Transporters and Drug-Metabolizing Enzymes in Drug Toxicity. Albert P. Li
events: Upon entry of a drug and its enteric metabolites into the portal circulation, the following events occur:
1 Entry into hepatocytes via diffusion or transporter‐mediated uptake.
2 Metabolism by drug‐metabolizing enzymes in the hepatocytes.
3 Excretion of parent drug and/or metabolites to the intestine as bile via diffusion or efflux transporters followed by excretion into the feces or reentry into the portal circulation (enterohepatic recirculation) via diffusion or uptake transporters.
4 Entry into the systemic circulation.
Extrahepatic events: Upon entry of a drug and its hepatic metabolites into the systemic circulation, the following events occur:
1 Entry into extrahepatic target cells via diffusion or transporter‐mediated uptake.
2 Metabolism by drug‐metabolizing enzymes in the target cells.
3 Exit of parent drug and/or metabolites from the target cells to the systemic circulation.
4 Excretion via urinary, perspiratory, respiratory pathways.
The manifestation of toxicity of the drug in question, either due to the parent molecule or its metabolites, is dependent on the concentration of the toxic moiety at the ultimate target. Metabolism (metabolic clearance, activation, and detoxification), uptake transport, and efflux transport can play critical roles on whether the toxic moiety will reach the critical concentration leading to the onset of toxicity.
1.3 The Multiple Determinant Hypothesis for Idiosyncratic Drug Toxicity
A perplexing observation in drug toxicity is the phenomenon of idiosyncratic drug toxicity which occurs at an incidence of less than 1 per 5000 patient‐years, thereby cannot be detected with conventional regulatory clinical trials but would be clearly identified after marketing of a drug with a large patient population being exposed (1–5). To explain this phenomenon and to provide a possible mitigating strategy, I have proposed the multiple determinant hypothesis (3) which states that idiosyncratic drug toxicity occurs due to a confluence of multiple discrete events in the individual succumbing to drug toxicity (Figure 1.1).
Figure 1.1 Schematic illustration of the Multiple Determinant Hypothesis of Idiosyncratic Drug Toxicity. Each circle represents a key determinant due to genetic and/or environmental factors. The hypothetical situation shown here is the confluence of the five determinants leading to severe liver toxicity in a patient at the time of drug administration: 1. Increased transporter‐mediated uptake, leading to increased liver drug concentration. 2. Increased metabolic activation due to the co‐exposure to enzyme inducers, leading to higher rate of formation of toxic/reactive metabolites. 3. Compromises metabolic detoxification due to co‐exposure to GSH depleting agents and/or inhibitors of phase 2 conjugating pathways. 4. Sensitized inflammatory response due to diseases or exposure to sensitizing agents. 5. Compromised efflux of bile salt due to genetic polymorphism of BSEP or co‐exposure to BSEP inhibitors.
I would like to illustrate the role of drug transport and metabolism in the manifestation of idiosyncratic drug toxicity based on the Multiple Determinant Hypothesis.
This hypothetical drug, T, is a substrate of an uptake transporter. Upon uptake into the hepatocytes, T is metabolized to a reactive metabolite, T*. T* is the ultimate toxicant which reacts with key cellular proteins, leading to cell death. T* also can form protein conjugates, leading to the formation of a neoantigen which would elicit a cytotoxic inflammatory event, leading to liver failure. However, T also undergoes metabolic clearance via glucuronidation and sulfation, with the conjugates excreted via transporter‐mediated efflux. T* can be detoxified via glutathione S‐transferase (GST)‐mediated glutathione (GSH) conjugation.
The following are key determinants and the respective probability (p) in the patient population for the manifestation of liver toxicity, resulting in liver failure:
1 Determinant 1 (p1): Elevated uptake transporter activity, leading to higher than normal intracellular concentration.
2 Determinant 2 (p2): Elevated drug metabolizing enzyme activity due to prior exposure to inducers or genetic polymorphism, resulting in higher than normal T* formation.
3 Determinant 3 (p3): Suppressed UDP‐glucuronosyltransferase (UGT) and sulfotransferase (SULT) detoxification activity.
4 Determinant 4 (p4): Suppressed GST activity and/or reduced cellular GSH level.
5 Determinant 5 (p5): Hypersensitized inflammatory reactions due to previous exposure to the same drug and the associated neoantigen.
6 Determinant 6 (p6): Reduced efflux transporter activity of glucuronide and sulfate conjugates, leading to feedback inhibition of the detoxifying pathways.
In this hypothetical example, Determinant 2 (metabolic activation) is the most crucial, while the other determinants would exacerbate the hepatotoxic events. The patient who will succumb to liver failure is one with the confluence of risk factors – simultaneous occurrence of the various determinants of drug toxicity at the time of drug administration (Figure 1.1).
Based on the theory that co‐occurrence of multiple determinants are required for the manifestation of severe drug‐induced liver injuries (DILIs), the incidence of DILI in the patient population will be a product of the incidence of patients with each of the determinants at the time of drug administration as shown in the following equation:
Considering a hypothetical case that the co‐occurrence of four determinants is required for DILI, and that each determinant has an incidence of 10% of the patient population, I(DILI) will be 10% × 10% × 10% × 10% or 1 in 10 000 patients. This event therefore cannot be readily detected in the regulatory clinical trials due to the limited number of patients, but will be manifested when the drug is administered to the general patient population upon marketing.
1.4 Concluding Remarks
Transporter‐mediated drug uptake and efflux are clear determinants of intracellular concentrations of drugs and metabolites which are substrates of the transporters. Individual variations in the expression of drug metabolism enzymes and transporters due to genetic polymorphism, exposure to drugs, foods and food supplements, and various diseases have been well‐established. A thorough understanding of the role of drug metabolism and transport in the disposition of a potentially toxic drug will aid the assessment of its toxic potential and the identification of the critical determinants of events leading to the formation and accumulation of the toxic moiety. Routine evaluation of the toxicological consequence of drug metabolism and transport may therefore aid the development of drugs with lower potential of causing idiosyncratic toxicity and may eventually lead to the identification of at‐risk populations.
1.4.1 A Comprehensive Approach to Safety Evaluation in Drug Development
While the conventional preclinical and clinical safety evaluation are essential to the assessment of drug safety, additional considerations are needed to avoid the occurrence of idiosyncratic drug toxicity. I have previously proposed that drug safety assessment requires a comprehensive, multidisciplinary approach (8, 9) with input from pharmacologists, toxicologists, epidemiologists, population geneticists, and especially drug metabolism/pharmacokinetics (DMPK) experts to provide insight toward the possible risk factors discussed above.