Cure of Disseminated Human Lymphoma with [¹⁷⁷Lu]Lu-Ofatumumab in a Preclinical Model

Although immunotherapies that target CD20 on most non-Hodgkin lymphoma (NHL) cells have improved patient outcomes, current therapies are inadequate because many cases are, or become, refractory or undergo relapse. Here, we labelled the third-generation human anti-CD20 antibody ofatumumab with ¹⁷⁷Lu, determined the in vitro characteristics of [¹⁷⁷Lu]Lu-ofatumumab, estimated human dosimetry, and assayed tumor targeting and therapeutic efficacy in a murine model of disseminated NHL. Methods: CHX-A″-diethylenetriaminepentaacetic acid-[¹⁷⁷Lu]Lu-ofatumumab was prepared. We evaluated radiochemical yield, purity, in vitro immunoreactivity, stability, (n = 7), affinity, and killing of CD20-expressing Raji cells (n = 3). Human dosimetry was estimated from biodistribution studies as percentage injected activity per gram using C57BL/6N mice. Tissue and organ biodistribution was determined in R2G2 immunodeficient mice with subcutaneous Raji-cell tumors. Therapy studies used R2G2 mice with disseminated human Raji-luc tumor cells (n = 10 mice/group). Four days after cell injection, the mice were left untreated or were treated with ofatumumab, 8.51 MBq of [¹⁷⁷Lu]Lu-IgG, or 0.74 or 8.51 MBq of [¹⁷⁷Lu]Lu-ofatumumab. Survival, weight, and bioluminescence were tracked. Results: Radiochemical yield was 93% ± 2%, radiochemical purity was 99% ± 1%, and specific activity was 401 ± 17 MBq/mg. Immunoreactivity was substantially preserved, and more than 75% of ¹⁷⁷Lu remained chelated after 7 d in serum. [¹⁷⁷Lu]Lu-ofatumumab specifically killed Raji-luc cells in vitro (P < 0.05). Dosimetry estimated that an effective dose for human administration is 0.36 mSv/MBq and that marrow may be the dose-limiting organ. Biodistribution in subcutaneous tumors 1, 3, and 7 d after [¹⁷⁷Lu]Lu-ofatumumab injection was 11, 15, and 14 percentage injected activity per gram, respectively. In the therapy study, median survival of untreated mice was 19 d, not statistically different from mice treated with 8.51 MBq of [¹⁷⁷Lu]Lu-IgG (25 d). Unlabeled ofatumumab increased survival to 46 d, similar to 0.74 MBq of [¹⁷⁷Lu]Lu-ofatumumab (59 d), with both being superior to no treatment (P < 0.0003). Weight loss and increased tumor burden preceded death or killing of the animal for cause. In contrast, treatment with 8.51 MBq of [¹⁷⁷Lu]Lu-ofatumumab dramatically increased median survival (>221 d), permitted weight gain, eliminated detectable tumors, and was curative in 9 of 10 mice. Conclusion: [¹⁷⁷Lu]Lu-ofatumumab shows favorable in vitro characteristics, localizes to tumor, and demonstrates curative therapeutic efficacy in a disseminated lymphoma model, showing potential for clinical translation to treat NHL.