An α-Particle Emitting Antibody ([213Bi]J591) for Radioimmunotherapy of Prostate Cancer
2000
A novel α-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-diamine- 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
(t 1/2 = 46 min) radionuclide that emits high
energy α-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 radiolabeled
construct-antigen complex was rapidly internalized into the cell,
carrying the radiometal inside. [ 213 Bi]J591 was
specifically cytotoxic to LNCaP. The LD 50 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 +/− 54
ng/ml ( n = 4, untreated animals), 66
ng/ml +/− 16 ng/ml ( n = 6, animals
treated with [ 213 Bi]HuM195), and 28 ng/ml +/− 22 ng/ml
( n = 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 animals 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 α-particles to prostate cancer cells for
potent and specific killing in vitro and in
vivo .
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