Interaction between a 3-nitrobenzothiazolo (3,2-a) quinolinium antitumour drug and deoxyribonucleic acid

Pharmacokinetic evaluation of C-3 modified 1,8-naphthyridine-3-carboxamide derivatives with potent anticancer activity: lead finding

To develop naphthyridine derivatives as anticancer candidates, pharmacokinetic (PK) evaluations of 10 novel derivatives of 1,4-dihydro-4-oxo-1-proparagyl-1,8-naphthyridine-3-carboxamide, with potent anticancer activity were done using in vitro ADME (absorption, distribution, metabolism, excretion) and pharmacokinetic--pharmcodynamic (PK/PD) assays. Only derivatives 5, 6, 9 and 10 showed better metabolic stability, solubility, permeability, partition coefficient and cytochrome P450 (CYP) inhibition values. PK of derivatives 5, 6, 9 and 10 in rat showed comparable PK profile for derivative 5 (C0 = 6.98 µg/mL) and 6 (C0 = 6.61 µg/mL) with no detectable plasma levels for derivatives 9 and 10 at 5.0 mg/kg i.v. dose. PK/PD assay of derivatives 5 and 6 in tumor-bearing mice (TBM) showed comparable PK but tumor plasma index (TPI) of derivative 6 (4.02) was better than derivative 5 (2.50), suggesting better tumor uptake of derivative 6. Derivative 6, as lead compound, showed highest tumor growth inhibition (TGI) value of 33.6% in human ovary cancer xenograft model.

Novel Nitrobenzazolo[3,2-a]quinolinium Salts Induce Cell Death through a Mechanism Involving DNA Damage, Cell Cycle Changes, and Mitochondrial Permeabilization

This study reports the capacity of three nitro substituted benzazolo[3,2-a]quinolinium salts NBQs: NBQ 95 (NSC-763304), NBQ 38 (NSC 763305), and NBQ 97 (NSC-763306) as potential antitumor agents. NBQ's are unnatural alkaloids possessing a positive charge that could facilitate interaction with cell organelles. The anticancer activities of these compounds were evaluated through the National Cancer Institute (NCI) 60 cell line screening which represents diverse histologies. The screening was performed at 10 µM on all cell lines. Results from the NCI screening indicated cytotoxicity activity on six cell lines. In order to explore a possible mechanism of action, a detailed biological activity study of NBQ 95 and NBQ 38 was performed on A431 human epidermoid carcinoma cells to determine an apoptotic pathway involving, cell cycle changes, DNA fragmentation, mutations, mitochondrial membrane permeabilization and caspases activation. DNA fragmentation, cell cycle effects, mutagenesis, mitochondrial permeabilization and activation of caspases were determined by fluorimetry and differential imaging. Our data showed that A431 growth was inhibited with an average IC50 of 30 µM. In terms of the mechanism, these compounds interacted with DNA causing fragmentation and cell cycle arrest at sub G0/G1 stage. Mutagenesis was higher for NBQ 38 and moderate for NBQ 95 Mitochon-drial permeabilization was observed with NBQ 38 and slightly for NBQ 95. Both compounds caused activation of Caspases 3 and 7 suggesting an apoptotic cell death pathway through an intrinsic mechanism. This study reports evidence of the toxicity of these novel compounds with overlapping structural and mechanistic similarities to ellipticine, a known anti-tumor compound.

Synthesis, antitumor, and DNA binding behavior of novel 4-(2-hydroxyquinolin-3-yl)-6-phenyl-5, 6 dihydropyrimidin derivatives in aqueous medium

This article deals with the synthesis of 4-(2-hydroxyquinolin-3-yl)-6-phenyl-5,6-dihydropyrimidin derivatives (2a-f), on condensation with various aromatic aldehydes and ketones in aqueous ethanolic NaOH solution yielding the corresponding chalcones (3). These chalcones were further reacted with thiourea/urea in the presence of a base, which led to the formation of the titled derivatives (2a-f). The newly synthesized heterocyles were characterized by elemental analysis, FTIR, (1)HNMR, and electronic and mass spectral data. The compounds (2a and 2b) were evaluated for in vitro cyctotoxicity against human breast adenocarcinoma cell (MCF-7). In MTT cytotoxicity studies, both quinolinde derivatives were found most effective. The binding interaction behavior of the compound (2a) and (2d) with calf thymus-DNA (CT-DNA) was studied by electronic spectra, viscosity measurements, and thermal denaturation studies. On binding to CT-DNA, the absorption spectrum underwent bathochromic and hypochromic shifts. The binding constant (K(b)) observed 4.3 x 10(5) M(-1) for (2a), and 3.8 x 10(5) M(-1) for (2d) suggested that compound (2a) binds more strongly with base pairs than (2d).

