The utility of trough mycophenolic acid levels for the management of lupus nephritis

ABSTRACT Background Monitoring of mycophenolic acid (MPA) levels may be useful for effective mycophenolate mofetil (MMF) dosing. However, whether commonly obtained trough levels are an acceptable method of surveillance remains debatable. We hypothesized that trough levels of MPA would be a poor predictor of area under the curve (AUC) for MPA. Methods A total of 51 patients with lupus nephritis who were on MMF 1500 mg twice a day and had a 4-h AUC done were included in this study. MPA levels were measured prior to (C0) and at 1 (C1), 2 (C2) and 4 (C4) h, followed by 1500 mg of MMF. The MPA AUC values were calculated using the linear trapezoidal rule. Regression analysis was used to examine the relationship between the MPA trough and AUC. Differences in the MPA trough and AUC between different clinical and demographic categories were compared using t-tests. Results When grouped by tertiles there was significant overlap in MPA, AUC 0-4 and MPA trough in all tertiles. Although there was a statistically significant correlation between MPA trough levels and AUC, this association was weak and accounted for only 30% of the variability in MPA trough levels. This relationship might be even more unreliable in men than women. The use of angiotensin-converting enzyme inhibitors or angiotensin receptor blockers was associated with increased MPA trough levels and AUC at 0-4 h (AUC0–4). Conclusion Trough levels of MPA do not show a strong correlation with AUC. In clinical situations where MPA levels are essential to guide therapy, an AUC0–4 would be a better indicator of the adequacy of treatment.

Pharmacokinetics of Tedizolid in Adults with Cystic Fibrosis

Background The presence of MRSA in the airways of patients with CF is associated with more rapid lung function decline and a higher mortality. Tedizolid (TDZ) is an oxazolidinone antibiotic with potent activity against MRSA; however, the pharmacokinetics (PK) in CF have not been described. The purpose of this study was to determine the PK of IV/PO tedizolid in plasma in patients being treated for acute pulmonary exacerbations. Methods We conducted a prospective, multiple dose, randomized, crossover study. TDZ phosphate was administered as 200 mg IV over 1h or PO once daily x 3 under fed conditions, with a 2-day washout, followed by crossover. Laboratory studies were performed throughout the study as routine clinical care. Blood samples were obtained prior to the Third dose of IV and PO and at 8 additional timepoints over 48 hours. TDZ concentrations in plasma were determined by LC-MS/MS. The maximum concentration (Cmax) and time to maximum (Tmax) were obtained from the measured data. Area-under-the concentration curve (AUC24) was determined using the linear trapezoidal rule. Compartmental PK was performed using ADAPT 5 software. Data are described by mean ± SD. Results The patients (4M, 2F) had a mean age of 27 years (22–32), BMI 22.0 ± 4.2 kg/m2, and predicted creatinine clearance of 128 ± 44 mL/minute. There was one report of diarrhea and hoarseness of voice that was unlikely related to the study drug. There were no clinically relevant laboratory changes. The Cmax, Tmax, and AUC24 following IV and PO administration were 2.87 ± 0.64 mg/L / 2.26 ± 0.70 mg/L, 1.32 ± 0.49 hours /2.06 ± 1.29 hours, and 27.1 ± 4.74 mg/L x hours / 25.8 ± 6.03 mg/L x hours respectively. The PK parameters for TDZ were described by a 1-compartment model: Elimination rate constant (Kel) = 0.088 ± 0.014 hour-1, Volume of distribution (V) = 96.4 ± 27.4 L, absorption rate constant (Ka) = 0.869 ± 0.978 hours−1, and bioavailability (F) = 1.00 ± 0.101. Conclusion The oral bioavailability of TDZ in patients with CF is complete and the pharmacokinetics are similar to that reported for healthy volunteers indicating no dose adjustments are needed in future studies evaluating the efficacy and safety in patients with CF. Disclosures All authors: No reported disclosures.

Pharmacokinetic study of paclitaxel and gefitinib in combination

13123 Background: Both gefitinib and paclitaxel are metabolized by CYP3A4, thus co-administration may result in a pharmacokinetic interaction (Miller V.A. et al. JCO 2003, 21:2094–2100). Paclitaxel is formulated in Cremophor EL (crEL), which also has the potential for pharmacokinetic interactions by either altering protein binding or inhibition of P-glycoprotein transporter systems (Gelderblom H. et al. EJC 2001, 37:1590–1598). Gefitinib is a high extraction ratio drug so changes in protein binding are not a concern, but inhibition of P-glycoprotein transporter systems could change gefitinib’s absorption profile. Methods: In a Phase II study, 17 patients (pts) with metastatic breast cancer received paclitaxel (100 mg/m2 on days 8, 15 q21) with gefitinib (250 mg daily, from day 1 to day 15). Blood samples were collected to measure plasma concentrations of gefitinib on days 7, 8, and 15 from six pts at pre-dose and at 3, 7 and 25 hours after gefitinib administration. The AUC at steady-state (AUCss) was calculated by the linear trapezoidal rule using WinNonlin and the minimum concentration at steady-state (Css, min) was taken directly from the data. Results: The effect of paclitaxel or crEL on the exposure to gefitinib was assessed by comparing the AUCss and Css,min on days 8 and 15 in the presence of paclitaxel to those on day 7 when gefitinib was alone. The geometric mean AUCss increased by 30 to 42% and Css,min by 28 to 58% in the presence of paclitaxel. Individual ratios of days 8 and 15 to day 7 also showed a trend to be greater than 1.0, further indicating an increase in exposure to gefitinib in the presence of paclitaxel. Conclusions: Steady state exposure to gefitinib increased by about 30 to 40% in the presence of paclitaxel. As the increase on day 15 was similar to that on day 8, the effect of paclitaxel appears to be transient, and would be unlikely to affect the safety profile of 250 mg gefitinib when in combination with paclitaxel compared to gefitinib monotherapy. [Table: see text]