Pharmacokinetics and pharmacogenetics of actinomycin D in children with cancer /

Abstract

Actinomycin D (Act D) has been used successfully to treat several cancers for over 50 years and continues to play a key role in the treatment of paediatric tumours. The aim of this thesis was to investigate Act D pharmacokinetic variability in children with cancer, alongside in vitro and in vivo drug transporter and pharmacogenetic studies. The transport of Act D by ABC transporter proteins was investigated in chapter 2. Growth inhibition and intracellular and cellular efflux assays in MDCKII cells demonstrated that Act D was a substrate for ABCB1, ABCC2 and to a lesser extent ABCC1. The in vivo relevance of these findings was assessed using knockout mouse models in chapter 3. Act D pharmacokinetics was investigated in Abcb1a/1b and Abcc2 knockout mice. Abcb1a/1b-/- mice had 1.6-fold higher AUC0-6h compared to wild-type (WT). Abcc2-/- mice exhibited an Act D AUC0-6h of 76% that seen in WT mice. Brain concentrations of Act D were also significantly higher in Abcb1a/1b-/- mice compared to WT, although no difference was observed in liver and kidney concentrations. These findings suggest that Abcb1a/1b and possibly Abcc2 play an important role in Act D pharmacokinetics but have only a minimal impact on tissue distribution. Following this the pharmacokinetics of Act D in children with cancer was investigated. Analysis of pharmacokinetic data from 74 patients showed large inter-patient variability in Act D pharmacokinetics. Clearance ranged from 15 – 341 ml/min and was positively correlated to patient age and body size. A 10-fold range in clearance normalised to body surface area was observed, indicating that factors other than body size may be involved in determining Act D pharmacokinetics. Finally, the relevance of ABCB1 and ABCC2 SNPs to Act D pharmacokinetics was investigated in 64 patients. Genotyping analysis of 3435C>T and 2677G>T/A, showed that patients with more than one variant allele in these two SNPs had 1.7-fold higher Act D clearance compared to patients who were WT at both SNPs (200 versus 115 ml/min/m2, respectively). The work presented in this thesis has advanced our understanding of Act D pharmacokinetics and the potential factors that influence patient exposure. Act D has been confirmed as a substrate for both ABCB1 and ABCC2, and these transporters have been shown to influence Act D pharmacokinetics in vivo. Patient variability in Act D exposure has been confirmed in a large patient cohort, and it has been demonstrated that some of this variability can be attributed to ABCB1 genotype.