What is a Human Mass Balance Study?

The goal of a human mass balance clinical study is to understand how drugs are absorbed, metabolized, and excreted (AME) after dosing. This information is important because it helps determine what other clinical investigations might be necessary for regulatory approval of a new drug. The AME process (also known as ADME when ‘distribution’ is included) is determined by attaching a radioactive isotope (or radiolabel), such as carbon 14 (14C) or tritium (3H) to a drug and following the attached radiolabel through the AME process of human subjects. The term “mass balance” refers to collecting as much of the administered radiolabel as possible from human subjects in the clinical study. The radiolabel is collected from excreta (urine, feces, expired air, sweat, etc.) while monitoring the exposure of radioactivity and the drug in whole blood, plasma, cerebrospinal fluid, or other accessible tissues of interest.

Why Conduct a Mass Balance Clinical Study?

A human mass balance/ADME study is required for New Drug Applications (NDA) and in certain circumstances, Biologics License Applications (BLA), unless there are reasons for not conducting the study as agreed upon with the FDA or other regulatory authorities. Sponsors also conduct human ADME studies to obtain valuable information to support other clinical studies in their drug development program. The following information can be collected from a human mass balance study:

  • Identifying metabolites
  • Determining the relative exposure of parent drug and metabolites
  • Determining the routes of elimination and clearance mechanisms of a drug
  • Confirming the human metabolite profile is covered by the metabolite profile in animals from toxicology studies

Identifying Metabolites

Comprehensively identifying and quantifying all metabolites of the parent drug is critical. In the “Safety Testing of Drug Metabolites” guidance for industry, the FDA requires identification and characterization of metabolites over the “10% threshold.” This means that human metabolites that comprise greater than 10% of the measured total exposure to drug and metabolites (usually based on group mean AUC), may require further clinical and nonclinical characterization.

Determining Relative Exposure

It is important to determine the relative exposure of the parent drug and all metabolites. For drugs over the 10% threshold, a bioanalytical method will need to be developed for analyzing and pharmacokinetically assessing the metabolite (PK concentration vs. time and other PK parameters) in subjects of future clinical studies. It is not enough to characterize the parent drug if humans are exposed to significant amounts of a metabolite that may contribute to the safety and efficacy profile. The first step is to identify the total human metabolite pool and determine which are important enough (over the 10% threshold) to require additional considerations in clinical studies.

Determining Routes of Elimination & Clearance Mechanisms

Understanding the routes of elimination and clearance mechanisms is important because this information helps determine other types of clinical pharmacology studies that may need to be conducted. For example, if a drug is excreted primarily in the urine and cleared via renal mechanisms, then a renal impairment study (a study in individuals with varying degrees of renal impairment) may be necessary. The same concept applies to drugs that are cleared via liver/hepatic mechanisms. Metabolite pathways that account for greater than 25% of drug clearance may need to be studied in a drug-drug interaction (DDI) study with co-medications that can inhibit or induce that pathway.

Comparing Clinical & Nonclinical Metabolite Profiles

It’s important to have toxicology coverage for all significant metabolites by confirming that the human metabolite profile is fully covered by the metabolite profile in animals from toxicology studies. If there are any novel/significant human metabolites that were not observed in nonclinical toxicology studies then separate toxicology studies with the unique human metabolite may be necessary.

When to Conduct a Mass Balance Clinical Study

Many Sponsors wonder when they should conduct a mass balance/ADME clinical study. The answer to this question depends on your overall drug development goals and strategy. In general, a mass balance/ADME study should be conducted prior to entering Phase 3 clinical studies. It is common to conduct the human ADME study after Phase 1 and during Phase 2 proof of concept (POC) studies. Companies who want to “de-risk” their program or generate more data for valuation discussions with potential investors or acquisition partners should conduct the human ADME study earlier in drug development. Some companies taking the drug to NDA may delay the human ADME study until Phase 2 POC data justifies a Phase 3 clinical program. The tradeoff is that trying to fit in a mass balance clinical study between Phase 2 and 3 can be challenging, especially if there are issues and delays in radiolabeling the drug. You should start planning for the ADME clinical study(ies) during early Phase 1 drug development. The initial planning could be limited to engaging a qualified radiochemist to start evaluating the strategy for radiolabeling your drug or something more extensive. Consider executing the human ADME study in parallel with the Phase 2 POC study(ies). Identifying metabolites earlier in the program will allow for more opportunities to collect metabolite data that could inform population PK (popPK) and modeling and simulation strategies for your drug/metabolite profile. For example, you can use modeling and simulation to support dose selection and dose justification instead of trying to backfill that information in Phase 3.

Key Steps: Conducting a Mass Balance Clinical Study

Synthesis of Radiolabeled Active Pharmaceutical Ingredient

Preparing your drug for a human mass balance study by attaching a radiolabel (radioactive isotope such as 14C or 3H) can be difficult and time-consuming. We recommended working with a radiochemist early in the development program and long before a clinical ADME study is needed. This is because efforts to radiolabel drugs may take up to one to two years. This requires the placement of a radioisotope (14C is preferred) on a moiety of your molecule that can be followed on the parent drug and all relevant metabolites.

