Preclinical antitumor activity of the azonafide series of anthracene-based DNA intercalators

The azonafides are a series of anthracene-based DNA intercalators which inhibit tumor cell growth in vitro at low nanomolar concentrations and are not affected by the multidrug resistance phenomenon (MDR). Prior studies have described antitumor efficacy in murine tumor models including L-1210 and P-388 leukemias, and B-16 melanoma. The current results extend these cell line observations to human tumors tested in the NCI panel of 56 cell lines, in freshly isolated tumors tested in colony-forming assays in soft agar and in several animal models. In the NCI panel, the overall mean 50% cell kill (LC50) for the unsubstituted azonafide, AMP-1, was 10(-5.53) M, with some selectivity noted in melanomas (10(-6.22) M). The mean LC50 for the 6-ethoxy substituted analog, AMP-53, was 10(-5.53) M, with some selectivity found in non-small cell lung cancer (10(-5.91)) and renal cell carcinoma (10(-5.84)). In freshly isolated human tumors tested in soft agar, there was marked activity (mean IC50 in microg/ml) for AMP-53 in four cell types: breast cancer (0.09), lung cancer (0.06), renal cell carcinomas (0.06) and multiple myeloma (0.03). These effects were superior to doxorubicin and to several other azonafides, including AMP-1, AMP-104 and the 6-hydroxyethoxy derivative, AMP-115. Compound AMP-1 was shown to be superior to amonafide in the mammary 16C breast cancer model in B6CF31 mice, but it had little activity in Colon-38 nor in M5076 ovarian sarcomas in vivo. Nine azonafides were evaluated in the Lewis lung cancer model in C57/bl mice, but only AMP-53 demonstrated significant efficacy with a treated/control x 100% (T/C) value of 30%. Because AMP-53 demonstrated the greatest breadth of activity, it was then evaluated in several human tumor cell lines growing in mice with severe combined immunodeficiency disease (SCID). Only three tumors were sensitive (T/C<42%), including HL-60 leukemia (T/C=39%), MCF-7 breast cancer (T/C=39%) and A549 non-small cell lung cancer (T/C=37%). Overall, these results demonstrate that the 6-ethoxy substituted azonafide, AMP-53, has consistent (in vitro and in vivo) experimental antitumor activity in human breast and lung cancer, and could be considered for clinical testing in patients with MDR tumors.

Analogues of amonafide and azonafide with novel ring systems

Three new types of amonafide and azonafide analogues were synthesized and screened in a panel of human solid tumor cells and murine L1210 leukemia cells. The structural types included tetrahydroazonafides, which have the naphthalene chromophore of amonafide within the anthracene nucleus of azonafide; phenanthrene analogues, in which the linear anthracene nucleus is replaced by the bent phenanthrene nucleus; and azaphenanthrenes. The tetrahydroazonafides were generally intermediate in potencies between amonafide and azonafide against the tumor cells, but some of them had high potencies against the L1210 cells and were more potent against the MDR strain than the sensitive strain. The phenanthrene and azaphenanthrene analogues showed no improvement on the potencies of the anthracenes.

Intracellular localization of 6- and 7-substituted 2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz[de,h]isoquino line-1,3-diones (azonafides) is not the limiting factor for their cytotoxicity: an in vitro confocal microscopy study

The intracellular localization of 14 structurally unique azonafide analogs was studied to determine if intracellular drug distribution is the limiting factor in azonafide cytotoxicity. Using scanning laser confocal microscopy, cytotoxicity of the azonafide analogs studies was observed in Chinese hamster ovary cells immediately after a 1 h exposure. The intracellular drug distribution patterns varied significantly for different analogs. Eight analogs showed primarily nuclear localization, five analogs showed primarily cytoplasmic localization and two analogs displayed perinuclear localization. In general, the type of chemical substitution on the anthracene nucleus determined the distribution pattern. For example, for each analog seven of eight nuclear-localizing analogs were amine-substituted agents, while four of five cytoplasmic-localized agents were ethoxy-substituted analogs. The individual exception within these groups was the 6-[(dimethylamino)ethoxy] agent that was nuclear localized. The two perinuclear-localized agents included the unsubstituted parent, azonafide, and its 6-methyl azonafide analog. Comparison of the cytotoxicity of the azonafides, based on intracellular localization, revealed that none of the localization patterns were associated with increased cytotoxicity. These results show that minor structural changes in the azonafide class of antitumor agents involving substitution along an anthracene chromophore result in substantially different intracellular drug distribution patterns. However, these distribution differences do not determine relative cytotoxic potency in vitro.

