Retroviral-mediated gene transduction of human alkyltransferase complementary DNA confers nitrosourea resistance to human hematopoietic progenitors

Myelosuppression is the dose-limiting toxicity for nitrosourea chemotherapy due to low levels of the DNA repair protein O6-alkylguanine-DNA alkyltransferase in myeloid precursors. We have shown that high-efficiency myeloproliferative sarcoma virus (vM5MGMT)-mediated transduction of the human MGMT cDNA into murine bone marrow (BM) cells leads to high MGMT expression and increased progenitor resistance to 1,3-bis-(2-chloroethyl) nitrosourea (BCNU) in vitro immediately after infection and after BM transplantation. These experiments were designed to increase MGMT expression in human hematopoietic progenitors. CD34(+) BM cells were isolated over an immunoaffinity column (CEPRATE, CellPro, Inc.), resulting in a mean 66-fold enrichment in clonogenic progenitors (colony-forming unit granulocyte-macrophage + burst-forming unit erythroid + colony-forming unit granulocyte erythroid macrophage = megakaryocyte), with an average progenitor yield of 58 +/- 11.5% and a final population that was 54% CD34(+). Seventy % of progenitors derived from CD34(+) cells were transduced after coculture with AM12-vM5MGMT retroviral producers. vM5MGMT-transduced progenitors were over 2-fold more resistant to concentrations of BCNU between 30 and 50 micrometer than were concurrently LacZ-transduced progenitors (P < 0.003). In vitro selection of transduced, cytokine-stimulated CD34(+) cells with 20 micrometer BCNU resulted in survival of 4.7% of MGMT+ clonogenic progenitors compared to 0.05% of LacZ+ progenitors. These studies indicate that MGMT-transduced human hematopoietic progenitors have increased resistance to nitrosoureas, and in a clinical transplant setting, this strategy may reduce alkylating agent myelosuppression.

Carmustine and streptozocin in refractory melanoma: an attempt at modulation of O-alkylguanine-DNA-alkyltransferase

The activity of the enzyme O6-alkylguanine-DNA-alkytransferase (AGAT) protects cells from the cytotoxic effects of alkylating agents. This Phase II trial was designed to assess the efficacy of a strategy designed to modulate the resistance to carmustine (BCNU) mediated by AGAT using streptozocin (STZ) in patients with advanced refractory melanoma. Seventeen patients who had failed prior chemotherapy were treated with STZ at 500 mg/m2 daily for 4 days with BCNU at 150 mg/m2 on day 3. Peripheral blood lymphocytes for assay of AGAT activity levels were collected prior to therapy and following the third dose of STZ. There were two partial responses in the 15 patients evaluable for response (13%). Most patients received only a single cycle of therapy due to rapidly progressive disease. Two patients developed fatal pulmonary toxicity, and one developed myelodysplasia. Other toxicities included transient rises in liver function tests. AGAT levels decreased by a mean of 53% in 9 patients but actually increased over baseline in 3 patients while on therapy. Based on these data, BCNU and STZ are not an effective combination for the therapy of advanced refractory melanoma, and pulmonary toxicity due to this combination appears to be increased compared with BCNU alone. STZ is not an effective modulator of AGAT activity when given on this schedule. New strategies designed to deplete AGAT activity using O6-benzylguanine or temozolomide should be explored with careful attention to the possibility that this approach may potentiate both the toxicity and efficacy of BCNU.

Determination of O6-benzylguanine in human plasma by reversed-phase high-performance liquid chromatography

A high-performance liquid chromatographic assay for O6-benzylguanine utilizing liquid-liquid extraction and reversed-phase chromatography has been developed. Plasma samples were alkalinized, extracted into ethyl acetate, evaporated, and the residues were reconstituted and chromatographed. Separation was accomplished by gradient elution with a mobile phase of methanol, acetonitrile, and phosphate buffer, pH 3.2. Eluted compounds were detected spectrophotometrically at 280 nm. Sample quantitation was obtained from the regression line of six-point standard curves ranging from 25 to 400 ng/ml. O6-Benzylguanine peak heights were compared to peak heights of O6-(p-chlorobenzyl)guanine (internal standard). The average regression coefficient was 0.999 (n = 4). High concentration (305 ng/ml) and low concentration (38 ng/ml) quality control samples were determined with a day-to-day relative standard deviation of 7 and 8%, respectively (n = 18). The within-day relative standard deviations were 2.7 and 3.0% (n = 18) for the high and low concentration quality control specimens, respectively. Sample quantitation was reliable to 25 ng/ml with a signal-to-noise ratio of 8:1. This method was applied to plasma samples obtained from patients in a clinical trial of O6-benzylguanine.

