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Sirolimus, an immunosuppressive agent used in renal transplantation, can prevent allograft rejection. Identification of the therapeutic index (the ratio of minimum toxic concentration to minimum therapeutic concentration) for immunosuppresants is necessary to optimize the care of patients and set standards for bioequivalence evaluation of sirolimus products. However, the therapeutic index for sirolimus has been inconsistently defined, potentially because of inconsistencies in sirolimus exposure-response relationships.The authors used retrospective therapeutic drug monitoring data from the electronic health records of patients treated in a tertiary health care system from 2008 to 2014 to (1) develop a population pharmacokinetic (PK) model, (2) use the model to simulate sirolimus concentrations, and (3) characterize the exposure-response relationship. Using Wilcoxon rank-sum and Fisher exact tests, the authors determined relationships between sirolimus exposure and adverse events (AEs) (anemia, leukopenia, thrombocytopenia, hyperlipidemia, and decline in renal function) and the composite efficacy end point of graft loss or rejection.The developed 2-compartment population PK model showed appropriate goodness of fit. In a late-phase (12 months), postrenal transplant population of 27 inpatients, the authors identified statistically significant relationships between 83 simulated peak and trough sirolimus concentrations and outcomes: graft loss or rejection (P = 0.018) and decline in renal function (P = 0.006), respectively.Use of therapeutic drug monitoring results and PK modeling permitted correlation of sirolimus concentrations with graft loss or rejection and decline in renal function. However, the method was limited in its assessment of other AEs. To better evaluate sirolimus exposure-response relationships, the method should be applied to a larger sample of newly transplanted patients with a higher propensity toward AEs or efficacy failure.

Recent concerns have been raised by neurologists and patients with epilepsy regarding the bioequivalence of generic lamotrigine to the brand Lamictal. Bioequivalence studies require the quantification of lamotrigine in human plasma, including in the presence of other drugs, for studies that will use patients with epilepsy rather than healthy volunteers.Lamotrigine was extracted from plasma through a simple protein precipitation and analyzed by fast liquid chromatography coupled to heated electrospray ionization with tandem mass spectrometric detection. A backflush step to remove interferent accumulation on column was included, and a stable isotope-labeled lamotrigine was used as an internal standard. The method was validated for accuracy, precision, interday and intraday coefficient of variation, specificity, lower limit of detection, lower limit of quantification, linearity, range, instrument precision, freeze-thaw, dilution integrity, and sample stability. Specificity evaluation included consideration of the impact of other antiepileptic drugs.The described method has a linear range of 8-10,000 ng/mL of lamotrigine (r = 0.9999) and a lower limit of detection of 1 ng/mL and a lower limit of quantification of 8 ng/mL. Intraday and interday reproducibility were less than 10.0% relative SD and 10.4% relative SD, respectively, and the percent recovery varied from 96.6% to 109.3% at various lamotrigine concentrations. A backflush step reduced matrix effects and no interference peak from plasma or other antiepileptic drugs were observed.A liquid chromatography-heated electrospray ionization-tandem mass spectrometry method including a backflush step was developed and validated to measure lamotrigine concentration in patient plasma. The method will be applied to bioequivalence studies that compare brand versus generic lamotrigine.