Dosing equation for tacrolimus using genetic variants and clinical factors

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT • Patients with low tacrolimus troughs are at a higher risk of rejection while those with high troughs are at an increased risk for toxicity. Therefore, achieving the therapeutic range is important. • CYP3A5 genotype and days post transplant have been previously shown individually to be associated with tacrolimus troughs. WHAT THIS STUDY ADDS • This paper presents the first dosing model for tacrolimus using a combination of genetic and clinical factors in adult kidney transplant recipients. It was developed from one of the largest tacrolimus pharmacogenetic studies conducted to date (681 subjects and 11 823 trough concentrations). • We found that CL/ F was significantly influenced by days post transplant, CYP3A5 genotype, transplantation at a steroid sparing centre, recipient age and the use of a calcium channel blocker. • Our large sample size enabled us to define the distinct differences in tacrolimus CL/ F between three CYP3A5 genotype groups (*1/*1, *1/*3 and *3/*3). • This study is an important step towards using pharmacogenetic information in the clinical setting. AIM To develop a dosing equation for tacrolimus, using genetic and clinical factors from a large cohort of kidney transplant recipients. Clinical factors and six genetic variants were screened for importance towards tacrolimus clearance (CL/ F ). METHODS Clinical data, tacrolimus troughs and corresponding doses were collected from 681 kidney transplant recipients in a multicentre observational study in the USA and Canada for the first 6 months post transplant. The patients were genotyped for 2 724 single nucleotide polymorphisms using a customized Affymetrix SNP chip. Clinical factors and the most important SNPs (rs776746, rs12114000, rs3734354, rs4926, rs3135506 and rs2608555) were analysed for their influence on tacrolimus CL/ F . RESULTS The CYP3A5*1 genotype, days post transplant, age, transplant at a steroid sparing centre and calcium channel blocker (CCB) use significantly influenced tacrolimus CL/ F . The final model describing CL/ F (l h −1 ) was: 38.4 ×(0.86, if days 6–10) or (0.71, if days 11–180)×(1.69, if CYP3A5*1/*3 genotype) or (2.00, if CYP3A5*1/*1 genotype)× (0.70, if receiving a transplant at a steroid sparing centre) × (age in years/50 −0.4 ) × (0.94, if CCB is present). The dose to achieve the desired trough is then prospectively determined using the individuals CL/ F estimate. CONCLUSIONS The CYP3A5*1 genotype and four clinical factors were important for tacrolimus CL/ F . An individualized dose is easily determined from the predicted CL/ F . This study is important towards individualization of dosing in the clinical setting and may increase the number of patients achieving the target concentration. This equation requires validation in an independent cohort of kidney transplant recipients.