An a-Particle Emitting Antibody (( 213 Bi)J591) for Radioimmunotherapy of Prostate Cancer 1

A novel a-particle emitting monoclonal antibody construct targeting the external domain of prostate-specific membrane antigen (PSMA) was prepared and evaluated in vitro and in vivo. The chelating agent, N-(2-amino-3-(p-isothiocyanatophen-yl)propyl)-trans-cyclohexane-1,2-dia- mine-N,N*,N*,N**,N**-pentaacetic acid, was appended to J591 monoclonal antibody to stably bind the 213 Bi radiometal ion. Bismuth-213 is a short- lived (t1/2 5 46 min) radionuclide that emits high energy a-particles with an effective range of 0.07- 0.10 mm that are ideally suited to treating single-celled neoplasms and micrometastatic carcinomas. The LNCaP prostate cancer cell line had an estimated 180,000 molecules of PSMA per cell; J591 bound to PSMA with a 3-nM affinity. After binding, the radio- labeled construct-antigen complex was rapidly internalized into the cell, carrying the radiometal inside. ( 213 Bi)J591 was specifically cytotoxic to LNCaP. The LD50 value of ( 213 Bi)J591 was 220 nCi/ml at a specific activity of 6.4 Ci/g. The potency and specificity of ( 213 Bi)J591 directed against LNCaP spheroids, an in vitro model for micrometastatic cancer, also was investigated. ( 213 Bi)J591 effectively stopped growth of LNCaP spheroids relative to an equivalent dose of the irrelevant control ( 213 Bi)HuM195 or unlabeled J591. Cytotoxicity experiments in vivo were carried out in an athymic nude mouse model with an i.m. xenograft of LNCaP cells. ( 213 Bi)J591 was able to significantly improve (P < 0.0031) median tumor- free survival (54 days) in these experiments relative to treatment with irrelevant control ( 213 Bi)HuM195 (33 days), or no treatment (31 days). Prostate-specific antigen (PSA) was also specifically reduced in treated animals. At day 51, mean PSA values were 104 ng/ml 1/2 54 ng/ml (n 5 4, untreated animals), 66 ng/ml 1/2 16 ng/ml (n 5 6, animals treated with ( 213 Bi)HuM195), and 28 ng/ml 1/2 22 ng/ml (n 5 6, animals treated with ( 213 Bi)J591). The reduction of PSA levels in mice treated with ( 213 Bi)J591 relative to mice treated with ( 213 Bi)HuM195 and untreated control ani- mals was significant with P < 0.007 and P < 0.0136, respectively. In conclusion, a novel ( 213 Bi)-radiolabeled J591 has been constructed that selectively delivers a-particles to prostate cancer cells for potent and specific killing in vitro and in vivo.