Volumetric absorptive microsampling for the quantification of tacrolimus in capillary blood by high performance liquid chromatography-tandem mass spectrometry.

Volumetric absorptive microsampling (VAMS) is an innovative alternative strategy to venipuncture for monitoring tacrolimus levels in transplant recipients. In this study, we aimed to validate a new high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method for quantifying tacrolimus in blood collected by VAMS. Tacrolimus was extracted from dried blood tips in an original process involving sonication, protein precipitation and salting out. The assay was validated in accordance with EMA and IATDMCT guidelines. For clinical validation, the tacrolimus concentrations measured in liquid venous whole blood (with the reference method) were compared with those measured in capillary whole blood collected simultaneously with VAMS by a nurse. The assay was then used to monitor tacrolimus exposure in transplant recipients. The method was linear, sensitive and fast. Within-day and between-day precisions and overall bias were within ±15%. No significant hematocrit effect was observed. The matrix effect was negligible and recovery exceeded 80% for every concentration and hematocrit levels. Tacrolimus was stable in blood collected by VAMS for 1 week at room temperature, 48 h at 60 °C and 4 °C and 1 month at -80 °C. Clinical validation (n = 42 paired samples) demonstrated a strong correlation between the two methods (r = 0.97 Pearson correlation). Bland-Altman analysis revealed that more than 90% of the differences between VAMS and liquid blood paired concentrations were within the ±20% acceptable range. The method had a satisfactory analytical performance and fulfilled clinical requirements. This minimally invasive VAMS-based assay appears reliable for the determination of tacrolimus levels in blood from transplanted patients.

Tacrolimus diffusion across the peripheral mononuclear blood cell membrane: impact of drug transporters.

Measuring tacrolimus (TAC) concentration in peripheral blood mononuclear cells (PBMCs) could better reflect the drug effect on its target (calcineurin (CaN) in lymphocytes) than whole blood concentrations. Mechanisms influencing TAC diffusion into PBMC are not well characterized. This work aimed at describing, ex vivo, TAC diffusion kinetics into PBMC and investigating the contribution of membrane transporters to regulate TAC intracellular concentration as well as the impact on CaN activity. PBMCs were incubated with TAC for 5 min to 4 h and under several experimental conditions: 37 °C (physiological conditions), 4 °C (inhibition of influx and efflux active transport), 37 °C + transporter inhibitors (verapamil, carvedilol, and probenecid and bromosulfophthalein, respectively, inhibitors of P-gp, OAT, and OATP). TAC concentration and CaN activity were measured in PBMC using liquid chromatography coupled with mass spectrometry. TAC intra-PBMC concentration was maximal after 1 h of incubation. Mean TAC PMBC concentrations were significantly lower in samples incubated at 4 °C compared to the 37 °C groups. Addition of verapamil slightly increased TAC accumulation in PBMC while other inhibitors had no effect. A significant correlation was found between TAC intra-PBMC concentration and the level of inhibition of CaN. Using an ex vivo cellular model, these results suggest that P-gp is involved in the drug efflux from PBMC while influx active transporters likely to regulate TAC intra-PBMC disposition remain to be identified. TAC concentration in PBMC is correlated with its pharmacodynamic effect. Then, TAC intra-PBMC concentration appears to be a promising biomarker to refine TAC therapeutic drug monitoring.