Ixabepilone Labelled; (1S,3S,7S,10R,11S,12S,16R)-7,11-dihydroxy-8,8,10,12,16-pentamethyl-3-[(1E)-1-methyl-2-(2-methyl-4-thiazolyl)ethenyl]-17-Oxa-4-azabicyclo[14.1.0]heptadecane-5,9-dione-13C2,15N; Azaepothilone B-13C2,15N,Ixabepilone Stable Isotopes