Pharmacodynamics of prolonged treatment with L,S-buthionine sulfoximine

L-buthionine sulfoximine potentiates the antitumor effect of 4-hydroperoxycyclophosphamide when administered locally in a rat glioma model

OBJECTIVE

L-buthionine sulfoximine (BSO) inhibits glutathione synthesis and may modulate tumor resistance to some alkylating agents, but it has not been proven effective in the treatment of intracranial neoplasms. To evaluate this drug for the treatment of brain tumors, we studied the use of BSO for potentiating the antineoplastic effect of 4-hydroxyperoxycyclophosphamide (4-HC) in the rat 9L glioma model.

METHODS

The survival of male Fischer 344 rats with intracranial 9L gliomas was measured after implantation of controlled-release polymers containing one of the following: no drug, BSO, 4-HC, or both BSO and 4-HC. The efficacy of intracranial 4-HC treatment was assessed with and without serial systemic intraperitoneal BSO injections. Tissue glutathione levels were measured in the brains, tumors, and livers of animals treated with intraperitoneal injections or local delivery of BSO.

RESULTS

The median survival of animals treated with intracranial polymers containing 4-HC was 2.3 times greater than that of controls. This survival benefit was doubled by local delivery of BSO. In contrast, systemic BSO therapy did not improve survival time. In animals that were treated systemically, both liver and tumor glutathione levels were significantly lower than they were in control animals. In the locally treated animals, glutathione levels were reduced in the brain tumor but not in the liver.

CONCLUSION

These results demonstrate that local but not systemic delivery of BSO enhances the antineoplastic effect of 4-HC in this rat 9L glioma model. In addition, because local delivery of BSO within the brain did not deplete glutathione levels systemically, this method of treatment may be safer than systemic administration of BSO.

Photodynamic therapy using Photofrin in combination with buthionine sulfoximine (BSO) to treat 9L gliosarcoma in rat brain

BACKGROUND AND OBJECTIVE

The reactive oxygen mechanisms associated with cell damage after photodynamic therapy (PDT) may be exploited to enhance tumor destruction. Pharmacological reduction of glutathione (GSH), an inhibitor of reactive oxygen species, can be induced by administration of buthionine sulfoximine (BSO).

STUDY DESIGN/MATERIALS AND METHODS

BSO was administered in combination with Photofrin as the photosensitizer in order to promote PDT induced cell damage. Photofrin (12.5 mg/kg) or Photofrin with BSO (440 mg/kg) were administered to male Fischer rats (n = 27) containing an intracerebral 9L gliosarcoma or to non tumored rats. Brain tumor or non tumored brain was treated with an optical (632 nm) irradiance of 140 J/cm2. Animals were sacrificed 24 h after PDT and the volume of tissue necrosis was measured. Brain Photofrin concentration was measured in tumor and in non tumor bearing animals administered either Photofrin or Photofrin with BSO. GSH was measured by high pressure liquid chromatography in tumor and homologous non tumor tissue in animals administered BSO or control solution.

RESULTS

The volume of tumor necrosis was significantly greater in animals administered Photofrin and BSO than in animals administered only Photofrin. No differences were detected in non tumored tissue damage between groups. No differences in Photofrin concentration were detected in tumored or nontumored animals between animals administered Photofrin and animals administered Photofrin and BSO. BSO administration preferentially and significantly reduced GSH in tumor compared to non tumor tissue.

CONCLUSIONS

Our data suggest that BSO administration preferentially augments tumor destruction without compromising non tumored tissue.

Cellular pharmacology of the D- and L-enantiomers of beta-5-o-carboranyl-2'-deoxyuridine

The cellular pharmacology of the D- and L-enantiomers of beta-5-o-carboranyl-2'-deoxyuridine (CDU), compounds designed for boron neutron capture therapy (BNCT), were studied using human CEM lymphoblast and U-251 glioblastoma cells, at a physiologically achievable concentration (1 microM). Accumulation of the enantiomers was rapid and indistinguishable, reaching cellular concentrations > 40-fold higher than extracellular levels, with approximately 5% persisting in cells after incubation in fresh medium for more than 2 hr. Uptake was not affected by nucleoside uptake inhibitors, but was inhibited by the purine base uptake inhibitor papaverine.

Expression and purification of human gamma-glutamylcysteine synthetase

gamma-Glutamylcysteine synthetase (gamma-GCS) catalyzes the ATP-dependent ligation of L-glutamate and L-cysteine to form L-gamma-glutamyl-L-cysteine; this is the first and rate-limiting step in glutathione biosynthesis. Inhibitors of gamma-GCS such as buthionine sulfoximine are widely used as tools for elucidating glutathione metabolism in vivo and as pharmacological agents for reversing glutathione-based resistance to chemotherapy and radiation therapy in certain cancers. Although gamma-GCS is readily isolated from rat kidneys, future drug design efforts are better based on structure-activity relationships established with the human enzyme. We report here the coexpression in Escherichia coli BL21(DE3) of the human gamma-GCS catalytic (heavy) subunit and regulatory (light) subunit using pET-3d and pET-9d vectors, respectively. Intracellular assembly of the holoenzyme occurred without difficulty, and levels of expression were acceptable (approximately 32 mg holoenzyme/100 g cells). Recombinant human gamma-GCS was purified to homogeneity in an overall yield of 45% by ammonium sulfate fractionation followed by sequential chromatography on Q-Sepharose ion-exchange, Superdex 200 gel filtration and ATP-affinity resins. Trace amounts of E. coli gamma-GCS were removed by immunoaffinity chromatography. The specific activity of the isolated enzyme was >1500 units/mg, comparable to the best preparations from rat kidney. The Km values for L-glutamate, L-cysteine, L-gamma-aminobutyrate (an L-cysteine surrogate), and ATP are 1.8, 0.1, 1.3, and 0.4 mM, respectively. Recombinant human gamma-GCS, like native rat gamma-GCS, is feedback inhibited by glutathione and is potently inhibited by buthionine sulfoximine and cystamine.

