Provided are thiadiazolidine compounds of formula I ##STR00001## wherein R.sub.1 is an organic group having at least 8 atoms selected from C or O, which is not linked directly to the N through a --C(O)-- and comprising at least an aromatic ring, and their pharmaceutical compositions. These compounds are selective GSK-3 inhibitors and have improved bioavailability. They are useful for the treatment of GSK-3 mediated diseases, among others Alzheimer's disease, type II diabetes, depression and brain injury.