Porfiromycin Research Articles

Abstract 453: Identification of human oxidoreductases involved in the hypoxia-dependent activation of bioreductive prodrugs

Abstract Hypoxia occurs in most human tumors and is associated with resistance to conventional therapies and poor patient outcomes. Eliminating hypoxic tumor cells therefore remains an important therapeutic goal. A number of bioreductive prodrugs are undergoing clinical evaluation as hypoxia selective cytotoxins, including PR-104, TH-302, tirapazamine (TPZ) and porfiromycin (POR). These prodrugs are activated in tumor cells by one-electron oxidoreductases (1eR) in a process that is reversed by molecular oxygen. Relatively few 1eR have been studied, with the notable exception of cytochrome P450 reductase (CYPOR), known to activate many prodrug substrates including the dinitrobenzamide mustard prodrug PR-104A (Guise et al., Biochem Pharmacol 2007;74:810). We now describe the identification of three additional PR-104A 1eR enzymes. Expression of twelve candidate 1eR in the HCT116 colorectal cancer cell line was demonstrated by induction of an occult V5-tag. Significant increases in hypoxic PR-104A metabolism were observed in cells expressing CYPOR, methionine synthase reductase (MTRR), novel diflavin oxidoreductase 1 (NDOR1) and inducible nitric oxide synthase (NOS2A). Hypoxic PR-104A metabolism was enhanced in the order CYPOR (3420 ± 250.5) > MTRR (1514.8 ± 240.1) > NOS2A (1018.1 ± 204.9) > NDOR1 (819.1 ± 9.8) as compared to HCT116 WT (411.5 ± 54.3 pmol metabolites/106 cells/hr). We further characterized the role of these 1eR in the cytotoxicity of PR-104A, TPZ, POR and TH-302, and complemented this analysis by determining the extent to which this catalytic activity can be detected by the hypoxia imaging agents pimonidazole (PIMO) and pentafluoro-etanidazole (EF5). Binding of the imaging agents was differentially enhanced by the expression of these 1eR in the order CYPOR >NDOR1 >MTRR >NOS2A. These data suggest it may be possible to utilise hypoxia PET probes as biomarkers for non-invasive detection of hypoxic cell metabolic capacity. A specific mAb was identified for each of the four 1eR and validated by immunoblot against matched V5-tag induced lysates. Next, mAb validation was extended to paraffin-embedded formalin-fixed (PEFF) cell plugs of each clonal cell population. The CYPOR mAb (1:50 dilution, Santa Cruz Biotech, sc-25263) was found to be of excellent specificity for IHC detection of CYPOR, and this validation was extended using PEFF tissue sections of parental (WT), CYPOR-overexpressing and CYPOR knockout HCT116 tumor xenografts. In order to translate these findings, tissue microarrays of various human tumors were evaluated for CYPOR expression. We identified several tumor types, including lung, breast, prostate and pancreas, where a proportion of cases displayed elevated levels of CYPOR protein in tumor epithelium. Thus CYPOR may be a suitable biomarker to identify diseases or individuals that may benefit from treatment with hypoxic cytotoxins. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 453.

Concurrent chemo-radiotherapy with mitomycin C compared with porfiromycin in squamous cell cancer of the head and neck: Final results of a randomized clinical trial

Previous randomized trials have shown a benefit with concurrent use of the hypoxic cell cytotoxin mitomycin C (MC) and radiation (RT) in the management of squamous cell cancer of the head and neck (SCCHN). We conducted a randomized trial comparing MC with porfiromycin (POR) in combination with RT in the management of SCCHN. Between 1992 and 1999, 128 patients with SCCHN were enrolled in this prospective randomized trial. Patients were stratified by management intent, and balanced with respect to stage and site of disease. They were randomized to receive MC (15 mg/M(2)) or POR (40 mg/M(2)) on Days 5 and 47 (or last day) of RT. Of 121 evaluable patients, 61 were randomized to MC and 60 to POR. Patients were treated with standard daily RT to a total median dose of 64 Gy over 47 days. Patients were well balanced with respect to management intent, stage, site, age, sex, hemoglobin levels, tumor grade, radiation dose, and days on treatment. There were no significant differences between the two arms with respect to acute hematologic or nonhematologic toxicities. As of January 2003 with a median follow-up of 6.3 years, there have been 19 local relapses (4 MC vs. 15 POR), 21 regional relapses (7 MC vs. 14 POR), 24 distant metastases (11 MC vs. 13 POR), and 66 deaths (33 MC vs. 33 POR). MC was superior to POR with respect to 5-year local relapse-free survival (91.6% vs. 72.7%, p = 0.01), local-regional relapse-free survival (82% vs. 65.3%, p = 0.05), and disease-free survival (72.8% vs. 52.9%, p = 0.026). There were no significant differences between the two arms with respect to overall survival (49.2% vs. 54.4%) or distant metastasis-free rate (79.9% vs. 75.9%). Despite promising preclinical data, and an acceptable toxicity profile, POR was inferior to MC as an adjunct to RT in the management of SCCHN. This randomized trial emphasizes the need for randomized studies to evaluate new agents in the management of SCCHN.

