DOTA-based rare earth complex, preparation method and application thereof

Abstract

The application discloses a DOTA rare earth complex and a preparation method and application thereof, and belongs to the technical field of contrast agents. The structure of the DOTA rare earth complex is as follows: wherein Ln includes Gd (III), Eu (III) and Yb (III). The DOTA Gd complex Gd-L2 disclosed in the application is a full alkyl-substituted chiral Gd-DOTA derivative, which comprises a tetraethyl-substituted cycloalkene skeleton and an alpha-arm methyl group. The double-site chiral design simultaneously enhances the kinetic inertness and the relaxation rate, and provides a structure-stable platform for liver MRI contrast enhancement. The Gd-L2 shows OATP-mediated liver uptake, can realize selective liver accumulation, and then is effectively removed by the kidney. In a hepatocellular carcinoma orthotopic mouse model, the complex realizes significant tumor margin delineation, and highlights the potential of the complex in early liver cancer detection.

Description

Technical Field

[0001] This invention belongs to the field of contrast agent technology, specifically relating to a DOTA-type rare earth complex, its preparation method, and its application. Background Technology

[0002] Gadolinium-based contrast agents play an indispensable role in improving diagnostic accuracy in the widespread application of magnetic resonance imaging (MRI). However, the clinical usability of linear GBCAs is significantly limited by their relatively low kinetic inertness, which may raise concerns about renal systemic fibrosis and trace gadolinium deposition in tissues after repeated administration. Given these risks, the European Medicines Agency (EMA) restricted the use of most linear GBCAs in 2017. Nevertheless, hepatobiliary imaging presents a unique challenge because the only available clinically targeted agents—the linear compounds Gd-EOB-DTPA and Gd-BOPTA—are not subject to this ban, thus necessitating reliance on a class of contrast agents with poor kinetic stability. Therefore, the development of novel macrocyclic contrast agents for hepatobiliary applications is crucial to ensuring high diagnostic performance and improving patient safety.

[0003] The macrocyclic chelator DOTA and its Gd-DOTA complex represent the benchmark for clinical stability in GBCA. Based on this exceptionally stable framework, introducing α-substituents on the acetic acid side chain of the DOTA backbone has become a key strategy for fine-tuning ligand properties. A representative example is Gd-DOTMA, where the chiral methyl group at the α-position confers kinetic stability comparable to Gd-DOTA, while promoting faster water exchange kinetics—a feature beneficial for improving MRI sensitivity.

[0004] Previous work extended this approach by introducing α-aryl substituents into the DOTA framework to further enhance kinetic inertness. This modification established a universal platform suitable for a variety of applications, including hepatobiliary targeting, tumor-targeting coupling, and high-performance vascular imaging. Wong and colleagues independently applied a similar α-aryl substitution strategy to develop multifunctional diagnostic and therapeutic probes. Botta, Woods, and colleagues further advanced this approach by developing the Gd-DOTBA platform with four α-aryl-substituted side chain arms. By combining optimized spatial control with exceptionally rapid water exchange kinetics, the system achieved a longitudinal relaxation rate value 2 to 3 times higher than currently approved clinical GBCAs. These results highlight the effectiveness of α-aryl substitution as a design strategy that simultaneously enhances kinetic stability and relaxation performance.

[0005] Meanwhile, structural modification of the DOTA macrocycle itself represents a unique and powerful strategy for optimizing kinetic inertness and MRI performance. The pioneering work of Woods and colleagues demonstrated that introducing a single chiral substituent (e.g., nitrobenzyl) onto the macrocyclic framework can effectively tune coordination geometry and accelerate water exchange kinetics. Dai et al. subsequently introduced a more sophisticated design, incorporating four symmetrically arranged chiral substituents onto the macrocycle. This higher-order, multi-site chiral structure significantly increased framework rigidity and fine-tuned water exchange kinetics, thereby further improving relaxation rate. Notably, when using ethyl as a substituent, the complex exhibited liver-selective metabolism. However, liver signal enhancement peaked only within 5 minutes post-injection, highlighting that prolonging liver retention time remains a key translational challenge for clinical hepatobiliary MRI.

[0006] In summary, these studies demonstrate that chiral modification of macrocycles is an effective method for optimizing DOTA-based GBCAs, simultaneously enhancing structural stability, relaxation efficiency, and organ selectivity. Importantly, synergistic combinations of substituents on the macrocycle and at the α-site of the acetic acid arm have been explored in related fields, such as for the development of circularly polarized luminescence (CPL) probes and pseudocontact displacement nuclear magnetic resonance (NMR) tags. However, their potential in designing advanced MRI contrast agents remains largely unexplored. Although compounds with substituents at both the macrocycle and arm sites have been reported, research in the MRI field has primarily focused on synthesis and basic relaxation rate characterization. Their biologically relevant properties—particularly their suitability as hepatobiliary-specific MRI contrast agents—have not been systematically evaluated. Summary of the Invention

[0007] To address the aforementioned issues, this application provides a DOTA-like rare-earth complex, specifically a chiral Gd-DOTA complex with dual-site chiral modification—an ethyl substituent on the macrocycle and an α-methyl group on the acetic acid arm—named Gd-L2, with Gd-L1 (Gd-DOTMA) as a reference compound. The molecular design of this application enhances kinetic inertness through increased steric rigidity, while optimizing water exchange kinetics through precise chiral modulation. Importantly, Gd-L2 differs structurally from clinically used hepatobiliary contrast agents (e.g., Gd-BOPTA and Gd-EOB-DTPA) and previously reported candidates (e.g., Gd-DOTA-POB and Gd-BOB-DOTA) because Gd-L2 possesses a fully aliphatic substituent framework, lacking an aromatic moiety. The intentional elimination of aromatic groups is expected to reduce nonspecific protein binding and related tissue retention, while mitigating potential toxicity risks associated with aromatic metabolism. Preliminary assessments indicate that Gd-L2 exhibits superior kinetic stability compared to current clinical standards and achieves efficient hepatic uptake mediated by OATP transporters, thereby enabling prolonged but physiologically appropriate retention in diagnostic imaging.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A DOTA-type rare earth complex, the structure of which is shown in Formula I:

[0009] Formula I Ln includes Gd(Ⅲ), Eu(Ⅲ), and Yb(Ⅲ).

[0010] The complexes described above, wherein the counter ion of the complexes is a pharmaceutically acceptable cation selected from hydrogen ions, sodium ions, potassium ions, or meglumine ions.