An unusual case of autoimmune hemolytic anemia with reticulocytopenia, erythroid dysplasia, and an IgG2 autoanti-U

BACKGROUND

Autoantibodies with anti-U specificity, usually in combination with autoantibodies of other specificities, have occasionally been identified in association with autoimmune hemolytic anemia. A case of life-threatening autoimmune hemolytic anemia, characterized by several atypical features, including apparent intravascular hemolysis associated with an IgG2 anti-U, reticulocytopenia, and bone marrow dyserythropoiesis is described.

CASE REPORT

A 36-year-old man with a severe case of acute-onset autoimmune hemolytic anemia was admitted to another hospital; he had a hematocrit of 15 percent, elevated bilirubin and lactate dehydrogenase, and positive direct and indirect antiglobulin tests. He received 7 units of incompatible red cells without improvement in hematocrit, and he was transferred to University Hospitals of Cleveland (OH). He was jaundiced and became syncopal in the sitting position. His serum was reddish pink; he had a hematocrit of 11.8 percent and a reticulocyte count of 2.5 percent. No spherocytes were observed in the peripheral blood smear. Shortly after admission, the hematocrit fell to 6.9 percent. He was given 3 units of "least-incompatible" red cells and was started on prednisone, with little improvement. An IgG2 autoanti-U was detected in his serum. Seven units of U- red cells were transfused over the next 4 days. The hematocrit improved to 23 percent and continued to rise without further transfusion. A bone marrow examination, initially revealing erythroid hyperplasia accompanied by dyserythropoiesis, became morphologically normal. Drug studies failed to show evidence of drug-related hemolysis. He remains well 2 years after discharge without evidence of recurrent hemolysis.

CONCLUSION

Severe life-threatening autoimmune hemolytic anemia, in this instance induced by an autoanti-U, may be associated with IgG2 autoantibody and characterized by apparent intravascular hemolysis and bone marrow dyserythropoiesis. Early treatment with U- blood, in addition to steroids, may be beneficial.

Differential sensitivity of human and mouse alkyltransferase to O6-benzylguanine using a transgenic model

O6-benzlguanine (O6-bG) potentiates nitrosourea cytotoxicity of human tumor xenografts in nude mice by inactivating O6-alkylguanine-DNA alkyltransferase (AGT). Recent reports dispute whether murine AGT, in cell-free systems, is less sensitive to O6-bG than the human AGT protein, raising the possibility that efficacy seen in the mouse host may not predict the therapeutic index observed in clinical trials. To establish whether mouse and human AGT have different sensitivity to O(6)-bG, we evaluated in vitro and in vivo models of O(6)-methylguanine-DNA methyltransferase gene (MGMT) expression in the same genetic background. The 50% inhibitory concentration of O6-bG for inactivation of mouse AGT was >10-fold higher than for the human protein in MGMT-transfected Chinese hamster ovary (CHO) cells. A dose of O6-bG, which inactivated human AGT, markedly sensitized human MGMT-transfected CHO cells to 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU), whereas mouse MGMT-transfected CHO cells were much more resistant. O6-bG inactivation of AGT in vivo was studied in livers of human MGMT-transgenic mice expressing both human and mouse AGT. After a single dose of O6-bG i.p., the 50% inhibitory concentration of AGT was higher for mouse than for human AGT. To reconcile our finding with those of others, we sequenced the mouse MGMT cDNA and found that mutation of amino acid residue Leu180 was associated with O6-bG resistance. These studies provide strong evidence that inactivation of AGT both in vivo and in vitro by O6-bG is species selective and impacts O6-bG-mediated enhancement of BCNU toxicity. This may influence the therapeutic index of O6-bG-BCNU combinations observed in human tumor xenograft-bearing mice.

