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Combination Therapy of Insulin-Like Growth Factor Binding Protein-3 and Retinoid X Receptor Ligands Synergize on Prostate Cancer Cell Apoptosis In vitro and In vivo

Authors' Affiliations: ¹ Division of Pediatric Endocrinology, Mattel Children's Hospital at UCLA, David Geffen School of Medicine, and ² Vitae Pharmaceuticals, Irvine, California

Requests for reprints: Pinchas Cohen, 10833 Le Conte Avenue, MDCC 22-315, Los Angeles, CA 90095. Phone: 310-825-6244; Fax: 310-206-5843; E-mail: hassy{at}mednet.ucla.edu.

We have previously identified the retinoid X receptor- (RXR) as an insulin-like growth factor binding protein-3 (IGFBP-3) nuclear binding partner, which is required for IGFBP-3-induced apoptosis. In the current study, we investigated the biological interactions of the RXR ligand, VTP194204 and rhIGFBP-3, in vitro and in vivo. In vitro, IGFBP-3 and VTP194204 individually induced apoptosis, and suppressed cell growth in prostate cancer cell lines in an additive manner. In vivo, LAPC-4 xenograft–bearing severe combined immunodeficiency mice treated daily with saline, IGFBP-3, and/or VTP194204 for 3 weeks showed no effect of individual treatments with IGFBP-3 or VTP194204 on tumor growth. However, the combination of IGFBP-3 and VTP194204 treatments inhibited tumor growth by 50% and induced a significant reduction in serum prostate-specific antigen levels. In terminal nucleotidyl transferase–mediated nick end labeling immunohistochemistry of LAPC-4 xenografts, there was modest induction of apoptosis with either IGFBP-3 or VTP194204 individual treatment, but combination therapy resulted in massive cell death, indicating that IGFBP-3 and VTP194204 have a synergistic effect in preventing tumor growth by apoptosis induction. In summary, this is an initial description of the successful therapeutic use of IGFBP-3 as a cancer therapy in vivo, and shows that combination treatment of IGFBP-3 and RXR ligand has a synergistic effect on apoptosis induction leading to substantial inhibition of prostate cancer xenograft growth. Taken together, these observations suggest that combination therapy with IGFBP-3 and RXR ligands may have therapeutic potential for prostate cancer treatment.

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