Oral targeted therapies in oncology: bioanalytical methods for therapeutic drug monitoring and pharmacokinetic investigations

Cancer is a major global health challenge with rising incidence and mortality rates. Advances in cancer treatment focus on oral targeted therapies, which are more effective and less toxic than traditional cytotoxic treatments. These new therapies include drugs that target proteins involved in DNA repair, such as poly(ADP-ribose) polymerase (PARP), and that regulate gene expression, like the enhancer of zeste homolog 2 (EZH2). Many of these therapies inhibit kinase proteins involved in various signaling pathways, such as cyclin-dependent kinases (CDK), Bcr-Abl, and c-Kit kinases. This thesis explores several aspects of these targeted therapies, from bioanalysis to pharmacology, focusing on PARP inhibitors (Part I), the EZH2 inhibitor tazemetostat (Part II), and kinase inhibitors such as imatinib and CDK4/6 inhibitors (Part III). Part I: PARP Inhibitors Chapter 1 reviews the clinical pharmacology of approved PARP inhibitors (olaparib, niraparib, rucaparib, and talazoparib). It highlights the potential benefit of therapeutic drug monitoring (TDM) to optimize dosing and manage variability in pharmacokinetics and drug-drug interactions. The chapter also discusses liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods for quantifying PARP inhibitors. Chapter 2 presents the development and validation of an LC-MS/MS method for the simultaneous quantification of olaparib, niraparib, and rucaparib in plasma and dried blood spot (DBS) matrices. The method was validated according to regulatory guidelines and proved reliable for TDM applications. The study also introduces a conversion strategy to estimate plasma concentrations from DBS samples, with good results for olaparib and niraparib, while preliminary but promising results were obtained for rucaparib. Chapter 3 analyzes the exposure-toxicity relationship for olaparib in cancer patients. Despite significant variability in drug exposure, no clear link between olaparib levels and toxicity was found, suggesting that routine TDM may not be beneficial during olaparib therapy. Further studies are suggested on this topic. Part II: Tazemetostat Chapter 4 reviews the pharmacology of tazemetostat, focusing on its pharmacokinetics (PK). The chapter discusses factors affecting tazemetostat absorption, distribution, metabolism, and excretion, highlighting the exposure-response relationships, drug-drug interactions and its behaviour in special populations. Chapter 5 details a UHPLC-MS/MS assay for quantifying tazemetostat in human plasma. This fast and validated method demonstrated excellent performance, and was successfully applied in a phase 1 clinical trial to evaluate the PK profile of tazemetostat when co-administered with belinostat in patients with lymphoma. Part III: Kinase Inhibitors Chapter 6 describes a novel LC-MS/MS method for quantifying imatinib and its metabolite in DBS samples. The method was validated using two volumetric devices and showed strong agreement between estimated and actual plasma concentrations, supporting the use of DBS-based TDM for imatinib, which is recommended by the authorities. Chapter 7 presents a DBS-based LC-MS/MS assay for CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) and the aromatase inhibitor letrozole. The method met regulatory guidelines and demonstrated stability under home-sampling conditions. Preliminary conversion models for estimating plasma concentrations from DBS samples were proposed, and the assay holds promise for TDM of CDK4/6 inhibitors and letrozole in clinical settings. In conclusion, this thesis provides valuable insights into the bioanalysis and pharmacology of several targeted anticancer therapies. It emphasizes the role of TDM in optimizing treatment and offers validated analytical methods for quantifying key drugs in both plasma and DBS matrices for clinical studies. These findings contribute to the ongoing development and refinement of precision oncology treatments.