Synopsis

Keywords: Contrast mechanisms: Contrast agents, Contrast mechanisms: Molecular imaging, Contrast mechanisms: Relaxometry

This educational session will focus on the use of small metal complexes as MRI contrast agents, and more particularly on responsive contrast agents. T₁, as well as ParaCEST, Parashift and non-proton (¹⁹F) contrast agents will be presented. Among all the physiological parameters that can be detected using such complexes, a particular emphasis will be put on enzymatically-activated, and zinc responsive contrast agents. Indeed, these two biomarkers are misregulated during pathological processes such as cancers, or neurodegenerative diseases for example. Monitoring their changes should allow, in principle, early diagnosis as they appear prior to the morphological changes they trigger.

Metal complexes are an important class of contrast agents, and more particularly lanthanide-based metal complexes. Indeed, with its high electron spin, and slow electronic relaxation, Gd³⁺ is an ideal candidate for the development of T₁ MRI contrast agents. The efficiency of a contrast agent is given by its proton relaxivity (r₁), defined as the longitudinal paramagnetic relaxation rate enhancement of water protons induced by one millimolar concentration of the agent. The relaxivity is related to a number of microscopic parameters of the Gd³⁺ complex.^[1]
Some years ago, two MRI approaches based on the paramagnetic-induced hyperfine shift properties of metal ions have appeared: (1) chemical exchange saturation transfer (CEST) and (2) Parashift. CEST agents contain protons in slow exchange with bulk water. To generate CEST contrast, the exchangeable protons are selectively saturated by radiofrequency pulses applied at their resonance frequency. Due to chemical exchange with bulk water protons, such saturation will result in a decrease of the water proton signal intensity, which is translated to an MR image.^[2] In order to observe a CEST effect, the exchange rate (k_ex) of the protons must be lower than the difference of chemical shift between the exchangeable proton and the bulk. The use of paramagnetic species such as Ln³⁺ complexes enables to achieve more selective saturation and to explore higher exchange rates. In addition, it leads to more important CEST effects. Parashift agents have non-exchangeable protons (such as methyl or tert-butyl groups) in close vicinity to a paramagnetic center. As a result, their resonance frequencies are shifted away from the diamagnetic window, allowing for their visualization without background signal.^[3] More recently ¹⁹F MRI has emerged as an important field due to the favorable properties of ¹⁹F (100% abundance ; I = ½ ; γ_19F/γ_1H = 0.94) and the absence of endogeneous signal, preventing interference with the anatomical MRI informations.^[4]
An important field in molecular imaging involves the in vivo detection of physico-chemical parameters of tissues, concentration of ions, metabolites… by using imaging probes that are responsive to the specific parameter to detect. These probes are called smart, activatable or responsive contrast agents. They allow, in principle, early detection of diseases as the changes in biomarkers are sought to appear prior to the morphological changes they will trigger. The efficacy of the probe has to be selectively influenced by the particular biomarker that we wish to detect. For T₁-based contrast agents, even if in principle all microscopic parameters influencing relaxivity could be modulated by the presence of a biomarker, most responsive probes are based on changes of the number of water molecules directly coordinated to Gd³⁺ or the rotational dynamics of the complexes.^[5] For PacaCEST or Parashift probes, the biomarker should influence the resonance frequency of the proton, and its exchange rate (for CEST).^[6] One of the great advantages of those two techniques is their frequency-encoded properties.
Among important biomarkers, enzymatic activity is particularly adapted to the low sensitivity of MRI as contrast agents are catalytically converted by the enzyme. Zinc is also an important biomarker involved in many biological processes, and for which the concentration of its « labile » pool is in the MRI detectable range.^[7] Examples of enzymatically-activated ParaCEST contrast agents, as well as T₁, ParaCEST/shift and ¹⁹F Zn-responsive contrast agents will be presented. The challenges associated to the in vivo translation of such systems, in particular the selectivity and the quantification of the biomarker will be discussed.
Finally, the replacement of lanthanide by transition metal ions will be briefly introduced.

Acknowledgements

Financial support from the French Agence Nationale pour la Recherche is acknowledged, as well as ITMO Cancer of Aviesan within the framework of 2021-2030 cancer control strategy, on funds administered by INSERM, INBS France Life Imaging, La Maison de la Chimie, and La Ligue Contre le Cancer.