Synthesis and Photochemical Studies on Gallium and Indium Complexes of DTPA-PEG₃-ArN₃ for Radiolabeling Antibodies

Abstract

Photochemistry is a rich source of inspiration for developing alternative methods to functionalize proteins with drug molecules, fluorophores, and radioactive probes. Here, we report the synthesis and photochemical reactivity of a modified diethylenediamine pentaacetic acid chelate that was derivatized with a light-responsive aryl azide group (DTPA-PEG3-ArN3, compound 1). The corresponding nonradioactive and radioactive nat/68Ga3+ and nat/111In3+ complexes of DTPA-PEG3-ArN3 were synthesized and their physical and photochemical properties were studied to evaluate the potential of employing this ligand system in the photochemical synthesis of radiolabeled antibodies. Photodegradation kinetics revealed that irradiation with ultraviolet light (365 nm) induced rapid photoactivation of compound 1 and the metal complexes nat/68Ga-1-and nat/111In-1-. Light-induced reactions were complete in <100 s, with measured first-order rate constants of 0.078 ± 0.045 s-1, 0.093 ± 0.009 s-1, and 0.117 ± 0.054 s-1 (n = 2, per species) for compound 1, natGa-1-, and natIn-1-, respectively. Photochemically induced bioconjugation reactions between DTPA-PEG3-ArN3 and the monoclonal antibody trastuzumab, as well as pre-and postconjugation 68Ga-and 111In-radiolabeling experiments, were performed using either a one-pot or two-step strategy. Both approaches yielded radiolabeled trastuzumab ([68Ga]GaDTPA-azepin-trastuzumab) with average radiochemical conversions of 3.9 ± 1.0% (n = 4, onepot), and 10.0 ± 1.0% (n = 3, two-step). One-pot radiolabeling reactions with [111In]InCl3 produced the corresponding [111In]InDTPA-azepin-trastuzumab radiotracer in a similar radiochemical conversion of 5.4 ± 0.8% (n = 3). Radiochemical conversions for the desired bimolecular coupling between the chelate and the protein were comparatively low. This observation is likely caused by the high photoinduced reactivity of the compounds and subsequent competition with background reactions. Nevertheless, access to DTPA-PEG3-ArN3 increases the scope of photoradiochemical methods to include metal ions like In3+ that form complexes with higher coordination numbers.

Light‐Activated Protein Conjugation and 89Zr‐Radiolabelling with Water‐Soluble Desferrioxamine Derivatives

“…We recently demonstrated that metal ion binding chelates functionalised with a photoactivatable ArN 3 group can be used to prepare radiolabelled antibodies by either a two‐step conjugation and radiolabelling process or by a novel one‐pot approach . Simultaneous 89 Zr‐radiolabelling and light‐induced conjugation using a photoactivatable DFO‐ArN 3 derivative produced viable PET radiotracers (Scheme ) .…”

Photochemical Reactions in the Synthesis of Protein–Drug Conjugates

“…[61] Our laboratory recently reported the synthesis, radiolabelling, and bimolecular couplingo fp hotoactivatable chelates bearingt he photoactive aryl azide (ArN 3 )g roup to antibodies for use in immuno-PET and RIT ( Figure 2). [41][42][43][44][45] Remarkably, switching the conjugation chemistry from traditional, thermally mediated reactions [10][11][12][13] to ap hoto-initiated reaction allowed us to produce radiolabelled mAbs in as imultaneous( one-pot) process directly from fully formulated antibodies (including Figure 1. Timelines howing the evolution of different photoactive groups used in PAL.…”

“…Eur.J. 2020,26,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] www.chemeurj.org Concept the photochemistry for bimolecular protein modification. [67,68] The conformational flexibility of an extended alkyl chain also means that aliphatic carbenes are more likely to undergo intramolecular reactions than the more conformationally rigid aryl carbenes.T hese competing intra-and intermolecularp rocesses are likely to restrict the use of aliphatic diazirines to systems that pre-associate.…”

“…[92] Chem.E ur.J. 2020,26,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] www.chemeurj.org ling of proteins with aliphatic azides. [92] They concluded that direct nitrene-insertionp roductsw ere not observed in the system and that site-specific and chemoselective labelling of cysteiner esidues through a1 ,2-alkyl shift mechanism wasf avoured.A liphatic azides have yet to be explored as reagents for bimolecular protein conjugation but their tendency to react with solvents and undergo facile intersystem crossing to give triplet speciesa re potentiald rawbacks.…”

“…Chem. Eur.J 2020,26,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. www.chemeurj.org2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim…”