Identification and characterization of in vitro metabolites of 2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz [de,h]isoquinoline-1,3-dione (Azonafide)

Azonafide (2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz[de, h]isoquinoline-1,3-dione) is the parent of a new series of anthracene-containing antitumor agents. Its structure is based on amonafide but lacks a primary amine and has an anthracene chromophore rather than a naphthalene chromophore. Using a rat liver cytosol incubation and HPLC/MS detection, we have identified four metabolites resulting from in vitro metabolism of azonafide. These alkyl-modified derivatives include a mono- and a di-N'-desmethyl metabolite, an N'-oxide metabolite, and a carboxylic acid metabolite. Purified samples of these metabolites were analyzed for cytotoxic activity using a 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl-2H-tetrazolium vital dye (mitochondrial reductase) assay and for inhibition of topoisomerase II (TOPO II) using a cell-free enzymatic system. Each metabolite had decreased cytotoxicity relative to azonafide with the following relative potencies in descending order: the mono-N'-desmethyl metabolite, di-N'-desmethyl metabolite, the N-oxide metabolite, and the carboxylic acid metabolite. Similarly, the N'-desmethyl metabolites retained TOPO II inhibitory activity but with lower potency than azonafide. The N-oxide and carboxylic acid metabolites did not inhibit TOPO II at 0. 05 and 0.5 microg/ml, respectively. Thus, metabolism of azonafide by rat liver cytosol represents a detoxification pathway rather than a bioactivation scheme for this DNA intercalator.

In vitro cytotoxicity and DNA damage production in Chinese hamster ovary cells and topoisomerase II inhibition by 2-[2'-(dimethylamino)ethyl]-1, 2-dihydro-3H-dibenz[de,h]isoquinoline-1,3-diones with substitutions at the 6 and 7 positions (azonafides)

The mechanism of action of a group of anthracene-containing analogs of amonafide was studied in Chinese hamster ovary (CHO) cells. These agents differ structurally from amonafide by the replacement of the naphthalene chromophore with an anthracene chromophore, the lack of a primary amine moiety in the 5 position, and substitutions at the 6 and 7 positions on the anthracene nucleus. In this study, five analogs with potent growth inhibitory activity and with low cardiotoxicity were chosen. Cytotoxicity analyses with tetrazolium dye assays (MTT) in vitro and continuous drug exposure revealed IC50 values in CHO cells in the nanomolar range. Intracellular scanning laser confocal microscopy of these drug-treated CHO cells showed that all analogs are able to enter cell nuclei with varying nuclear/cytoplasmic distribution: the more potent dimethylaminoethyl substituted analogs, 47 and 104, were primarily localized in the nucleus. Three analogs, including the unsubstituted parent (1), and numbers 35 (6-amino substituted) and 53 (6-aminoethyl substituted) inhibited DNA and RNA synthesis when assayed immediately after a 1 h exposure. In contrast, analogs 47 and 104 required 24 h post-drug exposure for 1 h to inhibit DNA and RNA synthesis. Using alkaline elution techniques, each analog also produced DNA single- and double-stranded breaks, as well as DNA protein cross-links. Interestingly, the most cytotoxic analogs, 47 and 104, produced minimal DNA strand damage in CHO cells at their IC90 concentrations, whereas the three other compounds with lower growth inhibitory potency produced marked and roughly equivalent DNA damage at equitoxic concentrations. Gel shift analysis of SV40 DNA exposed to the compounds demonstrated that these agents do not directly induce DNA strand breaks. However, catalytic studies with purified human topoisomerase II (Topo II) and plasmid DNA demonstrated that these drugs inhibit this enzyme. These results suggest that the azonafides inhibit Topo II to cause protein-associated strand breaks and impaired DNA and RNA synthesis. However, other mechanisms may also be operant, especially with the more potent dimethylamino ethyl substituted analogs.

2-[2'-(Dimethylamino)ethyl]-1,2-dihydro- 3H-dibenz[de,h]isoquinoline-1,3-diones with substituents at positions 4, 8, 9, 10, and 11. Synthesis, antitumor activity, and quantitative structure-activity relationships

New 2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz[de,h]isoquinoline-1,3- diones with substituents at the 4, 8, 9, 10, and 11 positions were synthesized. Diazonium salts prepared from aminoazonafides were key intermediates for many of the analogues. Six of the new compounds were more potent than azonafide in a panel of tumor cells including human melanoma and ovarian carcinoma and murine L1210 leukemias. Three of these compounds, the 10-OCH3, 10-OC2H5, and 10-F analogues, had better ratios of cardiotoxicity to tumor-cell toxicity than azonafide. Eight compounds were not cross-resistant with MDR L1210 leukemia, and the 10-CN analogue was more potent against solid tumor cells than leukemia cells. The 9-OH, 10-CN, and 10-F analogues had high potency against both sensitive and resistant cell lines of MFX 7 breast carcinoma and WiDr colon carcinoma and sensitivity A599 lung carcinoma. Advantages of the 10-Cl, 10-NH2, and 10-CN analogues over azonafide were apparent in P388 leukemia in mice, and the 10-CN analogue was more effective than doxorubicin in this assay. Quantitative structure-activity relationship studies revealed statistically significant correlations between DNA binding strength of 8- and 10-substituted azonafides, as measured by deltaTm, and toxicity to tumor cells. There also were correlations between substituent size, as measured by MR, and cytotoxicity for 9- and 10-substituted azonafides and between MR and deltaTm for 4- and 11-substituted azonafides. Lipophilicity of substituents (pi) correlated with cytotoxicity for 9-, 10-, and 11-substituted azonafides. These results lend support to a model in which DNA binding strength influences cytotoxic potency, and lipophilicity increases DNA binding whereas large substituents decrease it.

