2-amino-O4-benzylpteridine derivatives: potent inactivators of O6-alkylguanine-DNA alkyltransferase

2-amino-O4-benzylpteridine (1), 2-amino-O4-benzyl-6,7-dimethylpteridine (2), 2-amino-O4-benzyl-6-hydroxymethylpteridine (4), 2-amino-O4-benzylpteridine-6-carboxylic acid (5), 2-amino-O4-benzyl-6-formylpteridine (6), and O4-benzylfolic acid (7) are shown to be as potent or more potent inactivators of the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) in vitro than O6-benzylguanine, the prototype alkyltransferase inactivator currently in clinical trials. Additionally, the negatively charged (at physiological pH) inactivators 2-amino-O4-benzylpteridine-6-carboxylic acid (5) and O4-benzylfolate (7) are far more water soluble than O6-benzylguanine. The activity of O4-benzylfolic acid (7) is particularly noteworthy because it is roughly 30 times more active than O6-benzylguanine against the wild-type alkyltransferase and is even capable of inactivating the P140K mutant alkyltransferase that is resistant to inactivation by O6-benzylguanine. All the pteridine derivatives except 2-amino-O4-benzylpteridine-6-carboxylic acid are effective in enhancing cell killing by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). However, the effectiveness of O4-benzylfolate as an adjuvant for cell killing by BCNU appears to be a function of a cell's alpha-folate receptor expression. Thus, O4-benzylfolate is least effective as an adjuvant in A549 cells (which express little if any receptor), is moderately effective in HT29 cells (which express low levels of the receptor), but is very effective in KB cells (which are known to express high levels of the alpha-folate receptor). Therefore, O4-benzylfolic acid shows promise as an agent for possible tumor-selective alkyltransferase inactivation, which suggests it may prove to be superior to O6-benzylguanine as a chemotherapy adjuvant.     

Inactivation of O(6)-alkylguanine-DNA alkyltransferase by folate esters of O(6)-benzyl-2'-deoxyguanosine and of O(6)-[4-(hydroxymethyl)benzyl]guanine

O6-Alkylguanine-DNA alkyltransferase (alkyltransferase) provides an important source of resistance to some cancer chemotherapeutic alkylating agents. Folate ester derivatives of O6-benzyl-2'-deoxyguanosine and of O6-[4-(hydroxymethyl)benzyl]guanine were synthesized and tested for their ability to inactivate human alkyltransferase. Inactivation of alkyltransferase by the gamma-folate ester of O6-[4-(hydroxymethyl)benzyl]guanine was similar to that of the parent base. The gamma-folate esters of O6-benzyl-2'-deoxyguanosine were more potent alkyltransferase inactivators than the parent nucleoside. The 3'-ester was considerably more potent than the 5'-ester and was more than an order of magnitude more active than O6-benzylguanine, which is currently in clinical trials to enhance therapy with alkylating agents. They were also able to sensitize human tumor cells to killing by 1,3-bis(2-chloroethyl)-1-nitrosourea, with O6-benzyl-3'-O-(gamma-folyl)-2'-deoxyguanosine being most active. These compounds provide a new class of highly water-soluble alkyltransferase inactivators and form the basis to construct more tumor-specific and potent compounds targeting this DNA repair protein.     

Effect of DNA on the Inactivation of O6-alkylguanine-DNA alkyltransferase by 9-Substituted O6-benzylguanine derivatives

Studies were carried out on the inactivation of pure human O⁶-alkylguanine-DNA alkyltransferase by 9-substituted O⁶-benzylguanine derivatives in the presence and absence of DNA. The addition of DNA increased the rate of inactivation of the alkyltransferase by O⁶-benzylguanine and its 9-methyl derivative but had little effect on the rate of inactivation by the 9-cyanomethyl derivative. In contrast, when O⁶-benzylguanine derivatives with larger 9-substituents such as ribose, 2′-deoxyribose, dihydrotestosterone, or 2-hydroxy-3-(isopropoxy)propyl were used, the addition of DNA was strongly inhibitory to the inactivation. In the case of O⁶-benzylguanine, O⁶-benzylguanosine, and O⁶-benzyl-2′-deoxyguanosine, these results were confirmed by directly measuring the rate of formation by the alkyltransferase of guanine, guanosine, or 2′-deoxyguanosine, respectively. The data indicated that the presence of DNA activated the alkyltransferase, rendering it more reactive with O⁶-benzylguanine or O⁶-benzyl-9-methylguanine, but that DNA interferes with the binding of inhibitors with larger 9-substituents, presumably by competing for the same binding site. Since these inactivators readily inactivate alkyltransferase in cells, the amount of cellular alkyltransferase bound to DNA must be small or readily exchangeable with the free form.     

