Structure of the DNA interstrand cross-link of 4,5',8-trimethylpsoralen.

4,5',8-Trimethylpsoralen (TMP) cross-links a 5' TpA or a 5' ApT site by photoreacting with one thymine moiety in each DNA strand. We are interested in whether psoralen interstrand cross-links all share one structure or whether there are significant differences. In this paper, we employed a rapid method for probing the structure of the cross-link by making a series of TMP cross-linked duplexes containing specific base-pair mismatches. The relative stability provided by a base pair can be correlated with neighboring base pairs by comparing the extents of gel retardation when base-pair mismatches happen in each position. From our studies, we infer that with respect to the furan-side strand, the 5'T.A base pair of the two T.A base pairs in the TpA site is not hydrogen bonded. Immediately on each side of the cross-linked TpA site is a highly stabilized base pair. Next, a region of decreased stability occurs in each arm of a cross-linked duplex and these base pairs of least stability are located farther away from the cross-linked thymines as the lengths of the arms of the cross-linked helix increase. Finally, even in 7 M urea at 49 degrees C the cross-linked helix is hydrogen bonded at both ends of a duplex of 22 base pairs. We propose that the structures of interstrand cross-links in DNA vary appreciably with the DNA sequence, the length of the DNA duplex, and the structures of the DNA cross-linking agents.     

Sequence-specific DNA interstrand cross-linking by an aziridinomitosene in the absence of exogenous reductant

The aziridinomitosene derivative (1S,2S)-6-desmethyl(methylaziridino)mitosene (4) was shown to alkylate plasmid DNA at pH 7.4 in the absence of a reducing agent [Vedejs, E., Naidu, B. N., Klapars, A., Warner, D. L., Li, V. -s., Na, Y., and Kohn, H. (2003) J. Am. Chem. Soc. 125, 15796-15806], an activity not found in the parent mitomycins. We sought to evaluate aziridinomitosene 4 for the presence of DNA interstrand cross-linking activity using nonreductive reaction conditions. Radiolabeled DNA treated with 4 was analyzed by denaturing polyacrylamide gel electrophoresis (DPAGE), a technique that readily separates the less mobile cross-linked ds DNA from the more mobile ss DNA products. Nonreduced 4 produced an interstrand cross-link (ICL) in duplex DNA containing 5'-d(CG) sites, and the yield of ICL was comparable to that obtained from reduced MC under similar conditions. A ds DNA having the central tetranucleotide 5'-d(ACGT) provided the greatest ICL yield from both nonreduced 4 and reduced MC. Substitution of 5'-d(CG) with the inverted sequence 5'-d(GC) completely abolished interstrand cross-linking by 4, revealing 5'-d(CG) as its specific site of ICL formation. Replacement of dG at 5'-d(CG) with 2'-deoxyinosine (dI), which lacks the exocyclic C2 amino group present in dG, also prevented DNA ICL formation by 4, revealing an essential role for the dG C2 amino group in the interstrand cross-linking reaction between 4 and duplex DNA. This report directly demonstrates the presence of bifunctional alkylating activity in a nonreduced aziridinomitosene and clearly shows that unreduced 4 alkylates residues in the minor groove of ds DNA, cross-linking with the same 5'-d(CG) sequence specificity displayed by reduced MC.     

N(4)C-ethyl-N(4)C cross-linked DNA: synthesis and characterization of duplexes with interstrand cross-links of different orientations.

