Acivicin: a new glutamine antagonist in clinical trials

Targeting multidrug resistance-associated protein 1 (MRP1)-expressing cancers: Beyond pharmacological inhibition

Resistance to chemotherapy remains one of the most significant obstacles to successful cancer treatment. While inhibiting drug efflux mediated by ATP-binding cassette (ABC) transporters is a seemingly attractive and logical approach to combat multidrug resistance (MDR), small molecule inhibition of ABC transporters has so far failed to confer clinical benefit, despite considerable efforts by medicinal chemists, biologists, and clinicians. The long-sought treatment to eradicate cancers displaying ABC transporter overexpression may therefore lie within alternative targeting strategies. When aberrantly expressed, the ABC transporter multidrug resistance-associated protein 1 (MRP1, ABCC1) confers MDR, but can also shift cellular redox balance, leaving the cell vulnerable to select agents. Here, we explore the physiological roles of MRP1, the rational for targeting this transporter in cancer, the development of small molecule MRP1 inhibitors, and the most recent developments in alternative therapeutic approaches for targeting cancers with MRP1 overexpression. We discuss approaches that extend beyond simple MRP1 inhibition by exploiting the collateral sensitivity to glutathione depletion and ferroptosis, the rationale for targeting the shared transcriptional regulators of both MRP1 and glutathione biosynthesis, advances in gene silencing, and new molecules that modulate transporter activity to the detriment of the cancer cell. These strategies illustrate promising new approaches to address multidrug resistant disease that extend beyond the simple reversal of MDR and offer exciting routes for further research.

Novel Strategies for Disrupting Cancer-Cell Functions with Mitochondria-Targeted Antitumor Drug-Loaded Nanoformulations

Any variation in normal cellular function results in mitochondrial dysregulation that occurs in several diseases, including cancer. Such processes as oxidative stress, metabolism, signaling, and biogenesis play significant roles in cancer initiation and progression. Due to their central role in cellular metabolism, mitochondria are favorable therapeutic targets for the prevention and treatment of conditions like neurodegenerative diseases, diabetes, and cancer. Subcellular mitochondria-specific theranostic nanoformulations for simultaneous targeting, drug delivery, and imaging of these organelles are of immense interest in cancer therapy. It is a challenging task to cross multiple barriers to target mitochondria in diseased cells. To overcome these multiple barriers, several mitochondriotropic nanoformulations have been engineered for the transportation of mitochondria-specific drugs. These nanoformulations include liposomes, dendrimers, carbon nanotubes, polymeric nanoparticles (NPs), and inorganic NPs. These nanoformulations are made mitochondriotropic by conjugating them with moieties like dequalinium, Mito-Porter, triphenylphosphonium, and Mitochondria-penetrating peptides. Most of these nanoformulations are meticulously tailored to control their size, charge, shape, mitochondriotropic drug loading, and specific cell-membrane interactions. Recently, some novel mitochondria-selective antitumor compounds known as mitocans have shown high toxicity against cancer cells. These selective compounds form vicious oxidative stress and reactive oxygen species cycles within cancer cells and ultimately push them to cell death. Nanoformulations approved by the FDA and EMA for clinical applications in cancer patients include Doxil, NK105, and Abraxane. The novel use of these NPs still faces tremendous challenges and an immense amount of research is needed to understand the proper mechanisms of cancer progression and control by these NPs. Here in this review, we summarize current advancements and novel strategies of delivering different anticancer therapeutic agents to mitochondria with the help of various nanoformulations.

