What is clear from my reading of this document so far is that it...

What is clear from my reading of this document so far is that it needs to be read very closely and as a whole. For example, the authors say that the mechanism of action of rem'l is not clearly established. Okay. But then they go on to accept that determining the mechanism of action of cell based products in general is a complicated business and that this may make it difficult to define product quality attributes that can be related to clinical effectiveness. I see these more as general observations relating to cell based products coupled with an invitation to the experts to consider how best rem'l might be characterised at the point of manufacture to ensure that each batch has the qualities that are responsible for its clinical efficacy. The authors clearly recognise that this is easier said than done in the case of cell based products. They are looking for ideas as to other assays (in addition to those used by MSB) based on reasonable hypotheses as to mechanism of action.
See esp.page 9:
"As stated above, FDA’s position is that the analytical methods used for product characterizationof remestemcel-L do not have a demonstrated relationship with clinical outcomes. We ask thecommittee to consider the product attributes identified by the Applicant as CQAs and discusswhether they are adequate to ensure that the manufacturing process will produce lots ofconsistent quality. Additionally, given the limitations of the current CQAs, and the state ofknowledge in the field of MSC, we ask that the committee discuss other product characteristicsnot previously identified as CQAs for remestemcel-L that might provide more meaningfulmeasures of product quality and potency."
It's pretty clear they are looking to the experts here, just as you would expect given that the FDA is being asked to give its approval to the first allogeneic MSC therapy that, as @dolcevita reminds us, is not a drug. Product characterisation at point of manufacture is a challenge, but the FDA knows this and will be relatively accomodative. Just don't expect them to come out and say so in a public document of this nature.This is a big moment for the FDA and it is not going to be seen as having just waved the first therapy of this kind through without robust discussion and analysis. There is a whole lot of other cell based therapies waiting in line behind us.