In vitro bioassays for anticancer drug screening: effects of cell concentration and other assay parameters on growth inhibitory activity

In vitro growth inhibition assays were performed using human cancer cell lines at various concentrations with experimental anticancer drugs such as the cryptophycins and other cytotoxins. The effect of variations in assay parameters on the observed growth inhibition of these anticancer therapeutic agents was determined. The results demonstrated that the observed inhibitory activity of these compounds varied inversely with the cell concentrations used. The observed differences in activity between different cytotoxins were not necessarily proportionate. Thus, the relative activities of two toxins also varied with cell concentration. Furthermore, the sensitivity of these cell lines to the cytostatic purine analog, 6-mercaptopurine (used as a control), varied with cell concentration as well. The activity of this compound was dependent on the medium used for cell growth, yielding good activity in Eagle's minimum essential medium, but not in Ham's F-12 (Kaigin) medium. Moreover, growth inhibition by cryptophycin as well as 6-mercaptopurine was also dependent on the serum concentration in the medium. Finally, the sensitivity of the cancer cell lines to various organic solvents commonly used as drug vehicles for in vitro testing, such as ethanol, dimethylformamide, and dimethylsulfoxide, was likewise found to vary inversely with cell concentration.

Review article: the treatment of inflammatory bowel disease with 6-mercaptopurine or azathioprine

The thioguanine derivative, azathioprine, is a prodrug of 6-mercaptopurine that is further metabolized by various enzymes present in the liver and gut. Azathioprine and 6-mercaptopurine have been used in the treatment of inflammatory bowel disease, i.e. ulcerative colitis and Crohn's disease, for more than 30 years. However, widespread use of azathioprine or 6-mercaptopurine in inflammatory bowel disease is of more recent origin, the primary reason being a long-standing debate on the efficacy of these agents in inflammatory bowel disease. Both drugs are slow acting, which is why clinical efficacy cannot be expected until several weeks or even months of treatment have elapsed. Consequently, azathioprine and 6-mercaptopurine have no place as monotherapy in the treatment of acute relapsing inflammatory bowel disease. Today, azathioprine and 6-mercaptopurine are the most commonly used immunomodulatory drugs in the treatment of inflammatory bowel disease. Their clinical effects are probably identical, although their exact mode of action is still unknown. The mode of action of azathioprine is thought to be multifactorial, including conversion to 6-mercaptopurine (which acts as a purine antimetabolite), possible blockade of thiol groups by alkylation, inhibition of several pathways in nucleic acid biosynthesis (preventing proliferation of cells involved in the determination and amplification of the immune response) and damage to DNA through the incorporation of thiopurine analogues. However, 6-thioguanine nucleotides may accumulate in toxic doses in myeloid precursor cells, resulting in life-threatening myelosuppression. Azathioprine and 6-mercaptopurine are further known to alter lymphocyte function, reduce the number of lamina propria plasma cells and affect natural killer cell function. The purpose of this comprehensive review is to suggest guidelines for the application of azathioprine and 6-mercaptopurine in the treatment of inflammatory bowel disease.

Transport of cyclic nucleotides and estradiol 17-beta-D-glucuronide by multidrug resistance protein 4. Resistance to 6-mercaptopurine and 6-thioguanine

Human multidrug resistance protein 4 (MRP4) has recently been determined to confer resistance to the antiviral purine analog 9-(2-phosphonylmethoxyethyl)adenine and methotrexate. However, neither its substrate selectivity nor physiological functions have been determined. Here we report the results of investigations of the in vitro transport properties of MRP4 using membrane vesicles prepared from insect cells infected with MRP4 baculovirus. It is shown that expression of MRP4 is specifically associated with the MgATP-dependent transport of cGMP, cAMP, and estradiol 17-beta-D-glucuronide (E(2)17 beta G). cGMP, cAMP, and E(2)17 beta G are transported with K(m) and V(max) values of 9.7 +/- 2.3 microm and 2.0 +/- 0.3 pmol/mg/min, 44.5 +/- 5.8 microm and 4.1 +/- 0.4 pmol/mg/min, and 30.3 +/- 6.2 microm and 102 +/- 16 pmol/mg/min, respectively. Consistent with its ability to transport cyclic nucleotides, it is demonstrated that the MRP4 drug resistance profile extends to 6-mercaptopurine and 6-thioguanine, two anticancer purine analogs that are converted in the cell to nucleotide analogs. On the basis of its capacity to transport cyclic nucleotides and E(2)17 beta G, it is concluded that MRP4 may influence diverse cellular processes regulated by cAMP and cGMP and that its substrate range is distinct from that of any other characterized MRP family member.

