Characterization of the interaction of TZT-1027, a potent antitumor agent, with tubulin.

TZT-1027, a derivative of dolastatin 10 isolated from the Indian Ocean sea hare Dolabella auricularia in 1987 by Pettit et al., is a potent antimicrotubule agent. We have compared the activity of TZT-1027 with that of dolastatin 10 as well as the vinca alkaloids vinblastine (VLB), vincristine (VCR) and vindesine (VDS). TZT-1027 and dolastatin 10 inhibited microtubule polymerization concentration-dependently at 1 - 100 microM with IC50 values of 2.2 +/- 0.6 and 2.3 +/- 0.7 microM, respectively. VLB, VCR and VDS inhibited microtubule polymerization at 1 - 3 microM with IC50 values of 2.7 +/- 0.6, 1.6 +/- 0.4 and 1.6 +/- 0.2 microM, respectively, but showed a slight decrease in inhibitory effect at concentrations of 10 microM or more. TZT-1027 also inhibited monosodium glutamate-induced tubulin polymerization concentration-dependently at 0.3 - 10 microM, with an IC50 of 1.2 microM, whereas VLB was only effective at 0.3 - 3 microM, with an IC50 of 0.6 microM, and caused so-called "aggregation" of tubulin at 10 microM. Scatchard analysis of the binding data for [(3)H]VLB suggested one binding site (Kd 0.2 +/- 0.04 microM and Bmax 6.0 +/- 0.26 nM / mg protein), while that for [(3)H]TZT-1027 suggested two binding sites, one of high affinity (Kd 0.2 +/- 0.01 microM and Bmax 1.7 +/- 0.012 nM / mg protein) and the other of low affinity (Kd 10. 3 +/- 1.46 microM and Bmax 11.6 +/- 0.83 nM / mg protein). [(3)H]TZT-1027 was completely displaced by dolastatin 10 but only incompletely by VLB. [(3)H]VLB was completely displaced by dolastatin 10 and TZT-1027. Furthermore, TZT-1027 prevented [(3)H]VLB from binding to tubulin in a non-competitive manner according to Lineweaver-Burk analysis. TZT-1027 concentration-dependently inhibited both [(3)H]guanosine 5'-triphosphate (GTP) binding to and GTP hydrolysis on tubulin. VLB inhibited the hydrolysis of GTP on tubulin concentration-dependently to a lesser extent than TZT-1027, but no inhibitory effect of VLB on [(3)H]GTP binding to tubulin was evident even at 100 microM. Thus, TZT-1027 affected the binding of VLB to tubulin, but its binding site was not completely identical to that of VLB. TZT-1027 had a potent inhibitory effect on tubulin polymerization and differed from vinca alkaloids in its mode of action against tubulin polymerization.     

Antitumor activity of TZT-1027 (Soblidotin) against vascular endothelial growth factor-secreting human lung cancer in vivo

TZT-1027 (Soblidotin), an antimicrotubule agent, has been demonstrated to show potent antitumor effects, though the relationships among antitumor effect, cytotoxicity and anti-vascular effect of TZT-1027 have not been studied. We established in vivo human lung vascular-rich tumor models using a vascular endothelial growth factor-secreting tumor (SBC-3/VEGF). SBC-3/VEGF tumors exhibited a high degree of angiogenesis in comparison with the mock transfectant (SBC-3/Neo) tumors in a dorsal skinfold chamber model and grew much faster and larger than SBC-3/Neo tumors in the tumor growth study. The antitumor activity of antimicrotubule agents, including TZT-1027, was evaluated in both early- and advanced-stage SBC-3/Neo and SBC-3/VEGF tumor models to elucidate the relationship between the antitumor activity and anti-vascular effect of these agents. TZT-1027 exhibited potent antitumor activity against both early- and advanced-stage SBC-3/Neo and SBC-3/VEGF tumors, whereas combretastatin A4 phosphate did not. Vincristine and docetaxel exhibited potent antitumor activity against early-stage SBC-3/Neo and SBC-3/VEGF tumors, and advanced-stage SBC-3/Neo tumors, but did not exhibit activity against advanced-stage SBC-3/VEGF tumors. The difference in antitumor activity between these agents could be ascribed to differences in direct cytotoxicity and anti-vascular effect. Furthermore, a prominent accumulation of erythrocytes in the tumor vasculature, followed by leakage and scattering of these erythrocytes from the tumor vasculature, was observed after TZT-1027 administration to mice bearing advanced-stage SBC-3/VEGF tumors. These findings strongly suggest that TZT-1027 has a potent anti-vascular effect, in addition to direct cytotoxicity.     

