John R. Soglia

Minimising the potential for metabolic activation in drug discovery.

Investigations into the role of bioactivation in the pathogenesis of xenobiotic-induced toxicity have been a major area of research since the link between reactive metabolites and carcinogenesis was first reported in the 1930s. Circumstantial evidence suggests that bioactivation of relatively inert functional groups to reactive metabolites may contribute towards certain drug-induced adverse reactions. Reactive metabolites, if not detoxified, can covalently modify essential cellular targets. The identity of the susceptible biomacromolecule(s), and the physiological consequence of its covalent modification, will dictate the resulting toxicological response (e.g., covalent modification of DNA by reactive intermediates derived from procarcinogens that potentially leads to carcinogenesis). The formation of drug–protein adducts often carries a potential risk of clinical toxicities that may not be predicted from preclinical safety studies. Animal models used to reliably predict idiosyncratic drug toxicity are unavailable at present. Furthermore, considering that the frequency of occurrence of idiosyncratic adverse drug reactions (IADRs) is fairly rare (1 in 1000 to 1 in 10,000), it is impossible to detect such phenomena in early clinical trials. Thus, the occurrence of IADRs during late clinical trials or after a drug has been released can lead to an unanticipated restriction in its use and even in its withdrawal. Major themes explored in this review include a comprehensive cataloguing of bioactivation pathways of functional groups commonly utilised in drug design efforts with appropriate strategies towards detection of corresponding reactive intermediates. Several instances wherein replacement of putative structural alerts in drugs associated with IADRs with a latent functionality eliminates the underlying liability are also presented. Examples of where bioactivation phenomenon in drug candidates can be successfully abrogated via iterative chemical interventions are also discussed. Finally, appropriate strategies that aid in potentially mitigating the risk of IADRs are explored, especially in circumstances in which the structural alert is also responsible for the primary pharmacology of the drug candidate and cannot be replaced.

Trifluoromethylpyrimidine-based inhibitors of proline-rich tyrosine kinase 2 (PYK2): structure-activity relationships and strategies for the elimination of reactive metabolite formation.

The synthesis and SAR for a series of diaminopyrimidines as PYK2 inhibitors are described. Using a combination of library and traditional medicinal chemistry techniques, a FAK-selective chemical series was transformed into compounds possessing good PYK2 potency and 10- to 20-fold selectivity against FAK. Subsequent studies found that the majority of the compounds were positive in a reactive metabolite assay, an indicator for potential toxicological liabilities. Based on the proposed mechanism for bioactivation, as well as a combination of structure-based drug design and traditional medicinal chemistry techniques, a follow-up series of PYK2 inhibitors was identified that maintained PYK2 potency, FAK selectivity and HLM stability, yet were negative in the RM assay.

Importance of Early Assessment of Bioactivation in Drug Discovery

Publisher Summary Reactive metabolites have received considerable attention in drug discovery and development, and in vitro assays have been established to monitor the formation of reactive metabolites. Although each of these assays has relevance, none of the available assays provides an unambiguous link to a direct in vivo toxicological observation. A relatively simple screen to assess reactive metabolite formation is covalent binding to macromolecules using radiolabeled compounds. Alternatively, reactive, electrophilic metabolites can be trapped by the addition of exogenous nucleophiles, such as glutathione and cysteine, and the stable conjugates can be characterized by liquid chromatography–tandem mass spectrometry (LC–MS/MS) and/or nuclear magnetic resonance (NMR) spectroscopy. Advantages of the latter methodology are that no radiolabeled material is required and it is amenable to higher throughput screening. In addition, the elucidation of structure of these conjugates can provide indirect information about the structure of the electrophilic species, thereby providing insights into the bioactivation mechanism and hence a rationale on which to base subsequent chemical intervention strategies. The bioactivation of compounds by cytochrome P450 (CYP) enzymes is often accompanied by mechanism-based inactivation of these enzymes. The mechanism is distinct from that of competitive drug–drug interactions in that it results in prolonged reduction of the metabolic capacity. A context is provided to the general phenomenon of bioactivation and how to position its relevance in drug discovery and development.