Experimental allergic encephalomyelitis: the effect of 6-mercaptopurine

1. 6-Mercaptopurine (6-MP) prevents experimental allergic encephalomyelitis (EAE) during the period of drug administration in both rabbits and guinea pigs. The disease is suppressed even when treatment is started as late as the 5th day after antigenic stimulation in guinea pigs and the 12th day in rabbits. 2. After discontinuation of 6-MP treatment, there is a latent period before the disease is noted. The length of this latent period is not modified by the duration of 6-MP treatment. 3. The effect of 6-MP on EAE is not the result of leukopenia, non-specific toxicity and debilitation, anti-inflammatory activity, or mere masking of clinical signs of the disease. It is, rather, the result of 6-MP's specific anti-immunologic activity. 4. The effects of 6-MP on antibody production, delayed hypersensitivity, and EAE are compared. This provides indirect evidence for the importance of circulating antibody in the pathogenesis of EAE. 5. The important considerations in the use of 6-MP are discussed and the possible usefulness of 6-MP in human neurologic diseases is considered.

Clinical implications of thiopurine methyltransferase--optimization of drug dosage and potential drug interactions

Thiopurine methyltransferase (TPMT) is a cytoplasmic enzyme that preferentially catalyzes the S-methylation of aromatic and heterocyclic sulphydryl compounds, such as the thiopurine drugs azathioprine, mercaptopurine, and thioguanine. These drugs form the same terminal metabolites, the thioguanine nucleotides (TGNs). One major influence on thiopurine therapy is the inherited activity of TPMT. TPMT "deficiency" is associated with grossly elevated TGN concentrations and profound toxicity after a short course of thiopurine therapy. Variant alleles producing a functional loss of TPMT activity have now been described. Although all the ethnic groups investigated to date have the same wild-type enzyme, TPMT variant allele frequencies vary. Potentially, TPMT activity can influence a number of compounds that could be coadministered with thiopurine drugs. After a therapeutic dose of aspirin, plasma concentrations of salicylic acid are within the range for TPMT inhibition. Sulfasalazine and its metabolite 5-aminosalicylic acid inhibit TPMT, and concurrent furosemide therapy could influence the S-methylation of thiopurines. In addition, TPMT could interfere with disulfiram treatment in alcoholism. TPMT S-methylates the diethyldithiocarbamate metabolite involved in disulfiram activation.

Metabolic effects of thiopurine derivatives against human CCRF-CEM leukaemia cells

BACKGROUND and aims. To compare the metabolic effects induced by the anticancer drugs, 6-mercaptopurine (6-MP), 6-thioguanine (6-TG) and 6-methylmercaptopurine riboside (MMPR), which may inhibit the de novo biosynthesis of purine nucleotides or be mis-incorporated into DNA or RNA.

METHODS

Leukaemia cells were grown in culture, exposed to a thiopurine and cell extracts were analyzed for NTPs, dNTPs, drug metabolites and P-Rib-PP.

RESULTS

In leukaemia cells, 6-MP was converted to MPR-MP, thio-XMP, thio-GMP, thio-GDP and thio-GTP. Metabolites of 6-TG included thio-XMP, thio-GMP, thio-GDP and thio-GTP, while MMPR-MP was the only major metabolite of MMPR, MMPR (25 microM, 4 h) induced a 16-fold increase in P-Rib-PP and 6-MP (25 microM, 4 h) induced a delayed 5.2-fold increase. MPR-MP, thio-GMP and MMPR-MP are inhibitors of amido phosphoribosyltransferase from leukaemia cells with Ki values of 114 +/- 7.10 microM, 6.20 +/- 2.10 microM and 3.09 +/- 0.30 microM, respectively.

CONCLUSION

The nucleoside-5'-monophosphate derivatives of the 3 thiopurines inhibit amido phosphoribosyltransferase in growing leukaemia cells but there is also an initial inhibition of the further conversion of IMP in the pathway. In growing cells, MMPR acts solely as an inhibitor of de novo purine biosynthesis while 6-TG and to a lesser extent, 6-MP, are converted to significant concentrations of di- and tri-phosphate derivatives which may have other mechanisms of cytotoxicity.

