Transient activation of topoisomerase I in leukotriene D4 signal transduction in human cells.

U937 human monoblast cells incubated with leukotriene D4 (LTD4) rapidly released arachidonic acid metabolites into the culture medium. Release was suppressed by the high-affinity LTD4 receptor antagonist SK&F 104353. Arachidonic acid release induced by LTD4 has been linked to a rapid induction of gene expression, and the propagation of the receptor binding signal is probably associated with enzymes that regulate gene expression. We have studied the participation of DNA topoisomerase I in LTD4 signal transduction. LTD4-specific release of arachidonic acid metabolites was inhibited (60-80%) by the topoisomerase I inhibitor camptothecin. LTD4 increased protein-linked DNA strand breakage induced by camptothecin in U937 cells; this enhancement was prevented by coincubation of the cells with LTD4 plus the receptor antagonist SK&F 104353. In addition, LTD4 produced a rapid transient increase in extractable topoisomerase I activity, which was maximum within the first 10 min after addition of LTD4 to the culture medium. Incubation of cultures for greater than 10 min with LTD4 before the addition of camptothecin resulted in no enhancement of camptothecin-induced DNA strand breakage, consistent with a reversal of topoisomerase I activation. Staurosporine, an inhibitor of protein kinase C, blocked LTD4-induced arachidonic acid release and attenuated the effect of LTD4 on camptothecin-induced DNA strand breakage. These results are consistent with the view that the regulation of topoisomerase I activity is involved in the propagation of LTD4-mediated signals in U937 cells.

Evidence for the participation of topoisomerases I and II in cadmium-induced metallothionein expression in Chinese hamster ovary cells.

CHO-Cdr20 cells are 10-20 times more resistant to killing by cadmium than the parental CHO cells. Resistance has been linked to amplification of the metallothionein genes MT-I and MT-II and their coordinate induction by cadmium and other toxic metals. We studied the roles of the nuclear enzymes topoisomerase I and topoisomerase II in Cd-induced expression of MT-II. Camptothecin-induced DNA strand breakage, mediated by topoisomerase I in cells, increased by approximately 20% when the resistant cells were incubated first with 50 microM Cd and then with camptothecin. Short DNA fragments were enriched in MT-II-hybridizing sequences, indicating that topoisomerase I-associated breakage was directed in part toward the location of induced gene activity. Ten microM camptothecin inhibited Cd-induced accumulation of MT-II mRNA as well as induced and uninduced RNA synthesis in the resistant cells. These data are consistent with the notion that topoisomerase I participates in most or all forms of RNA synthesis. Topoisomerase II inhibitors which trap cleavable complexes (amsacrine, VM-26, VP-16) increased DNA strand breakage at very high concentrations (50-100 microM); the increased breakage appeared to be concentrated near the MT-II gene. This class of inhibitor did not block the accumulation of MT-II message. Novobiocin, a second type of topoisomerase II inhibitor blocked transcription at 300 microM. Merbarone, a novel, third type of topoisomerase II inhibitor, blocked MT-II transcription at 50-100 microM. The latter two inhibited total RNA synthesis in induced, but not uninduced cells. Thus, it is possible that topoisomerase II plays more than one role in transcription and that more than one form of this enzyme is involved.

In vitro and intracellular inhibition of topoisomerase II by the antitumor agent merbarone.

