Anti-CD45 Monoclonal Antibody (MAb) Dose Optimization For Astatine-211 (211At)-Radioimmunotherapy (RIT) Of Relapsed Non-Hodgkin Lymphoma (NHL) In a Canine Model

Abstract Background Despite numerous new and improved therapies, a majority of NHL patients eventually succumb to relapse. An approach to increase survival and decrease normal tissue toxicity in hematopoietic cell transplantation (HCT) regimens has been to replace high-dose total body irradiation with targeted RIT. Although radiolabeled MAbs targeting CD20 as conditioning prior to HCT have proven feasible and may provide curative radiation doses for NHL, targeting CD45 may be superior because of increased antigen expression, longer anti-CD45 MAb retention at target sites, and potentially less impact from antigen blocking by prior anti-CD20 treatment. In addition, replacing commonly employed beta emitters with the more radiobiologically favorable alpha (α) emitter 211At may be more effective for anti-CD45 RIT and HCT conditioning. This study aims to investigate the MAb dose of 211At-anti-CD45 RIT that provides efficient pharmacokinetics and optimal targeting of hematopoietic cells, with minimal normal tissue toxicity, using a canine model of relapsed NHL that closely resembles human disease in terms of clinical presentation, histology, and biological behavior. Methods Five normal dogs (7.7–12.8 kg) were injected i.v. with 211At-anti-CD45 MAb (B10-conjugated CA12.10C12) starting at a MAb dose of 0.75 mg/kg for dogs 1–4 (0.5 mg/kg was non-saturating in previous studies) and increasing to 1.0 mg/kg for dog 5. Blood was sampled from 5 min to 22 h post injection (p.i.) and measured for 211At activity. MAb clearance was assessed by ELISA. Dogs 1 and 2 were euthanized 22 and 19 h after 211At injection (0.395 and 0.745 mCi/kg) and tissues were harvested for comparing the % of injected radioactivity per gram (ID/g) in target organs and normal tissues. Dogs 3–5 received autologous HCT 3 days after 211At-anti-CD45 RIT (0.310–0.395 mCi/kg) and biopsies of lymph nodes and bone marrow were performed at 2 and/or 19 h p.i. Complete blood counts were obtained at baseline, daily for the first 2 months or until full hematopoietic recovery, then once a week until end of study. Electrolytes, BUN, creatinine, and liver enzymes were measured at baseline and then every other week for 3 months. Flow cytometry and immunohistochemistry (IHC) was used to assess CD45 binding and sub-organ MAb localization with saturation being determined by comparison of mean fluorescence intensity (MFI) of target cells stained with FITC-conjugated goat anti-mouse F(ab’)2 (FMF) or FITC-conjugated anti-CD45 (CD45F). In addition to IHC, localization of 211At-anti-CD45-MAb was evaluated using two novel digital autoradiography techniques: α-camera and iQID. Both systems enable quantitative ex vivo imaging of α-particles in tissue, allowing dosimetry and assessment of 211At distribution down to the cellular level. Results Biological half-lives for circulating 211At and anti-CD45 MAb were calculated using single-phase exponential fit to 2.8 ± 0.5 and 3.2 ± 1.3 h, respectively. CD45F staining of peripheral blood mononuclear cells showed that 0.75 mg/kg of MAb was saturating for blood lymphocytes, but gated on all cells the MFI was lower using 1.0 mg/kg, indicating a decrease in available CD45 sites at the higher dose. Lymph nodes remained sub-saturated at 1.0 mg/kg. A dramatic effect was seen on bone marrow lymphocytes with antigen saturation (low CD45F MFI) at 2 h p.i. and few cells (halved FMF MFI) remaining 19 h after treatment. Successful marrow targeting and ablation was supported by α-imaging of cryosectioned bone marrow, showing evenly distributed 211At in remaining cells. Lymph nodes showed efficient targeting of peripheral follicle-like structures at 19 and 22 h p.i., with lower activity seen in central areas (dogs 1–3). At 2 h p.i. (dog 4), the activity distribution was more homogeneous, indicating elimination of centrally located CD45-expressing cells between 2 and 19 h p.i. In dog 5, the 2-h lymph node biopsy displayed a heterogeneous 211At distribution as seen in dogs 1–3, hypothetically due to faster penetration and enhanced cell kill with increased 211At-MAb dose. Conclusion Administration of 1.0 mg/kg of 211At-anti-CD45 MAb resulted in saturation of CD45 sites in bone marrow with ablation of marrow cells. While the lymph nodes were not saturated, there was highly effective targeting of the lymph node follicles. Further studies are needed to assess if targeting 211At to CD45 may be effective for therapy of NHL. Disclosures: No relevant conflicts of interest to declare.