Synthesis and DNA binding studies of novel heterocyclic substituted quinoline schiff bases: a potent antimicrobial agent

The present article deals with the synthesis of 2-chloroquinoline-3-carbaldehyde [(2-hydroxy-1-naphthyl) methylene] hydrazone (CQCMH) (2a-c) and 2-chloroquinoline-3-carbaldehyde [4-(dimethylamino) benzylidene] hydrazone (CQCDBH) (3a-c) from quinoline derivatives under suitable experimental conditions. The synthesized compounds were characterized by elemental analysis, FTIR, (1)HNMR, and mass spectral data. The selected compounds were studied for interaction with calf thymus-DNA (CT-DNA) by electronic spectra, viscosity measurements as well as thermal denaturation studies. On binding to DNA, the absorption spectrum underwent bathochromic and hypochromic shifts. The binding constant (K(b)) had value of 2.3 x 10(3) M(-1) for (2a) and 2.5 x 10(4) M(-1) for (3a). The viscosity measurements indicated that the viscosity of sonicated rod like DNA fragments increased. The synthesized derivatives have been screened for antibacterial and antifungal activities.

Role of the nitro functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2-a]quinolinium chloride

Interest in DNA binding drugs has increased in recent years due to their importance in the treatment of genome-related diseases, like cancer. A new family of water-soluble DNA binding compounds, the benzothiazolo[3,2- a]quinolinium chlorides (BQCls), is studied here as potential candidates for chemical treatment of solid tumor cells that may encounter low-oxygen environments, a condition known as hypoxia. These compounds are good DNA intercalators; however, no studies have been made of these compounds under hypoxic conditions. This work demonstrates the importance of the nitro-functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2- a]quinolinium chloride (NBQ-91), which possesses nitro-functionality, and 10-methylbenzothiazolo[3,2- a]quinolinium chloride (BQ-106), which does not. Both NBQ-91 and BQ-106 have similar noncovalent binding affinity toward DNA. Dialysis experiments show that NBQ-91 binds DNA under N2-saturated conditions with increasing concentrations of reducing agent, presumably due to reduction of the nitro-functionality. Conversely, because of the lack of nitro-functionality, the presence of a reducing agent had no effect on BQ-106 binding to DNA under both aerobic and N2-saturated conditions. Clonogenic assays were performed to determine the quinolinium chloride cytotoxicities under both aerobic (95% air and 5% CO2) and hypoxic (80% N2 and 20% CO2) conditions. The calculated ratios of cellular toxicity under aerobic to hypoxic conditions caused by the same concentration of test agent (CTR values) show greater levels of cell death under hypoxia than under aerobic conditions for mitomycin C (MC) (CTR = 0.7 at 1 microM) and NBQ-91 (CTR = 0.4 at 200 microM) than for BQ-106 (CTR = 1.0 at 200 microM), which agreed with the previously reported data for MC and confirmed the importance of nitro-functionality for reactivity under hypoxic conditions. There was no correlation between noncovalent binding affinity constants and their cytotoxicity under hypoxic conditions. Adduct formation between the NBQ-91 and 2'-dG was also assessed by reacting 2'-dG or DNA with NBQ-91 and BQ-106 under N2-saturated conditions in the presence of hypoxanthine and xanthine oxidase (HX/XO). DNA covalent adduct formation was analyzed by two techniques: LC-ESI-MS and Sephadex size exclusion chromatography. LC-ESI-MS results clearly indicate the formation of a prominent molecular ion at masses of 266.0 and 530.58 Da, corresponding to the [M + H](+2) and [M](+) molecular ions of the monitored 2'-dG-NBQ-91 adduct. Results from the Sephadex size exclusion chromatography support these findings because the NBQ-91 elution percentage increases in the presence of HX/XO due to the reduction of the nitro-functionality, which results in covalent binding to DNA. This study reports evidence of the DNA binding capacity of this bioreductive drug. The preferential N2-saturated over aerobic conditions for DNA binding makes NBQ-91 a potential bioreductive compound for hypoxic cell killing.

