Patupilone induced vascular disruption in orthotopic rodent tumor models detected by magnetic resonance imaging and interstitial fluid pressure

Vessel size index magnetic resonance imaging to monitor the effect of antivascular treatment in a rodent tumor model

PURPOSE

Vascular disrupting agents are anticancer agents that typically produce a cytostatic tumor response. Vessel size index magnetic resonance imaging (MRI) allows for the estimation of the fractional blood volume (fBV) and blood vessel size (Rv). We assessed whether the vessel size index parameters provided imaging biomarkers for detecting early tumor response to a vascular disrupting agent.

METHODS AND MATERIALS

GH3 prolactinomas were grown subcutaneously in 12 rats. Vessel size index MRI was performed with Sinerem, an ultrasmall superparamagnetic iron oxide intravascular contrast agent, to determine the tumor fBV and Rv. MRI was performed before and at 24 h after treatment with either the vascular disrupting agent, 5,6-dimethylxanthenone 4-acetic acid (DMXAA) (n = 6) or with the drug vehicle (n = 6). After treatment, the tumors were analyzed histologically and correlates with the MRI findings sought.

RESULTS

Histogram analysis showed non-normal distributions of Rv and fBV. The 25th percentiles of the fBV and Rv were significantly reduced (p < 0.01) after treatment with DMXAA, with an increase in the regions of low-measured fBV. For the treated and control tumors, the fraction of tumor with an fBV of < or =1% correlated with the histologically determined percentage of necrosis (r = 0.77, p < 0.005). The fraction of tumor with an fBV of < or =1% in treated tumors was significantly increased compared with before treatment (p < 0.05) and with that in the controls (p < 0.05).

CONCLUSION

The vessel size index results were consistent with the known action of DMXAA to cause vascular collapse, with histogram analysis of the fBV providing the most sensitive indicator of response. In particular, the parameter, the fraction of tumor with an fBV of < or =1% is a potential biomarker that correlates with the histopathologic measure of tumor necrosis.

New tubulin targeting agents currently in clinical development

BACKGROUND

The first tubulin targeting agents were approved as cancer chemotherapeutics over 40 years ago and tubulin as an antitumor target continues to attract significant drug discovery and development attention. Mechanism of action as defined by tubulin binding sites and effect on microtubules distinguishes these agents, but the end result is equivalent in that microtubule disruption leads to cell cycle arrest at G2/M phase of the cell cycle and subsequent apoptotic cell death.

OBJECTIVES

The goal of this review is to describe the state of clinical development of tubulin targeting agents as of early 2008, with descriptions of clinical experience slanted toward the most advanced trials for each agent.

METHOD

Objective information in this review was obtained exclusively from public sources that included journals, scientific meeting abstracts, posters and oral presentations, websites and public presentations from companies. Opinions expressed in this review are exclusively from the author.

RESULTS/CONCLUSIONS

A large number of tubulin targeting agents are currently in clinical development, including microtubule stabilizing and destabilizing compounds acting through all three of the characterized tubulin binding sites. With the approval of ixabepilone for refractory breast cancer, the epothilones appear best positioned to make an impact among the new microtubule stabilizing compounds. There are 17 microtubule destabilizing agents under clinical assessment, with many only in Phase I and results to date include at best modest efficacy signals with no obvious indication trend.

Pharmacokinetic profile of the microtubule stabilizer patupilone in tumor-bearing rodents and comparison of anti-cancer activity with other MTS in vitro and in vivo

INTRODUCTION

Patupilone is a microtubule stabilizer (MTS) currently in clinical development. Here, we evaluate the anti-cancer activity in vitro and in vivo in comparison to paclitaxel and describe the pharmacokinetics (PK) of patupilone in tumor-bearing nude mice and rats.

METHODS

The potency in vitro of patupilone and two other MTS, paclitaxel and ixabepilone, was determined using human colon carcinoma cell lines with low (HCT-116, HT-29, RKO) and high (HCT-15) P-glycoprotein expression (P-gp), as well as two multi-drug resistance (MDR) model cell pairs, MCF7/ADR and KB-8511 cells and their respective drug-sensitive parental counterparts. The PK of patupilone was investigated in nude mice bearing HCT-15 or HT-29 xenografts and in rats bearing s.c. pancreatic CA20498 tumors or A15 glioma tumors. Anti-cancer activity in vivo was compared to that of paclitaxel using three different human tumor colon models. The retention and efficacy of patupilone was compared in small and large HT-29 xenografts whose vascularity was determined by non-invasive magnetic resonance imaging.

