¹⁷⁷Lu-Anti-CD25 Antibody for Interleukin-2 Receptor-α-Targeted Radioimmunotherapy of SUDHL1 Lymphomas in Mice

Antibodies (Abs) conjugated with particle-emitting radioisotopes are used for cancer therapy, and the CD25 receptor (IL-2Rα) is a promising target for lymphomas. We functionalized an anti-CD25 Ab with TCO-PEG₃-maleimide, specifically on sulfhydryl moieties, at 1.89 molecules per Ab. Radiosynthesis was achieved by efficiently prelabeling DOTA-PEG₄-tetrazine with ¹⁷⁷Lu at a high temperature and then linking it to the TCO-conjugated Abs via facile click chemistry under mild conditions. The [¹⁷⁷Lu]Lu-DOTA-PEG₄-Tz-TCO-PEG₃-anti-CD25 Ab (¹⁷⁷Lu-CD25 Ab) had a radiochemical purity of >99%, a specific activity of 707.9 ± 271.5 MBq/mg, an immunoreactive fraction of 77.6%, and high radiolabel stability in serum for up to 7 days. CD25-positive SUDHL1 human T lymphoma cells showed ¹⁷⁷Lu-CD25 Ab uptake that was completely blocked by pretreatment with unlabeled Ab. The ¹⁷⁷Lu-CD25 Ab dose-dependently suppressed SUDHL1 cell survival in vitro. In mice, ¹⁷⁷Lu-CD25 Ab uptake at 5 days was high in the SUDHL1 tumors (7.1 ± 1.6%ID/g), modest in the liver, kidneys, and spleen, and low in the blood, lungs, and bones. CD25-specific targeting was confirmed by 66.7% suppression of tumor uptake by pretreatment with unlabeled CD25 Ab. Treatment with 18.5 MBq of ¹⁷⁷Lu-CD25 Ab shrank the xenograft tumors, and they remained undetectable until study termination on day 61. In contrast, the tumors in all control and CD25 Ab-treated mice grew to exceed the end point criterion of 2,000 mm³. The standardized tumor growth rate and 19-day tumor volume were completely suppressed in the ¹⁷⁷Lu-CD25 Ab group (109.9 ± 73.5 and 132.6 ± 111.5 mm³), compared with the control (1053.9 ± 151.1 and 1804.5 ± 283.1 mm³) and CD25 Ab groups (1049.7 ± 212.2 and 1443.8 ± 839.4 mm³; all p < 0.001). A Kaplan-Meier survival analysis showed that ¹⁷⁷Lu-CD25 Ab-treated mice survived significantly longer than mice in the control and CD25 Ab groups. Tumors in a separate set of mice that were treated with ¹⁷⁷Lu-CD25 Ab displayed increased PARP1 cleavage fragments, a signature of apoptosis. Toxicity studies showed that white blood cell, red blood cell, and platelet counts in the ¹⁷⁷Lu-CD25 Ab group decreased from days 3-14, reaching a nadir on days 17-21 and returning to the normal range by days 24-31. Liver and renal function tests on day 28 did not differ from those of untreated mice. Thus, the ¹⁷⁷Lu-CD25 Ab prepared as described here could be useful for radioimmunotherapy of CD25-positive lymphomas.