Inhibition of growth of colon 38 adenocarcinoma by vinblastine and colchicine: evidence for a vascular mechanism

Comparison of the antivascular and cytotoxic activities of TZT-1027 (Soblidotin) with those of other anticancer agents

TZT-1027 (Soblidotin), a microtubule-depolymerizing agent exerts both a direct cytotoxic activity against cancer cells and an indirect antivascular activity against tumor vascular endothelial cells. We compared both activities of TZT-1027 with those of various anticancer agents having different mechanisms of action, including vinca alkaloids, a vascular targeting agent, a taxane and nonmicrotubule-binding agents. In the MTT assay, TZT-1027 most potently inhibited the growth of both murine colon C26 cancer cells and human umbilical vein endothelial cells, implying its potent antivascular activity against tumor vasculature in addition to its cytotoxic activity against cancer cells. Treatment with 0.1 microg/ml TZT-1027 significantly enhanced vascular permeability in human umbilical vein endothelial cell monolayers and a single intravenous administration of 2 mg/kg TZT-1027 significantly reduced the perfusion of Colon26 tumors implanted into mice, with efficacies superior to vinca alkaloids and comparable to a known vascular targeting agent. These results strongly suggest that TZT-1027 exerts marked antivascular activity. Next, to clarify the mechanism of the antivascular activity, we have taken a novel approach, and analyzed the relationships among human umbilical vein endothelial cells cytotoxicity, vascular permeability and tumor perfusion, on the basis of efficacies of each agent. Analyses revealed strong and significant correlations, and indicated that the vascular endothelial cell damage leads to endothelial barrier dysfunction and, thereby, tumor vascular shutdown. In summary, TZT-1027 was verified to have not only an excellent cytotoxic activity, but also an attractive antivascular activity through the induction of damage to vascular endothelial cells. We believe that these dual activities may make TZT-1027 useful for treating solid tumors.

A novel class of tubulin inhibitors that exhibit potent antiproliferation and in vitro vessel-disrupting activity

PURPOSE

Since anticancer agents that interfere with microtubule function are in widespread use and have a broad spectrum of activity against both hematological malignancies and solid tumors, there is an urgent need to develop novel tubulin inhibitors with broader activities and avoiding drug resistance.

METHODS AND RESULTS

In this study, we describe the characterization of select lead compounds from a novel class of indazole-based tubulin inhibitors. Three lead compounds, TH-337, TH-482 and TH-494, exhibit potent antiproliferative activity against cell lines derived from human pancreatic carcinoma, human breast adenocarcinoma and human colorectal adenocarcinoma cells. The three compounds were also tested for cytotoxicity against a panel of clinically relevant drug resistant cancer cell lines that either overexpress the drug resistance pumps MDR-1, MRP-1 and BCRP-1 or have altered Topoisomerase II activity. TH-482 and -494 retained cytotoxic activities against all of the resistant cell lines tested; however, TH-337 exhibited decreased cytotoxicity in the cell line overexpressing BCRP-1, indicating that TH-337 is a substrate of that pump. We show that TH-482's antiproliferative activity is due to cell cycle arrest at the G(2)/M phase. We demonstrate that TH-482 binds specifically to the colchicine site of tubulin and that it inhibits tubulin polymerization in vitro in a concentration-dependent manner. The in vitro anti-vascular activities of TH-482 were assessed using the HUVEC-C cell line. TH-482 inhibits in vitro neovessel formation and disrupts pre-established vessels using HUVEC-C cells. TH-482 also increases permeability of vascular endothelial cells in a concentration- and time-dependent manner.

CONCLUSIONS

TH-482 demonstrates potent in vitro efficacy as a novel tubulin-targeted anti-proliferative and anti-vascular agent and notably is more potent in antiproliferative assays than the benchmark compound combretastatin A-4. These results identify TH-482 as a potent tubulin inhibitor, and support the investigation of its in vivo efficacy and pharmacokinetic properties as the prototype of a new class of anti-tubulin agents.

Vascular damaging agents

To provide a comprehensive overview on vascular targeting agents and the application of radiobiological principles in pre-clinical and clinical studies, we completed a comprehensive review of published medical studies on vascular targeting agents using Pub Med. Vascular targeting agents are now divided into vascular disrupting agents (VDAs), which target the pre-existing tumour vasculature, and angiogenesis inhibitors (AIs), which prevent the formation of new blood vessels. Modest success has been seen when VDAs and AIs are used as single agents and therefore combination therapies that can work in a complimentary and synergistic manner, targeting both the tumour cells and endothelial cells, are needed. Radiobiological principles have been used to increase our understanding of these agents, and can explain the increased efficacy of combination treatments. In particular, the alteration of the tumour microenvironment by AIs and VDAs can lead to enhanced efficacy when combined with chemotherapy or radiotherapy, with phase II/III trials showing encouraging results. The optimal use and scheduling of AIs and VDAs remains to be determined. Further understanding of the mechanisms of action of these potentially very exciting anti-neoplastic agents is urgently required.

Emerging therapeutic options for Philadelphia-positive acute lymphocytic leukemia

Acute lymphocytic leukemia (ALL) is a heterogeneous group of disorders that are associated with a cure rate of > 80% in children. The prognosis in adults is considerably inferior, with age, disease bulk, leukemia karyotype and immune phenotype being prognostically relevant. Adult ALL treatment programs include induction, intensified consolidation and maintenance phases with CNS prophylaxis. The addition of imatinib in patients with BCR-ABL-positive ALL has improved the prognosis of this subgroup, but their survival is still poor. Initial data on the second-generation BCR-ABL inhibitors, dasatinib and nilotinib, indicate a potentially greater efficacy than imatinib, but the improvement is likely to be modest. The overall efforts in terms of developmental therapeutics in ALL are very modest and not in keeping with the urgent need for improvement. Most agents being investigated have mechanisms of action similar to those of existing agents for ALL therapy and thus represent modest opportunities to improve results. Of such agents, data on BCR-ABL inhibitors, sphingosomal vincristine, pemetrexed, talotrexin, annamycin and ABT-751 are reviewed.

Evaluation of ABT-751 against childhood cancer models in vivo

OBJECTIVE

ABT-751 is a novel antimitotic agent that binds tubulin at the colchicine binding site. ABT-751 is undergoing Phase I trials in children, but has not been evaluated against a range of pediatric tumor models in vivo.

MATERIALS AND METHODS

ABT-751 was evaluated against 27 subcutaneously implanted xenograft models of childhood cancer including neuroblastoma [4], osteosarcoma [4], Ewing sarcoma [2] rhabdomyosarcoma [8], medulloblastoma [1] and eight kidney cancer lines (six Wilms tumors, two rhabdoid). ABT-751 was administered at 100 mg/kg P.O. on a schedule of 5 days on, 5 days off, 5 days on, repeating the cycle at 21 days. Tumor diameters were measured at 7 day intervals for a period of 12 weeks. Three measures of antitumor activity were used: (1) clinical response criteria [e.g., partial response (PR), complete response (CR), etc.]; (2) treated to control (T/C) tumor volume at day 21; and (3) a time to event measure based on the median event free survival (EFS) of treated and control lines.

RESULTS

ABT-751 induced regression in 4 of 25 models (16%) including models of neuroblastoma that are refractory to vincristine and paclitaxel. Other regressions occurred in rhabdomyosarcoma and Wilms tumor models. ABT-751 significantly increased event free survival (EFS > 2.0) in eight models (33%) in addition to those with objective responses.

CONCLUSIONS

ABT-751 demonstrated intermediate activity against this tumor panel. Neuroblastoma models appear somewhat more sensitive to this agent, with objective regressions also in rhabdomyosarcoma and Wilms tumor. ABT-751 was also active in several tumor lines intrinsically refractory to vincristine or paclitaxel.

Drug insight: vascular disrupting agents and angiogenesis--novel approaches for drug delivery

Vascular disrupting agents (VDAs), or endothelial disrupting agents, attempt to exploit the vascular endothelium that supplies rapidly dividing neoplasms. Unlike antiangiogenesis agents (e.g. the monoclonal antibody bevacizumab; and tyrosine kinase inhibitors sorafenib and sunitinib) that disrupt endothelial cell survival mechanisms and the development of a new tumor blood supply, VDAs are designed to disrupt the already established abnormal vasculature that supports tumors, by targeting their dysmorphic endothelial cells. Tumor vascular endothelium is characterized by its increased permeability, abnormal morphology, disorganized vascular networks, and variable density. VDAs induce rapid shutdown of tumor blood supply, causing subsequent tumor death from hypoxia and nutrient deprivation. The safety profile of this class of compounds is more indicative of agents that are indeed 'vascularly' active. For example, VDAs can cause: acute coronary and other thrombophlebitic syndromes; alterations in blood pressure, heart rate, and ventricular conduction; transient flush and hot flashes; neuropathy; and tumor pain. Despite these cardiovascular concerns some patients have benefited from VDAs in early clinical trials. Further drug development of VDAs must include the combination of these agents with other novel biological agents, cytotoxic chemotherapy, and radiotherapy. Close monitoring of patients receiving VDAs for any cardiovascular toxicity is imperative.