Merbarone has previously been shown to have antitumor activity of unknown mechanism in P388 and L1210 tumor models (A. D. Brewer et al., Biochem. Pharmacol., 34:2047-2050, 1985) and is currently undergoing Phase I clinical trials. Here we report that merbarone is an inhibitor of topoisomerase II. Merbarone inhibited purified mammalian topoisomerase II with a 50% inhibitory concentration of 20 microM, as assessed by ATP-dependent unknotting of P4 phage DNA or relaxation of supercoiled pBR322 plasmid. In contrast to the type II enzyme, inhibition of catalytic activity of topoisomerase I required about 10-fold higher concentrations of merbarone, with a 50% inhibitory concentration of approximately 200 microM. Unlike epipodophyllotoxin analogues and certain DNA intercalative agents which stabilize the topoisomerase II-DNA "cleavable complex," merbarone did not cause detectable topoisomerase II-induced DNA cleavage. Furthermore, merbarone inhibited the production by amsacrine or teniposide of topoisomerase II-associated DNA strand breaks; under identical conditions novobiocin did not decrease these breaks, setting merbarone apart from a novobiocin-like class of topoisomerase II inhibitor. In L1210 cells, merbarone produced only small numbers of protein-associated DNA strand breaks, and only at very high concentrations. Merbarone reduced in a concentration-dependent manner the number of amsacrine- or teniposide-stimulated protein-associated DNA strand breaks in L1210 cells or their isolated nuclei. The data suggest that merbarone represents a novel type of topoisomerase II inhibitor.

Modification of the hydroxy lactone ring of camptothecin: inhibition of mammalian topoisomerase I and biological activity.

Several camptothecin derivatives containing a modified hydroxy lactone ring have been synthesized and evaluated for inhibition of topoisomerase I and cytotoxicity to mammalian cells. Each of the groups of the hydroxy lactone moiety, the carbonyl oxygen, the ring lactone oxygen, and the 20-hydroxy group, were shown to be critical for enzyme inhibition. For example the lactol, lactam, thiolactone, and 20-deoxy derivatives did not stabilize the covalent DNA-topoisomerase I complex. With a few exceptions, those compounds that did not inhibit topoisomerase I were not cytotoxic to mammalian cells. Two cytotoxic derivatives that did not inhibit topoisomerase I were shown to produce non-protein-associated DNA single-strand breaks and are likely to have a different mechanism of action. One of these compounds was tested for antitumor activity and was found to be inactive. The present findings, as well as other reports that the hydroxy lactone ring of camptothecin is critical for antitumor activity in vivo, correlate with the structure-activity relationships at the level of topoisomerase I and support the hypothesis that antitumor activity is related to inhibition of this target enzyme.

Synthesis, tubulin binding, antineoplastic evaluation, and structure-activity relationship of oncodazole analogues.

In an attempt to identify a soluble oncodazole analogue that could be easily formulated, a series of substituted oncodazoles was synthesized and evaluated for tubulin binding affinity, in vitro cytotoxicity against cultured mouse B-16 cells, and ability to prolong lifespan at the maximally tolerated dose in the P388 mouse leukemia model. Biological evaluation of all the isomeric methyloncodazoles demonstrated the thiophene 4'-position to be the only site of significant bulk tolerance, although substitution of this position with polar or charged functional groups abolished biological activity. Simple esters of the 4'-carboxymethyloncodazole were shown to have enhanced antitumor activity and tubulin binding affinity relative to oncodazole. Despite a failure of this study to identify a water-soluble oncodazole with antitumor activity, the structure-activity relationship developed led to a derivative with enhanced activity in the P388 leukemia model and facilitated the preparation of a biologically active photolabile analogue.

Pyrrolo[1,4]benzodiazepine antitumor antibiotics: relationship of DNA alkylation and sequence specificity to the biological activity of natural and synthetic compounds.

The DNA alkylation and sequence specificity of a group of natural and synthetic pyrrolo-[1,4]benzodiazepines [P(1,4)Bs] were evaluated by using an exonuclease III stop assay, and the results were compared with in vitro and in vivo biological potency and antitumor activity. The P(1,4)B antibiotics are potent antitumor agents produced by various Actinomycetes, which are believed to mediate their cytotoxic effects by covalent bonding through N-2 of guanine in the minor groove of DNA. In this article we describe the results of a sensitive DNA alkylation assay using exonuclease III which permits both estimation of the extent of DNA modification as well as location of the precise guanines to which the drugs are covalently bound. Using this assay, we have evaluated a series of natural and synthetic compounds of the P(1,4)B class for their ability to bind to DNA and also determined their DNA sequence preference. The compounds included in this study are P(1,4)Bs carrying different substituents in the aromatic ring, having varying degrees of saturation in the five-membered ring, or differing in the stereochemistry at C-11a. These same compounds were evaluated for in vitro cytotoxic activity against B16 melanoma cells, for potency in vivo in B6D2F1 mice (LD50), and for antitumor activity (ILSmax) against P388 leukemia cells. A good correlation was found between extent of DNA alkylation and in vitro and in vivo potency. Furthermore, on the basis of electronic and steric considerations, it was possible to rationalize why those compounds that showed negligible biological activity were unable to bond covalently to DNA. Last, we have determined that the degree of saturation in the five-membered ring of the P(1,4)Bs has a significant effect on the DNA bonding reactivity and biological activity of this class of compounds.

