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Targeted drug delivery for cancer therapy is an emerging area of research. Cancer cells overexpress certain biomarkers that can be exploited for their targeted therapy. Cyclic cell-penetrating peptides (cCPP) are increasingly assessed for intracellular cargo delivery in cancer cells. In this study, we have conjugated cabazitaxel (CBT) to the cCPP via an ester bond to assist CBT release in the tumor's acidic environment. Integrin targeting (RGDC, TP1) and extra domain B of fibronectin (EDB-Fn) targeting (CTVRTSAD, TP2) peptides were linked to the peptide-drug conjugate (cCPP-CBT) via a disulfide bond to provide targeting ability to the conjugates until they reach the tumor site. Conjugate 11 (TP1-cCPP-CBT) and conjugate 16 (TP2-cCPP-CBT) showed approximately 3-4-fold less antiproliferative activity on integrin and EDB-FN overexpressing cancer cell lines as compared to the CBT analogue used for comparison (CBT-GA, 5 ). Conjugates ( 11 and 16 ) were less toxic (31-34-fold less antiproliferative activity) to the normal human embryonic kidney (HEK-293) cells as compared to CBT. The flow cytometry and quantitative confocal microscopy data further confirm the selective efficacy of conjugates (TP1-cCPP-FAM ( 10 ) and TP1-cCPP-FAM ( 15 )) toward biomarker overexpressing cancer cells. Furthermore, the stability and release studies of conjugate 11 revealed its therapeutic potential under different conditions, such as human plasma, different pHs, and redox conditions. This conjugation strategy was proven to enhance chemotherapeutics agents' efficacy and targeting and can be applied to other chemotherapeutic agents.

Antimicrobial peptides (AMPs) are typical cell penetrating peptides (CPPs) that intercalate into biomembranes and exhibit broad activities. We designed a triple fusion protein consisting of an AMP, Ib-AMP4 at the N-terminus, a fluorescent GFP probe in the center, and the tumor-targeting peptide P1c at the other terminus. After purification from E. coli , the interaction between the Ib-AMP4-GFP-P1c fusion protein (IGP) and the lipid membrane was characterized. Experiments using isothermal titration calorimetry (ITC) and quartz crystal microbalance with dissipation (QCM-D) demonstrated that IGP proteins spontaneously bound the lipid bilayer with a maximal molar ratio of 1:52 (protein:lipid). Furthermore, transmission electron microscopy (TEM) confirmed that the IGP protein was present in the liposome membrane. After decoration with IGP proteins, the DOPC:DOPG liposomes were applied to cancer cells. Microscopy and flow cytometry reveal that the decorated liposomes selectively bound integrin αvβ3-positive A549 cells. In addition, compared with the common chemical conjugation method, the reported method seemed to be superior in certain aspects, such as simple sample preparation and cost-effectiveness. Next, the IGP protein was applied to decorate red blood cell (RBC) liposomes for targeted delivery in both in vitro and in vivo applications. The IGP-decorated RBC liposomes preferentially targeted integrin αvβ3 expressing A549 cancer cells. The in vivo imaging showed that IGP-decorated RBC liposomes were concentrated in tumor tissue and were primarily metabolized by the liver and kidney.

Cell-penetrating peptide [WR]₅ has been previously shown to be an efficient molecular transporter for various hydrophilic and hydrophobic molecules. The peptide was synthesized using Fmoc/tBu solid-phase chemistry, and one arginine was replaced with one lysine to enable the conjugation with the anticancer drugs. Paclitaxel (PTX) was functionalized with an esterification reaction at the C2' hydroxyl group of PTX with glutaric anhydride and conjugated with the cyclic peptide [W(WR)₄K( β Ala)] in DMF to obtain the peptide-drug conjugate PTX1. Furthermore, camptothecin (CPT) was modified at the C(20)-hydroxyl group through the reaction with triphosgene. Then, it was conjugated with two functionalized cyclic peptides through a formyl linker affording two different conjugates, namely CPT1 and CPT2. All the conjugates showed better water solubility as compared to the parent drug. The cytotoxicity assay of the drugs and their conjugates with the peptides were evaluated in the human breast cancer MCF-7 cell line. PTX inhibited cell proliferation by 39% while the PTX-peptide conjugate inhibited the proliferation by ~18% after 72 h incubation. On the other hand, CPT, CPT1, and CPT2 reduced the cell proliferation by 68%, 39%, and 62%, respectively, in the MCF-7 cell lines at 5 µM concentration after 72 h incubation.