OSTEOCALCIN PROMOTER-BASED TOXIC GENE-THERAPY FOR THE TREATMENT OF OSTEOSARCOMA IN EXPERIMENTAL-MODELS

Osteocalcin (OC), a noncollagenous bone matrix protein, is expressed i n high levels by osteoblasts. To determine whether the OC promoter med iates cell-specific gene expression in cells of osteoblast lineage, we constructed a recombinant adenovirus, Ad-OC-TK, which contains the OC promoter that drives the expression of herpes simplex virus thymidine kinase (TK). We tested the expression of TK by this virus in osteobla st cell lines as well as in non-osteoblastic cell lines by assessing t he enzyme activity of TK in vitro. Whereas the OC promoter failed to d rive the expression of the TK gene in several non osteoblastic cell li nes such as WH, a human bladder transitional carcinoma, and NIH 3T3, a n embryonic mouse fibroblast cell line, the OC promoter mediated high levels of expression in osteoblast cell lines including murine ROS and human MG-63 cells. The addition of acyclovir (ACV), a pro-drug for th e inhibition of cell proliferation, resulted in the induction of osteo blast-specific cell death in vitro. Intratumoral injection of Ad-OC-TK into murine ROS osteosarcoma abolished tumor growth in a host treated with subsequent i.p. ACV injection in vivo. The Ad-OC-TK virus plus A CV treatment appears to be highly selective in blocking the growth of both murine and human osteosarcoma cell lines in vitro and murine oste osarcoma in vivo.