Discovery of a highly specific radiolabeled antibody targeting B-cell maturation antigen: Applications in PET imaging of multiple myeloma

Multiple myeloma (MM) is characterized by the uncontrolled proliferation of monoclonal plasma cells (PC) in the bone marrow (BM). B-cell maturation antigen (BCMA) is predominantly expressed in malignant plasma cells, and associated with the proliferation, survival, and progression of various myeloma cells. Given these important roles, BCMA emerges as an ideal target antigen for MM therapy. However, effective stratification of patients who may benefit from targeted BCMA therapy and real-time monitoring the therapeutic efficacy poses significant clinical challenge. This study aims to develop a BCMA targeted diagnostic modality, and preliminarily explore its potential value in the radio-immunotherapy of MM.

Using zirconium-89 (⁸⁹Zr, t_1/2 = 78.4 h) for labeling the BCMA-specific antibody, the BCMA-targeting PET tracer [⁸⁹Zr]Zr-DFO-BCMAh230430 was prepared. The EC₅₀ values of BCMAh230430 and DFO-BCMAh230430 were determined by ELISA assay. BCMA expression was assessed in four different tumor cell lines (MM.1S, RPMI 8226, BxPC-3, and KYSE520) through Western blot and flow cytometry. In vitro binding affinity was determined by cell uptake studies of [⁸⁹Zr]Zr-DFO-BCMAh230430 in these tumor cell lines. For in vivo evaluation, PET imaging and ex vivo biodistribution studies were conducted in tumor-bearing mice to evaluate imaging performance and systemic distribution of [⁸⁹Zr]Zr-DFO-BCMAh230430. Immunochemistry analysis was performed to detect BCMA expression in tumor tissues, confirming the specificity of our probe. Furthermore, we explored the anti-tumor efficacy of Lutetium-177 labeled BCMA antibody, [¹⁷⁷Lu]Lu-DTPA-BCMAh230430, in tumor bearing-mice to validate its radioimmunotherapy potential.

The radiolabeling of [⁸⁹Zr]Zr-DFO-BCMAh230430 and [¹⁷⁷Lu]Lu-DTPA-BCMAh230430 showed satisfactory radiocharacteristics, with a radiochemical purity exceeding 99%. ELISA assay results revealed closely aligned EC₅₀ values for BCMAh230430 and DFO-BCMAh230430, which are 57 pM and 67 pM, respectively. Western blot and flow cytometry analyses confirmed the highest BCMA expression level. Cell uptake data indicated that MM.1S cells had a total cellular uptake (the sum of internalization and surface binding) of 38.3% ± 1.53% for [⁸⁹Zr]Zr-DFO-BCMAh230430 at 12 h. PET imaging of [⁸⁹Zr]Zr-DFO-BCMAh230430 displayed radioactive uptake of 7.71 ± 0.67%ID/g in MM.1S tumors and 4.13 ± 1.21%ID/g in KYSE520 tumors at 168 h post-injection (n = 4) (P < 0.05), consistent with ex vivo biodistribution studies. Immunohistochemical analysis of tumor tissues confirmed higher BCMA expression in MM.1S tumors xenograft compared to KYSE520 tumors. Notably, [¹⁷⁷Lu]Lu-DTPA-BCMAh230430 showed some anti-tumor efficacy, evidenced by slowed tumor growth. Furthermore, no significant difference in body weight was observed in MM.1S tumor-bearing mice over 14 days of administration with or without [¹⁷⁷Lu]Lu-DTPA-BCMAh230430.

Our study has successfully validated the essential role of [⁸⁹Zr]Zr-DFO-BCMAh230430 in non-invasively monitoring BCMA status in MM tumors, showing favorable tumor uptake and specific binding affinity to MM tumors. Furthermore, our research revealed, as a proof-of-concept, the effectiveness of [¹⁷⁷Lu]Lu-DTPA-BCMAh230430 in radioimmunotherapy for MM tumors. In conclusion, we present a novel BCMA antibody-based radiotheranostic modality that holds promise for achieving efficient and precise MM diagnostic and therapy.