Mitochondria Targeting as an Effective Strategy for Cancer Therapy

Mitochondria are well known for their role in ATP production and biosynthesis of macromolecules. Importantly, increasing experimental evidence points to the roles of mitochondrial bioenergetics, dynamics, and signaling in tumorigenesis. Recent studies have shown that many types of cancer cells, including metastatic tumor cells, therapy-resistant tumor cells, and cancer stem cells, are reliant on mitochondrial respiration, and upregulate oxidative phosphorylation (OXPHOS) activity to fuel tumorigenesis. Mitochondrial metabolism is crucial for tumor proliferation, tumor survival, and metastasis. Mitochondrial OXPHOS dependency of cancer has been shown to underlie the development of resistance to chemotherapy and radiotherapy. Furthermore, recent studies have demonstrated that elevated heme synthesis and uptake leads to intensified mitochondrial respiration and ATP generation, thereby promoting tumorigenic functions in non-small cell lung cancer (NSCLC) cells. Also, lowering heme uptake/synthesis inhibits mitochondrial OXPHOS and effectively reduces oxygen consumption, thereby inhibiting cancer cell proliferation, migration, and tumor growth in NSCLC. Besides metabolic changes, mitochondrial dynamics such as fission and fusion are also altered in cancer cells. These alterations render mitochondria a vulnerable target for cancer therapy. This review summarizes recent advances in the understanding of mitochondrial alterations in cancer cells that contribute to tumorigenesis and the development of drug resistance. It highlights novel approaches involving mitochondria targeting in cancer therapy.

Targeting Metabolism for Cancer Therapy

Metabolic reprogramming contributes to tumor development and introduces metabolic liabilities that can be exploited to treat cancer. Chemotherapies targeting metabolism have been effective cancer treatments for decades, and the success of these therapies demonstrates that a therapeutic window exists to target malignant metabolism. New insights into the differential metabolic dependencies of tumors have provided novel therapeutic strategies to exploit altered metabolism, some of which are being evaluated in preclinical models or clinical trials. Here, we review our current understanding of cancer metabolism and discuss how this might guide treatments targeting the metabolic requirements of tumor cells.

Phosphonate-based irreversible inhibitors of human γ-glutamyl transpeptidase (GGT). GGsTop is a non-toxic and highly selective inhibitor with critical electrostatic interaction with an active-site residue Lys562 for enhanced inhibitory activity

γ-Glutamyl transpeptidase (GGT, EC 2.3.2.2) that catalyzes the hydrolysis and transpeptidation of glutathione and its S-conjugates is involved in a number of physiological and pathological processes through glutathione metabolism and is an attractive pharmaceutical target. We report here the evaluation of a phosphonate-based irreversible inhibitor, 2-amino-4-{[3-(carboxymethyl)phenoxy](methoyl)phosphoryl}butanoic acid (GGsTop) and its analogues as a mechanism-based inhibitor of human GGT. GGsTop is a stable compound, but inactivated the human enzyme significantly faster than the other phosphonates, and importantly did not inhibit a glutamine amidotransferase. The structure-activity relationships, X-ray crystallography with Escherichia coli GGT, sequence alignment and site-directed mutagenesis of human GGT revealed a critical electrostatic interaction between the terminal carboxylate of GGsTop and the active-site residue Lys562 of human GGT for potent inhibition. GGsTop showed no cytotoxicity toward human fibroblasts and hepatic stellate cells up to 1mM. GGsTop serves as a non-toxic, selective and highly potent irreversible GGT inhibitor that could be used for various in vivo as well as in vitro biochemical studies.

Target discovery of acivicin in cancer cells elucidates its mechanism of growth inhibition†Electronic supplementary information (ESI) available: Synthesis, cloning, protein expression, purification and biochemical assays. See DOI: 10.1039/c4sc02339k

Using a chemical proteomic strategy we analyzed the targets of acivicin and provided a mechanistic explanation for its inhibition of cancer cell growth.

Acivicin is a natural product with diverse biological activities. Several decades ago its clinical application in cancer treatment was explored but failed due to unacceptable toxicity. The causes behind the desired and undesired biological effects have never been elucidated and only limited information about acivicin-specific targets is available. In order to elucidate the target spectrum of acivicin in more detail we prepared functionalized derivatives and applied them for activity based proteomic profiling (ABPP) in intact cancer cells. Target deconvolution by quantitative mass spectrometry (MS) revealed a preference for specific aldehyde dehydrogenases. Further in depth target validation confirmed that acivicin inhibits ALDH4A1 activity by binding to the catalytic site. In accordance with this, downregulation of ALDH4A1 by siRNA resulted in a severe inhibition of cell growth and might thus provide an explanation for the cytotoxic effects of acivicin.

