Structure–Activity Relationships of Photoswitchable Diarylethene-Based β-Hairpin Peptides as Membranolytic Antimicrobial and Anticancer Agents

Abstract

Five series (28 structures) of photoswitchable β-hairpin peptides were synthesized based on the cyclic scaffold of the natural antibiotic gramicidin S. Cell-type selectivity was compared for all activated (diarylethene "ringopen") and deactivated ("ring-closed") forms in terms of antibacterial activity (MIC against Escherichia coli and Bacillus subtilis), anticancer activity (IC 50 against HeLa cell line), and hemolytic cytotoxicity (HC 50 against human erythrocytes). Correlations between the conformational plasticity of the peptides, their hydrophobicity, and their bioactivity were also analyzed. Considerable improvements in selectivity were achieved compared to the reference compound. We found a dissociation of the anticancer activity from hemolysis. Phototherapeutic indices (PTI), HC 50 (closed)/MIC(open) and HC 50 (closed)/IC 50 (open), were introduced for the peptides as safety criteria. The highest PTI for HeLa-selective toxicity were observed among analogues containing hydroxyleucine on the hydrophobic face. For one compound, high PTIs were demonstrated across a range of different cancer cell lines, including a doxorubicin-resistant one.

Light-controllable dithienylethene-modified cyclic peptides: photoswitching the in vivo toxicity in zebrafish embryos

“…All peptides (Table 1) were synthesized as was previously described [30] and initially handled as ring-open photoforms under ambient (visual, vis) light conditions. Each of the ringopen photoforms was converted into the ring-closed photoforms by UV irradiation in a solution of denaturing agents, reproducing previously documented results [29,30,52]. All Table 1: Nomenclature, sequences, molecular masses of the peptides synthesized and explored in this study.…”

“…All Table 1: Nomenclature, sequences, molecular masses of the peptides synthesized and explored in this study. The photoswitchable peptides are grouped in series (i-v) according to their original design [30].…”

“…This natural antimicrobial agent has an amphipathic structure, whose functional mechanism is attributed to the permeabilization of bacterial membranes, with considerable side-effects also on eukaryotic cell membranes [27,28]. Compared to the ring-closed (deactivated, OFF) photoform, the ring-open (active, ON) isomer of 2 indeed displayed a much stronger in vitro toxicity against several tumor cell lines [29,30]. Using the photoswitchable analogue 2 in an allograft mouse model, the first in vivo photopharmacology application for a DAE-derived peptide as an anticancer agent has been demonstrated [29].…”

“…Using the photoswitchable analogue 2 in an allograft mouse model, the first in vivo photopharmacology application for a DAE-derived peptide as an anticancer agent has been demonstrated [29]. In order to optimize 2, we have recently performed a structure-activity relationship (SAR) study using a library of photoswitchable derivatives of 2 [30]. The library contained 29 compounds grouped into several series: with natural amino acid mutations affecting the amphipathicity (series i), with point mutations that modulate polarity and conformational stability of the β-structural elements (series ii), with backbone N-alkylation and side-chain hydroxylation modifications (series iii), with macrocycle ring-size variations (series iv), and with macrocycle homodimerization (series v).…”

“…In this study, we selected 19 photoswitchable DAE-modified cytotoxic peptides from our previous SAR evaluation [30] and established with them the D. rerio embryotoxicity assay. For each photoswitchable peptide, we determined the lethal dose of both photochromic forms, using two assay variations, and these in vivo toxicity values could then be compared with the known in vitro cytotoxicities.…”