Design, synthesis, and biological evaluation of pyrrolo[2,1-c][1,4]benzodiazepine and indole conjugates as anticancer agents

A series of novel pyrrolo[2,1-c][1,4]benzodiazepine (PBD) hybrids linked with indole carboxylates is described. These compounds were prepared by linking C-8 of 3 (DC-81) with an indole 2-carbonyl moiety (9) through carbon chain linkers to afford PBD hybrid agents 17-21 in good yields. Preliminary in vivo tests show that these hybrid agents have potent antitumor activity. The cytotoxic studies of the hybrid agents on human melanoma A2058 cells indicate most of the hybrids induced higher cytotoxicity, better DNA-binding ability, an increase in the apoptotic sub-G1 population, and a significant reduction in deltapsi(mt) relative to compound 3. In addition, DNA flow cytometric analysis shows that hybrids actively induce a marked loss of cells from the G2/M phase of the cell cycle, which progresses to early apoptosis as detected by flow cytometry after double-staining with annexin V and propidium iodide (PI). Thus, we suggest that the hybrid agents are potent inducers of cell apoptosis in A2058 cells.     

Bicyclic and tricyclic analogues of anthramycin

As analogues of pyrrolo[2,1-c][1,4]benzodiazepine antitumor antibiotics, such as anthramycin and tomaymycin, several benzo[1,4]diazepine imines and carbinolamine ethers were prepared and tested in vivo against P388 leukemia. Two different synthetic approaches, namely, a reduction of an aromatic nitro group with a concomitant cyclization and a reduction of a lactam, were employed to generate an imine or a carbinolamine moiety. Bicyclic analogues 6a', 6f, and 6g were found to be active, indicating that the pyrroline ring of anthramycin is not an absolute necessity for the antitumor activity. Compound 6g, 3,4-dihydro-9-hydroxy-4-propargyl-5H-1,4-benzodiazepin-5-one, was at least as active as neothramycin although it was 5 times less potent. Among the tricyclic analogues, compounds 5, 7a, and 8b were active against P388 leukemia, and they generally appear to be more potent than bicyclic analogues.     

DC-81-Indole conjugate agent induces mitochondria mediated apoptosis in human melanoma A375 cells

DC-81, an antitumor antibiotic produced by the Streptomyces species, belongs to pyrrolo[2,1-c] [1,4]benzodiazepine (PBD), which are potent inhibitors of nucleic acid synthesis. We previously reported an efficient synthesis of PBD hybrids linked with indole carboxylates. This is the first demonstration on the mechanism of the anticancer effect of PBD hybrid (IN6CPBD) agent on human melanoma A375 cells. IN6CPBD-treated cells exhibited higher cytotoxicity than DC-81 and displayed several features of apoptosis, including an increase in the sub-G1 population, a significantly increased annexin V binding, a degradation of caspase-3, and poly (ADP-ribose) polymerase (PARP) cleavage. Because degradative changes associated with apoptosis are often preceded by the disruption of mitochondrial function, the assessment of mitochondrial function in IN6CPBD-treated cells is worthy of investigation. Our data revealed that treatment of A375 cells with IN6CPBD resulted in the loss of mitochondrial membrane potential (DeltaPsimt), a decrease in intracellular pH (pHi), a reduction of ATP synthesis, increased reactive oxygen species (ROS) generation, and cytochrome c release. Collectively, our studies indicate that IN6CPBD induces apoptosis in A375 cells through a mitochondrial dysfunction pathway, leading to caspase-3 substrate PARP cleavage and subsequent apoptotic cell death.     

Synthesis and benzodiazepine receptor binding of 1H-pyrrolo[2,1-C][1,4]benzothiazines

The pyrrolo[2,1-c][1,4]benzothiazines 2a-n and the pyrrolo[2,1-c][1,4]thiazine 13 were prepared and tested for their ability to displace specific [3H]diazepam binding from rat brain membranes. Such compounds were found essentially devoid of binding affinity for the benzodiazepine receptor.     

Synthesis, in vitro antiproliferative activity, and DNA-binding properties of hybrid molecules containing pyrrolo[2,1-c][1, 4]benzodiazepine and minor-groove-binding oligopyrrole carriers

