Comparison of single- and dual-tracer pharmacokinetic modeling of dynamic contrast-enhanced MRI data using low, medium, and high molecular weight contrast agents

Improved Evaluation of Antivascular Cancer Therapy Using Constrained Tracer-Kinetic Modeling for Multiagent Dynamic Contrast-Enhanced MRI

Dynamic contrast-enhanced MRI (DCE-MRI) is a promising technique for assessing the response of tumor vasculature to antivascular therapies. Multiagent DCE-MRI employs a combination of low and high molecular weight contrast agents, which potentially improves the accuracy of estimation of tumor hemodynamic and vascular permeability parameters. In this study, we used multiagent DCE-MRI to assess changes in tumor hemodynamics and vascular permeability after vascular-disrupting therapy. Multiagent DCE-MRI (sequential injection of G5 dendrimer, G2 dendrimer, and Gd-DOTA) was performed in tumor-bearing mice before, 2 and 24 hours after treatment with vascular disrupting agent DMXAA or placebo. Constrained DCE-MRI gamma capillary transit time modeling was used to estimate flow F, blood volume fraction v_b, mean capillary transit time t_c, bolus arrival time t_d, extracellular extravascular fraction v_e, vascular heterogeneity index α^-1 (all identical between agents) and extraction fraction E (reflective of permeability), and transfer constant K^trans (both agent-specific) in perfused pixels. F, v_b, and α^-1 decreased at both time points after DMXAA, whereas t_c increased. E (G2 and G5) showed an initial increase, after which, both parameters restored. K^trans (G2 and Gd-DOTA) decreased at both time points after treatment. In the control, placebo-treated animals, only F, t_c, and K^trans Gd-DOTA showed significant changes. Histologic perfused tumor fraction was significantly lower in DMXAA-treated versus control animals. Our results show how multiagent tracer-kinetic modeling can accurately determine the effects of vascular-disrupting therapy by separating simultaneous changes in tumor hemodynamics and vascular permeability.Significance: These findings describe a new approach to measure separately the effects of antivascular therapy on tumor hemodynamics and vascular permeability, which could help more rapidly and accurately assess the efficacy of experimental therapy of this class. Cancer Res; 78(6); 1561-70. ©2018 AACR.

Flow versus permeability weighting in estimating the forward volumetric transfer constant (Ktrans) obtained by DCE-MRI with contrast agents of differing molecular sizes

PURPOSE

To quantify the differential plasma flow- (F_p-) and permeability surface area product per unit mass of tissue- (PS-) weighting in forward volumetric transfer constant (K^trans) estimates by using a low molecular (Gd-DTPA) versus high molecular (Gadomer) weight contrast agent in dynamic contrast enhanced (DCE) MRI.

MATERIALS AND METHODS

DCE MRI was performed using a 7T animal scanner in 14 C57BL/6J mice syngeneic for TRAMP tumors, by administering Gd-DTPA (0.9kD) in eight mice and Gadomer (35kD) in the remainder. The acquisition time was 10min with a sampling rate of one image every 2s. Pharmacokinetic modeling was performed to obtain K^trans by using Extended Tofts model (ETM). In addition, the adiabatic approximation to the tissue homogeneity (AATH) model was employed to obtain the relative contributions of F_p and PS.

RESULTS

The K^trans values derived from DCE-MRI with Gd-DTPA showed significant correlations with both PS (r²=0.64, p=0.009) and F_p (r²=0.57, p=0.016), whereas those with Gadomer were found only significantly correlated with PS (r²=0.96, p=0.0003) but not with F_p (r²=0.34, p=0.111). A voxel-based analysis showed that K^trans approximated PS (<30% difference) in 78.3% of perfused tumor volume for Gadomer, but only 37.3% for Gd-DTPA.

CONCLUSIONS

The differential contributions of F_p and PS in estimating K^trans values vary with the molecular weight of the contrast agent used. The macromolecular contrast agent resulted in K^trans values that were much less dependent on flow. These findings support the use of macromolecular contrast agents for estimating tumor vessel permeability with DCE-MRI.