Chemotherapeutic agents used in the treatment of childhood malignancies have direct effects on growth plate chondrocyte proliferation

Short stature is one of the most well recorded long term sequelae for adult survivors of childhood malignancies. It has become increasingly apparent that cytotoxic chemotherapy, as well as craniospinal irradiation, has a major impact on growth, but there are virtually no studies which explore the mechanisms by which these cytotoxic drugs affect growth. We have used an in vitro system to investigate the direct effects of a range of chemotherapeutic agents on the proliferative responses of rat tibial growth plate chondrocytes, both in suspension and monolayer culture. The glucocorticoids and purine anti-metabolites reduced chondrocyte proliferation both in monolayer and suspension cultures and this resulted from an increase in cell doubling times with a concomittant reduction in the numbers of S phase cells. DNA damaging agents (e.g. actinomycin-D) were also able to reduce chondrocyte proliferation, both in monolayer and suspension culture. This, however, was the result of a cell cycle arrest and subsequent cell death. In our studies, methotrexate had no significant effect on the proliferative responses of the chondrocytes either in monolayer or suspension culture. These results indicate direct effects of a range of chemotherapeutic agents on the proliferative responses of growth plate chondrocytes. Both cytostatic and cytotoxic effects were observed although the impact of either the potential loss of cells from the proliferative pool during chondrocyte differentiation, or the reduction in the rate of chondrocyte turnover on long bone growth remains to be elucidated.

Inhibition of DNA methylation in malignant MOLT F4 lymphoblasts by 6-mercaptopurine

Treatment of MOLT F4 lymphoblasts with 6-mercaptopurine (6-MP) resulted in a decrease of ATP and a depletion of S-adenosylmethionine (AdoMet). To investigate whether this might affect the methylation of DNA, we treated MOLT F4 lymphoblasts with increasing concentrations of 6-MP, followed by labeling with [methyl-14C]methionine and [methyl-3H]thymidine. After DNA isolation, we measured the incorporated radioactivity and determined the 14C/3H ratio as a measure for the methylation of newly formed DNA. The 14C/3H ratio was decreased by 17% with 1 mumol/L 6-MP; treatment with increasing concentrations of 6-MP up to 10 mumol/L showed a further decrease to 70%, in comparison with untreated cells. To demonstrate that the methylation of deoxycytidine residues in DNA was reduced, we quantified hydrolyzed DNA by HPLC. The 14C/3H ratio showed a decrease with increasing 6-MP concentrations, indicating that treatment with 6-MP resulted in hypomethylation of DNA.

Augmentation of sinecomitant immunity in mice by gamma-(9H-purine-6-yl)thiomethyl L-glutamate (6-MPG), a water-soluble derivative of 6-mercaptopurine

Effects of the administration of gamma-(9H-purine-6-yl)thiomethyl L-glutamate (6-MPG), a water-soluble derivative of 6-mercaptopurine, on concomitant and sinecomitant immunity against the implanted MethA tumor were studied in BALB/c mice. In the concomitant immunity experiments, mice were intradermally inoculated with 1x10(5) MethA cells at the right inguinal region on day 0. In sinecomitant immunity experiments, mice were similarly inoculated on day -21, and the grown tumor was excised on day -11. Both the tumor-bearing and tumor-ectomized animals were re-inoculated with 3x10(6) MethA cells intradermally at the left inguinal region on day 10. Administration of 6-MPG (100 mg/kg, i.p.) on days 3 through 7 significantly inhibited growth of the re-inoculated tumor in both series of experiments. Cyclophosphamide, adriamycin, mitomycin C and cis-diamminedichloroplatinum (II) had no significant effect on the growth of the re-inoculated tumor in the tumor-ectomized mice. Spleen cells harvested from the 6-MPG-treated tumor-ectomized mice showed a strong tumor-neutralizing activity (Winn assay).