The synthesis, biological activity, and DNA-binding properties of a series of four hybrids prepared by combining polypyrrole minor groove binders and pyrrolo[2,1-c][1,4]benzodiazepine (PBD) 13, related to the naturally occurring anthramycin (3) and DC-81 (4), have been described, and structure-activity relationships have been discussed. These hybrids 22-25 contain from one to four pyrrole units, respectively. To investigate sequence selectivity and stability of drug/DNA complexes, DNase I footprinting and arrested polymerase chain reaction (PCR) were performed on human c-myc oncogene, estrogen receptor gene, and human immunodeficiency virus type 1 long terminal repeat (HIV-1 LTR) gene sequences. The antiproliferative activity of the hybrids has been tested in vitro on human myeloid leukemia K562 and T-lymphoid Jurkat cell lines and compared to antiproliferative effects of the natural product distamycin A 1, its tetrapyrrole homologue 17, DC 81 (4), and the PBD methyl ester 12. The results obtained demonstrate that the hybrids 22-25 exhibit different DNA-binding activity with respect to both distamycin A 1 and PBD 12. In addition, a direct relationship was found between number of pyrrole rings present in the hybrids 22-25 and stability of drug/DNA complexes. With respect to antiproliferative effects, it was found that the increase in the length of the polypyrrole backbone leads to an increase of in vitro antiproliferative effects, i.e., the hybrid 25 containing the four pyrroles is more active than 22, 23, and 24 both against K562 and Jurkat cell lines.     

Design, synthesis, and evaluation of a novel sequence-selective epoxide-containing DNA cross-linking agent based on the pyrrolo[2, 1-c][1,4]benzodiazepine system.

Synthetic routes have been investigated to prepare a novel C8-epoxide-functionalized pyrrolo[2,1-c][1,4]benzodiazepine 6 as a potential sequence-selective DNA cross-linking agent (Wilson et al. Tetrahedron Lett. 1995, 36, 6333-6336). A successful synthesis was accomplished via a 10-step route involving a pro-N10-Fmoc cleavage method that should have general applicability to other pyrrolobenzodiazepine (PBD) molecules containing acid- or nucleophile-sensitive groups. During the course of this work, a one-pot reductive cyclization procedure for the synthesis of PBD N10-C11 imines from nitro dimethyl acetals was also discovered, although this method results in C11a racemization which can reduce DNA binding affinity and cytotoxicity. The target epoxide 6 was shown by thermal denaturation studies to have a significantly higher DNA-binding affinity than the parent DC-81 (3) or the C8-propenoxy-PBD (15), which is structurally similar but lacks the epoxide moiety. The time course of effects upon thermal denaturation indicated a rapid initial binding phase followed by a slower phase consistent with the stepwise cross-linking of DNA observed for a difunctional agent. This was confirmed by an electrophoretic assay which demonstrated efficient induction of interstrand cross-links in plasmid DNA at concentrations >1 microM. Higher levels of interstrand cross-linking were observed at 24 h compared to 6 h incubation. A Taq polymerase stop assay indicated a preference for binding to guanine-rich sequences as predicted for bis-alkylation in the minor groove of DNA by epoxide and imine moieties. The pattern of stop sites could be partly rationalized by molecular modeling studies which suggested low-energy models to account for the observed binding behavior. The epoxide PBD 6 was shown to have significant cytotoxicity (45-60 nM) in the A2780, CH1, and CH1cis(R) human ovarian carcinoma cell lines and an IC(50) of 0.2 microM in A2780cis(R). The significant activity of 6 in the cisplatin-resistant CH1cis(R) cell line (IC(50) = 47 nM) gave a resistance factor of 0.8 compared to the parent cell line, demonstrating no cross-resistance with the major groove cross-linking agent cisplatin.     

Derivatives of 1,2,3,11a-tetrahydro-5H-pyrrolo[2,1-c][1,4]benzodiazepine-5,11(10H)-dione as anxiolytic agents.

A study of the pharmacological properties of pyrrolo[2,1-c][1,4]benzodiazepine derivatives led to the choice of (+)-1,2,3,11a-tetrahydro-10-methyl-5H-pyrrolol[2,1-c][1,4]benzodiazepine-5,11)10H)-dione as a candidate for anxiolytic evaluation in a limited clinical trial in man. Metabolism studies in laboratory animals have pointed to rapid hydroxylation, possibly in the 3 and 11a positions. A series of compouds containing methyl groups in one or more of these positions has been prepared in an effort to block metabolism and thereby obtain more active or longer acting compounds. All of these derivatives were less active than the parent compound.     

Evaluation of the electrophilicity of DNA-binding pyrrolo[2,1-c][1,4]benzodiazepines by HPLC

An HPLC assay is described that can be used to study the covalent bonding interaction of carbinolamine-containing pyrrolo[2,1-c][1,4]benzodiazepines with the model nucleophile thiophenol, in order to evaluate electrophilicity at the C-11-position. Preliminary experiments with anthramycin, tomaymycin and neothramycin show that their reaction with thiophenol follows second-order kinetics, but the ranking order of reactivity (neothramycin greater than tomaymycin greater than anthramycin), does not correlate with either in vitro cytotoxicity or in vivo antitumour activity. This suggests that other factors such as non-covalent DNA-interaction or drug transport play a more crucial role in biological activity than simple alkylating ability. This assay should, however, prove a useful tool in the study of structure-activity relationships for this series of compounds and provide "C-11-electrophilicity" parameters for use in Hansch analysis and related studies.     

