Malaria is among the most serious infectious diseases worldwide, affecting people in tropical and sub-tropical countries including The Greater Mekong Subregion, particularly those in Cambodia, Southern China, Myanmar, Vietnam, Laos, and Thailand. The World Health Organization reported 249 million cases of malaria with 608,000 human deaths in 2022, especially in children under the age of five, with the African subcontinent accounting for the most patients with mosquito-borne diseases such as malaria [1], [2]. Nowadays, five Plasmodium parasite species (P. falciparum, P. malariae, P. vivax, P. ovale, and P. knowlesi) are known to cause malarial infections in humans, among which P. falciparum and P. vivax are the most important Plasmodium parasites responsible for approximately half a million deaths annually. The increase in multidrug-resistant (MDR) Plasmodium falciparum is a major challenge for the control and prevention of malaria worldwide, posing a severe threat to human health. Moreover, morbidity and mortality rates have significantly increased along with the prevalence of infections caused by MDR Plasmodium parasites [3]. The overuse and misuse of current anti-malarial drugs have promoted the evolution of resistant Plasmodium parasites with multiple mutations in their transmembrane proteins, effectively reducing the accumulation of anti-malarial drugs in their digestive vacuoles [2]. The emergence of Plasmodium parasite resistance to approved drugs has drastically increased in recent decades. P. falciparum (V1/S) strain is a MDR strain carrying the unique mutation of P. falciparum dihydrofolate reductase (DHFR) at Leu-164, Ile-51, Arg-59, and Asn-108, predominantly affecting the binding of cycloguanil and pyrimethamine [2]. Artemisinin, pyrimethamine, and quinolone are among the most effective anti-malarial drugs available in the market that are extensively prescribed for the treatment of malaria infections. Notably, Plasmodium parasites became drug-resistant owing to their extensive use [4], [5]. Currently, only a few effective anti-malarial drugs are available, endangering public health worldwide, especially in malaria-endemic regions. Therefore, overcoming the drug resistance of Plasmodium parasites is important for public health. Despite the urgent demand for effective anti-malarial drugs against MDR Plasmodium parasite strains, only a few effective drugs have been developed to treat malarial infections [6].

Natural resources are the most important sources for the discovery of novel effective therapeutic candidates for the treatment of various diseases, including pathogenic infections [7]. Mortiamide A belongs to a novel class of cyclic peptides with seven amino acid residues isolated from the marine sediment Mortierella sp. in the North Pole. It exerts significant inhibitory effects on P. falciparum strains 3D7 and Dd2 with half-maximal inhibitory concentration (IC50) values of 7.85 ± 0.97 and 5.31 ± 0.24 µM, respectively [8]. However, it has no significant inhibitory activity against a panel of bacterial strains, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium, Staphylococcus warneri, Pseudomonas aeruginosa, and Candida albicans [9]. Lugdunin, a thiazolidine-containing cyclic peptide with six amino acid residues isolated from Staphylococcus lugdunensis, effectively exhibits anti-bacterial activities against various gram-positive bacteria, including MRSA and vancomycin-resistant Enterococci (VRE) with minimum inhibitory concentration (MIC) values of 1.5–3 and 3–12 µg/mL, respectively [10]. The structures of mortiamide A and lugdunin are shown in Fig. 1.

Thorough investigations have revealed the similarity between mortiamide and lugdunin structures in terms of their size, characterized by 21-membered cyclic heptapeptides. However, they were slightly different in terms of their amino acid composition. In contrast to other anti-plasmodial peptides, several macrocyclic peptides typically consist of both D/L hydrophobic amino acids, such as Ile, Phe, Val, and Leu, and exhibit a size of approximately seven to eight amino acids, with at least one polar amino acid. However, it is noteworthy that certain cycloheptapeptides are composed entirely of hydrophobic amino acids [8], [11], [12], [13], [14], [15], [16], [17], [18]. Cyclogossine B is a cycloheptapeptide with hydrophobic properties, comprising two glycines, one tryptophane, two leucines, two alanines, and one isoleucine with all residues exhibiting a l-absolute configuration [19].

To broaden the structural diversity of mortiamide peptides, we selected an inhibitory epitope (-D-Phe1, L-Phe2, and D-Val3) of mortiamide peptides and grafted it onto the lugdunin scaffold to enhance its anti-Plasmodial activity against P. falciparum proliferation. The presence of d-Leu at position 7 played a key role in membrane association, directly affecting the biological activity of lugdunin [20], [21]. Notably, the occurrence of a large number of d-amino acid residues is relatively rare in natural resources, particularly cyclic peptides.19 In this study, by incorporating two distinct inherited properties of mortiamide and lugdunin, we synthesized and evaluated antimalarial activity of two series re-engineered mortiamide peptides that could potentially be developed further for therapeutic applications, especially for the treatment of malarial infections.