Antitumor agents 68: effects of a series of helenalin derivatives on P-388 lymphocytic leukemia nucleic acid and protein synthesis

In vitro inhibition of mouse hepatic mixed-function oxidase enzymes by helenalin and alantolactone

The sesquiterpene lactones (STL) helenalin and alantolactone were effective in vitro inhibitors of the mouse hepatic microsomal mixed-function oxidase (MFO) enzymes, aminopyrine demethylase (APD), aniline hydroxylase (ANH) and 7-ethoxyresorufin deethylase (ERD). Helenalin and alantolactone concentrations of 0.5 mM produced a 50-60% inhibition of APD and ERD, and a 20-30% inhibition of ANH. An increase in substrate (aminopyrine) concentration from 0.5 to 25 mM decreased STL inhibition of APD by 12-32%. APD was also inhibited at low aminopyrine concentrations (0.5 mM) by the helenalin derivative 2,3,11,13-tetrahydrohelenalin (tetrahydrohelenalin). The STL produced type I binding spectra with oxidized microsomes; Ks values for helenalin and alantolactone were 161 and 9 microM respectively. These results suggest that STL inhibition of the MFO system results, in part, from STL binding to the substrate-binding site of cytochrome P-450. It has been reported that the irreversible alkylation of protein cysteinyl residues is responsible for STL inhibition of several different enzymes, and second-order rate constants for the reaction of helenalin and alantolactone with glutathione were 25.1 and 1.80 mM-1.hr-1 respectively. Tetrahydrohelenalin did not react with glutathione. However, the subsequent addition of 3.0 mM thiols, i.e. L-cysteine, N-acetylcysteine or glutathione, to STL-treated (0.5 mM) microsomes reversed helenalin and alantolactone inhibition of APD and ERD by 50-80%. The ability of thiols to reverse STL inhibition of APD was decreased 20-43% by the coincubation of STL and microsomes with an NADPH-generating system. In addition, established effects of sulfhydryl-reactive compounds on the MFO system, i.e. inhibition of NADPH-cytochrome c reductase and conversion of cytochrome P-450 to cytochrome P-420, were not observed after addition of helenalin (1.0 mM) or alantolactone (0.5 mM) to mouse hepatic microsomes. These results suggest that STL inhibition of MFO enzymes may not be dependent upon the reactivity of the STL towards sulfhydryl groups. Instead, we suggest that STL binding to the substrate-binding site of cytochrome P-450 and subsequent metabolism of the STL may contribute to inhibition of the MFO system.

The effects of phthalimide and saccharin derivatives on low-density lipoprotein (LDL) and high-density lipoprotein (HDL) receptor activity and related enzyme activities

The hypolipidemic agents, phthalimide, saccharin, o-(N-phthalimido) acetophenone, N-(p-chlorobenzoyl) sulfamate, and o-chlorobenzylsulfonamide affected low-density lipoprotein (LDL) and high-density lipoprotein (HDL) receptor activity and lipoprotein degradation. In isolated rat hepatocytes, rat aorta foam cells, and human fibroblasts, LDL receptor activity, which is dependent on apo-B and -E, was inhibited by the drugs in a dose-dependent manner. LDL degradation was accelerated in the hepatocytes, while it was inhibited in aorta cells and fibroblasts. The drugs enhanced HDL receptor activity, dependent on apo-E and -A1, and HDL degradation in the hepatocytes, whereas in fibroblasts and aorta cells HDL receptor binding and degradation were suppressed. In parallel, activities of acyl CoA acyl transferase, sn-glycerol-3-phosphate acyl transferase, and heparin-induced lipoprotein lipase decreased and activities of HMG-CoA reductase and cholesterol oleate-ester hydrolase increased. In fibroblasts the presence of drugs enhanced HDL binding of intracellular cholesterol. In vivo studies demonstrated that phthalimide and saccharin treatment enhanced the clearance of HDL and decreased the clearance of LDL from the serum of rats. The results suggest that the mode of action of the agents is to modulate the lipoprotein receptor and, thereby, the clearance of lipids from peripheral tissue as part of the hypolipidemic activity.

Inhibition of nucleic acid synthesis in P-388 lymphocytic leukemia tumor cells by helenalin and bis(helenalinyl)malonate in vivo

Although the parent sesquiterpene lactone, helenalin, and its derivative, bis(helenalinyl)malonate, are structurally related chemically, they demonstrate differences in their antineoplastic activity, with bis(helenalinyl)malonate being much more active against P-388 lymphocytic leukemia cell growth (T/C% = 261) compared with helenalin (T/C% = 162). Previous studies have shown that both agents strongly inhibit protein synthesis in vivo by greater than 70% after 3 d of administration and in vitro by 50% at a 100 microM concentration of drug. This inhibition of protein synthesis of P-388 cells may be partially responsible for the cytotoxicity of the drug. These agents also inhibit nucleic acid synthesis in vivo, with DNA synthesis being suppressed by greater than 90% after 2 d of administration of drugs at the therapeutic dose. Of the sulfhydryl-bearing enzymes involved in nucleic acid synthesis that were assayed, only the activities of inosine-5'-monophosphate (IMP) dehydrogenase and the ribonucleotide reductase complex were inhibited by greater than 50% by these sulfhydryl-reactive drugs, which would account for the observed inhibition of nucleic acid synthesis in the P-388 cells. The inhibition of the activities of these enzymes lowered the deoxyribonucleotide levels in P-388 cells, which would explain the overall suppression of DNA synthesis by the sesquiterpene lactones.

Inhibition of inosine monophosphate dehydrogenase by sesquiterpene lactones

Inosine monophosphate (IMP) dehydrogenase had previously been determined to be a likely target enzyme for the sesquiterpene lactones, a class of potential anti-neoplastic drugs. IMP dehydrogenase was purified approx. 770-fold from the P-388 lymphocytic leukemia tumor cell line. The Km values for the substrates, IMP and NAD, were determined to be 12 microM and 25 microM, respectively. Xanthine monophosphate (XMP) was shown to be a competitive inhibitor with a Ki of 67 microM. Mycophenolic acid gave mixed-type inhibition with a Ki of 8 nM for the noncompetitive component and a Ki of 2 nM for the competitive component. Dissociation constants (Kd) and rate constants for inhibition of IMP dehydrogenase by nine different sesquiterpene lactones were determined. The highest Kd was seen with 2,3-dihydrohelenalin while the lowest Kd was observed with bis-helenalinyl malonate. Binding of the drugs by IMP dehydrogenase increased as the size of the drug increased. Also, changes in structure at position 6 had a relatively large effect on the Kd. There was no correlation with hydrophobicity, as determined by octanol/water partition. The first-order rate constants for the reaction of the sesquiterpene lactones with IMP dehydrogenase (k1) and the second-order rate constants for the reaction of the sesquiterpene lactones with glutathione (k2) were also determined. The rate constants for most of the sesquiterpene lactones with the alpha-methylene-gamma-lactone moiety were similar and were approximately twice as great as the rate constants for those sesquiterpene lactones with only the alpha, beta-unsaturated cyclopentenone ring. Microlenin had approximately 5-times the reactivity of the other sesquiterpene lactones towards IMP dehydrogenase, but had approximately the same reactivity towards glutathione, suggesting that it was bound to the enzyme in a way which facilitated its reaction with one or more essential sulfhydryls. The same procedure was used for a series of N-substituted maleimide compounds with the N-substituent ranging in size from a methyl group to a benzyl group. The binding of the maleimide compounds was generally tighter than for the sesquiterpene lactones and there was an increase in binding with size.