Effects of acivicin on growth, mycotoxin production and virulence of phytopathogenic fungi

Acivicin is an inhibitor of γ-glutamyl transpeptidase and glutamine amidotransferase. When grown on a synthetic minimal agar medium, acivicin strongly inhibited the growth of Magnaporthe oryzae and Alternaria brassicicola, and to a lesser extent, Botrytis cinerea. However, only partial or marginal growth inhibition was observed with regard to Fusarium sporotrichioides and Fusarium graminearum. The growth retardation caused by acivicin was significantly alleviated by cultivating the fungus on a nutrient-rich medium. The inhibition of M. oryzae growth caused by 1 μmol l(-1) of acivicin on minimal agar medium was subdued by the addition of specific single amino acids, including His, a branched-chain amino acid (Leu, Ile or Val), an aromatic amino acid (Trp, Tyr or Phe), Met or Gln, at a concentration of 0·4 mmol l(-1). Trichothecene production by F. graminearum in trichothecene-inducing liquid medium was reduced significantly in the presence of acivicin despite its inability to inhibit growth in the trichothecene-inducing liquid medium. Foliar application of conidia in the presence of acivicin reduced the severity of rice blast disease caused by M. oryzae. These results suggest the usefulness of this modified amino acid natural product to mitigate agricultural problems caused by some phytopathogenic fungi. Significance and impact of the study: Fusarium head blight or scab disease and rice blast, caused by Fusarium graminearum and Magnaporthe oryzae, respectively, are major diseases of cereal crops that cause a significant loss of yield and deterioration in the quality of the grain. The present study investigated the effects of acivicin, a glutamine amino acid analog, on the physiology of various phytopathogenic fungi. Application of acivicin to a fungal culture and conidial suspension reduced mycotoxin production by the wheat scab fungus and the severity of rice blast, respectively. These results suggest the possibility that acivicin may serve as a lead compound to develop agricultural chemicals for the control of some plant diseases.

Role of solute carriers in response to anticancer drugs

Membrane transporters play critical roles in moving a variety of anticancer drugs across cancer cell membrane, thereby determining chemotherapy efficacy and/or toxicity. The retention of anticancer drugs in cancer cells is the result of net function of efflux and influx transporters. The ATP-binding cassette (ABC) transporters are mainly the efflux transporters expressing at cancer cells, conferring the chemo-resistance in various malignant tumors, which has been well documented over the past decades. However, the function of influx transporters, in particular the solute carriers (SLC) in cancer cells, has only been recently well recognized to have significant impact on cancer therapy. The SLC transporters not only directly bring anticancer agents into cancer cells but also serve as the uptake mediators of essential nutrients for tumor growth and survival. In this review, we concentrate on the interaction of SLC transporters with anticancer drugs and nutrients, and their impact on chemo-sensitivity or -resistance of cancer cells. The differential expression patterns of SLC transporters between normal and tumor tissues may be well utilized to achieve specific delivery of chemotherapeutic agents.

Structure of Bacillus subtilis γ-glutamyltranspeptidase in complex with acivicin: diversity of the binding mode of a classical and electrophilic active-site-directed glutamate analogue

γ-Glutamyltranspeptidase (GGT) is an enzyme that plays a central role in glutathione metabolism, and acivicin is a classical inhibitor of GGT. Here, the structure of acivicin bound to Bacillus subtilis GGT determined by X-ray crystallography to 1.8 Å resolution is presented, in which it binds to the active site in a similar manner to that in Helicobacter pylori GGT, but in a different binding mode to that in Escherichia coli GGT. In B. subtilis GGT, acivicin is bound covalently through its C3 atom with sp2 hybridization to Thr403 Oγ, the catalytic nucleophile of the enzyme. The results show that acivicin-binding sites are common, but the binding manners and orientations of its five-membered dihydroisoxazole ring are diverse in the binding pockets of GGTs.

