Free‐base porphyrins as CEST MRI contrast agents with highly upfield shifted labile protons

Abstract

Purpose CEST has become a preeminent technology for the rapid detection and grading of tumors, securing its widespread use in both laboratory and clinical research. However, many existing CEST MRI agents exhibit a sensitivity limitation due to small chemical shifts between their exchangeable protons and water. We propose a new group of CEST MRI agents, free‐base porphyrins and chlorin, with large exchangeable proton chemical shifts from water for enhanced detection. Methods To test these newly identified CEST agents, we acquired a series of Z‐spectra at multiple pH values and saturation field strengths to determine their CEST properties. The data were analyzed using the quantifying exchange using saturation power method to quantify exchange rates. After identifying several promising candidates, a porphyrin solution was injected into tumor‐bearing mice, and MR images were acquired to assess detection feasibility in vivo. Results Based on the Z‐spectra, the inner nitrogen protons in free‐base porphyrins and chlorin resonate from −8 to −13.5 ppm from water, far shifted from the majority of endogenous metabolites (0‐4 ppm) and Nuclear Overhauser enhancements (−1 to −3.5 ppm) and far removed from the salicylates, imidazoles, and anthranillates (5‐12 ppm). The exchange rates are sufficiently slow to intermediate (500‐9000 s−1) to allow robust detection and were sensitive to substituents on the porphyrin ring. Conclusion These results highlight the capabilities of free‐base porphyrins and chlorin as highly upfield CEST MRI agents and provide a new scaffold that can be integrated into a variety of diagnostic or theranostic agents for biomedical applications.

Methylated tetra‐amide derivatives of paramagnetic complexes for magnetic resonance biosensing with both BIRDS and CEST

“…The 1 H NMR spectra of the complexes showed no chemical shift drifting, loss of intensity, or line broadening after several years with similarly sharp proton resonances, demonstrating their great thermodynamic stability. Integration of 1 H and 13 C NMR peaks depicts the chemical formulas for Tm-L1, Tm-L2, and Tm-L3 (Figure S1) in agreement with the high purity of the complexes (Figure S2).…”

“…While paramagnetic hyperfine shifts of the endogenous proton resonances from the chelating ligands have been explored in classical NMR studies, 7 utilization of these shifted proton signals from the ligands as responsive agents has great potential in molecular imaging. 8 These Ln 3+ complexes possess well resolved paramagnetically shifted proton resonances detected far beyond the diamagnetic range of proton NMR shifts, [9][10][11][12][13][14] and these non-exchangeable protons (e.g., -CH y , where 3 ≥ y ≥ 1) can be detected by high-speed MRSI to probe the microenvironment and this method is called biosensor imaging of redundant deviation in shifts (BIRDS). 6,15,16 These Ln 3+ complexes can also contain exchangeable protons (e.g., -OH or -NH x , where 2 ≥ x ≥ 1) and/or bound water protons (exchange rates that are much slower than Gd-based T 1 agents 17 ), where chemical exchange saturation transfer (CEST) can be used to generate MRI contrast from the same agent.…”

Repurposing Clinical Agents for Chemical Exchange Saturation Transfer Magnetic Resonance Imaging: Current Status and Future Perspectives

“…For example, as shown in Figure 6 C,D, when pyrimidine is replaced by triazine, the NH 2 protons of decitabine (Dec) and azacitidine (Aza) showed 2–3 times stronger CEST effects (i.e., MTR asym (2.3 ppm) = 0.23 and 0.31 per 20 mM agent, respectively) than that of deoxycytidine (i.e., MTR asym (2.1 ppm) = 0.12 per 20 mM agent) [ 59 ]. Since then, the list of CEST imageable drugs has been expanding, and anti-cancer drugs in different categories were also reported, such as DNA alkylating agent (melphalan) [ 57 ], DNA methylation inhibitor (olsalazine) [ 134 ], and photosensitizer (porphyrins and chlorin) [ 135 ]. Among them, some agents have very desirable CEST properties, i.e., offsets far from the majority of endogenous metabolites (0–4 ppm), which may allow more specific detection and longitudinal assessment of not only drug delivery and but also drug action (e.g., drug metabolism and the interaction of a drug with its targeted molecules and cells).…”

“…Among them, some agents have very desirable CEST properties, i.e., offsets far from the majority of endogenous metabolites (0–4 ppm), which may allow more specific detection and longitudinal assessment of not only drug delivery and but also drug action (e.g., drug metabolism and the interaction of a drug with its targeted molecules and cells). For example, olsalazine has a large downfield CEST contrast at ~9.8 ppm from the water resonance [ 134 ], and porphyrins and chlorins have unusual CEST peaks at −8 to −13.5 ppm [ 135 ]. As a result, in vivo CEST MRI detection is more specific and allows “multicolor” MRI detection of multiple agents [ 62 , 91 ].…”

Coloring ultrasensitive MRI with tunable metal–organic frameworks

“…11 Numerous CEST contrast agents with various irradiation frequencies (chemical shis) have been exploited. [12][13][14][15] Nevertheless, background signals from endogenous glucose, creatine, glutamate, nucleic acids, peptides, and other biomolecules make it difficult to extract target information due to overlapping signals.…”