The novel microtubule-interfering agent TZT-1027 enhances the anticancer effect of radiation in vitro and in vivo

TZT-1027 is a novel anticancer agent that inhibits microtubule polymerisation and manifests potent antitumour activity in preclinical models. We have examined the effect of TZT-1027 on cell cycle progression as well as the anticancer activity of this drug both in vitro and in vivo. With the use of tsFT210 cells, which express a temperature-sensitive mutant of Cdc2, we found that TZT-1027 arrests cell cycle progression in mitosis, the phase of the cell cycle most sensitive to radiation. A clonogenic assay indeed revealed that TZT-1027 increased the sensitivity of H460 cells to gamma-radiation, with a dose enhancement factor of 1.2. Furthermore, TZT-1027 increased the radiosensitivity of H460 and A549 cells in nude mice, as revealed by a marked delay in tumour growth and an enhancement factor of 3.0 and 2.2, respectively. TZT-1027 also potentiated the induction of apoptosis in H460 cells by radiation both in vitro and in vivo. Histological evaluation of H460 tumours revealed that TZT-1027 induced morphological damage to the vascular endothelium followed by extensive central tumour necrosis. Our results thus suggest that TZT-1027 enhances the antitumour effect of ionising radiation, and that this action is attributable in part to potentiation of apoptosis induction and to an antivascular effect. Combined treatment with TZT-1027 and radiation therefore warrants investigation in clinical trials as a potential anticancer strategy.     

Effect of oxygen on the cytotoxicity and antitumor activity of etoposide

The selective cytotoxicity of the epipodophyllotoxin etoposide toward normally oxygenated and hypoxic EMT6 mouse mammary tumor cells in culture was examined. Etoposide was much more toxic to normally oxygenated cells. The ratio (hypoxic to oxygenated) of drug concentrations producing 1 log of cell kill was approximately 30:1. Established FSa-11C fibrosarcomas of C3HeB/FeJ mice were treated with 10, 15, or 20 mg etoposide/kg body weight in a 6-day protocol. Fluosol-DA with or without breathing of carbogen (i.e., 95% O2-5% CO2) was added to the treatment program on days 1, 3, and 5. The combination of etoposide-Fluosol-DA-carbogen markedly enhanced tumor growth delay compared to the result with etoposide alone. The dose-modifying effect observed was 1.9 +/- 0.3. With the use of both single-dose and multiple-dose protocols for etoposide and Fluosol-DA with air or carbogen breathing, the survival of bone marrow cells was measured by colony formation in vitro (granulocyte-monocyte colony-forming units). Fluosol-DA and carbogen breathing did not increase the toxicity of etoposide to the bone marrow. Thus the enhancement in antitumor activity produced by the addition of Fluosol-DA and carbogen breathing to etoposide treatment was not accompanied by a concomitant increase in normal tissue toxicity and represents an increase in the therapeutic efficacy of etoposide.     

Natural antitumor cytotoxicity of the whole blood of donors and cancer patients

Natural antitumor resistance of cancer patients and healthy subjects was studied on the basis of whole blood natural cytotoxicity (WBNC) assay. A correlation between WBNC and mononuclear leukocyte cytotoxic activity in healthy donors was established (r = 0.912; p less than 0.001). WBNC levels in colonic, breast and gastric cancer patients were decreased as compared with healthy controls.     

