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Keywords:

  • alpha-particle emitter213 Bi;
  • radioimmunoconjugate;
  • spheroids;
  • prostate cancer;
  • tumor-associated antigen;
  • micrometastases

Abstract

BACKGROUND

Micrometastasis is a major problem for prostate cancer (CaP) patients. Our study investigated the therapeutic potential of multiple targeted α-therapy (MTAT) in the treatment of CaP micrometastases (spheroids) using 213Bi-labeled multiple targeted α-radioimmunoconjugates.

METHODS

The expression of multiple tumor-associated antigens (TAAs) on frozen sections of human fresh CaP tissues and spheroids cultured from DU 145 and LNCaP-LN3 CaP cell lines was detected by immunohistochemistry and flow cytometry. Targeting vectors were two monoclonal antibodies (MAbs), and plasminogen activator inhibitor type 2 (PAI2) that binds to cell surface urokinase plasminogen activator (uPA). These vectors were labeled with 213Bi using standard methodology. DU 145 and LNCaP-LN3 spheroids were incubated with different activities of test and control α-conjugates (ACs), and spheroid growth was measured for volume change and growth delay over a 50-day period using light microscopy.

RESULTS

TAAs were expressed heterogeneously on frozen sections from human CaP tissues and CaP spheroids. MTAT combining three ACs (one-third dose of each) with an activity of 6.4 MBq/ml completely targeted small DU 145 and LNCaP-LN3 spheroids (diameter <100 µm) and slightly regressed the growth of medium spheroids (180–200 µm); MTAT with 2.2 or 4.8 MBq/ml activities delayed the growth of tumor spheroids.

CONCLUSIONS

Our results suggest that the cytotoxicity of MTAT to CaP spheroids is highly dependent on antigenic expression, concentration of radioactivity and spheroid size. MTAT may be a potent therapeutic agent for micrometastases, effectively targeting small CaP cell clusters, and overcoming the heterogeneous expression of targeted antigens. Prostate © 2006 Wiley-Liss, Inc.