Thomas M. Polasek

In vitro approaches to investigate mechanism-based inactivation of CYP enzymes

Mechanism-based inactivation (MBI) of human drug-metabolising CYP enzymes is an important consideration in the preclinical ADME evaluation of new drug candidates. In this report, the in vitro approaches used to investigate MBI of CYP enzymes are described, with an emphasis on the characterisation required to assess potential drug–drug interactions. Recent disparities in MBI data between in vitro test systems are also reviewed, highlighting the limitations of Escherichia coli-expressed human recombinant CYP in the prediction of drug–drug interactions that arise via MBI.

Challenges and Limitations in the Interpretation of Systematic Reviews: Making Sense of Clopidogrel and CYP2C19 Pharmacogenetics

From 2010 to 2012, nine systematic reviews reported highly variable conclusions regarding the association between carriage of a cytochrome P450 2C19 loss‐of‐function allele and the risk of adverse cardiovascular (CV) events in individuals using clopidogrel. Possible contributors to the variable findings include differences in patient populations, CV end points, and statistical models utilized by the systematic reviews, as well as unexplained heterogeneity, inconsistent/incomplete reporting, and risk of publication bias with respect to the primary studies.

Pharmacokinetic evaluation of teriflunomide for the treatment of multiple sclerosis

Introduction: Teriflunomide is an immunomodulatory drug that received FDA approval for the treatment of relapsing forms of multiple sclerosis (MS) in September 2012. Its primary mode of action is inhibition of dihydroorotate dehydrogenase which inhibits the proliferation of activated T cells, but it also has a number of other actions that may be important contributors to its efficacy in MS. Areas covered: This review covers a basic pathophysiology of MS and the current treatment options, including a discussion of the needs for additional treatments. The main focus of the review is the pharmacokinetics and pharmacodynamics of teriflunomide, including a brief comparison with the disease-modifying antirheumatic drug leflunomide. The authors discuss the clinical efficacy and toxicity profile of teriflunomide and make some comparisons with treatments that are currently, or soon to be available. Expert opinion: While teriflunomide is no more effective than a number of other agents that are used in the treatment of MS, it has a favorable side-effect profile and the convenience of once a day oral administration. As such, it is likely to be a popular agent in the treatment of MS over the next 5 years.

Systematic review and meta-analysis of the association between cytochrome P450 2C19 genotype and bleeding

Systematic review and meta-analysis of the association between cytochrome P450 2C19 genotype and bleeding -

Metabolic activation of clopidogrel: in vitro data provide conflicting evidence for the contributions of CYP2C19 and PON1

The recent report that clopidogrel efficacy may be more dependent on paraoxonase-1 (PON1) than on cytochrome P450 2C19 (CYP2C19) activity raises questions about the roles of these and other enzymes in clopidogrel activation. To provide insight into the emerging PON1 versus CYP2C19 debate, this commentary summarizes the clinical evidence on the pharmacokinetic determinants of clopidogrel efficacy. We then review the in vitro studies investigating the enzymes involved in clopidogrel activation, and comment on their strengths and limitations. There is agreement amongst in vitro studies regarding the involvement of CYP1A2 and CYP2B6 in the metabolism of clopidogrel to 2-oxo-clopidogrel. However, the evidence for other CYP enzymes in the first activation step (e.g. CYP2C19 and CYP3A4) is inconsistent and dependent on the in vitro test system and laboratory. All major drug metabolizing CYP enzymes are capable of converting 2-oxo-clopidogrel to sulfenic acid intermediates that subsequently form the active thiol metabolite. However, the extent of CYP involvement in this second step has been challenged, and new evidence suggests that CYP-independent hydrolytic cleavage of the thioester bond may be more important than oxidative metabolism.

The In Vitro Characterization of Inhibitory Drug-Drug Interactions Involving UDP-Glucuronosyltransferase

Inhibition of UDP-glucuronosyltransferase (UGT) activity gives rise to both drug–drug (DDIs) and drug–endobiotic interactions in vivo. Furthermore, several glucuronides have been shown to reduce the metabolic clearances of cytochrome P4502C8 substrates. Experimental paradigms, based on the use of human liver microsomes (HLM), hepatocytes, and recombinant UGTs as the enzyme sources, are now available for the investigation of drug glucuronidation in vitro including the prediction and characterization of DDIs. The reaction phenotyping of drug glucuronidation is becoming increasingly feasible with the availability of ‘batteries’ of recombinant enzymes along with substrate and inhibitor probes for the major hepatic drug metabolizing UGTs. However, the occurrence of homo- and heterotropic activation potentially complicates screening for DDIs in vitro. Recent advances in knowledge of factors that influence UGT activity in vitro have also led to the development of experimental approaches that accurately predict the magnitude of known DDIs involving glucuronidated drugs, but further work in this area is required to demonstrate the generalizability of these models.

