Curcumin reduced the side effects of mitomycin C by inhibiting GRP58-mediated DNA cross-linking in MCF-7 breast cancer xenografts

Mitomycin C (MMC), a chemotherapeutic agent in breast cancer treatments, inhibits tumor growth through DNA cross-linking and breaking, but it has severe side effects. Here we examined whether and how curcumin reduced the side effects of MMC. We found that combination treatment with MMC and curcumin reduced tumor weight by 70% and 36% compared with saline and curcumin-treated groups, respectively. The combination treatment reduced weight loss and improved kidney function and bone marrow suppression compared with MMC treatment alone. Moreover, the combination treatment inhibited glucose regulatory protein (GRP58)-mediated DNA cross-linking. The combination treatment inhibited GRP58 through the ERK/p38 MAPK pathway. In conclusion, the current study provided evidence that MMC and curcumin combination treatment reduced MMC side effects by inhibiting GRP58-mediated DNA cross-linking through the ERK/p38 MAPK pathway. ( Cancer Sci 2009)     

pH dependence of mitomycin C-induced cross-linking activity in EMT6 tumor cells

Mitomycin C (MC), a quinone-containing bioreductive alkylating agent, is cytotoxic to aerobic EMT6 tumor cells despite the fact that little bioactivation of MC occurs in EMT6 cell homogenates in the presence of O2. Because spontaneous activation of MC at acidic pH has been reported in chemical systems, aerobic EMT6 tumor cells were incubated in serum-free 2-(N-morpholino)ethanesulfonic acid or N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer at pH 5.7, 6.4, and 7.5 and exposed to MC for 2 h. As the extracellular pH was lowered, MC-induced DNA-DNA cross-linking, as measured by alkaline elution techniques, was enhanced. This effect was dose dependent at the three pH values tested. Measurement of intracellular pH by flow cytometric analysis indicated that the decrease in extracellular pH was paralleled by a fall in intracellular pH. The alteration of the extracellular pH had no effect on the colony-forming ability of control cells. The survival of cells treated with MC, however, was decreased as the pH was lowered. These data suggest that the intracellular and/or the extracellular pH is an important determinant of MC activity in aerobic EMT6 tumor cells.     

Modulation of mitomycin C-induced multidrug resistance in vitro

Summary A series of in vitro cytotoxicity studies were performed to achieve pharmacologic reversal of resistance to the alkylating agent mitomycin (MMC) in L-1210 leukemia cells. A multidrug-resistant (MDR), P-glycoproteinpositive cell line, RL-1210/.1 [11], was exposed to potential MDR modulators in the absence or presence of MMC. The following compounds did not reverse MMC-induced MDR: quinine, quinidine, lidocaine, procaine, dimethylsulfoxide (DMSO), dexamethasone, hydrocortisone, prednisolone, estradiol, and testosterone. Three agents were capable of reversing MMC resistance: progesterone, cyclosporin A, and verapamil. The R- and S-isomers of verapamil were equipotent, although they showed a 10-fold difference in cardiovascular potency (S>R). Some agents produced cytotoxic effects in MDR cells in the absence of MMC, including progesterone, quinine, and quinidine. The results suggest that R-verapamil and progesterone may have clinical utility in reversing MMC resistance in human tumors. Progesterone may be uniquely efficacious due to (a) its low toxicity in normal cells, (b) its selective cytotoxicity in MDR cells (in the absence of MMC), and (c) its ability to reverse MMC resistance in vitro. The findings also suggest that the P-glycoprotein induced by MMC differs from that induced by doxorubicin, which is highly sensitive to modulation by lysosomotropic amines such as quinine and quinidine.Supported in part by grant CA-17094 (R.T.D.) from the Department of Health and Human Services, National Institute of Health, Bethesda, Maryland     

Metabolism of mitomycin C by DT-diaphorase: role in mitomycin C-induced DNA damage and cytotoxicity in human colon carcinoma cells.