Glutathione-analogous peptidyl phosphorus esters as mechanism-based inhibitors of γ-glutamyl transpeptidase for probing cysteinyl-glycine binding site

γ-Glutamyl transpeptidase (GGT) catalyzing the cleavage of γ-glutamyl bond of glutathione and its S-conjugates is involved in a number of physiological and pathological processes through glutathione homeostasis. Defining its Cys-Gly binding site is extremely important not only in defining the physiological function of GGT, but also in designing specific and effective inhibitors for pharmaceutical purposes. Here we report the synthesis and evaluation of a series of glutathione-analogous peptidyl phosphorus esters as mechanism-based inhibitors of human and Escherichia coli GGTs to probe the structural and stereochemical preferences in the Cys-Gly binding site. Both enzymes were inhibited strongly and irreversibly by the peptidyl phosphorus esters with a good leaving group (phenoxide). Human GGT was highly selective for l-aliphatic amino acid such as l-2-aminobutyrate (l-Cys mimic) at the Cys binding site, whereas E. coli GGT significantly preferred l-Phe mimic at this site. The C-terminal Gly and a l-amino acid analogue at the Cys binding site were necessary for inhibition, suggesting that human GGT was highly selective for glutathione (γ-Glu-l-Cys-Gly), whereas E. coli GGT are not selective for glutathione, but still retained the dipeptide (l-AA-Gly) binding site. The diastereoisomers with respect to the chiral phosphorus were separated. Both GGTs were inactivated by only one of the stereoisomers with the same stereochemistry at phosphorus. The strict recognition of phosphorus stereochemistry gave insights into the stereochemical course of the catalyzed reaction. Ion-spray mass analysis of the inhibited E. coli GGT confirmed the formation of a 1:1 covalent adduct with the catalytic subunit (small subunit) with concomitant loss of phenoxide, leaving the peptidyl moiety that presumably occupies the Cys-Gly binding site. The peptidyl phosphonate inhibitors are highly useful as a ligand for X-ray structural analysis of GGT for defining hitherto unidentified Cys-Gly binding site to design specific inhibitors.

Structure-based ligand discovery for the Large-neutral Amino Acid Transporter 1, LAT-1

The Large-neutral Amino Acid Transporter 1 (LAT-1)--a sodium-independent exchanger of amino acids, thyroid hormones, and prescription drugs--is highly expressed in the blood-brain barrier and various types of cancer. LAT-1 plays an important role in cancer development as well as in mediating drug and nutrient delivery across the blood-brain barrier, making it a key drug target. Here, we identify four LAT-1 ligands, including one chemically novel substrate, by comparative modeling, virtual screening, and experimental validation. These results may rationalize the enhanced brain permeability of two drugs, including the anticancer agent acivicin. Finally, two of our hits inhibited proliferation of a cancer cell line by distinct mechanisms, providing useful chemical tools to characterize the role of LAT-1 in cancer metabolism.

Phase II study of acivicin in patients with recurrent high grade astrocytoma

Acivicin, an antimetabolite which inhibits enzymes necessary for glutamine utilization, was administered to 16 patients with recurrent high grade astrocytoma. The dose was 12 mg/m(2)/day intravenously over 30 min, daily for 5 days to be repeated every 3 weeks. All patients had previously received cranial irradiation. There were no objective responders, two patients remained stable, two were not evaluable for response and the other 12 progressed on treatment. The median survival of the patients was 128 days. The major toxicity was reversible neurological toxicity, with episodes of WHO grade 3 symptoms in two patients, grade 2 in 19 and grade 1 in six patients. Non-neurological toxicities were infrequent with two patients with grade 2 vomiting and two patients developing infections on treatment, although no severe myelosuppression occurred. Three patients developed mild rashes. The lack of activity and the neurological toxicity makes a daily for 5 days schedule of acivicin unsuitable for further study in central nervous system tumours.