Association of p53 gene mutations with sensitivity to TZT-1027 in patients with clinical lung and renal carcinoma

BACKGROUND: It has been revealed that chemotherapy using DNA-damaging agents and radiotherapy were influenced by the p53 status of tumors; however, p53 status did not influence chemotherapy using antimicrotubule agents. To elucidate whether a novel antimicrotubule agent, TZT-1027, is influenced by the p53 status of tumors, the authors investigated the sensitivities of specimens obtained from patients with nonsmall cell lung carcinoma (NSCLC) and renal cell carcinoma (RCC) to various anticancer agents, including TZT-1027, and the status of the p53 gene in those specimens. METHODS: Twenty-nine NSCLC specimens and 22 RCC specimens were analyzed for their sensitivity to various anticancer agents and their p53 status. Sensitivities of the specimens to nine anticancer agents were determined by flow cytometric analysis. To determine p53 status, polymerase chain reaction amplification with primers for exons 5--9 was conducted, and the products were subjected to single-strand conformation polymorphism analysis. RESULTS: In the NSCLC specimens, anticancer agents, including TZT-1027, showed strong antitumor activity against 50--75% of specimens with the wild type p53 gene. TZT-1027 showed strong antitumor activity against 40% of specimens with the mutant type p53 gene, whereas DNA-damaging agents showed such activity only in 16--28% of specimens. In RCC specimens, TZT-1027 showed potent antitumor activity in 29% of specimens with the wild type gene, and DNA-damaging agents showed such activity in 6--18% of specimens. TZT-1027 showed strong antitumor activity in 40% of specimens with the mutant type p53 gene, whereas DNA-damaging agents showed such activity only in 0--20% of specimens. CONCLUSIONS: We found evidence to suggest that TZT-1027 was influenced less by the p53 status of specimens than DNA-damaging agents. Therefore, TZT-1027 is expected to show similar antitumor activity against tumors with a loss of p53 function as well as those with normal function of p53 in clinical fields.     

Tumor-specific antivascular effect of TZT-1027 (Soblidotin) elucidated by magnetic resonance imaging and confocal laser scanning microscopy

TZT-1027 (soblidotin), an antimicrotubule agent, has previously been evaluated in terms of its antivascular effects. In this study, Evans blue perfusion, magnetic resonance imaging (MRI), and confocal laser scanning microscopy (CLSM) were utilized to further elucidate the antivascular effect of TZT-1027 in female nude mice and rats bearing human breast tumor MX-1, as well as in female Sprague-Dawley rats that developed breast tumors induced by dimethylbenz(a)anthracene (DMBA). Therapeutic doses of TZT-1027 caused nearly complete regression of implanted MX-1 tumors in nude mice and rats as well as DMBA-induced tumors in rats. The perfusion in MX-1 tumor implanted in nude mice was drastically reduced within 30 min after TZT-1027 administration and was completely inhibited after 6 h or more, although not reduced in normal tissue of kidney. The study using MRI demonstrated that rich blood flow within tumors was remarkably reduced 1-3 h after TZT-1027 administration both in nude rats bearing MX-1 tumors and in rats with DMBA-induced tumors. Furthermore, the study with CLSM in nude mice bearing MX-1 tumors revealed a disruption of tumor microvessels at 1 h and a destruction of tumor microvessel network at 3 h after TZT-1027 administration. In contrast, these types of vascular disorders were not observed in heart and kidney. These results suggest that TZT-1027 specifically damages tumor vasculatures, leading to extensive tumor necrosis within tolerable dose range, and confirms earlier observations that TZT-1027 exerts a considerable antivascular effect in addition to an excellent cytotoxic effect.     

Correlation of in vitro cytotoxicity with preclinical in vivo antitumor activity

Several human and murine tumor cell lines were evaluated in an in vitro cytotoxicity assay as prescreens for fermentation extracts and pure materials subsequently tested in vivo against P388 leukemia or B16 melanoma. Each material, regardless of its in vitro cytotoxicity, was evaluated in vivo. At the criteria levels of in vitro positivity and in vivo activity invoked, a highly significant relationship between these two endpoints was demonstrated for each cell line. When cell lines were compared, most of them performed in a similar manner, with HCT-116 human colon carcinoma cells providing a modest advantage predicting for P388 activity in some comparisons. Using the data from any two cell lines in concert did not improve the acuity of the prescreen beyond that associated with the better cell lines used singularly and only a minority of active materials was predicted for uniquely. Overall, the in vitro cytotoxicity assay provided a useful prescreen for selecting P388 and B16 in vivo active materials.     