cis-4-Carboxy-6-(mercaptomethyl)-3,4,5,6-tetrahydropyrimidin-2(1 H)-one , a potent inhibitor of mammalian dihydroorotase.

A series of cis- and trans-4-carboxy-3,4,5,6-tetrahydropyrimidin-2(1H)-ones possessing either a carboxy, hydroxymethyl, or mercaptomethyl substituent at C-6 were prepared and tested for their ability to inhibit mammalian dihydroorotase. Of these compounds, only the cis-6-mercaptomethyl compound, cis-1, was found to be a potent competitive inhibitor of the enzyme (Ki = 140 nM at pH 7.4 and 8.5) when assayed in the direction of dihydro-L-orotate hydrolysis. These results suggest that the inhibition arises from the ligation of the thiolate to the zinc atom which is thought to be located in the enzyme's active site. Although analysis of cis-1 with 2,2'-dithiobis(5-nitrobenzoic acid) revealed significant loss of the free thiol group under enzymatic assay conditions, the addition of the reducing agent, dithiothreitol, to the enzymatic reaction mixtures afforded cis-1 complete protection against this chemical decomposition, as evidenced by lowering of the inhibition constant in the presence of dithiothreitol. Compound cis-1 had no significant antiproliferative activity against B16 melanoma cells in tissue culture, possibly due to the rapid decomposition of the compound or poor permeability into cells.

Relationship between the intracellular effects of camptothecin and the inhibition of DNA topoisomerase I in cultured L1210 cells.

Results of filter elution assays of lesions produced in the DNA of cultured L1210 cells by the antineoplastic alkaloid camptothecin support the notion that topoisomerase I is an intracellular target of this drug. One to 10 microM camptothecin induced DNA single-strand, but not double-strand, breaks when incubated with intact cells or with their isolated nuclei. Approximately one half of the strand breakage was protein concealed, as judged by filter elution. Camptothecin-induced, protein-concealed DNA strand breaks disappeared rapidly after drug removal. DNA-protein cross-links were generated by camptothecin with frequencies approximately equal to those of protein-concealed DNA strand breaks. It is likely that camptothecin can inhibit topoisomerase I in intact cells in a manner similar to that in which other antineoplastic agents such as amsacrine or teniposide inhibit topoisomerase II. DNA-breaking lesions other than those resulting from trapped topoisomerase I-DNA complexes may also be generated by camptothecin. The yields of DNA strand breaks induced by camptothecin, amsacrine, or teniposide were approximately doubled when cells were incubated for 16 h with 3-aminobenzamide, an inhibitor of poly(ADP ribosylation) of proteins, prior to 1-h exposure to the antineoplastic compounds. 3-Aminobenzamide also enhanced the cytotoxic action of camptothecin, amsacrine, and teniposide. These results suggest that protein-concealed strand breaks can be lethal lesions and that intracellular topoisomerase I and II activity may be regulated coordinately through poly(ADP ribosylation).

In vivo antitumor activity and in vitro cytotoxic properties of bis[1,2-bis(diphenylphosphino)ethane]gold(I) chloride.