Deoxythioguanosine triphosphate impairs HIV replication: a new mechanism for an old drug

Inhibition of HIV-1 reverse transcriptase (RT) and HIV protease are effective mechanisms for anti-retroviral agents, and the combined use of mechanistically different medications has markedly improved the treatment of HIV infected patients. The active metabolite of mercaptopurine and thioguanine (TG), deoxythioguanosine triphosphate, was shown to be incorporated into DNA by the polymerase function of HIV-1 RT and then to abrogate RNA cleavage by HIV-1 RNaseH. Treatment of human lymphocyte cultures with thioguanine produced substantial inhibition of HIV replication (IC(50)=0.035 microM, IC(95)=15.4 microM), with minimal toxicity to host lymphocytes (<10% at 15.4 microM TG, P<0.000005). Furthermore, low concentrations of TG and zidovudine were synergistic in inhibiting HIV replication in human lymphocytes (synergy volume=19 microM(2)%), without additive cytotoxicity to host lymphocytes. Thus, thiopurines are novel anti-retroviral agents that alter the DNA-RNA substrates for HIV RNaseH, thereby abrogating early stages of HIV replication.

Glucocorticoids and IL-10, but not 6-MP, 5-ASA or sulfasalazine block endothelial expression of MAdCAM-1: implications for inflammatory bowel disease therapy

BACKGROUND

Enhanced MAdCAM-1 (mucosal addressin cell adhesion molecule-1) expression is associated with the aetiology of inflammatory bowel disease, but little is known about MAdCAM-1: regulation, or how inflammatory bowel disease therapies modulate MAdCAM-1.

AIM

To examine how agents currently used to treat inflammatory bowel disease affect MAdCAM-1: induced by tnf-alpha in an in vitro model of inflammatory bowel disease.

METHODS

Endothelial monolayers were pretreated with dexamethasone (DEX): 5-aminosalicylic acid (5-ASA), 6-mercaptopurine (6-MP), sulfasalazine or interleukin-10: (IL-10: prior to TNF-alpha (20 ng/mL), and MAdCAM-1: measured by Western blotting, RT-PCR, EMSA and lymphocyte adhesion assays.

RESULTS

MAdCAM-1: was induced dose- and time-dependently by TNF-alpha on endothelial cells. Either dexamethasone or IL-10: reduced TNF-alpha-induced MAdCAM-1: protein, mRNA and lymphocyte adhesion. However, neither 5-ASA, sulfasalazine nor 6-MP blocked MAdCAM-1 induction.

CONCLUSIONS

Our data indicate that dexamethasone or IL-10 can exert therapeutic activity in inflammatory bowel disease through MAdCAM-1 inhibition. 5-ASA, sulfasalazine and 6-MP, while beneficial in inflammatory bowel disease, do not directly control MAdCAM-1, and are beneficial through inhibition of other inflammatory processes.

