In this study, it was aimed to develop drug delivery systems with high drug release rate, capable of overcoming multidrug resistance of cancer cells. The first generation drug delivery system (DDS) denoted as mPEG-AT3-DOX was prepared by methoxy polyethylene glycol (mPEG) and peptide conjugation, and the model anticancer drug, DOX, was attached to the mPEG-peptide carrier system using stable amide linkage. mPEG was used to increase blood circulation time of the DDS. Designed peptide (AT3 = CG3H6R2ALALG3E) controls release of the drug via RRALAL sequence, which is a substrate for a lysosomal enzyme, cathepsin B, overexpressed in most of the tumor cells. pH responsive histidines and reactive amino acids (glutamic acid and cysteine) for drug and mPEG conjugations were also added to the peptide sequence. The peptide synthesized by using Fmoc chemistry was conjugated to mPEG-maleimide via Michael addition reaction. DOX was attached to the carboxylic acid group present in the carrier system (mPEG-AT3) via amide linkage. Mass spectroscopy and HPLC were used to assess the purity of the AT3 and mPEG-AT3. At pH 7.4, mPEG-AT3-DOX exhibited bimodal size (hydrodynamic diameter) distribution at about 15 and 30 nm independent of pH. % DOX release from mPEG-AT3-DOX was observed to be below 10 % at neutral pH and pH 5.0 in the absence of cathepsin B, and increased to 17 ± 2 % in the presence of cathepsin B. Complete degradation of AT3 peptide within three hours in the presence of cathepsin B suggests lower than expected DOX release is due to aggregation tendency of the DDS. Cytotoxicity of the conjugates was evaluated using the lung cancer (A549) and prostate cancer (PC3) cell lines. At the end of 24 hours the absolute IC50 values of free DOX and mPEG-AT3-DOX were obtained as 1.37 ± 0.05 and 1.33 ± 0.11 for the A549 cell line, 1.51 + 0.07 and 1.63 + 0.80 μg equivalent DOX / ml for the PC3 cell line, respectively. Considering, these results, the second generation DDS will be designed to have more pronounced pH sensitive property by increasing the number of histidines in the peptide domain.