Antivascular effects of TZT-1027 (Soblidotin) on murine Colon26 adenocarcinoma

We investigated the ability of TZT-1027 (Soblidotin), a novel antimicrotubule agent, to induce antivascular effects, because most vascular targeting agents that selectively disrupt tumor vasculature also inhibit tubulin polymerization. Treatment with 10(-7) g/mL TZT-1027 rapidly disrupted the microtubule cytoskeleton in human umbilical vascular endothelial cells (HUVEC), and significantly enhanced vascular permeability in HUVEC monolayers. In addition, single intravenous administration of 2 mg/kg TZT-1027 to mice bearing Colon26 tumors significantly reduced tumor perfusion and caused extravascular leakage of erythrocytes 1 h after administration. Subsequently, thrombus formation with deposition of fibrin and tumor necrosis was observed 3 and 24 h after administration, respectively. These results strongly suggest that TZT-1027 possesses antivascular effects. TZT-1027 induced apoptosis not only in HUVEC but also in C26 cancer cells (cell line of Colon26 solid tumor) in vitro, suggesting it exerts direct cytotoxicity against tumor cells in addition to its antivascular effects. A single intravenous administration of 1, 2 and 4 mg/kg TZT-1027 significantly prolonged the survival of mice with advanced-stage Colon26 tumors in a dose-dependent manner. Furthermore, TZT-1027 itself less markedly enhanced the permeability of normal vessels, but was additive with vascular endothelial growth factor, indicating the possibility that TZT-1027 selectively exerts its activity on tumor vessels. In summary, these results suggest that TZT-1027 exerts both an indirect antivascular effect and a direct cytotoxic effect, resulting in strong antitumor activity against advanced-stage tumors, and that TZT-1027 may be useful clinically for treating solid tumors.

Inhibition of ovarian cancer growth and implantation by paclitaxel after laparoscopic surgery in a mouse model

OBJECTIVE

We used an established experimental model to evaluate the influence of intraperitoneal chemotherapy on the generation of laparoscopy-associated metastases and the effectiveness of chemotherapy.

STUDY DESIGN

Twenty-four nude mice underwent laparoscopy with carbon dioxide insufflation and the instillation of a tumor cell suspension with or without paclitaxel into the peritoneal cavity. Mice were allocated to 1 of the following groups (8 mice to each group): (1) controls; (2) paclitaxel given during the operation; (3) paclitaxel given after the operation. Mice were killed 30 days after the procedure, and the peritoneal cavity and port sites were examined for the presence of tumors.

RESULTS

Tumor implantation and port-site metastases were reduced more by the intraoperative intraperitoneal administration of paclitaxel during the operation than by administration after the operation.

CONCLUSION

Intraoperative intraperitoneal administration of paclitaxel may decrease significantly the occurrence of port-site metastasis and intraperitoneal dissemination in an animal study.

Metronomic trofosfamide inhibits progression of human lung cancer xenografts by exerting anti-angiogenic effects

BACKGROUND

Recent studies of conventional chemotherapeutic drugs administered in metronomic therapy schedules showed remarkable inhibitory effects on tumor angiogenesis. Subsequent and prolonged tumor regression was achieved moreover by circumventing acquired drug resistance. In this study, metronomic and conventional trofosfamide were compared on human NSCLC xenograft "LX-1."

MATERIALS AND METHODS

In vitro cytotoxicity of trofosfamide on tumor and human umbilical cord endothelial cells was determined under normoxic and hypoxic conditions. Additionally fractions and duration of cell cycles were analyzed by flow cytometry. In vivo LX-1 xenotransplanted nude mice were treated with trofosfamide in conventional and metronomic schedules (i.p./p.o.). Tumor sections were evaluated for microvessel density (MVD), relative growth fraction and apoptosis.

RESULTS

In contrast to the rapid growth of conventionally treated lung cancer, long lasting tumor growth retardation over the total treatment period was achieved with metronomic treatment. While growth fraction and apoptotic rate of LX-1 cells remained unchanged, the MVD was significantly reduced (50%).

CONCLUSION

Our results show advantages of a metronomic trofosfamide schedule compared to a conventional bolus therapy mainly due to inhibition of angiogenesis. In vitro data show that this mechanism works under normoxic and hypoxic conditions and suggest that this is in part a direct cytotoxic effect on endothelial cells.

Update on tubulin-binding agents

The clinical and commercial success of the taxanes and vinca alkaloids resulted in a drive to improve on current formulations and discover new compounds that target the microtubule. These strategies are all aimed at improving on (1) anti-tumour activity, (2) toxicity profile and (3) pharmacology. Drugs undergoing clinical development include the novel semi-synthetic taxane derivatives (DJ-927, XRP6258 and XRP9881), the epothilones, the dolastations, vinflunine and the combretastatin analogues. In several cases, some improvements in tumour response rates have been seen but randomised trials need to be completed before the role of specific novel tubulin-binding agents can be established.

Antiangiogenic effect of low-dose cyclophosphamide combined with ginsenoside Rg3 on Lewis lung carcinoma

Angiogenesis is now known to play an important role in both growth and metastasis of lung cancer. The intense interest in angiogenesis has led to a re-examination of the activity of many established cytotoxic agents. Some results of recent experimental studies have suggested that frequent administration of certain cytotoxic agents at low doses increases the antiangiogenic activity of the drugs. In the present study, we investigated the efficacy of the combination of low-dose cyclophosphamide and ginsenoside Rg3 for the antiangiogenic effect on Lewis lung carcinoma. Our findings suggest that continuous low-dose regimen of CTX increases the efficacy of targeting the tumor microvasculature, which produces therapeutic activity with decreased toxicity. The effects of the low-dose schedule of CTX may be further enhanced by concurrent administration of angiogenic inhibitor ginsenoside Rg3. As an antiangiogenic method, this regimen has the advantage of a reduced susceptibility to drug resistance mechanisms and improved animal survival.

Effects of platinum/taxane based chemotherapy on acute perfusion in human pelvic tumours measured by dynamic MRI

Dynamic contrast enhanced MRI (DCE-MRI) is being used increasingly in clinical trials to demonstrate that vascular disruptive and antiangiogenic agents target tumour microcirculation. Significant reductions in DCE-MRI kinetic parameters are seen within 4–24 and 48 h of treatment with vascular disruptive and antiangiogenic agents, respectively. It is important to know whether cytotoxic agents also cause significant acute reductions in these parameters, for reliable interpretation of results. This study investigated changes in transfer constant (K^trans) and the initial area under the gadolinium curve (IAUGC) following the first dose of chemotherapy in patients with mostly gynaecological tumours. A reproducibility analysis on 20 patients (using two scans performed on consecutive days) was used to determine the significance of DCE-MRI parameter changes 24 h after chemotherapy in 18 patients. In 11 patients who received platinum alone or with a taxane, there were no significant changes in K^trans or IAUGC in either group or individual patient analyses. When the remaining seven patients (treated with a variety of agents including platinum and taxanes) were included (n=18), there were also no significant changes in K^trans. Therefore, if combination therapy does show changes in DCE-MRI parameters then the effects can be attributed to antivascular therapy rather than chemotherapy.

Anti-angiogenic therapy and chemotherapy affect 99mTc sestamibi and 99mTc-HL91 accumulation differently in tumour xenografts

BACKGROUND

Favourable effects of cytotoxic chemotherapy for tumours are characterized by the reduced accumulation of radiotracers such as 99mTc sestamibi (MIBI). Anti-angiogenic therapy is primarily cytostatic; consequently, its influence on tracer accumulation may differ from that of cytotoxic treatments.

METHODS

Anti-angiogenic therapy employing 2-methoxyestradiol was administered in mice bearing subcutaneous xenografts of LS180 colon cancer cells. The effects of chemotherapy with 5-fluorouracil were examined as a cytotoxic counterpart. Treatments were conducted for 4 days from day 8. Distribution of 99mTc-MIBI and Tc-HL91, a hypoxic marker, was observed on days 8 and 12. Oxygen tension (PO2) in tumours was measured by a microelectrode. Cellular uptake of tracers was examined in vitro in normoxic and hypoxic conditions.