6- and 7-substituted 2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz[de,h] isoquinoline-1,3-diones: synthesis, nucleophilic displacements, antitumor activity, and quantitative structure-activity relationships

New 2-[2'-(dimethylamino)ethyl]-3H-dibenz[de,h]isoquinoline-1,3-diones with substituents at the 6- and 7-positions were prepared. Nucleophilic aromatic displacement was a key reaction in the syntheses. Ten of the new compounds were more potent than the unsubstituted compound, azonafide, in a panel of tumor cells including human melanoma and ovarian cancer and murine sensitive and MDR L1210 leukemia. They also were less cardiotoxic in cell culture. Four of these compounds were not cross-resistant with the MDR leukemia, and one of them, 6-ethoxyazonafide, was nearly as potent against solid tumor cells as leukemia cells. These compounds also had good potency against human breast, colon, and lung cancer cells, including doxorubicin and mitoxantrone resistant cell lines. Advantages of the new analogues over azonafide were less in vivo, but 6-ethoxyazonafide was more effective against L1210 leukemia and subcutaneous B16 melanoma in mice. Although correlations of antitumor potency in cells and physicochemical properties of substituents were not found, there were statistically significant correlations of DNA melt transition temperature (delta Tm) with potency in solid tumor cells and sensitive and MDR resistant L1210 leukemia cells for 6-substituted azonafides and with solid tumors for 7-substituted azonafides.

Amino-substituted 2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H- dibenz[de,h]isoquinoline-1,3-diones. Synthesis, antitumor activity, and quantitative structure--activity relationship

Sets of 2-[2-(dimethylamino)ethyl]-1,2-dihydro-3H- dibenz[de,h]isoquinoline-1,3-diones with amino and actylamino groups at each of the eight positions on the anthracene nucleus were synthesized from appropriately substituted anthracenes. Their evaluation in in vitro antitumor and cardiotoxicity assays revealed a very strong dependence of potency on the position of substitution. Certain compounds, including the 4-, 5-, 7-, and 9-amino derivatives, showed significantly higher potency than the unsubstituted parent compound, azonafide. Among them, 7-aminoazonafide had low cardiotoxicity relative to cytotoxicity. In general, the acetylamino analogues were less potent than the amino derivatives against tumor cells and neonatal rat heart myocytes; however, 5-(acetylamino)azonafide was highly cardiotoxic. 9-Aminoazonafide was more efficacious than azonafide or amonafide against P388 leukemia in mice. Statistically significant correlations were made between the ability of amino analogues to increase the transition melt temperature (delta Tm) of DNA and their potency against solid tumors, leukemia cells, or cardiac myocytes.

2-substituted 1,2-dihydro-3H-dibenz[de,h]isoquinoline-1,3-diones. A new class of antitumor agent

A new class of antitumor agents, having structural analogy to amonafide, but differing by the addition of a fourth ring in the nucleus, was synthesized conveniently from anthracene. Compounds with a variety of substituents, containing a basic nitrogen atom and located on the imide nitrogen, were prepared. Thirteen of 19 new compounds had greater growth inhibitory potency than amonafide in a panel of cultured murine and human tumor cells using the sulforhodamine B and MTT dye assays. The most active agents were similarly more toxic than amonafide to normal neonatal rat myocytes in vitro, but they had better chemotherapeutic indexes. From these compounds, the one with a 2-(dimethylamino)ethyl side chain (named azonafide) was chosen for further study. It showed high potency against a panel of cultured human colon cancer cells and it was active against ip P388 leukemia and subcutaneous B16 melanoma in mice. Preliminary structure-activity correlations suggest that the basicity of the side-chain nitrogen and the length of side chain are important determinants of antitumor potency in vitro. Steric hindrance and rigidity of the side chains might be other determinants.

