Identification of specific DNA lesions induced by three classes of chloroethylating agents: chloroethylnitrosoureas, chloroethylmethanesulfonates and chloroethylimidazotetrazines

Site-specific stabilization of DNA by a tethered major groove amine, 7-aminomethyl-7-deaza-2'-deoxyguanosine

A cationic 7-aminomethyl-7-deaza-2′-deoxyguanosine (7amG) was incorporated site-specifically into the self-complementary duplex d(G¹A²G³A⁴X⁵C⁶G⁷C⁸T⁹C¹⁰T¹¹C¹²)₂ (X = 7amG). This construct placed two positively charged amines adjacent to the major groove edges of two symmetry-related guanines, providing a model for probing how cation binding in the major groove modulates the structure and stability of DNA. Molecular dynamics calculations restrained by nuclear magnetic resonance (NMR) data revealed that the tethered cationic amines were in plane with the modified base pairs. The tethered amines did not form salt bridges to the phosphodiester backbone. There was also no indication of the amines being capable of hydrogen bonding to flanking DNA bases. NMR spectroscopy as a function of temperature revealed that the X⁵ imino resonance remained sharp at 55 °C. Additionally, two 5′-neighboring base pairs, A⁴:T⁹ and G³:C¹⁰, were stabilized with respect to the exchange of their imino protons with solvent. The equilibrium constant for base pair opening at the A⁴:T⁹ base pair determined by magnetization transfer from water in the absence and presence of added ammonia base catalyst decreased for the modified duplex compared to that of the A⁴:T⁹ base pair in the unmodified duplex, which confirmed that the overall fraction of the A⁴:T⁹ base pair in the open state of the modified duplex decreased. This was also observed for the G³:C¹⁰ base pair, where αK_op for the G³:C¹⁰ base pair in the modified duplex was 3.0 × 10⁶ versus 4.1 × 10⁶ for the same base pair in the unmodified duplex. In contrast, equilibrium constants for base pair opening at the X⁵:C⁸ and C⁶:G⁷ base pairs did not change at 15 °C. These results argue against the notion that electrostatic interactions with DNA are entirely entropic and suggest that major groove cations can stabilize DNA via enthalpic contributions to the free energy of duplex formation.

The labdane diterpene sclareol (labd-14-ene-8, 13-diol) induces apoptosis in human tumor cell lines and suppression of tumor growth in vivo via a p53-independent mechanism of action

The labdane diterpene sclareol has demonstrated significant cytotoxicity against human tumor cell lines and human colon cancer xenografts. Therefore, there is need to elucidate the mode of action of this compound as very little information is known for the anticancer activity of sclareol and other labdane diterpenes, in general. COMPARE analysis of GI(50) values for a number of human cancer cell lines was initially implicated in an effort to assign a putative mechanism of action to the compound. Sclareol-induced cell cycle arrest and apoptosis were assessed by flow cytometry and Western blot analyses. Finally, the anticancer ability of sclareol in vivo was assessed by using human colon cancer xenograft/mouse models. Sclareol arrested in vitro the growth of p53-deficient (HCT116(p53-/-)) human colon cancer cells and subsequently induced apoptosis by activating both caspases-8 and -9. Intraperitoneal administration of liposome-encapsulated sclareol at the maximum tolerated dose induced a marked growth suppression of HCT116(p53-/-) tumors established as xenografts in immunodeficient NOD/SCID mice. In conclusion, we demonstrate herein that sclareol kills human tumor cells by inducing arrest at the G(1)-phase of the cell cycle followed by apoptosis that involves activation of caspases-8, -9 and -3 via a p53-independent mechanism. These findings suggest that liposome-encapsulated sclareol possesses chemotherapeutic potential for the treatment of colorectal and other types of human cancer regardless of the p53-status.

A study of 7-deaza-2'-deoxyguanosine 2'-deoxycytidine base pairing in DNA

The incorporation of 7-deazaguanine modifications into DNA is frequently used to probe protein recognition of H-bonding information in the major groove of DNA. While it is generally assumed that 7-deazaguanine forms a normal Watson–Crick base pair with cytosine, detailed thermodynamic and structural analyses of this modification have not been reported. The replacement of the 7-N atom on guanine with a C–H, alters the electronic properties of the heterocycle and eliminates a major groove cation-binding site that could affect the organization of salts and water in the major groove. We report herein the characterization of synthetic DNA oligomers containing 7-deazaguanine using a variety of complementary approaches: UV thermal melting, differential scanning calorimetry (DSC), circular dichroism (CD), chemical probing and NMR. The results indicate that the incorporation of a 7-deazaguanine modification has a significant effect on the dynamic structure of the DNA at the flanking residue. This appears to be mediated by changes in hydration and cation organization.

