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Abstract
Both tacrolimus (TAC) and sirolimus (SRL) bind to the same immunophilin FKBP12; however,
their mechanisms of action are distinct. SRL inhibits mammalian target of rapamycin
(TOR), which is an enzyme critical to the immune function. TOR inhibition blocks the
signal that mediates T-cell proliferation by preventing cell-cycle progression from
G1 to S phase. Moreover, TOR inhibition results in a decrease in antibody production
by blocking B-cell proliferation and maturation into antibody producing cells. The
use of SRL has resulted in a decrease in the number of rejection episodes. As with
other immunosuppressive agents, SRL can cause dose-related side effects, the most
notable of which are hypercholesterolemia, hyperlipidemia, anemia, and thrombocytopenia.
Thus, therapeutic drug monitoring to assess efficacy and toxicity has became a necessity.
SRL blood levels do not correlate with its bioactivity and are affected by the concomitant
use of other immunosuppressive drugs. To determine the bioactivity of SRL we have
developed an assay to determine the level of Sirolimus per lymphocyte of transplant
patients. The levels were correlated with lymphocyte count.