Compositions are disclosed for releasing an FKBP-binding immunosuppressant drug from endogenous binding substances in a sample suspected of containing the FKBP-binding immunosuppressant drug. The compositions include sirolimus derivatives that are modified with a bulky organic radical in the triene portion of the sirolimus molecule. The compositions may be employed in conjunction with assays for an FKBP-binding immunosuppressant drug in a sample suspected of containing the drug.