Comparison of the nucleic acid covalent binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium salts upon enzymatic reduction

The DNA binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium chlorides (NBQs), NBQ-38 and NBQ-95, was evaluated upon their enzymatic reduction with hypoxanthine (HX)/xanthine oxidase (XO) under anaerobic conditions. In the presence of 2'-deoxyguanosine (2'-dG) or calf thymus DNA, covalent-addition products were monitored. Reactions of each NBQ with 2'-dG or DNA differed in the NBQ to HX molar ratio. Control reactions, one without HX/OX and another under aerobic conditions, were also analyzed. Adducts were isolated and characterized by high performance liquid chromatography (HPLC) and electrospray ionization-mass spectrometry (ESI-MS). Authentic samples of the reduced forms of these NBQs, identified as ABQ-38 and ABQ-95, were synthesized as standards to monitor bioreduction processes. HPLC analysis showed that the yield of formation of an unknown product (possibly, 2'-dG-NHBQ-38 adduct) from the reaction of NBQ-38 with 2'-dG and DNA was proportional to the HX to NBQ-38 molar ratio. ESI-MS analysis of the DNA hydrolysates showed evidence of an adduct formed upon bioreduction of NBQ-38 by the ions detection at m/z 528.3 and 454.8, consistent with chemical structures of a 2'-dG-NHBQ-38 adduct and a fragment ion. DNA adducts were not observed with NBQ-95, although the corresponding bioreduction product ABQ-95 was detected by ESI-MS. This study provides mechanistic information of these bioreductively-activated pro-drugs with potential therapeutic applications.

Reductive activation and thiol reactivity of benzazolo[3,2-a]quinolinium salts

Derivatives of benzazolo[3,2-a]quinolium salts (QSDs) are reductively activated by the enzymatic reducing agents hypoxanthine (or xanthine)/xanthine oxidase and NADH dehydrogenase as evidenced by the increase in rates of ferricytochrome c (Cyt(III)c) reduction and oxygen consumption, respectively. No correlation between Michaelis-Menten parameters and QSDs redox potentials was found regarding anaerobic or aerobic Cyt(III)c reduction, although maximum rates were observed for nitro-containing QSDs. However, oxygen consumption rates correlate with QSDs redox potentials when NADH dehydrogenase is used as reducing agent. QSDs bind covalently to bovine serum albumin (BSA) under anaerobic conditions, in the presence, and less in the absence, of HX/XO and only if the nitro group is present at the QSD. QSDs react with glutathione (GSH) in the presence of HX/XO but not in its absence, under anaerobic conditions. The amount of reacted GSH increases, and the relative amount of GSSG formed decreases, with an increase in the QSD reduction potential, thus indicating that GSH reacts with reduced nitro-containing QSDs mainly in a manner which does not involve the production of GSSG, presumably, through the formation of the nitroso-QSD-GSH conjugate. QSDs are, thus, novel nitro-containing heterocyclic compounds which could be bioreductively activated to react with oxygen and thiols.

Antitumor activity of 4-amino and 8-methyl-4-(3diethylamino propylamino)pyrimido[4′,5′:4,5]thieno (2,3-b) quinolines

The interaction of 4-aminopyrimido [4',5':4,5] thieno (2,3-b) quinoline and 8-methyl-4-(3-diethylaminopropylamino) pyrimido [4',5':4,5] thieno (2,3-b) quinoline with DNA was studied by UV-Vis and fluorescence spectrophotometry as well as by hydrodynamic methods. On binding to DNA, the absorption spectra underwent bathochromic and hypochromic shifts and the fluorescence was quenched. These compounds are able to bind to DNA with an affinity of about 10(6) M(-1) for calf thymus DNA at ionic strength 0.01 M and their intercalating characteristic (lengthening of the DNA) depends upon the length of the chain. Binding to the GC-rich DNA of Micrococcus lysodeikticus was stronger than the binding to calf thymus DNA at ionic strength 0.01 M. The cytotoxicities of these compounds on leukemia HL-60, melanoma B16F10 and neuro 2a cells are quite similar and inhibition (IC50) is in the range of 0.992-3.968 microM. The anticancer efficacy against B16 melanoma, has provided evidence of major antitumor activity for 8-methyl-4-(3diethylaminopropylamino) pyrimido [4',5':4,5] thieno(2,3-b)quinoline. Single or multiple intraperitonial (i.p) doses of drug proved high level activity against the subcutaneous (s.c) grafted B16 melanoma, significantly increasing survival (p<0.001) and inhibiting tumor growth (T/C of 4%). This study offers a new intercalation functional group to DNA-targeted drug design.