Structural features of substituted purine derivatives compatible with depletion of human O6-alkylguanine-DNA alkyltransferase.

A series of O6- and S6-substituted purine derivatives were tested for their ability to deplete the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) in cell-free extracts from HT29 colon tumor cells and intact HT29 cells. The order of potency was O6-(p-Y-benzyl)-guanine (Y = H, F, Cl, and CH3) > O6-benzyl-2'-deoxyguanosine > O6-(p-Y-benzyl)guanosine (Y = H, Cl, and CH3) > or = a series of 9-substituted O6-benzylguanine derivatives > or = O6-allylguanine > O6-benzylhypoxanthine > O6-methylguanine. A series of 7-substituted O6-benzylguanine derivatives, 2-amino-6-(p-Y-benzylthio)purine (Y = H, CH3), 2-amino-6-[(p-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine, and 7-benzylguanine were inactive. It is concluded that for efficient AGT depletion, an allyl or benzyl group attached through exocyclic oxygen at position 6 of a 2-aminopurine derivative is required. Activity is preserved with a variety of substituent groups attached to position 9 while substitution at position 7 leads to a complete loss of activity.     

Pharmacokinetics of O6-benzylguanine (NSC637037) and its metabolite, 8-oxo-O6-benzylguanine

O6-Benzylguanine and its metabolite, 8-oxo-O6-benzylguanine, are equally potent inhibitors of the DNA repair enzyme, O6-alkylguanine-DNA alkyltransferase. Pharmacokinetic values are derived from cancer patients participating in a phase I trial (10 or 20 mg/m2 of O6-benzylguanine in a single bolus dose or 10 to 120 mg/m2 as a 60-min constant infusion). A two-compartment model fits the plasma concentration versus time profile of O6-benzylguanine. O6-Benzylguanine is eliminated rapidly from the plasma compartment in humans (t1/2 alpha and t1/2 beta are 2 +/- 2 min and 26 +/- 15 min [mean +/- SD, n = 7], respectively), and its plasma clearance (513 +/- 148 mL/min/m2) is not dose dependent. Metabolite kinetics are evaluated using both a novel approach describing the relationship between O6-benzylguanine and 8-oxo-O6-benzylguanine and classical metabolite kinetics methods. With increasing doses of O6-benzylguanine, the plasma clearance of 8-oxo-O6-benzylguanine, decreases, prolonging elimination of the metabolite. This effect is not altered by coadministration of BCNU. The urinary excretion of drug and metabolites is minimal.     

Resistance-modifying agents. 8. Inhibition of O(6)-alkylguanine-DNA alkyltransferase by O(6)-alkenyl-, O(6)-cycloalkenyl-, and O(6)-(2-oxoalkyl)guanines and potentiation of temozolomide cytotoxicity in vitro by O(6)-(1-cyclopentenylmethyl)guanine

