Anticancer a-helical peptides and their structure / function relationships underpinning their interactions with tumour cell membranes

Over the last decade there has been an increase in premature deaths due to cancer. The identification of new chemotherapeutic agents has led to the discovery of toxic peptides with cancer cell specificity. Extensive research is therefore required to determine how these anticancer peptides (ACPs) function and how their structural characteristics relate to both specificity and efficacy. An ACP database comprising of 158 peptides was sub-divided into 3 datasets as follows: ACP 1 (inactive) containing 21 peptides, ACPAO (anticancer only) containing 14 peptides and ACPT (toxins) containing 123 peptides. Multivariate analysis of the physiochemical properties and toxicity indicated that no significant difference (Kruskal-Wallis = 8.82; p = 0.18) was observed in the median toxicity of the ACP datasets against the cancerous and non-cancerous cell lines. In contrast to the oligopeptides from Mtaldon and Argos control dataset, C residues were observed to be completely absent from all 3 ACP subsets with an abundance of the positively charged K residues (relative frequency >0.3) in the ACPAO and ACPTpeptides. This suggested that positive charge and the absence of C residues may facilitate peptide binding to anionic cancer cell membranes. Furthermore, lower levels of K residues were observed in the ACP 1 datasets, indicating that positive net charge may also be important for efficacy. A combination of several other physiochemical properties was analysed using box-plot analysis, linear regression and frequency distribution analysis. It was found that whilst sequence length, net charge, mean hydrophobicity () and amphiphilicity () of the peptides did not show a linear correlation with the toxicity, they may help increase efficacy in the case of some tumour cell lines. Three-dimensional clustering using net charge, and was carried out using the unweighted pair group method with arithmetic mean (UPGMA). The cluster analysis results showed that there was no optimal combination of these properties for high level efficacy and no specific residue arrangement was found to affect ACP specificity for cancer cells. Therefore, inter-quartile ranges were identified for key parameters but it was observed that no single optimal values could be obtained, emphasizing the complexity of these biological systems and the interdependence of these parameters. Further work in this study therefore, involved the investigation of secondary structural elements such as hydrophobicity gradients generated by residue arrangements. It was observed that although surface activity and amphiphilicity were important parameters, hydrophobic gradients were not essential for anticancer activity or selectivity

The effect of C-terminal amidation on the efficacy and selectivity of antimicrobial and anticancer peptides

Cationic defence peptides show high therapeutic potential as antimicrobial and anticancer agents. Some of these peptides carry a C-terminal amide moiety which has been shown to be required for antimicrobial activity. However, whether this is a general requirement or whether C-terminal amidation is required for the anticancer activity of defence peptides is unclear. In response, this study analyses the toxicity of a series of C-terminally amidated defence peptides and their non-amidated isoforms to normal fibroblast cells, a variety of tumour cells and bacterial cells. The toxicities of these peptides to microbial and cancer cells were generally <200 μM. Peptides were either unaffected by C-terminal amidation or showed up to 10-fold decreases or increases in efficacy. However, these peptides all showed toxicity to normal fibroblast cells with levels (generally <150 μM) that were comparable to those of their antimicrobial and anticancer activities. In contrast to previous claims which have been based on analysis of single amidation events, the results of this study clearly show that the C-terminal amidation of defence peptides has a variable effect on their antimicrobial and anticancer efficacy and no clear effect on their selectivity for these cell types