The preparation and physical properties of short DNA duplexes that contain a N(4)C-ethyl-N(4)C interstrand cross-link are described. Duplexes that contain an interstrand cross-link between mismatched C-C residues and duplexes in which the C residues of a -CG- or -GC- step are linked to give "staggered" interstrand cross-links were prepared using a novel N(4)C-ethyl-N(4)C phosphoramidite reagent. Duplexes with the C-C mismatch cross-link have UV thermal transition temperatures that are 25 degrees C higher than the melting temperatures of control duplexes in which the cross-link is replaced with a G-C base pair. It appears that this cross-link stabilizes adjacent base pairs and does not perturb the structure of the helix, a conclusion that is supported by the CD spectrum of this duplex and by molecular models. An even higher level of stabilization, 49 degrees C, is seen with the duplex that contains a -CG- staggered cross-link. Molecular models suggest that this cross-link may induce propeller twisting in the cross-linked base pairs, and the CD spectrum of this duplex exhibits an unusual negative band at 298 nm, although the remainder of the spectrum is similar to that of B-form DNA. Mismatched C-C or -CG- staggered cross-linked duplexes that have complementary overhanging ends can undergo self-ligation catalyzed by T4 DNA ligase. Analysis of the ligated oligomers by nondenaturing polyacrylamide gel electrophoresis shows that the resulting oligomers migrate in a manner similar to that of a mixture of non-cross-linked control oligomers and suggests that these cross-links do not result in significant bending of the helix. However, the orientation of the staggered cross-link can have a significant effect on the structure and stability of the cross-linked duplex. Thus, the thermal stability of the duplex that contains a -GC- staggered cross-link is 10 degrees C lower than the melting temperature of the control, non-cross-linked duplex. Unlike the -CG- staggered cross-link, in which the cross-linked base pairs can still maintain hydrogen bond contacts, molecular models suggest that formation of the -GC- staggered cross-link disrupts hydrogen bonding and may also perturb adjacent base pairs leading to an overall reduction in helix stability. Duplexes with specifically positioned and oriented cross-links can be used as substrates to study DNA repair mechanisms.     

Thermal and thermodynamic properties of duplex DNA containing site-specific interstrand cross-link of antitumor cisplatin or its clinically ineffective trans isomer

The effect of the single, site-specific interstrand cross-link formed by cisplatin or transplatin on the thermal stability and energetics of a 20-base pair DNA duplex is reported. The cross-linked or unplatinated 20-base pair duplexes were investigated with the aid of differential scanning calorimetry, temperature-dependent UV absorption, and circular dichroism. The cross-link of both platinum isomers increases the thermal stability of the modified duplexes by changing the molecularity of denaturation. The structural perturbation resulting from the interstrand cross-link of cisplatin increases entropy of the duplex and in this way entropically stabilizes the duplex. This entropic cross-link-induced stabilization of the duplex is partially but not completely compensated by the enthalpic destabilization of the duplex. The net result of these enthalpic and entropic effects is that the structural perturbation resulting from the formation of the interstrand cross-link by cisplatin induces a decrease in duplex thermodynamic stability, with this destabilization being enthalpic in origin. By contrast, the interstrand cross-link of transplatin is enthalpically almost neutral with the cross-link-induced destabilization entirely entropic in origin. These differences are consistent with distinct conformational distortions induced by the interstrand cross-links of the two isomers. Importantly, for the duplex cross-linked by cisplatin relative to that cross-linked by transplatin, the compensating enthalpic and entropic effects almost completely offset the difference in cross-link-induced energetic destabilization. It has been proposed that the results of the present work further support the view that the impact of the interstrand cross-links of cisplatin and transplatin on DNA is different for each and might also be associated with the distinctly different antitumor effects of these platinum compounds.     

Interstrand disulfide cross-linking of internal sugar residues in duplex RNA

Disulfide cross-linking is being used increasingly more to study the structure and dynamics of nucleic acids. We have previously developed a procedure for the formation of disulfide cross-links through the sugar-phosphate backbone of nucleic acids. Here we report the preparation and characterization of an RNA duplex containing a disulfide interstrand cross-link. A self-complementary oligoribonucleotide duplex containing an interstrand cross-link was prepared from the corresponding 2'-amino modified oligomer. Selective modification of the 2'-amino group with an aliphatic isocyanate, containing a protected disulfide, gave the corresponding 2'-urea derivative in excellent yield. An RNA duplex containing an intrahelical, interstrand disulfide cross-link was subsequently prepared by a thiol disulfide exchange reaction in nearly quantitative yield as judged by denaturing polyacrylamide gel electrophoresis (DPAGE). The cross-linked RNA was further characterized by enzymatic digestion and the Structure of the cross-link lesion was verified by comparison to an authentic sample, prepared by chemical synthesis. The effect of the chemical modifications on duplex stability was determined by UV thermal denaturation experiments. The intrahelical cross-link stabilized the duplex considerably: the disulfide cross-linked oligomer had a melting temperature that was ca. 40 degrees C higher than that of the noncross-linked oligomer.     

