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Specifically targeting compounds is an active area of research. Example: all oncotherapies are essentially just poison. The trick is to deliver the toxin to the 0.001% of cells in the body that are cancerous.

It is amusing you should say ways that avoid reaching the brain. The blood-brain barrier impedes a large number of therapeutics from entering the brain. It is incredibly difficult to engineer the constraints of something that: is potent enough to have an effect, can survive in the blood, cross into the brain, targets the cells of interest, has a meaningful half-life, does not decompose into something toxic, etc.