Interleukin-2 potentiation of cetuximab antitumor activity for epidermal growth factor receptor-overexpressing gastric cancer xenografts through antibody-dependent cellular cytotoxicity

Cetuximab, a chimeric monoclonal antibody to epidermal growth factor receptor (EGFR), has been proved to have clinically significant antitumor activity against advanced colorectal cancers, but its therapeutic activity for gastric cancers remains unclear. In the present study, we investigated the antitumor effect and action mechanism of cetuximab using EGFR high-expressing (MKN-28) and EGFR low-expressing (GLM-1) gastric cancer cell lines without gene amplification. Cetuximab showed neither significant growth inhibition nor induction of apoptosis in either cell line in vitro , and only slightly inhibited ligand-induced phosphorylation of protein kinase B and extracellular signal-regulated kinase in MKN-28 cells. In contrast, cetuximab significantly inhibited subcutaneous and intraperitoneal tumor growth of MKN-28 cells, but not GLM-1 cells, in nude mice. This antitumor activity was significantly enhanced and diminished in nude mice by treatment with interleukin-2 (IL-2) and antiasialo GM1 antibody, which can expand and deplete natural killer (NK) cells, respectively. Antibody-dependent cellular cytotoxicity (ADCC) of cetuximab, as measured by ⁵¹Cr release assay, was significantly higher in MKN-28 than in GLM-1 cells. This ADCC activity was enhanced by IL-2 and reduced by heat-aggregate of human immunoglobulin G, an inhibitor for FcR-III of NK cells. These results suggest that cetuximab in combination with IL-2 shows significant antitumor activity against EGFR high-expressing gastric cancer mainly through NK cell-mediated ADCC. Combination therapy with cetuximab and IL-2 would thus offer a new potential therapeutic approach for a subset of EGFR-overexpressing gastric cancers. ( Cancer Sci 2008; 99: 1471–1478)     

Mode of action of C-1027, a new macromolecular antitumor antibiotic with highly potent cytotoxicity,on human hepatoma BEL-7402 cells

Summary C-1027, a new macromolecular peptide antitumor antibiotic produced by aStreptomyces strain, was extremely cytotoxic to cultured cancer cells and markedly inhibited the growth of transplantable tumors in mice. As determined by tritium-labeled precursor-incorporation assay, C-1027 strongly inhibited DNA and RNA synthesis in hepatoma BEL-7402 cells without affecting protein synthesis. After incubation with the hepatoma cells for 4 h, IC₅₀ values for [³H]-thymidine and [³H]-uridine incorporation were 0.00012 and 0.00032 m, respectively. After 30 min incubation, C-1027 showed much stronger inhibition of [³H]-thymidine incorporation than did Adriamycin, mitomycin C or methotrexate, even at a concentration 10,000 times lower. The effect of C-1027 on pBR322 DNA suggested that the drug could cause single- or double-strand scission of DNA. As determined by flow cytometry, C-1027 delayed the progression of hepatoma cells through the S-phase and blocked the cells at G2+M. Cytological study showed that C-1027 caused a drastic reduction of the mitotic index within 1 h and that an overshot of the mitotic index occurred at 48 h. Our results indicate that C-1027 is an interesting compound with highly potent activity on cellular DNA.     

Epidermal growth factor receptor-mediated selective cytotoxicity of antitumor agents toward human xenografts and murine syngeneic solid tumors