[0011] This invention also provides a method for preparing DOTA-type rare earth complexes, comprising the following steps: Step S1: (R)-benzyl lactate and trifluoromethanesulfonic anhydride undergo a trifluoromethanesulfonation reaction of the hydroxyl groups; The products obtained in steps S2 and S1 were nucleophilically substituted with the R-configuration chiral ring tinning (structure shown in Formula II) to obtain compound 3;

[0012] Formula II In step S3, compound 3 undergoes a catalytic hydrogenation reaction to remove the benzyl protecting group, yielding the free carboxylic acid ligand L2; Step S4: The free carboxylic acid ligand L2 is heated and reacted with rare earth salt in a weakly acidic aqueous solution. After the reaction is completed, the pH is adjusted to weakly alkaline, filtered, the filtrate is adjusted to neutral, and freeze-dried to obtain DOTA-type rare earth complexes. .

[0013] In the preparation method described above, in step S1, under the protection of an inert gas, (R)-benzyl lactate and trifluoromethanesulfonic anhydride react in the presence of 2,6-dimethylpyridine.

[0014] In the preparation method described above, in step S1, (R)-benzyl lactate and trifluoromethanesulfonic anhydride are first cooled to 0°C, and then 2,6-dimethylpyridine is slowly added to react. After the reaction is completed, the mixture is quenched with ice water and extracted to obtain the product.

[0015] In the preparation method described above, in step S2, under the protection of an inert gas, the R-configuration chiral ring tinning and the product obtained in step S1 react in the presence of K2CO3.

[0016] In the preparation method described above, in step S3, compound 3 is hydrogenolyzed in an anhydrous solvent under the action of Pd-C to obtain L2.

[0017] In the preparation method described above, the rare earth salts mentioned in step S4 include GdCl3·6H2O, EuCl3·6H2O, and YbCl3·6H2O, and the corresponding products are DOTA-type Gd complex Gd-L2, DOTA-type Eu complex Eu-L2, and DOTA-type Yb complex Yb-L2.

[0018] This invention also provides the application of DOTA-type rare earth complexes in the preparation of magnetic resonance imaging contrast agents.

[0019] As described above, the DOTA-type rare earth complex is the DOTA-type Gd complex Gd-L2, and the MRI contrast agent is a hepatobiliary-specific contrast agent, used for diagnostic imaging of liver diseases.

[0020] The present invention also provides an MRI contrast agent composition comprising the above-described DOTA-type gadolinium complex and pharmaceutically acceptable excipients; said excipients are selected from one or more of buffer salts, osmotic pressure regulators, pH regulators, or water for injection.

[0021] Due to the adoption of the above technical solution, the technical effects achieved by the present invention are as follows: This application describes the design and synthesis of Gd-L2, a novel macrocyclic MRI contrast agent containing a dual-site chiral modification—a tetraethyl substitution on the cycloalkenyl ring and an α-methyl group on the side chain arm—that synergistically enhances kinetic inertness and relaxation properties. This fully alkyl-substituted, aromatic-free structure minimizes nonspecific protein interactions and potential metabolic risks, while simultaneously promoting selective hepatic uptake via the OATP transporter, as demonstrated by the Imaging and Pharmacological Inhibitions Study.

[0022] The superior performance of Gd-L2 was systematically verified. This complex exhibits a high relaxivity (5.6 mM in the presence of HSA). - ¹s - ¹, with approximately 50% higher uptake than Gd-DOTA, excellent kinetic stability under acidic and transmetallization conditions, and favorable biodistribution characteristics, characterized by rapid hepatic uptake (peak at approximately 10 minutes post-injection) followed by efficient clearance (<0.2% of the injected dose retained in the liver and kidneys after 24 hours). Importantly, in orthotopic HCC models, Gd-L2 produces strong and sustained tumor-liver contrast (CNR>8 for over 30 minutes), achieving clear tumor margin delineation.

[0023] These results demonstrate that synergistic chiral engineering of the macrocyclic core and side chain arms represents a powerful strategy for developing high-performance, hepatobiliary-specific MRI contrast agents. By integrating superior kinetic stability, enhanced imaging efficiency, and organ-selective targeting within a non-aromatic framework, Gd-L2 emerges as a promising and potentially safer candidate for clinical translation, establishing a new molecular design paradigm for next-generation GBCAs. Attached Figure Description

[0024] Figure 1 This section describes the longitudinal relaxation time variations of Gd-L1 and Gd-L2 under acidic and competitive ion conditions; where A represents the acid-catalyzed dissociation kinetics of Gd-DOTA, Gd-L1, and Gd-L2 in 1 M HCl at 50°C; and B represents the dissociation kinetics of Gd-DOTA, Gd-L1, and Gd-L2 in the presence of a 10-fold excess of Zn at 50°C. 2+ Transmetallization challenge tests of Gd-EOB-DTPA, Gd-DOTA, Gd-L1, and Gd-L2 were performed; all measurements were conducted at a fixed complex concentration of 1.0 mM. Figure 2 The ¹H NMRD curve of Gd-L2 and 17 O NMR data, where A is the ¹H NMRD curve of Gd-L2 (6.12 mM, pH 7.5) measured at 10°C (blue), 25°C (black), and 37°C (red); B is the NMR curve of Gd-L2 (13.7 mM, pH 7.5, 11.74 T).17 Temperature dependence of O transverse relaxation rate and (C) 17 O chemical shift; Figure 3 The in vitro cytotoxicity of Gd-L1 and Gd-L2; cell viability of LO2 hepatocytes and 293T renal epithelial cells after incubation in specified concentrations of Gd-L1 or Gd-L2 for 24 hours, data are expressed as mean ± standard deviation (n=3). Figure 4 The images show in vivo T1-weighted MRI and time-dependent signal enhancement at 3.0T; dynamic coronal images of the liver (top row) and kidney (bottom row) of mice before and after intravenous injection of (A) Gd-L2 or (B) Gd-L1 (0.1 mmol / kg); (C) the corresponding time-enhancement curves of the liver and (D) kidney, representing the relative signal changes over time; Figure 5 The data represent the biodistribution of Gd-L1 and Gd-L2 in normal mice following intravenous injection (0.1 mmol / kg); (A) shows the gadolinium concentration per gram of tissue in major organs (μg Gd / g) at 5 minutes and 24 hours; and (B) shows the percentage of injected dose recovered in the liver and kidney at the same time points (% ID). Data are expressed as mean ± standard deviation (n=3). Figure 6 This is OATP-mediated inhibition of hepatic Gd-L2 uptake, in which (A, B) showed no inhibition after injection of Gd-L2 (0.1 mmol / kg) (BSP). - ) or exist (BSP) + In the case of sodium bromosulfonylphthalein, representative coronal T1-weighted images (3.0T) of the liver (A) and kidney (B) obtained at specified time points, (C, D) corresponding normalized signal-to-noise ratio (nSNR) time processes of the liver (C) and kidney (D); Figure 7 This is an in vivo toxicity assessment of Gd-L1 and Gd-L2; Figures (A, B) show representative H&E-stained sections of liver and kidney tissues from mice 24 hours (A) (acute) and 60 days (B) (subacute) after intravenous injection of PBS, Gd-DOTA, Gd-L1, or Gd-L2 (0.1 mmol / kg), scale bar: 200 μm; Figures (C–F) show plasma biochemical parameters measured 24 hours (C, E) and 14 days (D, F) after administration; data are mean ± standard deviation (n=3); ALT (UL -1 ), alanine aminotransferase; AST (UL -1 ), aspartate aminotransferase; CK (UL) -1 ), creatine kinase; LDH (UL -1 ), lactate dehydrogenase; ALP (UL-1 ), alkaline phosphatase; BUN (mmol / L) -1 ), blood urea nitrogen; CR (μmol L -1 ), creatinine; ALB (g L) -1 ),albumin; Figure 8 This study used Gd-L2 to detect hepatocellular carcinoma (HCC) in situ. Figure (A) shows the transverse T1-weighted magnetic resonance imaging of H22 tumor and liver at different time points before and after intravenous injection of Gd-L2 (0.1 mmol / kg); the tumor region is marked in the figure. (B) is the time-varying curve of contrast-to-noise ratio (CNR) between liver parenchyma and tumor tissue. (C) shows the signal values ​​before injection and during the period of strongest signal (13 minutes). Figure 9 This is the hydrogen nuclear magnetic resonance spectrum of Eu-L2; Figure 10 This is the hydrogen nuclear magnetic resonance spectrum of Yb-L2; Figure 11 This is an in vivo toxicity assessment of Gd-L1 and Gd-L2; representative H&E stained sections of heart, spleen and lung tissue from mice 24 hours (acute) and 60 days (subacute) after intravenous injection of PBS, Gd-DOTA, Gd-L1 or Gd-L2 (0.1 mmol / kg), scale bar: 200 μm.

