Formulations which increase the size of lipoplexes prevent serum-associated inhibition of transfection

Background Cationic lipids are the most widely used nonviral vectors for ge ne delivery. Upon complexation to DNA, they offer a nonimmunogenic alternat ive to viral gene transfer. Unfortunately, their in vivo application has be en limited due to a serum-associated inhibition of transfection. As a resul t, significant research effort has focused on overcoming this deleterious e ffect of serum. Methods To better understand this phenomenon, we investigated the influence of lipoplex colloidal stability on gene transfection in the presence of se rum. In addition, conditions of the reaction medium were modulated and thei r effects on collidal stability and subsequent in vitro transfection effici ency were studied. Results The colloidal stability of the cationic lipid-DNA complexes, which depended on the charge ratio, determined the efficiency of in vitro transfe ction in the presence of serum. In particular, large-sized, colloidally uns table complexes of over 700 nm mean diameter induced efficient transfection in the presence or absence of serum. Conversely, colloidally stable comple xes of less than 250 nm in size resulted in efficient transfection only in the absence of serum. Furthermore, for the same charge ratio, both colloida lly stable and unstable lipoplexes could be obtained depending on the degre e to which various solution parameters (NaCl concentration, cationic lipid acyl chain length, pH and DNA concentration) were altered. In each case, on ly those complexes lacking colloidal stability resulted in high levels of i n vitro transfection in the presence of serum. This phenomenon was shown to be independent of both the percent DNA internalized and of the lamellar or ganization of the cationic lipid/DNA lipoplexes. Conclusions Through the modulation of various mixture conditions, large-siz ed lipoplexes can be formed which are resistant to the transfection-inhibit ing effect of serum. Copyright (C) 2000 John Wiley & Sons, Ltd.