Quantitative Whole-Body Autoradiography Study

Quantitative whole-body autoradiography (QWBA) is a study required in animals prior to conducting the human ADME clinical study(ies). This nonclinical study is typically completed in rodents and evaluates the tissue distribution of your drug. QWBA studies are used for dosimetry calculations that determine the allowable radiolabeled dose for human ADME studies. This is important to avoid the risk in a human study of your radiolabeled drug accumulating in sensitive tissues and causing toxicity or damage (e.g., accumulation in the eye or testes).

Formulation Development and Good Manufacturing Practice

Formulation development and good manufacturing practice (GMP) production of radiolabeled study medication is often a rate limiting step in conducting a human ADME study. The radiolabel does need to be GMP quality for a human ADME study which can be difficult to achieve. Proper planning at the beginning of the research and development program around formulation development and GMP production of sufficient quantities of radiolabeled drug is important.

Bioanalysis and Pharmacokinetic (PK) Parameters

Bioanalysis of a drug is conducted after the clinical portion of the study is complete using a validated bioanalytical method to provide concentration data for determining PK parameters for the parent drug and metabolites. Noncompartmental PK analysis (NCA) of concentration-time data from human ADME studies is important to understanding the PK parameters (Cmax, Tmax, AUC, half-life, etc.). It is a good idea to engage a qualified pharmacokineticist prior to study conduct to ensure a proper study design and PK sampling schedule are selected. It is also a good idea to engage a qualified pharmacokineticist after the data is collected to conduct a proper analysis using PK software, determine the PK parameters, and interpret the results.

Metabolic Pathway Analysis

It is critically important to understand the human metabolite profile of your drug and the structural identification of the metabolites (MetID). As noted above, the FDA requires identification and characterization of metabolites that comprise greater than 10% of the measured total exposure to drug and metabolites (other regulatory health authorities outside the US have similar requirements). In general, there should not be “unique” human metabolites that were not observed in toxicology species. If unique human metabolites exist, additional toxicology studies with those metabolite(s) may be warranted or may be expected by health authorities. Also, it is important to identify “major” metabolites (>25% of clearance) that may require reaction phenotyping for inclusion in a clinical DDI study strategy.

Clinical Pharmacology Plan

The results from the human mass balance/ADME study should be evaluated in the context of other clinical investigations that need to be conducted. Consider developing an overall clinical pharmacology plan to prepare next steps for your development program. It is recommended that Sponsors prepare a reasonable, prospective strategic clinical pharmacology plan that builds on prior data and is validated during FDA meetings (or during meetings with other regulatory health authorities).

Mass Balance Studies: Frequently Asked Questions

Can female subjects be included in mass balance clinical studies? The vast majority of subjects in human ADME clinical studies are male. However, in certain indications, it might be important to study females in a human ADME study, particularly if the disposition of the drug is different between females and males. In these circumstances, postmenopausal females of non-childbearing potential can be included in the study. Justification for including females in a mass balance study will need to be provided in the protocol and to the IRB because it is an uncommon study population for this type of study. Are mass balance studies required for oncology indications or cancer therapies? The answer to this question is that it depends. Some cancer drugs can be administered to healthy volunteers safely and at doses necessary to conduct a mass balance study. If your cancer drug cannot be administered to healthy volunteers, then you’ll need to determine if a mass balance study can be conducted in oncology patients. Are mass balance clinical studies required for biologics? Although radiolabeling biologics such as proteins, DNA, RNA, and antibody therapeutics is not challenging (as evidenced by the vast body of academic research that routinely radiolabels proteins and nucleic acids), conducting human ADME studies with biologics can be a challenge. The challenge comes down to the fundamental purpose of a mass balance study, which is to recover the drug and metabolites in excreta following administration. For example, with a DNA therapeutic, its metabolites are nucleosides and are taken up into the general nucleoside pool within the body. As such, dosimetry calculations to determine the amount of radionucleotide to dose in the clinical study is difficult and recovering the radiolabeled nucleosides in a mass balance equation is not informative. Similar issues are present for protein and antibody therapeutics. However, not all biologics are the same and it’s recommended to consider if a mass balance study may be needed and to validate that decision during regulatory meetings. It’s also recommended to evaluate the type of molecule and chemical modifications on your biologic and to determine if the biologic or metabolites can be recovered in appreciable amounts of excreta.


A human mass balance/ADME study is an important investigation needed to support the clinical pharmacology program for an NDA or BLA (CTD Section 2.7.2). This type of study is also important because it provides information that is needed for designing other clinical studies within a development program prior to NDA or BLA submission. A major pitfall that can delay an overall drug development program is not planning far enough ahead to conduct the ADME study or conducting it too late in a development program to utilize the important information that can be collected. Working with a radiochemist early-on in the drug development program is important as efforts to radiolabel drugs may take up to one to two years. Lastly, depending on your overall objectives, it is common to conduct the human ADME study in parallel with the Phase 2 proof of concept study(ies). Contact Allucent to discuss how and when to conduct a human mass balance/ADME study for your program.

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