Severe toxic side effects of antiproliferative agents limit their clinical usefulness as antitumor drugs. Recently we observed that the antitumor efficacy of various antitumor agents (5-fluorouracil, tegafur, adriamycin, mitomycin C, cyclophosphamide, and cisplatin) against experimental solid tumors was enhanced by prior or simultaneous administration of human epidermal growth factor (EGF). However, coadministration of EGF did not enhance the toxicity of antitumor agents as measured by LD50 and body weight loss. The above selective potentiation of efficacy of the antitumor agents by human EGF can be characterized as follows. In a dose-dependent manner, human EGF enhanced the efficacy of an antitumor agent (5-FU) treatment against human epidermoid carcinoma A431 transplanted sc in athymic nude mice [ED50 = 2.9 (0.2-49.7, 95% confidence interval) microgram/kg, sc]. Various degrees of enhancement were also observed against other experimental tumors transplanted sc. The degrees of enhancement were directly proportional to the numbers of human EGF binding sites present on tumor cell plasma membrane (threshold of binding site density = 1.5 X 10(3) sites/cell) using 5-FU or cisplatin as an antitumor agent, thus suggesting that the binding of EGF to the receptors on tumor cells is an essential process in enhancing the susceptibility of tumor cells to antitumor agents. Normal cells including intestinal epithelial and bone marrow cells are endowed with fewer EGF binding sites (less than 10(3) sites/cell). This may explain partially the absence of EGF-enhanced cytotoxicity by antitumor agents toward normal cells.     

Effect of various oxygenation conditions and fluosol-DA on cytotoxicity and antitumor activity of bleomycin in mice

In an attempt to improve the antitumor efficacy of bleomycin, the effects of the oxygen-carrying emulsion Fluosol-DA and increased levels of inspired oxygen were tested in the mouse FSaIIC fibrosarcoma system. The dose-dependent cytotoxicity of bleomycin toward the FSaIIC cells in vitro was significantly decreased under hypoxic conditions, but it increased in a 95% O2-5% CO2 (carbogen) atmosphere as compared with the cytotoxicity of bleomycin in normally oxygenated cells. Investigations on the FSaIIC tumor in vivo also demonstrated that growth delays induced by bleomycin (10 mg/kg ip given on days 6, 10, 13, and 16) were significantly increased when one of the following treatments was given with each bleomycin injection: carbogen breathing for 2 hours (4.7 days), carbogen breathing for 6 hours (5.7 days), and breathing 3 atm of hyperbaric oxygen (6.3 days) versus normal air (3.3 days). When Fluosol-DA (12 mL/kg iv) was administered just before each bleomycin injection, the following growth delays were produced: 4.8 days with air breathing, 14.6 days with carbogen breathing for 2 hours, 14.9 days with carbogen breathing for 6 hours, and 19.7 days with breathing 100% O2 at 3 atm for 1 hour. Excision studies on the FSaIIC tumor also demonstrated that the cytotoxicity increased approximately fivefold when Fluosol-DA and carbogen breathing for 2 hours were combined with a single treatment with 10 mg of bleomycin/kg. In contrast, no measurable bone marrow toxicity was evident with this combined regimen. These results suggest that the use of Fluosol-DA plus carbogen breathing could add substantially to the clinical antitumor effects of bleomycin.     

Monoclonal antibody blockade of the human Eag1 potassium channel function exerts antitumor activity

The potassium channel ether à go-go has been directly linked to cellular proliferation and transformation, although its physiologic role(s) are as of yet unknown. The specific blockade of human Eag1 (hEag1) may not only allow the dissection of the role of the channel in distinct physiologic processes, but because of the implication of hEag1 in tumor biology, it may also offer an opportunity for the treatment of cancer. However, members of the potassium channel superfamily are structurally very similar to one another, and it has been notoriously difficult to obtain specific blockers for any given channel. Here, we describe and validate the first rational design of a monoclonal antibody that selectively inhibits a potassium current in intact cells. Specifically blocking hEag1 function using this antibody inhibits tumor cell growth both in vitro and in vivo. Our data provide a proof of concept that enables the generation of functional antagonistic monoclonal antibodies against ion channels with therapeutic potential. The particular antibody described here, as well as the technique developed to make additional functional antibodies to Eag1, makes it possible to evaluate the potential of the channel as a target for cancer therapy.     

