Having a clinical drug-induced liver injury (DILI) signal is a surefire way to kill a drug candidate in non-serious indications like pain or chronic indications like diabetes, and will even give oncologists second thoughts as cancer patients are living longer. What can we do to prevent it?
Plan A: Small Pill and No Greasebombs
A widely accepted strategy for minimizing DILI risk is “the rule of two” – avoid lipophilic drugs (LogP > 3) which require high doses (> 100 mg) (Figure 1). (Chen, M. et al. “High lipophilicity and high daily dose of oral medications are associated with significant risk for drug-induced liver injury.” Hepatology, 2013, 58, 388-396. doi: 10.1002/hep.26208)
This makes sense as lipophilic compounds tend to be more promiscuous (For a great recent article worthy of a separate article, see: Brown, D. G. et al, “Promiscuity of in Vitro Secondary Pharmacology Assays and Implications for Lead Optimization Strategies.” J. Med. Chem. 2019, doi: 10.1021/acs.jmedchem.9b01625).
They’re also more susceptible to bioactivating P450 oxidations, and lower doses reduce the risk that the liver’s detoxifying pools of glutathione (5-10 mM) can be depleted. Compounds that can be given at a lower dose are also likely to be much higher quality as high lipophilic efficiencies are often necessary for a drug to provide full target engagement in whole blood with good PK, but this is a lengthly discussion I’ll save for another post!)
The “one rule to rule them all” in minimizing drug attrition due to off-target toxicity might be: target efficacious doses as low as possible (<10 mg ideally, Cmax <1 uM). A well-known analysis by Antonia Stepan and colleagues showed that low dose drugs (<100 mg) were less likely to have idiosyncratic adverse drug reactions with any target organ (Figure 2). (Stepan, A. F. et al. “Structural Alert/Reactive Metabolite Concept as Applied in Medicinal Chemistry to Mitigate the Risk of Idiosyncratic Drug Toxicity: A Perspective Based on the Critical Examination of Trends in the Top 200 Drugs Marketed in the United States.” Chem. Res. Toxicol. 2011, 24, 1345-1410. doi: 10.1021/tx200168d)
Unfortunately, binding pockets, biology, budgets, and timelines rarely make a 5 mg daily dose possible or practical. (With the multi-million-dollar-a-month $$ burn rate most project teams and startups deal with, perfect really is the enemy of good.) Plus, plenty of perfectly reasonable-looking, low-dose compounds have been withdrawn or given black box warnings due to observed hepatotoxicity (some examples in Figure 3, below). So what else can we do?
Plan B: De-risk Mechanistically
The mechanisms of DILI and some additional strategies scientists can use to minimize DILI risk in drug candidates were recently excellently summarized by an impressive consortium of scientists including from Servier, AbbVie, Merck, Lundbeck, GlaxoSmithKline, AstraZeneca, Janssen, Orion, and others including B. Kevin Park from the University of Liverpool (briefly summarized in Figure 4). (Weaver, R. J. et al. “Managing the challenge of drug-induced liver injury: a roadmap for the development and deployment of preclinical predictive models.” Nat. Rev. Drug Discov. 2019, https://www.nature.com/articles/s41573-019-0048-x) In this review they summarize the six main mechanisms by which drug-induced liver injury is thought to occur (Figure 4), along with a detailed description of assays and models available to assess whether any of these mechanisms are relevant to a molecule of interest.
The best human evidence for these mechanisms’ contribution to DILI is summarized in Table 1, below, along with a greatest-bang-for-your-buck assay for initially assessing whether any of these mechanisms are relevant to your molecule of interest. For example, polarized, functional hepatocyte assays where bile-canaliculi are recommended by the authors for assessing bile flow perturbation over assays measuring single transporters like BSEP. This is because not all drugs which cause cholestasis inhibit BSEP, and it was recently concluded that the inverted vesicle assay for BSEP inhibition were not predictive of the toxic potential of drugs. This is a rough guide for where to start an assessment, but more detailed risk assessment requires input and experimentation from a toxicologist. Getting a clean read in variants of these assays can definitely give a team peace of mind, but a signal in any of these assays is still just a starting point for a full risk evaluation discussion with the project team. Most of these key assays or variants of these are already incorporated in most companies’ late-stage compound evaluation cascades, but it’s worth checking some of these earlier if you have reason to suspect your lead series might be especially problematic (e.g. lipophilic amines w/ LogD ~ 4+).
For those newer to drug discovery, I hope this served as a helpful starting point for how to minimize DILI risk. For the more experienced, I hope this serves as a useful reminder for where to start your next conversation with your toxicologist.
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