Notes from the Journal Article:
Human Absorption, Distribution, Metabolism, and Excretion Studies: Conventional or Microtracer? Sean Xiaochun Zhu, Drug Metabolism and Disposition, DOI: https://doi.org/10.1016/j.dmd.2025.100067
This article provides a detailed review of a forthcoming publication in Drug Metabolism and Disposition that discusses the two primary types of human Absorption, Distribution, Metabolism, and Excretion (hADME) studies: conventional and microtracer. The manuscript analyzes the characteristics, advantages, and disadvantages of each approach based on published literature (reviewing 121 hADME studies published between 2010 and 2024), regulatory considerations, and practical implications. It offers insights into scenarios where a microtracer hADME study may be preferred and proposes a decision tree to guide the selection of the appropriate hADME study type for an investigational drug. The core distinction between the two methods lies in the radioactivity dose administered: >1 µCi for conventional and ≤1 µCi for microtracer studies. The manuscript highlights that while conventional studies have been the predominant approach, microtracer studies, particularly when coupled with Accelerator Mass Spectrometry (AMS), offer unique advantages in specific situations and are gaining increasing attention.
Definition and Differentiation of hADME Study Types:
- A hADME study is a Phase I clinical pharmacology study essential for small molecule drugs before large-scale Phase III trials. It is also known as a human mass balance or a human absorption, metabolism, and excretion study.
- The key differentiator between conventional and microtracer hADME studies is the radioactivity dose level. While regulatory guidelines do not specify a clear boundary, this manuscript sets it at 1 µCi.
- Conventional hADME studies utilize radioactivity doses > 1 µCi, typically around 100 µCi, but can range from 1 to several hundred µCi.
- Microtracer hADME studies use radioactivity doses ≤ 1 µCi.
- Beyond dosage, the two study types have distinct prerequisites and radiometric detection methods.
Prevalence of hADME Study Types in Literature:
- A review of 121 hADME studies published between 2010 and 2024 in Drug Metabolism and Disposition (DMD), British Journal of Clinical Pharmacology (BJCP), and Clinical Pharmacology & Therapeutics (CP) revealed that 88% (106 studies) were conventional, and 12% (15 studies) were microtracer.
- The vast majority (120 out of 121) of studies used carbon-14 (¹⁴C) as the radiolabel.
Radiometric Analysis Techniques:
- Conventional hADME studies generally employ traditional radiometric analyses such as radioflow detection or Liquid Chromatography (LC) fraction collection followed by microplate scintillation counting for metabolite profiling. Total Radioactivity (TRA) in blood and fecal samples is typically measured using Liquid Scintillation Counting (LSC) after combustion.
- Microtracer hADME studies typically require LC fraction collection and processing followed by Accelerator Mass Spectrometry (AMS) analysis to generate radioprofiles. TRA in plasma, whole blood, urine, and fecal samples is usually determined using AMS. This process is described as “lengthy, taking significantly longer time than traditional techniques.”
- There are exceptions: AMS was used in 15% of conventional hADME studies due to insufficient radioactivity levels for traditional methods. Conversely, LSC was used in some microtracer hADME studies (e.g., for excreta) where radioactivity concentrations were high enough.
Advantages and Disadvantages of Conventional vs. Microtracer hADME Studies:
- The manuscript explicitly states that “disadvantages of each study type are essentially the advantages of the other.” A detailed comparative list is implied but not fully enumerated in the provided excerpts. However, several key points are raised:
- Microtracer advantages (implying conventional disadvantages): Lower radiation exposure to participants, potentially allowing for multiple doses; may be feasible with non-GMP radiolabeled drug substance at impurity levels; can mitigate radiolysis issues associated with high specific activity.
- Conventional advantages (implying microtracer disadvantages): Radiometric sample analysis can often be performed internally or by most CROs with traditional techniques (LSC, radioflow), potentially faster and more readily available than AMS; GMP radiolabeled drug substance is typically used.
- Microtracer disadvantages: Requires specialized AMS equipment and expertise, potentially leading to delayed results, especially for excreta analysis if outsourced; can be more complex and time-consuming for metabolite profiling.
Scenarios Favoring Microtracer hADME Studies:
- The manuscript discusses several scenarios where a microtracer approach may be preferable:
- When a conventional hADME study might miss the development timeline.
- When a “human first/human only” hADME approach is desired, especially when preclinical Quantitative Whole-Body Autoradiography (QWBA) and human dosimetry estimation are unavailable. This is highlighted by the quote: “A preclinical QWBA study and human dosimetry estimation are unavailable.”
- When the sponsor has in-house AMS capabilities, as this “largely mitigate[s]” the disadvantages of microtracer studies. In this case, “the overall advantages of a microtracer hADME study would outweigh those of a conventional hADME study, making the microtracer approach the recommended option for the Sponsor.”
- When GMP ¹⁴C-labeled drug substance cannot be sourced, and the required quantity of ¹⁴C-labeled material for a microtracer study is below the impurity qualification threshold, allowing the use of non-GMP material spiked into GMP unlabeled drug. This is exemplified by the description of using “Compound X solution spiked with a trace level of non-GMP 14C-labeled Compound X solution” adhering to ICH Q3B (R2).
- When the ¹⁴C-labeled drug substance is unstable due to radiolytic degradation at a high specific activity (SA), which can be mitigated by the lower radioactivity dose in a microtracer study. The text notes that “Reducing the SA can mitigate radiolysis of the compound.”
- For drugs with a long terminal half-life or prolonged retention of radioactivity in nonclinical species, as AMS provides higher sensitivity to measure drug-related TRA without concerns for lingering radioactivity. The manuscript mentions “No concerns for lingering radioactivity in tissues; AMS provides higher sensitivity for measuring the drug-related TRA.”
- When there is time-dependent PK or metabolism observed in humans, allowing for a true or near steady-state radioactive metabolite profile with repeated microtracer doses. The example of emvododstat is provided: “a daily oral dose of 16 mg (0.3 µCi) of [14C]emvododstat was given to each healthy participant from Day 1 to 6.”
Recommended Decision Tree:
- The manuscript proposes a decision tree (Figure 2) for selecting the appropriate hADME study type.
- The default option is a conventional hADME study.
- A “yes” response to any of the first five questions (related to timeline, “human first,” in-house AMS, non-GMP material use, and radiolysis), with some additional conditions for questions 1 and 4, suggests a recommendation for a microtracer hADME study.
- A “yes” response to questions 6 (long half-life) or 7 (time-dependent PK/metabolism) suggests that either a microtracer or a conventional hADME study can be conducted on a case-by-case basis.
Coupling hADME with Absolute Bioavailability (ABA) Studies:
- The manuscript highlights the increasing popularity of conducting an ABA study alongside a hADME study, especially for non-IV dosing routes.
- A “microtracer ABA study” involves an unlabeled oral dose followed by an intravenous microdose containing a microtracer radioactivity at the anticipated Tmax. This approach is favorable as it avoids extensive IV toxicology studies and formulation work.
- Two main designs for a two-period study exist: microtracer hADME followed by microtracer ABA, or microtracer ABA followed by a conventional hADME study. The US FDA guidance recommends such combined studies for oral drugs.
Future Trends:
- The author notes that “Recently, at least one company has routinely conducted microtracer hADME studies.”
- The manuscript suggests that with advancements in AMS technology and increased accessibility, the industry may potentially shift towards microtracer hADME studies as the default design.