Given a suitable delivery system, one that localizes to the desired target tissues to a far greater degree than to all other undesirable off-target tissues, the big advantage of a gene therapy is it precisely achieves the manipulation desired. It dials up or dials down expression for selected genes, alters the amount of proteins produced from those genes, and thereby changes cell behavior as a consequence – and that is all it does. One doesn’t have the endless concern about off-target effects that characterize small molecule drug development.
There are, of course, different challenges. Setting aside some adventurous technologies that won’t be deployed in therapies any time soon, manipulating a few genes at a time is the present practical upper limit on gene therapy. Further, there is no viable delivery system for most target tissues in the body, if the goal is to maximize expression in a limited set of locations. Yes, a great many interesting technologies exist for use in animal studies, but the bounds of the possible are more limited when it comes to what is permitted in the clinic. Injecting a gene therapy vector into the bloodstream means that most of it will
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