Role of the semi-quinone free radical of the anti-tumour agent etoposide (VP-16-213) in the inactivation of single- and double-stranded phi X174 DNA

Re-evaluation of Brequinar sodium, a dihydroorotate dehydrogenase inhibitor

DUP-785 (Brequinar sodium) is a potent inhibitor of the mitochondrial dihydroorotate dehydrogenase (DHO-DH), a rate-limiting enzyme in the pyrimidine de novo nucleotide synthesis. In phase I clinical studies at the maximum tolerated dose (MTD) Brequinar induced a long-term inhibition of DHO-DH in white blood cells (WBC) and a long-term depletion of plasma uridine. These two parameters were related to severe myelosuppression, so that in Phase II studies the dose of Brequinar was decreased considerably. We further characterized the mechanism of DHO-DH enzyme inhibition while in blood samples of patients entered into Phase II studies we evaluated DHO-DH inhibition in WBC and plasma uridine depletion. With Electron Spin Resonance it was demonstrated that DHO-DH produced oxygen radical formation, which was inhibited by Brequinar. In the Phase II study depending on the dose (600 to 2000 mg/m²), uridine decreased to 20% (at the highest dose) or to 80-85% (at the middle dose) or did not change, which was associated with inhibition of DHO-DH (1% activity left vs 11 and 24% left). Inhibition of DHO-DH in the tumor of the latter patient was moderate as well (12% activity left). Brequinar was inactive in all tumor types evaluated possibly because of high uridine levels in the tumor. In conclusion, Brequinar was inactive against solid tumors, but DHO-DH inhibition was associated with myeloid toxicity, which may explain its potential for treatment of leukemia or inflammatory diseases.

Design and synthesis of novel isoxazole tethered quinone-amino Acid hybrids

A new series of isoxazole tethered quinone-amino acid hybrids has been designed and synthesized involving 1,3-dipolar cycloaddition reaction followed by an oxidation reaction using cerium ammonium nitrate (CAN). Using this method, for the first time various isoxazole tethered quinone-phenyl alanine and quinone-alanine hybrids were synthesized from simple commercially available 4-bromobenzyl bromide, propargyl bromide, and 2,5-dimethoxybenzaldehyde in good yield.

Oral treatment with etoposide in small cell lung cancer - dilemmas and solutions

Background

Etoposide is a chemotherapeutic agent, widely used for the treatment of various malignancies, including small cell lung cancer (SCLC), an aggressive disease with poor prognosis. Oral etoposide administration exhibits advantages for the quality of life of the patient as well as economic benefits. However, widespread use of oral etoposide is limited by incomplete and variable bioavailability. Variability in bioavailability was observed both within and between patients. This suggests that some patients may experience suboptimal tumor cytotoxicity, whereas other patients may be at risk for excess toxicity.

Conclusions

The article highlights dilemmas as well as solutions regarding oral treatment with etoposide by presenting and analyzing relevant literature data. Numerous studies have shown that bioavailability of etoposide is influenced by genetic, physiological and environmental factors. Several strategies were explored to improve bioavailability and to reduce pharmacokinetic variability of oral etoposide, including desired and undesired drug interactions (e.g. with ketoconazole), development of suitable drug delivery systems, use of more water-soluble prodrug of etoposide, and influence on gastric emptying. In addition to genotype-based dose administration, etoposide is suitable for pharmacokinetically guided dosing, which enables dose adjustments in individual patient.

Further, it is established that oral and intravenous schedules of etoposide in SCLC patients do not result in significant differences in treatment outcome, while results of toxicity are inconclusive. To conclude, the main message of the article is that better prediction of the pharmacokinetics of oral etoposide may encourage its wider use in routine clinical practice.

Carcinogenic effect of an antitumor drug etoposide in laboratory animals

Intravenous injection of antitumor drug etoposide in the maximum tolerated dose was followed by the development of breast cancer in 38.1% female Wistar rats and 6.7% female outbred mice.

