Table 1.

Previously discovered naturally occurring cyclopeptides and their medicinal significance.

Cyclic Peptide	Natural Source	Medicinal Significance	Mechanism of Action
Vancomycin	Amycolatopsis orientalis	An antibiotic mostly active against Gram-positive microorganisms, including MRSA but not vancomycin-resistant Enterococcus (VRE). It is used for bacterial prophylaxis in neurological, orthopedic, and vascular surgery. For patients who are allergic to penicillins and cephalosporins, it can be used as an alternative antibiotic [43].	Inhibiting cell wall synthesis of bacteria by binding to the building blocks of peptidoglycan monomers of N-acetylmuramic acid and N-acetylglucosamine and blocking cross-linking of the peptidoglycan layer [44].
Cyclosporin A	The fungus Tolypocladium inflatum	Has potent immunosuppressive properties. It prevents graft-versus-host disease following transplantation, rejection of kidney, heart, and liver transplants [45].	It blocks the transcription of cytokine genes in activated T cells via the calcineurin-phosphatase pathway [46]. It binds to the cytosolic protein cyclophilin of T cells and consequently leads to reduced T-cell function. It also binds to the cyclophilin D protein that constitutes part of the mitochondrial permeability transition pore (MPTP), which causes movement of calcium ions (Ca2+) into the mitochondria and causes the contraction of the muscle cells (heart) [47].
Romidepsin	Chromobacterium violaceum	A potent antitumor drug that reverses the malignancy of tumorigenic cell lines and induces apoptosis in malignant cell lines.	Promotes acetylated histones H3 and H4 in the peroxiredoxin 1 (Prdx1) promoter (as a tumor suppressor), thus activating Prdx1 expression in tumor tissues and inhibiting tumour growth [48].
Griselimycin	Streptomyces bacteria	Griselimycin derivatives showed antibiotic activity in addition to oral bioavailability, absorption, and antitubercular activity [40]. Griselimycin exhibited formidable pharmacokinetic properties for its chemical class and size.	Prevents DNA replication (known as sliding clamp) by inhibiting the interaction of the replicative DNA polymerase with the DNA polymerase beta subunit [41].