Tumour vascular disrupting agents: combating treatment resistance

Phase I trial of CYT997, a novel cytotoxic and vascular-disrupting agent

Background:

CYT997 is a novel microtubule inhibitor and vascular-disrupting agent with marked preclinical anti-tumour activity.

Methods:

This phase I dose-escalation study assessed the safety, tolerability, pharmacokinetics and pharmacodynamics of CYT997 administered by continuous intravenous infusion over 24 h every 3 weeks to patients with advanced solid tumours.

Results:

Thirty-one patients received CYT997 over 12 dose levels (7–358 mg m⁻²). Doses up to 202 mg m⁻² were well tolerated. Dose-limiting toxicities were observed at 269 and 358 mg m⁻², consisting of grade 3 prolonged corrected QT interval in two patients and grade 3 hypoxia and grade 4 dyspnea in one patient. All toxicities were reversible. The pharmacokinetics of CYT997 were linear over the entire dose range. Dynamic contrast-enhanced magnetic resonance imaging scans showed significant changes in tumour K^trans values consistent with vascular disruption in 7 out of 11 evaluable patients treated at CYT997 doses of ⩾65 mg m⁻². Moreover, plasma levels of von Willebrand factor and caspase-cleaved cytokeratin-18 increased post-treatment at higher dose levels. Among 22 patients evaluable for response, 18 achieved stable disease for >2 cycles.

Conclusions:

CYT997 was well tolerated at doses that were associated with pharmacodynamic evidence of vascular disruption in tumours.

Extracellular matrix proteins and tumor angiogenesis

Tumor development is a complex process that relies on interaction and communication between a number of cellular compartments. Much of the mass of a solid tumor is comprised of the stroma which is richly invested with extracellular matrix. Within this matrix are a host of matricellular proteins that regulate the expression and function of a myriad of proteins that regulate tumorigenic processes. One of the processes that is vital to tumor growth and progression is angiogenesis, or the formation of new blood vessels from preexisting vasculature. Within the extracellular matrix are structural proteins, a host of proteases, and resident pro- and antiangiogenic factors that control tumor angiogenesis in a tightly regulated fashion. This paper discusses the role that the extracellular matrix and ECM proteins play in the regulation of tumor angiogenesis.

Antivascular ultrasound therapy extends survival of mice with implanted melanomas

The goal of this murine investigation was to evaluate the effect of an antivascular ultrasound treatment on the growth of an implanted melanoma and the consequent survival rate. After the intravenous injection of 0.2 mL ultrasound contrast agent (Definity), therapy (n = 15) was performed on 1-mL tumors for 3 min with low-intensity continuous ultrasound (3 MHz; 2.4 +/- 0.1 W cm(-2) [I(SATA)]); control mice (n = 17) received a sham treatment. Mice were euthanized once the tumor had reached 3 mL, and then survival percentage vs. time curves were plotted. The median survival time (time for tumor to reach 3 mL) for the treated group was 23 d and for the control group was 18 d; the difference was statistically significant (p <or= 0.0001). Antivascular ultrasound therapy reduced the growth rate of an implanted melanoma and increased survival time. The ultrasound therapy provides a further example of tumor vascular disruption, and its future clinical potential should be investigated.

An efficient synthetic strategy for obtaining 4-methoxy carbon isotope labeled combretastatin A-4 phosphate and other Z-combretastatins

Human cancer and other clinical trials under development employing combretastatin A-4 phosphate (1b, CA4P) should benefit from the availability of a [(11)C]-labeled derivative for positron emission tomography (PET). In order to obtain a suitable precursor for addition of a [(11)C]methyl group at the penultimate step, several new synthetic pathways to CA4P were evaluated. Geometrical isomerization (Z to E) proved to be a challenge, but it was overcome by development of a new CA4P synthesis suitable for 4-methoxy isotope labeling.

An in vitro model that can distinguish between effects on angiogenesis and on established vasculature: actions of TNP-470, marimastat and the tubulin-binding agent Ang-510

In anti-cancer therapy, current investigations explore the possibility of two different strategies to target tumor vasculature; one aims at interfering with angiogenesis, the process involving the outgrowth of new blood vessels from pre-existing vessels, while the other directs at affecting the already established tumor vasculature. However, the majority of in vitro model systems currently available examine the process of angiogenesis, while the current focus in anti-vascular therapies moves towards exploring the benefit of targeting established vasculature as well. This urges the need for in vitro systems that are able to differentiate between the effects of compounds on angiogenesis as well as on established vasculature. To achieve this, we developed an in vitro model in which effects of compounds on different vascular targets can be studied specifically. Using this model, we examined the actions of the fumagillin derivate TNP-470, the MMP-inhibitor marimastat and the recently developed tubulin-binding agent Ang-510. We show that TNP-470 and marimastat solely inhibited angiogenesis, whereas Ang-510 potently inhibited angiogenesis and caused massive disruption of newly established vasculature. We show that the use of this in vitro model allows for specific and efficient screening of the effects of compounds on different vascular targets, which may facilitate the identification of agents with potential clinical benefit. The indicated differences in the mode of action between marimastat, TNP-470 and Ang-510 to target vasculature are illustrative for this approach.