A new acivicin prodrug designed for tumor-targeted delivery

Acivicin is an antitumor agent known to inhibit cell growth. A new prodrug 9b of acivicin 10 was synthesized, based on a p-hydroxybenzylcarbamate self-immolative spacer capable to release acivicin under esterase activity. The prodrug includes a maleimide-containing arm for linkage with thiol-containing macromolecules such as antibodies. This molecule is intended for the conception of bioconjugates to target an inactive acivicin precursor to tumor cells, when linked to a monoclonal antibody (mAb) which recognizes a tumor-specific antigen. Prodrug cleavage by plasmatic esterases will then restore the acivicin's activity toward tumor cells. We report here the synthesis and the in vitro characteristics of the prodrug. As expected, its inhibitory activity against the gamma-glutamyl transpeptidase (gamma-GT) enzyme and its cytotoxicity towards HL-60 cells were highly reduced compared to the parent drug. The chemical and plasmatic hydrolysis kinetics of the compound was studied by HPLC. The prodrug is stable, being slowly hydrolyzed in pH 7.6 buffer at 37 degrees C with a half-life of 37 h. It is converted into an active acivicin under the effect of pig liver esterase, and its half-life in human plasma is 3 h. These results indicate this compound may be further used as a prodrug-antibody conjugate, to target acivicin to malignant cells.

Phase I clinical trial of continuous infusion cyclopentenyl cytosine

Cyclopentenyl cytosine (CPE-C) is an investigational drug that is active against human solid tumor xenografts. The 5'-triphosphate of CPE-C inhibits CTP synthase, and depletes CTP and dCTP pools. We conducted a phase I clinical trial of CPE-C given as a 24-h continuous i.v. infusion every 3 weeks in 26 adults with solid tumors. The starting dose rate, 1 mg/m2 per h, was selected on the basis of both preclinical studies and pharmacokinetic data from two patients obtained after a test dose of 24 mg/m2 CPE-C as an i.v. bolus. Dose escalation was guided by clinical toxicity. A total of 87 cycles were given, and ten patients received four or more cycles. The mean CPE-C steady-state plasma levels (Cpss) increased linearly from 0.4 microM to 3.1 microM at dose levels ranging from 1 to 5.9 mg/m2 per h (actual body weight); the mean total body clearance was 146 +/- 38 ml/min per m2. CPE-C was eliminated by both renal excretion of intact drug and deamination to cyclopentenyl uracil in an apparent 2:1 ratio. CTP synthase activity in intact bone marrow mononuclear cells was inhibited by 58% to 100% at 22 h compared to matched pretreatment samples at all CPE-C dose levels. When all data were combined, flux through CTP synthase was decreased by 89.6% +/- 3.1% at 22 h (mean +/- SE, n = 16), and remained inhibited by 67.6% +/- 7.7% (n = 10) for at least 24 h post-CPE-C infusion. Granulocyte and platelet toxicities were dose-dependent, and dose-limiting myelosuppression occurred during the initial cycle in two of three patients treated with 5.9 mg/m2 per h. Four of 11 patients (4 of 20 cycles) who received 4.7 mg/m2 per h CPE-C experienced hypotension 24-48 h after completion of the CPE-C infusion during their first (n = 2), third (n = 1) and sixth cycles (n = 1), respectively. Two of these patients died with refractory hypotension despite aggressive hydration and cardiopulmonary resuscitation. One of 12 patients (28 total cycles) treated with 3.5 mg/m2 per h CPE-C experienced orthostatic hypotension during cycle 1, and this patient had a second episode of orthostatic hypotension at a lower dose (3.0 mg/m2 per h). Hypotension was not seen in patients receiving < or = 2.5 mg/m2 per h CPE-C.(ABSTRACT TRUNCATED AT 400 WORDS)

A phase I study of acivicin in refractory pediatric solid tumors. A Pediatric Oncology Group study

Forty-two patients with progressive solid tumors and brain tumors were entered in this Phase I study of the glutamine antagonist acivicin given intravenously over thirty minutes daily for five days. The major toxicities encountered were myelosuppression and central nervous system toxicity (nightmares and somnolence). The maximum tolerated dosage on this schedule was 26 mg/M2 daily for five days. Six patients including three patients with brain tumor had stable disease.