Synthesis and cytotoxicity on sensitive and doxorubicin-resistant cell lines of new pyrrolo[2,1-c][1,4]benzodiazepines related to anthramycin

A new 7,8-methylenedioxy analogue (4) of (+)-porothramycin B (2) and its water-soluble sodium bisulfite derivative (15) have been synthesized in high yields and have been shown to exhibit high cytotoxic activities against several tumor cell lines. The new pyrrolo[2,1-c][1,4]benzodiazepine 4 was as effective against the resistant cell lines as against the doxorubicin-sensitive cell lines tested.     

Synthesis of 11-aryl-5H-imidazo[2,1-c][1,4]benzodiazepines and their benzodiazepine and A1 adenosine binding activity.

In the context of a research program aimed at elucidating the properties of the 5H-imidazo[2,1-c][1.4]benzodiazepine system, a series of 11-aryl-5H-imidazo[2,1-c][1,4]benzodiazepines (3a-i) and their 10,11-dihydro-derivatives (4a-i) has been synthesized. The synthetic strategy includes the preparation of the aryl-[1-(2-nitrobenzyl)-1H-imidazol-2-yl]methanones (5a-i) followed by their reduction and subsequent cyclization. Affinities of compounds 3a-i and 4a-i for central benzodiazepine as well as for adenosine A1-receptors were determined by radioligand binding assays. Among the unsaturated analogues, the highest activity at both receptors is displayed by 1H-(2-thienyl) derivative 3e. The hydrogenated analogues 4a-i do not exhibit considerable binding affinity either for central benzodiazepine or for adenosine A1-receptors.     

Unsymmetrical DNA cross-linking agents: combination of the CBI and PBD pharmacophores

A set of 10 compounds, each combining the seco-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one (seco-CBI) and pyrrolo[2,1-c][1,4]benzodiazepine (PBD) pharmacophores, was designed and prepared. These compounds were anticipated to cross-link between N3 of adenine and N2 of guanine in the minor groove of DNA. The compounds, which differ in the chain length separating the two alkylation subunits, and the configuration of the CBI portion, showed great variation in cellular toxicity (over 4 orders of magnitude in a cell line panel) with the most potent example exhibiting IC50s in the pM range. Cytotoxicity correlated with the ability of the compounds to cross-link naked DNA. Cross-linking was also observed in living cells, at much lower concentrations than for a related symmetrical PBD dimer. A thermal cleavage assay was used to assess sequence selectivity, demonstrating that the CBI portion controlled the alkylation sites, while the PBD substituent increased the overall efficiency of alkylation. Several compounds were tested for in vivo activity using a tumor growth delay assay against WiDr human colon carcinoma xenografts, with one compound (the most cytotoxic and most efficient cross-linker) showing a statistically significant increase in survival time following a single iv dose.     

Synthesis and oral antiallergic activity of carboxylic acids derived from imidazo[2,1-c][1,4]benzoxazines, imidazo[1,2-a]quinolines, imidazo[1,2-a]quinoxalines, imidazo[1,2-a]quinoxalinones, pyrrolo[1,2-a]quinoxalinones, pyrrolo[2,3-a]quinoxalinones, and imidazo[2,1-b]benzothiazoles

4H-Imidazo[2,1-c][1,4]benzoxazine-2-carboxylic acid (3) was found to possess potent activity in the IgE-induced rat passive cutaneous anaphylaxis model which may be predictive of clinical antiallergic activity. Compared to disodium cromoglycate (DSCG, 1), 3 was less active following iv administration but unlike DSCG showed very significant oral activity. To explore the structural requirements for this activity, a range of tricyclic compounds was prepared and their activities were measured. Individual 2-carboxylic acids derived from imidazo[1,2-a]quinolines, imidazo[1,2-a]quinoxalines, imidazo[1,2-a]quinoxalinones, pyrrolo[1,2-a]quinoxalinones, pyrrolo[2,3-a]quinoxalinones, and imidazo[2,1-b]benzothiazoles showed iv activities up to 10(3) times as potent as DSCG and many of them showed significant oral activity. From these, imidazo[1,2-a]quinoxaline-2-carboxylic acid 114 has been chosen for further development.     

