Resistance of CHO cells expressing P-glycoprotein to cyclopropylpyrroloindole (CPI) alkylating agents

DNA minor groove binding ligands: a new class of anticancer agents

This paper gives an overview of the available pharmacological and clinical data of a new class of anticancer drugs which act by binding DNA in the minor groove, comprising cyclopropylpyrrolo-indole (CC-1065) derivatives and Distamycin derivatives. The emphasis of this review is placed on the distinctive mode of action of these drugs. Molecular pharmacology studies indicate that CC-1065 and its derivatives, and the benzoyl mustard derivative of Distamycin, Tallimustine, possess the most striking DNA sequence specificity of alkylation observed to date for an alkylating agent of relatively small molecular weight. The effects on the regulation of gene transcription, the perturbation of the cell cycle, and the mechanism involved in the repair of the DNA lesions induced by these drugs all strongly support the view that minor groove ligands act by a mechanism different from those previously described for other anticancer drugs. The CC-1065 derivatives, Adozelesin and its prodrug, Carzelesin, and Tallimustine were found to be very effective against several murine tumours and human xenografts, and were shown to be active against experimental tumours that were resistant to other antineoplastic agents, including conventional alkylating agents. The clinical studies performed so far do not confirm in humans the remarkable antitumour activity observed in mice. The major reason appears to be a very high susceptibility of human bone marrow to both CC-1065 derivatives and Tallimustine as compared to mouse bone marrow, which makes it impossible to administer these drugs at sufficiently high doses to exert antitumour effects. The search for new minor groove binders, which possess a different DNA sequence specificity of alkylation and which are less cytotoxic for human bone marrow cells, is still in progress. It will be several years before it is possible to draw firm conclusions on the clinical effectiveness of this class of drugs.

Cell-free and cellular activities of a DNA sequence selective hairpin polyamide-CBI conjugate

Alkylating agents are generally highly reactive with DNA but demonstrate limited DNA sequence selectivity. In contrast, synthetic pyrrole-imidazole polyamides recognize specific DNA sequences with high affinity but are unable to permanently damage DNA. An eight-ring hairpin polyamide conjugated to the alkylating moiety cyclopropylpyrroloindole, related to the natural product CC-1065, affords a conjugate 1-CBI (polyamide 1-CBI (1-(chloromethyl)-5-hydroxyl-1,2-dihydro-3H-benz[e]indole) conjugate), which binds to specific sequences in the minor groove of DNA and alkylates a single adenine flanking the polyamide binding site. In this study, we show that 1-CBI alkylates DNA in both plasmid and intracellular minichromosomal form and inhibits DNA replication under both cell-free and cellular conditions. In addition, it inhibits cell growth and arrests cells in the G2/M phase of the cell cycle.

Synthetic DNA minor groove-binding drugs

In this review, both cationic and neutral synthetic ligands that bind in the minor groove of DNA are discussed. Certain bis-distamycins and related lexitropsins show activities against human immunodeficiency virus (HIV)-1 and HIV-2 at low nanomolar concentrations. DAPI binds strongly to AT-containing polymers and is located in the minor groove of DNA. DAPI intercalates in DNA sequences that do not contain at least three consecutive AT bp. Berenil can also exhibit intercalative, as well as minor groove binding, properties depending on sequence. Furan-containing analogues of berenil play an important role in their activities against Pneumocystis carinii and Cryptosporidium parvuam infections in vivo. Pt(II)-berenil conjugates show a good activity profile against HL60 and U-937 human leukemic cells. Pt-pentamidine shows higher antiproliferative activity against small cell lung, non-small cell lung, and melanoma cancer cell lines compared with many other tumor cell lines. trans-Butenamidine shows good anti-P. carinii activity in rats. Pentamidine is used against P. carinii pneumonia in individuals infected with HIV who are at high risk from this infection. A comparison of the cytotoxic potencies of adozelesin, bizelesin, carzelesin, cisplatin, and doxorubicin indicates that adozelesin is a potent analog of CC-1065. Naturally occurring pyrrolo[2,1-c][l,4]benzodiazepines such as anthramycin have a 2- to 3-bp sequence specificity, but a synthetic PBD dimer spans 6 bp, actively recognizing a central 5'-GATC sequence. The crosslinking efficiency of PBD dimers is much greater than that of other major groove crosslinkers, such as cisplatin, melphalan, etc. Neothramycin is used clinically for the treatment of superficial carcinoma of the bladder.