[0025] Figure 12 This is a mouse biodistribution experimental species, showing the specific distribution values ​​of Gd in various major organs of mice; Figure 13 Plasma biochemical parameters were measured on day 1 (after a single injection) and day 60 (after four injections every two weeks): ALT (U / L), alanine aminotransferase; AST (U / L), aspartate aminotransferase; CK (U / L), creatine kinase; LDH (U / L), lactate dehydrogenase; ALP (U / L), alkaline phosphatase; BUN (mmol / L), blood urea nitrogen; CR (μmol / L), creatinine; and ALB (g / L), albumin. Detailed Implementation

[0026] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Example 1: Synthesis of Gd-L2

[0028] Synthesis of Compound 2 Under a nitrogen atmosphere, (R)-benzyl lactate (9.0 g, 50 mmol) and trifluoromethanesulfonic anhydride (9.0 mL, 52.5 mmol) were dissolved in anhydrous dichloromethane (100 mL). The mixture was cooled to 0°C in an ice bath, and then 2,6-dimethylpyridine (6 mL, 52.5 mmol) was slowly added to react with the mixture. The reaction was quenched with ice water, and the mixture was extracted twice with dichloromethane. The combined organic phases were washed with saturated brine. The solution was dried over anhydrous Na₂SO₄, filtered, and concentrated to dryness under reduced pressure. The residue was separated by silica gel column chromatography (DCM:MeOH = 20:1) to give the product (10.9 g, 70% yield). 1 H NMR (400 MHz, CDCl3) δ 7.4 (s, 5H), 5.28 (s, 3H), 1.72 (S, 3H). 13 C NMR (100 MHz, CDCl3) δ 167.61, 134.87,129.04, 123.61, 120.43, 117.26, 114.08, 80.52, 68.54, 18.17. Synthesis of Compound 3 Chiral cyclopentadiene (R configuration) (2.8 g, 10 mmol) was dissolved in anhydrous acetonitrile (40 mL), and K₂CO₃ (8.3 g, 60 mmol) and compound 2 (18.7 g, 60 mmol) were added. The reaction was carried out at room temperature for 24 hours under nitrogen protection. The reaction was monitored by mass spectrometry. After completion, the solvent was removed by filtration and centrifugation. The product was purified by silica gel column chromatography (PE:DCM = 1:2) to give compound 3 (4.7 g, 50% yield). ¹H NMR (400 MHz, CD3CN) δ 7.49-7.20 (m, 20H), 5.27-4.83 (m, 8H), 3.60 (q, J=6.8Hz, 4H), 3.08 (d, J=11.9 Hz, 3H), 2.82 (t, J=11.4 Hz, 4H), 2.10 (d, J=41.3 Hz, 6H), 1.57 (s, 4H), 1.13 (d, J=6.8 Hz, 13H), 1.08-0.88 (m, 4H), 0.71 (t, J=7.3 Hz, 12H). ¹³C NMR (100 MHz, CD3CN) δ 173.86, 136.57, 128.42, 128.08, 117.36, 65.86, 56.01, 52.74, 44.35, 22.54, 14.34, 10.72. ESI-MS calculated values ​​for C 57 H 76 N4O8 + [M+H] + : 933.5, Measured value: 933.5.

[0029] The chiral ring structure of the R configuration is as follows:

[0030] Reference article for synthesis method: Chiral DOTA chelators as an improved platform for biomedical imaging and therapy applications; Nature Communicaitons. 2018,9,857, DOI: 10.1038 / s41467-018-03315-8. L2 synthesis Compound 3 (3.0 g, 3.2 mmol) was dissolved in anhydrous EtOH (60 mL), followed by the addition of Pd-C (0.3 g). The mixture was reacted at 55°C for 24 hours under an oxygen-free hydrogen atmosphere. After the reaction was complete, the mixture was filtered and rotary evaporated to obtain ligand L2. ¹H NMR (400 MHz, MeOD) δ 3.93 (d, J=7.3 Hz, 4H), 3.50 (t, J=11.2Hz, 4H), 3.27 (s, 5H), 3.02 (t, J=13.1 Hz, 4H), 2.01 (s, 4H), 1.54 (d, J=7.2Hz, 12H), 1.30 (s, 5H), 1.04 (t, J=7.4 Hz, 12H). ¹³C NMR (100 MHz, CD3OD) δ182.80, 181.72, 78.04, 68.71, 57.40, 22.87, 20.59, 18.95. ESI-MS calculated values ​​for C 28 H 53 N4O8 + [M+H] + : 573.3, Measured value: 573.2.

