Regulated Expression of Plasmid-Based Gene Therapies

Following direct intramuscular injection of plasmids, transgene expression follows a biphasic pattern in which expression levels are maintained for approximately 1 month, and then decline to trace levels that persist for 1 year or more (1 ,2 ). However, even during the first month, transgene expression levels are too variable and too low to provide consistent therapeutic levels of circulating proteins. Thus, most intramuscular plasmid-based gene therapy efforts have focused on indications that require short-term, low-level, local expression. Examples include induction of collateral vessel growth following injection of vascular endothelial growth factor (VEGF) plasmids into regions of ischemic muscle (3 ) and induction of humoral and cell-based immunity following intramuscular injection of plasmid-based genetic vaccines (4 ). The use of protective, interactive, noncondensing (PINC^TM ) polymers, such as poly(vinyl) derivatives (5 ) or nonionic block copolymers made of ethylene oxide and pro-pylene oxide monomers (6 ), has led to the development of formulations that provided higher, more persistent transgene expression compared with plasmids in saline. For example, a single dose of an insulin-like growth factor-I (IGF-I) plasmid formulated in poly(vinylpyrrolidone) caused reinnervation of motor end plates and increased muscle fiber size when delivered to paralyzed adult rat laryngeal muscle (7 ).