New agents in the treatment of primary brain tumors

A review of single agent trials of cytotoxic agents in adults with high grade gliomas is presented. The rationale for testing these agents in patients with brain tumors was variable and is discussed. The criteria to evaluate responses were also variable ranging from subjective evaluation of clinical improvement with a stable radiographic assessment to the same objective response criteria utilized for solid tumors. Trials of agents specifically designed for brain tumors such as AZQ and spiromustine have been disappointing. There are encouraging results being seen in early trials of newer agents which await confirmation in larger trials but which hold promise for improving the disappointing results seen so far with chemotherapy in primary brain tumors.

Modulation of cytosine arabinoside toxicity by 3-deazauridine in a murine leukemia model

3-Deazauridine (DAUrd), a competitive inhibitor of CTP synthetase, inhibits both RNA and DNA synthesis. Murine leukemia cells resistant to cytosine arabinoside (ara-C) due to a deletion of deoxycytidine kinase are collaterally sensitive to DAUrd, which inhibits the de novo production of CTP and hence results in dCTP depletion. We evaluated DAUrd in combination with the palmitate derivative of ara-C (palmO-ara-C) in mice bearing L1210 leukemia cells with a subpopulation resistant to ara-C. Both simultaneous administration and a sequential schedule of palmO-ara-C at its maximally tolerated dose (MTD), followed by DAUrd treatment, failed to produce a therapeutic gain. We also studied whether non-toxic doses of DAUrd (15-250 mg/kg i.p. at h 0 and 6 on days 4 and 8) could modulate the antileukemic activity of palmO-ara-C (7.5-120 mg/kg i.p. at h 3 on days 4 and 8). The addition of DAUrd produced a modest (but statistically significant) prolongation of life span and a further 2-log10 reduction in tumor burden compared to the same dose of palmO-ara-C alone, and resulted in long-term survivors in five of 30 treated animals. Two-dimensional dose-response analysis of the survival data indicated a positive drug interaction (p less than or equal to 0.01) when the dosage of DAUrd was modeled to reflect an apparent threshold effect. Cyclopentenyl cytosine (CPE-C; 0.625-2.5 mg/kg i.p. at h 0 and 6 on days 4 and 8), a more potent inhibitor of CTP synthetase, was also given with palmO-ara-C. This combination resulted in an additional 2-6 log10 units of cell kill and occasional long-term survivors at palmO-ara-C dosages that alone resulted in no more than 2 log10 units of cell kill and no long-term survivors. However, DAUrd and CPE-C given with palmO-ara-C increased host toxicity, compromising the tolerable dose of palmO-ara-C. Single-agent palmO-ara-C given at its MTD produced a similar reduction in tumor burden and increase in life span compared to the highest palmO-ara-C dose that could be given in combination with either modulator.

Affinity of antineoplastic amino acid drugs for the large neutral amino acid transporter of the blood-brain barrier

The relative affinity of six anticancer amino acid drugs for the neutral amino acid carrier of the blood-brain barrier was examined in rats using an in situ brain perfusion technique. Affinity was evaluated from the concentration-dependent inhibition of L-[14C]-leucine uptake into rat brain during perfusion at tracer leucine concentrations and in the absence of competing amino acids. Of the six drugs tested, five, including melphalan, azaserine, acivicin, 6-diazo-5-oxo-L-norleucine, and buthionine sulfoximine, exhibited only low affinity for the carrier, displaying transport inhibition constants (Ki, concentrations producing 50% inhibition) ranging from 0.09 to 4.7 mM. However, one agent - D,L-2-amino-7-bis[(2-chloroethyl)amino]- 1,2,3,4-tetrahydro-2-naphthoic acid (D,L-NAM) - demonstrated remarkably high affinity for the carrier, showing a Ki value of approximately 0.2 microM. The relative affinity (1/Ki) of D,L-NAM was greater than 100-fold that of the other drugs and greater than 10-fold that of any compound previously tested. As the blood-brain barrier penetrability of most endogenous neutral amino acids is related to their carrier affinity, the results suggest that D,L-NAM may be a promising agent which may show enhanced uptake and distribution to brain tumors.

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.