[0031] Synthesis of Gd-L2 L2 (573 mg, 1.0 mmol) was dissolved in deionized water (3 mL), and the pH of the resulting solution was adjusted to approximately 6.5 using aqueous NaOH. Then, a solution of GdCl3·6H2O dissolved in 2 mL of deionized water was added, while maintaining the pH of the reaction system at approximately 6.5 throughout the process. The reaction mixture was then stirred under reflux and monitored by RP-HPLC. After the reaction was complete, the pH was adjusted to 10 using aqueous NaOH. The mixture was then filtered, and the filtrate was adjusted to neutral using aqueous HCl. The filtrate was then freeze-dried to give a white solid Gd-L2 (682 mg, yield 94%, RP-HPLC purity >99%). ESI-HRMS calculated values ​​for C 28 H 48 GdN4O8[M]: 726.2715, measured value: 726.2718. Eu-L2 was synthesized using the same method. ESI-HRMS calculated values ​​for C 28 H 48 EuN4O8[M]: 721.2681, Measured value: 721.2683.

[0032] RP-HPLC Method: Samples were analyzed using a Waters 2695 RP-HPLC system equipped with an XBridge C18 column (5 μm, 4.6 x 150 mm). Mobile phase A: deionized water containing 0.05% trifluoroacetic acid; Mobile phase B: CH3CN; Flow rate: 1.0 mL / min. Method A for L2 and its metal complexes: isocratic elution with 20% mobile phase B for 10 min at a flow rate of 1.0 mL / min. Method B for all other analyses: linearly increase the proportion of mobile phase B from 10% to 100% over 10 min, then return to 10% mobile phase B over 2 min and hold for 3 min with 10% mobile phase B.

[0033] Similarly, the same reaction (heating in a weakly acidic aqueous solution) was carried out with EuCl3·6H2O or YbCl3·6H2O and L2 to obtain Eu-L2 and Yb-L2, respectively, for ¹H NMR characterization (see...). Figure 9 and 10 ).

[0034] Experimental Example 1: Relaxation Rate Measurement The Gd-L1 complex and its corresponding ligand L1 (DOTMA) were synthesized according to previously reported methods (References: Properties, Solution State Behavior, and Crystal Structures of Chelates of DOTMA; DOI: 10.1021 / ic2012827).

[0035] The longitudinal (T1) and transverse (T2) relaxation times of all Gd(III) complexes were determined using a 1.41 T NMR relaxor (¹H frequency: 60 MHz; Huan Tong Nuclear Magnetic, China) under relaxation rate assessment conditions.

[0036] T1 measurements were performed using a standard inversion recovery pulse sequence with ten inversion times ranging from 0.05 x T1 to 5 x T1 to ensure accurate sampling of the relaxation curve. A repetition time of 10.0 seconds was applied to allow for full longitudinal relaxation. The 90° and 180° pulse widths were set to 37 and 75 μs, respectively. T2 relaxation time was measured using a Carr-Purcell-Meiboom-Gill pulse sequence with an echo interval of 3.0 ms and a repetition time of 5.0 s.

[0037] All complexes were dissolved in deionized water to prepare solutions of the appropriate concentrations. 200 μL aliquots were transferred to NMR tubes and equilibrated in a temperature-controlled water bath at 37°C for 10 minutes prior to relaxation measurements to ensure thermal stability and repeatability.

[0038] To assess the effect of serum protein binding on relaxation behavior, human serum albumin (HSA, 22.5% w / v in deionized water) was added to each NMR tube containing the complex solution. The resulting mixtures were incubated at 37°C for 30 min to allow binding equilibrium to be established, and T1 and T2 values ​​were recorded under the same conditions. Longitudinal and transverse relaxation rates (r1 and r2) were calculated from the slopes of linear plots of 1 / T1 and 1 / T2 against Gd(III) concentration, respectively, assuming a single-exponential relaxation mode. All measurements were performed triplicate, and reported values ​​represent the average of three independent experiments.

[0039] The diagnostic efficacy of gadolinium-based contrast agents depends primarily on their longitudinal relaxation rate (r1), a direct indicator of their ability to accelerate the longitudinal relaxation of water protons. To determine the contrast enhancement capability of the system designed in this application, the r1 values ​​of the Gd(III) chelate were measured at 1.41 T and 37°C and compared with the clinical standard Gd-DOTA. Consistent with literature reports, Gd-L1 and Gd-DOTA exhibited comparable longitudinal relaxation rates, with r1 values ​​of 3.3 and 3.2 mM, respectively. -1 s -1 According to the Solomon-Bloembergen-Morgan theory, the relaxation rate is mainly determined by the water exchange rate (1 / τ). M ) and rotation-related time (τ) R These factors, together, define the efficiency of inner sphere relaxation. Previous research has shown that chiral substitution on the macrocyclic framework can simultaneously accelerate water exchange (shorten τ). M and slowing down molecular tumbling (increasing τ) R This enhances the relaxation rate, thereby optimizing the kinetic and dynamic contributions to proton relaxation. Consistent with this mechanistic framework, Gd-L2 exhibits a 4.2 mM [value missing] at 1.41 T and 37°C. -1 s -1 The r1 value was approximately 20% higher than that of Gd-L1. To better approximate physiological conditions, relaxation rates were also measured in the presence of human serum albumin. Under these conditions, the r1 values ​​for Gd-DOTA, Gd-L1, and Gd-L2 were 3.7, 3.9, and 5.6 mM, respectively. -1 s -1 Notably, the relaxation rate of Gd-L2 increased by 30% compared to Gd-L1 and by 33.5% compared to Gd-DOTA, highlighting the beneficial effects of ligand design under biologically relevant conditions.

[0040] Table 1 Relaxation rates of gadolinium complexes a and log P

[0041] Conditions: 37°C, H₂O, 1.41T, mM -1 s -1 HSA: Human serum albumin, 4.5% (w / v); b. Reference: Efficiency, thermodynamic and kinetic stability of marketed gadolinium chelates and their possible clinical consequences: a critical review. Biometals [2008, 21, 469–490]. Experimental Example 2: Dynamic Stability Assessment The kinetic stability of gadolinium(III) chelates is a key determinant of their clinical safety, because the release of free Gd from unstable complexes... 3+ It is associated with serious adverse reactions. Extensive research has shown that the in vivo dissociation of gadolinium chelates occurs primarily through two mechanisms: proton-assisted dissociation under acidic conditions and transmetalation via metal ion displacement. Therefore, assessing stability under these representative challenging conditions is crucial for determining the safety of gadolinium-based contrast agents.