A review of the role of the sequence-dependent electrostatic landscape in DNA alkylation patterns

Alkylating agents, including environmental and endogenous carcinogens and DNA targeting antineoplastic agents, that adduct DNA via intermediates with significant cationic charge show a sequence selectively in their covalent bonding to nucleobases. The resulting patterns of alkylation eventually contribute to the agent-dependent distributions and types of mutations. The origin of the regioselective modification of DNA by electrophiles has been attributed to steric and/or electronic factors, but attempts to mechanistically model and predict alkylation patterns have had limited success. In this review, we present data consistent with the role of the intrinsic sequence-dependent electrostatic landscape (SDEL) in DNA that modulates the equilibrium binding of cations and the bonding of reactive charged alkylating agents to atoms that line the floor of the major groove of DNA.

Inactivated MGMT by O6-benzylguanine is associated with prolonged G2/M arrest in cancer cells treated with BCNU

HCT116 and HCT15 cells that highly express O(6)-methylguanine-DNA-methyltransferase (MGMT) displayed a transient cell cycle G2/M arrest in response to exposure to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) alone; however, 70-80% of cells were arrested in G2/M after treatment with O(6)-benzylguanine (BG) and BCNU. Cells accumulated in G2/M showed elevated levels of an inactive form of cyclin B1/p-Cdc2 (Tyr15) complex that was not associated with activation of Chk1/p-Cdc25C and was independent of p53/p21 status. The most prominent feature of cell death was the appearance of enlarged and multinucleated cells that was related to the inhibition of mitotic entry. In contrast, BG-resistant cell lines, HCT116 BBR and HCT15 BBR cells that contain mutations K165E and K165N of MGMT, respectively, displayed a normal cell cycle progression with a slight and transient increase in G2/M arrest at 24 h after treatments with either BCNU alone or BG combined with BCNU. The differences in the ability to progress toward G2/M after treatment with BG and BCNU between cells expressing wild-type MGMT and mutated MGMT were confirmed in CHO cells transfected with human wild type and K165E mutant MGMT cDNA, respectively. Thus, our findings suggest that BG-inactivated MGMT may be linked to cell signaling events, forcing cells into a permanent G2/M arrest in response to the DNA damages induced by BCNU.

Metabolism of the chloroethylnitrosoureas

1. The chemical degradation and metabolism of the 2-chloroethylnitrosoureas (CENUSs) have been critically reviewed with the objective of gaining a better understanding of factors that could aid in the design of new, more effective, anticancer drugs. 2. The CENUs are chemically unstable under normal physiological conditions and can rapidly degrade to give a variety of reactive intermediates capable of carbamoylating proteins and/or alkylating both proteins and DNA. 3. Carbamoylation is thought to make a minimal contribution to the cytotoxic effect of the CENUs, although it may be involved in some of the unwanted side-effects. It would seem desirable, therefore, to design new CENUs with low carbamoylating activity. 4. The main action of the CENUs is by alkylation of DNA via a chloroethyldiazo-hydroxide intermediate. Chloroethylation is important, as opposed to hydroxyethylation, since the former leads to inter-strand DNA cross-linking via an intramolecular rearrangement with the removal of chloride. It is this inter-strand cross-linking which prevents subsequent DNA repair and loss of cytotoxicity. 5. Metabolism usually, although not exclusively, leads to deactivation of the CENUs either by dechlorination or denitrosation of the molecule, generally with the former being the dominant route. These reactions occur very rapidly, and before chemical degradation can take place, and can be an important determinant in the final cytotoxicity. Moreover, both these pathways involve the cytochrome P-450 system and can be induced with phenobarbital.(ABSTRACT TRUNCATED AT 250 WORDS)