Antiangiogenic and antitumor activity of a selective PDGFR tyrosine kinase inhibitor, CP-673,451

CP-673,451 is a potent inhibitor of platelet-derived growth factor beta-receptor (PDGFR-beta) kinase- and PDGF-BB-stimulated autophosphorylation of PDGFR-beta in cells (IC(50) = 1 nmol/L) being more than 450-fold selective for PDGFR-beta versus other angiogenic receptors (e.g., vascular endothelial growth factor receptor 2, TIE-2, and fibroblast growth factor receptor 2). Multiple models have been used to evaluate in vivo activity of CP-673,451 and to understand the pharmacology of PDGFR-beta inhibition and the effect on tumor growth. These models include an ex vivo measure of PDGFR-beta phosphorylation in glioblastoma tumors, a sponge model to measure inhibition of angiogenesis, and multiple models of tumor growth inhibition. Inhibition of PDGFR-beta phosphorylation in tumors correlates with plasma and tumor levels of CP-673,451. A dose of 33 mg/kg was adequate to provide >50% inhibition of receptor for 4 hours corresponding to an EC(50) of 120 ng/mL in plasma at C(max). In a sponge angiogenesis model, CP-673,451 inhibited 70% of PDGF-BB-stimulated angiogenesis at a dose of 3 mg/kg (q.d. x 5, p.o., corresponding to 5.5 ng/mL at C(max)). The compound did not inhibit vascular endothelial growth factor- or basic fibroblast growth factor-induced angiogenesis at concentrations which inhibited tumor growth. The antitumor efficacy of CP-673,451 was evaluated in a number of human tumor xenografts grown s.c. in athymic mice, including H460 human lung carcinoma, Colo205 and LS174T human colon carcinomas, and U87MG human glioblastoma multiforme. Once-daily p.o. x 10 days dosing routinely inhibited tumor growth (ED(50) < or = 33 mg/kg). These data show that CP-673,451 is a pharmacologically selective PDGFR inhibitor, inhibits tumor PDGFR-beta phosphorylation, selectively inhibits PDGF-BB-stimulated angiogenesis in vivo, and causes significant tumor growth inhibition in multiple human xenograft models.     

Host and direct antitumor effects and profound reduction in tumor metastasis with selective EP4 receptor antagonism

Prostaglandin E(2) (PGE(2)), one of the major metabolites of cyclooxygenase-2, has been implicated in tumorigenesis and tumor progression in several human cancers, including colorectal and lung. Here, we show that one of the PGE(2) receptors, the EP4 receptor, plays an important role in metastasis in both of these tumor types. Using i.v. injected Lewis lung carcinoma (3LL), we found that tumor metastasis to lung was significantly reduced when mice were treated with a specific EP4 antagonist ONO-AE3-208 or when EP4 receptor expression was knocked down in the tumor cells using RNA interference technology. Host EP4 receptors also contributed to tumor metastasis and tumor growth with decreased metastasis and tumor growth observed in EP4 receptor knockout animals. In vitro tumor cell adhesion, motility, invasion, colony formation, and Akt phosphorylation were all significantly inhibited when 3LL cells were treated with the EP4 receptor-specific antagonist. When the cells were treated with an EP4-specific agonist (AE1-734), we observed a worsening of these same features in vitro. Treatment with ONO-AE3-208 also profoundly decreased liver metastases after intrasplenic injection of MC26 colon cancer cells. Our data show that selective antagonism of EP4 receptor signaling results in a profound reduction in lung and colon cancer metastasis. Selective antagonism of the EP4 receptor may thus represent a novel therapeutic approach for the treatment of cancer and especially its propensity to metastasize.     

Effect of red blood cells on the antitumor activity of oxycellulose

The antitumor activity of oxycellulose to MH134 tumor cells, evaluated with microscopic viable cell count, was suppressed by murine red blood cells (MRBCs) in a dose-dependent manner. The antitumor activity of oxycellulose was suppressed by MRBCs also when evaluated with 3H-TdR uptake. However, that of mitomycin C or 5-fluorouracil was not suppressed by MRBCs. On the other hand, the antitumor activity of oxycellulose was suppressed by MRBCs also when evaluated with 51Cr release assay. At that time, the antitumor activity of cisplatin to MH134 tumor cells were not suppressed by MRBCs. The antitumor activity of oxycellulose to MH134 tumor cells was suppressed by the membrane or the cytosol fraction of the lysate of MRBCs as well as intact MRBCs. When heated at 56 degrees C for 30 min, MRBCs were unable to suppress the activity of oxycellulose. It was examined whether or not proteins related to MRBCs (hemoglobin, catalase, superoxide dismutase or taurine) suppressed the activity of oxycellulose. They did not suppress the activity of oxycellulose.     