Lifespan of etoposide-treated human neutrophils is affected by antioxidant ability of quercetin

Neutropenia is the primary dose-limiting effect of etoposide toxicity resulting in a decreased efficiency of cancer treatment. Hence, the protection of neutrophils has important clinical implications. We investigated whether quercetin, due to its antioxidant properties, is able to modulate the damaging activity of etoposide. DNA damage, evaluated by the comet assay, and apoptosis, determined by FACScan flow cytometry using Annexin/PI, increased with etoposide doses. The intracellular level of reactive oxygen species (ROS) was enhanced in resting neutrophils incubated with etoposide at concentrations up to 25 microM; above this concentration etoposide revealed antioxidant properties. Only in latex-activated neutrophils, i.e. with latex-stimulated respiratory burst was the ROS production inhibited, as assessed by the luminol amplified chemiluminescence. The characteristic electron spin resonance (ESR) signal of etoposide phenoxyl radical, which occurs in the presence of myeloperoxidase, H2O2 and etoposide, was quenched by quercetin in a dose-dependent manner (0.1-0.5 microM). Quercetin also inhibited DNA damage induced by etoposide and enhanced the inhibitory action of etoposide on the ROS formation in neutrophils. However, quercetin (1 microM) lowered early and late apoptosis/necrosis only when apoptosis was induced by 25 microM etoposide; at higher etoposide concentration apoptosis was enhanced. Summing up, antioxidant adjuvant therapy using quercetin can be beneficial in prolonging neutrophils' lifespan in peripheral blood only when etoposide plasma concentration is low.

Interactions Between Antiretrovirals and Antineoplastic Drug Therapy

Despite the established impact of highly active antiretroviral therapy (HAART) in reducing HIV-related morbidity and mortality, malignancy remains an important cause of death. Patients who receive the combination of cancer chemotherapy and HAART may achieve better response rates and higher rates of survival than patients who receive antineoplastic therapy alone. However, the likelihood of drug interactions with combined therapy is high, since protease inhibitors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are substrates and potent inhibitors or inducers of the cytochrome P450 (CYP) system. Since many antineoplastic drugs are also metabolised by the CYP system, coadministration with HAART could result in either drug accumulation and possible toxicity, or decreased efficacy of one or both classes of drugs. Although formal, prospective pharmacokinetic interaction studies are not available in most instances, it is possible to infer the nature of drug interactions based on the metabolic fates of these agents. Paclitaxel and docetaxel are both metabolised by the CYP system, although differences exist in the nature of the isoenzymes involved. Case reports describing adverse consequences of concomitant taxane-antiretroviral therapy exist. Although other confounding factors may have been present, these cases serve as reminders of the vigilant monitoring necessary when taxanes and HAART are coadministered. Similarly, vinca alkaloids are substrates of CYP3A4 and are, thus, vulnerable to PI- or NNRTI-mediated changes in their pharmacokinetics. Interactions with the alkylating agents cyclophosphamide and ifosfamide are complicated as a result of the involvement of the CYP3A4 and CYP2B6 isoenzymes in both the metabolic activation of these drugs and the generation of potentially neurotoxic metabolites. Existing data regarding the metabolic fate of the anthracyclines doxorubicin and daunorubicin suggest that clinically detrimental interactions would not be expected with coadministered HAART. Commonly used endocrine therapies are largely substrates of the CYP system and may, therefore, be amenable to modulation by concomitant HAART. In addition, tamoxifen itself has been associated with reduced concentrations of both anastrozole and letrozole, raising the concern that similar inducing properties may adversely affect the outcome of PI- or NNRTI-based therapy. Similarly, dexamethasone is both a substrate and concentration-dependent inducer of CYP3A4; enhanced corticosteroid pharmacodynamics may result with CYP3A4 inhibitors, while the efficacy of concomitant HAART may be compromised with prolonged dexamethasone coadministration. Since PIs and NNRTIs may also induce or inhibit the expression of P-glycoprotein, the potential for additional interactions to arise via modulation of this transporter also exists. Further research delineating the combined safety and pharmacokinetics of antiretrovirals and antineoplastic therapy is necessary.

Plasma etoposide catechol increases in pediatric patients undergoing multiple-day chemotherapy with etoposide

PURPOSE

The purpose of this research was to determine inter- and intrapatient differences in the pharmacokinetic profiles of etoposide and its genotoxic catechol metabolite during conventional multiple-day dosing of etoposide in pediatric patients.

