Anthracycline binding to DNA. High-resolution structure of d(TGTACA) complexed with 4'-epiadriamycin

The effects of anthracycline drugs on the conformational distribution of mouse P-glycoprotein explains their transport rate differences

P-glycoprotein (Pgp) is an ATP-dependent efflux transporter and plays a major role in anti-cancer drug resistance by pumping a chemically diverse range of cytotoxic drugs from cancerous tumors. Despite numerous studies with the transporter, the molecular features that drive anti-cancer drug efflux are not well understood. Even subtle differences in the anti-cancer drug molecular structure can lead to dramatic differences in their transport rates. To unmask these structural differences, this study focused on two closely-related anthracycline drugs, daunorubicin (DNR), and doxorubicin (DOX), with mouse Pgp. While only differing by a single hydroxyl functional group, DNR has a 4 to 5-fold higher transport rate than DOX. They both non-competitively inhibited Pgp-mediated ATP hydrolysis below basal levels. The K_m of Pgp-mediated ATP hydrolysis extracted from the kinetics curves was lower for DOX than DNR. However, the dissociation constants (K_Ds) for these drugs determined by fluorescence quenching were virtually identical. Acrylamide quenching of Pgp tryptophan fluorescence to probe the tertiary structure of Pgp suggested that DNR shifts Pgp to a "closed" conformation, while DOX shifts Pgp to an "intermediate" conformation. The effects of these drugs on the Pgp conformational distributions in a lipid bilayer were also examined by atomic force microscopy (AFM). Analysis of AFM images revealed that DNR and DOX cause distinct and significant shifts in the conformational distribution of Pgp. The results were combined to build a conformational distribution model for anthracycline transport by Pgp.

Recent Advances in Developing Small Molecules Targeting Nucleic Acid

Nucleic acids participate in a large number of biological processes. However, current approaches for small molecules targeting protein are incompatible with nucleic acids. On the other hand, the lack of crystallization of nucleic acid is the limiting factor for nucleic acid drug design. Because of the improvements in crystallization in recent years, a great many structures of nucleic acids have been reported, providing basic information for nucleic acid drug discovery. This review focuses on the discovery and development of small molecules targeting nucleic acids.

Structure-based DNA-targeting strategies with small molecule ligands for drug discovery

Nucleic acids are the molecular targets of many clinical anticancer drugs. However, compared with proteins, nucleic acids have traditionally attracted much less attention as drug targets in structure-based drug design, partially because limited structural information of nucleic acids complexed with potential drugs is available. Over the past several years, enormous progresses in nucleic acid crystallization, heavy-atom derivatization, phasing, and structural biology have been made. Many complicated nucleic acid structures have been determined, providing new insights into the molecular functions and interactions of nucleic acids, especially DNAs complexed with small molecule ligands. Thus, opportunities have been created to further discover nucleic acid-targeting drugs for disease treatments. This review focuses on the structure studies of DNAs complexed with small molecule ligands for discovering lead compounds, drug candidates, and/or therapeutics.

Solution studies on the complex of 4'-epiadriamycin-d-(CGATCG)2 followed by time-resolved fluorescence measurement, diffusion ordered spectroscopy and restrained molecular dynamics simulations

4'-Epiadriamycin is a better-tolerated anthracycline drug, due to lesser cardiotoxicity. We report here a study of the 2:1 complex of 4'-epiadriamycin-d-(CGATCG)(2) by proton Nuclear Magnetic Resonance Spectroscopy which show the absence of sequential connectivities between C1pG2 and C5pG6 base pair steps and presence of intermolecular cross peaks of the drug and DNA. Our studies establish the role of 9OH, NH3+, 7O, 4OCH(3) groups in binding to DNA. Time-resolved fluorescence measurement and diffusion ordered spectroscopic studies reveal the formation of complex. The nonspecific interactions as well as those essential for biological activity are discussed along with its medicinal importance.