Preparation and antitumor activity of additional mitomycin A analogues

On the basis of qualitative structure-activity relationships developed in the preceding article, a series of 32 new mitomycin A analogues were prepared and tested in antitumor screens. Seven of them gave greater prolongation of life (ILS) than mitomycin C in the mouse P388 leukemia assay. They included examples with 7-O substituents such as cyclic ethers and nitrogen heterocycles. A Hansch analysis was attempted with log P and MR as the independent variables, but no statistically significant correlation could be made. Seven compounds, chosen mainly for their good potency (MED), were tested in the subcutaneous B16 melanoma assay in mice and four of them showed greater ILS than mitomycin C.

Preparation and antitumor activity of new mitomycin A analogues

A series of 26 mitomycin A analogues including 23 new ones was prepared by a variety of methods. The most useful methods were alkoxide exchange on mitomycin A and treatment of 7-hydroxymitosane with 3-substituted 1-phenyltriazenes. Many of the new analogues were superior to mitomycin C in the P388 leukemia assay and the more stringent subcutaneous B16 melanoma assay both in mice. Four of them gave long-term survivors in the latter assay. Quantitative correlations between log P and antitumor activity were not possible, but some guidelines for future analogue development are proposed.

Mitomycin C analogues with increased metal complexing ability

Twenty-three new mitomycin C analogues designed to have increased metal complexing ability were synthesized and tested against P388 leukemia in mice. Their ability to complex Cu(II) was revealed by the shifts in their UV absorption spectra caused by this metal. One analogue was clearly more active than mitomycin C in the antitumor assay and two others had good activity. Correlation between antitumor activity and Cu(II) complexing ability was ambiguous. The most active compounds were either not complexers or they were complexers limited to the 2-(2-pyridyl)alkyl type substituent on N7. A variety of amino acid substituents on N7 showed only weak antitumor activity.

Mitomycin C analogues with aryl substituents on the 7-amino group

A series of 30 different N7-phenyl-substituted mitomycin C analogues, including 25 new compounds, was prepared from mitomycin A. Seven of these compounds were clearly superior to mitomycin C in activity against P-388 murine leukemia. The para- and the meta-substituted derivatives were subjected to Hansch analysis, which revealed that the lipid-water distribution coefficient pi was the only significant factor in determining antitumor potency (MED). The substituent electronegativity factor sigma was statistically insignificant in determining potency, despite the good correlation of sigma p with the polarographic quinone-reduction potential. These results suggest that diffusion into the tumor cell or access to the receptor is more important than bioreductive activation in determining antitumor potency for this particular group of mitosanes . Fifteen new mitomycin C analogues with heterocycles on the 7-amino group also were prepared. Two of them, containing pyrazolyl and aminopyridyl substituents, were more active than mitomycin C against P-388 murine leukemia. No broad correlations could be made among the antitumor potencies and physicochemical properties for this type of analogue.

Mitomycin C analogues with secondary amines at position 7

A series of mitomycin C analogues with secondary amines at position 7 was prepared from mitomycin A. Eleven of the 20 new compounds in this series were more active than mitomycin C against P-388 murine leukemia, and 2 of these 11 were significantly less leukopenic. The two substituents conferring these superior properties were 4-formylpiperazine and 2-cyanoaziridine. No quantitative correlations could be made among antitumor activities and physicochemical properties of the analogues, although the relative ease of quinone reduction might be related to the good potencies (minimum effective doses) of many of them.

Mitomycin C and porfiromycin analogues with substituted ethylamines at position 7

A series of 7-(2-substituted-ethyl)amino analogues of mitomycin C and porfiromycin was prepared and screened in standard antitumor systems. Certain of these analogues showed better activity than mitomycin C against P-388 leukemia, L-1210 leukemia, and/or B-16 melanocarcinoma in mice. Compounds also tested for their leukopenic effects in mice, the limiting toxicity of mitomycin C. Some of them were less leukopenic and some were more leukopenic than this clinical agent. No statistically significant correlations could be made between physicochemical properties and antitumor activities of the analogues.

Cytotoxicity of modified indole alkaloids

Indole alkaloids of the iboga series were structurally modified by incorporation of a 3,4-dimethoxybenzyl or -benzoyl unit so that they contained the N-O-O triangle required for antileukemic activity according to the triangulation hypothesis. The cytotoxicities of the modified alkaloids in the in vitro P-388 system were not significantly increased over the unmodified alkaloids, suggesting that the triangulation hypothesis does not apply in this series at least.

Plant anticancer agents V: new bisindole alkaloids from Tabernaemontana johnstonii stem bark

The isolation and structure elucidation of the three new bisindole alkaloids, gabunamine, tabernamine, and 19,20-epoxyconoduramine, from Tabernaemontana johnstonii stem bark are described. The isolation of the seven known alkaloids, conodurine, conoduramine, gabunine, isovacangine, ibogamine, pericyclivine, and perivine, from the same source also is noted. The alkaloids gabunamine, gabunine, and tabernamine showed significant cytotoxicity against the P-388 cell culture system.