Cross-linking of the human DNA repair protein O6-alkylguanine DNA alkyltransferase to DNA in the presence of 1,2,3,4-diepoxybutane

1,2,3,4-Diepoxybutane (DEB) is a key carcinogenic metabolite of the important industrial chemical 1,3-butadiene. DEB is a bifunctional alkylating agent capable of reacting with DNA and proteins. Initial DNA alkylation by DEB produces N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-guanine monoadducts, which can react with another nucleophilic site to form cross-linked adducts. A recent report revealed a strong correlation between cellular expression of the DNA repair protein O6-alkylguanine DNA alkyltransferase (AGT) and the cytotoxic and mutagenic activity of DEB, suggesting that DEB induces AGT-DNA cross-links (Valadez, J. G., et al. (2004) Activation of bis-electrophiles to mutagenic conjugates by human O6-alkylguanine-DNA alkyltransferase. Chem. Res. Toxicol. 17, 972-982). The purpose of our study was to analyze the formation and structures of DEB-induced AGT-DNA conjugates and to identify specific amino acid residues within the protein involved in cross-linking. DNA-protein cross-link formation was detected by SDS-PAGE when 32P-labeled double-stranded oligodeoxynucleotides were exposed to DEB in the presence of either wild-type hAGT or a C145A hAGT mutant. Capillary HPLC-electrospray ionization mass spectrometry (ESI-MS) analysis of hAGT that had been treated with N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-deoxyguanosine (dG monoepoxide) revealed the ability of the protein to form either one or two butanediol-dG cross-links, corresponding to mass shifts of +353 and +706 Da, respectively. HPLC-ESI+ -MS/MS sequencing of the tryptic peptides obtained from dG monoepoxide-treated protein indicated that the two cross-linking sites were the alkyl acceptor site, Cys145, and a neighboring active site residue, Cys150. The same two amino acid residues of hAGT became covalently cross-linked to DNA following DEB treatment. Modification of Cys145 was further confirmed by HPLC-ESI+ -MS/MS analysis of dG monoepoxide-treated synthetic peptide GNPVPILIPCHR which represents the active site tryptic fragment of hAGT (C = Cys145). The replacement of the catalytic cysteine residue with alanine in the C145A hAGT mutant abolished DEB-induced cross-linking at this site, while the formation of conjugates via neighboring Cys150 was retained. The exact chemical structure of the cross-linked lesion was established as 1-(S-cysteinyl)-4-(guan-7-yl)-2,3-butanediol by HPLC-ESI+ -MS/MS analysis of the amino acids resulting from the total digestion of modified proteins analyzed in parallel with an authentic standard. AGT-DNA cross-linking is a likely mechanism of DEB-mediated cytotoxicity in cells expressing this important repair protein.     

Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.     

Guanine-adenine DNA cross-linking by 1,2,3,4-diepoxybutane: potential basis for biological activity

1,2,3,4-Diepoxybutane (DEB) is a prominent carcinogenic metabolite of 1,3-butadiene (1,3-BD), an important industrial chemical and an environmental pollutant found in cigarette smoke and automobile exhaust. DEB is capable of inducing a variety of genotoxic effects, including point mutations, large deletions, and chromosomal aberrations. The mutagenicity and carcinogenicity of DEB are thought to result from its ability to form bifunctional DNA-DNA adducts by sequentially alkylating two nucleobases within the DNA double helix. We recently reported that DEB-induced DNA-DNA cross-linking leads to the formation of 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) adducts [Park, S., and Tretyakova, N. (2004) Structural characterization of the major DNA-DNA cross-link of 1,2,3,4-diepoxybutane. Chem. Res. Toxicol. 17 (2), 129-136]. However, guanine-guanine cross-linking by DEB cannot explain the development of A:T base pair mutations following exposure to DEB and 1,3-BD. In the present work, four asymmetrical DNA-DNA cross-links involving both adenine and guanine nucleobases were identified in double-stranded DNA treated with racemic DEB. These novel lesions were assigned the structures of 1-(aden-1-yl)-4-(guan-7-yl)-2,3-butanediol (N1A-N7G-BD), 1-(aden-3-yl)-4-(guan-7-yl)-2,3-butanediol (N3A-N7G-BD), 1-(aden-7-yl)-4-(guan-7-yl)-2,3-butanediol (N7A-N7G-BD), and 1-(aden-N6-yl)-4-(guan-7-yl)-2,3-butanediol (N6A-N7G-BD), based on the comparison of their MS/MS spectra, HPLC retention times, and UV spectra with those of the corresponding authentic standards prepared independently. Although guanine-adenine lesions are approximately 10 times less abundant in DEB-treated double-stranded DNA than the corresponding bis-N7G cross-links, N1A-N7G-BD and N6A-N7G-BD are more hydrolytically stable and, if formed in vivo, may accumulate in target tissues. HPLC-ESI-MS/MS analysis of guanine-adenine DEB cross-links induced in synthetic DNA duplexes 5'-(GGT)5, 5'-(GT)7G, and 5'-(GAA)5 (+-strand) demonstrate that G-A cross-linking by DEB produces primarily 1,3-interstrand N1A-N7G lesions. The formation of bifunctional guanine-adenine adducts is likely to contribute to AT base pair substitutions and deletion mutations following DEB exposure.     

