DON, CONV and DONV-III. Pharmacologic and toxicologic studies

Therapeutic resurgence of 6-diazo-5-oxo-l-norleucine (DON) through tissue-targeted prodrugs

The recognition that rapidly proliferating cancer cells rely heavily on glutamine for their survival and growth has renewed interest in the development of glutamine antagonists for cancer therapy. Glutamine plays a pivotal role as a carbon source for synthesizing lipids and metabolites through the TCA cycle, as well as a nitrogen source for synthesis of amino acid and nucleotides. Numerous studies have explored the significance of glutamine metabolism in cancer, providing a robust rationale for targeting this metabolic pathway in cancer treatment. The glutamine antagonist 6-diazo-5-oxo-l-norleucine (DON) has been explored as an anticancer therapeutic for nearly six decades. Initial investigations revealed remarkable efficacy in preclinical studies and promising outcomes in early clinical trials. However, further advancement of DON was hindered due to dose-limiting gastrointestinal (GI) toxicities as the GI system is highly dependent on glutamine for regulating growth and repair. In an effort to repurpose DON and mitigate gastrointestinal (GI) toxicity concerns, prodrug strategies were utilized. These strategies aimed to enhance the delivery of DON to specific target tissues, such as tumors and the central nervous system (CNS), while sparing DON delivery to normal tissues, particularly the GI tract. When administered at low daily doses, optimized for metabolic inhibition, these prodrugs exhibit remarkable effectiveness without inducing significant toxicity to normal tissues. This approach holds promise for overcoming past challenges associated with DON, offering an avenue for its successful utilization in cancer treatment.

We're Not "DON" Yet: Optimal Dosing and Prodrug Delivery of 6-Diazo-5-oxo-L-norleucine

The broadly active glutamine antagonist 6-diazo-5-oxo-L-norleucine (DON) has been studied for 60 years as a potential anticancer therapeutic. Clinical studies of DON in the 1950s using low daily doses suggested antitumor activity, but later phase I and II trials of DON given intermittently at high doses were hampered by dose-limiting nausea and vomiting. Further clinical development of DON was abandoned. Recently, the recognition that multiple tumor types are glutamine-dependent has renewed interest in metabolic inhibitors such as DON. Here, we describe the prior experience with DON in humans. Evaluation of past studies suggests that the major impediments to successful clinical use included unacceptable gastrointestinal (GI) toxicities, inappropriate dosing schedules for a metabolic inhibitor, and lack of targeted patient selection. To circumvent GI toxicity, prodrug strategies for DON have been developed to enhance delivery of active compound to tumor tissues, including the CNS. When these prodrugs are administered in a low daily dosing regimen, appropriate for metabolic inhibition, they are robustly effective without significant toxicity. Patients whose tumors have genetic, metabolic, or imaging biomarker evidence of glutamine dependence should be prioritized as candidates for future clinical evaluations of novel DON prodrugs, given either as monotherapy or in rationally directed pharmacologic combinations. Mol Cancer Ther; 17(9); 1824-32. ©2018 AACR.

Targeting Metabolism as a Novel Therapeutic Approach to Autoimmunity, Inflammation, and Transplantation

Immune cell activation and differentiation occurs concurrently with metabolic reprogramming. This ensures that activated cells generate the energy and substrates necessary to perform their specified function. Likewise, the metabolic programs among different cells of the immune system vary. By targeting different metabolic pathways, these differences allow for selective regulation of immune responses. Further, the relative susceptibility of cells to a metabolic inhibitor is dictated by their metabolic demands; cellular selectivity is based on demand. Therefore, where differences exist in metabolic pathways between healthy and pathogenic cells, there is opportunity for selective regulation with agents lacking intrinsic specificity. There are now a host of studies demonstrating how inhibitors of metabolism (e.g., glycolysis, glutamine metabolism, and fatty acid oxidation) can regulate immune responses and treat immune-mediated pathogenesis. In this brief review we detail how inhibitors of metabolism can be employed to regulate immune responses in both autoimmunity and transplantation.