Nuclear factor kappa B dependent induction of gamma glutamylcysteine synthetase by ionizing radiation in T98G human glioblastoma cells

Glioblastoma is one of the most malignant of all neoplasms, and often shows resistance to chemotherapy and radiation therapy. Ionizing radiation activates transcriptional factors, such as nuclear factor kappa-B (NF-kappa B). Previously we found that glutathione (GSH) synthesis is induced by cytokines mediated by NF-kappa B (Urata et al. J. Biol. Chem., 1996). Here, we present direct evidence that NF-kappa B activated by ionizing radiation induces the expression of gamma-glutamylcysteine synthetase (gamma-GCS), the rate limiting enzyme of GSH synthesis, using T98G human glioblastoma cells. T98G cells have approximately 14-times the level of intracellular GSH of NB9 cells, radiation-sensitive neuroblastoma cells. In T98G cells, 30-Gy of ionizing radiation was required for the activation of NF-kappa B on an electrophoretic mobility shift assay and the induction of gamma-GCS mRNA on Northern blots and a nuclear run-on assay. However, when T98G cells were treated with buthionine sulfoximine, 3-Gy of ionizing radiation stimulated the DNA-binding activity of NF-kappa B and the expression of gamma-GCS. We constructed chimeric genes containing various regions of gamma-GCS promoter gene and the coding region for Luciferase. T98G cells transiently transfected with a plasmid containing the gamma-GCS promoter-luciferase construct showed increased luciferase activity when treated with ionizing radiation. The luciferase activity stimulated by ionizing radiation was found in the gamma-GCS promoter containing the NF-kappa B binding site, whereas not in that containing its mutated site. These results suggest that GSH synthesis is upregulated by ionizing radiation mediated by NF-kappa B and a high concentration of GSH in T98G cells causes downregulation of the NF-kappa B-DNA binding activity in response to ionizing radiation. The irresponsiveness of the intracellular signal transduction cascade to irradiation may be a factor in the resistance of T98G cells to radiation therapy.

L-S,R-buthionine sulfoximine: historical development and clinical issues

L-S,R-buthionine sulfoximine (L-S,R BSO) is a potent specific inhibitor of gamma-glutamylcysteine synthetase, the rate-limiting step in glutathione (GSH) biosynthesis. GSH is an important component of tumor drug resistance based on a strong association and recent transfection studies. Depletion of intracellular GSH by BSO significantly enhances the cytotoxicity of many cytotoxic agents, principally alkylating agents and platinating compounds but also irradiation and anthracyclines. Phase I clinical trials of BSO + melphalan (L-PAM)have been carried out and observed little toxicity with BSO alone and increased myelosuppression with BSO + L-PAM. Consistent and profound (< 10% of control) GSH depletion was observed in serial determinations of tumor GSH levels in patients receiving continuous infusion (CI) BSO. Evidence of clinical activity has been observed in patients with alkylating or platinating agent-refractory tumors. Phase II evaluation of CI BSO with L-PAM is in progress.

Phase I study of continuous-infusion L-S,R-buthionine sulfoximine with intravenous melphalan

BACKGROUND

Increased intracellular glutathione has long been associated with tumor cell resistance to various cytotoxic agents. An inhibitor of glutathione biosynthesis, L-S,R-buthionine sulfoximine (BSO), has been shown to enhance the cytotoxicity of chemotherapeutic agents in vitro and in vivo. We performed a phase I study of BSO administered with the anticancer drug melphalan to determine the combination's safety/tolerability and to determine clinically whether BSO produced the desired biochemical end point of glutathione depletion (<10% of pretreatment value).

METHODS

Twenty-one patients with advanced cancers received an initial 30-minute infusion of BSO totaling 3.0 g/m2 and immediately received a continuous infusion of BSO on one of the following schedules: 1) 0.75 g/m2 per hour for 24 hours (four patients); 2) the same dose rate for 48 hours (four patients); 3) the same dose rate for 72 hours (10 patients); or 4) 1.5 g/m2 per hour for 48 hours (three patients). During week 1, the patients received BSO alone; during weeks 2 or 3, they received BSO plus melphalan (15 mg/m2); thereafter, the patients received BSO plus melphalan every 4 weeks. Glutathione concentrations in peripheral blood lymphocytes were determined for all patients; in 10 patients on three of the administration schedules, these measurements were made in multiple sections from tumor biopsy specimens taken before, during, and after continuous-infusion BSO.

RESULTS

Continuous-infusion BSO alone produced minimal toxic effects, although BSO plus melphalan produced occasional severe myelosuppression (grade 4) and frequent low-grade nausea/vomiting (grade 1-2). This treatment also produced consistent, profound glutathione depletion (<10% of pretreatment value). The degree of glutathione depletion in peripheral lymphocytes was considerably less than that observed in tumor sections.

CONCLUSIONS

Continuous-infusion BSO is relatively nontoxic and results in depletion of tumor glutathione.