Quantitative immunohistochemical estimates of O6-alkylguanine-DNA alkyltransferase expression in normal and malignant human colon

A major mechanism of resistance to nitrosoureas is O6-alkylguanine-DNA-alkyltransferase. The alkyltransferase biochemical assay measures mean tissue activity but requires availability of fresh tissue and cannot assess tumor heterogeneity, an important component of tumor resistance to alkylating agents. We assessed the levels of alkyltransferase in human colon carcinoma and normal colon by biochemical assay, Western blot, conventional immunohistochemistry, and quantitative immunohistochemistry (using 5H7 and mT3.1 monoclonal IgGs) to correlate whole tissue levels with cell-specific expression. Alkyltransferase activity was 18.0 +/- 4.6 fmol/microgram DNA in normal colon and 15.0 +/- 6.5 fmol/microgram DNA in tumors. By Western blot estimates, alkyltransferase in normal colon was 14.8 +/- 4.2 fmol/microgram DNA and in tumors was 16.2 +/- 7.8 fmol/microgram DNA. Alkyltransferase estimates by biochemical and Western blots were correlated strongly (P < 0.0001). Conventional immunohistochemistry demonstrated that alkyltransferase was predominantly nuclear and in normal colon was concentrated in glandular epithelial mucosal cells close to the lumen, whereas in tumors, expression was heterogenous but localized to malignant epithelial cells. Two parameters of quantitative immunohistochemistry, integrated gray and mean gray, were correlated strongly with each other (P < 0.002) and with biochemical and Western blot estimates (P = 0.004-0.04). Thus, quantitative immunohistochemical estimates of alkyltransferase in fixed tissues are a reasonable alternative to biochemical analysis and have an added advantage of identifying heterogeneity of alkyltransferase expression in tumors.

Transfer of drug resistance genes into hematopoietic progenitors to improve chemotherapy tolerance

A number of drug resistance genes have been identified that may be useful in gene therapy approaches to ameliorate chemotherapy toxicity. Hematopoietic tissue is the most suitable target for drug resistance gene therapy because myelosuppression is the dose-limiting toxicity of the many chemotherapeutic agents. Recent studies have shown that murine and human hematopoietic progenitors can be transduced ex vivo using retroviral vectors to overexpress P-glycoprotein, dihydrofolate reductase, and O6-alkylguanine DNA alkyltransferase. In all instances, gene transfer results in significant drug resistance in hematopoietic progenitors both in vitro and in vivo. Clinical trials are underway to evaluate the role of MDR-1 gene therapy in amelioration of chemotherapy induced myelosuppression. Other genes being examined for their potential to transfer drug resistance to hematopoietic cells include genes encoding aldehyde dehydrogenase, nucleotide excision repair proteins, multidrug resistant protein, and superoxide dismutase. As a group these proteins could confer significant levels of chemotherapy drug resistance to bone marrow cells. When compared with other somatic gene therapy approaches, drug resistance gene therapy has the aim of protecting normal cells and preventing toxicity. In addition many of these genes could be used to select for cells carrying the drug resistance gene as well as cotransduced therapeutic gene. Thus, gene transfer of drug resistance genes will have broad applications in the field of gene therapy as well as in protecting hematopoietic cells from chemotherapy toxicity.

Thymidine kinase deficient cells with decreased TTP pools are hypersensitive to DNA alkylating agents

The effect of mutational loss of thymidine kinase (TK) on the sensitivity to alkylating agents was investigated in promyelocytic, HL-60, and T-lymphoblastoid, Molt-3, human leukemia cell lines. Although both cell lines exhibited approx. 1% residual TK activity, only HL-60 TK deficient cells had a decreased intracellular TTP pool, i.e., 20% of that of the wild-type. When treated with N-methyl-N'-nitronitrosoguanidine or ethyl methanesulfonate, HL-60 TK deficient cells showed significantly increased killing and mutation frequencies at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus relative than did wild-type. Pretreatment of cells with O6-benzylguanine, an inhibitor of O6-alkylguanine-DNA alkyltransferase, partially abolished those differences. Molt-3 wild-type and TK deficient cells had similar cell survivals and HGPRT mutation frequencies following treatment with alkylating agents. These results indicate that TK deficiency, only when a concomitant decrease of TTP pool is detected, plays a pivotal role in the sensitivity to the cytotoxic and mutagenic effects of alkylating agents.

Phase I/II study of combined granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor administration for the mobilization of hematopoietic progenitor cells

PURPOSE

To study the toxicity and efficacy of combined granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) administration for mobilization of hematopoietic progenitor cells (HPCs).

