Pancreatic enzymes are produced from pigs pancreas. Raw pancreatic enzymes for human use have 2 sources in the US. One is Creon’s manufacturer. The company produces his own pancreatic enzymes used in Creon. The second source is used by the other enzymes on the market (or seeking approval like Ultrase). So in fact, Zenpep and Ultrase are produced from the same raw pancreatic powder, coming from the same manufacturer. In fact, only the final packaging was made by Axcan, as Eurand also produced the bulk medication for Axcan in Milan (Italy).
The mix of Lipase, Protease, and Amylase may vary from batch to batch, however the Lipase content (used by the FDA and doctors for dosing purpose) has to remain constant.
Other factors and differences between final products (Creaon, Zenpep, Ultrase, etc) may have an impact on the outcome of the treatment. Just to give some examples: The size of the tablets or beads that are contained in the final capsule, the gastro-resistant coating of each tablet or bead, the real dose taken, since enzyme activity decay with time, so the manufacturing date of the product is also important, etc.
Symptoms and side effects are often driven by these subtle differences, as in fact the enzymes come form the same pancreas pigs and are bulk manufactured the same way for pretty much all the products on the market. Some patients may find a perfect mix in one product but not do well on the same declared dose of a different product. This does not mean that one product is bad and the other is good. It may require some adjusting of doses and dosing habits to switch from one product to another.
I am sure the FDA had their reasons for not approving Ultrase, and I know that patients will not do well if they do not take their enzymes to treat the pancreatic insufficiency. So I think that it is just a matter of finding another product that works well, maybe with some dose adjustment, because, in reality, all the enzymes in the US come from pretty much the same place.