RESULTS

Patupilone was highly potent in vitro against four different colon carcinoma cell lines including those showing multi-drug-resistance. In contrast, paclitaxel and ixabepilone displayed significantly reduced activity with markedly increased resistance factors. In both rats and mice, a single i.v. bolus injection of patupilone (1.5-4 mg/kg) rapidly distributed from plasma to all tissues and was slowly eliminated from muscle, liver and small intestine, but showed longer retention in tumor and brain with no apparent elimination over 24 h. Patupilone showed significant activity against three human colon tumor models in vivo, unlike paclitaxel, which only had activity against low P-gp expressing tumors. In HT-29 tumors, patupilone activity and retention were independent of tumor size, blood volume and flow.

CONCLUSIONS

The high potency of patupilone, which is not affected by P-gp expression either in vitro or in vivo, and favorable PK, independent of tumor vascularity, suggest that it should show significant activity in colorectal cancer and in other indications where high P-gp expression may compromise taxane activity.

The effects of tumor-derived platelet-derived growth factor on vascular morphology and function in vivo revealed by susceptibility MRI

Platelet-derived growth factors (PDGF) play a major role in pericyte recruitment in tumor capillaries. Pericytes are required for proper vessel development, and contribute to tumor angiogenesis by promoting stabilization and maturation of newly formed vessels. To investigate the effects of pericyte coverage on tumor vessel morphology and function in vivo, tumors derived from B16 melanoma cells transfected with either control plasmid (B16/ctr) or plasmid encoding full-length PDGF-BB (B16/PDGF), the latter previously shown to have enhanced blood vessel pericyte coverage and an increased tumor growth rate, were assessed using histopathological methods, Hoechst 33342-based perfusion analyses, and two noninvasive susceptibility magnetic resonance imaging (MRI) methods. Susceptibility-contrast MRI, incorporating the use of ultrasmall superparamagnetic iron oxide particles, revealed a significant (p < 0.05) reduction in vessel size index (R(v)) of B16/PDGF tumors, and which was validated histologically by the presence of significantly smaller (p < 0.001), more punctate blood vessels identified by fluorescence microscopy of the perfusion marker Hoechst 33342. Intrinsic-susceptibility MRI was used to measure the transverse MRI relaxation rate R(2)*, sensitive to changes in endogenous paramagnetic [deoxyhaemoglobin], and used to probe for vascular maturation and function. Hypercapnia (5% CO(2)/95% air) induced a negligible Delta R(2)* response in the B16/ctr and B16/PDGF tumors. In contrast, hyperoxia (5% CO(2)/95% O(2)) induced a significantly greater R(2)* reduction in the B16/PDGF tumors (p < 0.02). Together the susceptibility MRI-derived biomarkers reveal novel pericyte-dependent changes in the morphology and function of the perfused tumor vasculature in vivo.

Susceptibility contrast magnetic resonance imaging determination of fractional tumor blood volume: a noninvasive imaging biomarker of response to the vascular disrupting agent ZD6126

PURPOSE

To assess tumor fractional blood volume (xi), determined in vivo by susceptibility contrast magnetic resonance imaging (MRI) as a noninvasive imaging biomarker of tumor response to the vascular disrupting agent ZD6126.

METHODS AND MATERIALS

The transverse MRI relaxation rate R(2)( *) of rat GH3 prolactinomas was quantified prior to and following injection of 2.5 mgFe/kg feruglose, an ultrasmall superparamagnetic iron oxide intravascular contrast agent, and xi (%) was determined from the change in R(2)( *). The rats were then treated with either saline or 50 mg/kg ZD6126, and xi measured again 24 hours later. Following posttreatment MRI, Hoechst 33342 (15 mg/kg) was administered to the rats and histological correlates from composite images of tumor perfusion and necrosis sought.

