CD38-based Targeted Alpha Therapy for the treatment of multiple myeloma

Radioligand therapy holds great potential in cancer treatment. Targeted alpha therapy (TAT) offers the advantage of delivering cytotoxic alpha radiation selectively to tumour cells, while sparing surrounding healthy tissues, using a vector radiolabelled with a therapeutic radionuclide by means of a chelator. Single-domain antigen-binding fragments (sdAbs), derived from Camelidae heavy-chain antibodies, exhibit ideal pharmacokinetic properties, due to their small size (±15 kDa), and maintain high affinity for their target, making them suitable carrier molecules for TAT.
This project focuses on developing a sdAb-based radiopharmaceutical for TAT of multiple myeloma by radiolabeling the CD38-binding 2F8 sdAb with the therapeutic radiometals: lutetium-177 (177Lu; β--particle emitter) or actinium-225 (225Ac; ɑ-particle emitter).
The 2F8-sdAb was conjugated on its lysins with p-SCN-Bn-DOTA (20 eq.) at pH 8.8-9 in sodium carbonate buffer, at 4 mg/mL for 3 hours at room temperature. The 177Lu (25 MBq/µmol) and 225Ac (0.028 MBq/µmol) labellings were performed in ammonium acetate buffer 0.25 M at pH 5.2 at 50°C and 55°C respectively. Radiochemical purity (RCP) was evaluated by radio-instant thin layer chromatography (iTLC) and radio-size-exclusion chromatography (SEC). The median effective concentration (EC50) and the degree of internalization were evaluated on RPMI CD38+ tumour cells.
The DOTA-vector conjugation resulted in an average of 0.4 to 1.4 DOTA molecules per sdAb. The RCP was >97% after 45 min for 177Lu, while it was 75% after 1H30 for 225Ac. The 177Lu-labelled sdAb remained stable (RCP > 90%) in human serum for 24 hours. [177Lu]Lu-DOTA-2F8 did not internalize and displayed an EC50 of 40 nM (N=1).
The DOTA-conjugation and 225Ac-labelling will be further optimized. In vitro and preclinical studies with [225Ac]Ac-DOTA-2F8 will be carried out and compared to its 177Lu-labelled analogue to investigate advantage of TAT which could position this radiopharmaceutical as a significant therapeutic tool for future applications.