A series of O(6)-allyl- and O(6)-(2-oxoalkyl)guanines were synthesized and evaluated, in comparison with the corresponding O(6)-alkylguanines, as potential inhibitors of the DNA-repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). Simple O(6)-alkyl- and O(6)-cycloalkylguanines were weak AGT inactivators compared with O(6)-allylguanine (IC(50) = 8.5 +/- 0.6 microM) with IC(50) values ranging from 100 to 1000 microM. The introduction of substituents at C-2 of the allyl group of O(6)-allylguanine reduced activity compared with the parent compound, while analogous compounds in the O(6)-(2-oxoalkyl)guanine series exhibited very poor activity (150-1000 microM). O(6)-Cycloalkenylguanines proved to be excellent AGT inactivators, with 1-cyclobutenylmethylguanine (IC(50) = 0.55 +/- 0.02 microM) and 1-cyclopentenylmethylguanine (IC(50) = 0.39 +/- 0.04 microM) exhibiting potency approaching that of the benchmark AGT inhibitor O(6)-benzylguanine (IC(50) = 0.18 +/- 0.02 microM). 1-Cyclopentenylmethylguanine also inactivated AGT in intact HT29 human colorectal carcinoma cells (IC(50) = 0.20 +/- 0.07 microM) and potentiated the cytotoxicity of the monomethylating antitumor agent Temozolomide by approximately 3- and 10-fold, respectively, in the HT29 and Colo205 tumor cell lines. The observation that four mutant AGT enzymes resistant to O(6)-benzylguanine also proved strongly cross-resistant to 1-cyclopentenylmethylguanine indicates that the O(6)-substituent of each compound makes similar binding interactions within the active site of AGT.     

4-nitrobenzyloxycarbonyl derivatives of O(6)-benzylguanine as hypoxia-activated prodrug inhibitors of O(6)-alkylguanine-DNA alkyltransferase (AGT), which produces resistance to agents targeting the O-6 position of DNA guanine

A series of 4-nitrobenzyloxycarbonyl prodrug derivatives of O(6)-benzylguanine (O(6)-BG), conceived as prodrugs of O(6)-BG, an inhibitor of the resistance protein O(6)-alkylguanine-DNA alkyltransferase (AGT), were synthesized and evaluated for their ability to undergo bioreductive activation by reductase enzymes under oxygen deficiency. Three agents of this class, 4-nitrobenzyl (6-(benzyloxy)-9H-purin-2-yl)carbamate (1) and its monomethyl (2) and gem-dimethyl analogues (3), were tested for activation by reductase enzyme systems under oxygen deficient conditions. Compound 3, the most water-soluble of these agents, gave the highest yield of O(6)-BG following reduction of the nitro group trigger. Compound 3 was also evaluated for its ability to sensitize 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (laromustine)-resistant DU145 human prostate carcinoma cells, which express high levels of AGT, to the cytotoxic effects of this agent under normoxic and oxygen deficient conditions. While 3 had little or no effect on laromustine cytotoxicity under aerobic conditions, significant enhancement occurred under oxygen deficiency, providing evidence for the preferential release of the AGT inhibitor O(6)-BG under hypoxia.     

Differential inactivation of polymorphic variants of human O6-alkylguanine-DNA alkyltransferase

The human DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (hAGT) is an important source of resistance to some therapeutic alkylating agents and attempts to circumvent this resistance by the use of hAGT inhibitors have reached clinical trials. Several human polymorphisms in the MGMT gene that encodes hAGT have been described including L84F and the linked double alteration I143V/K178R. We have investigated the inactivation of these variants and the much rarer variant W65C by O(6)-benzylguanine, which is currently in clinical trials, and a number of other second generation hAGT inhibitors that contain folate derivatives (O(4)-benzylfolic acid, the 3' and 5' folate esters of O(6)-benzyl-2'-deoxyguanosine and the folic acid gamma ester of O(6)-(p-hydroxymethyl)benzylguanine). The I143V/K178R variant was resistant to all of these compounds. The resistance was due solely to the I143V change. These results suggest that the frequency of the I143V/K178R variant among patients in the clinical trials with hAGT inhibitors and the correlation with response should be considered.     