In Vitro Characterization of the Human Liver Microsomal Kinetics and Reaction Phenotyping of Olanzapine Metabolism

Olanzapine (OLZ) is an atypical antipsychotic used in the treatment of schizophrenia and related psychoses. The metabolism of OLZ is complex and incompletely characterized. This study aimed to elucidate the enzymes and pathways involved in the metabolism of OLZ and to determine the kinetics of OLZ oxidation and glucuronidation by human liver microsomes, recombinant cytochrome P450 (rP450) enzymes, and recombinant UDP-glucuronosyltransferase (rUGT) enzymes. An ultra-performance liquid chromatography–mass spectrometry method was developed and validated to quantify OLZ, its four oxidative metabolites (N-desmethyl-OLZ, 2-hydroxymethyl-OLZ, 7-hydroxy-OLZ, and OLZ-N-oxide), and two N-glucuronides (OLZ-10-N-glucuronide and OLZ-4′-N-glucuronide). Consistent with previous reports, UGT1A4, CYP1A2, and flavin-containing monooxygenase 3 play major roles in catalyzing the formation of OLZ-10-N-glucuronide, 7-hydroxy-OLZ, and OLZ-N-oxide, respectively. In addition, a previously uncharacterized major contribution of CYP2C8 to OLZ-N-demethylation was demonstrated. The kinetics of OLZ metabolite formation (Km and Vmax) by human liver microsomes, rP450 enzymes, and rUGT enzymes were characterized in the presence of bovine serum albumin [2% (w/v)]. Consistent with the known effect of bovine serum albumin on CYP1A2, CYP2C8, and UGT1A4 activities, Km values reported here are lower than previously reported values for OLZ metabolic pathways. In addition to CYP1A2-mediated OLZ-N-demethylation, these results suggest that other P450 enzymes, particularly CYP2C8, contribute significantly to oxidative OLZ metabolism through catalysis of OLZ-N-demethylation.

Prediction of olanzapine exposure in individual patients using physiologically based pharmacokinetic modelling and simulation

The aim of the present study was to predict olanzapine (OLZ) exposure in individual patients using physiologically based pharmacokinetic modelling and simulation (PBPK M&S).

Pharmacists in Australian general practice: an opportunity for expertise in precision medicine

At the end of May the Australian Medical Association (AMA) released a proposal for the Australian Commonwealth Government to establish a funding model to integrate nondispensing pharmacists into general practice – the Pharmacists in General Practice Incentive Programme (PGPIP) [Australian Medical Association, 2015]. This is built on joint work by the AMA and the Pharmaceutical Society of Australia. A similar collaboration exists in the UK between the Royal College of General Practitioners and the Royal Pharmaceutical Society, with a joint statement promoting the role of pharmacists in general practitioner (GP) surgeries released this February [Royal College of General Practitioners and Royal Pharmaceutical Society, 2015]. The Australian PGPIP proposal nicely summarizes the evidence to support improved quality use of medicines (QUM) and decreased adverse drug events when pharmacists work directly alongside GPs. Independent economic analysis showed cost savings of

Optimal sampling of antipsychotic medicines: a pharmacometric approach for clinical practice

A545 million to the Australian healthcare system over 4 years, primarily through fewer avoidable hospital admissions and a reduction in the utilization of medicines [Deloitte Access Economics, 2015]. The PGPIP funding proposal is a very important step in expanding the role of pharmacists in Australian primary care. ‘Practice pharmacists’ would become commonplace in GP clinics, like practice nurses, and would focus exclusively on high quality medicines management. As outlined in the proposal, nondispensing pharmacists in general practice would: conduct medication management reviews give patients medication advice to increase compliance and medication optimization support GP prescribing liaise with outreach services and hospitals when patients are discharged on complex medicine regimens update GPs on new drugs conduct quality and medication safety audits develop and manage drug-safety monitoring systems [Australian Medical Association, 2015]. In addition to these activities, we believe that pharmacists in general practice could become experts on the clinical utility of ‘precision medicine’, in particular, new technologies designed to improve drug efficacy and safety. Precision medicine has recently been defined as ‘treatments targeted to the needs of individual patients on the basis of genetic, biomarker, phenotypic or psychosocial characteristics that distinguish a given patient from other patients with similar clinical presentations’ [Jameson and Longo, 2015]. This concept is not new, but rapid advances in medical science, particularly in oncology with the molecular classification of solid tumours [Gillis et al. 2014], have accelerated research efforts to realize this concept more broadly in clinical practice. Importantly, next-generation DNA-sequencing costs are decreasing to a point that in the near future it will be affordable to sequence the whole genome and store it in an electronic health record, thus allowing real-time access to a wealth of information during consultations. In the State of the Union address in January, President Obama launched ‘a new Precision Medicine Initiative to bring us closer to curing diseases like cancer and diabetes’, pledging