EXPERIMENTAL DESIGN

Seven pediatric patients with various malignancies received etoposide at a dose of 100 mg/m(2) i.v. over 1 h daily for 5 days. Blood samples were taken at selected time points on days 1 and 5. Plasma and protein-free plasma concentrations of etoposide and etoposide catechol were determined using a validated liquid chromatography/tandem mass spectrometry assay. Pharmacokinetic parameters of both etoposide and etoposide catechol were calculated using the WinSAAM modeling program developed at NIH.

RESULTS

The mean maximum concentration (C(max)) for total (0.262 +/- 0.107 micro g/ml) and free catechol (0.0186 +/- 0.0082 micro g/ml) on day 5 were higher than the mean C(max) for total (0.114 +/- 0.028 micro g/ml) and free catechol (0.0120 +/- 0.0091 micro g/ml) on day 1. The mean area under the plasma concentration-time curve (AUC)(24h) for total (105.4 +/- 49.1 micro g.min/ml) and free catechol (4.89 +/- 2.23 micro g x min/ml) on day 5 were much greater (P < 0.05) than those for total (55.9 +/- 16.1 micro g x min/ml) and free catechol (3.04 +/- 1.04 micro g x min/ml) on day 1. In contrast, the AUC(24h) for etoposide was slightly lower on day 5 than on day 1.

CONCLUSIONS

The C(max) and AUC(24h) for etoposide catechol were significantly higher on day 5 than on day 1. This suggests that metabolism of etoposide to its catechol metabolite increases in pediatric patients receiving multiple-day bolus etoposide infusions. These findings may be relevant to future reduction of the risk of leukemia as a treatment complication, because etoposide and etoposide catechol are both genotoxins.

The oncogene Nup98-HOXA9 induces gene transcription in myeloid cells

The nucleoporin Nup98 gene is frequently rearranged in acute myelogenous leukemia (AML). In most cases this results in fusion of the N terminus of Nup98 to the DNA binding domain of a homeodomain transcription factor. The prototype of these fusions, Nup98-HOXA9, is associated with human AML and induces AML in mouse models. To understand the mechanisms by which Nup98-HOXA9 causes AML, we expressed it in myeloid cells and identified its target genes using high density oligonucleotide microarrays. The analysis was performed in triplicate and was confirmed by quantitative real time PCR. Of the 102 Nup98-HOXA9 target genes identified, 92 were up-regulated, and only 10 were down-regulated, suggesting a transcriptional activation function. A similar analysis of wild-type HOXA9 revealed 13 target genes, 12 of which were up-regulated, and 1 was down-regulated. In contrast, wild-type Nup98 had no effect on gene expression, demonstrating that the HOXA9 DNA binding domain is required for gene regulation. Co-transfection experiments using a luciferase reporter linked to the promoter of one of the Nup98-HOXA9 target genes confirmed up-regulation at the transcriptional level by Nup98-HOXA9 but not by either HOXA9 or Nup98. These data indicate that Nup98-HOXA9 is an aberrant transcription factor whose activity depends on the HOXA9 DNA binding domain but has a stronger and wider transcriptional effect than HOXA9. Several of the genes regulated by Nup98-HOXA9 are associated with increased cell proliferation and survival as well as drug metabolism, providing insights into the pathogenesis and epidemiology of Nup98-HOXA9-induced AML.

Near-precise interchromosomal recombination and functional DNA topoisomerase II cleavage sites at MLL and AF-4 genomic breakpoints in treatment-related acute lymphoblastic leukemia with t(4;11) translocation

We analyzed the der(11) and der(4) genomic breakpoint junctions of a t(4;11) in the leukemia of a patient previously administered etoposide and dactinomycin by molecular and biochemical approaches to gain insights about the translocation mechanism and the relevant drug exposure. The genomic breakpoint junctions were amplified by PCR. Cleavage of DNA substrates containing the normal homologues of the MLL and AF-4 translocation breakpoints was examined in vitro upon incubation with human DNA topoisomerase IIalpha and etoposide, etoposide catechol, etoposide quinone, or dactinomycin. The der(11) and der(4) genomic breakpoint junctions both involved MLL intron 6 and AF-4 intron 3. Recombination was precise at the sequence level except for the overall gain of a single templated nucleotide. The translocation breakpoints in MLL and AF-4 were DNA topoisomerase II cleavage sites. Etoposide and its metabolites, but not dactinomycin, enhanced cleavage at these sites. Assuming that DNA topoisomerase II was the mediator of the breakage, processing of the staggered nicks induced by DNA topoisomerase II, including exonucleolytic deletion and template-directed polymerization, would have been required before ligation of the ends to generate the observed genomic breakpoint junctions. These data are inconsistent with a translocation mechanism involving interchromosomal recombination by simple exchange of DNA topoisomerase II subunits and DNA-strand transfer; however, consistent with reciprocal DNA topoisomerase II cleavage events in MLL and AF-4 in which both breaks became stable, the DNA ends were processed and underwent ligation. Etoposide and/or its metabolites, but not dactinomycin, likely were the relevant exposures in this patient.

