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# Radiolabeled Mass Balance Study Comments
- URL: https://www.fdaweb.com/radiolabeled-mass-balance-study-comments/
- Published: 2022-08-17T12:00:00.000Z
- Updated: 2026-09-14T17:56:07.000Z
- Author: David McFarland
- Tags: Drugs, #legacy-id-D5152528

Pharmaron says it welcomes an FDA draft guidance on clinical pharmacology considerations for human radiolabeled mass balance studies because it clarifies several topics related to the design, execution, and sample analysis for a human mass balance study that were not previously outlined in this way. The [comment](https://www.regulations.gov/comment/FDA-2022-D-0113-0027?ref=fdaweb.com) also has specific questions and items needing clarification.

The Bayer comment [letter](https://www.regulations.gov/comment/FDA-2022-D-0113-0024?ref=fdaweb.com) says it would be beneficial to have examples for certain cases including the selection of subjects when polymorphism is known, or time-dependent pharmacokinetics.

Merck’s [comment](https://www.regulations.gov/comment/FDA-2022-D-0113-0026?ref=fdaweb.com) lists specific areas of the draft for which the company requests verification.

Vertex submitted three [questions](https://www.regulations.gov/comment/FDA-2022-D-0113-0020?ref=fdaweb.com) tied to specific lines in the draft.