We have previously reported the cytotoxicity and antitumor activity of bis(diphenylphosphino)ethane (DPPE) and a variety of its transition metal complexes. During studies of the chemistry of a gold complex of this group [(AuCl)2(DPPE)], it was observed that this complex readily underwent ring closure on reaction with DPPE to form the tetrahedral complex [Au(DPPE)2]+. Various counterion forms (e.g., Cl-) of this cation were isolated and were found to exhibit a remarkably high stability in solution. Evaluation of [Au(DPPE)2]Cl in mice bearing i.p. P388 leukemia demonstrated that the compound produced an average of 87% increase in life span at its maximally tolerated dose (2-3 mumol/kg/day for 5 days). Activity was also seen in i.p. M5076 reticulum cell sarcoma (60% increase in life span) and s.c. mammary adenocarcinoma 16/c. Modest activity was evident in i.p. B16 melanoma and L1210 leukemia. A subline of P388 leukemia resistant to cisplatin was not cross-resistant to [Au(DPPE)2]Cl. In addition, combination therapy of [Au(DPPE)2]Cl and cisplatin against i.p. P388 demonstrated an advantage over single-agent therapy. In vitro studies of [Au(DPPE)2]Cl showed that the compound: is cytotoxic to tumor cell lines; is only minimally inhibited in its cytotoxic activity by the presence of serum; produces DNA protein cross-links and DNA strand breaks in cells; and inhibits macromolecular synthesis with a preferential inhibitory effect on protein synthesis relative to DNA and RNA synthesis. 31P nuclear magnetic resonance spectroscopy indicated that the compound is stable in the presence of serum proteins, thiols, or disulfides and that it reacts with Cu(II) resulting in the formation of a Cu(I)DPPE complex. The results of these in vivo and in vitro experiments suggest that the contrasting pharmacological profile of [Au(DPPE)2]Cl with respect to other gold(I) phosphine complexes may be related to both the kinetic stability of the complex and its stability in the presence of thiols.

Cellular pharmacology of mu-[1,2-bis(diphenylphosphino)ethane]bis[(1-thio-beta-D-gluco pyranosato-S)gold(I)]: a novel antitumor agent.

SK&F 102912 (mu-[1,2-bis(diphenylphosphino)ethane]bis[(1-thio-beta-D- glucopyranosato-S)gold(I)], [(Autg)2(dppe)]) has shown reproducible and significant activity in transplantable murine tumor models and represents a structurally unique class of antineoplastic agents. A number of in vitro studies were performed to elucidate the cellular pharmacology of this gold-containing complex. [(Autg)2(dppe)] is a potent cytotoxic agent in vitro as demonstrated by its ability to inhibit the clonogenic capacity of a variety of tumor cell lines following a brief exposure to the drug. Cell-cycle analysis using HL-60 cells showed that low concentrations (2 microM) of [(Autg)2(dppe)] induced an S-phase block and higher concentrations induced a secondary block at the G1/S boundary. [(Autg)2(dppe)] had several effects on DNA metabolism and structure including preferential inhibition in cells of DNA synthesis (relative to RNA and protein synthesis) and the production of DNA single- and double-strand breaks as measured by alkaline elution. The cytoxic mechanism of this gold complex appears to be distinct from that of the monophosphine-gold complex auranofin.

Application of a tissue culture microtiter test for the detection of cytotoxic agents from natural products.

A method is described by which the growth inhibitory effects of cytotoxic compounds and fermentation broth cultures on adherent tumor cell lines can be quantitated. Cells are seeded into 96-well microtiter plates and 16 hours later the test compounds or broths are added to the wells. Cell growth is measured after three days (B16 mouse melanoma cells) or six days (HT-29, human colon carcinoma cells) by first fixing adherent cells, staining with Giemsa stain, washing away excess stain, then solubilizing stained cells with HCl. Absorbance is determined using a microELISA spectrophotometer and the data are transferred to and analyzed by a computer. The assay is rapid and reproducible and can be used to identify fermentation broths with cytotoxic components. Addition of DNA into the assay mixture (cells plus compound) inhibits the cytotoxic activities of certain DNA-reactive agents. The results of this study demonstrate the application of this assay system for primary and secondary evaluation of fermentation broths for in vitro antitumor activity.