The role of DT-diaphorase in bioreductive activation of mitomycin C was examined using HT-29 and BE human carcinoma cells which have high and low levels of DT-diaphorase activity, respectively. HT-29 cells were more sensitive to mitomycin C-induced cytotoxicity than the DT-diaphorase-deficient BE cell line. Mitomycin C induced DNA interstrand cross-linking in HT-29 cells but not in BE cells. Both mitomycin C-induced cytotoxicity and induction of DNA interstrand cross-links could be inhibited by pretreatment of HT-29 cells with dicoumarol. Metabolism of mitomycin C by HT-29 cell cytosol was pH dependent and increased as the pH was lowered to 5.8, the lowest pH tested. Metabolism of mitomycin C by HT-29 cytosol was inhibited by prior boiling of cytosol or by the inclusion of dicoumarol. Little metabolism was detected in BE cytosols. When purified rat hepatic DT-diaphorase was used, metabolism of mitomycin C increased as the pH was decreased and could be detected at pH 5.8, 6.4, 7.0, 7.4, but not at 7.8. Metabolism of mitomycin C was NADH dependent and inhibited by dicoumarol or by prior boiling of enzyme. An approximate 1:1 stoichiometry between NADH and mitomycin C removal was demonstrated and no oxygen consumption could be detected. Metabolism of mitomycin C by purified HT-29 DT-diaphorase was also dicoumarol inhibitable and pH dependent. The major metabolite formed during metabolism of mitomycin C by HT-29 cytosol, purified HT-29, and rat hepatic DT-diaphorase was characterized as 2,7-diaminomitosene. These data suggest that two-electron reduction of mitomycin C by DT-diaphorase may be an important determinant of mitomycin C-induced genotoxicity and cytotoxicity.     

Prospective study on the dose relationship of mitomycin C-induced interstitial pneumonitis

Lung damage after mitomycin C (MMC) was first reported in 1978. Although this side effect has been frequently reported since then, there are no data on dose dependency nor on incidence. Therefore, the authors initiated a prospective study to obtain more data on this subject. Forty-four patients treated with MMC entered the study; 37 were evaluable. All patients were subjected to repeated physical examinations, chest x-rays, chest computed tomography (CT) scan and pulmonary function tests. The results were evaluated per cumulative dose level. None of the patients had clinical pulmonary toxicity develop; one patient had pulmonary changes on CT scan, the significance of which remained unclarified. The world literature on this subject was also reviewed. Based on the combined data of the present study as well as the literature review, the authors concluded that MMC-related lung toxicity is a dose-dependent side effect, occurring at cumulative dose levels of 20 mg/m2 or more. The incidence is likely to be less than 10%.     

Recovery from mitomycin C-induced hemolytic uremic syndrome. A case report

Mitomycin C (MMC) is a cytotoxic agent that may induce a hemolytic uremic syndrome (HUS) with severe renal insufficiency. Of all reported patients with terminal renal failure only two survived with chronic hemodialysis. A patient with advanced gastric cancer in complete remission, who developed MMC-induced HUS, is reported; hemodialysis was necessary because of oliguria. Hemolysis subsided, and after addition of captopril renal function recovered partially. The patient is alive 6 months after discontinuation of hemodialysis. Recently she developed brain metastases. Symptoms of hemolysis did not recur. The pathogenesis and treatment of HUS are discussed.     

Inhibition of cyclophosphamide and mitomycin C-induced sister chromatid exchanges in mice by vitamin C

Ascorbic acid (vitamin C) is known to act as an antimutagen and anticarcinogen in several test systems. However, there is no report of its effect on carcinogen-induced chromosomal damage in vivo in animals. The present study was performed to determine whether or not ascorbic acid affects sister chromatid exchanges (SCEs) induced by cyclophosphamide (CPA) and mitomycin C (MMC) in bone marrow and spleen cells in mice. The results indicate that ascorbic acid per se did not cause a significant increase in SCEs in mice. However, increasing concentrations of ascorbic acid caused decreasing levels of CPA- and MMC-induced SCEs in both cell types in vivo. At the highest concentration of ascorbic acid, 6.68 g/kg, approximately 75 and 40% SCE inhibition in both cell types was noted for CPA and MMC, respectively. Likewise, under in vivo/in vitro conditions (exposure of animals to experimental chemicals followed by culturing of cells), ascorbic acid caused a dose-related decrease in CPA- and MMC-induced SCEs, up to a dose of 3.34 g/kg At this concentration, approximately 50% CPA- and MMC-induced SCE inhibition was observed in both cell types studied. Thus, ascorbic acid acts as an anti-SCE agent in both in vivo and in vivo/in vitro conditions in mice.     