Intercalation of daunomycin into d(CG)4 oligomer duplex containing G x T mismatches by vibrational circular dichroism and infrared absorption spectroscopy

The vibrational circular dichroism (VCD) and infrared absorption (IR) spectra of the mismatched octamer oligonucleotides d(CGTGCGCG)(2) (CGT) and d(CGCGTGCG)(2) (CGC) and their complexes with the antitumor drug daunomycin were measured in D(2)O, interpreted, and compared to the octamer d(CGCGCGCG)(2) (CG). The IR spectra of the mismatched octamers in the carbonyl-stretching region are similar to those of the parent CG, whereas the VCD spectra differ in several respects between each other. The main VCD feature due to carbonyl stretching is informative for the mismatches and CG. Vibrational modes in the sugar-phosphate region remain essentially unchanged especially for PO(2) (-) symmetric stretching. Differences between the free and complexed mismatch octamers occurred mainly in the carbonyl-stretching region (1,700-1,600 cm(-1)). The absorption intensity of the C==O peak of G is more prominent for CGC than CGT and resembles CG in this respect. The detailed composition of this doublet is clearly visible, indicating the geometric rearrangement of the base pairs in the presence of the mismatch and upon forming the daunomycin complex.

Searching for a new anti-HCV therapy: synthesis and properties of tropolone derivatives

Hepatitis C virus (HCV) is considered one of the most dangerous pathogens since about 3% of the world population is HCV-infected and the virus is a major cause of hepatitis, cirrhosis, and liver carcinoma. A need for a more efficient therapy prompted us to investigate new class of compounds, such as tropolone derivatives that possess antiviral, antibacterial, and antifungal activities. To synthesize bromo- and morpholinomethyl-analogues of tropolone, the previously reported methods were modified. The influence of new derivatives on the activity of the helicase and NTP-ase of HCV was investigated. The most potent inhibitory effect in the fluorometric helicase assay was exerted by 3,7-dibromo-5-morpholinomethyltropolone, for which the IC50 value was at low micromolar range. All the morpholino-derivatives had inhibitory activities higher than those of the non-modified analogues. Low toxicity in a yeast-based toxicity assay indicates that these compounds could be further modified to develop potent inhibitors of the HCV helicase and of viral replication.

Restrained molecular dynamics studies on complex of adriamycin with DNA hexamer sequence d-CGATCG

Adriamycin is an anthracycline anticancer drug used widely for solid tumors in spite of its adverse side effects. The solution structure of 2:1 adriamycin-d-(CGATCG)(2) complex has been studied by restrained molecular dynamics simulations. The restraint data set consists of several intramolecular and intermolecular nuclear Overhauser enhancement cross-peaks obtained from two-dimensional nuclear magnetic resonance spectroscopy data. The drug is found to intercalate between CG and GC base pairs at two d-CpG sites. The drug-DNA complex is stabilized via specific hydrogen bonding and van der Waal's interactions involving 4OCH(3), O5, 6OH, and NH(3)(+) moiety of daunosamine sugar, and rings A protons. The O-glycosidic bond C7-O7-C1'-C2' lies in the range 138 degrees -160 degrees during the course of simulations. The O6-H6...O5 hydrogen bond is stable while O11-H11...O12 hydrogen bond is not favored. The intercalating base pairs are buckled and minor groove is wider in the complex. The phosphate on one strand at intercalation site C1pG2 is in B(I) conformation and the phosphates directly lying on opposite strand is in B(II) conformation. The phosphorus on adjacent site G2pA3 is in B(II) conformation and hence a distinct pattern of B(I) and B(II) conformations is induced and stabilized. The role of various functional groups by which the molecular action is mediated has been discussed and correlated to the available biochemical evidence.

Structure of DNA sequence d-TGATCA by two-dimensional nuclear magnetic resonance spectroscopy and restrained molecular dynamics

The 5' d-TpG 3' element is a part of DNA sequences involved in regulation of gene expression and is also a site for intercalation of several anticancer drugs. Solution conformation of DNA duplex d-TGATCA containing this element has been investigated by two-dimensional NMR spectroscopy. Using a total of 12 torsional angles and 121 distance constraints, structural refinement has been carried out by restrained molecular dynamics (rMDs) in vacuum up to 100 ps. The structure is characterized by a large positive roll at TpG/CpA base pair step and large negative propeller twist for AT and TA base pairs. The backbone torsional angle, gamma(O5'-C5'-C4'-C3'), of T1 residue adopts a trans-conformation which is corroborated by short intra nucleotide T1H6-T1H5' (3.7A) distance in nuclear overhauser effect spectroscopy (NOESY) spectra while the backbone torsional angle, beta(P-O5'-C5'-C4'), exists in trans as well as gauche state for T1 and C5 residues. There is evidence of significant flexibility of the sugar-phosphate backbone with rapid inter-conversion between two different conformers at TpG/CpA base pair step. The base sequence dependent variations and local structural heterogeneity have important implications in specific recognition of DNA by ligands.