The cytotoxicity, DNA crosslinking ability and DNA sequence selectivity of the aniline mustards melphalan, chlorambucil and 4-[bis(2-chloroethyl)amino] benzoic acid.

Three aniline derivatives melphalan (L-PAM), chlorambucil (CHL) and 4-[bis(2-chloroethyl)amino] benzoic acid (BAM) have been compared on the basis of their in vitro cytotoxicities, DNA interstrand crosslinking ability and DNA sequence selectivity. Cytotoxicity was assessed in the human colonic adenocarcinoma LS174T and leukaemic K562 cell lines using the sulpho-rhodamine B and tetrazolium dye reduction assays. The order of cytotoxicities was L-PAM greater than CHL greater than BAM in both cell lines with K562 being less sensitive than LS174T. This was different from the order CHL greater than L-PAM greater than BAM which would be predicted from simple chemical reactivity or rate of hydrolysis, parameters which have been used previously as indicators of biological potency for aromatic nitrogen mustards. DNA interstrand crosslinking in cells as determined by alkaline elution showed a correlation with IC50 values. The ranking order of activity was further predicted by the ability of the agents to produce interstrand crosslinks in isolated DNA. The extent of guanine N-7 alkylation, assessed using a modified DNA sequencing technique, mirrored cytotoxicity and crosslinking ability, but at equivalent levels of alkylation there was no significant difference in DNA sequence selectivity. These data demonstrates that simple chemical reactivity or hydrolysis rate is not a good indicator of DNA reactivity or cytotoxicity for a number of aniline mustards, whereas DNA interstrand crosslinking ability either measured directly in cells or in isolated DNA, gives a good indication of biological activity.     

Design and synthesis of a nitrogen mustard derivative stabilized by apo-neocarzinostatin

Neocarzinostatin (NCS) is an antitumor antibiotic comprising a 1:1 protein-chromophore complex and exhibits cytotoxic action through DNA cleavage via H-abstraction. Cytotoxic activity resides with the chromophore 1 alone, while the protein (apoNCS) protects and transports labile 1. The naphthoate portion (2) of NCS chromophore (1) is important for binding to apoNCS and DNA intercalation. In this paper we describe our attempts to use apoNCS to improve the hydrolytic stability of novel bifunctional DNA alkylating agents. The nitrogen mustards, melphalan and chlorambucil, were both conjugated to 2, and the biological activities of these conjugates were assessed. Chlorambucil did not benefit from conjugation. The melphalan conjugate (6) formed covalent DNA adducts at guanine bases and exhibited greater in vitro cytotoxic activity than unmodified melphalan. Fluorescence and NMR spectroscopy showed that 6 binds to apoNCS. Binding to apoNCS-protected 6 reduced the extent of hydrolysis of the conjugate. This novel approach demonstrates for the first time that an enediyne apo-protein can be used to improve the stability of substances that are of potential interest in cancer chemotherapy.     