Inhibition of DNA repair as a means of increasing the antitumor activity of DNA reactive agents

Chemotherapeutic alkylnitrosoureas (BCNU, CCNU, streptozotocin) and alkyltriazenes (DTIC, temozolomide) produce a cytotoxic lesion at the O(6)-position of guanine. The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase removes damage from the O(6)-position in a single-step mechanism without co-factors. There is extensive evidence that this protein is one of the most important factors contributing to alkylnitrosourea and alkyltriazene treatment failure. There is an inverse correlation between the level of this protein and the sensitivity of cells to the cytotoxic effects of O(6)-alkylating agents. Attempts have been made to modulate AGT activity using anti-sense technology, methylating agents, O(6)-alkylguanines, and O(6)-benzylguanine analogs. O(6)-Benzylguanine and its analogs are clearly the most potent direct inactivators of the AGT protein. The mechanism involves O(6)-benzylguanine acting as a low-molecular weight substrate with transfer of the benzyl group to the cysteine residue within the active site of the repair protein. Pretreatment of cells with non-toxic doses of O(6)-benzylguanine results in an increase in the sensitivity to O(6)-alkylating agents. Animal studies revealed that the therapeutic index of BCNU increased when administered in combination with O(6)-benzylguanine. This drug is currently in phase I clinical trials. Evidence from animal studies indicates that myelosuppression may be the dose-limiting toxicity, thus, efforts are aimed at improving the therapeutic index by the stable expression of O(6)-benzylguanine-resistant AGT proteins into targeted normal tissue such as bone marrow. The successful modulation of alkyltransferases brings on an exciting new era for alkylnitrosoureas and alkyltriazenes.     

A strategy for selective O(6)-alkylguanine-DNA alkyltransferase depletion under hypoxic conditions

Cellular resistance to chemotherapeutics that alkylate the O-6 position of guanine residues in DNA correlates with their O(6)-alkylguanine-DNA alkyltransferase activity. In normal cells high [O(6)-alkylguanine-DNA alkyltransferase] is beneficial, sparing the host from toxicity, whereas in tumor cells high [O(6)-alkylguanine-DNA alkyltransferase] prevents chemotherapeutic response. Therefore, it is necessary to selectively inactivate O(6)-alkylguanine-DNA alkyltransferase in tumors. The oxygen-deficient compartment unique to solid tumors is conducive to reduction, and could be utilized to provide this selectivity. Therefore, we synthesized 2-nitro-6-benzyloxypurine, an analog of O(6)-benzylguanine in which the essential 2-amino group is replaced by a nitro moiety, and 2-nitro-6-benzyloxypurine is >2000-fold weaker than O(6)-benzylguanine as an O(6)-alkylguanine-DNA alkyltransferase inhibitor. We demonstrate oxygen concentration sensitive net reduction of 2-nitro-6-benzyloxypurine by cytochrome P450 reductase, xanthine oxidase, and EMT6, DU145, and HL-60 cells to yield O(6)-benzylguanine. We show that 2-nitro-6-benzyloxypurine treatment depletes O(6)-alkylguanine-DNA alkyltransferase in intact cells under oxygen-deficient conditions and selectively sensitizes cells to laromustine (an agent that chloroethylates the O-6 position of guanine) under oxygen-deficient but not normoxic conditions. 2-Nitro-6-benzyloxypurine represents a proof of concept lead compound; however, its facile reduction (E(1/2) - 177 mV versus Ag/AgCl) may result in excessive oxidative stress and/or the generation of O(6)-alkylguanine-DNA alkyltransferase inhibitors in normoxic regions in vivo.     

Modulation of nitrosourea resistance in human colon cancer by O6-methylguanine.

Human colon cancer is resistant to a variety of alkylating agents including the nitrosoureas. To specifically evaluate nitrosourea resistance, we studied the role of O6-alkylguanine-DNA alkyltransferase (alkyltransferase) which is known to repair nitrosourea-induced cytotoxic DNA damage. Alkyltransferase activity varied over a similar wide range in 25 colon cancer biopsies and 14 colon cancer cell lines but the activity was not correlated with differentiation status, Dukes' classification or in vitro growth characteristics. 1,3-Bis-(2-chloroethyl)-1-nitrosourea (BCNU) resistance and alkyltransferase activity were highly correlated (R2 = 0.929, P less than 0.001) in 7 different colon cancer cell lines, suggesting that the alkyltransferase is an important component of nitrosourea resistance in colon cancer cells. In the BCNU-resistant, high alkyltransferase VACO 6 cell line, inactivation of the alkyltransferase by O6-methylguanine caused a proportional decrease in the BCNU IC50, consistent with that predicted by the regression line. Enzyme inactivation was also associated with a marked increase in DNA cross-link formation. Because alkyltransferase correlates with BCNU resistance in colon cancer, and resistance can be reversed by inactivating the protein, the alkyltransferase may have an important role in nitrosourea resistance in human colon cancer cells. These data provide the rationale for clinical trials in colon cancer with biochemical modulators of the alkyltransferase to increase the therapeutic response to nitrosoureas.     