Simultaneous determination of etoposide and its catechol metabolite in the plasma of pediatric patients by liquid chromatography/tandem mass spectrometry

The anticancer drug etoposide is associated with leukemias with MLL gene translocations and other translocations as a treatment complication. The genotype of cytochrome P450 3A4 (CYP3A4), which converts etoposide to its catechol metabolite, influences the risk. In order to perform pharmacokinetic studies aimed at further elucidation of the translocation mechanism, we have developed and validated a liquid chromatography/electrospray/tandem mass spectrometry assay for the simultaneous analysis of etoposide and its catechol metabolite in human plasma. The etoposide analog teniposide was used as the internal standard. Liquid chromatography was performed on a YMC ODS-AQ column. Simultaneous determination of etoposide and its catechol metabolite was achieved using a small volume of plasma, so that the method is suitable for pediatric patients. The limits of detection were 200 ng ml(-1) etoposide and 10 ng ml(-1) catechol metabolite in human plasma and 25 ng ml(-1) etoposide and 2.5 ng ml(-1) catechol metabolite in protein-free plasma, respectively. Acceptable precision and accuracy were obtained for concentrations in the calibration curve ranges 0.2--100 microg ml(-1) etoposide and 10--5000 ng ml(-1) catechol metabolite in human plasma. Acceptable precision and accuracy for protein-free human plasma in the range 25--15 000 ng ml(-1) etoposide and 2.5--1500 ng ml(-1) etoposide catechol were also achieved. This method was selective and sensitive enough for the simultaneous quantitation of etoposide and its catechol as a total and protein-free fraction in small plasma volumes from pediatric cancer patients receiving etoposide chemotherapy. A pharmacokinetic model has been developed for future studies in large populations.

Leukemias related to treatment with DNA topoisomerase II inhibitors

The epipodophyllotoxins etoposide and teniposide and other DNA topoisomerase II inhibitors including anthracyclines and dactinomycin are highly efficacious anticancer drugs. All are associated with a distinct form of leukemia characterized by chromosomal translocations as a treatment complication. Most of the translocations disrupt a breakpoint cluster region (bcr) of the MLL gene at chromosome band 11q23. Other characteristic translocations also may occur. The normal function of the nuclear enzyme DNA topoisomerase II is to catalyze changes in DNA topology between relaxed and supercoiled states by transiently cleaving and re-ligating both strands of the double helix. Anticancer drugs that are DNA topoisomerase II inhibitors are cytotoxic because they form complexes with DNA and DNA topoisomerase II. The complexes decrease the re-ligation rate, disrupt the cleavage-re-ligation equilibrium, and have a net effect of increasing cleavage. The increased cleavage damages the DNA and leads to chromosomal breakage. Cells with irreparable DNA damage die by apoptosis. The association of DNA topoisomerase II inhibitors with leukemia suggests that the drug-induced, DNA topoisomerase II-mediated chromosomal breakage may be relevant to translocations in addition to this anti-neoplastic, cytotoxic action. Epidemiological studies, genomic translocation breakpoint cloning and in vitro DNA topoisomerase II cleavage assays together lead to a model for treatment-related leukemia in which DNA topoisomerase II causes chromosomal breakage and translocations form when the breakage is repaired.