Reduced DNA double strand breaks in chlorambucil resistant cells are related to high DNA-PKcs activity and low oxidative stress

Modulation of DNA repair represents a strategy to overcome acquired drug resistance of cells to genotoxic chemotherapeutic agents, including nitrogen mustards (NM). These agents induce DNA inter-strand cross-links, which in turn produce double strand breaks (dsbs). These breaks are primarily repaired via the nonhomologous end-joining (NHEJ) pathway. A DNA-dependent protein kinase (DNA-PK) complex plays an important role in NHEJ, and its increased level/activity is associated with acquired drug resistance of human tumors. We show in this report that the DNA-PK complex has comparable levels and kinase activity of DNA-PK catalytic subunit (DNA-PKcs) in a nearly isogenic pair of drug-sensitive (A2780) and resistant (A2780/100) cells; however, treatment with chlorambucil (Cbl), a NM-type of drug, induced differential effects in these cells. The kinase activity of DNA-PKcs was increased up to 2h after Cbl treatment in both cell types; however, it subsequently decreased only in sensitive cells, which is consistent with increased levels of DNA dsbs. The decreased kinase activity of DNA-PKcs was not due to a change in its amount or the levels of Ku70 and Ku86, their subcellular distribution, cell cycle progression or caspase-mediated degradation of DNA-PK. In addition to DNA cross-links, Cbl treatment of cells causes a 2.2-fold increase in the level of reactive oxygen species (ROS) in both cell types. However, the ROS in A2780/100 cells were reduced to the basal level after 3-4h, while sensitive cells continued to produce ROS and undergo apoptosis. Pre-treatment of A2780 cells with the glutathione (GSH) precursor, N-acetyl-L-cysteine prevented Cbl-induced increase in ROS, augmented the kinase activity of DNA-PKcs, decreased the levels of DNA dsbs and increased cell survival. Depletion in GSH from A2780/100 cells by L-buthionine sulfoximine (BSO) resulted in sustained production of ROS, lowered DNA-PKcs kinase activity, enhanced levels of DNA dsbs, and increased cell killing by Cbl. We propose that oxidative stress decreases repair of DNA dsbs via lowering kinase activity of DNA-PKcs and that induction of ROS could be the basis for adjuvant therapies for sensitizing tumor cells to nitrogen mustards and other DNA cross-linking drugs.     

The role of DNA repair in nitrogen mustard drug resistance

The nitrogen mustards are an important class of DNA cross-linking agents, which are utilized in the treatment of many types of cancer. Unfortunately, resistance often develops in the treatment of patients and the tumor either never responds to or becomes refractory to these agents. Resistance to the nitrogen mustards in murine and human tumor cells has been reported to be secondary to alterations in (i) the transport of these agents, (ii) their reactivity, (iii) apoptosis and (iv) altered DNA repair activity. In the present review, we will discuss the role of DNA repair in nitrogen mustard resistance in cancer. The nitrogen mustards' lethality is based on the induction of DNA interstrand cross-links (ICLs). Two DNA repair pathways are known to be involved in removal of ICLs: non-homologous DNA end-joining (NHEJ) and Rad51-related homologous recombinational repair (HRR). The reports discussed here lead us to hypothesize that low NHEJ activity defines a hypersensitive state, while high NHEJ activity, along with increased HRR activity, contributes to the resistant state in chronic lymphocytic leukemia. Studies on human epithelial tumor cell lines suggest that HRR rather than NHEJ plays a role in nitrogen mustard sensitivity.     

Kinetic studies of the binding of acridinecarboxamide topoisomerase poisons to DNA: implications for mode of binding of ligands with uncharged chromophores

