Structure-activity relationship studies of Longicalcynin A analogues, as anticancer cyclopeptides.

Cancer has emerged as the main cause of the highest rate of mortality in the world. Drugs used in cancer, although, show some beneficial effects on cancerous organs, demonstrate side effects on other normal tissues. On the other hand, anticancer peptides, being effective on target tissues, should be safe and less harmful on healthy organs, since peptides have several advantages, i.e., high activity, specificity, affinity, being less immunogenic and not accumulate in the body. In the present work, analogues of Longicalcynin A, a naturally occurring anticancer cyclopeptide, were synthesized and evaluated their cytotoxicity in order to gain information from structure-activity relationships of the such cyclopeptides which may lead to find novel and safer anticancer peptide compound(s) to be used in clinic. Peptides were prepared by the solid-phase peptide synthesis method using trityl-resin. Peptide cyclization was performed in liquid phase. To study anticancer activity of the peptide analogues of Longicalycinin A, several methods including MTT, flow cytometry analysis and Lysosomal membrane integrity assay were employed using two cell lines HepG2 and HT-29. Fibroblast cells were used to control the safety of the synthesized cyclopeptides on normal cells. Two cyclopeptides 11 and 17 with the sequences of cyclo-(Thr-Val-Pro-Phe-Ala) and cyclo-(Phe-Ser-Pro-Phe-Ala), respectively were cytotoxic against the colon as well as hepatic cancer cells with safety profile against fibroblast cells, probably with the mechanism of apoptosis as lysosomal membrane integrity damaged. These cyclopeptides showed to be more favorable compounds better than Longicalycinin A and good candidates to develop cyclopeptides as anticancer agents.

Synthesis of Linear and Cyclic Disulfide Heptapeptides of Longicalycinin A and Evaluation of Toxicity on Cancerous Cells HepG2 and HT-29.

In this work, linear and cyclic disulfide heptapeptides of Longicalycinin A have been successfully synthesized by solid phase methodology with Fmoc /t-Bu and solution phase, respectively. 2-Chlorotrityl chloride resin (2-CTC) was used as a solid support. The synthesized linear disulfide analogue of Longicalycinin A was cleaved from the resin as a protected peptide. The final deprotection was performed by treatment with TFA 95% containing scavengers to achieve the deprotected linear disulfide analogue of Longicalycinin A which was characterized by different instrumental methods using LC-MS and FT-IR. Macrocyclization of deprotected linear peptide was done by an oxidating reagent. Linear and cyclic disulfide heptapeptides of Longicalycinin A were evaluated their toxic activity against cell lines of HepG2 and HT-29 using 3- (4, 5-dimethylthiazol-2-yl) -2, -5-diphenyltetrazolium bromide reagent in MTT assay. The synthetic analogues showed a relative good activity against cell lines of HepG2 and HT-29 with IC50 values from 10.33 µg/mL to 12.45 µg/mL, in comparison to the standard drug 5-fluorouracil (5-FU). Safety profiles of the synthesized linear and cyclic disulfide analogues of Longicalycinin A were also examined on skin fibroblast cells. Between the linear and cyclic disulfide heptapeptides of Longicalycinin A, the cyclic peptide showed a considerable toxic activity on the cancerous cell lines along with a low safety result on normal cells. Therefore, the linear disulfide heptapeptide of Longicalycinin A would be encouraging to develop new anticancer agents.