DNA-protein cross-linking and DNA interstrand cross-linking by haloethylnitrosoureas in L1210 cells.

Bifunctional alkylating agents are known to produce cross-links between DNA and protein and between paired DNA strands. The possibility of discriminating these two classes of cross-links in L1210 cells treated with haloethylnitrosoureas or nitrogen mustard was explored with the alkaline elution technique. Two classes of cross-links were demonstrated, based on sensitivity to proteinase K; the proteinase-sensitive cross-links appear to be DNA-protein cross-links, and the proteinase-resistant class may include interstrand cross-links. Proteinase-sensitive cross-links form more rapidly than do proteinase-resistant cross-links in cells treated with chloroethylnitrosoureas, perhaps because these agents can chloroethylate protein sulfhydryl or amino groups followed by rapid reaction of these chloroethylated groups with DNA. Although both types of cross-links produced by nitrogen mustard disappeared or were repaired after 24 hr, the removal of cross-links produced by chloroethylnitrosoureas either did not occur or was incomplete in 24 hr. In addition to cross-links, cells treated with haloethylnitrosoureas exhibited DNA strand breaks; a method is suggested for estimating the apparent frequencies of strand breaks and cross-links in the DNA.     

16-16 dimethyl prostaglandin E2 reduces 5-fluorouracil-induced and mitomycin C-induced gastric mucosal injury in the dog

Gastric complications occur in 5% to 20% of patients treated with hepatic artery infusion of chemotherapeutic agents for hepatic metastatic lesions. Often these complications are due to catheter dislodgement from the common hepatic artery into the left gastric artery. These studies were designed to answer the following questions: (1) Will chronic infusion of 5-fluorouracil into the left gastric artery produce mucosal injury in dogs; and (2) if so, will 16-16 dimethyl prostaglandin E2 afford protection against such injury? Mongrel dogs, 20 kg, were prepared with a polyethylene catheter in the left gastric artery and a Thomas cannula in the antrum 5 days prior to the study. Daily intraarterial infusions of either 5-fluorouracil, 6.7 mgM-2 X h-1, (N = 5) or 5-fluorouracil + 16-16 dimethyl prostaglandin E2, 2 micrograms X kg-1 X h-1, (N = 5) were given 12 hours a day for 5 days. In 2 dogs, 0.15 M NaCl was infused for 12 hours a day for 5 days as controls. Daily endoscopic evaluation of the gastric mucosa was made through the Thomas cannula by an unbiased observer and scored 0 to +5 based on degree of erythema, edema, friability, exudate, and gross ulceration. Results of these studies demonstrated that this dose of 5-fluorouracil had no effect on histamine-stimulated acid output. This dose of 16-16 dimethyl prostaglandin E2 inhibited histamine-stimulated maximal acid output 65%. From the observations made it was concluded that infusion of this chemotherapeutic regimen into the left gastric artery produced significant mucosal injury, simultaneous intraarterial infusion of 16-16 dimethyl prostaglandin E2 provided significant protection against this damage, and, since 16-16 dimethyl prostaglandin E2, at this dose, inhibits stimulated gastric acid secretion, it cannot be determined whether this observed mucosal protection is due to its antisecretory effect or some other mechanism.     

DNA alkylation and interstrand cross-linking by treosulfan

The anti-tumour drug treosulfan (L-threitol 1,4-bismethanesulphonate, Ovastat) is a prodrug for epoxy compounds by converting non-enzymatically to L-diepoxybutane via the corresponding monoepoxide under physiological conditions. The present study supports the hypothesis that this conversion of treosulfan is required for cytotoxicity in vitro. DNA alkylation and interstrand cross-linking of plasmid DNA is observed after treosulfan treatment, but this is again produced via the epoxide species. Alkylation occurs at guanine bases with a sequence selectivity similar to other alkylating agents such as the nitrogen mustards. In treosulfan-treated K562 cells, cross-links form slowly, reaching a peak at approximately 24 h. Incubation of K562 cells with preformed epoxides shows faster and more efficient DNA cross-linking. © 1999 Cancer Research Campaign     

Enhancement of cis-diamminedichloroplatinum and mitomycin C-induced sister chromatid exchanges by dibutyryl cyclic AMP in a human salivary gland adenocarcinoma cell line in culture