MR reporter gene imaging of endostatin expression and therapy

PURPOSE

The aim of this study is to monitor endostatin gene expression and therapy using transferrin receptor (TfR) as reporter gene and transferrin conjugate of ultrasmall supramagnetic iron oxide nanoparticle (Tf-USPIO) as magnetic resonance (MR) reporter probe.

PROCEDURE

A retroviral plasmid (pLP-LNCX) encoding mouse endostatin and TfR was constructed, and packaged with a titer of 4 × 10(7)colony-forming units per millimeter. MDA-MB-231 breast tumors were established in BALB/c mice by subcutaneous injection of 2 × 10(6) MDA-MB-231 cells. Mice were intratumorally injected with recombinant retrovirus and imaged with MR using Tf-USPIO. Western blot, Prussian blue, and immunohistochemical staining were performed to validate the magnetic resonance imaging results. The antitumor effect of retro-endostatin (ES)-TfR was also evaluated by intratumoral injection of the viral vector.

RESULTS

The expression of both endostatin and TfR genes in MDA-MB-231 cells after retroviral transfection was confirmed by Western blot and flow cytometry. Tf-USPIO conjugate binds specifically to cells stably transfected with retro-ES-TfR. After intravenous injection of the Tf-USPIO conjugate, there was a more pronounced decrease in T2 relaxation time in tumors treated with retro-ES-TfR than in tumors treated with empty retrovirus retro-LNCX. The expression of ES gene significantly delayed the growth of MDA-MB-231 tumor and reduction of microvessel density and VEGF level as compared to those without viral transfection or transfected with empty retro-LNCX vector.

CONCLUSIONS

Endostatin therapeutic gene expression was visualized successfully using TfR reporter gene and Tf-USPIO MR reporter probe, which indicates that MR reporter gene imaging may be valuable in gene therapy to evaluate therapeutic gene expression and treatment efficacy.

Microtubule depolymerizing vascular disrupting agents: novel therapeutic agents for oncology and other pathologies

Vascular disrupting agents (VDAs) are a relatively new group of 'vascular targeting' agents that exhibit selective activity against established tumour vascular networks, causing severe interruption of tumour blood flow and necrosis to the tumour mass. Microtubule depolymerizing agents form by far the largest group of small molecular weight VDAs many of which, including lead compound disodium combretastatin A-4 3-O-phosphate (CA-4-P), are under clinical development for cancer. Although distinct from the angiogenesis inhibitors, VDAs can also interfere with angiogenesis and therefore constitute a potential group of novel drugs for the treatment of pathological conditions characterized by excessive angiogenesis, in addition to cancer. The endothelial cytoskeleton is the primary cellular target of this family of drugs, and some progress in understanding the molecular and signalling mechanisms associated with their endothelial disrupting activity has been made in the last few years. Susceptibility of tumour vessels to VDA damage is ascribed to their immature pericyte-defective nature, although the exact molecular mechanisms involved have not been clearly defined. Despite causing profound damage to tumours, VDAs fail to halt tumour growth unless used together with conventional treatments. This failure is attributed to resistance mechanisms, primarily associated with cells that remain viable within the tumour rim, and enhanced angiogenesis. The focus is now to understand mechanisms of susceptibility and resistance to identify novel molecular targets and develop strategies that are more effective.

A dose escalation, safety, and tolerability study of MN-029 in patients with advanced solid tumors

PURPOSE

To assess the maximum-tolerated dose (MTD), dose-limiting toxicity (DLT), safety, and tolerability of MN-209, a novel vascular disrupting agent, in patients with advanced solid tumors.

STUDY DESIGN

MN-029 was administered weekly for three consecutive weeks out of four; two cycles were planned. Dose escalation proceeded by 100% per toxicity criteria. Intra-patient dose escalation was permitted.

RESULTS

Twenty patients received a total of 151 infusions of MN-029. No DLTs or grade 4 toxicities occurred. The most common adverse events were nausea, vomiting, arthralgias, and headache. One patient developed acute substernal chest pain 4 days after his first dose of MN-029 and was removed from the study. An MTD was not determined. The recommended phase II dose was identified as 180 mg/m(2)/week. One patient with advanced pancreatic cancer attained a partial response lasting 10 weeks.

CONCLUSIONS

MN-029 was well tolerated in this schedule. Further development of this class of agents is warranted, especially in combination with other anti-cancer treatments.