N(4)C-alkyl-N(4)C cross-linked DNA: bending deformations in duplexes that contain a -CNG- interstrand cross-link

Short DNA duplexes containing a 1,3-N(4)C-alkyl-N(4)C interstrand cross-link that joins the two C residues of a -CNG- sequence were prepared using either a phosphoramidite or convertible nucleoside approach. The alkyl cross-link consists of 2, 4, or 7 methylene groups. The duplexes, which contain a seven-base-pair core and A(3)/T(3) complementary 3'-overhanging ends, were characterized by enzymatic digestion and MALDI-TOF mass spectrometry. Ultraviolet thermal denaturation studies showed that the duplexes denature in a cooperative manner and that the length of the cross-link affects the thermal stability. Thus, the transition temperature of the ethyl cross-linked duplex, 42 degrees C, is 16 degrees C higher than the melting temperature of the corresponding non-cross-linked control, whereas the transition temperatures of the butyl and heptyl cross-linked duplexes, 73 and 72 degrees C, respectively, are 46-47 degrees C higher. The reduced molecularity of denaturation of the cross-linked duplexes versus melting of the non-cross-linked duplex most likely accounts for these differences. Examination of molecular models suggests that the ethyl cross-link is too short to span the distance between the two C residues at the site of the cross-link in B-form DNA without causing distortion of the helix, whereas less and no distortion would be expected for the butyl and heptyl cross-links, respectively. The circular dichroism spectra, which show greatest deviation in the ethyl cross-linked duplex from B-form DNA, are consistent with this expectation. Anomalous mobilities on native polyacrylamide gels of multimers produced by self-ligation of each of the cross-linked duplexes suggest that the ethyl and butyl cross-linked duplexes undergo bending deformations, whereas multimers derived from the heptyl cross-linked duplex migrated normally. The bending angle was estimated to be 20 degrees, 13 degrees, and 0 degrees for the ethyl, butyl, and heptyl cross-linked duplexes, respectively. Thus, it appears that the degree of bending in these N(4)C-alkyl-N(4)C cross-linked duplexes is controlled by the length of the cross-link.     

Unique dynamic properties of DNA duplexes containing interstrand cross-links

Bifunctional DNA alkylating agents form a diverse assortment of covalent DNA interstrand cross-linked (ICL) structures that are potent cytotoxins. Because it is implausible that cells could possess distinct DNA repair systems for each individual ICL, it is believed that common structural and dynamic features of ICL damage are recognized, rather than specific structural characteristics of each cross-linking agent. Investigation of the structural and dynamic properties of ICLs that might be important for recognition has been complicated by heterogeneous incorporation of these lesions into DNA. To address this problem, we have synthesized and characterized several homogeneous ICL DNAs containing site-specific staggered N4-cytosine-ethyl-N4-cytosine cross-links. Staggered cross-links were introduced in two ways, in a manner that preserves the overall structure of B-form duplex DNA and in a manner that highly distorts the DNA structure, with the goal of understanding how structural and dynamic properties of diverse ICL duplexes might flag these sites for repair. Measurements of base pair opening dynamics in the B-form ICL duplex by (1)H NMR line width or imino proton solvent exchange showed that the guanine base opposite the cross-linked cytosine opened at least 1 order of magnitude more slowly than when in a control matched normal duplex. To a lesser degree, the B-form ICL also induced a decrease in base pair opening dynamics that extended from the site of the cross-link to adjacent base pairs. In contrast, the non-B-form ICL showed extensive conformational dynamics at the site of the cross-link, which extended over the entire DNA sequence. Because DNA duplexes containing the B-form and non-B-form ICL cross-links have both been shown to be incised when incubated in mammalian whole cell extracts, while a matched normal duplex is not, we conclude that intrinsic DNA dynamics is not a requirement for specific damage incision of these ICLs. Instead, we propose a general model in which destabilized ICL duplexes serve to energetically facilitate binding of DNA repair factors that must induce bubbles or other distortions in the duplex. However, the essential requirement for incision is an immobile Y-junction where the repair factors are stably bound at the site of the ICL, and the two DNA strands are unpaired.     

