**7. Conclusions**

We described herein that PCL composed of the low-molecular-weight polyamine conjugates, DCP–spermidine (**DCP-spd**) and DCP–spermine (**DCP-spm**), exhibit much higher gene transfer activity than PEI(1800) conjugate-based **DCP-PEI**(PCL). The former compounds generate 150–400 nm diameter lipoplexes whereas the latter gives rise to large aggregates. In the case of the former compounds, AFM images clearly reveal a morphological change upon acidification, indicating DNA release from the lipoplexes, whereas, in contrast, the morphology of micellar aggregates is insensitive to pH change. A pH-dependent transformation is crucial in gene transfer; and the chemical structure of the polyamine portion may therefore play an important role in the acidification-induced transformation. We have also described the relation between the N/P-dependence of transfection activity and the morphology of the lipoplexes as revealed by AFM. Morphological study with AFM provided useful information for understanding the basis of lipoplexes with superior activity and for design strategies leading to optimally efficient gene carriers. Our recent efforts have demonstrated that such a DCP-polyaimie-based liposomes are effective for systemic siRNA delivery (Asai et al., 2001).