Intercalation of trioxatriangulenium ion in DNA: binding, electron transfer, x-ray crystallography, and electronic structure

Trioxatriangulenium ion (TOTA(+)) is a flat, somewhat hydrophobic compound that has a low-energy unoccupied molecular orbital. It binds to duplex DNA by intercalation with a preference for G-C base pairs. Irradiation of intercalated TOTA(+) causes charge (radical cation) injection that results in strand cleavage (after piperidine treatment) primarily at GG steps. The X-ray crystal structure of TOTA(+) intercalated in the hexameric duplex d[CGATCG](2) described here reveals that intercalation of TOTA(+) results in an unusually large extension of the helical rise of the DNA and that the orientation of TOTA(+) is sensitive to hydrogen-bonding interactions with backbone atoms of the DNA. Electronic structure calculations reveal no meaningful charge transfer from DNA to TOTA(+) because the lowest unoccupied molecular orbital of TOTA(+), (LUMO)(T), falls in the gap between the highest occupied molecular orbital, (HOMO)(D), and the (LUMO)(D) of the DNA bases. These calculations reveal the importance of backbone, water, and counterion interactions, which shift the energy levels of the bases and the intercalated TOTA(+) orbitals significantly. The calculations also show that the inserted TOTA(+) strongly polarizes the intercalation cavity where a sheet of excess electron density surrounds the TOTA(+).

Inhibition of human topoisomerase II by anti-neoplastic benzazolo[3,2-alpha]quinolinium chlorides

Previously we reported [20] that there is no correlation between the cytotoxic activity of four new structural analogs of the antitumor DNA intercalator 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQ-2) and their interaction with DNA. In the present study, we present evidence suggesting that the molecular basis for the anti-proliferative activity of these drugs is the inhibition of topoisomerase II. The NBQ-2 derivatives inhibited the relaxation of supercoiled DNA plasmid pRYG mediated by purified human topoisomerase II. Inhibition of the decatenation of kinetoplast DNA mediated by partially purified topoisomerase II extracted from the human histiocytic lymphoma U937 (a cell line previously shown to be sensitive to the drugs) was also caused by these drugs. The potency of the benzazolo[3,2-a]quinolinium drugs against topoisomerase II in vitro was the following: 7-(1-propenyl)-3-nitrobenzimidazolo[3,2-a]quinolinium chloride (NBQ-59) > 4-chlorobenzothiazolo[3,2-a]quinolinium chloride (NBQ-76) > 7-ethyl-3-nitrobenzimidazolo[3,2-a]quinolinium chloride (NBQ-48) > 7-benzyl-3-nitrobenzimidazolol[3,2-a]quinolinium chloride (NBQ-38). This rank of potency for topoisomerase II inhibition correlated very well with the cytotoxicity elicited by these drugs. Furthermore, significant levels of topoisomerase II/DNA cleavage complex induced by these drugs in vivo were detected when U937 cells were treated with NBQ-59 and NBQ-76 whereas NBQ-38 and NBQ-48 produced negligible amounts of the cleavage complex. Our results strongly suggest that topoisomerase II is the major cellular target of this family of compounds.