Mutagenesis by O(6)-methyl-, O(6)-ethyl-, and O(6)-benzylguanine and O(4)-methylthymine in human cells: effects of O(6)-alkylguanine-DNA alkyltransferase and mismatch repair.

Double-stranded and gapped shuttle vectors were used to study mutagenesis in human cells by O(6)-methyl (m(6)G)-, O(6)-ethyl (e(6)G)-, and O(6)-benzylguanine (b(6)G), and O(4)-methylthymine (m(4)T) when these bases were incorporated site-specifically in the ATG initiation codon of a lacZ' gene. Vectors were transfected into either human kidney cells (293) or colon tumor cells (SO) or into mismatch repair defective human colon tumor cells (H6 and LoVo). Cellular O(6)-alkylguanine-DNA alkyltransferase (alkyltransferase) was optionally inactivated by treating cells with O(6)-benzylguanine prior to transfection. In alkyltransferase competent cells, the mutagenicity of all the modified bases was substantially higher in gapped plasmids than in double-stranded plasmids. Alkyltransferase inactivation increased mutagenesis by the three O(6)-substituted guanines in both double-stranded and gapped plasmids but did not affect m(4)T mutagenesis. In the absence of alkyltransferase, mutagenesis by m(6)G and to a lesser extent e(6)G in double-stranded vectors was higher in the mismatch repair defective H6 and LoVo cells than in SO or 293 cells indicating that e(6)G as well as m(6)G were subject to mismatch repair processing in these cells. The level of mutagenesis by m(4)T and b(6)G was not affected by mismatch repair status. When incorporated in gapped plasmids and in the absence of alkyltransferase, the order of mutagenicity for the modified bases was m(4)T > e(6)G congruent with m(6)G > b(6)G. The O(6)-substituted guanines primarily produced G-->A transitions while m(4)T primarily produced T-->C transitions. However, m(4)T also produced a significant number of T-->A transversion mutations in addition to T-->C transitions in mismatch repair deficient LoVo cells.     

以DNA修复蛋白AGT为靶向的PET显像剂前体O^6-苄基鸟嘌呤类似物的体外初步生物评价

合成一系列O^6-苄基鸟嘌呤（O^6-BG）类似物，并且采用MTT法评价其体外对DNA修复蛋白AGT的抑制作用，探讨其作为潜在的正电子发射断层成像技术（PET）显像剂前体的可能性。以鸟嘌呤作为起始原料分别合成了O^6-BG及其类似物HMBG，MOBG，MOMOBG，BABP和PEG。采用MTT方法，通过测定合成产物增强HeLa细胞对1，3-双（2-氯乙基）亚硝基脲（BCNU）药物敏感性的强弱来评价其对AGT的抑制作用。合成产物对AGT抑制活性强弱排序为HMBG≥O^6-BG≥MOBG≥MOMBG，而BABP和PEG基本未表现出任何的AGT抑制活性。HMBG，MOBG和MOMBG具有良好的体外活性，其正电子核素标记物可能成为有前景的用于肿瘤AGT显像的PET显像剂。     

O~6-苄基鸟嘌呤增强 1,3-二 (2 -氯乙基)-亚硝基脲对人肝癌细胞及其移植瘤的抗肿瘤作用(英文)