Bioanalysis of 6-diazo-5-oxo-l-norleucine in plasma and brain by ultra-performance liquid chromatography mass spectrometry

Glutamine is an abundant amino acid that plays pivotal roles in cell growth, cell metabolism, and neurotransmission. Dysregulation of glutamine-using pathways has been associated with pathological conditions such as cancer and neurodegenerative diseases. 6-Diazo-5-oxo-l-norleucine (DON) is a reactive glutamine analog that inhibits enzymes affecting glutamine metabolism such as glutaminase, 2-N-amidotransferase, l-asparaginase, and several enzymes involved in pyrimidine and purine de novo synthesis. As a result, DON is actively used in preclinical models of cancer and neurodegenerative disease. Moreover, there have been several clinical trials using DON to treat a variety of cancers. Considerations of dose and exposure are especially important with DON treatment due to its narrow therapeutic window and significant side effects. Consequently, a robust quantification bioassay is of interest. DON is a polar unstable molecule that has made quantification challenging. Here we report on the characterization of a bioanalytical method to quantify DON in tissue samples involving DON derivatization with 3 N HCl in butanol. The derivatized product is lipophilic and stable. Detection of this analyte by mass spectrometry is fast and specific and can be used to quantify DON in plasma and brain tissue with a limit of detection at the low nanomolar level.

ErbB2 activation upregulates glutaminase 1 expression which promotes breast cancer cell proliferation

Active glutamine utilization is critical for tumor cell proliferation. Glutaminolysis represents the first and rate-limiting step of glutamine utilization and is catalyzed by glutaminase (GLS). Activation of ErbB2 is one of the major causes of breast cancers, the second most common cause of death for women in many countries. However, it remains unclear whether ErbB2 signaling affects glutaminase expression in breast cancer cells. In this study, we show that MCF10A-NeuT cell line has higher GLS1 expression at both mRNA and protein levels than its parental line MCF10A, and knockdown of ErbB2 decreases GLS1 expression in MCF10A-NeuT cells. We further show that in these cells, ErbB2-mediated upregulation of GLS1 is not correlated to c-Myc expression. Moreover, activation of neither PI3K-Akt nor MAPK pathway is sufficient to upregulate GLS1 expression. Interestingly, inhibition of NF-κB blocks ErbB2-stimulated GLS1 expression, whereas stimulation of NF-κB is sufficient to enhance GLS1 levels in MCF10A cells, suggesting a PI3K-Akt-independent activation of NF-κB upregulates GLS1 in ErbB2-positive breast cancer cells. Finally, knockdown or inhibition of GLS1 significantly decreased the proliferation of breast cancer cells with high GLS1 levels. Taken together, our data indicate that ErbB2 activation promotes GLS1 expression via a PI3K-Akt-independent NF-κB pathway in breast cancer cells, identifying another oncogenic signaling pathway which stimulates GLS1 expression, and thus promoting glutamine utilization in cancer cells. These findings, if validated by in vivo model, may facilitate the identification of novel biochemical targets for cancer prevention and therapy.

Heat induction of heat shock protein 25 requires cellular glutamine in intestinal epithelial cells

Glutamine is considered a nonessential amino acid; however, it becomes conditionally essential during critical illness when consumption exceeds production. Glutamine may modulate the heat shock/stress response, an important adaptive cellular response for survival. Glutamine increases heat induction of heat shock protein (Hsp) 25 in both intestinal epithelial cells (IEC-18) and mesenchymal NIH/3T3 cells, an effect that is neither glucose nor serum dependent. Neither arginine, histidine, proline, leucine, asparagine, nor tyrosine acts as physiological substitutes for glutamine for heat induction of Hsp25. The lack of effect of these amino acids was not caused by deficient transport, although some amino acids, including glutamate (a major direct metabolite of glutamine), were transported poorly by IEC-18 cells. Glutamate uptake could be augmented in a concentration- and time-dependent manner by increasing either media concentration and/or duration of exposure. Under these conditions, glutamate promoted heat induction of Hsp25, albeit not as efficiently as glutamine. Further evidence for the role of glutamine conversion to glutamate was obtained with the glutaminase inhibitor 6-diazo-5-oxo-l-norleucine (DON), which inhibited the effect of glutamine on heat-induced Hsp25. DON inhibited phosphate-dependent glutaminase by 75% after 3 h, decreasing cell glutamate. Increased glutamine/glutamate conversion to glutathione was not involved, since the glutathione synthesis inhibitor, buthionine sulfoximine, did not block glutamine’s effect on heat induction of Hsp25. A large drop in ATP levels did not appear to account for the diminished Hsp25 induction during glutamine deficiency. In summary, glutamine is an important amino acid, and its requirement for heat-induced Hsp25 supports a role for glutamine supplementation to optimize cellular responses to pathophysiological stress.