RESULTS

99mTc-MIBI accumulation decreased with increasing tumour weight when no treatment was conducted. Tumour growth was suppressed by anti-angiogenic therapy and chemotherapy. 99mTc-MIBI accumulation in tumours decreased after chemotherapy as compared to pre-therapeutic values, whereas accumulation of 99mTc-HL91 increased. In contrast, accumulation of tracers did not significantly change after anti-angiogenic therapy as compared to that observed pre-therapeutically. Tumour PO2 decreased with increasing tumour volume when no treatment was conducted. Chemotherapy reduced PO2 in tumours. PO2 in tumours treated with anti-angiogenic therapy was as high as that observed before treatment. 2-Methoxyestradiol or 5-fluorouracil did not significantly affect tracer accumulation in cells under both normoxic and hypoxic conditions in vitro.

CONCLUSION

These findings indicate that scintigraphic assessment of therapeutic efficacy of anti-angiogenic therapy should be performed from a perspective distinct from that of cytotoxic treatment.

Phase 1 Study of ABT-751, a Novel Microtubule Inhibitor, in Patients with Refractory Hematologic Malignancies

PURPOSE

ABT-751 is an oral antimitotic agent that binds to the colchicine site on beta-tubulin. A phase 1 study was conducted to determine the maximum tolerated dose and toxicities of ABT-751 in patients with advanced myelodysplastic syndrome and relapsed or refractory acute leukemias.

STUDY DESIGN

Thirty-two patients were treated: nine with 100 (n = 3), 125 (n = 3), or 150 mg/m(2) (n = 3) of ABT-751 given orally once daily for 7 days every 3 weeks and 23 with 75 (n = 3), 100 (n = 3), 125 (n = 5), 150 (n = 5), 175 (n = 3), or 200 mg/m(2) (n = 4) of ABT-751 given orally once daily for 21 days every 4 weeks. Consenting patients had pharmacogenetic sampling and enumeration of circulating endothelial cells (CEC).

RESULTS

Dose-limiting toxicity consisted of ileus in one patient at 200 mg/m(2), with a subsequent patient developing grade 2 constipation at the same dose level. One patient with relapsed acute myelogenous leukemia achieved a complete remission that was sustained for 2 months. Four other patients had transient hematologic improvements, consisting of a decrease in peripheral blood blasts and improvements in platelet counts. CEC number was reduced in three patients with a concomitant reduction in peripheral blasts. A previously undescribed nonsynonymous single nucleotide polymorphism, encoding Ala(185)Thr, was identified in exon 4 of the beta-tubulin gene, TUBB, in three other patients. The recommended phase 2 dose in hematologic malignancies is 175 mg/m(2) daily orally for 21 days every 4 weeks.

CONCLUSION

Further assessment of ABT-751, especially in combination with other agents, in patients with acute leukemias is warranted.

Combretastatin A4 phosphate: background and current clinical status

Combretastatin A4 phosphate (CA4P) represents the lead compound in a group of novel tubulin depolymerising agents being developed as vascular targeting agents (VTAs). VTAs are drugs that induce rapid and selective vascular dysfunction in tumours. CA4P is a water-soluble prodrug of the cis-stilbene CA4 originally isolated from the tree Combretum caffrum. Preclinical studies have shown that CA4P induces blood flow reductions and subsequent tumour cell death in a variety of preclinical models. Moreover, this activity has been linked to its ability to rapidly alter the morphology of immature endothelial cells by disrupting their tubulin cytoskeleton. Phase I clinical trials have established a maximum tolerated dose in the range 60-68 mg/m2 and in addition have established that significant changes to tumour perfusion can be achieved across a wide range of doses. The dose-limiting toxicities include tumour pain, ataxia and cardiovascular changes. The maximum tolerated dose was independent of schedule, indicating the absence of cumulative toxicity. Although unexpected from preclinical studies, some evidence of clinical response was seen using CA4P as a single modality. Based on the Phase I data, combination studies of CA4P with established therapies are in progress and should determine whether the exciting preclinical data obtained when VTAs are used in combination with cytotoxic chemotherapy, radiation, radioimmunotherapy and even antiangiogenic agents, can be translated into man.

Disrupting tumour blood vessels

Low-molecular-weight vascular-disrupting agents (VDAs) cause a pronounced shutdown in blood flow to solid tumours, resulting in extensive tumour-cell necrosis, while they leave the blood flow in normal tissues relatively intact. The largest group of VDAs is the tubulin-binding combretastatins, several of which are now being tested in clinical trials. DMXAA (5,6-dimethylxanthenone-4-acetic acid) - one of a structurally distinct group of drugs - is also being tested in clinical trials. A full understanding of the action of these and other VDAs will provide insights into mechanisms that control tumour blood flow and will be the basis for the development of new therapeutic drugs for targeting the established tumour vasculature for therapy.

Antiangiogenic concentrations of paclitaxel induce an increase in microtubule dynamics in endothelial cells but not in cancer cells

Microtubule-targeted drugs such as paclitaxel exhibit potent antiangiogenic activity at very low concentrations, but the mechanism underlying such an effect remains unknown. To understand the involvement of microtubules in angiogenesis, we analyzed the dynamic instability behavior of microtubules in living endothelial cells [human microvascular endothelial cells (HMEC-1) and human umbilical vascular endothelial cells (HUVEC)] following 4 hours of paclitaxel treatment. Unexpectedly, antiangiogenic concentrations of paclitaxel (0.1-5 nmol/L) strongly increased microtubule overall dynamicity in both HMEC-1 (86-193%) and HUVEC (54-83%). This increase was associated with increased microtubule growth and shortening rates and extents and decreased mean duration of pauses. The enhancement of microtubule dynamics by paclitaxel seemed to be specific to antiangiogenic concentrations and to endothelial cells. Indeed, cytotoxic concentration (100 nmol/L) of paclitaxel suppressed microtubule dynamics by 40% and 54% in HMEC-1 and HUVECs, respectively, as observed for all tested concentrations in A549 tumor cells. After 4 hours of drug incubation, antiangiogenic concentrations of paclitaxel that inhibited endothelial cell proliferation without apoptosis (1-5 nmol/L) induced a slight decrease in anaphase/metaphase ratio, which was more pronounced and associated with increased mitotic index after 24 hours of incubation. Interestingly, the in vitro antiangiogenic effect also occurred at 0.1 nmol/L paclitaxel, a concentration that did not alter mitotic progression and endothelial cell proliferation but was sufficient to increase interphase microtubule dynamics. Altogether, our results show that paclitaxel mediates antiangiogenesis by an increase in microtubule dynamics in living endothelial cells and suggest that the impairment of interphase microtubule functions is responsible for the inhibition of angiogenesis.

The tubulin-binding agent combretastatin A-4-phosphate arrests endothelial cells in mitosis and induces mitotic cell death

The tubulin-binding agent combretastatin A-4-phosphate (CA-4-P), rapidly disrupts the vascular network of tumors leading to secondary tumor cell death. In vitro, CA-4-P destabilizes microtubules and causes endothelial cell death. In this study we analyze the mechanisms by which CA-4-P induces the death of proliferating endothelial cells. We demonstrate that at >/=7.5 nmol/L, CA-4-P damages mitotic spindles, arrests cells at metaphase, and leads to the death of mitotic cells with characteristic G(2)/M DNA content. Mitotic arrest was associated with elevated levels of cyclin B1 protein and p34(cdc2) activity. Inhibition of p34(cdc2) activity by purvalanol A caused mitotic-arrested cells to rapidly exit mitosis, suggesting that sustained p34(cdc2) activity was responsible for metaphase arrest. Pharmacological prevention of entry into mitosis protected cells from undergoing cell death, further establishing the link between mitosis and cell death induction by CA-4-P. CA-4-P-mediated cell death shared characteristics of apoptosis but was independent of caspase activation suggesting the involvement of a non-caspase pathway(s). These data suggest that induction of apoptosis in endothelial cells by CA-4-P is associated with prolonged mitotic arrest. Therefore, by activating cell death pathways, CA-4-P, in addition to being an effective anti-vascular agent, may also interfere with regrowth of blood vessels in the tumor.

Antivascular and antitumor evaluation of 2-amino-4-(3-bromo-4,5-dimethoxy-phenyl)-3-cyano-4H-chromenes, a novel series of anticancer agents

A novel series of 2-amino-4-(3-bromo-4,5-dimethoxy-phenyl)-3-cyano-4H-chromenes was identified as potent apoptosis inducers through a cell-based high throughput screening assay. Six compounds from this series, MX-58151, MX-58276, MX-76747, MX-116214, MX-116407, and MX-126303, were further profiled and shown to have potent in vitro cytotoxic activity toward proliferating cells only and to interact with tubulin at the colchicine-binding site, thereby inhibiting tubulin polymerization and leading to cell cycle arrest and apoptosis. Furthermore, these compounds were shown to disrupt newly formed capillary tubes in vitro at low nanomolar concentrations. These data suggested that the compounds might have vascular targeting activity. In this study, we have evaluated the ability of these compounds to disrupt tumor vasculature and to induce tumor necrosis. We investigated the pharmacokinetic and toxicity profiles of all six compounds and examined their ability to induce tumor necrosis. We next examined the antitumor efficacy of a subset of compounds in three different human solid tumor xenografts. In the human lung tumor xenograft (Calu-6), MX-116407 was highly active, producing tumor regressions in all 10 animals. Moreover, MX-116407 significantly enhanced the antitumor activity of cisplatin, resulting in 40% tumor-free animals at time of sacrifice. Our results identify MX-116407 as the lead candidate and strongly support its continued development as a novel anticancer agent for human use.