Glutathione S-transferase genotypes in children who develop treatment-related acute myeloid malignancies

Epipodophyllotoxin-associated secondary myeloid leukemia is a devastating complication of acute lymphoblastic leukemia (ALL) therapy. The risk factors for treatment-related myeloid leukemia remain incompletely defined. Genetic deficiencies in glutathione S-transferase (GST) activities have been linked to higher frequencies of a number of human malignancies. Our objective was to determine whether the null genotype for GSTM1, GSTT1, or both, was more frequent in children with ALL who developed treatment-related myeloid malignancies as compared to those who did not. A PCR technique was used to assay for the null genotype for GSTM1 and GSTT1 in 302 children with ALL, 57 of whom also subsequently developed treatment-related acute myeloid leukemia or myelodysplastic syndrome. Among children with ALL who did not develop treatment-related myeloid malignancies, the frequencies of GSTM1 and GSTT1 wild-type, GSTM1 null-GSTT1 wild-type, GSTM1 wild-type-GSTT1 null, and GSTM1 and GSTT1 null genotypes were 40%, 42%, 9% and 9%, respectively. The corresponding frequencies for patients who developed acute myeloid malignancies were 42%, 32%, 11% and 16%, respectively (P = 0.26). A statistically significant increase in the frequency of the GST null genotype was observed in male patients who developed myeloid malignancies as compared to male ALL control patients (P = 0.036), but was not observed in female patients (P = 0.51). Moreover, a logistic regression analysis of possible predictors for myeloid malignancies, controlling for gender and race, did not reveal an association of GSTM1 or GSTT1 null genotypes (P = 0.62 and 0.11, respectively) with treatment-related malignancies. Our data suggest that GSTM1 and GSTT1 null genotypes may not predispose to epipodophyllotoxin-associated myeloid malignancies.

Simultaneous quantitation of etoposide and its catechol metabolite in human plasma using high-performance liquid chromatography with electrochemical detection

Etoposide, a highly active and widely used antineoplastic agent, is O-demethylated to its active catechol metabolite. A high-performance liquid chromatographic assay method for the simultaneous quantitation of etoposide and etoposide catechol in human plasma was established. Etoposide and etoposide catechol were extracted from plasma using chloroform and methanol followed by phase separation, evaporation of the organic phase, and reconstitution of the residue. Chromatography was accomplished using a reversed-phase phenyl analytical column (390 mm x 3.9 mm I.D.) with a mobile phase of 76.6% 25 mM citric acid-50 mM sodium phosphate (pH 2.4)-23.4% acetonitrile pumped isocratically at 1 ml/min with electrochemical detection. The limit of detection for etoposide was 1.2 nM and for etoposide catechol was 0.2 nM. The precision (CV) for etoposide ranged from 0.7 to 3% and for the catechol metabolite from 1 to 6%; accuracy of predicted values ranged from 97 to 106% and 94 to 103%, respectively. The assay was linear from 0.1 to 10 microM for etoposide and from 0.005 to 0.5 microM for etoposide catechol in plasma. Recovery of etoposide and etoposide catechol ranged from 93 to 95% and 90 to 98%, respectively. Stability of etoposide and etoposide catechol in human plasma containing ascorbic acid stored at -70 degrees C for one year was demonstrated. This assay procedure is suitable for evaluation of etoposide and etoposide catechol pharmacokinetics in plasma following etoposide administration.

Association of CYP3A4 genotype with treatment-related leukemia

Epipodophyllotoxins are associated with leukemias characterized by translocations of the MLL gene at chromosome band 11q23 and other translocations. Cytochrome P450 (CYP) 3A metabolizes epipodophyllotoxins and other chemotherapeutic agents. CYP3A metabolism generates epipodophyllotoxin catechol and quinone metabolites, which could damage DNA. There is a polymorphism in the 5' promoter region of the CYP3A4 gene (CYP3A4-V) that might alter the metabolism of anticancer drugs. We examined 99 de novo and 30 treatment-related leukemias with a conformation-sensitive gel electrophoresis assay for the presence of the CYP3A4-V. In all treatment-related cases, there was prior exposure to one or more anticancer drugs metabolized by CYP3A. Nineteen of 99 de novo (19%) and 1 of 30 treatment-related (3%) leukemias carried the CYP3A4-V (P = 0.026; Fisher's Exact Test, FET). Nine of 42 de novo leukemias with MLL gene translocations (21%), and 0 of 22 treatment-related leukemias with MLL gene translocations carried the CYP3A4-V (P = 0. 016, FET). This relationship remained significant when 19 treatment-related leukemias with MLL gene translocations that followed epipodophyllotoxin exposure were compared with the same 42 de novo cases (P = 0.026, FET). These data suggest that individuals with CYP3A4-W genotype may be at increased risk for treatment-related leukemia and that epipodophyllotoxin metabolism by CYP3A4 may contribute to the secondary cancer risk. The CYP3A4-W genotype may increase production of potentially DNA-damaging reactive intermediates. The variant may decrease production of the epipodophyllotoxin catechol metabolite, which is the precursor of the potentially DNA-damaging quinone.

