Nimbolide is the Principal Cytotoxic Component of Neem-Seed Insecticide Preparations

Several neem-seed extracts, some used for preparing commercial azadirachtin-containing insecticides, are cytotoxic to N1E-115 murine neuroblastoma cells with IC₅₀ values of 20–200 μg extract ml⁻¹ culture medium. Bioassay-directed fractionation by reversed-phase HPLC shows that the toxicity to N1E-115 cells is associated primarily with a single minor component identified by isolation and NMR and MS as nimbolide with an IC₅₀ of 1·5 μg ml⁻¹ (3·2 μM ). The difference in quantity of nimbolide in seven neem extract sources generally correlates with their overall cytotoxicity. Three other limonoids (epoxyazadiradione, salannin and possibly deacetylsalannin) but not azadirachtin, nimbin and deacetylnimbin contribute in small part to the cytotoxicity. Reconstituted neem extract with only nimbolide removed is less cytotoxic than the original extract. It therefore appears that nimbolide is the principal cytotoxic component of the neem extracts examined and that such minor constituents may warrant consideration in safety evaluations.