Connexin 43-targeted T1 contrast agent for MRI diagnosis of glioma

Glioblastoma multiforme is the most aggressive form of brain tumor. Early and accurate diagnosis of glioma and its borders is an important step for its successful treatment. One of the promising targets for selective visualization of glioma and its margins is connexin 43 (Cx43), which is highly expressed in reactive astrocytes and migrating glioma cells. The purpose of this study was to synthesize a Gd-based contrast agent conjugated with specific antibodies to Cx43 for efficient visualization of glioma C6 in vivo. We have prepared stable nontoxic conjugates of monoclonal antibody to Cx43 and polylysine-DTPA ligands complexed with Gd(III), which are characterized by higher T1 relaxivity (6.5 mM(-1) s(-1) at 7 T) than the commercial agent Magnevist® (3.4 mM(-1) s(-1)). Cellular uptake of Cx43-specific T1 contrast agent in glioma C6 cells was more than four times higher than the nonspecific IgG-contrast agent, as detected by flow cytometry and confocal analysis. MRI experiments showed that the obtained agents could markedly enhance visualization of glioma C6 in vivo after their intravenous administration. Significant accumulation of Cx43-targeted contrast agents in glioma and the peritumoral zone led not only to enhanced contrast but also to improved detection of the tumor periphery. Fluorescence imaging confirmed notable accumulation of Cx43-specific conjugates in the peritumoral zone compared with nonspecific IgG conjugates at 24 h after intravenous injection. All these features of Cx43-targeted contrast agents might be useful for more precise diagnosis of glioma and its borders by MRI.

A novel approach to tracer-kinetic modeling for (macromolecular) dynamic contrast-enhanced MRI

PURPOSE

To develop a novel tracer-kinetic modeling approach for multi-agent dynamic contrast-enhanced MRI (DCE-MRI) that facilitates separate estimation of parameters characterizing blood flow and microvascular permeability within one individual.

METHODS

Monte Carlo simulations were performed to investigate the performance of the constrained multi-agent model. Subsequently, multi-agent DCE-MRI was performed on tumor-bearing mice (n = 5) on a 7T Bruker scanner on three measurement days, in which two dendrimer-based contrast agents having high and intermediate molecular weight, respectively, along with gadoterate meglumine, were sequentially injected within one imaging session. Multi-agent data were simultaneously fit with the gamma capillary transit time model. Blood flow, mean capillary transit time, and bolus arrival time were constrained to be identical between the boluses, while extraction fractions and washout rate constants were separately determined for each agent.

RESULTS

Simulations showed that constrained multi-agent model regressions led to less uncertainty and bias in estimated tracer-kinetic parameters compared with single-bolus modeling. The approach was successfully applied in vivo, and significant differences in the extraction fraction and washout rate constant between the agents, dependent on their molecular weight, were consistently observed.

CONCLUSION

A novel multi-agent tracer-kinetic modeling approach that enforces self-consistency of model parameters and can robustly characterize tumor vascular status was demonstrated.

Convergent synthesis of degradable dendrons based on l-malic acid

Dendron synthesis using malic acid derivatives in a stepwise manner leads to the preparation of polyfunctional dendrons, degradable by hydrolysis.

Practical dynamic contrast enhanced MRI in small animal models of cancer: data acquisition, data analysis, and interpretation

Dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) consists of the continuous acquisition of images before, during, and after the injection of a contrast agent. DCE-MRI allows for noninvasive evaluation of tumor parameters related to vascular perfusion and permeability and tissue volume fractions, and is frequently employed in both preclinical and clinical investigations. However, the experimental and analytical subtleties of the technique are not frequently discussed in the literature, nor are its relationships to other commonly used quantitative imaging techniques. This review aims to provide practical information on the development, implementation, and validation of a DCE-MRI study in the context of a preclinical study (though we do frequently refer to clinical studies that are related to these topics).

Vascular priming enhances chemotherapeutic efficacy against head and neck cancer

PURPOSE

The need to improve chemotherapeutic efficacy against head and neck squamous cell carcinomas (HNSCC) is well recognized. In this study, we investigated the potential of targeting the established tumor vasculature in combination with chemotherapy in head and neck cancer.

METHODS

Experimental studies were carried out in multiple human HNSCC xenograft models to examine the activity of the vascular disrupting agent (VDA) 5,6-dimethylxanthenone-4-acetic acid (DMXAA) in combination with chemotherapy. Multimodality imaging (magnetic resonance imaging, bioluminescence) in conjunction with drug delivery assessment (fluorescence microscopy), histopathology and microarray analysis was performed to characterize tumor response to therapy. Long-term treatment outcome was assessed using clinically-relevant end points of efficacy.

