Systemic clearance, denoting how much drug is cleared from a volume of blood per unit of time, is of critical importance to a drug candidate’s pharmacokinetic profile. Tools such as the Extended Clearance Classification System (ECCS) and in vitro-in vivo extrapolation (IVIVE) can help drug developers predict the rate at which a drug compound is eliminated. Information gleaned from these applications support a risk-based approach to pharmacokinetic evaluation of a drug candidate and may inform dosing considerations and clinical study design.
There can be a lot of complexity involved in your drug’s mechanisms and rate of clearance. Factors include turnover by drug-metabolizing enzymes, transport to and away from tissues, biliary excretion and renal filtration, and more. Fortunately, many of these factors can be estimated using in vitro and in vivo absorption, distribution, metabolism, and excretion (ADME) and pharmacokinetic (PK) studies early in the development pipeline.
Extended clearance classification system (ECCS)
The Extended Clearance Classification System (ECCS) has been proposed as a tool to predict the rate-limiting step in a drug’s clearance using physicochemical properties. Depending on molecular weight, permeability, and ionization, drugs can be categorized into one of four classes which predict whether drug transport, renal elimination, or hepatic metabolism is most likely to critically impact clearance. Equipped with a sound prioritization strategy, drug developers can make informed decisions regarding timing of preclinical investigations into their drug compound’s interaction with drug-metabolizing enzymes and transporters.
Plasma protein binding (PPB)
Only free (unbound) drug is available for therapeutic action and clearance from the body, so experiments to determine your drug’s affinity to plasma proteins, e.g. albumin, provide insight to clearance rate as well as exposure. Plasma Protein Binding (PPB) studies use techniques like equilibrium dialysis, ultrafiltration, and ultracentrifugation to calculate fraction unbound (fu).
Drug transporter studies
Transporters represent a critical component of a drug’s ADCE—No, that isn’t a typo for ADME. ADCE stands for absorption, distribution, clearance, and elimination. While a drug’s Metabolism (the M in ADME) is important in understanding the fundamental pharmacokinetic principle of how a drug is changed, it’s equally important to consider how it navigates the body by considering drug transport as well. Transporter proteins can affect the elimination of a drug compound just as much as drug metabolizing enzymes. Even if a drug is rapidly metabolized by a high-turnover enzyme, its clearance may be slowed by slow transport to the site of metabolism. For example, MAO-A is an enzyme which can metabolize drugs very quickly, but some drugs (e.g., sumatriptan) only get to MAO-A through the transporter OCT1. In people with low OCT1 activity, OCT1 is expressed very differently between individuals. Metabolism is slowed and clearance is reduced, so exposure (concentration of drug compound in plasma) is increased, which could lead to negative effects, such as toxicity.
“We are seeing more and more compounds where OCT1 is important for hepatic uptake.” Dr. Brian Ogilvie, VP of Consulting Transporters of Emerging Importance Webinar.
Due to complexity created by protein expression differences, different routes of administration, interplay between transporters and enzymes, inhibition potential, and other factors, in vitro transporter studies don’t have a lot of the prescriptive guidance that some other safety studies have to create a roadmap for development. Drug developers must then consider current science published by other academic authorities, like the International Consortium of Transporters (ITC), to make decisions. Drug developers can navigate this difficult terrain with intimate familiarity of the rapidly growing trove of current scholarship or by forging a relationship with a specialty CRO known for its extensive experience with drug transport studies, ability to meet (and scholastically influence) regulatory guidance, and access to a wide selection of non-standard enzymes and transporters to plan and execute carefully designed assays to better understand your drug candidate’s disposition.
Drug metabolism, often by well-characterized enzyme families such as CYP and UGT, plays a critical role in a drug’s clearance from the body by way of biotransformation into metabolites which can be ultimately excreted through urine, feces, or other routes. Intrinsic clearance should be explored early in the development pipeline. Metabolic stability in hepatocytes or microsomes first predicts a drug’s affinity to be metabolized by enzymes, then metabolite characterization and identification studies allow a drug developer to find out which metabolites may be formed and if any are unique to humans or disproportionately higher in human than preclinical species. Reaction phenotyping studies then provide drug developers with insight to which enzymes are responsible for metabolism of a drug candidate. Definitive reaction phenotyping uses recombinant enzymes to identify potential activity with specific isoforms in addition to human liver microsomes (HLM), which may have lower expression of relevant enzymes but provide a system more reflective of in vivo concentrations. These studies can definitively validate major players in your drug’s biotransformation, identify risk factors for drug-drug interactions, and in some cases predict population-specific issues.
Transport roles dictate how drugs may be eliminated from the body via clearance routes, including urine, feces, and bile. In vivo ADME, such as mass balance and bile cannulation excretion studies, can explore clearance routes in relevant preclinical species over time, using radiolabeled or unlabeled compound and specialized bioanalytical instrumentation. Recommended before radiolabeled human AME studies in the clinic, these studies can provide critical inputs for dosimetry calculations and inform the design of clinical trials.
Methods and models incorporating preclinical data equip drug developers with a predicted pharmacokinetic profile for a drug compound, to provide regulatory authorities with a strong data package for IND (or equivalent) submission. In vitro-in vivo extrapolation (IVIVE) helps to contextualize in vitro and preclinical results to better predict in-patient drug behavior. A fine-tuned or customized examination is sometimes needed to explain unexpected results or justify decisions in a drug’s development.
Every drug is unique, and especially for drugs with non-standard behaviors, partnership with a CRO with specific expertise and experience, a capacity for flexible assay design, and access to a large profile of drug metabolizing enzymes and drug transporters can translate to better understanding of your drug compound and improve prediction of what could happen when it is administered to patients.
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