Interstrand cross-linking of DNA by FK317 and its deacetylated metabolites FR70496 and FR157471

FR900482 (1) and FR66979 (2) are structurally novel natural products isolated by Fujisawa in 1987 and have been shown to be highly potent antitumor antibiotics structurally related to the mitomycins. Studies on the mode of action have established that these new agents form covalent DNA interstrand cross-links both in vitro and in vivo as a result of the reactive mitosene intermediate generated upon bioreductive activation. Semisynthetic analogues such as FK973 (3) and FK317 (4) were developed in the search for potentially superior clinical candidates. Although FK317 has been shown to be a potent compound, to date no direct evidence of DNA interstrand cross-link sequence specificity has been reported. In this study, DNA interstrand cross-links were generated by treatment of a synthetic duplex DNA substrate with FK317 (4) and its deacetylated metabolites FR70496 (5) and FR157471 (6). Analysis by gel electrophoresis revealed the formation of orientation isomers displaying electrophoretic mobility vastly greater than the mobilities of those generated from FR900482 (1). Despite these differences, it was established by Fe(II)-EDTA footprinting that FK317 (4) as well as 5 and 6 forms DNA interstrand cross-links within the expected 5'CpG3' step, clearly demonstrating that the phenolic hydrogen in 1 and 2 is not a prerequisite for efficient DNA interstrand cross-linking by the FR class of compounds.     

An assay combining high-performance liquid chromatography and mass spectrometry to measure DNA interstrand cross-linking efficiency in oligonucleotides of varying sequences

The main method of evaluating the DNA interstrand cross-linking ability of cancer chemotherapeutic agents in naked DNA currently involves the electrophoresis of relatively long radiolabeled duplex DNA fragments (typically approximately 2000 bp) on neutral gels after incubation with the agent of interest. Denaturation by heating is carried out prior to loading, and a neutral gel allows reannealing of cross-linked DNA. To avoid the use of radioactivity we have developed a new method based on ion pair reversed phase liquid chromatography (RPLC) and mass spectrometry (MS) that allows characterization and quantitation of drug-DNA interstrand cross-links formed within short oligonucleotide duplexes (i.e., 12 bp). Advantages of this assay include rapid throughput, as compared to electrophoretic methods, and the use of readily available short nonradiolabeled oligonucleotides of any sequence, thereby facilitating investigation of sequence selectivity. A further advantage is that all species separated by the chromatographic process can be positively identified by MS. Using this new method, we have investigated the rate of DNA cross-linking and sequence selectivity of the interstrand cross-linking agent SJG-136, a pyrrolobenzodiazepine (PBD) dimer currently in phase I clinical trials. The assay was found to be sufficiently sensitive and selective to allow separation of the unbound and drug-bound oligonucleotide species by high-performance liquid chromatography (HPLC) and to allow positive identification of these individual species by MS. A further benefit, as compared with electrophoretic methods, is that kinetic information can be obtained and compared for different binding sequences.     

Monoalkylation of DNA by reductively activated FR66979

The antitumor antibiotic FR66979 has previously been shown to form interstrand cross-links in duplex DNA at the sequence [5'-d(CG)]2, linking the exocyclic amino groups (N2) of deoxyguanosine (dG) residues. During the reaction of reductively activated FR66979 with DNA. products are formed which have electrophoretic mobility in denaturing polyacrylamide gels which is intermediate between that of unmodified and interstrand cross-linked DNA. We show here that these products are monoadducts between FR66979 and DNA and provide strong evidence for the site of alkylation being N2 of dG. Moreover, the sequence selectivity of monoalkylation reactions between FR66979 and DNA containing either 5'-d(CG).5'-d(CI) or [5'-d(CG)]2 was observed to be ca. 5-fold less than for the related antitumor antibiotic mitomycin C (MC). The mechanistic implications of this result are discussed. Furthermore, it was demonstrated that contrary to a previous report, FR66979 requires DNA to be in duplex form for efficient monoadduct formation.     

Diepoxybutane and diepoxyoctane interstrand cross-linking of the 5S DNA nucleosomal core particle

Diepoxyalkanes form interstrand cross-links in DNA oligomers preferentially at 5'-GNC sites. We have examined cross-linking by 1,2,3,4-diepoxybutane (DEB) and 1,2,7,8-diepoxyoctane (DEO) within a fragment of the 5S RNA gene of Xenopus borealis in both the free and nucleosomal states. Sites and efficiencies of interstrand cross-linking were probed through denaturing polyacrylamide gel electrophoresis and quantitative phosphorimagery. Both agents targeted 5'-GNC sites for cross-linking in the restriction fragment in its free state, and DEO also targeted 5'-GNNC sites. Monoalkylation occurred at all deoxyguanosines. The sites for both monoalkylation and interstrand cross-linking were similar in nucleosomal and free DNA, and cross-linked DNA was cleanly incorporated into the core particle structure. These findings suggest that the 5S core particle is able to tolerate any structural abnormalities induced by diepoxide cross-linking.     

