1-Methylene Ondansetron is derived from 1,2,3,9-Tetrahydro-9-methyl-4H-carbazol-4-one (T293920), which is used in the preparation of 5-HT3 modulators useful in the treatment and prevention of 5-HT3 receptor-mediated diseases. An Ondansetron (O655000) impurity. Molecular Formula: C19H19N3O Molecular Weight: 305.37