Insulin and acivicin improve host nutrition and prevent tumor growth during total parenteral nutrition

The effect that a 14-day treatment program of total parenteral nutrition (TPN) combined with the glutamine antimetabolite, acivicin, and anabolic hormone, insulin, has on carcass weight and muscle sparing was investigated in tumor-bearing rats. Although TPN resulted in increased carcass weight gain as compared to chow-fed tumor-bearing rats, no savings in gastrocnemius muscle could be demonstrated. The combination of TPN with daily insulin treatment elicited significant increases in both carcass weight and muscle savings, with no alteration in tumor growth. Although combining acivicin with TPN halted tumor growth and increased carcass weight, the change in carcass weight was less than that observed with the insulin-TPN combination. No muscle savings were observed in the acivicin-TPN-treated rats. Yet when acivicin and insulin were combined with TPN, tumor growth was stopped, carcass weight was gained, and muscle mass was saved. Therefore, these experiments suggest that it is possible to add lean body tissue and stabilize tumor growth in rats that receive TPN through anabolic hormone treatment combined with an inhibitor of tumor metabolism.

Uptake of glutamine antimetabolites 6-diazo-5-oxo-L-norleucine (DON) and acivicin in sensitive and resistant tumor cell lines

The uptake system for 6-diazo-5-oxo-L-norleucine (DON) was studied in mouse P388 leukemia cells. The DON transport system was found to resemble that of another glutamine antimetabolite, Acivicin, in its strong temperature dependence, utilization of the "L" transport system, inhibition by glutamine but not by glutamate, potent inhibition by p-chloromercuribenzene sulfonate, Na+, and only minimal inhibition by various energy poisons. A Km of approximately 70 microM and a Vmax of 3.4 nmoles/10(6) cells/min was calculated for this cell line. The accumulated DON was not metabolized by P388 cells and moderate efflux occurred at 37 degrees C. The DON transport characteristics of a DON-resistant P388 cell line (100 times ID50 of parent line) were similar to those of the DON-sensitive parent line, indicating that altered drug transport may not be involved in development of resistance to this antimetabolite. The finding that an Acivicin-resistant subline of P388 cells which exhibited good transport of DON showed negligible transport of Acivicin suggests different modes of resistance towards the two glutamine antimetabolites.

Phase I-II trial of acivicin in adult acute leukemia

Six patients with relapsed or refractory acute leukemia were treated with 9 mg/m2 or 11 mg/m2 of acivicin daily for seven days in a phase I-II trial. No responses were attained and further dose escalation was prohibited by neurotoxicity in 2 of 3 patients who received 11 mg/m2/day. Although acivicin appears to have limited potential as a single agent, laboratory evaluation of leukemic blasts in one patient revealed cell cycle (S-phase accumulation) and metabolic effects which suggest that acivicin may be effective as a modulator of other antileukemia agents such as cytosine arabinoside.

Reduction of tumor growth following treatment with a glutamine antimetabolite

Assessment of arterial-venous differences across transplanted methylcholanthrene-induced sarcomas in rats revealed significant decreases in plasma concentrations of glutamine, serine and glucose. Treatment with the glutamine antimetabolite, acivicin, significantly reduced tumor weights by 65% at the conclusion of the experiment 34 days after tumor induction. These results suggest that glutamine is an essential metabolic substrate for tumor growth and that blockade of glutamine utilization can inhibit the growth of these transplantable sarcomas.

Phase II trial of acivicin in patients with advanced epithelial ovarian carcinoma. A Gynecologic Oncology Group Study

Twenty-four evaluable patients with epithelial ovarian cancer resistant to primary therapy, or relapsing after response to initial therapy, were treated with acivicin utilizing a daily X 5 schedule. One patient achieved a partial response lasting five months; the remaining 23 patients showed no objective response. Profound and dose-limiting neurological toxicity was seen in 11 patients. Acivicin is inactive in patients with previously treated ovarian cancer and has a poor therapeutic index due to neurologic adverse effects in this patient population. The profound neurotoxicity may be related to prior therapy with cisplatin, to protein binding in ascitic fluid accumulations or to yet undefined parameters.