Immuno-positron emission tomography (immuno-PET) is a versatile diagnostic technique, most notably used in oncological imaging. The radionuclide 89Zr possesses a half-life that is well matched to the circulation time of monoclonal antibodies, making it well suited to immuno-PET imaging. However, clinical data of 89Zr radiopharmaceuticals suggest that the stability of metal-ligand complexes could be improved.[1] One group of attractive clinical agents are macrocyclic chelators facilitating octadentate Zr(IV) coordination containing a chemical handle capable of mild chemistry for targeting vector conjugation. Access to hydroxamate macrocyclic chelators such as DFOT1 has been difficult due to multi-step syntheses.[2] Bacterial species such as Erwinia amylovora and Salinispora tropica enzymatically assemble these macrocyclic hydroxamate chelators through the enzyme cluster DesABCD.[3,4] The final enzyme in the cluster DesD is responsible for the oligomerisation and macrocyclisation of chelators.[5] This molecular machinery could be leveraged for the production of hydroxamate macrocyclic chelators. This project aims to utilise DesD to incorporate non-native substrates bearing a chemical handle to generate novel hydroxamate macrocyclic chelators. DesD is known to accept monomeric analogues of the native substrate N-hydroxy-N-succinylcadaverine (HSC) (Fig 1).[5] We are evaluating the potential of DesD to generate clinically relevant chelators through the incubation of DesD with novel monomeric and dimeric HSC analogues with an added chemical handle. We are examining the metal binding capabilities of the chelator profile of DesD when incubated with the amine bearing non-native substrates N-hydroxy-N-aspartylcadaverine (HDC) or N-hydroxy-N-glutamylcadaverine (HEC). This approach could delivery biocombinatorial libraries of macrocyclic chelators from small substrate pools and demonstrates the potential of this system as a synthetic and discovery tool.

References:
[1] La, M. T.; Tran, V. H.; Kim, H.K. Nucl. Med. Mol. Imaging. 2019, 53, 115-124.
[2] Tieu, W.; Lifa, T.; Katsifis, A.; Codd, R. Inorg. Chem. 2017, 56 (6), 3719-3728.
[3] Feistner, G. J.; Stahl, D. C.; Gabrik, A. H. Org. Mass Spectrom. 1993, 28 (3), 163-175.
[4] Ejje, N.; Soe, C. Z.; Gu, J.; Codd, R. Metallomics. 2013, 5 (11), 1519-1528.
[5] Rütschlin, S.; Böttcher, T. Chem. Eur J. 2018, 24 (60), 16044-16051.

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