Developing Responsive MRI Contrast Agents to Study Tumor Biology

Abstract

Enzymes are important biomarkers for determining tumor growth and progression. We have developed two molecules to image enzyme response by catalyCEST MRI. This technology allows for non-invasive detection of enzymes. A background of importance of measuring enzyme activity and MRI agents developed for this purpose have been covered in Chapter 1. We have synthesized a responsive paramagnetic Chemical Exchange Saturation Transfer (CEST) agent, called Tm-DO3A-cadaverine. This contrast agents has been successfully cross-linked to the protein albumin by the enzyme transglutaminase leading to the appearance of CEST at -9.2 ppm. The enzyme catalysis has been validated by measuring chemical exchange rates. We have shown that the position of the CEST peak is influenced by the conformation of the molecule depending on the neighboring amino acids to glutamine. This is the first example to show the appearance of CEST due to formation of a covalent bond. We have also synthesized a diamagnetic CEST agent with a large chemical shift dispersion to detect cathespin B activity. Upon enzyme mediated cleavage of PheArgSal, the aryl amide CEST peak at 5.3 ppm disappears. Taking a ratio of the CEST effects from salicylic acid at 9.5 ppm and aryl amide at 5.3 ppm we can detect enzyme activity. The salicylic acid moiety also undergoes some slow response due to enzyme action, as evident by the disappearance of CEST at 9.5 ppm. However, this proof of concept study is the first example of a DIACEST agent designed to measure enzyme activity using a ratio of two CEST effects from the same substrate. The last chapter highlights suggests improvements to the catalyCEST research. The appendix shows the use of bulk magnetic susceptibility measurements by NMR to determine bio-distribution of lanthanides in ex-vivo tissue.