Liposomal compositions comprising of liposomes of guanidinylated cationic amphiphiles as the main lipid and cholesterol/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/aminopropyl polyethyleneglycol carbamyl-distearoylphosphatidyl-ethanolamine (DSPE-peg-NH2) as co-lipids are described. These liposomal compositions containing encapsulated or electrostatically complexed therapeutic small interfering RNAs (siRNAs) against Cdc20, a key cell cycle regulator, inhibit solid tumor growth and melanoma tumor growth on lung in C57BL/6J mice.