All sandacrabins showed moderate activity against Bacillus subtilis and Staphylococcus aureus in the antimicrobial assays, with sandacrabin A exhibiting a MIC of 32 µg/mL against B. subtilis. In contrast to the three sandacrabins isolated from S. defensii MSr10575, the two sandacrabins D and E accessible by synthetic chemistry revealed an additional weak activity against Mycobacterium smegmatis at 64 µg/mL. All sandacrabins except for sandacrabin Awhich did not show insecticidal activity up to 5 µg/insect-were found to be toxic to A. pisum at 0.5 µg/insect. Sandacrabin B and D displayed a 100% death rate, whereas sandacrabin C and E both showed a 90% death rate at this concentration. The simultaneous effects on Huh-7.5 cells, however, most likely impede insecticidal use of the sandacrabins. Most interestingly, we observed a significant reduction in viral replication when cells infected with the human pathogenic coronavirus HCoV229E were treated with sandacrabin B-E (see Figure ). As observed for the insecticidal activities, sandacrabin A did not show any effect on viral replication at the concentrations tested. Although sandacrabin B and C exhibited cytotoxic effects at the highest test concentration, a potential application window for development of antiviral pharmaceuticals is present (Figure ). Due to the reduction in viral load, we even observe an increase in cell viability for cells treated with concentrations higher than 1 μM of sandacrabin B and C. At the two highest concentrations tested, cell viability is again reduced indicating that at this point the cytotoxic effects predominate the advantage of viral inhibition. Notably, the cytotoxic activity and antiviral reduction differ by a factor of 11 when comparing IC50 values against HCoV229E and Huh-7.5 for sandacrabin C. For Sandacrabin B, D and E we observe a smaller window represented by only a 3-4fold difference in cytotoxic and antiviral activities, making sandacrabin C the most promising derivative for further optimisation among the five sandacrabins. Driven by the intriguing antiviral activities of the sandacrabins, we moved on investigating their mode of action by analyzing their potential as inhibitors of viral RNA synthesis using the purified SARS-CoV-2 RNA-dependent RNA polymerase complex consisting of the proteins Nsp7, Nsp8, and Nsp12 (RdRp). The results of this assay are depicted in Figure and Figure . While all five sandacrabins inhibited RNA synthesis in vitro, sandacrabins B and C exhibited most promising IC50 values in the lower micromolar range with 3.5 µM and 6.8 µM, respectively (see Table ). As visualized in Figure , inhibition of the RNA synthesis nicely correlates with the antiviral activity, whereby interaction with the RdRp complex likely represents at least part of the sandacrabin antiviral mode of action.