Daunomycin Intercalation Stabilizes Distinct Backbone Conformations of DNA

Daunomycin is a widely used antibiotic of the anthracycline family. In the present study we reveal the structural properties and important intercalator-DNA interactions by means of molecular dynamics. As most of the X-ray structures of DNA-daunomycin intercalated complexes are short hexamers or octamers of DNA with two drug molecules per doublehelix we calculated a self complementary 14-mer oligodeoxyribonucleotide duplex d(CGCGCGATCGCGCG)2 in the B-form with two putative intercalation sites at the 5'-CGA-3' step on both strands. Consequently we are able to look at the structure of a 1:1 complex and exclude crystal packing effects normally encountered in most of the X-ray crystallographic studies conducted so far. We performed different 10 to 20 ns long molecular dynamics simulations of the uncomplexed DNA structure, the DNA-daunomycin complex and a 1:2 complex of DNA-daunomycin where the two intercalator molecules are stacked into the two opposing 5'-CGA-3' steps. Thereby--in contrast to X-ray structures--a comparison of a complex of only one with a complex of two intercalators per doublehelix is possible. The chromophore of daunomycin is intercalated between the 5'-CG-3' bases while the daunosamine sugar moiety is placed in the minor groove. We observe a flexibility of the dihedral angle at the glycosidic bond, leading to three different positions of the ammonium group responsible for important contacts in the minor groove. Furthermore a distinct pattern of BI and BII around the intercalation site is induced and stabilized. This indicates a transfer of changes in the DNA geometry caused by intercalation to the DNA backbone.

Surprising roles of electrostatic interactions in DNA-ligand complexes

The positions of cations in x-ray structures are modulated by sequence, conformation, and ligand interactions. The goal here is to use x-ray diffraction to help resolve structural and thermodynamic roles of specifically localized cations in DNA-anthracycline complexes. We describe a 1.34 A resolution structure of a CGATCG(2)-adriamycin(2) complex obtained from crystals grown in the presence of thallium (I) ions. Tl(+) can substitute for biological monovalent cations, but is readily detected by distinctive x-ray scattering, obviating analysis of subtle differences in coordination geometry and x-ray scattering of water, sodium, potassium, and ammonium. Six localized Tl(+) sites are observable adjacent to each CGATCG(2)-adriamycin(2) complex. Each of these localized monovalent cations are found within the G-tract major groove of the intercalated DNA-drug complex. Adriamycin appears to be designed by nature to interact favorably with the electrostatic landscape of DNA, and to conserve the distribution of localized cationic charge. Localized inorganic cations in the major groove are conserved upon binding of adriamycin. In the minor groove, inorganic cations are substituted by a cationic functional group of adriamycin. This partitioning of cationic charge by adriamycin into the major groove of CG base pairs and the minor groove of AT base pairs may be a general feature of sequence-specific DNA-small molecule interactions and a potentially useful important factor in ligand design.

X-ray crystallography of DNA-drug complexes

Here we have stressed important differences between protein and DNA crystallography. Crystal growth and data collection methodologies are not directly transferable between the two subfields. In addition, we note that analysis of symmetry and packing of DNA crystals can be useful and a uniquely aesthetic exercise.

A survey of the sequence-specific interaction of damaging agents with DNA: emphasis on antitumor agents

This article reviews the literature concerning the sequence specificity of DNA-damaging agents. DNA-damaging agents are widely used in cancer chemotherapy. It is important to understand fully the determinants of DNA sequence specificity so that more effective DNA-damaging agents can be developed as antitumor drugs. There are five main methods of DNA sequence specificity analysis: cleavage of end-labeled fragments, linear amplification with Taq DNA polymerase, ligation-mediated polymerase chain reaction (PCR), single-strand ligation PCR, and footprinting. The DNA sequence specificity in purified DNA and in intact mammalian cells is reviewed for several classes of DNA-damaging agent. These include agents that form covalent adducts with DNA, free radical generators, topoisomerase inhibitors, intercalators and minor groove binders, enzymes, and electromagnetic radiation. The main sites of adduct formation are at the N-7 of guanine in the major groove of DNA and the N-3 of adenine in the minor groove, whereas free radical generators abstract hydrogen from the deoxyribose sugar and topoisomerase inhibitors cause enzyme-DNA cross-links to form. Several issues involved in the determination of the DNA sequence specificity are discussed. The future directions of the field, with respect to cancer chemotherapy, are also examined.

