You are right that a limit of ERT is it’s penetration to particular tissues/cells.

However, transporters are not the only mechanism for cellular entry. Have you heard of receptor-mediated endocytosis?

You can check out this paper if you want to see the mechanism in more detail

You can read the paper if you’re interested, from my skim she has to receive injections every couple weeks for the rest of her life.

I don’t see how a single dose would work for enzyme replacement therapy, but I am not an expert in this topic.

Many genes encode enzymes. A mutation in a gene may result in a defective enzyme (protein) product. Enzymes catalyze biochemical reactions , so not having a particular enzyme can have dire consequences physiologically. In enzyme replacement therapy, a functional version of the enzyme is given to the patient like a medication.