An Assay for the Determination of Sirolimus Levels in the Lymphocyte of Transplant Patients

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. Methods Whole blood samples from patients on SRL were collected in Ethylene Diamine Tetra-acetic acid (EDTA) vacutainer tubes. Immediately the lymphocytes from 2 mL of blood were separated using 1.5 mL of Ficoll gradient, by centrifugation for 30 minutes at 2500 RPM. The lymphocytes were washed three times with phosphate-bufferd saline and the pellet suspended in 150 μL of Middle East research institute (MERI) drug extraction solution (Beirut, Lebanon), which was then added to 300 μL of IMx solublizing reagent. The cytoplasmic SRL concentrations in lymphocytes were measured using kits supplied from Abbott diagnostics or by high-performance liquid tomography. A corresponding whole blood sample from each patient was used to measure blood levels. To determine the level per lymphocyte, the value obtained was divided by the number of lymphocytes and expressed as Pg/cell. A pharmacokinetic profile for both blood and lymphocytes was constructed for each patient using data corresponding to predose C 0 , 1 hour (C 1 ) and 2 hours (C 2 ) after the dose. The lymphocyte enumeration for C 0 , C 1 , and C 2 was performed using the FACS Calibur Flow Cytometer from Becton Dickinson. The average dose was 2.86 ± 1.27 mg/d with a C 0 = 8.05 ± 4.24, C 1 = 21.9 ± 8.9 ng/mL, and C 2 = 23 ± 0.03 ng/mL. Although there was a significant correlation ( P = .0975) between the dose and C 0 , there was no correlation between the dose and C 0 level on the lymphocyte count P = .897. However, there was a strong correlation between SRL lymphocyte levels (pg/cell) and the lymphocyte count (r 2 = .6.06). The higher the concentration of the drug the lower the lymphocyte counts. The assay is sensitive to within 0.45 pg/cell, reproducible with a coffcient of variance (CV) of 6.4% within assay and 7.5% for intraassay.