MOLT-4 lymphoblasts.

Topics: Aminobiphenyl Compounds; Antineoplastic Agents; Binding Sites; Biphenyl Compounds; Cell Division; Dihydroorotate Dehydrogenase; Humans; Imidazoles; Naphthoquinones; Oxidoreductases; Oxidoreductases Acting on CH-CH Group Donors; Pyrimidines; Software; Structure-Activity Relationship; Tumor Cells, Cultured

The novel anticancer drug candidate brequinar sodium (DuP 785, NSC 368390, 6-fluoro-2-(2'-fluoro-1,1'-biphenyl-4-yl)-3-methyl-4-quinoline- carboxylic acid sodium salt) was shown previously to be an inhibitor of dihydroorotate dehydrogenase, the fourth enzyme of the de novo pyrimidine biosynthetic pathway. Brequinar sodium inhibits the activity of this enzyme isolated from mammalian sources only but not those forms isolated from yeast or bacteria, which also use ubiquinone as the cofactor. Brequinar sodium also does not inhibit the activity of a soluble Zymobacterium oroticum dihydroorotate dehydrogenase which uses NAD+ as a cofactor. Brequinar sodium inhibits L1210 dihydroorotate dehydrogenase with mixed inhibition kinetics with respect to either the substrate (dihydroorotate) or the cofactor (ubiquinone Q6) with Ki' values in the 5-8 nM range. Our results suggest that brequinar sodium inhibits dihydroorotate dehydrogenase by binding to the enzyme at a unique site that is distinct from the dihydroorotate or the ubiquinone-binding site. This binding site appears to be unique to the mammalian enzyme, because brequinar sodium does not inhibit the yeast, Escherichia coli, or Z. oroticum forms of the enzyme.

Topics: Animals; Antineoplastic Agents; Biphenyl Compounds; Dihydroorotate Dehydrogenase; Kinetics; Leukemia L1210; Mice; Naphthoquinones; Orotic Acid; Oxidoreductases; Oxidoreductases Acting on CH-CH Group Donors