DNA binding-independent anti-proliferative action of benzazolo[3,2-alpha]quinolinium DNA intercalators

The proposed mechanism of action of the antineoplastic drug 3-nitrobenzothiazolo[3,2-alpha]quinolinium chloride (NBQ-2) involves its interaction with DNA by intercalation and inhibition of topoisomerase II activity by arresting the enzyme in a covalent cleavage complex. In an attempt to identify some structural determinants for activity and develop a molecular structure/cytotoxicity correlation, four new structural analogs of the antitumor NBQ-2 were prepared and their cytotoxic activity and DNA binding properties were investigated. The cytotoxic activity was evaluated against six different human tumor cell lines: U937, K-562, HL-60, HT-29, HeLa, and A431. The results showed that these new drugs elicit pronounced cytotoxic effects against U937, K-562, HL-60 and A431 while HeLa and HT-29 were less sensitive to the new drugs. This apparent selectivity was different to that of m-AMSA, a drug currently used for cancer treatment. Since the interaction of NBQ-2 to DNA by intercalation has been proposed as the initial step leading to its antineoplastic activity, DNA binding and changes in DNA contour length induced by the new NBQ-2 structural analogs were also investigated using calf thymus and human DNA. The drug, 7-(1-propenyl)-3-nitrobenzimidazolo[3,2-alpha]quinolinium chloride (NBQ-59) was the most cytotoxic agent of the analog series (IC50 = 16 microM for HL-60 cells), however, it demonstrated the weakest binding to DNA (Kint = 0.9 x 10[5] M-1 for calf thymus DNA). NBQ-59 was also found to be a poor intercalator into the DNA double helix. Therefore, our results suggest that DNA binding is not the primary mechanism of drug action for this family of compounds. In addition structural determinants important for cytotoxicity of the benzazolo quinolinium chlorides were suggested by our results. In particular, the nitro group in the 3 position does not seem to be necessary for bioactivity, while substitutions in the benzazolo moiety have striking effects on the biological activity of the drugs.

Effects of 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQ) and doxorubicin on lens regeneration in the adult newt: a morphological study

The antitumor drug 3-nitrobenzothiazolo [3,2-a] quinolinium chloride (NBQ) stimulates the in vivo lens regeneration in the adult newt Notophthalmus viridescens and induces a differentiated state in HL-60 leukemia cells. Because the cytotoxic drug doxorubicin (Adriamycin) induces differentiation of HL-60 cells in vitro we decided to compare the effect(s) of doxorubicin with NBQ on lens regeneration in vivo. Both drugs were injected intraperitoneally at six different schedules. Morphological criteria of the different regeneration stages were used in the analysis of the regenerates. NBQ stimulated lens regeneration independently of the time intervals and the stage of regeneration at which the drug was administered. There was an increase in the mean number of mitoses suggesting that NBQ stimulated cell proliferation. Doxorubicin administered for five days did not modify the regenerative process. On the other hand, doxorubicin given for periods of nine or more days after lentectomy, strongly inhibited the formation of a new lens. Thus, the inhibitory effect of doxorubicin is dependent on the continuous long term contact with the tissue. Although NBQ and doxorubicin are both DNA intercalators, they induced the effects on lens regeneration through different mechanisms.

Myeloid differentiation of HL-60 cells induced by anti-tumor drug 3-nitrobenzothiazolo[3,2-a]quinolinium

Drugs which elicit cell differentiation might have an important role in the treatment of leukemias and other neoplasias. Various chemotherapeutic agents promote leukemic cell differentiation. The HL-60 cell line is a useful model to study in vitro myeloid differentiation. Sublethal concentrations of 3-nitrobenzothiazolo[3,2-a]quinolinium (NBQ), an antitopoisomerase II drug, were given to HL-60 cells from one to five days to evaluate its capacity to induce differentiation. NBQ-induced HL-60 cells reduced nitroblue tetrazolium (NBT), increased MY-4 receptors, increased phagocytic activity and displayed the granulocytic morphology. Flow cytometric DNA analysis of NBQ-induced cells revealed an arrest in the G1 phase a reduction in the relative percentage of cells in S and G2+M phases. Our results suggest that NBQ induces an S-phase specific differentiation of HL-60 cells comparable to that previously described with dimethyl sulfoxide and retinoic acid. NBQ and its analogs, as differentiation inducers, may have potential utility as a novel therapeutic modality for leukemias.

Absence of mutagenicity of the antitumor drug 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQ) in the germ cells of Drosophila melanogaster males

The antitumor drug, 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQ) was tested for genotoxicity with the sex-linked recessive lethal test by feeding Drosophila melanogaster males. Although toxic to adults, the drug tested negative at the concentrations studied.