A human salivary gland adenocarcinoma cell line was cultivated in the presence of dibutyryl cyclic AMP (dB-cAMP), cis-diammine dichloroplatinum (cisplatin) or mitomycin C (MMC) only, or of the combination of dB-cAMP and each of the antineoplastic drugs. Then the treated cells were examined for the induction of sister chromatid exchanges (SCEs), colony-forming efficiency (CFE) in semisolid agar medium and cell survival as measured by counting for viable cells. It has been found that the frequency of SCE induction is significantly increased by treatment of cells with cisplatin or MMC under the presence of dB-cAMP, when compared to that in the cells treated with cisplatin or MMC only. Moreover, marked reduction in CFE and cell survival was found to occur in the cells treated with the combination of dB-cAMP and cisplatin or MMC. These finding indicate that treatment of a human salivary gland adenocarcinoma cell line with dB-cAMP results in modification of SCE induction, CFE and cell survival in the cells exposed to cisplatin or MMC.     

Role of xanthine oxidase in the potentiation of doxorubicin-induced cardiotoxicity by mitomycin C

Clinical evidence has suggested that mitomycin C (MMC) potentiates doxorubicin (DOX) induced cardiotoxicity. In this study a mouse model was used to examine the effect of DOX on the ability of cardiac tissue to bioactivate MMC to generate oxygen radicals. Cardiac damage was assessed by measuring serum CPK-MB isoenzyme levels and thiobarbituric acid reactive substances (TBARS) in the cardiac tissue. The exposure of animals to DOX or DOX and MMC over a three week period led to an increase in serum CPK-MB isoenzyme levels as well as TBARS. Treatment with DOX led to an increase in MMC-dependent, NADH-dependent, cyanide insensitive oxygen consumption, compared to control animals, thereby suggesting increased MMC-dependent oxygen radical generation. Levels of xanthine oxidase (XO; EC 1.1.3.22) and NADPH:cytochrome C reductase, two enzymes known to bioactivate MMC with subsequent oxygen radical generation, were measured in cardiac tissue with a 4.5 x increase in XO activity seen in DOX treated animals vs controls and no change in NADPH:cytochrome C reductase activity. Cardiac levels of xanthine dehydrogenase (XDH; EC 1.1.1.204) activity in DOX treated animals decreased while the XO/XDH ratio increased, suggesting a conversion of XDH to XO following DOX treatment.     

Sequence-selective depurination, DNA interstrand cross-linking and DNA strand break formation associated with alkylated DNA.

Alkylation of DNA by chloroethylnitrosourea (CNU) at the guanine N7 position has been shown to occur in a sequence-selective fashion. In this report we find that the depurination of these alkylated sites occurs with two distinct kinetic components--GG sequences depurinate within 30 min of exposure to CNU, while depurination at GT sequences is first observed after 1 h and continues to increase 16 h after drug exposure. These apurinic sites are converted to DNA strand breaks and constitute less than 10% of the total sites of guanine N7 alkylation. Spermidine was found to decrease alkylation in 5'-GG-3' sequences but increases alkylation at 5'-GTC-3' sequences. These findings suggest that the majority of the guanine N7 alkylations formed by CNU are stable, with a minor adduct being responsible for the slow depurination event. We propose that the rapid depurination induced by CNU occurs from an initial guanine O6 alkylation, which then depurinates via a guanine O6-N7 cyclized intermediate. We also propose that the resulting apurinic sites may lead to DNA interstrand cross-linking (ISC). In support of these hypotheses we show that (i) DNA modified with the monoalkylating agent dimethylsulfate forms DNA ISC upon depurination; (ii) ellagic acid enhances the level of guanine N7 alkylation and alters the pattern of sequence selectivity shown by three bifunctional chloroethylating agents CNU, mitozolomide and methyl 3-(2-chloroethyl)-4-oxoimidazo[5,1-d]-1,2,3,5-tetrazine-8-ca rboxylate but not with nitrogen mustard; (iii) ellagic acid has no effect upon the frequency of alkylation observed with the monofunctional alkylators N-methyl-N-nitrosourea, N-ethyl-N-nitrosourea and methylmethanesulfonate; (iv) ellagic acid increases the frequency of depurination and strand break formation induced by CNU without affecting the sequence-selective pattern of depurination.     

