The FDA just released a new draft guidance document, which explains homologous use of therapeutic cell and tissue products (HCT/P) in details.
The reason for such clarification was “many inquiries from manufacturers about whether their HCT/Ps meet the homologous use criterion”. I agree, determination of homologous/ non-homologous for some HCT/P applications could be confusing. Unfortunately, in my opinion, this guidance does not clarifies all confusing cases – yes, I’m talking about adipose tissue, stromal vascular fraction (SVF) and mesenchymal stromal cells (MSC).
One big confusion is related to definition of “basic function”. HCT/P considered “homologous” if it “performs the same basic function or functions in the recipient as in the donor”. So, at least one basic function must be the same. If we look at adipose tissue-derived SVF and “any adult tissue”-derived MSC, we can see an example of multiple basic functions – mainly structural and metabolic. So, if I’m transferring auto- bone marrow MSC with intent to mechanically support cerebral blood vessels (one of MSCs basic function) into the brain, is it homologous use? It should be, because I’m claiming that my MSCs are performing the same function (mechanical support of blood vessels) in 2 different anatomical locations (which also fits definition of homologous use).
It is not necessary for the HCT/P in the recipient to perform all of the basic functions it performed in the donor, in order to meet the definition of homologous use.
An HCT/P may perform the same basic function or functions even when it is not used in the same anatomic location where it existed in the donor.
Let’s say, I’m isolating auto- MSCs from dental pulp and transferring them into injured eye with intent to stimulate angiogenesis. If I have an experimental evidence (or if it’s published), that dental pulp MSC have a basic agiogenic function in situ (among many others) and perform the same angiogenic function in damaged eye, would FDA clear it as homologous use? I doubt it. What if FDA will say “basic function of MSC is to differentiate into skeletal tissues”. I’d pick then pericytes, which exist virtually in any tissue and perform quite similar mechanical and metabolic functions in all tissues. Will I pass “homologous use” definition?
The problem with this guidance is that examples are so obvious. Seem like FDA is trying to avoid tough ones – auto/ fat/ MSC/ SVF/ pericytes… I’ve picked the most interesting example from the document:
A manufacturer provides HPCs derived from cord blood with a package insert stating that cord blood may be infused intravenously to differentiate into neuronal cells for treatment of cerebral palsy. This is not homologous use because there is insufficient evidence to support that such differentiation is a basic function of these cells in the donor.
Would FDA change its opinion if somebody will provide sufficient evidence? I know that such literature exist, but neural differentiation will never be a basic function of hematopoietic cord blood cells.
The position of FDA on MSC/ SVF seem like expressed in this sentence (emphases mine):
Generally, if an HCT/P is intended for use as an unproven treatment for a myriad of diseases or conditions, the HCT/P is likely not intended for homologous use only.
Generally, myriad, likely not… Is it really FDA’s language? How many is myriad, by the way? What is unproven? What if company picked 4 conditions and have 2 publications with evidence level of 4 and 6? Is it myriad yet and proven yet?
Finally, my favorite example where, I think, FDA is getting it totally wrong:
… if adipose tissue used for breast plastic and augmentation by subcutaneous injection for re-shaping (structure), it is not considered as “homologous use”, since primary function of breast tissue is producing a milk.
Dear FDA, I’m not replacing breast glandular tissue here, but I’m re-shaping it! My intent is not milk production! I’m supplementing existent breast fatty tissue with additional adipose tissue to make it prettier. The same basic function!
If I’m mistakenly mixing up here function of whole organ with function of particular tissue/ cell type withing the organ, how to explain this –
Pericardium is intended to be used as a wound coveringfor dura mater defects. This is homologous use because the pericardium is intended to repair or reconstructthe dura mater and serve as a covering in the recipient, which is one of the basic functions it performs in the donor.
Pericardium is a part of heart, dura mater is a part of brain – heart and brain are performing different functions. But as parts of different organs these tissues perform the same basic function. The same thing with a breast and subcutaneous fat.
Overall, In my opinion, this guidance is less valuable then previous “clarifying documents”, but still good to have in your library. The bottom line, even without any guidance, if you’re not sure – consult with FDA!
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