Quantifying exchange in host-guest systems for hyperpolarized xenon
Sergey Korchak1, Wolfgang Kilian1, Leif Schröder2, and Lorenz Mitschang1

1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, 2Leibniz-Institut für Molekulare Pharmakologie (FMP), Berlin, Germany

Synopsis

The reversible binding of xenon to host structures is fundamental to the development of novel contrast agents employing hyperpolarized xenon and chemical exchange saturation transfer (HyperCEST) for molecular imaging. The rates for entering and leaving the host depend on atomic details and affect the obtainable contrast rendering them pivotal for the selection of hosts and optimization of imaging methods. However, different exchange processes may apply whose contributions are difficult to assign. Exchange spectroscopy experiments are proposed which enable straightforward disentanglement of the exchange kinetics and quantification of individual contributions. The approaches are exemplified for the cryptophane-xenon host-guest system.

Purpose

Determination and quantification of the kinetics for the reversible binding of xenon to host molecules to enable development of effective contrast agents and optimization of contrast.

Method and Results

The remarkable MR properties of xenon and its ability to reversible binding to suitable host structures like container molecules or nanostructures is currently heavily explored for novel approaches to molecular imaging. Upon functionalization of the host biomarkers or tissue can be targeted specifically while the NMR readout can report about the state of the system under investigation with sensitivity enhanced by orders of magnitude because of hyperpolarization of xenon and application of HyperCEST$$$^{1,2}$$$. The binding kinetics underlying the method and its application comprises contributions from different exchange mechanisms: a simple dissociative process, where xenon enters a free host or leaves one behind, and a degenerate process where bound and unbound xenon interchange their binding status directly without detour of an intermediate free host$$$^{3,4}$$$. Their differentiation by hand of a single NMR spectrum is not possible. However, by employing exchange spectroscopy in combination with variation of xenon concentration the overall kinetics can be disentangled and individual contributions quantified. For demonstration the binding kinetics of xenon to the host molecule cryptophane-A (CrA) in pure water at room temperature was analyzed. In a gas mixture with He and N$$$_2$$$ hyperpolarized xenon was bubbled through the aqueous CrA solution ($$$6.4\,\mu$$$M) until xenon saturation to an extent given by Henry´s Law was achieved, i.e. by xenon partial pressure the amount of unbound xenon in the solution, [Xe], could be controlled. Subsequently, either of the pulse sequences in Fig.$$$\,1$$$ was applied to achieve saturation or magnetization transfer in between the pools of unbound and CrA-bound xenon, $$$M_{\text{Xe}}$$$ and $$$M_{\text{CXe}}$$$, respectively. In all cases, perturbation of the one pool (saturation, or magnetization inversion and subsequent free exchange) leads to depletion of the other pool because of exchange and the accompanied signal decay can be determined by a series of experiments with variable perturbation period. In dynamic equilibrium and under the mild condition $$$M_{\text{Xe}}\,\gg\,M_{\text{CXe}}$$$ in case of experimental schemes II and III which can be easily achieved by a sufficiently high xenon partial pressure, linear dependencies of the depletion rate (I and II) or a related quantity (III) in the unbound xenon concentration are obtained. In Fig.$$$\,2$$$ data from experiments I to III are displayed together with linear fittings according to the linear relations in Fig.$$$\,1$$$. While saturation as well as magnetization transfer from the unbound to CrA-bound xenon pools (schemes I and II) allow for direct quantification of the rates of degenerate exchange, $$$k$$$, and complex dissociation, $$$c$$$, by determination of slope and intersection of the fit, in case of HyperCEST (scheme III) prior knowledge of the relaxation rate, R$$$_{1\text{Xe}}$$$, and CrA concentration, [C$$$^{tot}$$$], are required for that purpose. The rates obtained in this manor are displayed in Tab.$$$\,1$$$, the values and error estimates indicate high accuracy for every of the proposed experiments and their respective precision. By the weighted average of the results of experiments I to III the kinetic rate coefficients for complex dissociation and for degenerate exchange are for aqueous solution at 298 K determined as $$$c\,=\,(\,20.0\,\pm\,0.3\,)\,$$$s$$$^{-1}$$$ and $$$k\,=\,(\,5800\,\pm\,280\,)\,$$$M$$$^{-1}\,$$$s$$$^{-1}$$$, respectively. The identification of exchange mechanisms and the quantification of their individual contributions to the overall kinetics as proposed here may, because of simplicity in interpretation and data evaluation and by the numerical quality of the results, prove helpful in efforts towards a complete characterization and optimization of parameters governing molecular imaging approaches using hyperpolarized xenon$$$^5$$$.

Acknowledgements

This work was funded by the Bundesministerium für Bildung und Forschung (01EZ1010A) and the European Metrology Research Programme participating countries within EURAMET and the European Union (EMRP grant HLT-10).The authors thank C. Witte at FMP for generous support in the experimentation.

References

1. L. Taratula, I. J. Dmochowski, Curr. Opin. Chem. Biol. 14, 97 (2010).

2. L. Schröder, T. J. Lowery, C. Hilty, D. E. Wemmer, A. Pines, Science 314, 446 (2006).

3. K. Bartik, M. Luhmer, J.-P. Dutasta, A. Collet, J. Reisse, J. Am. Chem. Soc.120, 784 (1998).

4. S. Korchak, W. Kilian, L. Mitschang, Chem. Comm. 51, 1721 (2015).

5. M. Kunth, C. Witte, L. Schroeder, J. Chem. Phys. 141, 194202 (2014).

Figures

Figure 1: Schemes of saturation and magnetization transfer experiments numbered I to III which differ in the spin state preparation (first column) together with expressions for the respective depletion rates (second column) and their linear approximations (third column).

Figure 2: Depletion rates (dots) $$$R_{\text{CXe}}$$$, $$$R^{\prime}_{CXe}$$$, and quantity $$$R_{Xe}[\text{Xe}]$$$ for experiments I, II, and III, respectively, in dependence of free xenon concentration with linear fits for quantitative evaluation of exchange rates (presented in Tab.$$$\,1$$$).

Table 1: Kinetic rate coefficients from experiments I to III.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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