Evidence for enzymatic activation and oxygen involvement in cytotoxicity and antitumor activity of N,N',N'-triethylenethiophosphoramide

The cytotoxicity of N,N',N'-triethylenethiophosphoramide (thiotepa) was studied in vitro in the MCF-7 human breast carcinoma cell line and in vivo using the EMT6 mouse mammary tumor model, under various conditions of oxygenation and in the presence and absence of Aroclor 1254-induced liver preparations. The cytotoxicity of thiotepa toward exponentially growing MCF-7 cells was markedly dependent on the presence of oxygen during the period of drug exposure, with 3 log greater cell kill at 500 microM thiotepa being observed when the cells were normally oxygenated compared with hypoxic cells. Incubation of thiotepa with an Aroclor 1254-induced rat liver S-9 homogenate, in the presence of a NADPH-regenerating system, resulted in an 8-fold increase in cytotoxicity towards the MCF-7 cells over a wide range of drug concentrations. Thiotepa was shown to be metabolized under these conditions in a NADPH- and O2-dependent reaction that was catalyzed by one or more microsomal cytochrome P-450 enzymes that were present in the S-9 fraction. The thiotepa metabolite triethylene phosphoramide, which hydrolyzes significantly faster than thiotepa, was significantly less cytotoxic toward the MCF-7 cells than was thiotepa itself, suggesting that it is unlikely to be the S-9 metabolite responsible for the observed increase in drug cytotoxicity. Moreover, triethylene phosphoramide cytotoxicity was only partially O2 dependent and was largely unaffected by incubation in the presence of the S-9 preparation, indicating a mechanism of action distinct from that of thiotepa. Tumor cell survival experiments with the EMT6 mouse mammary carcinoma system revealed that a 3.6-fold increase in thiotepa cytotoxicity was obtained by prior administration of the liver inducer Aroclor 1254 to the tumor-bearing animals, 5 days before drug treatment. Finally, the therapeutic effectiveness of thiotepa was significantly enhanced (3- to 5.8-fold increase in tumor growth delay) when an increase in oxygenation was achieved, by carbogen breathing, in animals given the perfluorochemical emulsion Fluosol-DA. These findings establish that the cytotoxic effects of thiotepa are oxygen dependent and may involve, at least in part, metabolic processes catalyzed by cytochrome P-450 enzymes.     

The structure and antitumor activity of antitumor antibiotics--recent progress

Effort looking for new antitumor antibiotics useful for the treatment of curing cancer resulted to the discovery of a number of new compounds with newer action mechanism as well as newer structural feature. The antibiotics which have been discovered since 1984 are discussed under classifications of action mechanism and structural feature, as well. The first group, which belong to a novel class of antibiotics containing a bicyclodiynene carbon skeleton in the molecules exhibited the most strong anti-tumor activity comparing with the antitumor antibiotics so far discovered. The action mechanism of this was explained by the diradical formation of diynene-cyclization, which led to the scission of double strand DNA. Amongst, esperamicin A seems of great interest in view of the therapeutic development. Moreover, elsamicin A, a member of chatarin antibiotics, and FR-900482 compound, an antibiotic having a polycyclic alkalodal skeleton are under development for the new chemotherapeutic agents. Rhizoxin, the metabolite of Rhizopus chinensis is also a promising candidate as anticancer agent. Its action mechanism was classified as an inhibitor of mitosis by binding to the microtibline proteins. Rhizoxin A shows no cross resistance with vincristine. MX2 (KRN 8602), the morpholino derivative of 13-deoxo-10-hydroxy-carminomycin, shows anticancer activity against tumor cells resistant to P388/ADM as well as low cardial toxicity. Miscellaneous compounds whose action mechanism are unknown are described.