Development of Non-Invasive Responsive Agents for Magnetic Resonance Imaging

Molecular imaging involves visualizing bioactive molecules or biological parameters in vivo at the molecular level in a live organism. In Magnetic Resonance Imaging (MRI), inorganic complexes with paramagnetic ions, are commonly used to enhance the intrinsic image contrast of the soft tissues—contrast agents (CAs). Gd-based MR probes paved their way towards medical imaging and clinical application. CAs could change their r1 relaxivity in response to the local environment are known as “smart” probes. This dissertation reports the development and optimization of “smart” Magnetic Resonance Imaging agents to detect in-vivo copper ion levels and extracellular pH–two important extracellular biomarkers. Copper is the third most abundant transition metal in the body and a required dietary nutrient. Although the relationship between copper dynamics and its physiological or pathological roles have been extensively studied, information about its extracellular behavior in biologically relevant conditions remains insufficiently understood due to the lack of real-time non-invasive copper detecting techniques. Chapter 2 presents the design and organic synthesis of novel copperresponsive MR sensors. These sensors included a copper-selective bis(benzoic acid)methylamine recognition motif (GdL). We studied the physicochemical properties of the newly developed agents and devised a comprehensive study to understand the possible coordination of GdL1 with copper and HAS. GdL1 shows high selectivity to copper ions and exhibits an increase in relaxivity by 47% upon binding to 1 equivalent of Cu²⁺.Interestingly, when fully bound to Cu²⁺ sensor presents a 270% increase in relaxivity(r1) in the presence of a physiological concentration of human serum albumin. We performed in vivo imaging with healthy mice and visualized extracellular exchangeable copper in the liver for the first-time by MRI. These results will pave the way for unique opportunities to explore the role of copper in the progression of many neurological disorders, including Wilson’s disease. pH is a fundamental physiological parameter tightly regulated by endogenous buffers at the intracellular and extracellular level. Disruption of regulation of pH is associated with pathological conditions such as cancer, acidosis and kidney disease. GdDOTA-4AmP is a T₁ agent for MRI that has been applied non-invasively to image in-vivo tissue. This approach was limited by the use of a dual-contrast agent strategy and with lower elimination time of the agents in vivo due to possible deposition in the bones. In Chapter 3, we describe a set of novel Gd-based T₁ agents that present optimized pH-responsive MRI properties to GdDOTA-1AmP, GdDOTA-2AmP, and GdDOTA-3AmP. The GdDOTA-1AmP, exhibits a surprisingly large increase in r₁ relaxivity from 3.0 to 6.3 mM⁻¹s⁻¹ as the pH is reduced from 9 to 2.5. The origin of this unique pH sensitivity was traced to protonation of the single phosphonate side-chain, which, upon protonation, catalyzes exchange of protons between a Gd-bound water molecule and bulk water. T₁-weighted images of phantoms showed that MR image intensity increased 12-fold between a physiological pH of 7.4 and pH 6. This demonstrates it is possible to design simple, small molecule MRI contrast agents that respond to pH using simple acid-base principles.