Parametric Diffusion and Perfusion MRI in Monitoring Therapy Response

PL-20

Following the seminal discoveries of nuclear magnetic resonance and magnetic resonance imaging, numerous new techniques and applications, as well as continued technical improvements, have been developed and exploited. Today, magnetic resonance imaging is a leading modality of choice for evaluation of most malignant tumors including diagnosis, staging, prognosis and follow up of treatment. We present here the application of MRI to measure perfusion of contrast agents via the microcapillarty network of tumors and self diffusion of water in tumors. We also describe initial molecular MRI studies of the estrogen receptor using novel contrast agents. Application of dynamic contrast enhanced MRI to measure vascular perfusion. The diverse capabilities of MRI led to the development of several techniques that generate vascular contrast and measure various parameters associated with tumor microvasculature. Most of these techniques are based on contrast generated by exogenous agents administered intravenously. The most common types of contrast agents currently in research use may be divided into three major categories: (i) Blood pool agents that remain in the vascular system for relatively long periods and permeate slowly into the interstitium (ii) Low molecular weight agents (gadolinium chelates of organic molecules) characterized by fairly rapid exchange rate between the blood vessels and the interstitial space, thus enabling ready extraction of the agent into the tumor tissue. (iii) Small, highly diffusible molecules with high vascular permeability, whose perfusion is flow-limited, such deuterated or ¹⁷O-labeled water.We present here the application of low molecular weight gadolinium complexes to monitor vascular volume fraction, transcapillary transfer constant, and interstitial fluid pressure (IFP) 1-7. The unique capability to non invasively map tumor IFP provides a novel mean to predict barriers to drug delivery.Tumor blood vessels are highly disorganized, tortuous and dilated and are often leaky due to discontinuous walls and fenestrated endothelial lining. Consequently, blood flow in some tumor regions is impaired and delivery of nutrients and oxygen is inefficient. In addition, the high vascular permeability of tumor capillaries and impaired drainage of fluid by the lymphatic network lead to water accumulation in the tissue and increased interstitial fluid pressure (IFP). This pressure reduces the driving force for extravasation and leads to outward convection forces. Hence, the flux of blood borne molecules from the intravascular compartment to the tissue interstitial space is determined by both diffusion in the direction of the concentration gradients and convection due to hydrostatic pressure gradients.We employed high resolution DCE-MRI to study the vascular perfusion of orthotropic MDA-MB-231 tumors as well as of lymph node metastases in SCID mice. Pressure gradients were estimated from the measured disparity between the influx and outflux transcapillary transfer. In these tumors, a large fraction exhibited higher outflux than influx transfer constants, with the difference between these constants increasing with tumor growth. Furthermore, the spatial distribution of the disparity suggested a steep increase in IFP from the rim towards the center.Recently, using a slow GdDTPA infusion protocol, which enabled reaching steady state conditions for the contrast agent concentration, and measure T₁ relaxation rates at steady state, it was shown that IFP can be estimated at pixel resolution across the entire tumor. This methodology was tested in a mouse model of ectopic human non-small cell lung cancer which exhibited high IFP of ~28 mmHg (measured with the "wick-in-needle" technique) and of orthotopic MCF7 human breast cancer which exhibited low IFP of ~ 7 mmHg. Image analysis yielded parametric images of steady state tissue concentration of the contrast agent with high values of this concentration outside the tumor boundaries declining in the tumor periphery to and then steeply decreasing to low or null concentrations depending on IFP. We also describe the results of a pilot clinical study using high resolution parametric contrast enhanced MRI to monitor response of infiltrating ductal carcinoma of the breast to neoadjuvent systemic treatment (NST) 8. In tumors responding to chemotherapy, tumor angiogenesis is halted and the development of necrosis and fibrosis leads to the establishment of a microcapillary network with properties different from that of the cancer network. These changes can be quantified by analyzing enhancement parameters in dynamic contrast enhanced MRI images using high resolution parametric MRI methods. We present the application of the method termed the three time point method to quantify response to NST. It enabled us to detect very small residual tumor foci (<1mm) and apply an objective segmentation procedure that estimated changes in the volume and spread of the disease in the breast, before and after NST as well as variations in the microvascular function of viable neoplastic regions Application of diffusion MRI Commonly, water molecules in the extracellular matrix move by following winding paths around the cells, colliding with cells' membrane and walls of blood capillaries. The mobility of the water inside the cells is highly hindered and is further restricted by the cell membrane. Diffusion MRI techniques were applied to investigate water mobility, compartmentation and anistropic movement in animal models as well as in cancer patients. As the diffusion parameters are sensitive to changes in tumor cellularity they can serve as markers for assessing cell death and tumor regression during anticancer treatment. We present as an example the application of diffusion MRI to monitor response to a selective estrogen receptor modulator tamoxifen methiodide (TMI) 9. We measured the diffusion parameters in control (placebo) and TMI treated MCF7 tumors and evaluated the changes in the water diffusion characteristics due to the treatment. Diffusion studies were performed at high spatial resolution using various protocols with constant diffusion times or constant diffusion gradient strengths, reaching high strength of 20 G/cm. Analysis of the diffusion weighed images yielded an estimation of the intracellular volume fraction, which reflects the tumor cellularity, and the intracellular and extracellular apparent diffusion coefficients. The distribution of these two parameters was heterogeneous and asymmetric with respect to the mean. TMI treatment induced a transient increase in the fraction of intracellular water as compared to control tumors, indicating a cytostatic response in the form of cell swelling.MRI of the estrogen receptorNovel imaging techniques can now be developed leading to the targeting of hormones and drugs and simultaneously monitoring their distribution in the body. We focused on developing molecular MRI imaging of the estrogen receptor (ER) 10. Estrogen receptors (α or β form) act as potent transcription factors regulating many cellular functions. They are also major therapeutic targets for the treatment and prevention of breast cancer. We have synthesized two selective estrogen receptor modulators (SERMs) tagged to a gadolihium chelate that can enhance the MRI signal in their vicinity. The first is a gadolinium chelate of estradiol pyridine tetrakis acetic acid (EPTA-Gd.) and the second of tamoxifen pyridine tetrakis acetic acid (TPTA-Gd). Both chelates showed competitive binding affinities with estradiol with IC₅₀ in the micromolar range. Their biological activities were tested in ER positive human breast cancer cells and in the ovariectomized rat uterus demonstrating agonistic effects at micromolar concentrations. The utilization of EPTA-Gd as a contrast agent for in vivo application was tested in the uterine tissue which serves as a classical target organ for estrogen. Intravenous injection of EPTA-Gd (0.024 mmol/kg wt) to ovariectomized rats elicited an agonistic effect in the uterus demonstrated in density and T₂ weighted MR images by increased water fraction and endometrial volume indicating estrogenic like response to EPTA-Gd. Parallel, sequential T₁ weighted images demonstrated that EPTA-Gd induced a distinct enhancement in the uterine tissue that remained constant for three hours and then declined steeply as expected for 17β- estradiol induced degradation of ER (shown previously by radio labeling methods). 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