1,2,3,4-Cyclobutanetetracarboxylic dianhydride treated BSA 1,2,3,4-Cyclobutanetetracarboxylic dianhydride treated bovine serum albumin AIDS-189625 AIDS189625 Tetrahydro-2,5-dioxa-cyclobutadicyclopentene-1,3,4,6-tetraone