This presentation will review Paramagnetic Chemical Exchange Saturation Transfer (ParaCEST) MRI contrast agents. These agents should be thoroughly characterized with regard to their dependence on saturation time, saturation power, concentration, pH and temperature. Responsive ParaCEST agents can detect or measure enzyme activity, metabolites, metal ions, pH, redox state, temperature, and light. Some ParaCEST agents can also exhibit T2-Exchange relaxation. The intermediate exchange rate of a T2ex agent does not affect the T1 relaxivity of the agent. Therefore, the T2/T1 ratio of a T2ex agent can be employed to detect a biomarker.
Paramagnetic Chemical Exchange Saturation Transfer (ParaCEST) MRI contrast agents were first reported in 2001 and 2002 [1-3]. These agents can have a large saturation frequency (chemical shift of a labile proton), which allows for a chemical exchange rate as fast as 10,000 Hz that can generate a CEST signal with srong amplitude [4]. A ParaCEST agent typically consists of a macrocyclic chelate that holds a lanthanide ion, such as Eu(III), Tm(III), or Yb(III) [5]. Linear chelates and other lathanide ions have also been used as ParaCEST agents. To avoid the potential toxicity of lanthanide ions, chelates with iron, nickel and chromium have also been developed as paraCEST agents [6]. The labile proton that undergoes chemical exchange can be an amide, amine, or hydroxyl group of a ligand on the agent, or can be a water molecule that is bound to the metal of the agent.
ParaCEST agents should be thoroughly characterized before they are used in experiments. The effect of saturation time on the CEST signal amplitude can be assessed using the non-linear QUEST method [7] or the linear RL-QUEST method [8]. The effect of saturation power on the CEST signal amplitude can also be evaluated, and the average chemical exchange rate of the agent can be measured by using the non-linear QUESP method [7], a linear Omega Plot [9], or the HW-QUESP method [10]. The concentration dependence of the CEST signal amplitude under steady state saturation conditions can be evaluated using the linear HW-conc method [11]. In addition, the effects of pH on the CEST signal amplitude should be evaluated from 6.0 to 8.0 in small increments to assess a range of physiological conditions [12]. The effects of temperature should be evaluated between 22-52 °C [13]. Other conditions such as the effects of salt, proteins, cell homogenates or plasma can also be evaluated.
ParaCEST agents that are responsive to a biomarker comprise 20% (24/117) of all responsive MRI contrast agents reported as of 2014, demonstrating the creativity in the development of these agents [14,15]. ParaCEST agents have been developed that respond to enzyme activity [16]; metabolites such as sugars, lactate, phosphates, and nitric oxide [17]; metals such as zinc, calcium, and copper; pH [18]; redox state; temperature; and light. Some ParaCEST agents have been developed that have a biomarker-responsive CEST signal and an unresponsive CEST signal [13], or two CEST signals that respond differently to the biomarker [12]. A ratio of these two CEST signals can be used to detect or measure the biomarker in a manner that is independent of agent’s concentration, the T1 relaxation time constant of the sample, and B1 inhomogeneity of the saturation period [12,13]. This ratiometric approach facilitates in vivo studies with responsive ParaCEST agents [19,20].
Some ParaCEST agents can also exhibit T2-Exchange relaxation [21]. A physical exchange of protons between a T2ex agent and water causes the MR frequency of the exchanging proton to take a weighted average value between the frequency of the agent and the frequency of water. Because chemical exchange is stochastic, each proton experiences a different weighted average for its MR frequency. These different MR frequencies result in the decay of net coherent magnetization, which manifests as T2 relaxation. T2ex agents typically have chemical exchange rates of 100,000-1,000,000 Hz that are intermediate between the exchange rates of 10,000,000-1,000,000,000 Hz for T1 agents and 10-10,000 Hz for CEST agents [4]. This intermediate rate greatly improves detection sensitivity of T2ex agents relative to CEST agents, but T2ex agents have less sensitivity than T1 agents. The chemical exchange rate of bound water in a T2ex agent can be estimated by measuring the linewidth of 17O NMR spectra over a range of temperatures [22]. The chemical shift of bound water can be estimated based on the chemical shift of a proton on the macrocyclic chelate [22]. Finally, the temperature dependence of the T2ex relaxation should be evaluated to further confirm that the chemical exchange rate of the agent is in the intermediate rate regime [23].
T2ex relaxation can cause the CEST signal amplitude to decrease, so that paraCEST agents should be designed to avoid chemical exchange rates that create T2ex [24]. This apparent disadvantage can be exploited as an advantage to create responsive contrast agents that change from slow to intermediate chemical exchange rates (or vice-versa), which causes the agent to change from a CEST agent to a T2ex agent (or vice-versa) [23]. Furthermore, the intermediate exchange rate of a T2ex agent does not affect the T1 relaxivity of the agent that depends on faster chemical exchange rates. Therefore, a change in intermediate chemical exchange rate by a biomarker can alter T2 relaxation without altering T1 relaxation [23]. The T2/T1 ratio can then be employed to detect the biomarker without a dependence on the concentration of the agent. This approach has been used to develop a responsive T2ex agent that detects nitric oxide [23].
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