Involvement of cytotoxicity and variation of the mitochondrial membrane potential induced by hybrid agent

The pyrrolo[2,1‐c][1,4]benzodiazepine (PBD) derivative DC‐81 is a potent antitumor antibiotic produced by Streptomyces species. The marked cytotoxic potential of this drug may be the result of its interaction with DNA. Because DC‐81 only recognizes three DNA base pairs this has precluded its clinical utility. Combination of DC‐81 with an indole carboxylate moiety was a hybrid designed to have much higher sequence selectivity in DNA Interactivity. In this paper, the association between cytotoxicity and the changes of mitochondria membrane potential ΔΨ_mt after exposing human melanoma cell line A375 to the hybrid agent was examined using MTS cell proliferation assay and flow cytometry using the fluorochrome rhodamine 123. Our results indicated that the hybrid induced cytotoxity and a significant reduction in ΔΨ_mt of A375 cells. We suggest that the hybrid agent is a potent inducer of cell apoptosis in A375 cells. Drug Dev. Res. 61: 1–5, 2004. © 2004 Wiley‐Liss, Inc.     

Synthesis and benzodiazepine receptor binding of 5H-pyrido[2,1-C][1,4]benzothiazines

The pyrido[2,1-c][1,4]benzothiazines 1a-t and the pyrido[2,1-c][1,4]thiazines 7a,b were prepared and tested for their ability to displace specific [3H]diazepam binding from rat brain membranes. Some of the examined compounds of type 1 and 7b showed moderate binding affinity for the benzodiazepine receptor.     

Design, synthesis and in vitro cytotoxicity studies of novel pyrrolo[2,1-c][1,4]benzodiazepine (PBD)--polymade conjugates and 2,2'-PBD dimers

A series of novel pyrrolo[2,1-c][l,4]benzodiazepine (PBD)-polyamide conjugates (1 and 2) and 2,2'-PBD dimers (3, 4 and 5) were synthesized and evaluated for cytotoxicity in >60 human tumor cell lines. In general PBD-polyamide conjugates (1 and 2) exhibit higher cytotoxic potency compared with 2,2'-PBD dimers (3, 4 and 5). Compound 2 exhibits a wide spectrum of anticancer activities against 17 cell lines in six cancer panels with LC50 values of <9 microM, and is especially effective against colon cancer, melanoma, renal cancer and breast cancer. Compound 1 selectively affects cell growth against renal cancer A 498 cell line and compound 4 affects cell growth against breast cancer MDA-MB-231/ATCC cell line with an LC50 value 0.06 microM. Increases in the chain length of the linker in 2,2'-PBD dimers significantly increase the cytotoxic potency and increases in the number of pyrrole groups in the PBD-polyamide conjugates similarly increase the cytotoxic potency.     

Derivatives of 11-(1-piperazinyl)-5H-pyrrolo[2,1-c][1,4]benzodiazepine as central nervous system agents

Four 11-(1-piperazinyl)-5H-pyrrolo[2,1-c][1,4]benzodiazepines were prepared and evaluated as central nervous system agents. All were active psychotropic agents as determined by animal screening tests. The most interesting compound, 11-(1-piperazinyl)-5H-pyrrolo[2,1-c][1,4]benzodiazepine, showed dual activity as an antidepressant against tetrabenazine depression and as a neuroleptic as measured by protection vs. amphetamine lethality in grouped mice.     

Design, synthesis, and evaluation of orally active 4-(2,4-difluoro-5-(methoxycarbamoyl)phenylamino)pyrrolo[2,1-f][1,2,4]triazines as dual vascular endothelial growth factor receptor-2 and fibroblast growth factor receptor-1 inhibitors

A series of substituted 4-(2,4-difluoro-5-(methoxycarbamoyl)phenylamino)pyrrolo[2,1-f][1,2,4]triazines was identified as potent and selective inhibitors of the tyrosine kinase activity of the growth factor receptors VEGFR-2 (Flk-1, KDR) and FGFR-1. The enzyme kinetics associated with the VEGFR-2 inhibition of compound 50 (K(i) = 52 +/- 3 nM) confirmed that the pyrrolo[2,1-f][1,2,4]triazine analogues are competitive with ATP. Several analogues demonstrated low-nanomolar inhibition of VEGF- and FGF-dependent human umbilical vein endothelial cell (HUVEC) proliferation. Replacement of the C6-ester substituent of the pyrrolo[2,1-f][1,2,4]triazine core with heterocyclic bioisosteres, such as substituted 1,3,5-oxadiazoles, afforded compounds with excellent oral bioavailability in mice (i.e., 50 F(po) = 79%). Significant antitumor efficacy was observed with compounds 44, 49, and 50 against established L2987 human lung carcinoma xenografts implanted in athymic mice. A full account of the synthesis, structure-activity relationships, pharmacology, and pharmacokinetic properties of analogues within the series is presented.