RESULTS

Pretreatment of tumors with VDA prior to administration of chemotherapy increased intratumoral drug delivery and treatment efficacy. Enhancement of therapeutic efficacy was dependent on the dose and duration of VDA treatment but was independent of the chemotherapeutic agent evaluated. Combination treatment resulted in increased tumor cell kill and improvement in progression-free survival and overall survival in both ectopic and orthotopic HNSCC models.

CONCLUSION

Our results show that preconditioning of the tumor microenvironment with an antivascular agent primes the tumor vasculature and results in enhancement of chemotherapeutic delivery and efficacy in vivo. Further investigation into the activity of antivascular agents in combination with chemotherapy against HNSCC is warranted.

Molecular imaging of fibrin in a breast cancer xenograft mouse model

RATIONALE AND OBJECTIVES

Fibrin deposition has been indicated within the stroma of a majority of solid tumors. Here we assess the feasibility of using the established fibrin-specific probe EP-2104R for noninvasive imaging of fibrin in the context of breast cancer.

METHODS

EP-2104R, untargeted gadopentetate dimeglumine (Gd-DTPA), and a newly synthesized nonfibrin binding control linear peptide (CLP) were compared using steady-state and dynamic contrast-enhanced magnetic resonance imaging in a breast cancer xenograft mouse model at 9.4 T.

RESULTS

EP-2104R transiently enhanced both tumor core and tumor periphery, but only the enhancement in the tumor periphery persisted even 90 minutes after EP-2104R administration. However, untargeted Gd-DTPA and CLP are not retained in the tumor periphery. The half-life of EP-2104R in the tumor periphery (103 ± 18 minutes) is significantly longer (P < 0.05) than that of either Gd-DTPA (29.6 ± 2.4 minutes) or CLP (42.4 ± 1.5 minutes), but the rate of clearance is similar for all the 3 probes from the tumor core. The presence of high concentrations of fibrin in the tumor periphery was corroborated using immunohistochemistry with a fibrin-specific antibody.

CONCLUSIONS

The persistent enhancement observed in the tumor periphery with EP-2104R is likely a result of its fibrin-specific binding rather than its size and demonstrates the feasibility of EP-2104R for molecular imaging of fibrin in tumor stroma.

Quantitative analysis of neovascular permeability in glioma by dynamic contrast-enhanced MR imaging

This study was designed to quantitatively analyse neovascular permeability in glioma by dynamic contrast-enhanced MRI (DCE-MRI). Forty-four patients with glioma were included in this study. The highest value of volume transfer constant (K(trans)) and volume of extravascular extracellular space per unit volume of tissue (V(e)) were obtained and the differences in K(trans) and V(e) between low-grade glioma (LGG) and high-grade glioma (HGG) were analyzed. The correlations between K(trans), V(e) and glioma grade were performed. Receiver operating characteristic (ROC) curve analyses were conducted. The values of K(trans) and V(e) of LGG were significantly lower than those of HGG. The correlation analysis demonstrated statistically significant relationships between K(trans) and glioma grade, between V(e) and glioma grade, and between K(trans) and V(e). The ROC curve analyses of K(trans) (0.035/min) and V(e) (0.130) for differentiating LGG from HGG were statistically significant. Thus, DCE-MRI can be used to estimate neovascular permeability and for pre-operative grading of glioma.

HIF-1-dependent stromal adaptation to ischemia mediates in vivo tumor radiation resistance

PURPOSE

Hypoxia-inducible factor 1 (HIF-1) promotes cancer cell survival and tumor progression. The specific role played by HIF-1 and tumor-stromal interactions toward determining tumor resistance to radiation treatment remains undefined. We applied a multimodality preclinical imaging platform to mechanistically characterize tumor response to radiation, with a focus on HIF-1-dependent resistance pathways.

METHODS

C6 glioma and HN5 human squamous carcinoma cells were stably transfected with a dual HIF-1 signaling reporter construct (dxHRE-tk/eGFP-cmvRed2XPRT). Reporter cells were serially interrogated in vitro before and after irradiation as monolayer and multicellular spheroid cultures and as subcutaneous xenografts in nu/nu mice.