MATERIALS AND METHODS

Cohorts of a minimum of five patients each were treated subcutaneously as follows: G-CSF 5 micrograms/kg on days 1 to 12 and GM-CSF at .5, 1, or 5 micrograms/kg on days 7 to 12 (cohorts 1, 2, and 3); GM-CSF 5 micrograms/kg on days 1 to 12 and G-CSF 5 micrograms/kg on days 7 to 12 (cohort 4); and G-CSF and GM-CSF 5 micrograms/kg each on days 1 to 12 (cohort 5). Ten-liter aphereses were performed on days 1 (baseline, pre-CSF), 5, 7, 11, and 13. Colony assays for granulocyte-macrophage colony-forming units (CFU-GM) and erythroid burst-forming units (BFU-E) were performed on each harvest.

RESULTS

The principal toxicities were myalgias, bone pain, fever, nausea, and mild thrombocytopenia, but none was dose-limiting. Four days of treatment with either G-CSF or GM-CSF resulted in dramatic and sustained increases in the numbers of CFU-GM per kilogram collected per harvest that represented 35.6 +/- 8.9- and 33.7 +/- 13.0-fold increases over baseline, respectively. This increment was attributable both to increased numbers of mononuclear cells collected per 10-L apheresis and to increased concentrations of progenitors within each collection. The administration of G-CSF to patients already receiving GM-CSF (cohort 4) caused the HPC content to surge to nearly 80-fold the baseline (P = .024); the reverse sequence, ie, the addition of GM-CSF to G-CSF, was less effective. The CFU-GM content of the baseline aphereses correlated with the maximal mobilization achieved (r = .74, P = .001).

CONCLUSION

Combined G-CSF and GM-CSF administration effectively and predictably mobilizes HPCs and facilitates apheresis.

Role of DNA repair in resistance to drugs that alkylate O6 of guanine

The mechanism of cytotoxicity of a number of chemotherapeutic agents involves alkylation at the O6 position of guanine, a site that strongly influences cytotoxicity. Repair of these lesions by the alkyltransferase protects from cytotoxicity and is a major mechanism of resistance to these agents. O6-benzylguanine inhibition of alkyltransferase sensitizes tumor cells, and clinical trials are underway to determine its efficacy. The use of gene therapy to enhance the expression of alkyltransferase in hematopoietic cells may prevent dose-limiting myelosuppression and may enhance the utility of this class of chemotherapeutic agents.

Plasma and cerebrospinal fluid pharmacokinetics of O6-benzylguanine and time course of peripheral blood mononuclear cell O6-methylguanine-DNA methyltransferase inhibition in the nonhuman primate

O6-Benzylguanine (O6BG) enhances the cytotoxicity of the nitrosoureas by irreversibly binding and inhibiting the DNA repair enzyme O6-methyl-guanine-DNA methyltransferase (MGMT). The plasma and cerebrospinal fluid (CSF) pharmacokinetics of O6BG and its active metabolite, O6-benzyl-8-oxoguanine, were studied in a nonhuman primate model after 200 mg/m2 had been injected i.v. The parent drug and the metabolite were measured with a reverse-phase HPLC assay. A pharmacokinetic model incorporating separate compartments for O6BG and the O6-benzyl-8-oxoguanine metabolite, first-order conversion of O6BG to the metabolite, and additional first-order elimination rate constants for each compound, was simultaneously fitted to the parent drug and metabolite plasma concentration time data. Elimination of O6BG from plasma was rapid; it had a half-life of 1.6 h and a clearance of 68 ml/min/m2. On the basis of the pharmacokinetic model, essentially all of the O6BG was converted to O6-benzyl-8-oxoguanine. The plasma pharmacokinetic profile of the metabolite differed considerably from that the parent drug. The half-life (14 h) was 10-fold longer and the area under the curve (2420 microM/h) was 11-fold higher than that of O6BG (212 microM/h). The clearance rate of O6-benzyl-8-oxoguanine was 6.4 ml/min/m2. The CSF:plasma ratio was 4.3% for O6BG and 36% for O6-benzyl-8-oxoguanine, and the metabolite area under the curve was 90-fold higher than that of O6BG in CSF. The excellent CSF penetration of the active metabolite provides a rationale for the use of O6BG as a chemosensitizing agent for brain tumors. We also studied the duration of MGMT inhibition in peripheral blood mononuclear cells. By 2 h after a 200 mg/m2 dose of O6BG, > 98% of MGMT activity was suppressed, and > 95% suppression of enzyme activity persisted at 18 and 48 h after the dose. By 2 weeks after the treatment, MGMT levels had returned to baseline. Persistent high concentrations of the active metabolite appear to provide a pharmacological explanation for the prolonged suppression of MGMT activity.

