New microplate disc volatilization assay for in vitro evaluation of toxicity and antimicrobial activity of volatiles in vapour phase.

In this study, a novel microplate disc volatilization method for in vitro screening of cytotoxic and antibacterial potential of volatile agents in the vapour phase was designed with the aim to assess their therapeutic potential for development of new inhalation therapies. The toxicity and antibacterial activity of plant-derived volatiles (allyl isothiocyanate, 8-quinolinol, thymohydroquinone, thymoquinone) and essential oils (EOs) from Colebrookea oppositifolia leaf and Neolitsea cassia bark were evaluated against human lung fibroblasts MRC-5 and respiratory pathogens. Allyl isothiocyanate and C. oppositifolia EO did not affect the cell viability (IC50 > 160 μg/cm3), on the other hand thymohydroquinone exhibited the highest cytotoxic effect (IC50 = 5.85 μg/cm3) followed by thymoquinone and N. cassia EO (IC50 = 7.95 and 18.58 μg/cm3, respectively). 8-Quinolinol possessed the strongest antibacterial effect against Staphylococcus aureus and Heamophilus influenzae with respective minimum inhibitory concentrations of 0.625 and 1.25 μg/cm3. Calculated therapeutic index (>21.05) suggests relative inhalation safety of this compound. Using a gas chromatography/mass spectrometry analysis, E-cinnmaldehyde, benzaldehyde, and eucalyptol were identified as major components of N. cassia bark oil, while leaf oil of C. oppositifolia consisted predominantly of thunbergene, β-caryophyllene, and m-camphorene. Moreover, chemical profile of N. cassia EO in the vapour phase showed E-cinnamaldehyde, o-cymene, and α-pinene as the most abundant volatiles. The results suggest 8-quinolinol for further research focused on development of inhalation therapies. Additionally, the results demonstrate validity of the microplate disc volatilization method, which allows cost and labour effective high-throughput screening of cytotoxicity and antimicrobial activity of volatiles in vapour phase.