RESULTS

In vitro, single-dose irradiation of C6 and HN5 reporter cells modestly impacted HIF-1 signaling in normoxic monolayers and inhibited HIF-1 signaling in maturing spheroids. In contrast, irradiation of C6 or HN5 reporter xenografts with 8 Gy in vivo elicited marked upregulation of HIF-1 signaling and downstream proangiogenic signaling at 48 hours which preceded recovery of tumor growth. In situ ultrasound imaging and dynamic contrast-enhanced (DCE) MRI indicated that HIF-1 signaling followed acute disruption of stromal vascular function. High-resolution positron emission tomography and dual-contrast DCE-MRI of immobilized dorsal skin window tumors confirmed postradiotherapy HIF-1 signaling to spatiotemporally coincide with impaired stromal vascular function. Targeted disruption of HIF-1 signaling established this pathway to be a determinant of tumor radioresistance.

CONCLUSIONS

Our results illustrate that tumor radioresistance is mediated by a capacity to compensate for stromal vascular disruption through HIF-1-dependent proangiogenic signaling and that clinically relevant vascular imaging techniques can spatially define mechanisms associated with tumor irradiation.

Radiosensitization and stromal imaging response correlates for the HIF-1 inhibitor PX-478 given with or without chemotherapy in pancreatic cancer

Growing tumors are hypoxic and respond to microenvironmental stress through increased expression of the hypoxia inducible factor-1alpha (HIF-1alpha) transcription factor, resulting in an adaptive switch to glycolytic metabolism, angiogenic signaling, survival, and metastasis. HIF-1alpha expression is associated with tumor resistance to cytotoxic therapy and inferior patient outcomes. Pancreatic cancer is the most hypoxic of all solid tumors and remains refractory to current chemoradiotherapy. We have seen nuclear HIF-1alpha in 88% of human pancreatic ductal carcinoma but in only 16% of normal pancreas. Stroma adjacent to the pancreatic ductal carcinoma also showed HIF-1alpha in 43% of cases. We investigated the novel selective HIF-1alpha inhibitor PX-478 on in vitro and in vivo radiation response of human pancreatic cancer models. Inhibition of HIF-1alpha by PX-478 increased cell killing by radiation. In mice with Panc-1, CF-PAC-1, or SU.86.86 pancreatic xenografts, concurrent administration of PX-478 potentiated the antitumor effects of fractionated radiation, with or without combined treatment with 5-fluorouracil or gemcitabine. Alternative sequencing of PX-478 with fractionated radiotherapy suggests optimal radiosensitization with concurrent or neoadjuvant administration of drug. Early tumor responses to combined PX-478/radiation treatment could be rapidly and repeatedly quantified by vascular imaging biomarkers. Dual-tracer dynamic contrast enhanced-magnetic resonance imaging and ultrasound imaging discriminated response to combined treatment prior to detection of differences in anatomic tumor size at 10 days posttreatment. Therefore, PX-478 is a mechanistically appealing and potentially clinically relevant enhancer of pancreatic cancer radiosensitivity, inhibiting tumor and stromal HIF-1 proangiogenic signaling and reducing the innate radiation resistance of hypoxic tumor cells.

Chapter 7. Molecular imaging of tumor vasculature

Cancer, with more than 10 million new cases a year worldwide, is the third leading cause of death in developed countries. One critical requirement during cancer progression is angiogenesis, the formation of new blood vessels. Structural and functional imaging of tumor vasculature has been studied using various imaging modalities such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound. Molecular imaging, a key component of the 21st-century cancer-patient management strategy, takes advantage of these traditional imaging techniques and introduces molecular probes to determine the expression of indicative molecular markers at different stages of cancer development. In this chapter, we will focus on two tumor vasculature-related targets: integrin alpha(v)beta(3) and vascular endothelial growth factor receptor (VEGFR). For imaging of integrin alpha(v)beta(3) on the tumor vasculature, only nanoparticle-based probes will be discussed. VEGFR imaging will be discussed in depth, and we will give a detailed example of positron emission tomography (PET) imaging of VEGFR expression using radio-labeled VEGF(121) protein. Future clinical translation will be critical for maximum patient benefit from these agents. To achieve this goal, multidisciplinary approaches and cooperative efforts from many individuals, institutions, industries, and organizations are needed to quickly translate multimodality tumor vasculature imaging into multiple facets of cancer patient management.