Tacrolimus exhibits significant inter- and intra-individual variability of its absorption and metabolism. Because of this variability, standard dosing is not an accurate predictor of drug exposure. In clinical use, tacrolimus whole blood trough concentrations are measured to ensure efficacy and safety. Furthermore, the relatively low bioavailability of tacrolimus is thought to be a result of the combination of poor water-solubility, pre-systemic metabolism of tacrolimus in the gastrointestinal tract and activity of the P-glycoprotein efflux pump found in the enterocytes of the GI tract. Tacrolimus is extensively metabolized by the cytochrome P-450 system (CYP3A). The plasma protein binding of tacrolimus is approximately 99%. Tacrolimus is bound mainly to albumin and alpha-1-acid glycoprotein. The distribution of tacrolimus between blood and plasma depends on several factors including hematocrit, temperature at the time of plasma separation, drug concentration, and plasma protein concentration.

Pharmacodynamic studies have revealed that, depending on time following transplantation, maintaining whole blood trough levels between 5 and 20 ng/mL provides adequate protection against acute rejection and limits the occurrence of adverse events. The management of tacrolimus blood levels is complicated by variable intra- and inter-patient absorption, interaction with food and concomitant medications, and the relatively low bioavailability of tacrolimus from the Prograf formulation (17 ± 10% in adult kidney transplant patients).

Previous studies examining immunosuppressants have shown that drug levels in the immediate post-transplant period are a major determinant of subsequent acute cellular rejection. It is known that tacrolimus (TAC) \< 10 ng/mL is associated with increased rates of acute cellular rejection by one month post-transplant.

There is controversy regarding the appropriate dosing weight to use for immunosuppressants (IS). Weights use range from ideal body weight (IBW) to total body weight (TBW) depending on the institution and drug being dosed. This becomes particularly important in the obese population when there are significant differences between IBW and TBW. Our institution has always used IBW for the dosing of all IS due to concerns for nephrotoxicity with initial high blood levels of tacrolimus. The concern in obese patients is that the investigators are underdosing this population that could be at higher risk for rejection due to higher circulating concentrations of pro-inflammatory cytokines. The introduction of the novel use of a robotic transplantation procedure at our institution for this patient population has led to increasing numbers of transplant in obese recipients; therefore, the investigators decided to re-evaluate our dosing protocol. Data from an internal study at UIC show that our use of IBW for tacrolimus dosing is not sufficient for the obese population (body mass index \[BMI\] ≥30). The dose used through month 3 was closer to 0.1 mg/kg/day when total body weight was utilized. However, the use of an adjusted body weight (aBW) is common for medication dosing in obese patients. Adjusted body weight is calculated if the TBW is greater than 30% of the calculated IBW. aBW = IBW + 0.4(TBW - IBW). There is limited data available supporting the use of either IBW or aBW in dosing tacrolimus within obese patients as these patients are typically excluded from most clinical trials, particularly the pharmacokinetic trials. In addition, no literature is available comparing the two dosing weights to determine which leads to therapeutic concentrations most effectively.

Summary and Present Study Tacrolimus extended release (Astagraf) has recently been approved by the FDA as a once a day dosing regimen. This formulation has the potential to improve compliance. Current dosing recommendation for the extended release formulation in renal transplant is 0.15 mg/kg/day administered once daily in the morning. There are no specifications on appropriate dosing in obese patients or on whether to use actual, ideal or and adjusted weight. It will be advantageous to understand the pharmacokinetics of this medication in the obese to determine the appropriate dosing regimen. In this study, obese patients will be randomized to receive tacrolimus extended release 0.15 mg/kg/day based on either ideal body weight (IBW) or adjusted body weight (aBW).