Concurrent chemo-radiotherapy with mitomycinc (MC) compared to porfiromycin (PO) in squamous cell carcinoma of the head and neck (SCCHN): final results of a randomized clinical trial

Purpose/Objective: Previous randomized trials have shown a benefit with concurrent use of the hypoxic cell cytotoxin MitomycinC (MC) and radiation (RT) in the management of SCCHN. Based on laboratory data demonstrating superior hypoxic cell cytotoxicity with Porfiromycin (PO, Methyl-mitomycinC), and a Phase I trial demonstrating acceptable toxicity with concurrent RT and PO at 40mg/M2, we conducted a randomized trial comparing MC to PO in combination with RT in the management of SCCHN.

Bioactivation of mitomycin antibiotics by aerobic and hypoxic Chinese hamster ovary cells overexpressing DT-diaphorase

DT-Diaphorase catalyzes a two-electron reduction of mitomycin C (MC) and porfiromycin (POR) to reactive species. Many cell lines that overexpress DT-diaphorase and are sensitive to the mitomycins are protected from the aerobic cytotoxicity of these drugs by the DT-diaphorase inhibitor dicumarol. The cytoprotective properties of this relatively non-specific inhibitor, however, vanish under hypoxic conditions. To ascertain the role of DT-diaphorase in mitomycin bioactivation and cytotoxicity in living cells, a rat liver DT-diaphorase cDNA was transfected into Chinese hamster ovary cells. MC was equitoxic to the parental cells under oxygenated and hypoxic conditions. In contrast, POR was less toxic than MC to these cells under aerobic conditions, but significantly more toxic than MC under hypoxia. Two DT-diaphorase-transfected clones displayed increases in DT-diaphorase activity of 126- and 133-fold over parental cells. The activities of other oxidoreductases implicated in mitomycin bioreduction were unchanged. MC was more toxic to both DT-diaphorase-transfected lines than to parental cells; the toxicity of MC to the transfected lines was similar in air and hypoxia. POR was also more toxic to the DT-diaphorase-elevated clones than to parental cells under oxygenated conditions. Under hypoxia, however, the toxicity of POR to the transfected clones was unchanged from that of parental cells. The findings implicate DT-diaphorase in mitomycin bioactivation in living cells, but suggest that this enzyme does not contribute to the differential toxicity of MC or POR in air and hypoxia.

Porfiromycin as an adjunct to radiotherapy in young and old mice

Radiobiological data and measurements with O 2 microelectrodes show that EMT6 tumors implanted into aged mice have a higher proportion of radioresistant, hypoxic cells than do tumors implanted into young adult animals; radiation is less effective in killing cells in tumors in old mice than in tumors in young adult mice. The studies reported here examine the effects of porfiromycin (POR), a bioreductive alkylating agent shown previously to be preferentially toxic to hypoxic EMT6 cells in vitro and in solid tumors in young adult mice. POR was effective in attacking the hypoxic cells of tumors in aged mice; regimens combining POR with x-rays overcame the radioresistance of tumors in the old animals. Comparisons of the distribution of 3H-labeled POR in young and old mice showed that tumors in aged mice had a slightly larger proportion of areas with necrotic features. which bound higher levels of tritiated POR than did healthy tumor regions without necrotic features. Studies of histology, lissamine green distributions, binding of tritiated POR, and radiation and POR cytotoxicity suggested that tumors in old mice contained a larger proportion of poorly perfused tumor cells, and that cells in these regions were resistant to radiation and sensitive to POR. Studies of the distribution of POR in normal tissues and of the toxicity of POR to bone marrow progenitor cells (CFU-GM) revealed no differences between young and old animals, showing that the differences observed in tumors reflected differences in the microenvironments within the tumors, rather than differences in the processing of drug in young and old animals

The role of NAD(P)H:quinone oxidoreductase in mitomycin C- and porfiromycin-resistant HCT 116 human colon-cancer cells.