GRP78在蛋白酶体抑制剂诱导甲状腺癌细胞凋亡中的作用

目的:探讨葡萄糖调节蛋白78(GRP78)在蛋白酶体抑制剂诱导甲状腺癌细胞凋亡中的作用.方法:对2种人甲状腺未分化癌细胞系FRO、AR0分别设空白对照组和蛋白酶体抑制荆处理组;及siGRP78、随机序列核酸siRNA和错位型siGRP78组.利用蛋白酶体抑制荆MG132(1μtmol/L)、PSI(10 nmol/L)和EPOX(5 nmol/L)作用2种细胞系,实时定量RT-PCR和蛋白质印迹法检测各组细胞GRP78 mRNA、蛋白表达.流式细胞仪检测细胞凋亡.结果:蛋白酶体抑制剂处理24 h后,2种细胞系中GRP78mRNA及蛋白水平保持相似增长高峰,为空白对照组2～3倍(P<0.01);siGRP78转染24 h,细胞总GRP78 mRNA丢失>80%,转染48 h,GRP78蛋白总量丢失>75%.蛋白酶体抑制剂MG132作用后,ARO、FRO细胞凋亡率分别为8.3%和78.3%;当siGRP78靶向沉默GRP78后,蛋白酶体抑制剂诱导2种细胞凋亡率显著增加,AR0细胞凋亡率提高5倍左右(P<0.01).结论:2种甲状腺癌细胞系存在GRP78基因表达,不同程度干扰蛋白酶体抑制荆凋亡诱导作用;沉默GRP78基因表达可提高蛋白酶体抑制荆对肿瘤细胞杀伤率.     

Quenching of DNA cross-link precursors of chloroethylnitrosoureas and attenuation of DNA interstrand cross-linking by glutathione

Interstrand DNA cross-linking is essential for the antitumor activity of chloroethylnitrosoureas (CENUs). The critical cross-links have been proposed to involve a rapid O6-guanine chloroethylation on one DNA strand, followed by a rearrangement of the O6-(2-chloroethyl)guanine and slow alkylation of the second DNA strand. In view of the relative intracellular abundance of glutathione (GSH) and nucleophilicity of its thiolate ion, the ability of GSH to react with and to inactivate 2-chlorethylated DNA and the possibility that this interaction decreases net DNA cross-linking by CENUs were investigated. Chloroethylated calf thymus DNA was reacted with GSH, the DNA was precipitated and redissolved, and subsequent DNA interstrand cross-linking was determined. The DNA cross-link index was compared for both GSH-treated and 2-chloroethylated untreated DNA. Simultaneously, Col E1 plasmid DNA was chloroethylated and reacted with GSH, and the extent of DNA interstrand cross-linking was determined by agarose gel electrophoresis and compared with controls. The results show both a time- and GSH concentration-dependent quenching of chloroethylated DNA, with a corresponding decrease in the DNA cross-link index. Using [methyl-3H]GSH, it was also demonstrated that 56% of the total GSH was bound to quenched 2-chloroethylated Col E1 DNA and 25% to quenched 2-chloroethylated calf thymus DNA. GSH binding to cross-linked DNA and native DNA was insignificant. It is concluded that, in addition to direct inactivation of reactive cytotoxic CENU species, GSH may also modulate cellular response to CENUs by quenching chloroethylated DNA, thereby decreasing the formation of potentially lethal DNA cross-links.     

GRP78在肺腺癌细胞顺铂耐药中的作用

背景与目的糖调节蛋白78(GRP78)在肺癌细胞中表达增高并与其对化疗药物VP-16的耐药性有关,而GRP78的表达与其对化疗药物顺铂的耐药性是否相关,研究较少。本研究旨在通过检测GRP78在人肺腺癌细胞SPCA-1中的表达及顺铂作用后的细胞生存率,研究GRP78在肺腺癌细胞顺铂耐药中的作用。方法依据是否应用诱导剂A23187将细胞分为实验组和对照组,采用RT-PCR、Western blot方法检测各组细胞中GRP78在核酸和蛋白水平的表达,利用MTT法测定细胞在顺铂作用下的生存率,分析GRP78在SPCA-1细胞顺铂耐药中的作用。结果A23187可以明显诱导SPCA-1细胞中GRP78在核酸和蛋白水平的表达,且其表达水平与A23187具有一定的浓度依赖性,MTT检测显示:实验组细胞生存率明显低于对照组,亦呈一定的A23187浓度依赖性。结论在人肺腺癌细胞SPCA-1中,诱导剂A23187可以诱导GRP78核酸和蛋白水平的表达,其表达与肺癌细胞在顺铂作用下的生存率呈负相关,表明GRP78可提高SPCA-1细胞对顺铂的敏感性,与肺癌细胞化疗耐药有一定的相关性。     

Enhancement of the DNA cross-linking activity of melphalan by misonidazole in vivo.

