"Low sirolimus" generally means that the sirolimus level in your blood is too low to be within therapeutic levels. This means that your dosage of sirolimus will probably be increased, so that your blood concentration of sirolimus falls within therapeutic levels.

No, they don't need to be fasting. However they need to be done before you've had your dosage of Sirolimus for the day (since taking Sirolimus on the day of the blood test, before the blood, test messes up the test).

No, it is not. Sirolimus is a newer and far stronger drug compared to tacrolimus; it is a once per day dosing regime. It also works in a slightly different way, which means that you can end up on both tacrolimus and sirolimus at the same time . Sirolimus, at present is usually only used to prevent organ rejection in kidney transplants (however it's range of uses is increasesing).

No, it does not. However it is considered that Sirolimus (rapamyacin) may have antiproliferative properties.

cyclosporine, prednisone, azathioprine, tacrolimus or FK506, mycophenolate mofetil, sirolimus, and OKT3, as well as ATGAM and Thymoglobulin.