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What is a Human Mass Balance Study?

A human mass balance clinical study aims to understand how drugs are absorbed, metabolized, and excreted after dosing. This information is important because it helps determine other clinical investigations that might need to be conducted in support of regulatory approval for a new drug.

The absorption, metabolism, and excretion (AME, also known as ADME to include ‘distribution’) is determined by attaching a radioactive isotope (radiolabel), such as carbon 14 (14C) or tritium (3H) to a drug and following the radiolabel in human subjects.

The term “mass balance” refers to attempting to collect as much of the radiolabel administered to subjects in a clinical study as possible. The radiolabel is collected in excreta, including urine, feces, and other excreta as needed (e.g., expired air, sweat, etc.), while monitoring the exposure of radioactivity and drug in whole blood, plasma, cerebrospinal fluid (CSF) or other accessible tissues of interest.

Why Conduct a Human Mass Balance Clinical Study?

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

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

Determining the Routes of Elimination and Clearance Mechanisms

This information is important in determining other types of clinical pharmacology investigations or studies that may need to be conducted. For example, if a drug is primarily excreted in urine and cleared via renal mechanisms, then a renal impairment study may be necessary (i.e., a study in individuals with various degrees of renal impairment). The same concept applies for drugs that are cleared via hepatic mechanisms.

Identifying Metabolites and Determining the Relative Exposure of Parent Drug and Metabolites

A comprehensive metabolite ID (MetID) approach to identifying and quantifying 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 (e.g., AUC 0-inf), may require further clinical and nonclinical characterization.

For drugs over the 10% threshold, the Sponsor will need to develop a bioanalytical method for analyzing the metabolite and pharmacokinetically assess that metabolite (PK concentration vs. time and PK parameters) in subjects of future clinical studies. It’s not enough to characterize parent drug if humans are exposed to significant amount of a metabolite(s) 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 to require additional considerations in clinical studies.

Confirming that the Human Metabolite Profile is Covered by the Metabolite Profile in Animals from Toxicology Studies

It’s important to have toxicology coverage for all significant metabolites. If there are novel significant human metabolites that were not observed in nonclinical toxicology studies then separate toxicology studies with that novel human metabolite may be necessary.

When to Conduct a Human ADME Clinical Study

The answer to this question depends on your overall company goals and strategy. In general, a human ADME clinical 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 will conduct the human ADME 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 human ADME clinical study between Phase 2 and 3 can be a challenge, especially if there are issues and delays in radiolabeling the drug.

Start planning for human ADME studies during early Phase 1 drug development. This 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 data on those metabolites that could inform a population PK and modeling and simulation strategy for your drug/metabolite profile (for example to support dose selection/dose justification) instead of trying to back fill that information in Phase 3.

Key Steps in Conducting a Human Mass Balance Clinical Study

Synthesis of Radiolabeled API

Preparing your drug for an ADME clinical study by attaching a radiolabel (radioactive isotope such as 14C or 3H) can be difficult and time consuming. It is recommended to work with a radiochemist early on in a development program and long before a clinical ADME study is needed. This is because it’s possible that efforts to radiolabel drugs may take up to 1 to 2 years. This effort requires the placement of a radioisotope (14C is preferred) on a section of your molecule that can be followed on the parent drug (i.e., intact drug) and all relevant metabolites.

Quantitative Whole-Body Autoradiography (QWBA)

QWBA is a required study in animals prior to conducting the human ADME study. This nonclinical study is typically completed in rodents and evaluates the tissue distribution of your drug. A QWBA study is used for dosimetry calculations that determines the allowable radiolabeled dose for a human ADME study. 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 GMP Production of Radiolabeled Study Medication

This is often a rate limiting step in the conduct of a human ADME study. Proper planning at the beginning of the R&D program around formulation development and GMP production of sufficient quantities of radiolabeled drug is important.


Bioanalysis of a drug will be conducted after the clinical portion of the study is complete using a validated bioanalytical method to provide concentration data for determination of PK parameters.

Pharmacokinetics (PK Parameters)

Noncompartmental PK analysis of concentration-time data from a human ADME study is important to understand the pharmacokinetic parameters (Cmax, Tmax, AUC, half-life, etc.). It is important to engage a qualified pharmacokineticist prior to study conduct to ensure a proper study design and PK sampling schedule is selected. Also, it is important 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 (Metabolite ID)

It is critically important to understand the human metabolite profile of your drug. 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 drug-drug interaction (DDI) study strategy.

Clinical Pharmacology Plan

The results from the human 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).

Human Mass Balance Studies: FAQs

Can Female Subjects be Included in Human 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 inclusion of 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.

Are Human Mass Balance Clinical Studies Required for Biologics?

Although radiolabeling biologics (protein, DNA, RNA, 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. It 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 so it’s advisable to consider if a mass balance study is needed for biologics and validate that decision during FDA meetings (or during meetings with other regulatory health authorities).

It’s recommended to evaluate the type of molecule and chemical modifications on your biologic and determine if parent or metabolites can be recovered in appreciable amounts in excreta.

Are Human Mass Balance Clinical Studies Required for Oncology Indications / Cancer Therapeutics?

The answer 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.


A Human Mass Balance study (ADME/AME study) is an important investigation needed to support the clinical pharmacology program for an NDA/BLA (CTD Section 2.7.2). A Mass Balance 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.

A major pitfall that can delay an overall 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. Working with a radiochemist early in a development program is important as efforts to radiolabel drugs may take up to 1 to 2 years. Lastly, depending on your overall objectives for a development program, it is common to conduct the human ADME study in parallel with the Phase 2 proof of concept study(ies).

Contact Nuventra to discuss a human mass balance study for your program.

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