Trypanosoma brucei CTP synthetase: a target for the treatment of African sleeping sickness

The drugs in clinical use against African sleeping sickness are toxic, costly, or inefficient. We show that Trypanosoma brucei, which causes this disease, has very low levels of CTP, which are due to a limited capacity for de novo synthesis and the lack of salvage pathways. The CTP synthetase inhibitors 6-diazo-5-oxo-l-norleucine (DON) and alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (acivicin) reduced the parasite CTP levels even further and inhibited trypanosome proliferation in vitro and in T. brucei-infected mice. In mammalian cells, DON mainly inhibits de novo purine biosynthesis, a pathway lacking in trypanosomes. We could rescue DON-treated human and mouse fibroblasts by the addition of the purine base hypoxanthine to the growth medium. For treatment of sleeping sickness, we propose the use of CTP synthetase inhibitors alone or in combination with appropriate nucleosides or bases.

Bovine pancreatic asparagine synthetase explored with substrate analogs and specific monoclonal antibodies

Several substrate analogs were tested for their ability to inhibit bovine pancreatic asparagine synthetase. Of the substrate analogs tested both 6-diazo-5-oxo-L-norleucine (DON) and 5-chloro-4-oxo-L-norvaline (CONV) were shown to inhibit the enzyme strongly. DON inhibited the glutaminase and glutamine-dependent asparagine synthetase activities and CONV inhibited the ammonia-dependent activity as well. Both of these inhibitors appeared to be relatively tight binding since desalting failed to remove the inhibition. The inactivation of bovine pancreatic asparagine synthetase by DON is accompanied by a shift from a 47,000 molecular weight monomer to a 96,000 molecular weight dimer as observed by HPLC gel filtration chromatography. This DON-induced shift is prevented by the presence of the substrate glutamine. A monoclonal antibody known to inhibit specifically the ammonia-dependent and glutamine-dependent asparagine synthetase activities but not glutaminase (monoclonal antibody 2B4) binds to both the monomer and the dimer forms of untreated enzyme, as well as to the dimer form of the DON-inactivated enzyme. On the other hand, a monoclonal antibody known to inhibit specifically the glutaminase and glutamine-dependent activities and not the ammonia-dependent asparagine synthetase (monoclonal antibody 5A6) binds to both forms of untreated enzyme but cannot bind to the DON-inactivated enzyme. These data are used to describe the relation of regions of the active site of asparagine synthetase in relation to antibody binding sites.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Phase I study and clinical pharmacology of 6-diazo-5-oxo-L-norleucine (DON)

The toxicity of the glutamine antagonist 6-diazo-5-oxo-L-norleucine (DON) administered as a 24 hour infusion has been evaluated. Studies of the clinical pharmacology of the drug have also been performed in 3 patients. The limiting toxicity of the drug was acute nausea, vomiting and diarrhea that was dose dependent in its severity and duration. The maximum tolerated dose was 600 mg/m2 over 24 hours. The other major toxicity was thrombocytopenia that was maximal 7-10 days after the completion of the infusion. The drug does not exhibit renal, hepatic or central nervous system toxicity. DON achieves steady state levels during these infusions and is eliminated by first order kinetics when the infusion is completed (t1/2 alpha = 1.81 h). The principal route of excretion is renal. A starting dose of 400 mg/m2 would be acceptable for Phase II studies of this drug administered on this schedule.

The rediscovery of DON (6-diazo-5-oxo-L-norleucine)

DON (6-diazo-5-oxo-L-norleucine) and azotomycin are glutamine antagonists that were tested in human malignancies in the 1950s. Azotomycin demonstrated significant activity in colorectal cancer. DON is probably the active form of azotomycin. Recent impressive results for both of these agents in human tumor xenografts (especially the CX-2 colon tumor) have stimulated renewed clinical interest in DON, the more readily available agent. DON mechanism of action, clinical pharmacology, previous clinical data, and current phase I studies are discussed.