Persistent effects of doxorubicin on cardiac gene expression

During administration of the anthracycline antitumour agents, their cardiotoxicity can progress from cardiac dysfunction to heart failure. Cardiomyopathy can also develop years after receiving anthracyclines. To determine if persistent and/or progressive anthracycline effect(s) are referable to anthracycline effects on cardiac gene expression, steady-state mRNA levels were determined 4 days (n=8), 4 weeks (n=7) and 10 weeks (n=7) after doxorubicin (DOX; 2 mg/kg IV) in a well-characterized rabbit model. Levels of mRNA for alpha -actin, beta -myosin heavy chain and the calcium pump of the sarcoplasmic reticulum (SERCA2a) in the left ventricle (LV) were determined by Northern blot hybridization and expressed relative to an 18S constitutive marker. The mRNA levels for the high molecular weight subunit (cardiac isoform) of the ryanodine receptor (RyR2), sarcolemmal calcium channel (dihydropyridine receptor; DHPR), angiotensin-converting enzyme (ACE), angiotensin II receptor (ATR) and atrial naturetic peptide prohormone (ANP) were determined by reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot analysis, and expressed relative to GAPDH, a constitutive marker. Histopathologic evidence for anthracycline-induced myocardial cell injury was absent (score <1) in all hearts examined except one (score=1.1; 4 weeks post-DOX), which was considered separately. Relative mRNA levels for beta -myosin heavy chain 4 days after DOX increased 1.9-fold compared to the vehicle-treated group, but by 4 weeks levels had returned to baseline. Relative mRNA levels for DHPR were increased 1.2-fold 4 days after DOX and were persistently increased 1.9- and 2.2-fold 4 and 10 weeks after DOX, respectively. The mRNA levels for ANP were first decreased (4.5-fold) 4 days after DOX. Four weeks after DOX, ANP message levels approached Control in seven out of eight rabbits. The one rabbit with early LV histopathology 4 weeks post-DOX had increased mRNA for DHPR (2.7-fold) and ANP (80-fold). Between 4 and 10 weeks after DOX, mRNA levels for ANP increased C 16-fold: evidence for late progression. In situ hybridization with specific riboprobes localized the persistent increase in DHPR and the progressive increase in ANP to myocytes. Thus, DOX alters steady-state mRNA levels in LV that are referable to both persistent and progressive anthracycline effects on myocellular gene expression.

Structure of a DNA-bisdaunomycin complex

The application of detailed structural data bases has now culminated in the successful design of a new generation of bisanthracyclines that form ultratight DNA complexes [Chaires, J. B., Leng, F., Przewloka, T., Fokt, I., Ling, Y. H., Perez-Soler, R., & Priebe, W. (1997) J. Med. Chem. 40, 261-266]. Daunomycin dimers were designed to bind to DNA in complexes resembling those of monomers intercalated at adjacent sites. The goal of the work described here was to determine, with X-ray crystallography, if a potent member of this newly designed and synthesized class of bisanthracyclines (WP631) binds as intended. WP631 is composed of two daunomycin molecules, linked N3' to N3' by a xylyl group. We have solved the 2.2 A X-ray crystal structure of a complex of WP631 bound to [d(CGATCG)]2. We demonstrate, on a detailed molecular level, that the WP631 design strategy is a success. The structures of WP631 and two daunomycin molecules bound to [d(CGATCG)]2 provide the unprecedented opportunity for detailed comparison of mono- and bis-intercalated complexes of the same chromophore, allowing us to distinguish effects of mono-intercalation from those of bis-intercalation. Differences are focused primarily in the centers of the complexes. DNA unwinding and other helical distortions propagate more efficiently to the center of the WP631 complex than to the center of the daunomycin complex.