We have used stopped-flow spectrophotometry and the sodium dodecyl sulfate sequestration technique to study the kinetics of dissociation of DNA complexes of the mixed topoisomerase I/II poison N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (termed DACA) and a range of related linear tricyclic carboxamides with neutral chromophores. Complexes of DACA and related acridine and phenazinecarboxamides bearing an N,N-dimethylaminoethyl side chain dissociate from calf thymus DNA by a kinetic pathway involving four discernible steps in a manner similar to complexes of N-[(2-dimethylamino)ethyl]-9-aminoacridine-4-carboxamide (termed 9-amino-DACA). We infer from these findings that the side chains of DACA, its phenazine homologue, and 9-amino-DACA make comparable interactions with the DNA base pairs. In the case of 9-amino-DACA, a selective topoisomerase II poison, these are known, by crystallographic analysis, to involve hydrogen-bonding interactions between the protonated dimethylammonium group of the side chain and the O6/N7 atoms of guanine and to include a bridging water molecule hydrogen bonded to the carboxamide group and a phosphate oxygen. By contrast, we find that other linear tricyclic carboxamides with neutral chromophores which lack a peri nitrogen atom and are biologically inactive dissociate from DNA by a different mechanism in which it appears their side chains fail to interact with guanine. We conclude that the ability of the carboxamide group to lie preferentially in the plane of the chromophore, so facilitating the dimethylammonium-guanine hydrogen bond and ensuring maintenance of the water-bridged carboxamide-phosphate interaction, is a critical requirement for antitumor activity among ligands of the linear tricyclic carboxamide class. However, unlike the situation for 9-amino-DACA, for ligands with uncharged chromophores containing peri nitrogen atoms such as DACA, this outcome is possible with the 4-carboxamide group rotated cis or trans with respect to the ring nitrogen. This difference may have relevance to the ability of DACA to be a dual poison of both topoisomerases I and II.     

Role of DNA minor groove alkylation and DNA cross-linking in the cytotoxicity of polybenzamide mustards

Interstrand DNA cross-links have been considered essential to the activity of current clinical DNA-alkylating antitumour drugs, which generally alkylate in the major groove. However, the relationship between cross-linking adducts located in the minor groove of DNA with cytotoxicity and antitumour activity has not been extensively investigated. Previous studies have shown that cross-linking ability is not correlated with cytotoxicity in a novel series of polybenzamide-linked nitrogen mustard compounds which alkylate DNA at adenines in the minor groove. In the present study the nature of these cross-linking adducts was explored for a related pair of compounds which are both highly effective cross-linkers but which differ in antitumour potential. Both of these drugs effectively interact with adenines in the minor groove, although their sequence specificity differs. However, the cross-linking event was not inhibited by pre-treatment with Hoechst 33258, although this pre-treatment effectively prevented adenine alkylation. The primary cross-links detected may thus represent guanine N7 alkylations in the major groove. Whether minor groove cross-linking adducts can be formed is uncertain, since the effect of background guanine N7 alkylation may complicate analysis. The cytotoxicity of the polybenzamides may therefore be related to other factors such as their interaction with cellular repair systems.     

Synthesis and in vitro evaluation of quaternary ammonium derivatives of chlorambucil and melphalan, anticancer drugs designed for the chemotherapy of chondrosarcoma

To enhance affinity for malignant cartilaginous tumors (chondrosarcomas), quaternary ammonium (QA) conjugates of chlorambucil and melphalan were prepared by linking the QA moiety to nitrogen mustards via an amide bond. They exhibited closely similar and sometimes more favorable values than their parent compounds. In the cell lines tested, the two QA conjugates displayed appreciable cytotoxicity, the QA conjugate of chlorambucil even showing an enhanced efficiency against chondrosarcoma compared with chlorambucil.     

Structural elucidation of a novel DNA-DNA cross-link of 1,2,3,4-diepoxybutane

DNA-DNA cross-linking by 1,2,3,4-diepoxybutane (DEB) is considered the molecular basis for its potent cytotoxic and genotoxic effects. DEB reactions with DNA initially lead to N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-guanine monoadducts, which can then alkylate neighboring DNA bases to form bifunctional lesions. We recently reported the structures of four regioisomeric guanine-adenine adducts of DEB involving the N7 position of guanine and the N1, N3, N6, and N7 positions of adenine (Park, S., et al. (2004) Chemical Research in Toxicology 17, 1638-1651). In the present work, a novel bifunctional DNA lesion of DEB was identified as 1-(hypoxanth-1-yl)-4-(guan-7-yl)-2,3-butanediol (N1HX-N7G-BD). An authentic standard of N1HX-N7G-BD was prepared and structurally characterized by proton NMR, UV, and mass spectrometry. HPLC-ESI-MS/MS analyses of acid hydrolysates of DEB-treated calf thymus DNA revealed a peak that had the same retention time, MS/MS fragmentation, and UV spectrum as the authentic standard of N1HX-N7G-BD. We propose that N1HX-N7G-BD is formed by the hydrolytic deamination of previously reported 1-(aden-1-yl)-4-(guan-7-yl)-2,3-butanediol. Although N1HX-N7G-BD adducts are less abundant in DEB-treated DNA than the corresponding guanine-guanine cross-links, they may play a role in the induction of both AT and GC base pair mutations.     