Differing contribution of thiopurine methyltransferase to mercaptopurine versus thioguanine effects in human leukemic cells

Thioguanine and mercaptopurine are prodrugs requiring conversion into thiopurine nucleotides to exert cytotoxicity. Thiopurine S-methyltransferase (TPMT), an enzyme subject to genetic polymorphism, catabolizes thiopurines into inactive methylated bases, but also produces methylthioguanine nucleotides and methylmercaptopurine nucleotides from thioguanine and mercaptopurine nucleotides, respectively. To study the effect of TPMT on activation versus inactivation of mercaptopurine and thioguanine, we used a retroviral gene transfer technique to develop human CCRF-CEM cell lines that did (TPMT+) and did not (MOCK) overexpress TPMT. After transduction, TPMT activities were 14-fold higher in the TPMT+ versus the MOCK cell lines (P < 0.001). TPMT+ cells were less sensitive to thioguanine than MOCK cells (IC(50) = 1.10+/- 0.12 microM versus 0.55 +/- 0.19 microM; P = 0.02); in contrast, TPMT+ cells were more sensitive to mercaptopurine than MOCK cells (IC(50) = 0.52 +/- 0.20 microM versus 1.50 +/- 0.23 microM; P < 0.01). The lower sensitivity of TPMT+ versus MOCK cells to thioguanine was associated with lower thioguanine nucleotide concentrations (917 +/- 282 versus 1515 +/- 183 pmol/5 x 10(6) cells; P = 0.01), higher methylthioguanine nucleotide concentrations (252 +/- 34 versus 27 +/- 10 pmol/5 x 10(6) cells; P = 0.01), less inhibition of de novo purine synthesis (13 versus 95%; P < 0.01), and lower deoxythioguanosine incorporation into DNA (2.0 +/- 0.6% versus 7.2 +/- 2.0%; P < 0.001). The higher sensitivity of TPMT+ cells to mercaptopurine was associated with higher concentrations of methylmercaptopurine nucleotide (2601 +/- 1055 versus 174 +/- 77 pmol/5 x 10(6) cells; P = 0.01) and greater inhibition of de novo purine synthesis (>99% versus 74%; P < 0.01) compared with MOCK cells. We conclude that methylation of mercaptopurine contributes to the antiproliferative properties of the drug, probably through inhibition of de novo purine synthesis by methylmercaptopurine nucleotides, whereas thioguanine is inactivated primarily by TPMT.

Antimycobacterial activity of 9-sulfonylated/sulfenylated-6-mercaptopurine derivatives

A series of 9-sulfonylated/sulfenylated-6-mercaptopurines has been prepared by reaction of 6-mercaptopurine with sulfonyl/sulfenyl halides. These compounds constitute a new class of potent antimycobacterial agents, possessing MIC values against Mycobacterium tuberculosis H37Rv in the range of 0.39-3.39 microg/mL, as well as appreciable activity against Mycobacterium avium. Furthermore, a compound of this small series exhibited good activity (MIC under 1 microg/mL) against several drug resistant strains of M. tuberculosis.

A novel protein complex distinct from mismatch repair binds thioguanylated DNA

To elucidate molecular mechanism(s) of cellular response to mercaptopurine, a widely used antileukemic agent, we assessed mercaptopurine (MP) sensitivity in mismatch repair (MMR) proficient and MMR deficient human acute lymphoblastic leukemia (ALL) cells. Sensitivity to thiopurine cytotoxicity was not dependent on MMR (i.e., MutSalpha) competence among six cell lines tested. Using electrophoretic mobility shift assay analysis, we found that the incubation of nuclear extracts from ALL cells with synthetic 34-mer DNA duplexes containing deoxythioguanosine (G(S)) within either G(S).T or G(S).C pairs, resulted in formation of a DNA-protein complex distinct from the DNA-MutSalpha complex and unaffected by ATP. Isolation and sequence analysis of proteins involved in this DNA-protein complex identified glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as a component. Western blot analysis of nuclear extracts from a panel of human lymphoblastic leukemia cell lines revealed markedly different basal levels of GAPDH in nuclei, which was significantly related to thiopurine sensitivity (p = 0.001). Confocal analysis revealed markedly different intracellular distribution of GAPDH between nucleus and cytosol in six human ALL cell lines. Redistribution of GAPDH from cytosol to nucleus was evident after MP treatment. These findings indicate that a new DNA-protein complex containing GAPDH and distinct from known MMR protein-DNA complexes binds directly to thioguanylated DNA, suggesting that this may act as a sensor of structural alterations in DNA and serve as an interface between these DNA modifications and apoptosis.