[0042] Therefore, this application first investigated the acid-mediated dissociation of Gd-L1 and Gd-L2 in 1M HCl, using the clinically established macrocyclic reagent Gd-DOTA as a benchmark. Figure 1 A). Dissociation was monitored by changes in longitudinal relaxation time (T1) over 192 hours. Throughout the observation period, both Gd-L1 and Gd-L2 exhibited negligible dissociation, indicating excellent tolerance to acid-catalyzed dechelation. In contrast, Gd-DOTA showed significant dissociation in the first 12 hours before reaching equilibrium. For comparison, the linear chelate Gd-EOB-DTPA, known to be unstable under acidic conditions, exhibited an extremely short half-life (t1 / 2 < 5 seconds) even in 0.1 M HCl. This significant difference stems from the acid-catalyzed protonation of the ligand at low pH (≤ 2), which weakens metal-ligand coordination and accelerates decomposition, a process particularly pronounced for acyclic chelates.

[0043] Next, use a 10-fold molar excess of Zn 2+The kinetic stability of a physiologically relevant and highly compatible macrocyclic ligand-competitive ion under transmetallation challenge conditions was evaluated. Figure 1 B). As expected, Gd-EOB-DTPA in Zn 2+ The presence of rapid dissociation is consistent with its linear structure and finite kinetic inertia. In contrast, Gd-L1 and Gd-L2 show minimal change in T1 over 192 hours, exhibiting resistance to Zn. 2+ Strong tolerance to induced metal exchange.

[0044] In summary, these findings confirm that the kinetic stability of clinically used linear hepatobiliary contrast agents, such as Gd-EOB-DTPA, is significantly lower than that of DOTA-based macrocyclic chelates. Notably, the chiral-modified complexes Gd-L1 and Gd-L2 exhibit enhanced overall inertness compared to Gd-DOTA, highlighting their potential in applications requiring high relaxation rates and superior kinetic robustness.

[0045] Experimental Example 3: Study on Relaxation Characteristics Sample preparation: Several milligrams of sample were dissolved in 3.0 mL of ultrapure water, and the pH was adjusted to 6.5–7.5. The concentration of metal ions in the solution was determined by volumetric magnetic susceptibility displacement measurement performed at 11.7 T.

[0046] The magnetic field dependence of the longitudinal relaxation rate (R1) of protons in solvent by ¹H NMRD analysis (¹H NMRD curves) was measured in aqueous solution using a variable field relaxor equipped with an HTS-110 3T metrology-free cryogenic superconducting magnet (Mede, Italy), which operates over the entire proton Larmor frequency range of 20–120 MHz (0.47–3.00 T). Measurements were performed using a standard inversion recovery sequence (20 experiments, 2 scans) with a typical 90° pulse width of 3.5 μs, and data reproducibility was within ±0.5%. Temperature was controlled by a Stelar VTC-91 heating gas flow. Other points in the 0.01–10 MHz frequency range were collected on a Stelar SmarTracer fast field cyclic relaxor. ¹H NMRD curves were measured at 283, 298, and 310 K. All experiments were repeated three times, with data reproducibility within ±0.5%.

[0047] 17 O NMR was recorded on a Bruker Avance III spectrometer (11.7 T) equipped with a 5 mm dual-resonance Z-gradient broadband probe and a Bruker BVT-3000 unit for temperature control. 17O NMR measurements were performed. Samples were prepared in 3 mm NMR tubes by mixing 188 μL of a complex solution at physiological pH, 22 μL of D₂O containing 10% tert-butanol, and 10 μL of H₂. 17 O (Cambridge Isotope, 2% isotope enriched). Transverse relaxation rate was calculated based on the full width at half maximum (FWHM) of the signal. The ¹H NMR chemical shift of the tert-butanol signal was used as an internal standard from... 17 The contribution of volumetric magnetic susceptibility is subtracted from the NMR displacement data.

[0048] The relaxation properties of paramagnetic metallic complexes are controlled by a variety of structural and dynamic molecular parameters. The most comprehensive and reliable method for determining the physicochemical parameters controlling NMR relaxation is to measure the relaxation rate as a function of magnetic field strength over a wide frequency range (approximately 0.01–100 MHz). This dataset constitutes NMR relaxation dispersion curves, obtained using a variable-field relaxor (up to 3 T) combined with a conventional high-field NMR spectrometer (above 3 T). These measurements exhibit temperature dependence. 17 O NMR studies (including transverse relaxation rate (R²) and paramagnetic chemical shift (Δω)) effectively complement each other, providing further insights into water exchange kinetics. Figure 2 C).

[0049] In aqueous solution, the ¹H NMRD curves of Gd-L2 were recorded at near physiological pH and three temperatures (10, 25, and 37°C). Figure 2 A). These curves show a characteristic low-frequency plateau with a nearly constant relaxation rate, followed by a dispersive region between 1 and 10 MHz, and finally a second quasi-plateau at higher frequencies. This behavior is typical of low molecular weight Gd(III) chelates in aqueous solution undergoing rapid tumbling. The observed decrease in r1 with increasing temperature (from 10 to 37°C) indicates that Gd-L2 is in a rapid water exchange state, where molecular tumbling rather than exchange kinetics dominates the relaxation rate. Therefore, the water exchange rate is not a limiting factor for the overall relaxation rate. 17 O NMR data further support this conclusion.

[0050] Specifically, from 17 The ln(R²) value obtained by O NMR measurement showed a significant and nearly linear increase with decreasing temperature. Figure 2 (B), which is consistent with fast swap behavior.

[0051] Experimental data were analyzed using the Solomon-Bloembergen-Morgan and Freed equations to describe proton relaxation in the inner and outer spheres, while the Swift-Connick form was used to analyze... 17O NMR data. From the ¹H NMRD curve of Gd-L2 and 17 The parameters obtained from simultaneous best-fitting of ONMR data are summarized in Table 2 and compared with previously reported Gd-L1 parameters. During the fitting process, the hydration number q=1 of Gd-L2 was assumed, consistent with its coordination chemistry, similar to its parent monohydrated Gd-L1 complex. The Gd-H distance of the inner spherical water molecule was fixed at r=3.1 Å. During optimization, the activation energy (E_V) modulated by electronic relaxation, the water diffusivity at 25°C, and the parameters were also considered. 25 D) and the closest distance (a) to the outer sphere water molecules remained constant. At 25°C, the Gd-L2 complex exhibited a higher r1 value than Gd-L1 across the entire magnetic field range studied. This enhancement is primarily attributed to the slower rotational dynamics (τ_R = 130 ps, ​​approximately 60% longer than Gd-L1), reflecting the increased molecular weight of the complex and favorable electronic relaxation parameters (A² and τ_V), the effects of which were particularly pronounced at low magnetic fields. Furthermore, as shown in Table 2, Gd-L2 exhibited a significantly shorter water molecule residence lifetime (τ_M = 3 ns) and an activation enthalpy of 23.6 kJ mol compared to Gd-L1. -1 This accelerated water exchange may be due to the increased steric constraint imposed by macrocyclic substituents within the ligands of the metal ions.

