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Background aims The aim was to evaluate cartilage regeneration in animal models involving induced knee joint damage. Through cell-mediated gene therapy methods, a cell mixture comprising a 3:1 ratio of genetically unmodified human chondrocytes and transforming growth factor beta-1 (TGF-β1)-secreting human chondrocytes (TG-C), generated via retroviral transduction, resulted in successful cartilage proliferation in damaged regions. Methods Non-clinical toxicology assessments for efficacy, biodistribution and local/systemic toxicity of single intra-articular administration of the cell mixture in mice, rabbits and goats was conducted. Results Administration of the mixture was tolerated well in all of the species. There was evidence of cartilage proliferation in rabbits and goats. As an additional precautionary step, the efficacy of TGF-β1 secretion in irradiated human chondrocytes was also demonstrated. Conclusions Four studies in rabbits and goats demonstrated the safety and efficacy of TG-C following direct intra-articular administration in animal models involving induced knee joint damage. Based on these pre-clinical studies authorization has been received from the USA Food and Drug Administration (FDA) to proceed with an initial phase I clinical study of TG-C for degenerative arthritis.

Bone morphogenetic protein 2 (BMP2) was selected as a transgene to regenerate osteoporotic bone defects after several BMPs were tested using a bone formation study in nude mice. Human chondrocytes were transduced with a BMP2-containing retroviral vector, and single clones were selected. The cells were characterized over numerous passages for growth and BMP2 expression. The single clones were irradiated and tested for viability. BMP2 expression lasted for 3 weeks before dying off completely after approximately 1 month. Irradiated and non-irradiated transduced chondrocytes successfully healed fractures in osteoporotic rats induced by ovariectomy. The osteoinducing effect of irradiated cells was better than that of their non-irradiated counterparts or a chondrocytes-only control. This study showed that delivering BMP2 from the transduced and irradiated chondrocytes could be an effective and safe method of repairing osteoporotic bone fractures.

One of the most important factors in the production of cartilage is transforming growth factor beta1 (TGF-beta1). To obtain sustained release of TGF-beta1, a cell-mediated gene therapy technique was introduced. We infected chondrocytes with a retroviral vector carrying the TGF-beta1 gene. The single clone derivative showed sustained TGF-beta1 secretion. It also showed constitutive type II collagen expression. Whereas the TGF-beta1 protein itself is unable to induce formation of cartilage in vivo, human chondrocytes engineered to express a retroviral vector encoding TGF-beta1 showed cartilage formation in vivo when cells were injected into nude mice intradermally. These data suggest that cell-mediated gene therapy using TGF-beta1 as a transgene would be a promising treatment for osteoarthritis.

Retroviral vectors have been widely used in gene therapy due to their simple genomic structure and high transduction efficiency. We report a construction of Moloney murine sarcoma virus (MoMSV) and Moloney murine leukemia virus (MoMLV) hybrid-based retroviral vectors with significantly improved efficiency of transgene expression after stable incorporation into the host genome. In these vectors, the residual gag gene coding sequence located in the extended region of packaging signal was removed. These vectors, therefore, contain no coding sequence for the gag, pol, or env gene that can be used for homologous recombination with sequences introduced in the packaging system for a recombinant competent retrovirus (RCR) generation. A strong splice acceptor site obtained from the exon/intron junction of either the chimpanzee EF1-alpha gene or the human CMV major immediate early gene was placed downstream of the MoMSV packaging signal (Psi), significantly improving the efficiency of transgene expression. The 5' LTR U3 sequence was replaced with an extended human CMV major immediate early gene enhancer/promoter for a strong expression of full-length messages from the viral backbone, helping to maintain high levels of viral titer. These newly developed retroviral vectors should facilitate RCR-free gene transfer with significantly improved efficacy in clinical gene therapy trials.