Antibacterial and Anticancer Properties of L-Cyclic Dipeptides from Streptomyces sanyensis ARR153, an Endophyte in Caulerpa lentillifera J. Agardh.

<b>Background and

Objective

</b> Marine algal endophytes remain a highly promising yet underexplored frontier for discovering unique bioactive scaffolds. This study aimed to isolate endophytic actinomycetes from the tissues of the green macroalga <i>Caulerpa lentillifera</i> in Phetchaburi Province, Thailand and evaluate their antibacterial and selective anticancer properties. <b>Materials and

Methods

</b> Actinomycetes were isolated from surface-sterilized <i>C. lentillifera</i> tissues using humic acid-vitamins agar. The most antagonistic strain, ARR153, was identified via morphological and 16S rDNA phylogenetic analysis. Following large-scale cultivation on ISP-2 agar, the secondary metabolites were extracted using ethyl acetate and purified via column and thin-layer chromatography. Purified compounds were evaluated using MIC microdilution assays, <i>in vitro</i> MTT cytotoxicity tests against human cancer lines (HeLa, HepG2, MDA-MB-231) and normal Vero cells and <i>in silico</i> molecular docking/ADMET simulations targeting the EGFR kinase domain. <b>

Results

</b> Isolate ARR153 was identified as <i>Streptomyces sanyensis</i>. Chromatographic purification yielded four L-cyclic dipeptides (diketopiperazines): Cyclo-(L-Leu-L-Val), Cyclo-(L-Leu-L-Thr), Cyclo-(L-Leu-L-Ile) and Cyclo-(L-Leu-L-Leu). These molecules exhibited broad-spectrum bactericidal activity against Gram-positive pathogens, including clinical MRSA (MICs: 16-128 μg/mL). In cytotoxicity assays, the dipeptides induced strong, dose-dependent growth inhibition against MDA-MB-231 and HeLa cells, while showing no toxicity toward normal Vero cells (IC<sub>50</sub>>400 μg/mL). Notably, Cyclo-(L-Leu-L-Leu) demonstrated a higher selectivity index for breast cancer cells than doxorubicin. <i>In silico</i> modeling confirmed that the compounds effectively bind the EGFR active site via essential hydrogen bonds at residue Thr830 with favorable predictive pharmacokinetic profiles. <b>

Conclusion

</b> This study demonstrates that <i>C. lentillifera</i>-associated <i>S. sanyensis</i> ARR153 is a valuable source of bioactive L-cyclic dipeptides. These natural products offer potent structural platforms for the design of targeted biopharmaceuticals to counter multidrug-resistant bacterial infections and specific human cancers.