DNA sequence specificity of 4,5',8-trimethylpsoralen cross-linking. Effect of neighboring bases on cross-linking the 5'-TA dinucleotide

The DNA sequence specificity for 4,5',8-trimethylpsoralen cross-linking of DNA has been examined using chemically synthesized DNA fragments containing all possible pyrimidine and purine base pair combinations. We confirm our previous findings that the 5'-TA dinucleotide represents a preferred cross-link site. Other dinucleotides that form cross-links are 5'-AT much greater than 5'-TG greater than 5'-GT. Although 5'-TA represents a preferred cross-link site, the rate of cross-linking can vary 3-4-fold depending on the base composition flanking the cross-linkable 5'-TA dinucleotide. In some cases, changes in DNA sequence 3 base pairs (bp) away from 5'-TA resulted in significant changes in the rate of cross-linking. We also see evidence for a long-range sequence context effect on the rate of cross-linking. A 30-bp fragment cross-linked at a relative rate of 2.9 compared to the rate of cross-linking of a 20-bp fragment when cloned contiguously in plasmid DNA. When cross-linked as separate fragments, the 30-bp fragment cross-linked at a relative rate of 1.9 compared to the 20-bp fragment. The results suggest that the reactivity of DNA to psoralens, and perhaps other intercalating drugs, is dependent on the dinucleotide sequence, the bases flanking the dinucleotide, and the long-range sequence context of the DNA.     

Interstrand cross-linking reaction in triplexes containing a monofunctional transplatin-adduct.

Our aim was to determine whether a single transplatin monofunctional adduct, either trans-[Pt(NH3)2(dC)Cl]+ or trans-[Pt(NH3)2(dG)Cl]+ within a homopyrimidine oligonucleotide, could further react and form an interstrand cross-link once the platinated oligonucleotide was bound to the complementary duplex. The single monofunctional adduct was located at either the 5' end or in the middle of the platinated oligonucleotide. In all the triplexes, specific interstrand cross-links were formed between the platinated Hoogsteen strand and the complementary purine-rich strand. No interstrand cross-links were detected between the platinated oligonucleotides and non-complementary DNA. The yield and the rate of the cross-linking reaction depend upon the nature and location of the monofunctional adducts. Half-lives of the monofunctional adducts within the triplexes were in the range 2-6 h. The potential use of the platinated oligonucleotides to modulate gene expression is discussed.     

Thermal stability and energetics of 15-mer DNA duplex interstrand crosslinked by trans-diamminedichloroplatinum(II)

The effect of the location of the interstrand cross-link formed by trans-diamminedichloroplatinum(II) (transplatin) on the thermal stability and energetics of 15-mer DNA duplex has been investigated. The duplex containing single, site-specific cross-link, thermodynamically equivalent model structures (hairpins) and nonmodified duplexes were characterized by differential scanning calorimetry, temperature-dependent uv absorption, and circular dichroism. The results demonstrate that the formation of the interstrand cross-link of transplatin does not affect pronouncedly thermodynamic stability of DNA: the cross-link induces no marked changes not only in enthalpy, but also in "reduced" (concentration independent) monomolecular transition entropy. These results are consistent with the previous observations that interstrand cross-links of transplatin structurally perturb DNA only to a relatively small extent. On the other hand, constraining the duplex with the interstrand cross-link of transplatin results in a significant increase in thermal stability that is primarily due to entropic effects: the cross-link reduces the molecularity of the oligomer system from bimolecular to monomolecular. Importantly, the position of the interstrand cross-link within the duplex modulates cooperativity of the melting transition of the duplex and consequently its thermal stability.     

