Objectives: We have previously identified a minor immunophilin of 52 kDa mo
lecular weight capable of binding tacrolimus and sirolimus. Because immunop
hilins are capable of binding both parent drug and metabolites and HPLC ass
ays are typically used to assess parent drug in clinical situations, we use
d this immunophilin in a radioreceptor assay (RRA) to determine if any meta
bolites not included in the HPLC measurement would bind to the immunophilin
and be associated with thrombocytopenia in patients receiving sirolimus.
Design and methods: We tested 51 steady-state trough whole blood samples fr
om non-thrombocytopenic patients and 51 steady-state trough samples from th
rombocytopenic patients and compared them to HPLC measurements of parent dr
ug in the same samples. We also tested whole blood samples spiked with auth
entic sirolimus metabolites using RRA to ascertain the effect these metabol
ites have on the technique.
Results: We found minimal cross-reactivity in this assay for sirolimus meta
bolites (binding ranged from <10% to 26%), and good correlation of the radi
oreceptor assay with HPLC (linear regression slope 0.92, y-intercept 0.79).
There was no statistically significant difference between the RRA and HPLC
results' in two patient groups-thrombocytopenic and non-thrombocytopenic-u
sing the paired t-test p < 0.005) and Bland-Altman analysis.
Conclusions: These findings indicate that although the RRA could be substit
uted for HPLC in therapeutic drug monitoring, the 52 kDa immunophilin does
not offer an advantage in terms of detecting metabolites associated with th
rombocytopenia. However, the RRA offers the advantages of shorter turnaroun
d time, smaller sample volume and potential for automation. Copyright (C) 2
000 The Canadian Society of Clinical Chemists.