Biochemistry of topoisomerase I and II inhibition by anthracenyl-amino acid conjugates

Mono-conjugation of an anthraquinone nucleus with a range of naturally occurring amino acids chemically modified at their C-terminus has been adopted as a synthetic approach in the rational design of novel topoisomerase (topo) inhibitors. The biochemistry of topo I and II inhibition has been investigated for a series of 16 new compounds (NU/ICRF 500-515) from which structure-activity relationships have been investigated. Only three compounds could be demonstrated to bind to DNA: two serine derivatives (NU/ICRFs 500 and 506) and an arginine derivative (NU/ICRF 510). In decatenation and relaxation assays with purified enzyme, several compounds were shown to be potent catalytic inhibitors of topo II (100% inhibition at 5 micrograms/mL (10-15 microM) or less) without stabilizing cleavable complex formation. These included the three DNA binding species (of which NU/ICRF 506 was the most active) and a dihydroxyphenylalanine analogue (NU/ICRF 513). Both NU/ICRFs 500 and 506 were further shown to antagonize DNA cleavage induced by amsacrine. Only NU/ICRF 506 unequivocally inhibited the catalytic activity of topo I without induction of DNA cleavage, and was the only combined topo I and II catalytic inhibitor. One compound, NU/ICRF 505 (tyrosine conjugate), stabilized topo I cleavable complexes without inhibiting the catalytic activity of topo I and II. Modifications to the structure of NU/ICRF 505 revealed that the presence of an unhindered hydroxyl on the tyrosine ring and a more hydrophobic ethyl ester at the amino acid C-terminal were both essential, suggesting a highly specific interaction between drug, enzyme and DNA in the ternary complex. Molecular modelling studies suggested that the observed differences in topo inhibition are a consequence of major conformational alterations brought about by small changes in the amino acid substituent, and confirmed a rigid structural requirement for the induction of topo I cleavage, in addition to a less rigid structural requirement for topo II inhibition. A strong correlation was observed between topo inhibition and in vitro cytotoxicity against the human ovarian cancer cell line A2780 (IC50 range 3.4-11.6 microM), suggesting a mechanism of cell kill, at least in part, involving topo inhibition.

A trigonal form of the idarubicin:d(CGATCG) complex; crystal and molecular structure at 2.0 A resolution

The X-ray crystal structure of the complex between the anthracycline idarubicin and d(CGATCG) has been solved by molecular replacement and refined to a resolution of 2.0 A. The final R-factor is 0.19 for 3768 reflections with Fo > or = 2 sigma (Fo). The complex crystallizes in the trigonal space group P31 with unit cell parameters a = b = 52.996(4), c = 33.065(2) A, alpha = beta = 90 degree, gamma = 120 degree. The asymmetric unit consists of two duplexes, each one being complexed with two idarubicin drugs intercalated at the CpG steps, one spermine and 160 water molecules. The molecular packing underlines major groove-major groove interactions between neighbouring helices, and an unusually low value of the occupied fraction of the unit cell due to a large solvent channel of approximately 30 A diameter. This is the first trigonal crystal form of a DNA-anthracycline complex. The structure is compared with the previously reported structure of the same complex crystallizing in a tetragonal form. The geometry of both the double helices and the intercalation site are conserved as are the intramolecular interactions despite the different crystal forms.

DNA-nogalamycin interactions: the crystal structure of d(TGATCA) complexed with nogalamycin

The structure of the self-complementary deoxyoligonucleotide d5'(TGATCA) complexed with nogalamycin, an antitumor anthracycline, has been solved to 1.8 A resolution using X-ray crystallographic methods. The technique of single isomorphous replacement, utilizing the anomalous signal of bromine in derivative data collected at three different wavelengths, Cu K alpha, Mo K alpha, and 0.91 A synchroton radiation, was used. The complex crystallized in space group P4(1)2(1)2 with unit cell dimensions a = 37.2 A and c = 70.1 A. The final structure including 116 water molecules has an overall R factor of 19.5% for the 4767 reflections with F > or = 1 sigma F in the resolution range 10.0-1.8 A. One nogalamycin molecule intercalates between each of the d5'(TpG) steps at both ends of a distorted B DNA double helix. This structure provides the first three-dimensional picture of nogalamycin bound to the triplet sequence d5'(TGA), one of its favorable natural binding sites. The drug exhibits a strict requirement for binding to the 3' side of a pyrimidine and the 5' side of a purine. Nogalamycin has bulky sugar groups at either end of a planar aglycon chromophore; therefore, in order for intercalation to occur, the DNA must either transiently open or flex along the helix axis to allow insertion of the chromophore between the base pairs. Conformational change in nogalamycin is observed in the drug-DNA complex with respect to free nogalamycin. Nogalamycin binding to DNA induces severe deformation to the intercalation site base pairs. In comparison to previously reported anthracycline-DNA structures significant differences in base-pair geometry, drug hydrogen-bonding patterns, and the extent of hydration are observed. The position of the drug in this complex is stabilized by a number of nonbonded forces including van der Waals interactions and extensive direct and solvent-mediated hydrogen bonds to the DNA duplex.

