Radiolabeling of a polypeptide polymer for intratumoral delivery of alpha-particle emitter, Ac, and beta-particle emitter, Lu.

INTRODUCTION

Radiotherapy of cancer requires both alpha- and beta-particle emitting radionuclides, as these radionuclide 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 radionuclide. In this study, we report the radiolabeling of TCO-modified pGlu with either lutetium-177 (Lu), a beta-particle emitter, or actinium-225 (Ac), 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 and Ac, and click to pGlu-TCO. Radiolabeled Lu-pGlu and Ac-pGlu were isolated by size exclusion chromatography. The retention of Lu or Ac by the obtained conjugates was investigated in vitro in human serum.

RESULTS

All DOTA-modified Tzs efficiently chelated Lu resulting in average radiochemical conversions (RCC) of >75%. Isolated radiochemical yields (RCY) for Lu-pGlu prepared from Lu-Tzs ranged from 31% to 55%. TLC analyses detected <5% unchelated Lu for all Lu-pGlu preparations over six days in human serum. For Ac 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-pGlu prepared from Ac-Tzs ranged from 28% to 51%. For 3 out of 5 Ac-pGlu conjugates prepared from DOTA-Tzs, the amount of unchelated Ac stayed below 10% over six days in human serum, while Ac-pGlu prepared from macropa-Tz showed a steady release of up to 37% Ac.

CONCLUSION

We labeled TCO-modified pGlu polymers with alpha- and beta-emitting radionuclides in acceptable RCYs. All Lu-pGlu preparations and some Ac-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.