Utilising Genotype Informed Bayesian Dosing of Tacrolimus in Children Post Solid Organ Transplantation.

This study aims to evaluate the efficacy of genotype-informed Bayesian dosing of tacrolimus in optimising drug exposure among paediatric solid organ transplant recipients. By tailoring tacrolimus dosage based on individual genetic makeup and using Bayesian modeling to predict drug levels, the researchers hope to increase the likelihood of achieving therapeutic drug concentrations while minimising the risk of adverse events associated with subtherapeutic or supratherapeutic exposure.

Tacrolimus, a calcineurin inhibitor is an effective immunosuppressant for solid organ transplants (SOT). Due to its narrow therapeutic index and individual variability in its pharmacokinetics (PK), this can lead to inefficacy, toxicities and suboptimal outcomes.

Tacrolimus is typically administered orally twice daily, with a starting dose scaled linearly to body weight (mg/kg). Dose is then adjusted based on measured steady-state trough (pre-dose) whole blood tacrolimus concentrations, to bring to within a desired "therapeutic range". However, this dosing strategy remains associated with incomplete effectiveness and toxicities in a substantial proportion of recipients, related to under- or over-exposure respectively.

Cytochrome P450 CYP3A4 and CYP3A5 enzymes metabolise tacrolimus, with research suggesting a link between the CYP3A5 genetic makeup and achieving tacrolimus target levels. Genotyping for the CYP3A5 gene prior to SOT can identify individuals who are at risk of high or low tacrolimus levels, and guide tacrolimus dosing prior to transplantation. Bayesian prediction is a pharmaco-statistical technique that uses population pharmacokinetic data and individual patient characteristics to accurately predict the tacrolimus dose required to achieve a target concentration. Subtherapeutic levels post-transplant, increases the risk of acute rejection. Furthermore, failure to maintain the target tacrolimus range for the first 6 months significantly raises the chance of rejection, donor-specific antibody formation and graft loss.

Genotype informed dosing algorithms may optimise and ameliorate sub-therapeutic levels, thus potentially reducing the risk of rejection or toxicity.

To determine if implementing a genotype-informed Bayesian dosing of tacrolimus is superior to standard weight-based dosing and empiric dose adjustment to trough concentrations post SOT, a combined retrospective/prospective cohort study in Solid Organ Transplant recipients will be undertaken at The Royal Children's Hospital Melbourne.

The outcomes from the Retrospective cohort (n=45) using clinician-led therapeutic drug monitoring will be compared with the Prospective cohort (n=45), using genotype to predict initial tacrolimus doses and predictive Bayesian dosing for ongoing tacrolimus dosing over a 12-week period.

Eligibility Criteria:
Participants will be assigned to the prospective arm if treated at Royal Children's Hospital who are receiving a solid organ transplant (SOT) (excluding repeat graft in liver transplant recipients) and who will be on tacrolimus as one of the main immunosuppressants post-transplant.

Inclusion Criteria:

* 1 to 18years old
* SOT transplant (planned or on waiting list) excluding repeat liver transplant.
* Heart OR Liver OR Renal Transplant
* Amenable to venepuncture and blood draw
* Patient and/or parent consented to the study.

Exclusion Criteria:

* older than 18 year old
* younger than 1 year old
* Previous liver transplant.
* Lung OR Intestinal transplant.
* For prospective arm, patient has had a SOT prior to receiving genotyping results.
* Patient has a known hypersensitivity to tacrolimus and/or its formulation.
* On a slow release preparation of Tacrolimus (e.g Advagraf extended release Brand)
* Patient has a life expectancy estimated to be less than 12-weeks by the treating clinical team.
* Patient and/or parent is unable to consent to the study.
* Patient and/or parent is unwilling to take part in the study.