Studies on the molecular pharmacology of GR63178A. A novel pentacyclic pyrolloquinone anticancer drug

GR63178A (NSC D611615) is the second pentacyclic pyrolloquinone to be evaluated clinically as an anticancer drug. Its mechanism of action is unknown but may be related either to its quinone group or planar ring system. In this report we have investigated the ability of GR63178A to bind non-covalently to DNA, inhibit topoisomerase II and undergo reduction to reactive free radical species. Using two DNA duplexes, a 12-mer oligonucleotide which is a preferred sequence for minor groove binders and a hexamer which is a preferred sequence for intercalators, no evidence of significant binding with GR63178A was found. Neither GR63178A nor GR54374X (its 9-hydroxy metabolite) inhibited purified human topoisomerase II in a decatenation assay. Free radical chemistry was studied by both pulse radiolysis and ESR spectroscopy as well as by in vitro drug incubations with NADPH-fortified rat liver microsomes and purified cytochrome P450 reductase. The one-electron reduction potential of GR63178A was -207 mV +/- 10 which is much more positive than other quinone-containing anticancer drugs such as doxorubicin, mitomycin C and mitozantrone. GR63178A underwent enzyme-catalysed quinone reduction more readily than doxorubicin but produced significantly fewer reactive oxygen species. No evidence was detected of drug-induced, radical-mediated DNA damage in vitro using pBR322 plasmid DNA. Disproportionation of the GR63178A semi-quinone free radical proceeded with a rate constant of 1 x 10(9) M-1 sec-1 under anaerobic conditions, one order of magnitude faster than doxorubicin. The preferential disproportionation of the semi-quinone may explain our inability to detect a free radical signal by ESR. The hydroquinone of GR63178A was stable and exhibited strong visible absorption with a bathochromic shift of 120 nm over the parent drug. These unusual properties may be due to the hydroquinone undergoing a form of keto-enol tautomerization. Thus, GR63178A free radical formation does not appear to result in significant drug activation. In conclusion, GR63178A is unlikely to mediate its antitumour activity by DNA binding, topoisomerase II inhibition or free radical formation in direct contrast to similar anthracycline- and anthraquinone-based anticancer drugs.

The molecular structure of a 4'-epiadriamycin complex with d(TGATCA) at 1.7A resolution: comparison with the structure of 4'-epiadriamycin d(TGTACA) and d(CGATCG) complexes

The structure of the complex between d(TGATCA) and the anthracycline 4'-epiadriamycin has been determined by crystallographic methods. The crystals are tetragonal, space group P4(1)2(1)2 with unit cell dimensions of a = 28.01, c = 52.95A. The asymmetric unit consists of one strand of hexanucleotide, one molecule of 4'-epiadriamycin and 34 waters. The R-factor is 20.2% for 1694 reflections with F greater than or equal to 2 sigma F to 1.7A. Two asymmetric units associate to generate a duplex complexed with two drug molecules at the d(TpG) steps of the duplex. The chromophore intercalates between these base pairs with the anthracycline amino-sugar positioned in the minor groove. The double helix is a distorted B-DNA type structure. Our structure determination of d(TGATCA) complexed to 4'-epiadriamycin allows for comparison with the previously reported structures of 4'-epiadriamycin bound to d(TGTACA) and to d(CGATCG). The three complexes are similar in gross features and the intercalation geometry is the same irrespective of whether a d(CpG) or d(TpG) sequence is involved. However, the orientation of the amino-sugar displays a dependence on the sequence adjacent to the intercalation site. The flexibility of this amino-sugar may help explain why this class of antibiotics displays a relative insensitivity to base sequence when they bind to DNA.