[0052] Table 2 is from ¹H NMRD and 17 Parameters analyzed simultaneously with O NMR data

[0053] a is fixed during the fitting process. b References (Properties, Solution State Behavior, and Crystal Structures of Chelates of DOTMA; DOI: 10.1021 / ic2012827) Experimental Example 4: Cytotoxicity Cytotoxicity assays provide a preliminary screening tool for assessing the biocompatibility of GBCAs under controlled conditions, offering initial insights into their cellular tolerance. While such assays do not directly measure complex stability, they help identify formulations that may induce significant cellular stress or damage—typically associated with free GdO2. 3+The release of ions or the inherent toxicity of ligands are relevant. In this application, normal human hepatocytes (LO2, purchased from Mirror Image (Shanghai) Cell Technology Co., Ltd., catalog number: iCell-h054) and renal epithelial cells (293T, purchased from Mirror Image (Shanghai) Cell Technology Co., Ltd., catalog number: iCell-h237) were selected as relevant models to assess basic cytotoxicity. This selection is physiologically reasonable because the liver and kidneys are the main pathways for the uptake, metabolism, and elimination of most GBCAs. Therefore, these organs are potential Gd... 3+ The most susceptible target for inducing toxicity.

[0054] Human normal hepatocytes (LO2) and human embryonic kidney cells (HEK-293T) were cultured as representative normal cell lines for cell compatibility assessment. Cells were collected, prepared into single-cell suspensions, and seeded into 96-well plates at a density of 5,000 cells per well. The plates were incubated in a humidified incubator at 37°C and 5% CO2 for 24 hours to allow cell adhesion and recovery. After initial incubation, the culture medium was replaced with Dulbecco modified Eagle medium containing different concentrations of Gd-L1 or Gd-L2. Cells were then incubated for another 24 hours to assess the concentration-dependent cytotoxic effects of gadolinium complexes. After drug treatment, the culture medium was removed, and cells were gently washed with phosphate-buffered saline. Fresh culture medium supplemented with 10% CCK-8 reagent was added to each well, and the plates were incubated for approximately 4 hours to allow metabolically active cells to form water-soluble formazan. The absorbance at 450 nm was measured using an ELISA reader, and cell viability was quantified based on the measured optical density value relative to untreated control cells.

[0055] like Figure 3 As shown, both Gd-L1 and Gd-L2 exhibited good cytocompatibility in LO2 and 293T cells, maintaining high cell viability across concentrations up to 1.0 mM. A concentration-dependent decrease in cell viability was observed for both complexes; however, the decrease was limited. Even at the highest concentration tested (1.0 mM), cell viability treated with Gd-L2 remained above 90%. Notably, Gd-L2 consistently showed slightly higher, but systematically higher, cell viability than Gd-L1 across all tested concentrations and both cell lines. Furthermore, the lower variability observed in the dose-response curves of Gd-L2 suggests more reproducible and controllable biological interactions, which may be attributed to its optimized bipedal structure and enhanced kinetic inertness.

[0056] Experimental Example 5: In vivo MR imaging All gadolinium-based chelates used in in vivo magnetic resonance imaging were validated by HPLC for purity exceeding 95%, ensuring their suitability for biological evaluation. Normal male BALB / c mice (6–8 weeks old, weighing 20 ± 3 g; Charles River, China) were used as the animal model for MRI studies. Mice were anesthetized with 2% sodium pentobarbital at a dose of 40 μL / 10 g body weight prior to imaging. Contrast agent was then administered intravenously at a dose of 0.1 mmol Gd / kg body weight. T1-weighted MR images were acquired using a clinical 3.0T MRI scanner. Imaging parameters were as follows: echo time = 9.71 ms, repetition time = 191.1 ms, flip angle = 50°, field of view = 50 x 50 mm, matrix size = 200 x 135, slice thickness = 1.50 mm, slice interval = 0.165 mm, 12 consecutive slices, pixel bandwidth = 216.0 Hz, number of excitations = 4. Quantitative image analysis was subsequently performed using MicroDicom Viewer software. Signal intensity was measured from manually defined regions of interest in the liver and kidneys. In vivo uptake and enhancement of the contrast agent were assessed using relative enhancement, calculated according to the following formula: Relative enhancement = (signal intensity after injection / signal intensity before injection) × 100%, The pre-injection signal intensity and post-injection signal intensity represent the signal intensities measured before and after contrast agent administration, respectively.

[0057] The in vivo performance of Gd-L1 and Gd-L2 was evaluated in normal BALB / c mice by dynamic contrast-enhanced MRI at a clinical field strength of 3.0T following intravenous injection of 0.1 mmol / kg. Coronal T1-weighted images were acquired at multiple time points before and after injection. Figure 4 As shown in Figure A, Gd-L2 induced significant liver signal enhancement approximately 10 minutes post-injection, resulting in excellent contrast between the liver and surrounding tissues. The enhancement remained significant at 27 minutes, providing a sustained imaging window, before significantly decreasing at 33 minutes. Simultaneously, the gallbladder showed marked signal enhancement at both 27 and 33 minutes, indicating active excretion of Gd-L2 via the hepatobiliary pathway. Kidney enhancement was also observed concurrently. Figure 4 A. The following diagram and Figure 4 D), characterized by an initial increase in signal followed by gradual clearance, reflects an accompanying renal clearance pathway. In contrast, mice injected with Gd-L1 did not show significant liver-specific enhancement ( Figure 4 (Figure B above). The corresponding time-signal curves show that the liver signal intensity did not increase significantly over time. Figure 4 C). Conversely, the signal enhancement is primarily confined to the kidneys, manifesting as rapid uptake followed by gradual elimination (C). Figure 4 B. The following diagram and Figure 4(D), which is consistent with the main renal clearance pathway. These results indicate that Gd-L2 has effective hepatocellular targeting ability and is cleared via a dual hepatobiliary-renal pathway, while Gd-L1 is mainly cleared by the kidneys with no significant hepatic uptake.