Solution structure of a nitrous acid induced DNA interstrand cross-link

Nitrous acid is a mutagenic agent. It can induce interstrand cross-links in duplex DNA, preferentially at d(CpG) steps: two guanines on opposite strands are linked via a single shared exocyclic imino group. Recent synthetic advances have led to the production of large quantities of such structurally homogenous cross-linked duplex DNA. Here we present the high resolution solution structure of the cross-linked dodecamer [d(GCATCCGGATGC)]2 (the cross-linked guanines are underlined), determined by 2D NMR spectroscopy, distance geometry, restrained molecular dynamics and iterative NOE refinement. The cross-linked guanines form a nearly planar covalently linked 'G:G base pair' with only minor propeller twisting, while the cytidine bases of their normal base pairing partners have been flipped out of the helix and adopt well defined extrahelical positions in the minor groove. On the 5'-side of the cross-link, the minor groove is widened to accommodate these extrahelical bases, and the major groove becomes quite narrow at the cross-link. The cross-linked 'G:G base pair' is well stacked on the spatially adjacent C:G base pairs, particularly on the 3'-side guanines. In addition to providing the first structure of a nitrous acid cross-link in DNA, these studies could be of major importance to the understanding of the mechanisms of nitrous acid cross-linking and mutagenicity, as well as the mechanisms responsible for its repair in intracellular environments. It is also the shortest DNA cross-link structure to be described.     

4-vinyl-substituted pyrimidine nucleosides exhibit the efficient and selective formation of interstrand cross-links with RNA and duplex DNA

The formation of interstrand cross-links in nucleic acids can have a strong impact on biological function of nucleic acids; therefore, many cross-linking agents have been developed for biological applications. Despite numerous studies, there remains a need for cross-linking agents that exhibit both efficiency and selectivity. In this study, a 4-vinyl-substituted analog of thymidine (T-vinyl derivative) was designed as a new cross-linking agent, in which the vinyl group is oriented towards the Watson–Crick face to react with the amino group of an adenine base. The interstrand cross-link formed rapidly and selectively with a uridine on the RNA substrate at the site opposite to the T-vinyl derivative. A detailed analysis of cross-link formation while varying the flanking bases of the RNA substrates indicated that interstrand cross-link formation is preferential for the adenine base on the 5′-side of the opposing uridine. In the absence of a 5′-adenine, a uridine at the opposite position underwent cross-linking. The oligodeoxynucleotides probe incorporating the T-vinyl derivative efficiently formed interstrand cross-links in parallel-type triplex DNA with high selectivity for dA in the homopurine strand. The efficiency and selectivity of the T-vinyl derivative illustrate its potential use as a unique tool in biological and materials research.     

Repair of an interstrand DNA cross-link initiated by ERCC1-XPF repair/recombination nuclease

Interstrand DNA cross-link damage is a severe challenge to genomic integrity. Nucleotide excision repair plays some role in the repair of DNA cross-links caused by psoralens and other agents. However, in mammalian cells there is evidence that the ERCC1-XPF nuclease has a specialized additional function during interstrand DNA cross-link repair, beyond its role in nucleotide excision repair. We placed a psoralen monoadduct or interstrand cross-link in a duplex, 4-6 bases from a junction with unpaired DNA. ERCC1-XPF endonucleolytically cleaved within the duplex on either side of the adduct, on the strand having an unpaired 3' tail. Cross-links that were cleaved only on the 5' side were purified and reincubated with ERCC1-XPF. A second cleavage was then observed on the 3' side. Relevant partially unwound structures near a cross-link may be expected to arise frequently, for example at stalled DNA replication forks. The results show that the single enzyme ERCC1-XPF can release one arm of a cross-link and suggest a novel mechanism for interstrand cross-link repair.     

Interstrand cross-linking by adriamycin in nuclear and mitochondrial DNA of MCF-7 cells

Activation of Adriamycin by formaldehyde leads to the formation of drug–DNA adducts in vitro and these adducts stabilise the DNA to such a degree that they function as virtual interstrand cross-links. The formation of these virtual interstrand cross-links by Adriamycin was investigated in MCF-7 cells using a gene-specific interstrand cross-linking assay. Cross-linking was measured in both the nuclear-encoded DHFR gene and in mitochondrial DNA (mtDNA). Cross-link formation increased linearly with Adriamycin concentration following a 4 h exposure to the drug. The rate of formation of Adriamycin cross-links in each of the genomes was similar, reaching maximal levels of 0.55 and 0.4 cross-links/10 kb in the DHFR gene and mtDNA respectively, following exposure to 20 µM Adriamycin for 8 h. The interstrand cross-link was short lived in both DNA compartments, with a half-life of 4.5 and 3.3 h in the DHFR gene and mtDNA respectively. The kinetics of total Adriamycin adduct formation, detected using [¹⁴C]Adriamycin, was similar to that of cross-link formation. Maximal adduct levels (30 lesions/10 kb) were observed following incubation at 20 µM drug for 8 h. The formation of such high levels of adducts and cross-links could therefore be expected to contribute to the mechanism of action of Adriamycin.