应用噻唑蓝 (MTT)法检测 O6-苄基鸟嘌呤(O6- BG)与 1 ,3-二 (2 -氯乙基 ) -亚硝基脲 (BCNU)合用的细胞毒作用及透射电镜检测凋亡细胞的方法研究了 O6- BG对 O6-烷基鸟嘌呤 - DNA烷基转移酶(O6- AGT )阳性的人肝癌细胞 SMMC- 772 1对BCNU细胞毒作用敏感性的影响及其与 BCNU合用治疗移植瘤的协同效果 .结果显示 :1 .5- 6.0 mg· L-1的 O6- BG预先作用 2 h后 ,SMMC- 772 1细胞对 BCNU的敏感性明显增加 ;0 .75- 6.0 mg· L-1的 O6- BG可完全快速地抑制肿瘤细胞的 AGT活性并持续 1 2 h;ip 90 mg· kg-1的 O6- BG预处理 2 h后给予 2 5mg·kg-1的 BCNU治疗 ,可使动物 sc接种的人肝癌移植瘤生长延迟 38.6d,诱导肿瘤细胞凋亡 ,并且可明显抑制肿瘤组织的转移酶活性 .说明 O6- BG与 BCNU合用于 AGT阳性的肿瘤将具有明显的治疗效果     

Influence of O6-methylguanine on DNA damage and cytotoxicity of temozolomide in L1210 mouse leukemia sensitive and resistant to chloroethylnitrosoureas.

Temozolomide is a new anticancer agent which in the early clinical investigation has shown promising antitumor activity. It decomposes spontaneously to the active metabolite of DTIC (MTIC). Temozolomide is more cytotoxic against L1210 than against a subline L1210/BCNU, resistant to chloroethylnitrosoureas. Using [methyl-3H] temozolomide we found that after 1 h exposure the amount of O6-methylguanine (O6mGua) was twice as high in L1210 than in L1210/BCNU whereas the amount of N7 mGua was approximately the same in the two cell lines. O6-alkylguanine DNA alkyltransferase (AT) levels were higher in L1210/BCNU than in L1210, supporting the view that the resistance to methyltriazenes is probably related to the efficient repair of O6mGua in L1210/BCNU. Exposure of L1210/BCNU cells to 0.4 mM O6mGua for 24 h resulted in a depletion of AT and in a higher temozolomide-induced cytotoxicity. In the sensitive cell line L1210, temozolomide activity was not potentiated by O6mGua pretreatment. Moreover, in L1210/BCNU, O6mGua increased DNA single-strand breaks caused by temozolomide, suggesting that O6-guanine alkylation induces an excision repair mechanism in cells depleted in AT.     

The therapeutic potential of O6-alkylguanine DNA alkyltransferase inhibitors

Resistance to O(6-)alkylating agents can be overcome by depletion of the DNA repair protein, O(6)-alkylguanine DNA alkyltransferase. Inhibitors of this protein act as pseudosubstrates and, so far, O(6)-benzylguanine and lomeguatrib have been tested in clinical trials. Inherently non-toxic, optimum doses for protein depletion have been established for both agents. Myelosuppression of alkylating agents is significantly enhanced when used in combination with these agents, necessitating significant reductions in standard doses. Consequently, no improvement in efficacy is seen. Strategies to limit myelotoxicity are complex and will be very difficult to apply clinically. O(6)-alkylguanine DNA alkyltransferase inhibition may also potentiate the toxicity of other agents such as cyclophosphamide and irinotecan. Other mechanisms of DNA repair are also important and drugs targeting some of these systems are in early phase clinical trials.     

Protection of CHO cells by mutant forms of O6-alkylguanine-DNA alkyltransferase from killing by 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) plus O6-benzylguanine or O6-benzyl-8-oxoguanine.

