Doxorubicin (Dox) is used for breast cancer, leukemia, and lymphoma treatment as an effective chemotherapeutic agent. However, Dox use is restricted due to inherent and acquired resistance and an 8-fold increase in the risk of potentially fatal cardiotoxicity. Hybrid cyclic-linear peptide [R<sub>5</sub>K]W<sub>7</sub>A and linear peptide R<sub>5</sub>KW<sub>7</sub>A were conjugated with Dox through a glutarate linker to afford [R<sub>5</sub>K]W<sub>7</sub>A-Dox and R<sub>5</sub>KW<sub>7</sub>A-Dox conjugates to generate Dox derivatives. Alternatively, [R<sub>5</sub>K]W<sub>7</sub>C was conjugated with Dox via a disulfide linker to generate [R<sub>5</sub>K]W<sub>7</sub>C-S-S-Dox conjugate, where S-S is a disulfide bond. Comparative antiproliferative assays between conjugates [R<sub>5</sub>K]W<sub>7</sub>A-Dox, [R<sub>5</sub>K]W<sub>7</sub>C-S-S-Dox, linear R<sub>5</sub>KW<sub>7</sub>A-Dox, the corresponding physical mixtures of the peptides, and Dox were performed in normal and cancer cells. [R<sub>5</sub>K]W<sub>7</sub>A-Dox conjugate was 2-fold more efficient than R<sub>5</sub>KW<sub>7</sub>A-Dox, and [R<sub>5</sub>K]W<sub>7</sub>C-S-S-Dox conjugates in inhibiting the cell proliferation of human leukemia cells (CCRF-CEM). Therefore, hybrid cyclic-linear [R<sub>5</sub>K]W<sub>7</sub>A-Dox conjugate was selected for further studies and inhibited the cell viability of CCRF-CEM (84%), ovarian adenocarcinoma (SK-OV-3, 39%), and gastric carcinoma (AGS, 73%) at a concentration of 5 μM after 72 h of incubation, which was comparable to Dox (5 μM) efficacy (CCRF-CEM (85%), SK-OV-3 (33%), and AGS (87%)). While [R<sub>5</sub>K]W<sub>7</sub>A-Dox had a significant effect on the viability of cancer cells, it exhibited minimal cytotoxicity to normal kidney (LLC-PK1, 5-7%) and heart cells (H9C2, <9%) at concentrations of 5-10 μM (compared to free Dox at 5 μM that reduced the viability of kidney and heart cells by 85% and 44%, respectively). The fluorescence microscopy images were consistent with the cytotoxicity studies, indicating minimal uptake of the cyclic-linear [R<sub>5</sub>K]W<sub>7</sub>A-Dox (5 μM) in H9C2 cells. In comparison, Dox (5 μM) showed significant uptake, reduced cell viability, and changed the morphology of the cells after 24 h. [R<sub>5</sub>K]W<sub>7</sub>A-Dox showed 16-fold and 9.5-fold higher activity against Dox-resistant cells MDA231R and MES-SA/MX2 (lethal dose for 50% cell death or LC<sub>50</sub> of 2.3 and 4.3 μM, respectively) compared to free Dox (LC<sub>50</sub> of 36-41 μM, respectively). These data, along with the results obtained from the cell viability tests, indicate comparable efficiency of [R<sub>5</sub>K]W<sub>7</sub>A-Dox to free Dox in leukemia, ovarian, and gastric cancer cells, significantly reduced toxicity in normal kidney LLC-PK1 and heart H9C2 cells, and significantly higher efficiency in Dox-resistant cells. A number of endocytosis inhibitors did not affect the cellular uptake of [R<sub>5</sub>K]W<sub>7</sub>A-Dox.