Vascular targeting agents as cancer therapeutics

Vascular targeting agents (VTAs) for the treatment of cancer are designed to cause a rapid and selective shutdown of the blood vessels of tumors. Unlike antiangiogenic drugs that inhibit the formation of new vessels, VTAs occlude the pre-existing blood vessels of tumors to cause tumor cell death from ischemia and extensive hemorrhagic necrosis. Tumor selectivity is conferred by differences in the pathophysiology of tumor versus normal tissue vessels (e.g., increased proliferation and fragility, and up-regulated proteins). VTAs can kill indirectly the tumor cells that are resistant to conventional antiproliferative cancer therapies, i.e., cells in areas distant from blood vessels where drug penetration is poor, and hypoxia can lead to radiation and drug resistance. VTAs are expected to show the greatest therapeutic benefit as part of combined modality regimens. Preclinical studies have shown VTA-induced enhancement of the effects of conventional chemotherapeutic agents, radiation, hyperthermia, radioimmunotherapy, and antiangiogenic agents. There are broadly two types of VTAs, small molecules and ligand-based, which are grouped together, because they both cause acute vascular shutdown in tumors leading to massive necrosis. The small molecules include the microtubulin destabilizing drugs, combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and Oxi 4503, and the flavonoid, DMXAA. Ligand-based VTAs use antibodies, peptides, or growth factors that bind selectively to tumor versus normal vessels to target tumors with agents that occlude blood vessels. The ligand-based VTAs include fusion proteins (e.g., vascular endothelial growth factor linked to the plant toxin gelonin), immunotoxins (e.g., monoclonal antibodies to endoglin conjugated to ricin A), antibodies linked to cytokines, liposomally encapsulated drugs, and gene therapy approaches. Combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and DMXAA are undergoing clinical evaluation. Phase I monotherapy studies have shown that the agents are tolerated with some demonstration of single agent efficacy. Because efficacy is expected when the agents are used with conventional chemotherapeutic drugs or radiation, the results of Phase II combination studies are eagerly awaited.

Therapeutic strategies that selectively target and disrupt established tumor vasculature

This articles discusses therapeutic strategies that selectively target and disrupt established tumor vasculature.

Antiangiogenic activity of paclitaxel is associated with its cytostatic effect, mediated by the initiation but not completion of a mitochondrial apoptotic signaling pathway

Angiogenesis is a critical event in tumor growth and metastasis, which can be inhibited by conventional anticancer drugs such as the microtubule-damaging agent paclitaxel (Taxol). In this study, we investigate the mechanism of action of paclitaxel on human endothelial cells. We characterize two distinct effects of paclitaxel on human umbilical vein endothelial cell and human microvascular endothelial cell-1 proliferation according to drug concentration: a cytostatic effect at low concentrations and a cytotoxic effect at concentrations 10 nmol/L. The cytotoxic effect involves signaling pathways similar to those described in tumor cells (i.e., microtubule network disturbance, G2-M arrest, increase in Bax/Bcl-2 ratio, and mitochondria permeabilization) that result in apoptosis. In sharp contrast, the cytostatic effect involves an inhibition of endothelial cell proliferation without apoptosis induction and without any structural modification of the microtubule network. This cytostatic effect is due to a slowing of the cell cycle rather than to an arrest in a specific phase of the cell cycle. In addition, paclitaxel, at cytostatic concentrations, early initiates an apoptotic signaling pathway associated with increases in the mitochondrial reducing potential, mitochondrial membrane potential, p53 expression, and Bax/Bcl-2 ratio. However, this apoptotic pathway is stopped upstream of mitochondria permeabilization and it does not lead to endothelial cell death. Finally, we found that paclitaxel inhibits endothelial cell morphogenesis on Matrigel at all tested concentrations. In conclusion, we describe the mechanism of action of low concentrations of paclitaxel related to the antiangiogenic properties of this drug.

Combretastatin A4 phosphate

Combretastatin A4 phosphate (CA4P) is a water-soluble prodrug of combretastatin A4 (CA4). The vascular targeting agent CA4 is a microtubule depolymerizing agent. The mechanism of action of the drug is thought to involve the binding of CA4 to tubulin leading to cytoskeletal and then morphological changes in endothelial cells. These changes increase vascular permeability and disrupt tumor blood flow. In experimental tumors, anti-vascular effects are seen within minutes of drug administration and rapidly lead to extensive ischemic necrosis in areas that are often resistant to conventional anti-cancer treatments. Following single-dose administration a viable tumor rim typically remains from which tumor regrowth occurs. When given in combination with therapies targeted at the proliferating viable rim, enhanced tumor responses are seen and in some cases cures. Results from the first clinical trials have shown that CA4P monotherapy is safe and reduces tumor blood flow. There has been some promising demonstration of efficacy. CA4P in combination with cisplatin is also safe. Functional imaging studies have been used to aid the selection of doses for phase II trials. Both dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and positron emission tomography can measure the anti-vascular effects of CA4P in humans. This review describes the background to the development of CA4P, its proposed mechanism of action, the results from the first clinical trials with CA4P and the role of imaging techniques in its clinical development.

Vascular-targeting therapies for treatment of malignant disease

BACKGROUND

Tumor endothelium represents a valuable target for cancer therapy. The vasculature plays a critical role in the survival and continued growth of solid tumor masses; in addition, the inherent differences between tumor blood vessels and blood vessels associated with normal tissue make the tumor vasculature a unique target on which to base the design of novel therapeutics, which may allow highly selective treatment of malignant disease. Therapeutic strategies that target and disrupt the already formed vessel networks of growing tumors are actively being pursued. The goal of these approaches is to induce a rapid and catastrophic shutdown of the vascular function of the tumor so that blood flow is arrested and tumor cell death due to the resulting oxygen and nutrient deprivation and buildup of waste products occurs.

METHODS

Biologic approaches and small-molecule drugs that can be used to damage tumor vasculature have been identified. Physiologic, histologic/morphologic, and immunohistochemical assessments have demonstrated that profound disruption of the tumor vessel network can be observed minutes to hours after treatment. The small-molecule agents that have made the greatest advances in the clinical setting (5,6-dimethylxanthenone-4-acetic acid [DMXAA], combretastatin A4 disodium phosphate [CA4DP], and ZD6126) are the focus of the current review.

RESULTS

Loss of patent blood vessels, decreased tumor blood flow, extensive necrosis, and secondary ischemia-induced tumor cell death have been well documented in a variety of preclinical tumor models treated with agents such as DMXAA, CA4DP, and ZD6126. The use of such agents in conjunction with irradiation and other chemotherapeutic agents has led to improved treatment outcomes.

CONCLUSIONS

The targeting of tumors' supportive blood vessel networks could lead to improvements in cancer cure rates. It is likely that this approach will prove to be most efficacious when used in concert with conventional treatment strategies.

Vascular targeting effects of ZD6126 in a C3H mouse mammary carcinoma and the enhancement of radiation response

PURPOSE

The aim of this study was to investigate the pathophysiologic effects induced by the novel vascular targeting agent ZD6126 in a C3H mouse mammary carcinoma and to evaluate the agent's ability to inhibit tumor growth either when given alone or in combination with radiation.

METHODS AND MATERIALS

A C3H mammary carcinoma grown in the right rear foot of female CDF1 mice was treated when at 200 mm(3) in size. ZD6126 was dissolved in saline and injected intraperitoneally. Blood perfusion was measured using the RbCl extraction procedure, tumor oxygen (pO(2)) status was assessed with the Eppendorf electrode, and tumor necrosis was estimated from histologic sections. Radiation (240-kV X-rays) was locally administered to tumors of restrained nonanesthetized mice, and response was assessed using a tumor growth assay.

