Hapten, Immunogens and Derivatives of Ascomycin Useful for Preparation of Antibodies and Immunoassays

The invention teaches derivatives of ascomycin and methods of preparing immunogens and other conjugates useful in immunoassays for quantitatively measuring concentrations of tacrolimus in patient specimens. Antibodies produced from the disclosed immunogens capable of binding to tacrolimus with cross-reactivity of no more than 5% with each of 15-O-demethyl tacrolimus, 31-O-demethyl tacrolimus, and 13,31-O-didemethyl tacrolimus, less than 40% with 13-O-demethyl tacrolimus, and less than 1% with cyclosporin, rapamycin, mycophenolic acid, prednisone, hydrocortisol, and prednisolone are described. Further, immunoassays for measuring the concentration of tacrolimus using such antibodies are taught.