Study on the mechanism of immunopotentiating antitumor effect of 6-MPG, a water-soluble derivative of 6-mercaptopurine

We investigated possible mechanisms of the antitumor action of gamma-(9H-purine-6-yl) thiomethyl L-glutamate (6-MPG), a water-soluble derivative of 6-MP. In the double grafted tumor system, BALB/c mice were inoculated intradermally with 10(6) cells of MethA fibrosarcoma at the right inguinal region on day 0 (the primary tumor) and later with 3 x 10(6) cells at the left on day 10 (the secondary tumor). Intraperitoneal administration of 6-MPG at a dose of 100 mg/kg/day from day 3 through 7 completely prevented growth of the secondary tumor. 6-MPG showed no effect on growth of colon 26 adenocarcinoma cells inoculated in place of the secondary MethA cells (antigen specificity). 6-MPG did not inhibit the secondary MethA growth in the BALB/c (nu/nu) mouse. The inhibitory effect of 6-MPG on the secondary tumor growth was diminished by prior treatment of the primed animals with cyclosporin A and anti-Thy antibody. Spleen cells from the tumor-bearing mice treated with 6-MPG showed a tumor-neutralizing activity (Winn assay). Treatment of the spleen cells with anti-CD8 antibody plus complement diminished the tumor-neutralizing effect but that with anti-CD4 antibody plus complement did not, indicating that CD8-positive cells are responsible for potentiation of the tumor immunity. These results suggest that the antitumor effect of 6-MPG against the secondary tumor is elicited by augmenting tumor specific T-cell production.

Final height and puberty in 40 patients after antileukaemic treatment during childhood

Endocrine dysfunction and damage of the epiphysial growth plates have been reported as late effects of antileukaemic treatment during childhood. It is a common opinion that cranial irradiation (CI) is the most important factor for blunted growth. Accordingly, recent therapeutic strategies in acute lymphoblastic leukaemia (ALL) avoid cranial irradiation. Here we analysed longitudinal data on growth and puberty of 54 children in first complete remission, who were treated with 18 Gy CI or not submitted to radiotherapy. Two chemotherapeutic protocols were compared which were similar during the induction period but differed in the intensity of maintenance therapy. In cranial irradiated patients both in males and females the pubertal growth spurt started at a mean age of 1.2 years (SD: 0.93 years) earlier than controls. Age at diagnosis and age at pubertal growth spurt were significantly correlated (r = 0.35, P = 0.017). Similarly, menarche occurred at a mean age (n = 22) of 12.1 years and was correlated with the age at start of therapy in girls who were treated with 18 Gy CI (r = 0.61, P = 0.01). Adult height was reached spontaneously in 30 patients treated during prepubertal age and in 10 treated shortly before or during puberty. In all prepubertal patients treated for 2-3 years with intensive maintenance therapy blunted growth resulted in a significant loss of -1.85 H-SDS (median, P = 0.0051) compared to height at diagnosis. However, if continuation treatment used only methotrexate and 6-mercaptopurine (i.e. BFM protocol) final height equalled projected adult height, despite 18 Gy CI.

CONCLUSIONS

(1) multiagent chemotherapy is of major impact for growth and puberty; (2) 18 Gy cranial irradiation is below the critical dosage responsible for blunted growth; (3) loss in potential growth might be prevented by current CT strategies; (4) onset of puberty depends on age when antileukaemic therapy is applied.

6-Mercaptopurine decreases the Bcl-2/Bax ratio and induces apoptosis in activated splenic B lymphocytes

6-Mercaptopurine and related purine antimetabolites are used in the treatment of several B cell disorders. These drugs inhibited the proliferation of mature splenic B cells after being triggered with polyclonal mitogens. In addition to the antiproliferative effects, 6-mercaptopurine, 2-mercaptopurine, and aminoguanidine evoked a rapid apoptotic cell death in activated B cells that started at 6 hr after drug treatment and therefore preceded DNA synthesis. Incubation of activated B lymphocytes with 6-mercaptopurine blocked the low but sustained nitric oxide release observed in these cells that contributes to the prevention of apoptotic cell death; the addition of chemical nitric oxide donors significantly antagonized the apoptosis elicited by these drugs. The inhibition of nitric oxide synthesis elicited by mercaptopurines correlated with a decrease in the release of nitric oxide-derived species to the culture medium and in the intracellular levels of cGMP. The ratio between the amounts of Bcl-2 and Bax, two proteins involved in the control of apoptosis in mature B cells, markedly decreased as result of mercaptopurine treatment.