RESULTS

ZD6126 induced a significant dose- and time-dependent decrease in tumor perfusion, reaching a maximal 70% reduction around 3 h after injecting 150-300 mg/kg. However, full recovery was seen within 6 h. A 200 mg/kg dose significantly decreased tumor oxygenation status at 3 h (median pO(2) decreased from 7 to 3 mm Hg; % pO(2) values <or=2.5 mm Hg increased from 30% to 55%) and by 24 h had significantly increased necrotic fraction from 14.5% to 25.2%. This ZD6126 dose resulted in a small, yet significant, 1.4 days inhibition of tumor growth when given alone. It also enhanced the tumor response to radiation, giving rise to a significant 1.3-fold increase in the slope of the radiation dose-response curve. Of the normal tissues, only muscle (at 3 h) and spleen (at 6 h) showed any significant reduction in perfusion after injecting 200 mg/kg, but these transient decreases were only 32% and 49%, respectively.

CONCLUSIONS

Our preclinical studies clearly demonstrate a tumor-specific reduction in blood perfusion by ZD6126. Although these changes were transient, they were sufficient to increase tumor necrosis, inhibit tumor growth, and enhance radiation response.

Arsenic Trioxide Enhances Radiation Response of 9L Glioma in the Rat Brain

Arsenic trioxide (ATO) at low doses induces leukemia cells to undergo apoptosis and at higher doses causes blood flow to solid tumors to shut down. To determine whether a potential synergistic interaction exists between ATO at the non-toxic dose level in the rat and radiation, the present study was carried out with orthotopic 9L malignant gliomas growing in the brains of rats. Animals died within 50 days of treatment when 12-day-old 9L gliomas growing in the brain of Fischer rats were treated with either the drug alone (8 mg/kg) or radiation alone (25 Gy). In contrast, the overall tumor cure rate exceeded 50% at a follow-up time of 120 days after the combined treatment with radiation and ATO. Long-term surviving animals showed no clinical or disproportionately enhanced histopathological changes in the brain parenchyma. Early changes in tumor physiology showed that the vascular leakage of FITC-dextran conjugates was apparent within 8 h of drug administration. Last, the use of diffusion magnetic resonance imaging as an early surrogate marker of therapeutic efficacy corroborated the effects of drug with and without radiation on brain histology and animal survival.

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.

Combined effects of docetaxel and angiostatin gene therapy in prostate tumor model

The anti-tumor effects of adenovirus-delivered angiostatin (AdK3) in association with docetaxel (Taxotere) have been evaluated in a human-origin prostate tumor model. In vitro, human endothelial cells were 50- to 100-fold more sensitive to docetaxel than prostate cell lines (PC3, LNCaP, and DU145), and the combination regimen of docetaxel and AdK3 was significantly more cytotoxic for endothelial cells than either treatment alone. PC3 cells, which display the highest sensitivity to docetaxel, were then grafted onto athymic mice for an evaluation of the combined regimen as a therapy. The combination of a single intratumoral injection of AdK3 (2 x 10(9) pfu) and a single intravenous injection of docetaxel (15 mg/kg) was compared with the injection of AdK3 alone on preestablished mice bearing PC3-derived tumors with a mean tumor volume of 60 +/- 11 or 205 +/- 46 mm3. Significant antitumoral effects were observed only in mice receiving the combined treatment. We showed that all PC3 tumors regressed in the AdK3-docetaxel combination group and that 40 to 83% totally regressed. In all cases, this regimen was tightly correlated with a marked decrease in intratumoral vascularization. Our experimental data show that attacking both endothelial and tumoral compartments is an efficient and logical strategy, making this bitherapy approach clinically promising.

Antivascular therapy of cancer: DMXAA

The vascular endothelium of tumour tissue, which differs in several ways from that of normal tissues, is a potential target for selective anticancer therapy. By contrast with antiangiogenic agents, antivascular agents target the endothelial cells of existing tumour blood vessels, causing distortion or damage and consequently decreasing tumour blood flow. DMXAA (5,6-dimethylxanthenone-4-acetic acid), a low-molecular-weight drug, has a striking antivascular and in some cases curative effect in experimental tumours. Its action on vascular endothelial cells seems to involve a cascade of events leading to induction of tumour haemorrhagic necrosis. These events include both direct and indirect effects, the latter involving the release of further vasoactive agents, such as serotonin, tumour necrosis factor, other cytokines, and nitric oxide from host cells. Phase I clinical trials of DMXAA have been completed and the next challenge to face is how the antivascular effect of this drug should be exploited for the treatment of human cancer.

Paclitaxel at ultra low concentrations inhibits angiogenesis without affecting cellular microtubule assembly

Many conventional chemotherapeutics, such as the microtubule-stabilizing anticancer drug paclitaxel (Taxol), have been shown to have anti-angiogenic activity and clinical application of a continuous low dose of these agents has been suggested for cancer therapy. In this study, we show that paclitaxel selectively inhibits the proliferation of human endothelial cells (ECs) at ultra low concentrations (0.1-100 pM), with an IC50 = 0.1 pM, while it inhibits non-endothelial type human cells at 10(4) - to 10(5) -fold higher concentrations, with IC50 = 1-10 nM. The selectivity of paclitaxel inhibition of cell proliferation is also species specific, as mouse ECs are not sensitive to paclitaxel at ultra low concentrations. They are inhibited by higher concentrations of paclitaxel with IC50 = 1-10 nM. Inhibition of human ECs by paclitaxel at ultra low concentrations does not affect the cellular microtubule structure, and the treated cells do not show G2/M cell cycle arrest and apoptosis, suggesting a novel but as yet unidentified mechanism of action. In an in vitro angiogenesis assay, paclitaxel at ultra low concentrations blocks human ECs from forming sprouts and tubes in the three-dimensional fibrin matrix. In summary, paclitaxel selectively inhibits human EC proliferation and in vitro angiogenesis at low picomolar concentrations. The data support a clinical application of continuous ultra-low-dose paclitaxel to treat cancer.

Farnesyltransferase inhibitors in breast cancer therapy

Farnesyltransferase inhibitors (FTIs) belong to a group of agents originally designed to prevent membrane attachment of Ras protein by inhibiting a key step in its post-translational processing. It was thus hypothesized that FTIs would curtail the oncogenic ras-mediated proliferative and antiapoptotic signals that are activated in human tumors. Although the Ras protein is mutated in only < 5% of breast cancers, there are multiple aberrant pathways that lead to activation of wild-type ras signaling. Moreover, FTIs have consistently demonstrated efficacy in tumors regardless of their ras mutational status. Thus, the role of other protein targets in mediating the antitumor effect of FTIs is being elucidated. This article reviews current data on the use of FTIs in breast cancer.

DMXAA: an antivascular agent with multiple host responses

PURPOSE

To measure host responses to the antivascular agent DMXAA (5,6-dimethylxanthenone-4-acetic acid) and to compare them with those of other antivascular agents.

METHODS

Induction of tumor necrosis was measured in s.c. murine Colon 38 carcinomas growing in normal or tumor necrosis factor (TNF) receptor-1 knockout mice. Plasma and tumor tissue TNF concentrations were measured by ELISA. Plasma concentrations of 5-hydroxyindoleacetic acid (as a measure of serotonin release) and nitrite (as a measure of nitric oxide release) were measured by high-performance liquid chromatography.

RESULTS

Administration of DMXAA to tumor-bearing mice increased plasma and tumor tissue-associated TNF, in addition to increasing plasma nitric oxide, distinguishing its action from that of mitotic poisons that had an antivascular action. Results from TNF receptor-1 knockout mice showed that TNF played an important role in both its antitumor action and its host toxicity. Release of serotonin occurred in response to mitotic poisons, as well as to DMXAA. CONCLUCIONS: The antivascular action of DMXAA involves in situ production in tumor tissue of a cascade of vasoactive events, including a direct effect on vascular endothelial cells and indirect vascular effects involving TNF, other cytokines, serotonin, and nitric oxide. Now that Phase I clinical trials of DMXAA are completed, the optimization of this cascade in cancer patients is a major challenge. Plasma 5-hydroxyindoleacetic acid concentrations may provide a useful surrogate marker for the antivascular effects of DMXAA and other antivascular agents.

Combination of vascular targeting agents with thermal or radiation therapy

PURPOSE

The most likely clinical application of vascular targeting agents (VTAs) is in combination with more conventional therapies. In this study, we report on preclinical studies in which VTAs were combined with hyperthermia and/or radiation.

METHODS AND MATERIALS

A C3H mammary carcinoma grown in the right rear foot of female CDF1 mice was treated when at 200 mm3 in size. The VTAs were combretastatin A-4 disodium phosphate (CA4DP, 25 mg/kg), flavone acetic acid (FAA, 150 mg/kg), and 5,6-dimethylxanthenone-4-acetic acid (DMXAA, 20 mg/kg), and were all injected i.p. Hyperthermia and radiation were locally administered to tumors of restrained, nonanesthetized mice, and response was assessed using either a tumor growth or tumor control assay.