RESULTS

Irrespective of treatment, tumor volume significantly increased over 24 hours. Saline-treated tumors showed no statistically significant change in xi, whereas a significant (p = 0.002) 70% reduction in xi of the ZD6126-treated cohort was determined. Hoechst 33342 uptake was associated with viable tumor tissue and was significantly (p = 0.004) reduced and restricted to the rim of the ZD6126-treated tumors. A significant positive correlation between posttreatment xi and Hoechst 33342 uptake was obtained (r = 0.83, p = 0.002), providing validation of the MRI-derived measurements of fractional tumor blood volume.

CONCLUSIONS

These data clearly highlight the potential of susceptibility contrast MRI with ultrasmall superparamagnetic iron oxide contrast agents to provide quantitative imaging biomarkers of fractional tumor blood volume at high spatial resolution to assess tumor vascular status and response to vascular disrupting agents.

In vivo mouse imaging and spectroscopy in drug discovery

Imaging modalities such as micro-computed tomography (micro-CT), micro-positron emission tomography (micro-PET), high-resolution MRI, optical imaging, and high-resolution ultrasound have become invaluable tools in preclinical pharmaceutical research. They can be used to non-invasively investigate, in vivo, rodent biology and metabolism, disease models, and pharmacokinetics and pharmacodynamics of drugs. The advantages and limitations of each approach usually determine its application, and therefore a small-rodent imaging laboratory in a pharmaceutical environment should ideally provide access to several techniques. In this paper we aim to illustrate how these techniques may be used to obtain meaningful information for the phenotyping of transgenic mice and for the analysis of compounds in murine models of disease.

Pathophysiologic Effects of Vascular-Targeting Agents and the Implications for Combination with Conventional Therapies

A functional vascular supply is critical for the continued growth and development of solid tumors. It also plays a major role in metastatic spread of tumor cells. This importance has led to the concept of targeting the vasculature of the tumor as a form of cancer therapy. Two major types of vascular-targeting agent (VTA) have now emerged: those that prevent the angiogenic development of the neovasculature of the tumor and those that specifically damage the already established tumor vascular supply. When used alone neither approach readily leads to tumor control, and so, for VTAs to be most successful in the clinic they will need to be combined with more conventional therapies. However, by affecting the tumor vascular supply, these VTAs should induce pathophysiologic changes in variables, such as blood flow, pH, and oxygenation. Such changes could have negative or positive influences on the tumor response to more conventional therapies. This review aims to discuss the pathophysiologic changes induced by VTAs and the implications of these effects on the potential use of VTAs in combined modality therapy.

Integration of quantitative DCE-MRI and ADC mapping to monitor treatment response in human breast cancer: initial results

PURPOSE

The objective of this study was to assess changes in the water apparent diffusion coefficient (ADC) and in pharmacokinetic parameters obtained from the fast-exchange regime (FXR) modeling of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) during neoadjuvant chemotherapy in breast cancer.

MATERIALS AND METHODS

Eleven patients with locally advanced breast cancer underwent MRI examination prior to and after chemotherapy but prior to surgery. A 1.5-T scanner was used to obtain T1, ADC and DCE-MRI data. DCE-MRI data were analyzed by the FXR model returning estimates of K(trans) (volume transfer constant), v(e) (extravascular extracellular volume fraction) and tau(i) (average intracellular water lifetime). Histogram and correlation analyses assessed parameter changes post-treatment.

RESULTS

Significant (P < .05) changes or trends towards significance (P < .10) were seen in all parameters except tau(i), although there was qualitative reduction in tau(i) values post-treatment. In particular, there was reduction (P < .035) in voxels with K(trans) values in the range 0.2-0.5 min(-1) and a decrease (P < .05) in voxels with ADC values in the range 0.99 x 10(-3) to 1.35 x 10(-3) mm2/s. ADC and v(e) were negatively correlated (r = -.60, P < .02). Parameters sensitive to water distribution and geometry (T(1), v(e), tau(i) and ADC) correlated with a multivariable linear regression model.

CONCLUSION

The analysis presented here is sensitive to longitudinal changes in breast tumor status; K(trans) and ADC are most sensitive to these changes. Relationships between parameters provide information on water distribution and geometry in the tumor environment.