Methotrexate increases red blood cell concentrations of 6-methylmercaptopurine ribonucleotide in rats in vivo

PURPOSE

To elucidate the effect of methotrexate (MTX) on 6-mercaptopurine (6-MP) metabolism in rats.

METHODS

Fourteen rats were given 6-MP 20 mg/kg daily for 7 days. Seven of the rats were also given MTX 20 mg/kg on days 5 and 7. Blood samples were obtained from all rats on days 0.5 and 8, and red blood cell (RBC) lysates were analysed for thiopurine methyltransferase (TPMT) activity and the concentration of methylated 6-MP metabolites [methyl mercaptopurine ribonucleotides (MMPRP)] and 6-thioguanine nucleotides (6-TGN).

RESULTS

The concentration of MMPRP increased 2.4 times from day 5 to day 8 in RBCs from rats given MTX in addition to 6-MP, as against 1.2 times in rats given 6-MP alone (P = 0.003). 6-TGN levels increased and TPMT activity decreased from day 5 to day 8, with no difference between the 6-MP and the 6-MP plus MTX groups.

CONCLUSIONS

Single bolus doses of MTX increase the concentration of MMPRP in rats given daily s.c. doses of 6-MP, with no effect on 6-TGN concentration or TPMT activity.

Induction of cell killing, mutation and umu gene expression by 6-mercaptopurine or 2-thiouracil with UVA irradiation

When Escherichia coli cells were irradiated by UVA in the presence of 6-mercaptopurine (6-MP) or 2-thiouracil (S2Ura), two kinds of repair-deficient strains of recA- and uvrA- were killed more efficiently than the parental wild-type strain having normal repair capacities. In addition, these agents with UVA exposure greatly induced the incidence of mutations in the uvrA- strain as compared with the wild-type strain but not the recA- strain. Furthermore, the induction of expression of umuDC genes was investigated in two Salmonella typhimurium strains, TA1535 and TA1538, carrying a pSK1002 plasmid. In these systems, it is easy to measure beta-galactosidase activities for the induced activities of SOS responses. These agents with UVA exposure also induced expression of the umuDC genes. These results suggest that 6-MP and S2Ura with UVA induce DNA damage which is repairable by the excision repair mechanism.

[Growth inhibitory effects of ubenimex on leukemic cell lines resistant to chemotherapeutic agents]

Ubenimex (Bestatin, Ubx) has been shown to have anti-tumor activity and immuno-modulating activities. Ubx has been used in immuno-therapy in combination with remission maintenance chemotherapy after induction of complete remission for adult acute non-lymphocytic leukemia (ANLL, AML). Daunomycin (DNR), arabinosylcytosine (Ara-C) and 6-mercaptopurine (6-MP) are used for the standard chemotherapy for ANLL. It is, however, believed that emergence of resistant cells to chemotherapy cause minimal residual leukemia resulting in poor prognosis. Ubx has been administered in combination with these chemotherapeutic agents. We examined the combinatorial effect of Ubx with DNR, Ara-C, 6-MP and etoposide on K562 leukemic cell line and the chemotherapeutic agent resistant cells derived from K562 cell line. Ubx showed growth inhibitory effects on these cell lines. A synergistic effect was observed on growth inhibition and with colony formation of parent k562 cell line when DNR and Ubx were used in combination. A combination of Ubx with Ara-C or etoposide showed additional effects on parent cells and other resistant cell lines. The combined growth inhibitory effect of 6-MP and Ubx was stronger than the effect of 6-MP alone. These results show that Ubx has a direct growth inhibitory effect on leukemic cells and additional or synergistic effects are obtained on K562 leukemic cell line and on chemotherapeutic agent resistant cells derived from the K562 cell line when Ubx is used combination with the above chemotherapeutic agents.