RESULTS

Heating tumors at 41.5 degrees C gave rise to a linear relationship between the heating time and tumor growth with a slope of 0.02. This slope was increased to 0.06, 0.09, and 0.08, respectively, by injecting the VTAs either 30 min (CA4DP), 3 h (FAA), or 6 h (DMXAA) before heating. The radiation dose (+/-95% confidence interval) that controls 50% of treated tumors (the TCD(50) value) was estimated to be 53 Gy (51-55 Gy) for radiation alone. This was decreased to 48 Gy (46-51 Gy), 45 Gy (41-49 Gy), and 42 Gy (39-45 Gy), respectively, by injecting CA4DP, DMXAA, or FAA 30-60 min after irradiating. These values were further decreased to around 28-33 Gy if the tumors of VTA-treated mice were also heated to 41.5 degrees C for 1 h, starting 4 h after irradiation, and this effect was much larger than the enhancement seen with either 41.5 degrees C or even 43 degrees C alone.

CONCLUSIONS

Our preclinical studies and those of others clearly demonstrate that VTAs can enhance tumor response to hyperthermia and/or radiation and support the concept that such combination studies should be undertaken clinically for the full potential of VTAs to be realized.

The development of combretastatin A4 phosphate as a vascular targeting agent

PURPOSE

This overview summarizes the preclinical development of tubulin-depolymerizing agents as vascular targeting agents, leading to the identification of combretastatin A4P (CA4P).

METHODS AND MATERIALS

The murine tumor CaNT was implanted s.c. in the dorsum of CBA mice. Vascular function was determined after treatment using the perfusion marker Hoechst 33342 and fluorescence microscopy. Tumor cell response was assessed by using an excision assay and by measuring the delay in growth of treated tumors.

RESULTS

At doses that approximated one-half the maximum tolerated dose (MTD) in CBA mice, none of the agents evaluated-i.e., taxol, melphalan, 5-fluorouracil, doxorubicin, cisplatin, gemcitabine, and irinotecan-induced any significant reduction in perfused vascular volume within the tumor mass. In contrast, CA4P at a dose of 100 mg/kg, which approximates one-fifth the MTD, induced a greater than 80% reduction in vascular function. Although colchicine did induce vascular shutdown, this occurred only at doses approximating the MTD. Histologic evaluation demonstrated that continued growth and repopulation of the tumor mass was the result of a surviving rim of viable tumor cells at the tumor periphery.

CONCLUSION

These results confirm the ability of CA4P to selectively compromise vascular function in experimental tumors, inducing extensive tumor cell death at well-tolerated doses. However, despite these effects, no growth retardation is obtained when CA4P is administered alone in a single dose. The continued growth and repopulation of the tumor mass occurs from a narrow rim of viable cells at the periphery. If, as is believed, these remaining cells are the ones most sensitive to conventional cytotoxic and macromolecular approaches, CA4P and other vascular targeting agents offer considerable potential for enhancing the effectiveness of existing and emerging cancer therapies.

Schedule dependence of combretastatin A4 phosphate in transplanted and spontaneous tumour models

Tubulin depolymerizing drugs that selectively disrupt tumour-associated vasculature have recently been identified. The lead drug in this class, combretastatin A4 phosphate (CA4P), has just completed Phase I clinical trial. Previous studies have focussed on the effects of single drug doses and have demonstrated little or no retardation of tumour growth when CA4P is used alone, but significant benefit when it is combined with conventional treatment. We have investigated the effects of multiple daily or twice daily dosing with CA4P on the vascular function, cell survival and growth of syngeneic and spontaneous breast cancers in mice. In both transplanted and spontaneous tumours significant growth retardation is observed if CA4P is administered daily (10 doses x 50 mg/kg), whereas no significant effects are seen if the same total dose (500 mg/kg) is administered as a single bolus injection. This effect is attributed, at least in part, to anti-proliferative effects on the tumour and endothelial cells, which retard the revascularisation and repopulation of the tumour core that is initially necrosed by the drug treatment. Further investigation of dose scheduling showed that the initial anti-vascular effects of CA4P are enhanced by administering the drug in 2 equal doses separated between 2 and 6 hr. The twice daily dosing schedule (25 mg/kg twice a day) produced increased growth retardation compared to the 50 mg/kg once a day schedule in the transplanted CaNT tumour. It did not do so in the spontaneous T138 tumour model. These studies indicate that the potential anti-tumour activity of CA4P when used as a single agent in clinical trials may be enhanced when used in multiple dose schedules.

Topotecan is anti-angiogenic in experimental hepatoblastoma

BACKGROUND

Advanced hepatoblastoma often is lethal despite current therapies, yet development of novel approaches has been hampered by the lack of biologically relevant models. One new strategy selectively targets endothelium rather than tumor cells using frequently administered, low-dose ("metronome") chemotherapy. Metronome topotecan has antiangiogenic activity in some experimental tumors. The authors developed a xenograft model of human hepatoblastoma to test the effect of metronome topotecan in this system.

METHODS

Xenografts resulted from intrarenal injection of cultured human hepatoblastoma cells in athymic mice. Topotecan (0.36 mg/kg/dose) or vehicle was injected intraperitoneally 5 times per week. At week 6, 10 control/treated mice were killed, and remaining animals were maintained without treatment until week 8. Tumor weights were compared by Kruskal-Wallis analysis, and vascular alterations were ascertained by specific immunostaining.

RESULTS

Metronome topotecan affected tumor weights in a delayed fashion: weights were diminished significantly only at 8 weeks (treated v control: 6 weeks, 0.59 g v 82 g, P value, not significant; 8 weeks, 1.13 g v 3.82; P <.02). Decreased vascularity and increased endothelial cell apoptosis were observed in treated xenografts.

CONCLUSIONS

Metronome topotecan inhibits growth and neovascularization in experimental hepatoblastoma. The durability of this effect is novel and has not been observed in other xenograft tumor models. Cytotoxic targeting of endothelial cells may hold particular promise for therapy of children with advanced hepatoblastoma. .

A novel combretastatin A-4 derivative, AC7700, strongly stanches tumour blood flow and inhibits growth of tumours developing in various tissues and organs

In a previous study, we used subcutaneous LY80 tumours (a subline of Yoshida sarcoma), Sato lung carcinoma, and methylcholanthrene-induced primary tumours, to demonstrate that a novel water-soluble combretastatin A-4 derivative, AC7700, abruptly and irreversibly stopped tumour blood flow. As a result of this interrupted supply of nutrients, extensive necrosis was induced within the tumour. In the present study, we investigated whether AC7700 acts in the same way against solid tumours growing in the liver, stomach, kidney, muscle, and lymph nodes. Tumour blood flow and the change in tumour blood flow induced by AC7700 were measured by the hydrogen clearance method. In a model of cancer chemotherapy against metastases, LY80 cells (2x10(6)) were injected into the lateral tail vein, and AC7700 at 10 mg x kg(-1) was injected i.v. five times at intervals of 2 days, starting on day 7 after tumour cell injection. The number and size of tumours were compared with those in the control group. The change in tumour blood flow and the therapeutic effect of AC7700 on microtumours were observed directly by using Sato lung carcinoma implanted in a rat transparent chamber. AC7700 caused a marked decrease in the tumour blood flow of all LY80 tumours developing in various tissues and organs and growth of all tumours including lymph node metastases and microtumours was inhibited. In every tumour, tumour blood flow began to decrease immediately after AC7700 administration and reached a minimum at approximately 30 min after injection. In many tumour capillaries, blood flow completely stopped within 3 min after AC7700 administration. These results demonstrate that AC7700 is effective for tumours growing in various tissues and organs and for metastases. We conclude that tumour blood flow stanching induced by AC7700 may become an effective therapeutic strategy for all cancers, including refractory cancers because the therapeutic effect is independent of tumour site and specific type of cancer.

Enhancement of radiation therapy by the novel vascular targeting agent ZD6126

PURPOSE

The aim of this study was to evaluate the antitumor efficacy of the novel vascular targeting agent ZD6126 (N-acetylcochinol-O-phosphate) in the rodent KHT sarcoma model, either alone or in combination with single- or fractionated-dose radiation therapy.

METHODS

C3H/HeJ mice bearing i.m. KHT tumors were injected i.p. with ZD6126 doses ranging from 10 to 150 mg/kg. Tumors were irradiated locally in unanesthetized mice using a linear accelerator. Tumor response to ZD6126 administered alone or in combination with radiation was assessed by clonogenic cell survival assay or tumor growth delay.

