Disclosure: I am in IT, so the following is just my amateur understanding of a mass balance study. A labeled* sample of the drug is given to the subject. Almost everything that comes out of the subject is collected. When substantially all of the label is collected the study is over.
Assuming that the drug was labeled correctly, the collected material will have any non metabolized drug and it's metabolites. Almost all of the collected material will be metabolites. Without any analysis you know the drug and its metabolites life span in the subject. I assume that using mass cytometry or other techniques will separate the different metabolites. If the drug has any unexpected activity or extended action, the metabolites can be tested for activity.
Going into the mass balance study, all of the data that Arena had shared showed that there was no unexpected activity. That is unlike other drugs in the same class.
I agree that the mass balance study has reduced the risk. I just thought the other data showed that the risk of a long lived metabolites was near zero. I think I was right about that. It looks like I was wrong thinking other investors had the same opinion.
* The label is a radioactive tag. Not sure way a lanthanide tag would not work as well?