Induction of AT-specific DNA-interstrand crosslinks by bizelesin in genomic and simian virus 40 DNA

Bizelesin is a bifunctional AT-specific DNA alkylating drug. Our study characterized the ability of bizelesin to induce interstrand crosslinks, a potential lethal lesion. In genomic DNA of BSC-1 cells, bizelesin formed from approx. 0.3 to 6.03+/-0.85 interstrand crosslinks per 106 base pairs, at 5-100 nM drug concentration, respectively, comparable to the number of total adducts previously determined in the same system (J.M. Woynarowski, M.M. McHugh, L.S. Gawron, T.A. Beerman, Biochemistry 34 (1995) 13042-13050). Bizelesin did not induce DNA-protein crosslinks or strand breaks. A model defined target, intracellular simian virus 40 (SV40) DNA, was employed to map at the nucleotide level sites of bizelesin adducts, including potential interstrand crosslinks. Preferential adduct formation was observed at AT tracts which are abundant in the SV40 matrix associated region and the origin of replication. Many sites, including each occurrence of 5'-T(A/T)4A-3', co-mapped on both DNA strands suggesting interstrand crosslinks, although monoadducts were also formed. Bizelesin adducts in naked SV40 DNA were found at similar sites. The localization of bizelesin-induced crosslinks in AT-rich tracts of replication-related regions is consistent with the potent anti-replicative properties of bizelesin. Given the apparent lack of other types of lesions in genomic DNA, interstrand crosslinks localized in AT-rich tracts, and to some extent perhaps also monoadducts, are likely to be lethal effects of bizelesin.

Sequence selectivity of DNA alkylation by adozelesin and carzelesin

Adozelesin and carzelesin are synthetic analogues of the extremely potent antitumor antibiotic CC-1065, which alkylates N3 of adenine in a consensus sequence 5'-(A/T)(A/T)A* (A* is the site of alkylation). We have investigated the DNA sequence selectivity of adozelesin and carzelesin by thermally induced DNA strand cleavage assay using radiolabeled restriction DNA fragments. An analysis of alkylation patterns shows that the consensus sequences for carzelesin and adozelesin have been found to be 5'-(A/T)(A/T)A* and 5'-(A/T)(G/C)(A/T)A*. A new consensus sequence, 5'-(A/T)(A/T)CA*, has been observed to display an additional alkylation site for adozelesin but not for carzelesin. These results indicate that the pattern of sequence selectivity induced by carzelesin is similar but not identical to those induced by adozelesin.

Effects of bizelesin (U-77,779), a bifunctional alkylating minor groove binder, on replication of genomic and simian virus 40 DNA in BSC-1 cells

Bizelesin, an AT-specific DNA-alkylating antitumor drug, is a potent inhibitor of genomic DNA replication in BSC-1 cells. Fifty percent inhibition of DNA synthesis was observed at 10 nM bizelesin compared to 160 nM needed for 50% inhibition of RNA synthesis while no inhibition of protein synthesis was observed up to 200 nM. Sedimentation analysis of nascent genomic DNA showed that bizelesin inhibited new replicon initiation and had significantly less effect on replicon maturation. Bizelesin also suppressed the intracellular synthesis of simian virus 40 (SV40) DNA in virus-infected BSC-1 cells. The analysis of nascent SV40 intermediates synthesized after bizelesin treatment confirmed an initiation-specific inhibition. The inhibitory effects on cellular DNA replication occurred at bizelesin levels resulting in infrequent adducts (one adduct per several replicons). Only one bizelesin adduct per several SV40 molecules was needed for a potent inhibition of intracellular SV40 replication. In contrast, only partial inhibition of SV40 replication in vitro was observed with bizelesin-treated naked SV40 DNA as a template. Overall, the results indicate that infrequent bizelesin lesions impede the cellular replication apparatus at the level of the initiation of new replicons.