[0058] Experimental Example 6: Biological Distribution Study Twenty healthy male BALB / c mice (6–8 weeks old, weighing 20 ± 3 g; Charles River, China) were randomly assigned to four groups. Blood samples were collected from the mice at predetermined time points for subsequent analysis. The animals were then humanely euthanized, and major organs, including the heart, liver, spleen, lungs, kidneys, small intestine, and muscle, were carefully collected. Approximately 0.1 g of each tissue sample was collected for elemental analysis. To assess biodistribution and residual metal accumulation, all tissue samples were digested with nitrate under controlled conditions. The concentration of metal ions in the digested samples was determined using inductively coupled plasma mass spectrometry (ICP-MS).

[0059] The in vivo biodistribution of Gd-L1 and Gd-L2 was quantitatively assessed after intravenous injection (0.1 mmol / kg) into normal mice. Animals were sacrificed at 5 minutes and 24 hours post-injection, and major organs (heart, liver, spleen, lung, kidney, brain), intestinal muscle, and blood were collected. Gadolinium content in each sample was determined by inductively coupled plasma mass spectrometry, and the results are summarized in […]. Figure 5 .

[0060] like Figure 5 As shown, Gd-L1 and Gd-L2 exhibited significantly different biodistribution profiles. At 5 minutes post-injection, Gd-L2 showed significant hepatic accumulation, reaching 14.81 μg Gd / g tissue. This corresponds to 7.54% of the injected dose, approximately three times that observed with Gd-L1. This enhanced hepatic uptake is consistent with previous findings, suggesting that the introduction of four symmetrically arranged chiral ethyl groups on the DOTA macrocycle promotes hepatocyte targeting, possibly mediated by an active transport mechanism. In contrast, both agents showed considerably greater renal uptake at 5 minutes. Figure 5 (B) This confirms that renal excretion remains the primary route of clearance. By 24 hours, gadolinium levels in the liver and kidneys of both complexes had decreased to below 0.2% of the injected dose, indicating efficient systemic clearance and minimal long-term retention.

[0061] Experimental Case 7: Liver Uptake Mechanism The observed significant hepatic accumulation of Gd-L2 prompted this application to investigate its potential molecular mechanism. It was hypothesized that this enhanced hepatic uptake is mediated by organic anion transport peptides (OATP), a major family of transport proteins responsible for the uptake of various anion substrates by hepatocytes. To evaluate this hypothesis, dynamic MRI studies were performed in male mice (6–8 weeks old, 20 ± 3 g; Charles River, China) pretreated with sodium bromosulfonylphthalein (BSP), a recognized competitive inhibitor of OATP-mediated transport. BSP was administered before and during imaging to maintain a stable plasma concentration of approximately 0.1 mM. Figure 6 As shown in Figure A, under BSP inhibition, the characteristic liver enhancement of Gd-L2 was significantly reduced compared to the control group. Quantitative analysis confirmed a significant and sustained decrease in liver signal intensity over time. Figure 6 C). Simultaneously, when the hepatobiliary pathway is blocked, the enhancement of renal signaling becomes more pronounced (C). Figure 6 B). The corresponding kidney signal-time curves showed a significant increase in both peak intensity and duration. Figure 6 (D) indicates a compensatory shift in renal clearance when OATP-mediated hepatic uptake is inhibited. These pharmacological inhibition studies suggest that the favorable hepatic targeting of Gd-L2 is primarily mediated by OATP transporters. This result highlights how strategic chiral modification of the macrocyclic cytoskeleton can effectively utilize specific transport pathways to achieve tissue-selective contrast enhancement.

[0062] Case 8: In vivo toxicity assessment Forty healthy male BALB / c mice (6–8 weeks old, weighing 20 ± 3 g; Charles River, China) were randomly assigned to acute and subacute toxicity groups, with 20 animals in each group. Within each group, mice treated with phosphate-buffered saline and Gd-DOTA served as separate control groups. All contrast agents were administered intravenously at a dose of 0.1 mmol Gd / kg body weight. In the acute toxicity study, mice were assessed after a single administration of the appropriate agent. In contrast, mice in the subacute toxicity group received repeated administrations over 60 days to assess potential cumulative toxic effects. At the designated experimental endpoint, blood samples were collected from the mice under appropriate conditions, and the animals were then sacrificed. Major organs, including the heart, liver, spleen, lungs, and kidneys, were harvested for further analysis. Blood samples were centrifuged to obtain plasma, which was analyzed using a commercial enzyme-linked immunosorbent assay (ELISA) kit to determine the levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), albumin, alkaline phosphatase (ALP), blood urea nitrogen (BUN), creatinine, lactate dehydrogenase (LDH), and creatine kinase (CK) to assess liver and kidney function and potential systemic tissue damage. Simultaneously, the collected organs were fixed, sectioned, and stained with hematoxylin and eosin for histopathological evaluation. Tissue sections were examined and documented under an optical microscope to identify any treatment-related morphological changes.

[0063] A comprehensive in vivo toxicity assessment was performed to evaluate systemic safety and potential tissue damage associated with Gd-L1 and Gd-L2. Healthy mice were intravenously injected with the test agent and sacrificed at 24 hours to assess acute toxicity and at 60 days to assess subacute effects (following a two-week dosing interval). Phosphate-buffered saline (PBS) and clinical standard Gd-DOTA served as negative and reference controls, respectively. Major organs—including heart, liver, spleen, lung, and kidney—were collected, fixed, and stained with hematoxylin and eosin for detailed histopathological analysis.

[0064] Microscopic examination revealed normal tissue structure and intact cell morphology in all treatment groups. In the liver, lobular structure and hepatocyte integrity were well preserved, with no signs of degeneration, necrosis, or inflammatory infiltration. Kidney tissue showed intact glomeruli and tubules, indicating no structural damage. Figure 7 A, B). Similar findings were observed in heart, spleen, and lung sections, all of which were indistinguishable from the control group. Figure 11 To assess functional toxicity, key plasma biochemical markers of hepatic and renal function impairment were quantified. For example... Figure 7 As shown in CF, the levels of alanine aminotransferase, aspartate aminotransferase, creatinine, blood urea nitrogen, and other related parameters were not statistically significantly different between mice treated with Gd-L1 or Gd-L2 and mice in the PBS or Gd-DOTA control group at both acute and subacute time points.