Secondary leukemias induced by topoisomerase-targeted drugs

The major established cause of acute myeloid leukemia (AML) in the young is cancer chemotherapy. There are two forms of treatment-related AML (t-AML). Each form has a de novo counterpart. Alkylating agents cause t-AML characterized by antecedent myelodysplasia, a mean latency period of 5-7 years and complete or partial deletion of chromosome 5 or 7. The risk is related to cumulative alkylating agent dose. Germline NF-1 and p53 gene mutations and the GSTT1 null genotype may increase the risk. Epipodophyllotoxins and other DNA topoisomerase II inhibitors cause leukemias with translocations of the MLL gene at chromosome band 11q23 or, less often, t(8;21), t(3;21), inv(16), t(8;16), t(15;17) or t(9;22). The mean latency period is about 2 years. While most cases are of French-American-British (FAB) M4 or FAB M5 morphology, other FAB AML subtypes, myelodysplastic syndrome (MDS), acute lymphoblastic leukemia (ALL) and chronic myelogenous leukemia (CML) occur. Between 2 and 12% of patients who receive epipodophyllotoxin have developed t-AML. There is no relationship with higher cumulative epipodophyllotoxin dose and genetic predisposition has not been identified, but weekly or twice-weekly schedules and preceding l-asparaginase administration may potentiate the risk. The translocation breakpoints in MLL are heterogeneously distributed within a breakpoint cluster region (bcr) and the MLL gene translocations involve one of many partner genes. DNA topoisomerase II cleavage assays demonstrate a correspondence between DNA topoisomerase II cleavage sites and the translocation breakpoints. DNA topoisomerase II catalyzes transient double-stranded DNA cleavage and rejoining. Epipodophyllotoxins form a complex with the DNA and DNA topoisomerase II, decrease DNA rejoining and cause chromosomal breakage. Furthermore, epipodophyllotoxin metabolism generates reactive oxygen species and hydroxyl radicals that could create abasic sites, potent position-specific enhancers of DNA topoisomerase II cleavage. One proposed mechanism for the translocations entails chromosomal breakage by DNA topoisomerase II and recombination of DNA free ends from different chromosomes through DNA repair. With few exceptions, treatment-related leukemias respond less well to either chemotherapy or bone marrow transplantation than their de novo counterparts, necessitating more innovative treatments, a better mechanistic understanding of the pathogenesis, and strategies for prevention.

Inhibition of the topoisomerase II-DNA cleavable complex by the ortho-quinone derivative of the antitumor drug etoposide (VP-16)

Etoposide (VP-16) is a widely used anticancer drug whose toxicity involves poisoning of topoisomerase II. VP-16 undergoes enzymatic oxido-reductive transformations in cells, resulting in the formation of the ortho-quinone derivative (VPQ) as a major product. The actions of VP-16 and VPQ on purified human topoisomerase II have been compared. Both the parent drug and VPQ are very efficient at trapping the topoisomerase II-DNA cleavable complex, suggesting that methoxy groups on the E-ring are not a prerequisite for activity. Our data also imply that VPQ has more effect than VP-16 on the breakage-reunion equilibrium of topoisomerase II and DNA. The stronger inhibition of the religation of the second strand observed with VPQ suggests it interacts asymmetrically with the two homodimers of topoisomerase II bound to DNA.