A mitomycin C (MMC)- and porfiromycin (PFM)-resistant subline of the HCT 116 human colon-cancer cell line was isolated after repeated exposure of HCT 116 cells to increasing concentrations of MMC under aerobic conditions. The MMC-resistant subline (designated HCT 116-R30A) was 5 times more resistant than the parent cells to MMC and PFM under aerobic conditions. Both the MMC-resistant cells and the parent HCT 116 cells accumulated similar amounts of PFM by passive diffusion, but levels of macromolecule-bound PFM were about 50% lower in the resistant cell line, implying a decrease in PFM reductive activation in the resistant cells. The finding that microsomes from either sensitive or resistant cells showed an equal ability to reduce MMC and PFM indicated that the activity of NADPH cytochrome P-450 reductase (EC 1.6.2.4) was not changed in the resistant subline. Soluble extracts of HCT 116 cells reduced MMC and PFM more effectively at pH 6.1, and NADH and NADPH were utilized equally well as electron donors under both aerobic and anaerobic conditions. These data suggest that quinone reductase (EC 1.6.99.2; DT-diaphorase) in soluble extracts is responsible for the reduction of MMC. Quinone reductase activities in soluble extracts of HCT 116-R30A cells for the reduction of dichlorophenol indophenol (DCPIP) and menadione-cytochrome c at optimal pHs were decreased by 95% as compared with those obtained in parent cells. However, the MMC-reducing activity of HCT 116-R30A soluble extracts was only 50% lower than that of the parent cell extracts. The kinetic constants (Km, Vmax) found for quinone reductase in the two cell lines with respect to the substrates DCPIP and menadione differed. Two species of mRNA for quinone reductase (2.7 and 1.2 kb) were detected in both cell lines, and there was no detectable difference between parent and resistant cells in the steady-state level of either of these mRNA species. Furthermore, incubation with the quinone reductase inhibitor dicoumarol rendered HCT 116 cells more resistant to MMC. Alteration of the quinone reductase activity in HCT 116-R30A cells appears to be the mechanism responsible for their resistance to MMC and PFM.

Isolation, Identification, and Assay of [3H]-Porfiromycin Adducts of EMT6 Mouse Mammary Tumor Cell DNA: Effects of Hypoxia and Dicumarol on Adduct Patterns

[3H]-(N-la-methyl) Porfiromycin (POR) was employed to detect and identify the radiolabeled mono- and bis-adducts formed in living EMT6 mouse mammary tumor cells under different conditions. To provide authentic standard adducts, calf-thymus DNA was treated with POR under reductive activation, then digested to nucleosides and POR-nucleoside adducts. The three major adducts formed were isolated by HPLC and authenticated. Two were mono-adducts, composed of deoxyguanosine linked at its N2-position to C-1 of POR and of 10-decarbamoyl POR. The third was a bis-adduct, in which POR was crosslinked to two deoxyguanosines at their N2-positions. DNA from [3H]-POR treated EMT6 cells was digested an analyzed by HPLC. DNA-associated label was located in thymidine and in two mono-adducts and one bis-adduct identical to those described above. Label in thymidine resulted from N-demethylation of POR and reincorporation of label into new thymidylate residues. Adducts were formed more abundantly in hypoxia than in air. In addition, the mono-adduct to crosslink ratios were different, approximately 1:1 and 2:1 for hypoxic and aerobic cells, respectively. The different patterns of alkylation in air and hypoxia may be related to the greater toxicity of POR in hypoxia. When cells were treated simultaneously with POR and dicumarol, adduct levels were lower, and a new, unknown adduct was observed primarily under hypoxia; these changes may be related to the altered toxicity of POR in the presence of dicumarol. The HPLC assay detected simultaneously the full array of stable mono- and bis-adducts in DNA with good sensitivity (greater than or equal to 2 x 10(6) adducts/nucleotide) and excellent reproducibility. This assay should be generally applicable to all cells and tissues when MC or POR with high specific radioactivity can be employed.

