Functionalized Lipopeptide Micelles as Highly Efficient NMR Depolarization Seed Points for Targeted Cell Labelling in Xenon MRI

Abstract

Improving diagnostic imaging and therapy by targeted compound delivery to pathological areas and across biological barriers is of urgent need. A lipopeptide, P‐CrA‐A2, composed of a highly cationic peptide sequence (A2), an N‐terminally attached palmitoyl chain (P) and cryptophane molecule (CrA) for preferred uptake into blood–brain barrier (BBB) capillary endothelial cells, was generated. CrA allows reversible binding of Xe for NMR detection with hyperpolarized nuclei. The lipopeptide forms size‐optimized micelles with a diameter of about 11 nm at low micromolar concentration. Their high local CrA payload has a strong and switchable impact on the bulk magnetization through Hyper‐CEST detection. Covalent fixation of CrA does not impede micelle formation and does not hamper its host functionality but simplifies Xe access to hosts for inducing saturation transfer. Xe Hyper‐CEST magnetic resonance imaging (MRI) allows for distinguishing BBB endothelial cells from control aortic endothelial cells, and the small micelle volume with a sevenfold improved CrA‐loading density compared to liposomal carriers allows preferred cell labelling with a minimally invasive volume (≈16 000‐fold more efficient than 19F cell labelling). Thus, these nanoscopic particles combine selectivity for human brain capillary endothelial cells with great sensitivity of Xe Hyper‐CEST MRI and might be a potential MRI tool in brain diagnostics.

Cryptophane–xenon complexes for 129Xe MRI applications

“…We have highlighted instances of innovative biosensor design, such as the use of a singular moiety for both uorescence and 129 Xe MRI readout, 82 modulation of solubility aer target binding, 72 and loading up to thousands of individual cryptophane host molecules onto a single carrier. 69,[93][94][95][96][97][98] Finally, we have shown examples of selective and sensitive 129 Xe MRI contrast attainable with these biosensors. This past decade of expanding the scope of 129 Xe MRI using cryptophane-based biosensing has been extremely productive, and the future of this eld remains bright.…”

“…This technique was later improved by using a lipopeptide comprised of a cationic peptide, a cryptophane-A unit, and a palmitoyl chain. 94 Aer micelle formation, it was estimated that the average cryptophane content per unit volume was ca. 7-fold higher in the micelles than in LUVs.…”

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

“…Since the membrane volume can carry only a limited number of cages, such particles can be further optimized in terms of their invasive cell labelling volume. A recent study introduced a lipopeptide that forms size-optimized micelles with a diameter of about 11 nm at low micromolar concentration [ 351 ]. It provides a high local CryA payload that is ca.…”

“…However, 19 F MRI of such compounds relies on the detection of (a) thermally polarized spins and (b) does not benefit from CEST enhancement. It is thus significantly less efficient than the HyperCEST approach as demonstrated in the above mentioned recent study with micelles carrying a high local Xe host load [ 351 ].…”

<sup>129</sup>Xe Ultrafast Z-spectroscopy enables micromolar detection of biosensors on a 1T benchtop spectrometer

“…The approach of 129 Xe NMR-based biosensing using functionalized cryptophanes has yet been successfully applied in vitro for detection of small analytes (Tassali et al, 2014;Dubost et al, 2014;Jeong et al, 2015;Yang et al, 2016;Guo et al, 2016;Yang et al, 2017), of large biosystems (Wei et al, 2006;Chambers et al, 2009;Boutin et al, 2011;Rose et al, 2014;Taratula et al, 2015;Khan et al, 2015;Riggle et al, 2017;Milanole et al, 2017;Schnurr et al, 2020), or of change of physiological conditions: temperature (Schröder et al, 2008;Schilling et al, 2010) or pH (Léonce et al, 2018). To date, it has however never been used in vivo; only a proof of concept has been performed on rats using a non-functionalized cucurbituril (Hane et al, 2017).…”