Combination toxicity of etoposide (VP-16) and photosensitisation with a water-soluble aluminium phthalocyanine in K562 human leukaemic cells

Etoposide (VP-16) is an anti-cancer drug commonly used against several types of tumours and leukaemia, either alone or in combination chemotherapy. Photodynamic therapy (PDT) is another, relatively new modality for treatment of various malignancies. The interactions between VP-16 and PDT, using aluminium tetrasulphophthalocyanine as photosensitiser, in K562 human leukaemic cells were investigated. Cell responses to individual and combined drug treatment under different experimental conditions revealed synergistic drug toxicity. The latter was evident from various events of cell response, including supra-additive accumulation of cells in G2/M cell cycle phase and endonucleolytic DNA fragmentation (apoptosis). The involvement of the cellular antioxidant system in the synergistic interactions of photosensitisation and VP-16 is proposed.

High-dose Ara-C for remission induction and consolidation of previously untreated adults with ALL or lymphoblastic lymphoma

Thirty-two patients with untreated ALL (n = 26) or lymphoblastic lymphoma (n = 6) between 17 and 65 years of age were treated with a short remission induction course with VP16-213, amsacrine, intermediate dose Ara-C for 6 days, prednisone and intrathecal methotrexate, followed by a consolidation course with vincristine, amsacrine, high dose Ara-C for 4 days, prednisone and intrathecal methotrexate. After subsequent cranial irradiation, no further maintenance was planned. However, some patients underwent an allogenic (n = 5) or autologous (n = 5) bone marrow transplantation after the consolidation treatment. Twenty-three of 32 patients (72%) achieved a complete remission. Ten of 13 patients with T-ALL or lymphoma, six of eight patients with pre-B or common ALL, and seven of 11 patients with B-ALL or Burkitt's lymphoma achieved a complete remission. The median duration of remission was 24 months. Overall survival for the whole group was 35% at 5 years. The disease-free survival was 45% at 5 years. Long-term survival for patients with B or T-ALL was approximately 60%, compared with 15% for those with common or pre B-ALL. Short term intensive courses including intermediate or high dose Ara-C during remission and consolidation treatment lead to results comparable to those obtained with long-term maintenance regimens. Our regimen may be sufficient for patients with T or B-ALL. Larger randomized studies are needed to investigate the relative importance of our observations.

Tyrosinase-induced phenoxyl radicals of etoposide (VP-16): interaction with reductants in model systems, K562 leukemic cell and nuclear homogenates

Etoposide (VP-16) is an antitumor drug currently in use for the treatment of a number of human cancers. Mechanisms of VP-16 cytotoxicity involve DNA breakage secondary to inhibition of DNA topoisomerase II and/or direct drug-induced DNA strand cleavage. The VP-16 molecule contains a hindered phenolic group which is crucial for its antitumor activity because its oxidation yields reactive metabolites (quinones) capable of irreversible binding to macromolecular targets. VP-16 phenoxyl radical is an essential intermediate in VP-16 oxidative activation and can be either converted to oxidation products or reduced by intracellular reductants to its initial phenolic form. In the present paper we demonstrate that the tyrosinase-induced VP-16 phenoxyl radical could be reduced by ascorbate, glutathione (GSH) and dihydrolipoic acid. These reductants caused a transient disappearance of a characteristic VP-16 phenoxyl radical ESR signal which reappeared after depletion of the reductant. The reductants completely prevented VP-16 oxidation by tyrosinase during the lag-period as measured by high performance liquid chromatography; after the lag-period VP-16 oxidation proceeded with the rate observed in the absence of reductants. In homogenates of human K562 leukemic cells, the tyrosinase-induced VP-16 phenoxyl radical ESR signal could be observed only after a lag-period whose duration was dependent on cell concentration; VP-16 oxidation proceeded in cell homogenates after this lag-period. In homogenates of isolated nuclei, the VP-16 phenoxyl radical and VP-16 oxidation were also detected after a lag-period, which was significantly shorter than that observed for an equivalent amount of cells. In both cell homogenates and in nuclear homogenates, the duration of the lag period could be increased by exogenously added reductants. The duration of the lag-period for the appearance of the VP-16 phenoxyl radical signal in the ESR spectrum can be used as a convenient measure of cellular reductive capacity. Interaction of the VP-16 phenoxyl radical with intracellular reductants may be critical for its metabolic activation and cytotoxic effects.

