Sodium azide is less suitable as a positive control of drug-induced lethality for in vitro clonogenic assays

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Abstract
Sodium azide (6 mg/ml) was used as a positive control for drug-induced lethality in an in vitro clonogenic assay. Petri dishes containing control and sodium azide treated cultures of WiDr cells were placed together in a large Petri dish and incubated at 37°C in an atmosphere of 10% CO2 in air. No growth was observed. Control cells formed colonies only when the dishes were separated from the sodium azide dishes. Using a microtiter plate the toxic effect was inversely related to the distance of the test cultures from the sodium azide treated cultures. These results suggested the formation of a toxic gas or vapour from sodium azide under cell culture conditions, probably an azide. Chemical analysis was based on characteristic reactions, such as the production of a precipitate with silver ions or formation of a red-coloured complex with ferric salts. On a microtiter plate, a gradient of the expected precipitate or red colour was observed, the highest amounts adjacent to the wells containing sodium azide. These results show that sodium azide acts as a positive control of drug-induced lethality for in vitro clonogenic assays. However, the formation of a highly toxic vapour, most likely hydrazaic acid, makes it a less suitable standard.