Ever since its introduction in the '50s, the CFC pMDI has been the main formulation type of aerosols. CFCs were linked to ozone depletion and are now being phased out. This phase-out of CFCs forces reformulation and development of new technologies for aerosol delivery.
The regulatory requirements for delivered-dose uniformity evolved mainly based on FDA's experience with these CFC pMDI products. Over time, the DDU testing requirements became more stringent. Now, even for the mature technology of CFCs, this poses challenges, and even more so with the new technologies where formulation options are more limited.
I don't think I need to go through this slide in any detail because Wally did that for me. Thank you. I just want to highlight this undesirable characteristic of a zero tolerance requirement; namely, that the stringentness of that requirement is completely correlated to the sample size. So the more you look, the more certainty you have in failing that requirement. Therefore, it is unsuitable for situations where you do a lot of testing, for example, in stability testing, in validations, and as Ajaz pointed out, in PAT.