update 2013-01-31: this post has been deprecated; please see Fluphenazine, trimipramine and two styryl compounds for a more complete treatment of this subject.
@s just noticed today this study from late 2011. Thomas Wisniewski’s lab at NYU tested two already FDA-approved drugs, trimiprimine (an antidepressant) and fluphenazine (an antipsychotic) in mice and found significant delays (about 20%) in disease onset as well as reduction in severity. Since they’re both psychiatric-indicated drugs, they obviously both know how to cross the blood-brain barrier. The mouse model was scrapie mice infected via intraperitoneal injection.
The study in PLoS ONE:
And see also the NYU press release.