O6-Benzylguanine (BG) is an inactivator of human O6-alkylguanine-DNA alkyltransferase (AGT) currently undergoing clinical trials to enhance cancer chemotherapy by alkylating agents. Mutant forms of AGT resistant to BG in vitro were expressed in CHO cells to determine if they could impart resistance to killing by the combination of BG and 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU). All the BG-resistant mutant proteins tested (P140A, P140K, P138M/V139L/P140K, G156A, P140A/G160R, and G160R) showed a reduced rate of reaction with methylated DNA substrates in vitro. However, when expressed in equal amounts in CHO cells, mutants P140A, P140K, P138M/V139L/P140K, and G160R gave levels of protection from the chloroethylating agent BCNU equivalent to that of wild-type AGT. This indicates that a 10-fold reduction in rate constant did not prevent their ability to repair chloroethylated DNA in the cell. AGT activity was readily lost when CHO cells expressing wild-type AGT were exposed to BG or its 8-oxo metabolite (O6-benzyl-8-oxoguanine), but cells expressing mutants P140A or G160R required 30-fold higher concentrations and cells expressing mutants P140K or P138M/V139L/P140K were totally resistant. When cells were treated with 80 microM BCNU plus BG or 8-oxo-BG, those expressing wild-type AGT were killed when inhibitor concentrations of up to 500 microM were used, whereas cells expressing P140K or P138M/V139L/P140K showed no effect, and cells expressing P140A or G160R showed an intermediate resistance. These results suggest that: (i) appearance of BG-resistant mutant AGTs may be a problem during therapy, and (ii) the P140K mutant AGT is an excellent candidate for gene therapy approaches where expression of a BG-resistant AGT in hematopoietic cells is used to reduce toxicity.     

Mammalian cells expressing Escherichia coli O6-alkylguanine-DNA alkyltransferases are hypersensitive to dibromoalkanes.

The effect of expression of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase, on the growth inhibitory effects of the dibromoalkanes (DBA) dibromomethane (DBM) and dibromoethane (DBE) was determined in Chinese hamster lung fibroblasts transfected with and expressing high levels of the Escherichia coli alkyltransferase (ATase) genes. These included the ogt gene and complete or truncated versions of the E. coli ada gene encoding either O6-alkylguanine (O6-alkG) or alkylphosphotriester (alkPT) ATase activities. The functional activity of the ATase in these cells was demonstrated by in vitro assay of cell extracts using 3H-methylated DNA as a substrate, and by the protection they provided against the growth inhibitory effects of methylating agents N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU) and the chloroethylating agent 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU). However, cells expressing the full length or the O6-alkG ATase region, but not the alkPT ATase region, of Ada were found to be more sensitive to the growth inhibitory effects of the DBA; Ogt expression sensitized cells to DBM but not significantly to DBE. Addition of DBA to cell extracts depleted O6-alkG ATase activity on the methylated DNA substrate, but had no effect on alkPT ATase activity. This suggests that ATase-mediated sensitization of the intact cells may be related to the inactivation of the ATase protein. Addition to the cell culture medium of GSH or buthionine sulfoximine in attempts to augment or deplete cellular levels of GSH had no marked effect on the ATase-mediated sensitization to DBA. This suggests that rather than GSH-mediated DNA damage, the effect may be mediated by a DNA adduct caused by the oxidative metabolic pathway. These observations indicate that expression of ATase may have a detrimental effect on cellular sensitivity to environmentally relevant alkylating agents.     

Synthesis and antitumor activity of methyltriazene prodrugs simultaneously releasing DNA-methylating agents and the antiresistance drug O(6)-benzylguanine

Active resistance of tumor cells against DNA alkylating agents arises by the production of high levels of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). This resistance during treatment with, for example, the anticancer agent temozolomide can be reversed by administration of O(6)-benzylguanine, a purine that transfers its benzyl group to AGT and irreversibly inactivates it. Stimulated by the favorable therapeutic properties of temozolomide we designed and synthesized DNA-methylating triazenes built on the antiresistance benzylguanine ring system. The condensation reaction between 2-nitrosopurines and acylhydrazines proved to be very suitable to prepare acylated methyltriazenes. Fine-tuning of the release rate of both the methylating agent (diazomethane) and of O(6)-benzylguanine was accomplished by variation of the hydrolysis-sensitive acyl substituent in 5. Hydrolysis studies were performed with (1)H NMR and revealed that the p-nitrophenyl substituted triazene 26 showed an optimal hydrolysis rate (t(1/2) = 23 min) and almost 100% selectivity for the desired fragmentation route. In vitro antitumor studies in the 60 human tumor cell line panel of the National Cancer Institute confirmed the superior properties of p-nitrophenyl-protected methyl triazene 26, showing mean IC(50) values of 10 microM compared to 100 microM for temozolomide. In analogy with temozolomide, triazene 26 showed however low preference for each of the cancer subpanels, with IC(50) values between 8 and 14 microM.