RESULTS

Treatment with ZD6126 led to a rapid tumor vascular shutdown as determined by Hoechst 33342 diffusion. Histologic evaluation showed morphologic damage of tumor cells within a few hours after drug exposure, followed by extensive central tumor necrosis and neoplastic cell death as a result of prolonged ischemia. When combined with radiation, a 150 mg/kg dose of ZD6126 reduced tumor cell survival 10-500-fold compared with radiation alone. These enhancements in tumor cell killing could be achieved for ZD6126 given both before and after radiation exposure. Further, the shape of the cell survival curve observed after the combination therapy suggested that including ZD6126 in the treatment had a major effect on the radiation-resistant hypoxic cell subpopulation associated with this tumor. Finally, when given on a once-weekly basis in conjunction with fractionated radiotherapy, ZD6126 treatment was found to significantly increase the tumor response to daily 2.5 Gy fractions.

CONCLUSION

The present results demonstrated that in the KHT sarcoma, ZD6126 caused rapid tumor vascular shutdown, induction of central tumor necrosis, tumor cell death secondary to ischemia, and enhancement of the antitumor effects of radiation therapy.

The influence of combretastatin A-4 and vinblastine on interstitial fluid pressure in BT4An rat gliomas

The influence of combretastatin A-4 disodium phosphate (CA-4, 50mg/kg intraperitoneally (i.p.)) and vinblastine (2mg/kg i.p.) on interstitial fluid pressure (IFP) was assessed in BT4An rat gliomas implanted subcutaneously in the neck. Furthermore the growth inhibitory effect of vinblastine and the distribution of fluorescence-conjugated vinblastine (BODIPY-vinblastine) were investigated. Tumors at different volumes were compared. Whereas CA-4 had no major influence on IFP, independent of tumor size, vinblastine increased the IFP in neoplasms above 8 cm(3) (P=0.03). Vinblastine yielded a significant tumor response only in tumors below 2.1 cm(3) (P=0.03). The distribution of BODIPY-vinblastine was heterogeneous and comparable despite tumor volume differences. We conclude that the influence of vinblastine on IFP is more pronounced than that of CA-4 in BT4An neck tumors, and that vinblastine may reduce subsequent drug delivery to solid tumors by increasing the IFP.

The biology of the combretastatins as tumour vascular targeting agents

The tumour vasculature is an attractive target for therapy. Combretastatin A-4 (CA-4) and A-1 (CA-1) are tubulin binding agents, structurally related to colchicine, which induce vascular-mediated tumour necrosis in animal models. CA-1 and CA-4 were isolated from the African bush willow, Combretum caffrum, and several synthetic analogues are also now available, such as the Aventis Pharma compound, AVE8062. More soluble, phosphated, forms of CA-4 (CA-4-P) and CA-1 (CA-1-P) are commonly used for in vitro and in vivo studies. These are cleaved to the natural forms by endogenous phosphatases and are taken up into cells. The lead compound, CA-4-P, is currently in clinical trial as a tumour vascular targeting agent. In animal models, CA-4-P causes a prolonged and extensive shut-down of blood flow in established tumour blood vessels, with much less effect in normal tissues. This paper reviews the current understanding of the mechanism of action of the combretastatins and their therapeutic potential.

Hypoxia as a target for combined modality treatments

There is overwhelming evidence that solid human tumours grow within a unique micro-environment. This environment is characterised by an abnormal vasculature, which leads to an insufficient supply of oxygen and nutrients to the tumour cells. These characteristics of the environment limit the effectiveness of both radiotherapy and chemotherapy. Measurement of the oxygenation status of human tumours has unequivocally demonstrated the importance of this parameter on patient prognosis. Tumour hypoxia has been shown to be an independent prognostic indicator of poor outcome in prostate, head and neck and cervical cancers. Recent laboratory and clinical data have shown that hypoxia is also associated with a more malignant phenotype, affecting genomic stability, apoptosis, angiogenesis and metastasis. Several years ago, scientists realised that the unique properties within the tumour micro-environment could provide the basis for tumour-specific therapies. Efforts that are underway to develop therapies that exploit the tumour micro-environment can be categorised into three groups. The first includes agents that exploit the environmental changes that occur within the micro-environment such as hypoxia and reduced pH. This includes bioreductive drugs that are specifically toxic to hypoxic cells, as well as hypoxia-specific gene delivery systems. The second category includes therapies designed to exploit the unique properties of the tumour vasculature and include both angiogenesis inhibitors and vascular targeting agents. The final category includes agents that exploit the molecular and cellular responses to hypoxia. For example, many genes are induced by hypoxia and promoter elements from these genes can be used for the selective expression of therapeutic proteins in hypoxic tumour cells. An overview of the various properties ascribed to tumour hypoxia and the current efforts underway to exploit hypoxia for improving cancer treatment will be discussed.

Measurement of plasma 5-hydroxyindoleacetic acid as a possible clinical surrogate marker for the action of antivascular agents

BACKGROUND

Serotonin (5HT), a naturally occurring vasoactive substance, is released from platelets into plasma under various pathological conditions. Recently, anticancer drugs that act by selectively disrupting tumour blood flow have been found to increase plasma 5HT concentrations in mice. Two such antivascular agents, flavone acetic acid (FAA) and 5,6-dimethylxanthenone-4-acetic acid (DMXAA), have completed Phase I clinical trial and raise the important question of whether suitable surrogate markers for antivascular effects can be identified.

METHODS

5HT is unstable to storage, precluding routine clinical assay, but the 5HT metabolite, 5-hydroxyindoleacetic acid (5HIAA) accumulates in plasma following 5HT release and is a more suitable marker because of its greater stability. We have developed an automated procedure for the assay of the low concentrations of 5HIAA found in humans by combining solid-phase extraction with high-performance liquid chromatography (HPLC).

RESULTS

Efficient separation of 5HIAA from possible interfering substances in human plasma, including a variety of pharmaceutical agents, was achieved on C18 columns using cetyltrimethylammonium bromide (CETAB) as an organic modifier. Adequate precision, accuracy and sensitivity were achieved by electrochemical detection (ECD) at +400 mV. Analysis of plasma from two patients treated with DMXAA in a Phase I trial demonstrated DMXAA-induced elevation of plasma 5HIAA with a time course similar to that previously described in mice.

CONCLUSIONS

Measurement of changes in plasma 5HIAA provides a new approach to the monitoring of therapies with an antivascular effect. The assay is sensitive to dietary sources of 5HT, which should be minimised.

Combretastatin A-4 disodium phosphate: a vascular targeting agent that improves that improves the anti-tumor effects of hyperthermia, radiation, and mild thermoradiotherapy

PURPOSE

To investigate the effect of combining the vascular targeting drug combretastatin A-4 disodium phosphate (CA4DP) with hyperthermia, radiation, or mild thermoradiotherapy in a transplanted C3H mouse mammary carcinoma.

METHODS AND MATERIALS

The C3H mammary carcinoma was grown on the rear foot of female CDF1 mice and treated when at 200 mm(3) in size. CA4DP was dissolved in saline and injected i.p. Hyperthermia and/or radiation were locally given to tumors in restrained nonanesthetized mice. Tumor response was assessed using either a tumor growth or a tumor control assay. Mouse foot skin was used to assess normal tissue damage.

RESULTS

CA4DP significantly enhanced thermal damage in this tumor model. This effect was independent of drug doses between 25-400 mg/kg, but was strongly dependent on the time interval between drug injection and heating, with the greatest improvement seen when CA4DP preceded the heating by 1 h or less. There was also a suggestion of a temperature dependency with a 1.9-fold increase in heat damage at 42.5 degrees C and a 2.6-fold increase at 41.5 and 40.5 degrees C. Heat-induced normal tissue damage was also enhanced by combining CA4DP with heat, but the degree of enhancement was less than that seen in tumors. CA4DP (25 mg/kg) significantly increased radiation-induced local tumor control and this was further enhanced by combining CA4DP with mild temperature (41.5 degrees C, 60 min) heating.

CONCLUSIONS

CA4DP improved the anti-tumor effect of hyperthermia, especially at mild temperatures. More importantly, it also increased the tumor response to mild hyperthermia and radiation, which suggests that CA4DP may ultimately have an important application in clinical thermoradiotherapy.

