Genetics of tacrolimus pharmacokinetics and kidney transplant outcomes

Abstract

Tacrolimus (TAC) is the mainstay of current immunosuppressive therapy following kidney transplantation. However, TAC pharmacokinetics (PK) are highly variable and excessive high and low whole blood trough TAC concentrations (TAC C0) have been associated with toxicity and acute rejection, respectively. Therefore, therapeutic drug monitoring (TDM), targeting TAC C0 between 5-15 ng/mL or a slightly varied range, has been adopted in most transplant centres to maximise immunosuppression and minimise toxicity. Nonetheless, a TAC C0-rejection relationship has not been adequately shown under TDM. In addition, TAC pharmacogenetic (PGx) and innate immunogenetic studies reported conflicting findings (except for CYP3A5*3) regarding their impact on dose-adjusted TAC C0 (TAC C0/D), acute rejection and kidney function. Therefore, this thesis hypothesised that: 1) TAC dispositional genes would affect TAC C0/D inter-individual variability, biopsy-proven acute rejection (BPAR) and estimated glomerular filtration rate (eGFR); 2) there would be a temporal relationship between TAC C0 and BPAR; and 3) innate immunogenetics would predict BPAR incidence. Chapters 2 and 3 investigated if CYP3A4/5, POR, ABCB1 and NR1I2 genetics affect TAC C0/D inter-individual variability, BPAR and eGFR in a retrospective cohort of 165 Australian kidney transplant recipients in the first 3 months post-transplantation. CYP3A5 expressors (*1/*1 + *1/*3) (P = 5.5×10-16) and ABCB1 61G allele carriers (P = 0.001) had lower log-transformed TAC C0/D (56% and 26% lower geometric mean TAC C0/D, respectively) and accounted for approximately 30% and 4%, respectively, of log10-transformed TAC C0/D variability in the first 3 months post-transplantation. However, CYP3A4, POR and NR1I2 genotypes did not significantly affect TAC C0/D. In addition, none of these TAC PK genes significantly affected BPAR incidence in the first 14 days, or eGFR in the first 3 months, post-transplantation. Notably, an incidental finding was that there was no significant difference in BPAR incidence between the groups with 1) TAC C0 < and ≥ 8 ng/mL, or 2) TAC C0 < and ≥ 5 ng/mL for 3 consecutive days in the first 14 days post-transplantation, prompting further investigation. Chapter 4 explored the relationship between TAC C0 and BPAR incidence in the first 14 days post-transplantation and confirmed that TAC C0 (log10-transformed) were lower on the day of (mean difference [95% confidence interval] = -0.13 [-0.24 to - 0.025], post-hoc P = 0.013), and 1 day prior to (-0.13 [-0.21 to -0.048], post-hoc P = 0.002), BPAR. Adjusting for haematocrit variability assisted to identify this temporal TAC C0-response relationship. Chapter 5 found pro- and anti-inflammatory mediator (CASP1, CRP, IL1B, IL2, IL6, IL6R, IL10, TGFB and TNF) and MyD88-dependent TLR signalling pathway (LY96, MYD88, TLR2 and TLR4) genetics did not significantly affect BPAR incidence in the first 14 days post-transplantation. Notably, IL6 -6331C/C genotype had a higher incidence of BPAR compared to T/T genotype (Odds Ratio [95% confidence interval] = 6.6 [1.7 to 25.8], likelihood-ratio test P = 0.02), whereas it was non-significant after correction for multiple comparisons (P-value threshold = 0.0038). In summary, this thesis provides the first evidence that ABCB1 61A>G, along with confirming CYP3A5*3, affects TAC C0/D variability. However, the known single nucleotide polymorphisms (SNP) in TAC dispositional genes did not affect kidney transplant outcomes, likely due to TAC TDM substantially reducing the risk of suband supra-exposure of TAC. Notably, a temporal TAC C0-rejection relationship was identified, and it was shown for the first time that accounting for haematocrit variability assisted in identifying this response relationship. None of the investigated innate immunogenetic factors predicted BPAR incidence in a relatively limited cohort of TAC-treated kidney transplant recipients. Therefore, future studies are still needed to confirm current findings and explore novel factors to predict kidney transplant outcomes.