A comparison of molecular and enzyme-based assays for the detection of thiopurine methyltransferase mutations

S-Methylation by thiopurine methyltransferase (TPMT) is an important route of metabolism for the thiopurine drugs. About one in 300 individuals are homozygous for a TPMT mutation associated with very low enzyme activity and severe myelosuppression if treated with standard doses of drug. To validate the use of molecular genetic techniques for the detection of TPMT deficiency, we have determined red blood cell TPMT activity in 240 adult blood donors and 55 normal children. Genotype was determined by restriction fragment length analysis of polymerase chain reaction products in a cohort of 79 of the blood donors and five cases of azathioprine-induced myelosupression, and this confirmed a close relationship between genotype and phenotype. In 17 of the 24 cases in which mutations were found, DNA was also available from remission bone marrow. In one of these cases, DNA from the remission marrow sample indicated the presence of a non-mutated allele that had not been seen in the blast DNA sample obtained at presentation. These results indicate that polymerase chain reaction-based assays give reliable and robust results for the detection of TPMT deficiency, but that caution should be exercised in relying exclusively on DNA obtained from lymphoblasts in childhood leukaemia.

Azathioprine or 6-mercaptopurine for inflammatory bowel disease: do risks outweigh benefits?

The treatment of Crohn's disease and ulcerative colitis has evolved and has improved the quality of life of patients afflicted with these disorders. Immune modulators such as azathioprine and 6-mercaptopurine are an important class of medications used for the treatment of patients with inflammatory bowel disease. Controlled studies have demonstrated their efficacy in both induction and maintenance of remission in Crohn's disease, and similarly, for the induction and maintenance of remission in patients with ulcerative colitis. These agents have had an increasing importance in the management of steroid-resistant, steroid-dependent diseases, and fistulizing Crohn's disease. The primary limitations to these agents have been their slow onset of action and their side effect profile. Despite these limitations, these agents have demonstrated efficacy and have become paramount to the management of patients with these incurable potentially disabling disorders. The precise role of azathioprine/6-mercaptopurine, their limitations and their safety are reviewed in this paper.

[The possibilities of sensitized photomutagenesis for the induction of genetic variability in plants]

It was found that combined influence of sensitizers and laser emission on vegetation under some conditions induces marked mutagenesis. The observed synergism was analysed. The possible mechanisms of the phenomenon was considered. A unified approach was proposed to assess efficacy of sensitizer-laser pairs by fragmentation DNA. The recommendations were given for application of combined action of sensitizers and laser emission for experimental mutagenesis.

Modification of cardiac Na(+) current by RWJ 24517 and its enantiomers in guinea pig ventricular myocytes

We examined the effects of the cardiotonic agent RWJ 24517 (Carsatrin, racemate) and its (S)- and (R)-enantiomers on action potential duration, Na(+) current (I(Na)), and delayed rectifier K(+) current (I(K)) of guinea pig ventricular myocytes. RWJ 24517 (0. 1 and 1 microM) prolongation of action potential duration could not be accounted for by suppression of either the rapid (I(Kr)) or slow (I(Ks),) component of I(K), although RWJ 24517 did reduce I(Kr) at concentrations of 1 microM. A more dramatic effect of RWJ 24517 (0.1-1 microM) and the (S)-enantiomer of RWJ 24517 (0.1-3 microM) was an increase in peak I(Na) and slowing of the rate of I(Na) decay, eliciting a large steady-state current. Neither RWJ 24517 nor the (S)-enantiomer affected the fast time constant for I(Na) decay, but both significantly increased the slow time constant, in addition to increasing the proportion of I(Na) decaying at the slow rate. Both agents elicited a use-dependent decrease of peak I(Na) (3-10 microM), which probably resulted from a slowing of both fast and slow rates of recovery from inactivation. In contrast, the (R)-enantiomer of RWJ 24517 did not induce a steady-state component I(Na) or increase peak I(Na) up to 10 microM, but it decreased peak I(Na) at 30 microM. The (R)-enantiomer displayed little use-dependent reduction of I(Na) during trains of repetitive pulses and had no effect on rates of inactivation or recovery from inactivation. These actions of the racemate and the (S)-stereoisomer to slow inactivation and to prolong both Na(+) influx and action potential duration may contribute to the positive inotropic actions of these agents because the resulting accumulation of intracellular Na(+) would increase intracellular Ca(2+) via Na(+)/Ca(2+) exchange.