Mass spectrometry based approach to study the kinetics of O6-alkylguanine DNA alkyltransferase-mediated repair of O6-pyridyloxobutyl-2'-deoxyguanosine adducts in DNA

O(6)-POB-dG (O(6)-[4-oxo-4-(3-pyridyl)but-1-yl]deoxyguanosine) are promutagenic nucleobase adducts that arise from DNA alkylation by metabolically activated tobacco-specific nitrosamines such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonicotine (NNN). If not repaired, O(6)-POB-dG adducts cause mispairing during DNA replication, leading to G → A and G → T mutations. A specialized DNA repair protein, O(6)-alkylguanine-DNA-alkyltransferase (AGT), transfers the POB group from O(6)-POB-dG in DNA to a cysteine residue within the protein (Cys145), thus restoring normal guanine and preventing mutagenesis. The rates of AGT-mediated repair of O(6)-POB-dG may be affected by local DNA sequence context, potentially leading to adduct accumulation and increased mutagenesis at specific sites within the genome. In the present work, isotope dilution high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI(+)-MS/MS)-based methodology was developed to investigate the influence of DNA sequence on the kinetics of AGT-mediated repair of O(6)-POB-dG adducts. In our approach, synthetic DNA duplexes containing O(6)-POB-dG at a specified site are incubated with recombinant human AGT protein for defined periods of time. Following spiking with D(4)-O(6)-POB-dG internal standard and mild acid hydrolysis to release O(6)-POB-guanine (O(6)-POB-G) and D(4)-O(6)-POB-guanine (D(4)-O(6)-POB-G), samples are purified by solid phase extraction (SPE), and O(6)-POB-G adducts remaining in DNA are quantified by capillary HPLC-ESI(+)-MS/MS. The new method was validated by analyzing mixtures containing known amounts of O(6)-POB-G-containig DNA and the corresponding unmodified DNA duplexes and by examining the kinetics of alkyl transfer in the presence of increasing amounts of AGT protein. The disappearance of O(6)-POB-dG from DNA was accompanied by pyridyloxobutylation of AGT Cys-145 as determined by HPLC-ESI(+)-MS/MS of tryptic peptides. The applicability of the new approach was shown by determining the second order kinetics of AGT-mediated repair of O(6)-POB-dG adducts placed within a DNA duplex representing modified rat H-ras sequence (5'-AATAGTATCT[O(6)-POB-G]GAGCC-3') opposite either C or T. Faster rates of alkyl transfer were observed when O(6)-POB-dG was paired with T rather than with C (k = 1.74 × 10(6) M(-1) s(-1) vs 1.17 × 10(6) M(-1) s(-1)).     

Hemisynthesis of antineoplastic conjugates with nitrogen-mustard proteins

The synthesis of new conjugates with inhibitory action on tumour growth is investigated by linking amino functions of proteins compounds (lysozyme and alpha s-casein) through an amide linkage at the carboxylic function of nitrogen mustards (chlorambucil and melphalan). The polychlorambucil amides of lysozyme and alpha s-casein derivatives prepared showed experimental antitumour activity when these conjugates were screened against the experimental P388 leukemia. In the case of the conjugates lysozyme-melphalan, an antitumour activity is observed when the amino function of the drug is combined with the carboxylic functions of the protein contrary to the situation of the free amino function of the drug described into the literature.     