Photochemistry of aqueous solutions of benzazolo[3,2-a]quinolinium salts. A spin-trapping study using 17O-enriched water and oxygen

The photolysis of buffered aqueous solutions containing the quinolinium salts, 3-nitro-7-ethyl-benzimidazolo[3,2-a]quinolinium perchlorate (NEBQClO4) and 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQCl), at 344 and 365 nm, respectively, was studied in the presence of the spin trap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). Only a water-derived DMPO-OH.spin-adduct was obtained for both of these salts, at a DMPO concentration of 14 mM, as confirmed by H2 17O-enriched water experiments. A photosolvated intermediate is postulated as the OH donating species. Lower steady-state concentrations of the spin adduct were obtained in argon-saturated solutions, implying that oxygen gas is at least partially necessary in the water-derived DMPO-OH formation. Evidence for superoxide ion formation was obtained by the DMPO-17OH spin-adduct formation during the photolysis of NBQCl in an 17O-enriched oxygen atmosphere in the presence of 150 mM DMPO. An increase in the DMPO-OH steady-state concentration was observed if the photolysis of NBQCl was performed in the presence of superoxide dismutase (SOD). Our results suggest that this effect is due to the SOD inhibition of the destruction of DMPO-OH.by superoxide ion.

The antitumor drug 3-nitrobenzothiazolo(3,2-a)quinolinium chloride (NBQ): effects on lens regeneration and interaction with DNA of Notophthalmus viridescens

We have studied the effect of 3-nitrobenzothiazolo(3,2-a)quinolinium (NBQ) on the regeneration of the lens in adult newt Notophthalmus viridescens. NBQ has marked cytotoxic effects in tumor cells, intercalates DNA, and was found to enhance lens regeneration. Newt liver DNA was isolated, and the thermal denaturation temperature (Tm) determined to be 76.6% +/- 0.8%. The G-C content was determined to be 44.0% +/- 0.4% and 45.0% +/- 0.1%. Parameters of NBQ binding to newt DNA were determined by spectrophotometric methods and compared with those obtained for calf thymus and Micrococcus lysodeikticus. The association constant, K(o), was found to be 1.1 x 10(+5) M-1 with a site-size parameter, n, of 8.7 nucleotides. No explanation is apparent for the paradoxical stimulation of lens regeneration.

Effects of the antitumor drugs 3-nitrobenzothiazolo[3,2-alpha]quinolinium and fagaronine on nucleic acid and protein synthesis

3-Nitrobenzothiazolo[3,2-alpha]quinolinium perchlorate (NBQ) has been shown to be active against in vivo experimental tumors of P388 and Ehrlich ascites cells. Furthermore, it has been established that NBQ binds to DNA by intercalation. In this work we describe its effects on DNA, RNA and protein syntheses both in KB cells and in cell-free synthesizing systems. Fagaronine, an alkaloid structurally related to NBQ, was studied also in an attempt to establish the basis for future studies on structure-activity relationships. Both NBQ and fagaronine inhibited DNA, RNA and protein syntheses in KB cells, with essentially equal effectiveness. Exposure of KB cells to NBQ for 2 hr caused irreversible inhibition of DNA, RNA and protein syntheses. Studies in cell-free systems showed that NBQ strongly inhibited Escherichia coli DNA polymerase I, whereas RNA polymerase activities were moderately affected. Furthermore, both drugs inhibited protein synthesis in cell-free systems derived from rabbit reticulocytes and Saccharomyces cerevisiae. Our results indicate that NBQ and fagaronine exert their cytotoxic activity by at least two independent mechanisms: inhibition of DNA activity by binding to this molecule, and inhibition of protein synthesis probably by interacting with the ribosomal system.

Binding of sanguinarine to deoxyribonucleic acids of differing base composition

The binding of the alkaloid sanguinarine to natural DNAs of differing GC content has been studied by spectrophotometry and viscometry techniques. Binding parameters determined from spectrophotometric measurements by Scatchard analysis, according to an excluded-site model, indicate a very high specificity of sanguinarine binding to GC rich DNA. In the strong binding region, the increase of contour length of DNA depends strongly on its base composition, being larger with GC rich DNA than with AT rich DNA. It is concluded that the alkaloid binds preferentially to the GC pairs in DNA template.