[0065] Example 9: In vivo MR imaging of mice with orthotopic hepatocellular carcinoma H22 hepatocellular carcinoma cells (purchased from Mirror Image (Shanghai) Cell Technology Co., Ltd., catalog number: iCell-m074) from a mouse tumor model were cultured in DMEM medium supplemented with 10% fetal bovine serum and 1% penicillin-streptomycin solution at 37°C under a 95% air / 5% carbon dioxide atmosphere. The cell suspension was mixed with basement membrane extract and injected into the liver of healthy male BALB / c mice (6-8 weeks old, weighing 20 ± 3 g; Charles River, China) (2 x 10⁻⁶ cells per injection). 5 (cells). After routine postoperative culture, magnetic resonance imaging (MRI) was performed on day 7. The MR imaging protocol for tumor-bearing mice was the same as for normal mice, with the only modification being the formula for quantifying signal enhancement: Relative enhancement = signal intensity after injection (tumor-liver) - signal intensity before injection (tumor-liver).

[0066] Monitoring high-risk populations using ultrasound combined with serum biomarkers is fundamental for the early detection of hepatocellular carcinoma (HCC), while final diagnosis relies on multi-phase contrast-enhanced imaging, in which hepatocellular-specific MRI plays a central role. To evaluate the diagnostic potential of Gd-L2 for HCC, dynamic T1-weighted MRI was performed on mice carrying orthotopic H22 tumors after intravenous injection of Gd-L2 (0.1 mmol / kg).

[0067] The contrasting behavior of Gd-L2 stems from the difference in transporter protein expression between normal and malignant hepatocytes. OATP expression is significantly downregulated or absent in HCC cells, while normal hepatocytes retain robust OATP activity. Therefore, Gd-L2 uptake, primarily mediated by OATP, selectively accumulates in normal liver parenchyma, with minimal uptake in tumor tissue. Figure 8 As shown in the cross-sectional image of A, the tumor margins became clearly discernible relative to the enhanced liver background as early as 8 minutes post-injection, and the contrast persisted throughout the 42-minute imaging period. Quantitative analysis of the liver-tumor contrast-to-noise ratio further confirmed these observations. Figure 8 (B) Following Gd-L2 administration, the CNR increased rapidly, peaking at approximately 13 minutes and remaining at a high level for over 30 minutes, providing a wide diagnostic time window. In summary, these imaging and quantitative data demonstrate that Gd-L2 can effectively distinguish HCC from surrounding liver tissue, highlighting its ability as a contrast agent for tumor detection in hepatocellular-specific MRI.

[0068] All of the above animal experiments were approved by the Animal Experiment Ethics Committee of the Wenzhou Institute of Animal Sciences, Chinese Academy of Sciences (Ethics Approval Numbers: WIUCAS23013103, WIUCAS22110104).

[0069] Clinically used gadolinium-based contrast agents release trace amounts of gadolinium ions (Gd). 3+ The resulting safety concerns highlight the need for ultra-stable alternatives for magnetic resonance imaging (MRI). This application describes the development of Gd-L2, a fully alkyl-substituted chiral Gd-DOTA derivative comprising a tetraethyl-substituted cycloene backbone and an α-arm methyl group. This dual-site chiral design enhances both kinetic inertness and relaxation rate, providing a structurally robust platform for contrast enhancement in hepatobiliary MRI. Gd-L2 exhibits OATP-mediated hepatic uptake, enabling selective hepatic accumulation followed by efficient renal clearance. In an orthotopic mouse model of hepatocellular carcinoma, the complex achieved significant tumor margin delineation, highlighting its potential for early hepatocellular carcinoma detection. This application establishes a non-aromatic molecular engineering strategy for developing next-generation hepatobiliary MRI contrast agents that combine high stability, enhanced performance, and targeting capabilities.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A DOTA-like rare earth complex, characterized in that: The structure of the complex is shown in Formula I: Formula I Ln includes Gd(Ⅲ), Eu(Ⅲ), and Yb(Ⅲ).

2. The DOTA-type rare earth complex according to claim 1, characterized in that: The counterion of the complex is a pharmaceutically acceptable cation selected from hydrogen ions, sodium ions, potassium ions, or meglumine ions.

3. A method for preparing DOTA-type rare earth complexes, characterized in that, Includes the following steps: Step S1: (R)-benzyl lactate and trifluoromethanesulfonic anhydride undergo a trifluoromethanesulfonation reaction of the hydroxyl groups; The products obtained in steps S2 and S1 were nucleophilically substituted with the R-configuration chiral ring tinning to obtain compound 3, the structure of which is shown in Formula II. Formula II; In step S3, compound 3 undergoes a catalytic hydrogenation reaction to remove the benzyl protecting group, yielding the free carboxylic acid ligand L2; Step S4: The free carboxylic acid ligand L2 is heated and reacted with rare earth salt in a weakly acidic aqueous solution. After the reaction is completed, the pH is adjusted to weakly alkaline, filtered, the filtrate is adjusted to neutral, and freeze-dried to obtain DOTA-type rare earth complexes. 。 4. The method for preparing a DOTA-type rare earth complex according to claim 3, characterized in that: In step S1, under the protection of an inert gas, (R)-benzyl lactate and trifluoromethanesulfonic anhydride react at low temperature in the presence of 2,6-dimethylpyridine.

5. The method for preparing a DOTA-type rare earth complex according to claim 3, characterized in that: In step S2, under the protection of an inert gas, the R-configuration chiral ring tinning and the product obtained in step S1 react in the presence of K2CO3.

6. The method for preparing a DOTA-type rare earth complex according to claim 3, characterized in that: In step S3, compound 3 undergoes hydrogenolysis in an anhydrous solvent under the action of Pd-C to yield L2.

7. The method for preparing a DOTA-type rare earth complex according to claim 3, characterized in that: The rare earth salts mentioned in step S4 include GdCl3·6H2O, EuCl3·6H2O, and YbCl3·6H2O, and the corresponding products are DOTA-type Gd complex Gd-L2, DOTA-type Eu complex Eu-L2, and DOTA-type Yb complex Yb-L2.

8. The use of the DOTA-type rare earth complex according to any one of claims 1-2 in the preparation of magnetic resonance imaging contrast agents.

9. The application according to claim 8, characterized in that: The DOTA-type rare earth complex is the DOTA-type Gd complex Gd-L2, and the MRI contrast agent is a hepatobiliary-specific contrast agent used for diagnostic imaging of liver diseases.

10. An MRI contrast agent composition, characterized in that: Includes the DOTA-type gadolinium complex as described in any one of claims 1-2 and pharmaceutically acceptable excipients; said excipients are selected from one or more of buffer salts, osmotic pressure regulators, pH regulators or water for injection.

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