**4. Genocompatibility and toxicogenomics of polycationic nanostructures**

To pursue the genomic impacts of any gene based medicine, it is necessary to exploit high throughput screening methodologies (e.g., DNA microarray) for evaluation of global gene changes induced by the gene medicine or any other chemicals/compounds. Such genome based impact could be termed as "genotoxicity" or "toxicogenomics".

The DNA microarray technology combines standard molecular techniques with highthroughput screening to monitor the expression of up to ~40000 genes, which may provide a means for toxicity prediction prior to classical toxicological endpoints such as histopathology or clinical chemistry (Goldsmith & Dhanasekaran, 2004). In gene silencing experiments, such approach may allow a genomic characterization of delivery systems leading to identification of possible incompatibilities with intended target genes or biological effects of the gene based medicine. This may allow screening of compatible or useful delivery systems early in drug development that could subsequently save time and money in pre-clinical and clinical studies (Fielden & Kolaja, 2006; Lettieri, 2006).

Cytotoxicity and genotoxicity potentials of CPs and CLs are going to be well acknowledged, and accordingly these cationic nanosystems should undergo a rigorous genocompatibility evaluation prior to *in vitro* and *in vivo* exploitation (Kabanov, 2006; Omidi et al., 2005a). These systems alone or in combination with biologically active molecules (e.g., siRNA, antisense, aptamer) are able to alter cell signaling and biological responses in cells and organisms, emerging a cluster of genomic and post genomic consequences. In general, toxic responses to these kinds of nanomaterials are deemed to be very profound, in which various signaling pathways such as oxidative stress, immune responses and apoptosis pathways may be involved in response to generation of reactive oxygen species in the membranes (Kabanov, 2006). Cationic liposomes, irrespective of complexation with DNA, can downregulate the synthesis of pro-inflammatory mediators such as nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) in lipopolysaccharide (LPS)/interferon-gamma (IFN-gamma)-activated macrophages (Filion & Phillips, 1997a; Filion & Phillips, 1997b). Under the oxidative stress, cells may undergo the Nrf-2 signaling or the proinflammatory signaling cascades such as mitogen-activated protein kinase (MAPK) and nuclear factor kB (NFkB) cascades and eventually a programmed cell death may occur (Kabanov, 2006). Certain proteins such as protein kinase C (PKC) may also be affected detrimentally by cationic amphiphiles (Aberle et al., 1998), which function as PKC inhibitors and may inevitably result in inadvertent toxicity. It seems that the cationic amphiphiles with steroid backbones can exert more potent inhibitors of PKC than their straight-chain analogues, resulting in greater toxic impacts (Bottega & Epand, 1992). Polycations such as PEI formulated with plasmid DNA and administered to mouse lungs was reported to activate the p38 pathway involved in endocytosis, phagocytosis and hydrogen peroxide production. The observed *in vitro* and *in vivo* toxicity of such PEI polyplex formulations appeared to link to a general stress reaction, inflammatory responses, cell cycle regulation and DNA damage repair (Regnstrom et al., 2006). To obtain a complete image, it is essential to recruit high throughput screening methods such as DNA microarray.
