Radiolabeling of a polypeptide polymer for intratumoral delivery of emitter, 225Ac, and emitter, 177Lu

Introduction: Radiotherapy of cancer requires both alpha- and beta-particle emitting radionuclides, as these radio-nuclide types are efficient at destroying different types of tumors. Both classes of radionuclides require a vehicle, such as an antibody or a polymer, to be delivered and retained within the tumor. Polyglutamic acid (pGlu) is a polymer that has proven itself effective as a basis of drug-polymer conjugates in the clinic, while its derivatives have been used for pretargeted tumor imaging in a research setup. trans-Cyclooctene (TCO) modified pGlu is suitable for pretargeted imaging or therapy, as well as for intratumoral radionuclide therapy. In all cases, it becomes indirectly radiolabeled via the bioorthogonal click reaction with the tetrazine (Tz) molecule carrying the radionu-clide. In this study, we report the radiolabeling of TCO-modified pGlu with either lutetium-177 (Lu-177), a beta-particle emitter, or actinium-225 (Ac-225), an alpha-particle emitter, using the click reaction between TCO and Tz. Methods: A panel of Tz derivatives containing a metal ion binding chelator (DOTA or macropa) connected to the Tz moiety directly or through a polyethylene glycol (PEG) linker was synthesized and tested for their ability to chelate Lu-177 and Ac-225, and click to pGlu-TCO. Radiolabeled Lu-177-pGlu and Ac-225-pGlu were isolated by size exclusion chromatography. The retention of Lu-177 or Ac-225 by the obtained conjugates was investigated in vitro in human serum. Results: All DOTA-modified Tzs efficiently chelated Lu-177 resulting in average radiochemical conversions (RCC) of 5% unchelated Lu-177 for all Lu-177-pGlu preparations over six days in human serum. For Ac-225 chelation, optimized RCCs ranged from 61 +/- 34% to quantitative for DOTA-Tzs and were quantitative for the macropa-modified Tz (98%). Isolated radiochemical yields (RCY) for Ac-225-pGlu prepared from Ac-225-Tzs ranged from 28% to 51%. For 3 out of 5 Ac-225-pGlu conjugates prepared from DOTA-Tzs, the amount of unchelated Ac-225 stayed below 10% over six days in human serum, while Ac-225-pGlu prepared from macropa-Tz showed a steady release of up to 37% Ac-225. Conclusion: We labeled TCO-modified pGlu polymers with alpha- and beta-emitting radionuclides in acceptable RCYs. All Lu-177-pGlu preparations and some Ac-225-pGlu preparations showed excellent stability in human plasma. Our work shows the potential of pGlu as a vehicle for alpha- and beta-radiotherapy of tumors and demonstrated the usefulness of Tz ligation for indirect radiolabeling. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

In this study, TCO-modified pGlu polymers were successfully labeled with alpha- and beta-emitting radionuclides with acceptable radiochemical yields (RCYs). The DOTA-modified Tzs efficiently chelated Lu-177, while the macropa-modified Tz showed quantitative chelation for Ac-225. The potential of pGlu as a vehicle for alpha- and beta-radiotherapy of tumors was demonstrated, along with the usefulness of Tz ligation for indirect radiolabeling.