Semi-mechanistic pharmacodynamic modelling of gene expression and silencing processes

Background: Suppressed expression of transgene is a major obstacle in gene therapy. Understanding of the mechanisms involved in expression and silencing of exogenous genes is required to overcome gene therapy hurdles. Purpose: To develop a semi-mechanistic model describing the effects of transgenes over the activity of an expression cassette. Methods:Twelve Balb/c mice received 40 mu g of plasmid DNA. Animals were assigned to one of the following treatments: (I)20 mu g of the plasmid expressing luciferase(pEF-Luc) and 20 mu g of "empty" plasmid; (II) pEF-Luc (20 mu g) and 20 mu g of plasmid expressing murine interferon alpha (IFN alpha); and (III) pEF-Luc(20 mu g), and 20 mu g of plasmid expressing P-galactosidase (pCMV beta). The expression of luciferase over time, quantified by a noninvasive method, was used as a measured of pEF-Luc activity and modelled using NONMEM. Results: The selected model suggests the co-existence of two forms of active DNA differing in their transcription efficiencies. The core model was expanded to describe reversible and irreversible silencing processes, induced by the coexpression of IFN alpha or beta-galactosidase, respectively. Conclusion: Coupling noninvasive in vivo imaging and mathematical modelling allows quantitative description of gene transfer, providing a tool to select the best regulatory elements to construct a therapeutic expression cassette. (C) 2009 Elsevier B.V. All rights reserved.