The present invention relates to the use of 2-thioxo-l,2, 3, 5- tetrahydro-pyrrolo[3,2-d]pyrimidine-4-one, thioxanthine and 2- thioxo-l,2,3,7-tetrahydro-6H-purin-6-one derivatives as MPO inhibitors for the treatment of multiple system atrophy (MSA), Huntington' s disease (HD).