Drug-related killings: a case of mistaken identity

Abortive attempts at DNA repair can contribute to the effects of DNA damage inflicted by cytotoxic drugs. DNA methylation damage, 6-thioguanine and cisplatin adducts all owe their cytotoxicity in part to the intervention of DNA mismatch repair.

Induction or suppression of SV40 amplification by genotoxic carcinogens, non-genotoxic carcinogens or tumor promoters

Carcinogens are generally classified into two groups: genotoxic and non-genotoxic. As the final product of genotoxic and non-genotoxic carcinogens is the same, i.e., a clone of genetically altered cells, it could be possible that non-genotoxic carcinogens may yield genotoxic events as a secondary result of cell toxicity having led to mitogenesis/cellular proliferation, or that genetic alterations are induced that are normally neglected in genotoxicity tests. A genetic effect with possible relevance for the ultimate mechanism of carcinogenicity is the amplification of oncogenes. In the present experiments, amplification of SV40-virus DNA in transformed CO631-cells has been measured. In this system, two genotoxic carcinogens (positive control), two non-carcinogens (negative control), and eight non-genotoxic carcinogens have been tested. With the exception of testosterone and BVDU, all substances were tested when given alone. Testosterone and BVDU were tested in combination with either MTX or AMP in the presence or absence of S9-mix. The genotoxic carcinogens TEM and 4-NQO as well as the tumor promoters TPA, coumarin, mezerein and chrysarobin were able to induce SV40 amplification when given alone. The same was for acetamide and thioacetamide. The non-carcinogens acetone and dimethylformamide were ineffective. In case of testosterone, a co-amplification effect was seen in the absence of S9-mix, and an anti-amplification effect in presence of S9-mix. The anti-recombinogen BVDU reduced the SV40 amplification effect of AMP in experiments without addition of S9-mix. In the presence of activating S9-mix, this substance was effective at lower concentrations than without S9-mix, and showed an enhanced anti-amplification effect. Similar effects of testosterone and BVDU had been observed before in respect to stimulation or suppression of recombination. These results can be used as supportive evidence for the assumption that gene amplification is mechanistically related to recombination.

Individualizing therapy with 6-mercaptopurine and 6-thioguanine related to the thiopurine methyltransferase genetic polymorphism

The formation of intracellular thionucleotides are a prerequisite for mercaptopurine (MP) cytotoxicity, and interindividual variations in the inherited level of thiopurine methyltransferase (TPMT) activity regulate their formation. Measurement of pretreatment TPMT activities can identify the TPMT "deficient" patient and, conversely, the individual with very high enzyme activities. The former are at higher risk of acute toxicity and potentially fatal bone marrow failure and the latter of suboptimal treatment. Leukaemic children taking MP therapy who form inadequate amounts of thioguanine nucleotides (TGNs) do not experience drug toxicity and are at an increased risk of disease relapse. When low TGNs are due to very high TPMT activities, thioguanine may be a more appropriate thiopurine. Another cause of inadequate TGN concentrations is partial or noncompliance with oral chemotherapy. Compliance problems can be identified by the measurement of both TGNs and methylated drug metabolites.

Augmentation of tumor immunity by 6-MPG, a water-soluble derivative of 6-mercaptopurine, in mice

Inhibitory effects on some immunological responses and MethA fibrosarcoma in the double grafted tumor system in mice were compared between 6-mercaptopurine (6-MP) and its novel water-soluble derivative, gamma-(9H-purine-6-yl)thiomethyl L-glutamate (6-MPG). The dose-dependent inhibitory effects by 6-MPG on the hemagglutinin response to SRBC, DTH reaction to MBSA, contact sensitivity to oxazolone, GVH response and growth of the primary tumor were 3-10 times weaker than those by 6-MP, probably reflecting the difference in their cytotoxicities antimetabolites. However, the two drugs were nearly equipotent in reproducing inhibition of the secondary tumor growth, which is a host-mediated immunological response to tumor antigen as shown by its dependency on the primary inoculation with 1 x 10(4) or more MethA cells and by the production of anti-tumor splenocytes in tumor-bearing animals (the Winn assay). Thus, 6-MPG may point to the direction of derivatization towards anti-tumor immunopotentiators with an improved therapeutic index.