Activity of C-7 Substituted Cyclic Acetal Derivatives of Mitomycin C and Porfiromycin Against Hypoxic and Oxygenated EMT6 Carcinoma Cells In Vitro and In Vivo

A series of cyclic acetal derivatives of mitomycin C (MC) and porfiromycin (POR) were tested for their ability to kill hypoxic and oxygenated EMT6 tumor cells. Amino methyl acetal and thioacetal substitutions at C-7 of MC and POR dramatically increased the cytotoxicity of the compounds to hypoxic EMT6 tumor cells in vitro but had little effect on the aerobic toxicities. In contrast, a methyl substitution at N1a markedly decreased the aerobic cytotoxicities of the compounds but did not alter the hypoxic cytotoxicities. The POR acetal, BMY-42355, had the largest differential between hypoxic and aerobic cytotoxicities yet observed among MC analogs. Preliminary studies in mice showed that BMY-42355 had good antineoplastic activity when used alone or in combination with radiation and was less toxic than POR; the therapeutic ratio of this compound in these initial studies was higher than those of either MC or POR.

Studies on the mechanism of resistance to mitomycin C and porfiromycin in a human cell strain derived from a cancer-prone individual

The mechanism of aerobic resistance to the quinone-containing anti-tumour agents mitomycin C (MMC) and porfiromycin (PM) has been investigated using non-transformed human cells. One of the cell strains used (3437T) was derived from an afflicted member of a cancer-prone family. This cell strain had been shown previously to be six times more resistant to the cytotoxic effects of these agents under aerobic but not hypoxic conditions when compared to a cell strain derived from an unrelated, normal donor (GM38). Differences could not be detected in the ability of cell sonicates prepared from either cell strain to produce alkylating species under aerobic conditions using a 4-( p-nitrobenzyl)pyridine assay. However, using 3H-labelled PM to monitor rapid drug uptake and subsequent accumulation due to drug metabolism, results were obtained indicating that the resistant cell strain (3437T) was deficient in an enzymatic pathway capable of metabolizing these compounds under aerobic but not hypoxic conditions. Dicumarol, an inhibitor of the quinone reductase DT-diaphorase (EC 1.6.99.2), decreased aerobic drug accumulation and cytotoxicity in the control cell strain, but did not alter the lack of accumulation noted in the resistant cell strain. Under hypoxic conditions, dicumarol increased cytotoxicity and drug accumulation in both cell strains. The mechanism of this enhanced cytotoxicity remains unclear. These results suggested that the resistant cells were deficient in the enzyme DT-diaphorase, a potential activator of PM. Enzymatic assays confirmed this and revealed no alterations in cytochrome P450 reductase (EC 1.6.2.4) activity or glutathione content. No protein characteristic of DT-diaphorase was detected in the resistant cell strain using a polyclonal rabbit-anti-rat antibody raised against this enzyme. Southern blot analysis using a rat DT-diaphorase cDNA probe demonstrated differences between the normal and resistant cell strains in the restriction fragment patterns. The present results are consistent with the hypothesis that decreased DT-diaphorase levels are causally associated with PM and MMC resistance in these cells under aerobic exposure conditions.

Inhibition of mitomycin C's aerobic toxicity by the seleno-organic antioxidant PZ-51

Mitomycin C (MMC) is a bioreductive alkylating agent that is capable of generating oxygen radicals. Porfiromycin (PM) is an analog to MMC that generates oxygen radicals at a significantly lower level than the parent compound. Under aerobic conditions, the toxicity of MMC to EMT6 cells is 2.5-fold that of PM, whereas hypoxically the two are equitoxic. In the present studies, the protective effect of PZ-51 in combination with NAC was assessed against the dose-dependent toxicity of either MMC or PM under both aerobic and hypoxic conditions. Aerobically, the PZ-51 and NAC combination inhibited the toxicity of MMC at concentrations of between 0.25 and 2 microM but had no effect on PM toxicity. Under hypoxic conditions, the PZ-51 and NAC combination had no effect on either MMC or PM toxicity. These findings support a role for oxygen radical generation in the aerobic toxicity of MMC at clinically relevant doses.

