This therapy maybe the simplest of all the therapies, but it is not proving effective in disease with neurological components – so far. The plan is simple – if the body has low levels of some enzymes – let's produce some and inject them into the body. Since the cells are adept at picking up enzymes from outside the body and absorbing them – it seems like it should work.
This therapy has been very effective with some lysosomal storage diseases (Gaucher disease, Fabry disease, or MP1) - but not with the disease that involve enzymes needed in the brain. The blood brain barrier is a factor, the foreign object response of the white blood cells is a factor, and the quality of enzymes needed is a factor. Additionally, ERT treats the symptoms temporarily – the injected enzymes will eventually die off.
In therapies like gene therapy the number of cells injected is lower because the cells duplicate once they arrive on site. In cord blood transplant therapies the blood continually regenerates Hex A. With a therapy like ERT- we need to put in as many as we need, and the process may need to be repeated several times – it might be able to slow the progression – but it cannot be a permanent solution (at least not yet).