Ex vivo expansion and subsequent infusion of human bone marrow-derived stromal progenitor cells (mesenchymal progenitor cells): implications for therapeutic use

We report a phase I trial to determine the feasibility of collection, ex vivo culture-expansion and intravneous infusion of human bone marrow-derived progenitor stromal cells (mesenchymal progenitor cells (MPCs)). Ten milliliter bone marrow samples were obtained from 23 patients with hematologic malignancies in complete remission. Bone marrow mononuclear cells were separated and adherent cells were culture-expanded in vitro for 4-7 weeks. Autologous MPCs were reinfused intravenously and a bone marrow examination repeated 2 weeks later for histologic assessment and in vitro hematopoietic cultures. Patient age ranged from 18 to 68 years and 12 subjects previously had undergone an autologous or syngeneic bone marrow transplant 4-52 months prior to collection of MPCs. A median of 364 x 10(6) nucleated bone marrow cells (range: 103 to 1004 x 10(6)) were used for ex vivo expansion. Median number of MPCs which were obtained after ex vivo culture expansion was 59.0 (range: 1.1 to 347 x 10(6)) representing a median cell doubling of 16,000-fold (13 doublings). Fifteen of 23 patients completed the ex vivo expansion and underwent MPC infusion. Time to infusion of MPCs after collection ranged from 28 to 49 days. Five patients in each of three groups were given 1, 10 and 50 x 10(6) MPCs. No adverse reactions were observed with the infusion of the MPCs. MPCs obtained from cancer patients can be collected, expanded in vitro and infused intravenously without toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of O6-alkylguanine alkyltransferase-directed DNA repair in metastatic colon cancers

PURPOSE

Carmustine (BCNU) resistance has been correlated with tumor expression of the DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AT). It has been shown that streptozotocin will deplete AT activity of human colon cancer cells in vitro and potentiate BCNU cytotoxicity. This clinical trial was conducted to determine whether streptozotocin can be used as a modulator of AT in metastatic colorectal cancers and thereby overcome clinical resistance to BCNU.

PATIENTS AND METHODS

Fifteen patients with fluorouracil-resistant metastatic colon or rectal cancers were treated sequentially with 2 g/m2 of streptozotocin followed 5 1/2 hours later by BCNU. Sequential biopsies of metastases before and after streptozotocin were conducted to determine whether streptozotocin depletes tumor AT. Peripheral-blood mononuclear cells (PBMCs) were evaluated as a surrogate tissue for prediction of baseline AT levels and streptozotocin posttreatment modulation of the AT in metastases.

RESULTS

Streptozotocin treatment led to a 78% (range, 69% to 89%) decrease in the AT levels in colon cancer metastases; however, myelosuppression and hepatic toxicity limited the BCNU dose to 130 mg/m2. A similar decrease in AT levels of PBMCs was found; however, the absolute levels of AT in PBMCs at baseline and following streptozotocin were not predictive of the levels expressed in metastases from the same patient. Despite the decrease in tumor levels of AT, no clinical responses were observed.

CONCLUSION

Streptozotocin decreases but does not fully deplete AT activity in metastatic colorectal cancers and the residual AT level in metastases is sufficient to maintain clinical resistance to BCNU. We have also demonstrated that sequential computed tomography (CT)-directed biopsies of colorectal cancer metastases can be used to evaluate strategies to investigate modulators of AT-directed repair. AT levels of PBMCs do not predict for the AT level or degree of modulation achieved in the metastatic tumor.