Inhibition of initiation of simian virus 40 DNA replication in infected BSC-1 cells by the DNA alkylating drug adozelesin

Adozelesin is a member of a family of extraordinarily cytotoxic DNA damaging agents that bind to the DNA minor groove in a sequence-specific manner and form covalent adducts with adenines. Previous studies employing purified enzymes and adozelesin-modified template DNAs suggested that adozelesin-DNA adducts inhibit DNA replication at the level of nascent DNA chain elongation. In this study, neutral/neutral two-dimensional agarose gel electrophoresis was employed to analyze simian virus 40 (SV40) DNA replication intermediates recovered from adozelesin-treated SV40 virus-infected cells. SV40 replication intermediates rapidly disappeared from infected cells when they were treated with adozelesin, but not when the cells were also treated with aphidicolin to block maturation of replicating SV40 DNA. We conclude that the disappearance of SV40 replication intermediates induced by adozelesin treatment was a consequence of maturation of these intermediates in the absence of new initiation events. Adozelesin inhibition of nascent chain elongation is first observed at concentrations above those needed to block initiation. Adozelesin treatment inhibits SV40 DNA replication at concentrations that produce adducts on just a small fraction of the intracellular population of SV40 DNA molecules.

Therapeutic efficacy of the cyclopropylpyrroloindole, carzelesin, against xenografts derived from adult and childhood solid tumors

The therapeutic efficacy of the sequence-selective, DNA minor-groove-binding alkylating agent carzelesin was evaluated against a series of human tumor xenografts growing at the s.c. site. The model consisted of seven colon adenocarcinomas, and six pediatric rhabdomyosarcomas. In addition, carzelesin was evaluated against xenografts selected in situ for resistance to vincristine, melphalan, and topotecan. Carzelesin was given as a single i.v. injection, and tumor volumes were determined at 7-day intervals. At the highest dose [0.5 mg/kg, the dose producing 10% lethality (LD10)]), carzelesin significantly inhibited growth in four of six colon tumor lines, causing a high proportion of partial regressions in one of seven lines and complete regressions of VRC5 colon tumors. At 0.25 mg/kg, significant growth inhibition was determined in only two of seven colon tumor lines with infrequent volume regressions. Carzelesin given at the highest nonlethal dose level significantly inhibited the growth of each of six rhabdomyosarcomas, causing a high frequency of partial or complete regressions in four of six tumor lines. There was no apparent cross-resistance to carzelesin in two rhabdomyosarcomas selected for vincristine resistance (Rh12/VCR, Rh18/VCR) or in Rh28/LPAM xenografts selected for primary resistance to the bifunctional alkylating agent melphalan. Interestingly, carzelesin maintained full activity against Rh18/TOPO tumors selected in situ for resistance to topotecan, whereas the colon tumor VRC5/TOPO, selected in a similar manner, was completely resistant to this agent.

V79 Chinese hamster lung cells resistant to the bis-alkylator bizelesin are multidrug-resistant

Bizelesin (U-77779) is a highly potent bis-alkylating antitumor agent that is effective against several tumor systems in vitro and in vivo. V79 cells that were 125- to 250-fold resistant to bizelesin developed after constant exposure to gradually increasing concentrations of the drug. Resistant cells exhibited a multidrug-resistant phenotype and genotype as indicated by cross-resistant to several structurally and functionally unrelated drugs, e.g., colchicine, actinomycin D, and Adriamycin, and overexpression of mdr mRNA. Very low levels of cross-resistance to the alkylating agents cisplatin and melphalan were seen. Multidrug-resistant mouse leukemia (P388/Adriamycin-resistant) and human (KB/vinblastine-resistant) cells were also resistant to bizelesin. Bizelesin resistance was unstable and decreased when cells were grown in the absence of the drug. Resistant and sensitive cell lines had similar levels of glutathione, and bizelesin cytotoxicity for resistant cells was not markedly affected by treatment with buthionine sulfoximine. Cross-resistance between bizelesin and several of its analogs is reported.