Importance of hematocrit for a tacrolimus target concentration strategy.
OBJECTIVE
To identify patient characteristics that influence tacrolimus individual dose requirement in kidney transplant recipients.


METHODS
Data on forty-four 12-h pharmacokinetic profiles from 29 patients and trough concentrations in 44 patients measured during the first 70 days after transplantation (1,546 tacrolimus whole blood concentrations) were analyzed. Population pharmacokinetic modeling was performed using NONMEM 7.2®.


RESULTS
Standardization of tacrolimus whole blood concentrations to a hematocrit value of 45 % improved the model fit significantly (p<0.001). Fat-free mass was the best body size metric to predict tacrolimus clearance and volume of distribution. Bioavailability was 49 % lower in expressers of cytochrome P450 3A5 (CYP3A5) than in CYP3A5 nonexpressers. Younger females (<40 years) showed a 35 % lower bioavailability than younger males. Bioavailability increased with age for both males and females towards a common value at age >55 years that was 47 % higher than the male value at age <40 years. Bioavailability was highest immediately after transplantation, decreasing steeply thereafter to reach its nadir at day 5, following which it increased during the next 55 days towards an asymptotic value that was 28 % higher than that on day 5.


CONCLUSIONS
Hematocrit predicts variability in tacrolimus whole blood concentrations but is not expected to influence unbound (therapeutically active) concentrations. Fat-free mass, CYP3A5 genotype, sex, age and time after transplant influence the tacrolimus individual dose requirement. Because hematocrit is highly variable in kidney transplant patients and increases substantially after kidney transplantation, hematocrit is a key factor in the interpretation of tacrolimus whole blood concentrations.
Department of Transplant Medicine, Oslo University Hospital, Rikshospitalet, Oslo, Norway, elisabet.storset@gmail.com.
Elisabet
E
Størset
Nick
N
Holford
Karsten
K
Midtvedt
Sara
S
Bremer
Stein
S
Bergan
Anders
A
Åsberg
D000328:Adult; D000368:Aged; D001682:Biological Availability; D051544:Cytochrome P-450 CYP3A / Q000235:genetics*; D005260:Female; D005838:Genotype; D006400:Hematocrit; D006801:Humans; D007166:Immunosuppressive Agents / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D016030:Kidney Transplantation; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D016559:Tacrolimus / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D055815:Young Adult
D007166:Immunosuppressive Agents; C510164:CYP3A5 protein, human; D051544:Cytochrome P-450 CYP3A; D016559:Tacrolimus