Myeloproliferative sarcoma virus directed expression of beta-galactosidase following retroviral transduction of murine hematopoietic cells

The introduction of genetic sequences into hematopoietic stem cells (HSC) has allowed study of HSC proliferation in vivo by proviral-sequence molecular analysis in the DNA of progeny. Analysis of HSC proliferation could be enhanced by development of a retroviral vector that encodes a reporter gene that allows sensitive detection of transduced cells. We developed a recombinant retrovirus vector encoding the reporter gene lacZ under the transcriptional control of the myeloproliferative sarcoma virus long-terminal repeat (LTR). Bone marrow cells from C3H mice were co-cultured on retrovirus producer cell lines and cultured for growth of colony-forming unit granulocyte/macrophage (CFU-GM) and high proliferative potential colony-forming cells (HPP-CFC) in semisolid media or were transplanted into irradiated recipients. In other experiments, recombinant retrovirus was injected in vivo into the liver of developing fetal rat pups, and circulating hematopoietic cells of the postnatal rats were analyzed for evidence of proviral integration and expression of beta-galactosidase. Expression of lacZ was detected in both CFU-GM and HPP-CFC that were cultured immediately following in vitro infection of mouse bone marrow. Beta-galactosidase activity from the retrovirus was also detected in both marrow cells isolated from reconstituted mice 22 weeks following transplantation as well as in blood cells of postnatal rats transduced in utero with the recombinant retrovirus. This strategy may be especially useful for characterizing proliferation of transduced populations of hematopoietic cells and in the development of protocols for somatic gene therapy.

Retroviral transduction and expression of the human alkyltransferase cDNA provides nitrosourea resistance to hematopoietic cells

Myelosuppression is the dose-limiting toxicity for nitrosourea chemotherapy. This toxicity predominantly involves modification of the O6 position of guanine with an alkyl moiety. The enzyme responsible for repair of O6-alkylguanine adducts, O6-alkylguanine-DNA alkyltransferase (alkyltransferase), is expressed at low levels in bone marrow (BM) cells. High alkyltransferase expression prevents the cytotoxicity and carcinogenicity of nitrosoureas in several transgenic and in vitro gene transfer models. We used gene transfer using a novel myeloproliferative sarcoma virus (MPSV) based retrovirus (vM5MGMT) to express the human alkyltransferase cDNA (MGMT) in human and murine hematopoietic cells. Transduced K562 cells had very high levels of alkyltransferase expression and significantly increased resistance to 1,3-bis (2-chloroethyl) nitrosourea (BCNU) as compared with untransduced K562 cells. Primary murine BM progenitors showed a high transduction efficiency with vM5MGMT and have increased BCNU resistance in vitro. After BM transplantation with vM5MGMT-transduced BM cells and BCNU treatment of these mice, BM, spleen and thymus had a 10- to 40-fold increase in alkyltransferase expression that persisted for at least 23 weeks posttransplantation. Progenitor cells procured from mice expressing high levels of alkyltransferase also had increased resistance to BCNU. Thus, an MPSV-based retroviral vector transduces mouse and human hematopoietic cells at high efficiency and results in high levels of gene expression both in vitro and in vivo. Overexpression of the alkyltransferase protein may protect hematopoietic progenitors from nitrosourea-induced myelosuppression.

The immature thymocyte is protected from N-methylnitrosourea-induced lymphoma by the human MGMT-CD2 transgene

N-methylnitrosourea (MNU) induces thymic lymphoma in a high proportion of susceptible C57BL/6xSJL (C57/SJL) mice. Expression of the human DNA repair gene, MGMT cDNA, which encodes O6-methylguanine-DNA-methyltransferase, in transgenic mice effectively prevents MNU-induced thymic lymphomas. In this study, we determined the phenotype of thymocytes expressing the transgene and defined whether the target cell population for MNU induced lymphomas were actually those that expressed the transgene. Transgene expression was characterized by in situ hybridization for MGMT mRNA and immunohistochemistry for the human alkyltransferase protein and was compared to the phenotype of the MNU induced lymphomas. The MGMT transgene was expressed uniformly in immature cortical thymocytes that were CD4+CD8+J11d+ and to a lesser extent in the medullary thymocyte. Lymphomas were induced by single [50 or 80 mg/kg] or multiple doses [30 mg/kg x 5] of MNU to evaluate the dose response of tumor induction and protection by the MGMT-CD2 transgene. Forty-seven of the 108 treated mice developed lymphomas: 38 of 58 nontransgenic and 9 of 50 MGMT+ mice. The T-cell phenotype of thymic lymphomas was established by immunohistochemistry and FACS analysis. Most of the lymphomas were J11d+ (98%), 70% of the tumors were CD4+CD8+, 21% were CD4-CD8+, 9% were CD4-CD8-, and none were CD4-CD8-. All lymphomas in MGMT+ transgenic mice were CD4+CD8+. Since the main phenotype of MNU induced lymphomas in these mice, CD4+CD8+J11d+, is also the cell phenotype which expresses the MGMT-CD2 transgene at high levels, it appears that MGMT-induced protection has occurred in the cell target for MNU induced transformation.

