Comparative Molecular Transporter Properties of Cyclic Peptides Containing Tryptophan and Arginine Residues Formed through Disulfide Cyclization

We have previously reported cyclic cell-penetrating peptides [WR](5) and [WR](4) as molecular transporters. To optimize further the utility of our developed peptides for targeted therapy in cancer cells using the redox condition, we designed a new generation of peptides and evaluated their cytotoxicity as well as uptake behavior against different cancer cell lines. Thus, cyclic [C(WR)(x)C] and linear counterparts (C(WR)(x)C), where x = 4–5, were synthesized using Fmoc/tBu solid-phase peptide synthesis, purified, and characterized. The compounds did not show any significant cytotoxicity (at 25 µM) against ovarian (SK-OV-3), leukemia (CCRF-CEM), gastric adenocarcinoma (CRL-1739), breast carcinoma (MDA-MB-231), and normal kidney (LLCPK) cells after 24 and 72 h incubation. Both cyclic [C(WR)(5)C] and linear (C(WR)(5)C) demonstrated comparable molecular transporter properties versus [WR](5) in the delivery of a phosphopeptide (F′-GpYEEI) in CCRF-CEM cells. The uptake of F′-GpYEEI in the presence of 1,4-dithiothreitol (DTT) as the reducing agent was significantly improved in case of l(C(WR)(5)C), while it was not changed by [C(WR)(5)C]. Fluorescence microscopy also demonstrated a significant uptake of F′-GpYEEI in the presence of l(C(WR)(5)C). Cyclic [C(WR)(5)C] improved the uptake of the fluorescent-labeled anti-HIV drugs F′-d4T, F′-3TC, and F′-FTC by 3.0–4.9-fold. These data indicate that both [C(WR)(5)C] and linear (C(WR)(5)C) peptides can act as molecular transporters.