It's mentioned separately, but not giving any further reasons:
> A global data safety monitoring board will look at interim results at regular intervals and decide whether any member of the quartet has a clear effect, or whether one can be dropped because it clearly does not. Several other drugs, including the influenza drug favipiravir, produced by Japan’s Toyama Chemical, may be added to the trial.
I'm not finding the original statement/source from WHO, I'm sure they gives more reasons there around why some drugs are not included (yet)
> A global data safety monitoring board will look at interim results at regular intervals and decide whether any member of the quartet has a clear effect, or whether one can be dropped because it clearly does not. Several other drugs, including the influenza drug favipiravir, produced by Japan’s Toyama Chemical, may be added to the trial.
I'm not finding the original statement/source from WHO, I'm sure they gives more reasons there around why some drugs are not included (yet)