Determinants of O6-alkylguanine-DNA alkyltransferase activity in human colon cancer

The DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) repairs cytotoxic DNA O6-alkylguanine adducts induced by the nitrosoureas, triazines, and tetrazines. In this study, we determined whether there was a relationship between alkyltransferase activity in colon cancer and that of adjacent normal mucosa, and whether there were demographic patient characteristics which correlated with alkyltransferase expression in either tissue. Alkyltransferase activity and expression of the alkyltransferase gene, MGMT, were measured in 49 paired primary colon cancer samples and adjacent normal appearing mucosa. Alkyltransferase activity was found in all samples. The mean activity was higher in the tumor than the mucosa (r = 0.374, P < 0. 01), although the low correlation coefficient suggested that multiple factors influence the alkyltransferase activity. MGMT mRNA could also be detected in all samples and was highly correlated with alkyltransferase activity (r = 0.64, P < 0.001). No correlation was found between alkyltransferase activity and age, or gender of the patient, or location of the tumor, although activity tended to be higher in patients with lower stage disease. Thus, alkyltransferase activity is present in most, if not all, colon cancer samples, suggesting that it could play an important role in chemotherapeutic resistance of human colon cancer. Patients with colon cancer would appear to be prime candidates for studies utilizing O6-benzylguanine to deplete alkyltransferase prior to therapy with a nitrosourea, triazine, or tetrazine.

O6-alkylguanine-DNA alkyltransferase. A target for the modulation of drug resistance

The DNA repair protein, O6-alkylguanine DNA alkyltransferase, is a major contributor to the resistance to the nitrosourea, triazine, and tetrazine class of alkylating agents. Many tumor cells and primary tumor samples contain high levels of this protein, although a great deal of heterogeneity exists between and within tumors. Inhibition of the alkyl-transferase by O6-benzylguanine results in significant potentiation of the cytotoxic effects of these chemotherapeutic agents, generating responses in human tumor xenografts that are completely resistant to nitrosoureas alone. These studies may rekindle the interest in the nitrosourea class of alkylating agents and stimulate the search for inhibitors of other mechanisms of chemotherapy resistance.

Transgenic expression of human MGMT protects against azoxymethane-induced aberrant crypt foci and G to A mutations in the K-ras oncogene of mouse colon

Transgenic mice over-expressing MGMT, which codes for the human protein O6-alkylguanine-DNA alkyltransferase, are protected from methylating agent-induced thymic lymphomas. In this study we evaluated the ability of transgenic overexpression of MGMT in the colon to protect mice from the development of azoxymethane(AOM)-induced aberrant crypt foci (ACF) and mutations in K-ras. Colonic alkyltransferase in MGMT+ transgenic mice was > 5-fold higher than in nontransgenics: 10.5 +/- 1.1 vs 2.2 +/- 1.1 fmol/micrograms DNA, P = < 0.0001. Mice received 20 mg AOM/kg i.p. at 6 weeks or 15 mg AOM/kg at 6 and 7 weeks of age, and 8 wks later colons were examined for ACF. A significant protective effect of MGMT was seen in mice given single dose of 20 mg AOM/kg. The incidence of ACF/colon was lower in MGMT+ mice (2.0 +/- 1.2) than in nontransgenic mice (3.9 +/- 1.8, P = 0.02). G to A mutations in codon 12 of K-ras were detected by PCR-RFLP in ACF and in random samples of normal appearing mucosa. The incidence of ACF with mutant K-ras in MGMT transgenic mice (0.6 +/- 0.7/colon) was significantly reduced compared to nontransgenic mice (2.3 +/- 1.7/colon, P = 0.02). We propose that AOM induces at least two overlapping but not identical premalignant lesions (aberrant crypt foci and K-ras mutations) which can be prevented by over-expression of MGMT. Thus, MGMT may protect colonic mucosa from carcinogenesis involving methylating agents such as AOM.

