Use of a composition for releasing an FKBP-binding immunosuppressant drug from endogenous binding substances in a sample suspected of containing the FKBP-binding immunosuppressant drug, wherein the FKBP-binding immunosuppressant drug is rapamycin, tacrolimus, or a derivative thereof, wherein derivatives of rapamycin and of tacrolimus are: derivatives prepared through esterification of one or more hydroxyl groups into a carboxylic ester, a carbamate, a sulfonate ester, or an amide, compounds resulting from the reduction of one or more carbonyl carbons to a hydroxyl group or reduction of one or more of the double bonds; the composition comprising: (a) a compound of the formula: or of the formula: or a mixture thereof; wherein: R 1, R 2 and R 3 are each independently H or lower alkyl; R 4 and R 5 are each independently H, lower hydrocarbyl or lower hydrocarbyloxy or taken together to form a double bond to O, CH 2, N-alkyl, or N-O-alkyl; R 6 and R 7 are each independently H, lower hydrocarbyl or lower hydrocarbyloxy or taken together to form a double bond to O, CH 2, N-alkyl, or N-O-alkyl; R 8 and R 9 are each independently H, non-bulky organic radical or a bulky organic radical, or are taken together to form a double bond to O or CH 2; R 10 and R 11 are each independently H, non-bulky organic radical or a bulky organic radical, or are taken together to form a double bond to O or CH 2; R 12 is H, non-bulky hydrocarbyl, or a bulky organic radical; wherein at least one of R 8, R 9, R 10, R 11 or R 12 is a bulky organic radical; p is 1, 2 or 3; a is 0 or 1; and D is N, O, or CH, with the proviso that a is 0 when D is O; and (b) a buffered medium. derivatives prepared through esterification of one or more hydroxyl groups into a carboxylic ester, a carbamate, a sulfonate ester, or an amide, compounds resulting from the reduction of one or more carbonyl carbons to a hydroxyl group or reduction of one or more of the double bonds; (a) a compound of the formula: or of the formula: or a mixture thereof; wherein: R 1, R 2 and R 3 are each independently H or lower alkyl; R 4 and R 5 are each independently H, lower hydrocarbyl or lower hydrocarbyloxy or taken together to form a double bond to O, CH 2, N-alkyl, or N-O-alkyl; R 6 and R 7 are each independently H, lower hydrocarbyl or lower hydrocarbyloxy or taken together to form a double bond to O, CH 2, N-alkyl, or N-O-alkyl; R 8 and R 9 are each independently H, non-bulky organic radical or a bulky organic radical, or are taken together to form a double bond to O or CH 2; R 10 and R 11 are each independently H, non-bulky organic radical or a bulky organic radical, or are taken together to form a double bond to O or CH 2; R 12 is H, non-bulky hydrocarbyl, or a bulky organic radical; wherein at least one of R 8, R 9, R 10, R 11 or R 12 is a bulky organic radical; p is 1, 2 or 3; a is 0 or 1; and D is N, O, or CH, with the proviso that a is 0 when D is O; and R 1, R 2 and R 3 are each independently H or lower alkyl; R 4 and R 5 are each independently H, lower hydrocarbyl or lower hydrocarbyloxy or taken together to form a double bond to O, CH 2, N-alkyl, or N-O-alkyl; R 6 and R 7 are each independently H, lower hydrocarbyl or lower hydrocarbyloxy or taken together to form a double bond to O, CH 2, N-alkyl, or N-O-alkyl; R 8 and R 9 are each independently H, non-bulky organic radical or a bulky organic radical, or are taken together to form a double bond to O or CH 2; R 10 and R 11 are each independently H, non-bulky organic radical or a bulky organic radical, or are taken together to form a double bond to O or CH 2; R 12 is H, non-bulky hydrocarbyl, or a bulky organic radical; wherein at least one of R 8, R 9, R 10, R 11 or R 12 is a bulky organic radical; p is 1, 2 or 3; a is 0 or 1; and D is N, O, or CH, with the proviso that a is 0 when D is O; and (b) a buffered medium.