Hypoxia-selective antitumor agents. 16. Nitroarylmethyl quaternary salts as bioreductive prodrugs of the alkylating agent mechlorethamine

Nitrobenzyl quaternary salts of nitrogen mustards have been previously reported as hypoxia-selective cytotoxins. In this paper we describe the synthesis and evaluation of a series of heterocyclic analogues, including pyrrole, imidazole, thiophene, and pyrazole examples, chosen to cover a range of one-electron reduction potentials (from -277 to -511 mV) and substitution patterns. All quaternary salt compounds were less toxic in vitro than mechlorethamine, and all were more toxic under hypoxic than aerobic conditions, although the differentials were highly variable within the series. The most promising analogue, imidazole 2, demonstrated DNA cross-linking selectively in hypoxic RIF-1 cells, and was active in vivo in combination with radiation or cisplatin. However, 2 also produced unpredictable toxicity in vivo, suggestive of nonspecific nitrogen mustard release, and this has restricted further development of these compounds as hypoxia-selective cytotoxins.     

Flanking sequences modulate diepoxide and mustard cross-linking efficiencies at the 5'-GNC site

Diepoxybutane, diepoxyoctane, and mechlorethamine are cytotoxic agents that induce interstrand cross-links between the N7 positions of deoxyguanosine residues on opposite strands of the DNA duplex preferentially at 5'-GNC sequences. We have systematically varied the identity of either the base 5' to the cross-linked deoxyguanosine residues or the intervening base pair to determine flanking sequence effects on cross-linking efficiency. We used synthetic DNA oligomers containing four 5'-N(1)GN(2)C sites that varied either N(1) or N(2). Interstrand cross-links were purified through denaturing polyacrylamide gel electrophoresis and then subjected to piperidine cleavage. The amount of cleavage at each deoxyguanosine residue, representative of cross-linking efficiency at that site, was determined by sequencing gel analysis. Our data suggest that cross-linking efficiency varies with the identity of N(1) similarly (purines > pyrimidines) for diepoxybutane, diepoxyoctane, and mechlorethamine but that the effects of N(2) differ for the three compounds.     

The effects of nitrogen mustards on the proliferative and Ig synthetic response of human peripheral blood lymphocytes

Maximal inhibition of pokeweed mitogen-stimulated Ig production and [3H]thymidine incorporation was shown to occur when unfractionated human peripheral blood mononuclear cells were cultured with concentrations of the nitrogen mustards melphalan, mechlorethamine or chlorambucil in the 20-100-microM range, whereas concentrations of microsome-activated cyclophosphamide (A-Cy) in the 2-mM range were required for equivalent inhibition. Around 400 microM A-Cy, IgM secretion was not inhibited, but secretion of IgA and IgG was. The [3H]thymidine incorporation of enriched populations of both large and small B and T cells all showed about 20-50-fold greater sensitivity to melphalan than to A-Cy, despite a difference of only 6-fold in alkylating activity between these drugs. Large (250 micron 3) B and T cells were only marginally more sensitive to melphalan and A-Cy than small (210 micron 3) T and B cells. Kinetic studies showed that IgG and IgA secreted by day 7 could be maximally inhibited by melphalan added as late as day 3, and IgM synthesis as late as day 2. In contrast, inhibition of Ig production by A-Cy steadily declined after the first day, especially IgM, which was no longer inhibitable by A-Cy on day 3. Inhibition of cumulative Ig production did not occur when A-Cy or melphalan was added on day 5 or later. Cell recombination experiments performed with drug pulsed and untreated monocytes plus B cells and irradiated T cells showed that inhibition of [3H]thymidine or Ig production was most striking when monocytes + B cells (rather than T cells) were exposed to melphalan in the first 16 h. When A-Cy was used in the first 16 h, inhibition of Ig production was partial and inconsistent, and inhibition of monocytes + B cell or T cell [3H]thymidine incorporation was not evident. We conclude that the nitrogen mustards melphalan and A-Cy can inhibit pokeweed mitogen-stimulated DNA synthesis by human T or B cells and Ig production in vitro, but that their mechanisms of action differ.     

