Radioimmunotherapy using Anti-CD20 Antibody, 177Lu-Ofatumumab, Cures Lymphoma in a Preclinical Model

Introduction: Ofatumumab is a fully human anti-CD20 antibody approved for use by the FDA. Despite greater potential for redosing than with chimeric or mouse anti-CD20 antibodies, ofatumumab has not been investigated previously for lymphoma radioimmunotherapy. Here, we determine if ofatumumab radiolabeled with the beta-particle-emitting radioisotope, lutetium 177 (¹⁷⁷Lu), has therapeutic potential.

Methods: Ofatumumab and human IgG1 were conjugated to CHX-A”-DTPA and ¹⁷⁷Lu chelated. ¹⁷⁷Lu-antibody complexes were purified by size exclusion chromatography and radiochemical yield, purity and stability in serum were analyzed by thin-layer chromatography. Immunoreactivity was assayed by binding to CD20-positive Raji cells expressing luciferase (Raji-luc). ¹⁷⁷Lu-ofatumumab biodistribution was determined as percent injected activity/gram (% IA/g) at 7 days (d) post-injection into immunodeficient mice with subcutaneous Raji tumors. For therapeutic studies, immunodeficient mice were injected intravenously (IV) with 1 x 10⁶ Raji-luc cells (d 0). On d 4, groups were injected IV with 20 μg unlabeled ofatumumab, 8.51 MBq/mouse ¹⁷⁷Lu-IgG1 or 0.74 MBq/mouse or 8.51 MBq/mouse ¹⁷⁷Lu-ofatumumab, while one group received no therapy (n = 10 mice/group, 5 groups). Tumor-cell growth was monitored by bioluminescence imaging and survival was tracked.

Results: Radiochemical yields were >85% with >98% radiochemical purity and specific activities of 458 ± 31 MBq/mg (n = 3). ¹⁷⁷Lu-ofatumumab retained 52 ± 5 % (n = 3) immunoreactivity and 74 ± 5% (n = 3) chelation in serum after 7 days. Biodistribution of ¹⁷⁷Lu-ofatumumab at 7 d post-injection indicated 17 ± 5% IA/g in tumor and 7 ± 3, 5 ± 1 and 11 ± 4% IA/g in blood, liver and spleen, respectively. Median survival for mice with no therapy was 15 d, with no mice surviving by 18 d. Unlabeled ofatumumab extended the median survival to 41.5 d, with 2 mice surviving at 90 d p.i. Injection with 8.51 MBq ¹⁷⁷Lu-IgG resulted in a 20 d median survival, with 2 mice surviving at 90 d p.i. Injections with 0.74 and 8.51 MBq ¹⁷⁷Lu-ofatumumab yielded 54 d and undefined (but >90 days) median survivals, with 2 and all 10 mice alive, respectively, at 90 d p.i. Bioluminescent imaging results corresponded well with survival. Log-rank (Mantel-Cox) tests of the survival data show p <0.001 when comparing the 8.51 MBq ¹⁷⁷Lu-ofatumumab group with each of the 4 other groups.

Conclusions: Radiolabeling of ofatumumab with ¹⁷⁷Lu was accomplished with high yield, purity, in vitro stability, in vitro cell binding activity and efficient tumor targeting in vivo. Survival time and suppression of tumor growth was greatly improved in mice treated with 8.51 MBq ¹⁷⁷Lu-ofatumumab, compared to all other groups with mice appearing to be cured on long term follow up. Together, these indicate ¹⁷⁷Lu-ofatumumab has strong therapeutic efficacy in mice and is a promising candidate for radioimmunotherapy in patients with non-Hodgkin lymphoma.

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