Design Strategies for Developing Chiral Macrocyclic Chelators as a Diagnostic Tool

Macrocyclic chelators have become an indispensable part of the field of molecular imaging due to their ability to form highly stable complexes with various metal ions for diverse diagnostic and therapeutic applications. Among them, DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), which serves as a gold standard in both stability and versatility, is extensively utilized in clinical contexts as contrast agents for a series of imaging modalities, including MRI (magnetic resonance imaging), PET (positron emission tomography), SPECT (single-photon emission computed tomography), and optical imaging. Despite its advantageous properties, in certain cases, DOTA still has limitations to be solved, such as slow radiolabeling kinetics and possible in vivo decomplexation, which have led to increased research interest in structural modifications of the DOTA ligand. To date, the most popular design strategy for DOTA analogues focuses on the modification of pendant arms, which can be simply achieved through the N-derivatization of the tetraaza macrocycle. Another strategy is to modify the macrocyclic ring structure, with the most successful example being the macrocycle pyclen, which contains a pyridine moiety. In 2022, pyclen-based Gd3+ complex gadopiclenol was approved by the U.S. FDA (United States Food and Drug Administration) as a next-generation MRI contrast agent for clinical use. However, reported studies on similar structural modifications of macrocyclic rings are relatively rare, as this requires the redesign of synthetic procedures that can be highly complicated. In this Account, we discuss our design strategies of various macrocyclic chelators with chiral substituents on the macrocyclic ring for different biomedical imaging applications. We start with structural modifications to the frequently used DOTA chelator through the introduction of four symmetrical chiral substituents on its macrocyclic backbone. Results of NMR studies confirmed that the incorporated chiral substituents can prevent the interconversion between two TSAP/SAP regioisomers formed in the complexation process of DOTA with lanthanides. The chiral substituents also provide increased ligand rigidity and can enhance the coordination geometry for better encapsulation of the metal, preventing possible dechelation and transmetalation, thereby achieving higher coordination stability. A series of DOTA analogues with different chiral substituents were studied, and we have revealed that the TSAP/SAP isomeric ratios and pharmacokinetics of chiral DOTA varied with the steric hindrance and polarity of attached chiral groups. In particular, the Gd3+ complexes of chiral DOTAs with symmetric aliphatic groups exhibit enhanced relaxivity over that of achiral DOTA, making them ideal contrast agents for MR imaging. Through the subsequent optimization of introduced chiral groups, various improved chelators were then developed for photoluminescence or for MRI. Next, we focused on further pendant arm modifications of these chiral DOTA platforms. Multiple functionalized pendant arms were attached to the chiral macrocycle as chromophores or targeting vectors, resulting in versatile chiral bifunctional chelators for targeted circularly polarized luminescence (CPL), PET/SPECT, and radiotherapy applications. Moreover, our chiral modifications are not limited to the 12-membered DOTA macrocycle. Recently, we broadened our design strategy to alter the macrocyclic ring sizes, an approach with relatively few literature examples. The chiral analogues of 9-membered NOTA and 10-membered DETA systems were synthesized for 68Ga radiolabeling, with the influence of cavity sizes and positions of chiral substitution investigated. Follow-up studies will go deeper into the metal ion selectivity effects of differently sized chiral macrocycles to pave the way for future designs on novel macrocyclic chelators.