Synthesis and evaluation of new pyrrolo[2,1-c ][1,4]benzodiazepine hybrids linked to a flavone moiety

Pyrrolo[2,1-c ][1,4]benzodiazepine hybrids linked to a flavone ring system have been designed, synthesized and evaluated for their biological activity. Flavone has been tethered to the C8 position of pyrrolobenzodiazepine through alkanedioxy spacers of varying lengths. These new type of hybrids exhibit significant DNA minor groove binding ability in comparison to the naturally occurring DC-81 and appreciable in vitro cytotoxicity.


Introduction
In recent years there is much interest in the design and development of DNA interactive molecules that can bind to DNA in a sequence selective manner.In spite of enormous efforts to design synthetic DNA intercalators, very few molecules are in clinical usage that exhibit sequence selectivity.The synthesis of small molecules which exhibit DNA sequence selectivity is of immense importance for targeting the rapid growth of tumour cells.
The pyrrolo[2,1-c] [1,4]benzodazepines (PBDs) are a group of naturally occurring antitumour antibiotics produced by various Streptomyces species and examples of which include anthramycin, tomaymycin, sibiromycin and DC-81.The formation of a covalent bond in the minor groove of DNA by nucleophilic attack of 2-amino group of a guanine base to form an aminal linkage to C-11 is responsible for the biological activities of PBDs. 1 Studies like X-ray and DNA foot-printing on the covalent DNA-PBD adducts have demonstrated a high sequence specificity for GC-rich DNA regions in particular, for Pu-G-Pu motifs. 2 A large number of structurally modified PBDs have been prepared and evaluated for their biological activity, particularly their antitumour and DNA binding potential. 3A recently developed PBD dimer with C2-exomethylene substitution (SJG-136) has been selected for clinical trials (Figure 1). 4 The PBDs have been used to attach ethylenediamine tetra acetic acid (EDTA), 5 epoxide, 6 pyrrole and imidazole polyamide [7][8][9] and cyclopropaindole 10,11 moieties, which have exhibited sequence selective DNA-cleavating and cross linking properties.Recently, we have been involved in the development on new synthetic strategies 12 for the preparation of PBD ring systems and also in the design of structurally modified PBDs and their hybrids. 13In this pursuit some novel PBD hybrids and conjugates have been prepared by linking the PBD moiety to other DNA interactive compounds through suitable linkers.
Several flavonoid natural products have recently attracted attention as novel inhibitors of virus-associated reverse transcriptase. 14These flavonoid derivatives also inhibit various cellular DNA polymerases by binding to the double stranded nucleic acid template-primer or by competing with it for a site on the enzyme.The DNA intercalation contributes to the cytotoxicity of the flavonoids. 15In literature some flavonoids have been examined to induce enzymatic breakage by mammalian topoisomerase II. 16Additionally the cytotoxic effects of flavonoids have demonstrated promising cytotoxicity against some selected human tumour cell lines.Taking advantage of the DNA interactive property of flavone moiety, it was considered of interest to link it to the PBD ring system through different alkane spacers for the preparation of new hybrids of PBD and evaluation of their DNA binding property as well as in vitro anticancer potential.

Results and Discussion
The precursor (2S)-N-[4-(n-bromoalkyloxy)-5-methoxy-2-nitrobenzoyl]-pyrrolidine-2-carboxaldehyde diethyl thioacetal 1a-c has been prepared following our previously described procedure.13a Etherification of 6-hydroxy flavone by 1a-c gives the desired intermediate 2a-c.This nitrothioacetal has been reduced with SnCl 2 .2H 2 O to give 3a-c.The deprotection of the thioacetal group afforded the desired compounds 4a-c (Scheme).The DNA binding ability for these hybrids has been examined by using thermal denaturation assay with calf thymus (CT) DNA. 17Studies have been carried out at [DNA]/[ligand] molar ratio of 5:1.The increase in helix melting temperature (∆T m ) has been examined after 0 h and 18 h of incubation at 37 o C (Table 1).Interestingly, the compounds 4a and 4c elevates the helix melting temperature of CT-DNA by 6.2 o C and 2.6 o C after incubation for 18 h at 37 o C. On the other hand, the naturally occurring DC-81 exhibits a ∆T m of 0.7 o C.This demonstrates that these PBDflavone conjugates have significant DNA binding ability as illustrated in Table 1.The results also demonstrate that increasing chain length from three to five reduces the DNA binding affinity.Primarily, compounds 4a-c have been evaluated for their in vitro cytotoxicity in selected human cancer cell lines of colon (HT-29, HCT-15), lung (A-549, HOP-62), cervix (SiHa) origin by using sulforhodamine B (SRB) method. 18The results are expressed as percent of cell growth inhibition determined relative to that of untreated control cells (Table 2).Usually, when the concentration of the compound solution is 10 -6 mol/L, the inhibition of the solution is more than 50%, and then compound is considered as an effective agent.According to this standard, it has been observed from Table 2 that 4a exhibit a strong effect to HT-29, HCT-15, HOP-62 cell lines and compound 4c suppressing HCT-15 and A-549 cell growth by 69% and 63%.The comparison of the data in Table 2 reveals the importance of an alkane spacer.As the alkane spacer increases from three to four the growth inhibition activity has decreased.This observation has been supported by thermal denaturation studies.In conclusion, we report the synthesis, DNA binding ability and in vitro cytotoxicity of the new PBD-flavone hybrids.These results show that an increase in linker chain length reduces the DNA binding affinity and in vitro growth inhibition of certain cancer cell lines.One of the hybrids with three carbon chain spacer (4a) exhibited significant DNA binding affinity as well as in vitro cytotoxicicty.