Rapid repair of O6-methylguanine-DNA adducts protects transgenic mice from N-methylnitrosourea-induced thymic lymphomas

The methylating agent N-methylnitrosourea (MNU) is biased, surprisingly, in its carcinogenic potential toward the mouse thymus. Previous studies have shown that single doses of MNU administered to adult mice induced thymic lymphomas in over 80% of mice, while transgenic mice expressing high levels of the human O6-alkylguanine-DNA alkyltransferase gene in the thymus (MGMT-CD2 transgenics) were protected from developing MNU-induced lymphomas. The mechanism of this protection was examined in this report. In nontransgenic mice given a lymphomagenic dose of 80 mg/kg MNU, depletion of thymic alkyltransferase activity occurred within 3 h and remained undetectable for the subsequent 192 h; whereas in MGMT-CD2-transgenic mice, this dose of MNU did not deplete thymic alkyltransferase, and the lowest level of alkyltransferase was still 10-fold higher than the constitutive level of thymic alkyltransferase in nontransgenic mice. Likewise, the level of O6-methylguanine adducts detected in the thymus of nontransgenic mice was 96 pg/micrograms guanine 3 h after MNU compared to only 8 pg/micrograms guanine in transgenic mice. By 18 h, the level of O6-methylguanine in MGMT-CD2-transgenic mice was below 2 pg/micrograms guanine, compared to over 70 pg/micrograms guanine in nontransgenic mice. In contrast, no differences were noted in the liver between groups because the MGMT transgene is not expressed in the liver of this strain of mouse. Our data establish that rapid O6-methylguanine-DNA adduct repair due to enhanced levels of alkyltransferase in MGMT-CD2-transgenic mice blocks the initiation of MNU-induced carcinogenesis.

G-CSF and the management of clozapine-induced agranulocytosis

The agranulocytosis associated with clozapine is, indeed, a serious medical disorder. Patients experience prolonged and profound severe granulocytopenia--often with absolute neutrophil counts of less than 100/cu mm. Patients suffer neutropenic sepsis and often are as sick as patients undergoing induction chemotherapy for lymphoma or leukemia. Thus, it is important to evaluate the state-of-the-art management of such patients and to define the role of growth factors such as granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Early use of G-CSF or GM-CSF can shorten the duration of granulocytopenia from a mean of 16 to 8 days and reduce the morbidity of the disorder. Such intervention can potentially decrease the total cost of agranulocytosis. Further issues under consideration are the early use of hematopoietic growth factors prior to the onset of agranulocytosis and the use of these factors for the outpatient management of this disorder.

Combined depletion of O6-alkylguanine-DNA alkyltransferase and glutathione to modulate nitrosourea resistance in breast cancer

MCF-7 human breast cancer cells possess high levels of O6-alkylguanine-DNA alkyltransferase and moderate levels of glutathione, and are more resistant to chloroethylnitrosoureas (CNUs) than cells with low levels of either molecule. The role of each as a component of CNU resistance was assessed using O6-benzylguanine (O6-bG) or O6-methylguanine (O6-mG) to deplete the alkyltransferase and L-buthionine sulfoxamine (L-BSO) to deplete glutathione. O6-bG and O6-mG potentiated 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) cytotoxicity, resulting in a dose modification factor of 5.4 and 2.3, respectively, which reflected the more potent inhibitory effect of O6-bG. L-BSO alone had little effect on the survival of MCF-7 cells following BCNU exposure, but when combined with O6-mG, BCNU cytotoxicity was additive, yielding a dose modification factor of 3.2. O6-bG or O6-mG and L-BSO acted independently, as neither class of inhibitor affected the other's mechanism of CNU resistance. Furthermore, MCF-7 cells overexpressing GST mu were not more resistant to BCNU than the parent cell line in either the presence or absence of O6-bG or L-BSO. These results indicate that on a relative basis in MCF-7 cells, the alkyltransferase is the cell's first line of defense against CNUs. This suggests that therapeutic trials based on O6-bG-induced biochemical modulation of CNU resistance may increase the efficacy of these chemotherapeutic agents against human malignant cells and that L-BSO may have little additive effect when used with these agents.