Clinical chronotolerance to anticancer drugs: relevance for dose-intensity

There is increasing experimental and clinical data to suggest that the timing of chemotherapy may be important for both toxicity and response. The availability of programmable infusion pumps has made the clinical testing and application of circadian scheduling possible. This paper provides an overview regarding the current clinical data with some references to the experimental studies. Further progress in this area is dependent on a better understanding of the mechanisms involved and continued high quality clinical trials.

Augmentation of tumor immunity by 6-mercaptopurine (6-MP) and its analogs in the double grafted tumor system in mice

We investigated the antitumor effect of 6-mercaptopurine (6-MP) and its analogs using the double grafted tumor technique. BALB/c mice were inoculated intradermally with MethA fibrosarcoma cells at the right inguinal region on day 0 (the primary tumor) and at the left on day 10 (the secondary tumor). Intraperitoneal or intra-lesional administration of 6-MP, 6-mercaptopurine riboside (6-MP-r) and 6-mercaptopurine riboside triacetate (6-MPRTA) from day 3 to 7 dose-dependently inhibited growth of the secondary tumor. Without the primary inoculation, 6-MP showed no effect on growth of the tumor inoculated on day 10, indicating that the antitumor effect of 6-MP could not be attributable to its direct antimetabolic or tumoricidal action only, and that the primary tumor inoculation is necessary for these compounds to inhibits growth of the challenging tumor. 6-MP did not inhibit the secondary MethA growth in the BALB/c (nu/nu) mouse. Both CD4+ and CD8+ T cells increased in the spleen of mice treated with 6-MP. Meanwhile, delayed-type hypersensitivity (DTH) reaction to the methylated bovine serum albumin (MBSA) antigen at the footpad was not augmented but inhibited by 6-MP-r and 6-MPRTA in both normal and tumor-bearing mice. Thus, the immunomodulatory activity of 6-MP could be observed in two opposite directions, augmentation of tumor immunity and inhibition of DTH to MBSA. This indicates that the immune mechanism and/or the type of effector cells induced in these two cell-mediated immune systems are different from each other.

Water-soluble antitumor agents. I. Synthesis and biological activity of 6-S-aminoacyloxymethyl mercaptopurine derivatives

In an attempt to improve the effectiveness and bioavailability of 6-mercaptopurine, various kinds of water-soluble analogues, such as 6-S-aminoacyloxymethyl mercaptopurine derivatives (3a--m) and 6-S,9-disubstituted derivates (7a,b and 9a,b), were synthesized. These compounds were evaluated for activity to augment antitumor immunity by using a double grated tumor system. Antitumor activities against solid tumors (sarcoma 180 and colon 26) were also evaluated. Many compounds exhibited potent activities in both test systems. In particular, the aminopropionate derivative (3a) and the L-glutamate derivative (3f) showed significant enhancement of antitumor immunity together with potent antitumor activities.

Microcalorimetric evaluation of the effect of combined chemotherapeutic drugs

A study of the effect of combined antineoplastic drugs in vitro was carried out by microcalorimetric monitoring of the metabolic activity of treated cells. Power-time curves of growing T-lymphoma cell suspensions, treated with single or combined drugs, were recorded. The extent of the effect was evaluated by changes in the slopes of the microcalorimetric curves and the kinetics of the drug action were interpreted from the time at which these changes reached their maximum value. The method was validated using two well-established drug combinations, the potentiatory effect of dipyridamole on methotrexate cytotoxicity, and the synergism between methotrexate and 6-thioguanine. In the first case, where one drug is not toxic, the modulation may be evaluated by comparing the inhibition produced by the toxic drug alone and in combination with its modulator. Otherwise, when both drugs are toxic, the combined effect must be evaluated by means of their combination index. The measurement procedure is simple, the electric signal is well suited to automation of data acquisition and the response may be evaluated within 5 to 6 h of drug administration. Moreover, we demonstrate that microcalorimetry is a reliable method for the detection of modulatory effects in combination chemotherapy.