4,8-Ethenopyrrolo[3',4':3,4]cyclobut[1,2-f]isoindole- 1,3,5,7-tetrone, 1,3,3a,3b,4,4a,5,7,7a,8,8a,8b-dodecahydro- 2,6-bis(hydroxymethyl)-, bis(dihydrogen phosphate) (ester), tetrasodium salt NSC600267