N-(2-hydroxyethyl)-N-[2-(7-guaninyl)ethyl]amine, the putative major DNA adduct of cyclophosphamide in vitro and in vivo in the rat

The anti-cancer agent, cyclophosphamide, metabolises to the cytotoxic alkylating agent phosphoramide mustard, which can be dephosphoramidated to give nornitrogen mustard. A rat liver mitochondrial supernatant system was used to study the binding of [chloroethyl 3H]cyclophosphamide to DNA. The reacted DNA was acid-hydrolysed and one major adduct was identified using Sephadex G-10 chromatography, followed by HPLC, using reversed-phase or ion-exchange systems. Further studies, using [14C]guanine as reaction substrate for [chloroethyl 3H]cyclophosphamide, phosphoramide mustard or nornitrogen mustard, demonstrated the main adduct from each reaction had identical chromatographic properties in these systems. The radiolabelled ratio in the [3H]cyclophosphamide-[14C]guanine reaction demonstrated a monoadducted product. From this evidence and from 1H NMR data, the common adduct was putatively identified as a hydroxylated nornitrogen mustard adduct (N-(2-hydroxyethyl)-N-[2-(7-guaninyl)ethyl]amine). In in vivo studies, rats were injected intraperitoneally with 2.775 MBq [3H]cyclophosphamide. Total organ [3H] content and DNA binding levels were ascertained. Maximal levels of [3H] binding to DNA were seen between 1-4 hr with the highest binding levels observed in the bladder. The in vivo adduct was shown, using various HPLC systems, to co-chromatograph with the in vitro adduct and thus the main in vivo adduct was putatively identified as N-(2-hydroxyethyl)-N-[2-(7-guaninyl)ethyl]amine.     

DNA-directed alkylating agents. 3. Structure-activity relationships for acridine-linked aniline mustards: consequences of varying the length of the linker chain

Four series of acridine-linked aniline mustards have been prepared and evaluated for in vitro cytotoxicity, in vivo antitumor activity, and DNA cross-linking ability. The anilines were attached to the DNA-intercalating acridine chromophores by link groups (-O-, -CH2-, -S-, and -SO2-) of widely varying electronic properties, providing four series of widely differing mustard reactivity where the alkyl chain linking the acridine and mustard moieties was varied from two to five carbons. Relationships were sought between chain length and biological properties. Within each series, increasing the chain length did not alter the reactivity of the alkylating moiety but did appear to position it differently on the DNA, since cross-linking ability (measured by agarose gel assay) altered with chain length, being maximal with the C4 analogue. The in vivo antitumor activities of the compounds depended to some extent on the reactivity of the mustard, with the least reactive SO2 compounds being inactive. However, DNA-targeting did appear to allow the use of less reactive mustards, since the S-linked acridine mustards showed significant activity whereas the parent S-mustard did not. Within each active series, the most active compound was the C4 homologue, suggesting some relationship between activity and extent of DNA alkylation.     

Sequence specificity for DNA interstrand cross-linking induced by anticancer drug chlorambucil

Chlorambucil is known to alkylate primarily N7 of guanine and N3 of adenine to induce DNA monofunctional adducts and interstrand cross-links (ISC). We have investigated the sequence specificity for DNA ISC induced by chlorambucil using duplex oligomers containing a difined cross-linkable sequences 5′-A^*TT, 5′-G^*TT, or 5′-G^*CC in which asterisk indicates the potential cross-linking site and underlined base indicates the potential cross-linking site on the opposite strand. An analysis of 20% denaturing polyacrylamide gel electrophoresis showed that chlorambucil was able to induce DNA ISC in the duplex oligomers containing a sequence 5′-GCC. The formation of DNA ISC was not observed in the duplex oligomers containing sequences 5′-ATT or 5′-GTT. These results indicate that chlorambucil induces guanineguanine DNA ISC but not guanine-adenine or adenine-adenine DNA ISC. In addition, we have tested the ability of chlorambucil to induce DNA ISC within 5′-GNNC or 5′-GC sequences using duplex oligomers containing the sequence 5′-G⁴G³G²C. The result of DNA strand cleavage assay showed that DNA ISC was formed at the 5′-GGC sequence (an 1,3 cross-link, G₁-G₃) but not at 5′-GGGC (an 1,4 cross-link, G₁-G₄) or 5′-GC sequence (an 1,2 cross-link, G₁-G₂).