Reactions of glutathione with the catechol, the ortho-quinone and the semi-quinone free radical of etoposide. Consequences for DNA inactivation

Etoposide [4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta- D-glucopyranoside)] can be metabolized to DNA-inactivating catechol, ortho-quinone and semi-quinone free radical derivatives which may contribute to its cytotoxicity. In this paper, we examined in vitro whether glutathione (GSH), which is known to react easily with quinoid compounds, could interact with the active etoposide intermediates and in this way influence the cytotoxicity of the parent compound. To this end, reactions of GSH with the etoposide intermediates were studied, using HPLC and ESR measurements, together with the effects of GSH on the biological inactivation of single-stranded (ss) and double-stranded (RF) phi X174 DNA by these compounds. From the results it could be determined that: (a) GSH does not react with the catechol and, as a consequence, has no effect on the reaction of this intermediate of etoposide with ss and RF phi X174 DNA; (b) GSH reacts with the ortho-quinone most likely by formation of a conjugate and by two-electron reduction to the catechol, resulting in a partial protection of ss and RF phi X174 DNA against inactivation by this species; and (c) GSH protects ss phi X174 DNA against inactivation by the semi-quinone free radical of etoposide probably by conjugation with this species.

Modulation by D,L-buthionine-S,R-sulphoximine of etoposide cytotoxicity on human non-small cell lung, ovarian and breast carcinoma cell lines

Treatment with 25 mumol/l D,L-buthionine-S,R-sulphoximine (BSO) for at least 24 h depleted glutathione (GSH) in human non-small cell lung (SW-1573), ovarian (A2780) and breast carcinoma (MCF-7) cell lines to about 20% of control, and was accompanied by a 2-fold potentiation of the cytotoxicity of etoposide, doxorubicin and cisplatin. Cellular etoposide, but not doxorubicin or cisplatin, concentrations were increased 2-fold due to decreased efflux. This occurred independently of the presence of BSO during 1 h of etoposide exposure, but required prolonged exposure to BSO (at least 24 h). Energy depletion as well as cotreatment, but not pretreatment, of the cells with daunomycin, doxorubicin, vinblastine or vincristine increased cellular etoposide accumulation. Treatment of control cells with verapamil caused similar changes in etoposide cytotoxicity and cellular pharmacokinetics as GSH depletion, but did not further increase etoposide cytotoxicity and accumulation in GSH-depleted cells. Etoposide efflux may have been inhibited, not because of (competitive) inhibition by BSO or disturbance of the energy required for this process, but probably because of plasma membrane alterations.

Formation of different reaction products with single- and double-stranded DNA by the ortho-quinone and the semi-quinone free radical of etoposide (VP-16-213)

In this report, the types of DNA damage introduced by the ortho-quinone and the semiquinone free radical of 4'-demethylepipodophyllotoxin-9-(4-6-O-ethylidene-beta-D- glucopyranoside) (etoposide) and their relevance for the inactivation of single-stranded (ss) and double-stranded (ds) replicative form (RF) phi X174 DNA have been examined in vitro. The ortho-quinone yielded in both ss and ds DNA only chemical adducts, of which on the average about 1 out of 3 and 1 out of 12 per DNA molecule led to inactivation of ss or RF phi X174 DNA, respectively. The semi-quinone free radical, on the other hand, generated both frank and alkali-labile strand-breaks in ss and in ds DNA which, however, did not contribute significantly to DNA inactivation. The radical introduced, in addition, chemical DNA adducts. Unlike the ortho-quinone adducts, however, each of the semi-quinone adducts was lethal in ss phi X174 DNA, while more than 40 were required for the inactivation of RF DNA. The excision repair system of Escherichia coli did not operate on semi-quinone-modified RF DNA but removed about half of the ortho-quinone adducts [van Maanen JMS, Lafleur MVM, Mans DRA, van den Akker E, de Ruiter C, Koostra PR, Pappie D, de Vries J, Retèl J and Pinedo HM, Biochem Pharmacol 37: 3579-3589, 1988]. When ortho-quinone-modified ss or ds DNA was subjected to a post-alkaline treatment, the adducts remained stably bound to the DNA and the degree of biological inactivation was not influenced. In contrast, post-alkaline treatment removed about 70 and 60% of the semi-quinone adducts from ss and ds DNA, respectively, which, in the case of ss phi X174 DNA, resulted in a partial restoration of the biological activity. It is concluded that the ortho-quinone and the semi-quinone free radical of etoposide produce different types of damage in DNA which have different effects on the biological activity.