[Immunosuppressive treatment of ulcerative colitis]

5-aminosalicylic compounds and corticosteroids are standard drugs, which have been used for treatment of ulcerative colitis (UC). Although unusually we experience severe active UC, such as refractory cases to these drugs. Immunosuppressive agents are introduced and effective for these cases. In the treatment for UC, 6-mercaptopurine and azathioprine are effective, but it takes a few months until their optimum effect. So these drugs are not suitable for severe active UC which needs rapid treatment. On the other hand, newer immunosuppressive agents such as cyclosporin, tacrolimus have a rapid onset of action. These drugs are very effective, but have side effects. We will review immunosuppressive treatment strategy of UC.

Delayed cytotoxicity of 6-mercaptopurine is compatible with mitotic death caused by DNA damage due to incorporation of 6-thioguanine into DNA as 6-thioguanine nucleotide

Many protocol studies have shown that low dose 6-mercaptopurine (6MP) in maintenance chemotherapy for childhood acute lymphoblastic leukemia (ALL) can be utilized to cure the disease. Mitotic or reproductive cell death has been recognized after G2 arrest when cells are treated with antitumor agents. The precise mechanism of mode of action of 6MP still remains unclear. We found delayed cytotoxic effect of 6MP in P388 murine leukemic cells. Morphological study showed that 6MP induced delayed death was characterized by an enlargement of cell size and multinucleated nuclei. Agarose gel electrophoresis of fragmented DNA from cells treated with 6MP showed the typical ladder pattern. These findings were compatible with mitotic death. Our results make us hypothesize that the delayed cytotoxicity of 6MP is one of the drug induced mitotic deaths caused by DNA damage due to incorporation of 6-thioguanine (6TG) into DNA as thioguanine nucleotide (TGN). Mitotic death may be a mechanism for killing the cycling cells from residual leukemic cells in G0 or long G1 phases in the treatment of childhood ALL.

Purine analogue 6-methylmercaptopurine riboside inhibits early and late phases of the angiogenesis process

Angiogenesis has been identified as an important target for antineoplastic therapy. The use of purine analogue antimetabolites in combination chemotherapy of solid tumors has been proposed. To assess the possibility that selected purine analogues may affect tumor neovascularization, 6-methylmercaptopurine riboside (6-MMPR), 6-methylmercaptopurine, 2-aminopurine, and adenosine were evaluated for the capacity to inhibit angiogenesis in vitro and in vivo. 6-MMPR inhibited fibroblast growth factor-2 (FGF2)-induced proliferation and delayed the repair of mechanically wounded monolayer in endothelial GM 7373 cell cultures. 6-MMPR also inhibited the formation of solid sprouts within fibrin gel by FGF2-treated murine brain microvascular endothelial cells and the formation of capillary-like structures on Matrigel by murine aortic endothelial cells transfected with FGF2 cDNA. 6-MMPR affected FGF2-induced intracellular signaling in murine aortic endothelial cells by inhibiting the phosphorylation of extracellular signal-regulated kinase-2. The other molecules were ineffective in all of the assays. In vivo, 6-MMPR inhibited vascularization in the chick embryo chorioallantoic membrane and prevented blood vessel formation induced by human endometrial adenocarcinoma specimens grafted onto the chorioallantoic membrane. Also, topical administration of 6-MMPR caused the regression of newly formed blood vessels in the rabbit cornea. Thus, 6-MMPR specifically inhibits both the early and the late phases of the angiogenesis process in vitro and exerts a potent anti-angiogenic activity in vivo. These results provide a new rationale for the use of selected purine analogues in combination therapy of solid cancer.

