Long-term, drug-dependent regulation of erythropoietin and hematocrit in mice following delivery of plasmids to skeletal muscle

As gene therapy advances, the need for regulated gene therapy will become paramount for safety and efficacy. The GeneSwitchTM is a gene regulation system that utilizes hormonal levels of antiprogestins, like mifepristone (MFP), to specifically induce transgene expression. A 1:1 mixture of two plasmids (one for the GeneSwitch regulator protein, one for the inducible murine Epo transgene) was delivered to the hind-limb muscles of mice by direct injection followed by electroporation. We observed 9 rounds of induction of Epo expression by administering MFP (0.3 mg/kg) at different times during a 9-month period. Each time, serum Epo was induced from an undetectable level to 15–30 mU/mL. Peak expression occurred 24 hours following MFP administration, and returned to near basal levels after 72 hrs. We also observed five cycles of induction of hematocrit over a 9-month period. Each cycle, in which the hematocrit increased for 4–5 weeks, occurred in response to 5 successive days of MFP administration. Our data demonstrate that gene therapy with formulated plasmids delivered by electroporation can be used to provide a long-term, drug-controllable therapeutic benefit in a manner that had previously been achieved only with viral systems. The increased safety margin associated with nonviral gene therapy makes our plasmid-based GeneSwitch system extremely attractive for the controlled expression of systemic proteins of therapeutic interest.