Base sequence specificity of three 2-chloroethylnitrosoureas

Chemical modifications of guanine are some of the most common results of interactions of DNA with many carcinogens and anti-cancer drugs, including nitrosoureas, nitrogen mustards, triazenes, polycyclic aromatics, and aflatoxins. The base sequence specificity for alkylation of guanines by three 2-chloroethylnitrosoureas has been determined. Guanines in the midst of a run of guanines are more susceptible than guanines in other base sequences. We have shown that certain 2-chloroethylnitrosoureas (BCNU, CCNU and methyl-CCNU) follow this same pattern. However, the quantitative degree of higher specificity for guanine with guanines as nearest neighbors depended on both the guanine position alkylated and the structure of the alkyl group attached. For example, when hydroxyethylation of runs of guanine occurred at N-7, a 6- to 11-fold increase of alkylation occurred compared to that found in the random base sequences of DNA, while hydroxyethylation at O-6 increased 1.2 to 3.5-fold and chloroethylation at N-7 was 2-to 4-fold higher than in DNA. Guanines with thymines on both the 3' and 5' sides were much less susceptible, most notably in N-7-hydroxyethylation and N-7-chloroethylation. Since guanine-rich regions are found in regulatory regions of the genome, knowledge concerning the effect of base sequence upon the production of each of the potential DNA lesions is vital to gaining an understanding of the roles of these lesions in the anti-tumor activity of a drug.

Inhibition of cellular esterases by the antitumour imidazotetrazines mitozolomide and temozolomide: demonstration by flow cytometry and conventional spectrofluorimetry

Using flow cytometry and conventional spectrofluorimetry we have previously shown that chloroethylnitrosoureas (CNUs) can exhibit marked inhibition of cellular enzymes catalysing hydrolysis of fluorescein diacetate (FDA). More potent inhibition was seen for the carbamoylating CNUs, whereas alkylating agents were largely inactive. We now report results obtained with the developmental imidazotetrazines mitozolomide and temozolomide in comparison with BCNU, the novel alkylating agents clomesome and cyclodisone, and the active mitozolomide metabonate MCTIC. Inhibition of EMT6 mouse mammary-tumour esterases was seen for mitozolomide and temozolomide, and activity against purified porcine carboxylesterase was demonstrated. Flow cytometric analysis showed that inhibition occurred across the entire EMT6 cell population, with no evidence of a subpopulation resistant to enzyme inhibition. Inhibitory potency for the imidazotetrazines was much weaker than for BCNU. With EMT6 cells, I50 values from flow cytometry were 9.7 x 10(-3) M and 1.5 x 10(-3) M for mitozolomide and temozolomide compared with 3.7 x 10(-4) M for BCNU. These were higher than the ID50 values for in vitro antitumour activity (MTT assay), 8.5 x 10(-6) M in the case of mitozolomide and 1.2 x 10(-5) M for BCNU, but similar to that of 5.6 x 10(-4) M for the less toxic temozolomide. MCTIC and cyclodisone showed very low activity, but significant inhibition was seen for clomesome. The results are consistent with the view that the imidazotetrazines do not exhibit major carbamoylating ability, although significant effects are seen at cytotoxic concentrations of temozolomide. In addition, the potential for the generation of carbamoylating species at the enzyme active site cannot be ruled out.

Inhibition of intracellular esterases by antitumour chloroethylnitrosoureas. Measurement by flow cytometry and correlation with molecular carbamoylation activity

Antitumour chloroethylnitrosoureas (Cnus) decompose in physiological conditions yielding alkylating species and organic isocyanates. While antitumour activity is mainly attributed to the alkylation of DNA, carbamoylation of intracellular proteins by isocyanates may also have pharmacological and toxicological relevance. We previously reported a novel dynamic flow cytoenzymological assay for esterase inhibition in intact murine cells by BCNU and related isocyanates, and proposed that this might form the basis of an assay for intracellular carbamoylation. We have now examined a wide range of Cnus, isocyanates, and alkylating agents for their ability to inhibit cellular esterases. BCNU, CCNU, their derived isocyanates, and the 4-OH metabolites of CCNU exhibited potent inhibitory activity (I50 values 5.5 x 10(-5)-7.3 x 10(-4) M). Chlorozotocin and GANU were relatively inactive (I50 much greater than 10(-2) M). ACNU, TCNU and the 2-OH metabolites of CCNU exhibited intermediate activity (I50 values 1.1 x 10(-3)-2.3 x 10(-2) M). Compounds not able to form isocyanates were essentially inactive. Poor membrane permeability was also implicated in the weak activity of chlorozotocin and GANU. There was overall a good correlation between esterase inhibition and chemical carbamoylating activity, but some particular differences were identified. We concluded that flow cytoenzymological assay appears to have the potential to provide useful measurement of intracellular protein carbamoylation by existing Cnus and novel derivatives, and also offers the advantage of cell subpopulation identification for in vivo evaluation of these agents.