Choice of vincristine or 6-mercaptopurine on the basis of polyamine level in tumor-bearing regions of the brain

The effects of vincristine (VC) and 6-mercaptopurine (6-MP) on body weight, regional weights, and the contents of putrescine, spermidine, and spermine in six regions of the brain were examined in rats that had been given these drugs for 5 consecutive days. VC is recommended for management of tumors in the corpus striatum and/or hippocampus, and cortex although its efficacy is dependent on the doubling time of the tumor cells, whereas 6-MP is recommended for the management of tumors in the cortex, thalamus and/or hypothalamus, and diencephalon. VC and 6-MI are chosen for treatment of the brain tumors because they reduce polyamines which are associated with the reduction of drug-sensitive cells and an inhibition of tumor growth.

Azathioprine and 6-mercaptopurine alter the nucleotide balance in endothelial cells

Graft vascular disease after solid organ transplantation is a main complication that limits long-term survival of the graft. An injury of the endothelium and subsequent vascular response is considered to be responsible for smooth muscle cell hyperplasia with resulting luminal narrowing. What is less certain are the precise steps leading to endothelial injury and subsequent vessel disease. Since the immunosuppressive drug azathioprine is in clinical use due to its antiproliferative effect on lymphocytes, we were interested in how far it exerts effects on the vascular endothelium. Azathioprine and its metabolite 6-mercaptopurine, a potent inhibitor of purine salvage pathway enzymes, dose dependently led to decreased endothelial cell proliferation as well as to decreases in intracellular purine nucleotides adenosine-triphosphate and guanosine-triphosphate. By increasing the formation of the pyrimidine nucleotide uridine-triphosphate within 24 hours, azathioprine and its metabolite altered the endothelial nucleotide balance. Since not only the formation of toxic thio- and methylthiopurines (thio-guanosine-monophosphate, methyl-thio-inosine-monophosphate) was measured, the activity of the enzyme thiopurinemethyltransferase was induced (3.21+/-2.04 U per 10(9) cells, mean+/-SD). These findings indicate that the vascular endothelium plays an active role in the metabolization of the established immunosuppressant azathioprine that then exerts specific toxic effects on endothelial cells.

Ethanol's effect on rat pituitary adrenal axis is prevented by purine metabolic pathway inhibitors

This study investigated the in vitro dose effects of ethanol (EtOH), adenosine (ADO), and urate (URA) on the basal and CRF stimulated ACTH production of pituitary tissue culture (PTC). Furthermore, the effects of low (LE = 0.5 g/kg bodyweight) and high (HE = 2.5 g/kg bodyweight) concentrations of EtOH were tested in rats on plasma ACTH and corticosterone concentration (PCC), with or without the following ADO metabolic pathway inhibitors: 6-mercaptopurine (6MP) (salvage pathway) and purpurogallin (PPG) (xanthine dehydrogenase). EtOH at 0-50 mM does not increase the in vitro basal or CRF (10-7 M) stimulated ACTH secretion in PTC; in fact doses up to 20 mM tended to be inhibitory. ADO significantly increased only basal ACTH secretion whereas URA increased both basal and CRF-stimulated ACTH secretion. Pretreatment with PPG or 6MP + PPG significantly increased both in vivo ACTH and PCC over control values in rats. HE versus LE significantly increased ACTH and PCC in the control (H2O) and 6MP pretreated groups whereas in the PPG pretreated animals, only ACTH was increased significantly by HE. However, combined pretreatment with 6MP + PPG prevented the effect of HE on ACTH and PCC. The current experiment suggests that purine metabolism is involved in ethanol's effect on the hypothalamic-pituitary-adrenal axis.