Vinflunine, the latest Vinca alkaloid in clinical development. A review of its preclinical anticancer properties

Vinflunine is a new Vinca alkaloid uniquely fluorinated, by the use of superacid chemistry, in a little exploited region of the catharanthine moiety. In vitro investigations have confirmed the mitotic-arresting and tubulin-interacting properties of vinflunine shared by other Vinca alkaloids. However, differences in terms of the inhibitory effects of vinflunine on microtubules dynamics and its tubulin binding affinities have been identified which appear to distinguish it from the other Vinca alkaloids. Vinflunine induced smaller spirals with a shorter relaxation time, effects, which might be associated with reduced neurotoxicity. Studies investigating the in vitro cytotoxicity of vinflunine in combination therapy have revealed a high level of synergy when vinflunine was combined with either cisplatin, mitomycin C, doxorubicin or 5-fluorouracil. Furthermore, although vinflunine appears to participate in P-glycoprotein-mediated drug resistance mechanisms, it has proved only a weak substrate for this protein and a far less potent inducer of resistance than vinorelbine. Vinflunine was identified in preclinical studies as having marked antitumour activity in vivo against a large panel of experimental tumour models, with tumour regressions being recorded in human renal and small cell lung cancer tumour xenografts. Overall its level of activity was superior to that of vinorelbine in many of the experimental models used. Interestingly, an in vivo study using a well vascularised adenocarcinoma of the colon has suggested that vinflunine mediates its antitumour activity at least in part via an antivascular mechanism, even at sub-cytotoxic doses. Therefore, these data provide a favourable preclinical profile for vinflunine, supporting its promising candidacy for clinical development. Phase I evaluations of vinflunine have been completed in Europe and phase II clinical trials are now ongoing.

Vinblastine and hyperthermia target the neovasculature in BT(4)AN rat gliomas: therapeutic implications of the vascular phenotype

PURPOSE

The antivascular and antitumor activity of vinblastine and hyperthermia at different tumor volumes were examined in the subcutaneous (s.c.) BT(4)An rat glioma model.

METHODS AND MATERIALS

The influence of vinblastine (3 mg/kg) and hyperthermia (44 degrees C/60 min) on tumor growth was assessed in small (100 mm(3)) and large (200 mm(3)) BT(4)An tumors. To disclose how vinblastine and hyperthermia interacted in the neoplasms, tumor blood flow and the extent of vascular damage, hypoxia, cell proliferation, and apoptosis were assessed after treatment. The content of smooth muscle cells/pericytes in the tumor vasculature was examined in small and large tumors to assess how the vascular phenotype changed during tumor growth.

RESULTS

In the large tumors, vinblastine reduced the blood flow, but the tumor growth was not affected. The combination of drug and local heating yielded massive vascular damage and a significant tumor response. The small neoplasms had a higher content of smooth muscle cells/pericytes in the vessel walls (host vasculature), and the tumor vasculature displayed a higher resistance to vascular damage than the large neoplasms. Yet, vinblastine alone exhibited a potent antiproliferative activity and induced massive apoptosis in the small tumors, and the drug significantly inhibited tumor growth. The addition of hyperthermia yielded no additional growth delay in the small tumors.

CONCLUSION

The antivascular properties of vinblastine and hyperthermia can be exploited to facilitate vascular damage in BT(4)An solid tumors with a low content of host vasculature.

Combretastatin A-4 and hyperthermia;a potent combination for the treatment of solid tumors

BACKGROUND AND PURPOSE

Attacking tumor vasculature is a promising approach for the treatment of solid tumors. The tubulin inhibitor combretastatin A-4 disodium phosphate (CA-4) is a new vascular targeting drug which displays a low toxicity profile. We wanted to investigate how CA-4 influences tumor perfusion in the BT4An rat glioma and how the vascular targeting properties of CA-4 could be exploited to augment hyperthermic damage towards tumor vasculature.

MATERIAL AND METHODS

We used the (86)RbCl extraction technique to assess how CA-4 influences tumor perfusion, and the tumor endothelium was examined for morphological changes induced by the drug. We combined CA-4 (50 mg/kg i.p.) with hyperthermia (44 degrees C, 60 min) at different time intervals to evaluate how therapy should be designed to affect tumor growth, and we studied the tumors histologically to assess tissue viability.

RESULTS

We found that CA-4 induced a profound, but transient reduction in tumor perfusion 3-6 h postinjection. If hyperthermia was administered 3-6 h after injecting CA-4, massive hemorrhagic necrosis developed, and tumor response was significantly enhanced compared to simultaneous administration of the two treatment modalities (P<0.005). CA-4 alone had no influence on tumor growth and failed to disrupt the vasculature of the BT4An solid tumors. Interestingly though, a mild endothelial edema was observed in some tumor areas 3 h after injecting CA-4.

CONCLUSIONS

We conclude that the combination of CA-4 and hyperthermia is a potent therapeutic option for BT4An tumors, but the selection of adequate time intervals between CA-4 and hyperthermia are imperative to obtain tumor response.

Anti-vascular effects of vinflunine in the MAC 15A transplantable adenocarcinoma model

Anti-vascular effects of the novel Vinca alkaloid, vinflunine have been investigated in the MAC 15A transplantable murine colon adenocarcinoma model and compared with those induced by the most recently identified clinically useful third generation Vinca. Administration of the maximum tolerated dose of either vinflunine (50 mg kg(-1)) or vinorelbine (8 mg kg(-1)) resulted in significant tumour growth delay with subsequent histological analysis revealing substantial haemorrhagic necrosis. This suggested possible anti-vascular effects and these were confirmed by Hoechst 33342 perfusion studies. Vinflunine, currently undergoing Phase I trials in Europe, was found to be at least as effective as the clinically active vincristine and vinorelbine in this model and, remarkably, produced anti-vascular effects at doses much lower than the maximum tolerated dose. Although vinflunine caused apoptosis in HUVEC monolayer cultures this event did not occur within the first 8 hours of exposure whereas vascular shutdown in vivo was observed within the first 4 hours.

Vascular targeting of solid tumours: a major 'inverse' volume-response relationship following combretastatin A-4 phosphate treatment of rat rhabdomyosarcomas

Tumour-specific vascularisation may be therapeutically approached in two different ways: by antiangiogenic treatments specifically directed to dividing and migrating endothelial cells, or by agents that target principally the inadequate and ill-structured tumour vasculature. Combretastatin A-4 phosphate (combreAp), a recently synthesised prodrug (OXiGENE, Lund, Sweden), is a vascular targeting agent of the latter kind. We evaluated the effect of a single intraperitoneal (i.p.) combreAp injection on the growth of rhabdomyosarcomas syngeneic in WAG/Rij rats. Different tumour volume groups, ranging between 0.1 and 27 cm(3), were selected to assess the relationship between the size at treatment time and the response to combreAp. A double combreAp treatment (2x25 mg/kg) was investigated within the same overall aim: the relationship between growth delay and tumour size. Our results show that the systemic administration of combreAp induces a clear-cut differential growth delay in the solid rat rhabdomyosarcomas: with very large tumours (>/= 14 cm(3)), a 17.6-fold stronger effect was measured than with very small tumours (<1 cm(3)). This is the 'inverse' of the volume-response seen with the conventional therapeutic approaches (radiotherapy, chemotherapy or surgery). These combreAp antitumour responses were observed without treatment limiting systemic toxicity in the rats. With clinical digital subtraction angiography, using microsurgical cannulation of a major tumour draining vessel, and with histopathology, we demonstrate that growth delay is related to an early (within 3-6 h) and extensive breakdown of tumour blood vessels. The experiments involving a second injection also indicate a volume-dependent effect of combreAp in reducing the regrowth rate of small or large rhabdomyosarcomas. This significant differential volume-response obtained with 'selective' vascular targeting, stronger in larger tumours than smaller ones, suggests the potential of broadening the therapeutic window.

TZT-1027, an antimicrotubule agent, attacks tumor vasculature and induces tumor cell death

TZT-1027, a dolastatin 10 derivative, is an antimicrotubule agent with potent antitumor activity both in vitro and in vivo. In this study, we performed biochemical and histopathological examinations, and evaluated TZT-1027-induced tumoral vascular collapse and tumor cell death in an advanced tumor model, murine colon 26 adenocarcinoma. In addition, we studied the effects of TZT-1027 on cultured human umbilical vein endothelial cells (HUVEC). Tolerable doses of TZT-1027 induced tumor-selective hemorrhage within 1 h. This hemorrhage occurred mainly in the peripheral area of the tumor mass. Measurements of tumoral hemoglobin content and dye permeation revealed that the hemorrhage occurred firstly and tumor blood flow stopped secondarily. The vascular damage was followed by continuous induction of apoptosis of the tumor cells, tumor tissue necrosis, and tumor regression. In cultured HUVEC, TZT-1027 induced marked cell contraction with membrane blebbing in 30 min. These cell changes were completely inhibited by K252a, a broad-spectrum inhibitor of protein kinases. These effects of TZT-1027 on both tumor vasculature and HUVEC were greater than those of vincristine. In conclusion, TZT-1027 quickly attacked the well-developed vascular system of advanced tumors by a putative protein kinase-dependent mechanism, and then blocked tumor blood flow. Therefore, TZT-1027 has both a conventional antitumor activity and a unique anti-tumoral vascular activity, making it a potentially powerful tool for clinical cancer therapy.