Experimental Section
General Procedures.Reaction progress was monitored by thin-layer chromatography (TLC) using GF 254 silica gel with fluorescent indicator on glass plates.Visualization was achieved with UV light and iodine vapour unless otherwise stated.Chromatography was performed using Acme silica gel (100-200 and 60-120 mesh).The majority of reaction solvents were purified by distillation under nitrogen from the indicated drying agent and used fresh: dichloromethane (calcium hydride), tetrahydrofuran (sodium benzophenone ketyl), methanol (magnesium methoxide) and acetonitrile (calcium hydride). 1H NMR spectra were recorded on Varian Gemini 200 MHz spectrometer using tetramethylsilane (TMS) as an internal standard.Chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane.Spin multiplicities are described as s (singlet), br s (broad singlet), d (doublet), t (triplet), q (quartet), m (multiplet).Coupling constants are reported in Hertz (Hz).Mass spectra obtained using a FAB-MS spectrophotometer.
(593 mg, 84%).13 gr, 5 mmol) was refluxed for 5 h.The reaction mixture was cooled and the methanol was evaporated under vacuum and the residue was carefully adjusted to pH 8 with saturated NaHCO 3 solution and then extracted with ethyl acetate (2x30 mL).The combined organic phase was washed with brine (15 mL), dried over anhydrous Na 2 SO 4 and evaporated under vacuum to afford the amino diethyl thioacetal 3a as yellow oil (506 mg, 78%) and this was directly used in the next step.

7-Methoxy-8-{3-[2-phenyl-4-oxo-4H-[1]benzopyran-6-yloxy]propoxy}-(11aS)-1,2,3,11atetrahydro-5H-pyrrolo[2,1-c][1,4]benzodiazepin-5-one (4a).
A solution of amino thioacetal 3a (649 mg, 1 mmol), HgCl 2 (597 mg, 2.2 mmol) and CaCO 3 (240 mg, 2.4 mmol) in acetonitrilewater (4:1) was stirred slowly at room temperature for 12 h, until TLC indicate the complete disappearance of starting material.The reaction mixture was diluted with ethyl acetate (30 mL) and filtered through a celite.The clear yellow organic supernatant was extracted with ethyl acetate (2x20 mL).The organic layer was washed with saturated NaHCO 3 (20 mL), brine (20 mL) and the combined organic phase was dried over anhydrous Na 2 SO 4 .The organic layer was evaporated under vacuum and the crude product was purified by column chromatography (5% CHCl 3 -MeOH) to afford the compound 4a as a pale yellow solid (273 mg, 52%).This material was repeatedly evaporated from CHCl The DNA-PBD solutions were incubated at 37 °C for 0, 18, 36 and 48 h prior to analysis.Samples were monitored at 260 nm using a Beckman DU-7400 spectrophotometer fitted with high performance temperature controller, and heating was applied at 1 °C min −1 in the 40-90 °C range.DNA helix coil transition temperatures (T m ) were obtained from the maxima in the (dA 260 )/dT derivative plots.Results are given as the mean ± standard deviation from three determinations and are corrected for the effects of MeOH co-solvent using a linear correction term.Druginduced alterations in DNA melting behaviour are given by: ∆T m =T m (DNA+PBD)−T m (DNA alone), where the T m value for the PBD-free CT-DNA is 69.0 ± 0.01.The fixed [PBD]/[DNA] ratio used did not result in binding saturation of the host DNA duplex for any compound examined.
In vitro evaluation of cytotoxic activity.In vitro cytotoxicity was evaluated using human cancer cell lines HT-29, HCT-15, A-549,HOP-62 and SiHa.Viable cells were seeded in growth medium into 96-well microtiter plates and allowed to attach overnight.A protocol of 48 h continuous drug exposure was used, 18a and a sulforhodamine B (SRB) protein assay was used to estimate cell viability or growth.18b

Table 2 .
The percentage growth inhibition data for flavone-PBD hybrids