Porfiromycin disposition in oxygen-modulated P388 cells

The cytotoxicity, metabolism, and DNA alkylation of porfiromycin (PFM) under aerobic and hypoxic conditions were evaluated in P388 murine leukemia cells. Clonogenic assays showed that the IC50 value for a 1-h exposure to PFM was 4 microM for aerobic cells and 0.5 microM for hypoxic cells. After a 1-h exposure to concentrations of 1, 5, and 10 microM [14C]-PFM, the accumulation of total radioactivity in hypoxic cells was 10 to 20 times that in aerobic cells. The disposition of radioactivity in cells that had been treated for 1 h with 5 microM PFM under aerobic or hypoxic conditions showed that (a) under either condition, internal free-PFM concentration equalled the external drug concentration; (b) DNA-, RNA-, and protein-bound radioactivity were at least 10 times greater in hypoxic cells than in aerobic cells; and (c) known metabolites and unidentified radioactive products were also generated in greater amounts in hypoxic cells than in aerobic cells. Thus, the increased amounts of radioactivity accumulated by hypoxic P388 cells after exposure to [14C]-PFM resulted from the accumulation of nonexchangeable protein and nucleic-acid adducts and metabolites rather than free PFM. Determinations of DNA adducts formed in P388 cells revealed five possible adducts: (1) N2-(2'-deoxyguanosyl)-7-methylaminomitosene, (2) a second monofunctional PFM-guanine adduct, (3) a PFM cross-linked dinucleotide, (4) possibly a nucleoprotein-related adduct, and (5) an unknown. We conclude that the enhancement of PFM-induced cytotoxicity by hypoxia appears to be primarily due to increased alkylation of macromolecules.

Deficient activation by a human cell strain leads to mitomycin resistance under aerobic but not hypoxic conditions

Two non-transformed human skin fibroblast strains, GM38 and 3437T, were found to be more sensitive to the bioreductive alkylating agents mitomycin C (MMC) and porfiromycin (PM) under hypoxic compared to aerobic conditions. One of these strains, 3437T, was 6-7 times more resistant to these agents under aerobic exposure conditions, but was identical in sensitivity to the normal strain, GM38, under hypoxic conditions. Aerobic 3437T cells demonstrated no increased resistance to cisplatin compared to the normal strain, arguing against enhanced ability to repair DNA interstrand cross-links as the underlying explanation for the mitomycin resistance. The aerobic resistance of 3437T was not altered by dicumarol, an inhibitor of the enzyme DT-diaphorase which is believed to be involved in aerobic activation of MMC and PM. Dicumarol did increase the resistance of GM38, but not to the same level of resistance demonstrated by 3437T. These results suggest that the aerobic MMC and PM resistance of 3437T may arise, in part, from a deficiency in DT-diaphorase activity. The identical sensitivities under hypoxic conditions indicate that drug activation pathways operative in the absence of oxygen are similar in both the normal and 3437T cells.

Preclinical Studies of Porfiromycin as an Adjunct to Radiotherapy

The bioreductive alkylating agent porfiromycin (POR) is more toxic to EMT6 cells that are hypoxic at the time of treatment than to aerobic cells. The toxicity of POR to hypoxic EMT6 cells in vitro was similar to that of mitomycin C (MC): the aerobic toxicity of POR was considerably less than that of MC. Treatment of cells in vitro with POR before and during irradiation did not sensitize either hypoxic or aerobic cells to X rays; instead, only additive cytotoxicity was produced. In contrast, treatment of solid EMT6 tumors in vivo with POR plus radiation produced supra-additive cytotoxicity, as assessed by analyses of the complete dose-response curves for the killing of tumor cells by radiation alone or by POR alone. The supra-additivity of the combination regimens appeared to reflect the preferential killing by each agent of those tumor cells which were in an environment conferring resistance to the other agent. In contrast, combinations of POR and X rays produced only additive cytotoxicities to marrow CFU-GM. Supra-additive antineoplastic effects were obtained at doses of POR which produced little hematologic or other host toxicity. The complementary cytotoxicities of radiation and POR to cells in different microenvironments in solid tumors and the absence of a similar effect in normal tissue make optimized regimens combining radiotherapy and POR unusually promising for the treatment of solid tumors.

DNA crosslinking, sister-chromatid exchange and specific-locus mutations

Chinese hamster overy cells were treated with the DNA-crosslinking chemicals, mitomycin C (MMC) and porfiromycin (POR), and their monofunctional derivative decarbamoyl mitomycin C (DCMMC). After exposure, the cells were studied for the induction of sister-chromatid exchanges (SCEs) and mutations at the hypoxanthine phosphoribosyltransferase and adenine phosphoribosyltransferase loc. The frequency of SCEs varied significantly in successive sampling intervals, requiring the weighting of each interval by the percentage of second-divison mitosis in that interval to obtain the mean SCE frequency for each dose. All 3 compounds were potent induceers of SCEs but weakly mutagenic. All 3 chemicals by concentration were approximately equally effective in inducing SCEs or mutations. When the induced SCEs and mutations were com-compared at equal levels of survival, DCMMC was slightly more effective than MMC or POR in inducing SCEs and somewhat less mutagenic. These results indicate that the DNA interstrand crosslink is not the major lesion responsible for the induction of SCE or mutation by these compounds.