The technique of alkaline elution has been adapted for the study of drug-induced DNA cross-link formation in vivo. Pretreatment with misonidazole (MISO) enhances the number of cross-links formed in a fibrosarcoma and in the spleen and gut of mice for periods up to 48 h following a single injection of melphalan (MEL). The tumour was sensitized by a greater factor (2.05) than either of the normal tissues (enhancement factor 1.4-1.5). This enhancement did not appear to be related to inhibition of the repair of actual cross-links. Rather, the effect was explicable in terms of one of two alternative models. Firstly, MISO pretreatment could result in a greater amount of binding of MEL to DNA at early times after injection. This may be the result of altered pharmacokinetics of MEL, or of enhanced intracellular uptake of MEL due to MISO pretreatment. Secondly, MISO may exert its affect by inhibition of the repair of cross-links or monoadducts at early times post-injection, which would not be observed in this study. The possible involvement of glutathione depletion in chemosensitization by MISO was investigated by comparison with the effect of diethyl maleate (DEM), a known thiol-depleting reagent. Glutathione depletion, while perhaps being important, could not account for all of the effects observed.     

Interactions of mitomycin C with mammalian DNA detected by alkaline elution

The antitumor antibiotic mitomycin C (MMC) was studied in vitro using L1210 leukemia and 8226 human myeloma cells. Cytotoxicity was evaluated by colony formation in soft agar, and DNA damage was analyzed using alkaline elution filter assays. The purposes of these studies were: (a) to characterize the time course of MMC-DNA damage; (b) to characterize the type of DNA damage [DNA-DNA interstrand cross-links (ISC), DNA-protein cross-links (DPC), single and double strand breaks (SSBs, DSBs)]; and (c) to correlate this damage with cytotoxicity in vitro. Colony-forming assays showed the D0 value for 1 h MMC to be 15.0 microM for L1210 cells and 17 microM for 8226 cells. Alkaline elution studies showed that dose-dependent ISCs and DPCs formed rapidly following MMC exposure. Removal of cross-links was delayed, with only 50% repaired 32 h after exposure. There was a good correlation between ISCs and cytotoxicity in dose-response studies in each cell line. ISCs appeared to comprise most of the MMC-DNA lesions in both cell lines. No DNA SSBs or DSBs were observed following MMC exposure. Nuclei isolated from both cell lines and exposed to MMC produced less MMC alkylation than whole cells but, again, no strand breaks were evident. These results demonstrate that MMC is principally an alkylating agent when used at pharmacological (cytotoxic) concentrations in vitro. The lack of evidence for DNA strand breaks discounts a significant role for putative quinone-generated oxygen free radicals in the production of MMC cytotoxicity.     

Oxidation-mediated DNA cross-linking contributes to the toxicity of 6-thioguanine in human cells

The thiopurines azathioprine and 6-mercaptopurine have been extensively prescribed as immunosuppressant and anticancer agents for several decades. A third member of the thiopurine family, 6-thioguanine (6-TG), has been used less widely. Although known to be partly dependent on DNA mismatch repair (MMR), the cytotoxicity of 6-TG remains incompletely understood. Here, we describe a novel MMR-independent pathway of 6-TG toxicity. Cell killing depended on two properties of 6-TG: its incorporation into DNA and its ability to act as a source of reactive oxygen species (ROS). ROS targeted DNA 6-TG to generate potentially lethal replication-arresting DNA lesions including interstrand cross-links. These triggered processing by the Fanconi anemia and homologous recombination DNA repair pathways. Allopurinol protected against 6-TG toxicity by acting as a ROS scavenger and preventing DNA damage. Together, our findings provide mechanistic evidence to support the proposed use of thiopurines to treat HR-defective tumors and for the coadministration of 6-TG and allopurinol as an immunomodulation strategy in inflammatory disorders. Cancer Res; 72(18); 4787-95. ?2012 AACR.