QSPR modelling of in vitro degradation half-life of acyl glucuronides

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Acyl Glucuronide and Coenzyme A Thioester Metabolites of Carboxylic Acid-Containing Drug Molecules: Layering Chemistry with Reactive Metabolism and Toxicology

Glucuronidation and CoA (coenzyme A) conjugation are common pathways for the elimination of carboxylic acid-containing drug molecules. In some instances, these biotransformations have been associated with toxicity (such as idiosyncratic hepatic injury, renal impairment, hemolytic anemia, gastrointestinal inflammation, and bladder cancer) attributed to, in part, the propensity of acyl glucuronides and acyl CoA thioesters to covalently modify biological macromolecules such as proteins and DNA. It is to be noted that, while acyl glucuronidation and CoA conjugation are indeed implicated in adverse effects, there are many safe drugs in the market that are cleared by these reactive pathways. It is therefore important that new molecular entities with carboxylic acid groups are evaluated for toxicity in a manner that is not unreasonably risk-averse. In the absence of truly predictable methods, therefore, the general approach is to apply a set of end points to generate a weight-of-evidence evaluation. In practice, the focus is to identify structural liabilities and provide structure-activity recommendations early in the program, at a stage where an attempt to improve reactive metabolism does not deoptimize other critical drug-quality criteria. This review will present a high-level overview of the chemistry of glucuronidation and CoA conjugation and provide a discussion of the possible mechanisms of adverse effects that have been associated with these pathways, as well as how such potential hazards are addressed while delivering a new chemical entity for clinical evaluation.

Relationship between the risk of idiosyncratic drug toxicity and formation and degradation profiles of acyl-glucuronide metabolites of nonsteroidal anti-inflammatory drugs in rat liver microsomes

Acyl glucuronides (AGs) are considered to cause idiosyncratic drug toxicity (IDT), and evaluating the chemical instability of AGs may be useful for predicting the IDT risk of novel drug candidates. However, AGs show variations in their chemical instability, degree of formation, and enzymatic hydrolysis. Therefore, we evaluated the degree of AG formation, enzymatic hydrolysis, and chemical instability in liver microsomes and their relationship with IDT risk. Nonsteroidal anti-inflammatory drugs (NSAIDs) were classified into three categories in terms of their IDT risk as parent drugs: safe (SA), warning (WA), and withdrawn (WDN). To evaluate the enzymatic and non-enzymatic degradation of AG, the parent drugs were incubated with rat liver microsomes in the absence or presence of AG hydrolase inhibitors. The degree of AG formation and disappearance was considered as the rate constant. For all NSAIDs investigated, the number of AGs formed notably increased following addition of AG hydrolase inhibitors. Particularly, AG was produced by WDN drugs at a lower level than that produced by WA and SA drugs in the absence of AG hydrolase inhibitors but was significantly increased after adding AG hydrolase inhibitors. The rate constants of AG formation and non-enzymatic AG disappearance did not significantly differ among the WDN, WA, and SA drugs, whereas the rate constant of enzymatic AG disappearance of WDN drugs tended to be higher than those of WA and SA drugs. In conclusion, we evaluated the enzymatic degradation and chemical instability of AG by simultaneously producing it in liver microsomes. This method enables evaluation of AG degradation without preparing AG. Moreover, we determined the relationship between enzymatic AG degradation in rat liver microsomes and IDT risk.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

Acyl glucuronide reactivity in perspective

Acyl glucuronidation is a common metabolic fate for acidic drugs and their metabolites and, because these metabolites are reactive, they have been linked to adverse drug reactions (ADRs) and drug withdrawals. However, alternative routes of metabolism leading to reactive metabolites (e.g., oxidations and acyl-CoA thioesters) mean that unambiguous proof that acyl glucuronides are toxic is lacking. Here, we review the synthesis and reactivity of these metabolites, and describe the use of molecular modelling and in vitro and in vivo reactivity assessment of acyl glucuronide reactivity. Based on the emerging structure-dependent differences in reactivity and protein adduction methods for risk assessment for acyl glucuronide-forming acid drugs or drug candidates in drug discovery/development are suggested.