There is disclosed the use of a compound of formula (Ia) or (Ib) wherein R1, R2, R3, R4, X and Y are as defined in the specification, and pharmaceutically acceptable salts thereof, in the manufacture of a medicament, for the treatment or prophylaxis of diseases or conditions in which inhibition of the enzyme myeloperoxidase (MPO) is beneficial. Certain novel compounds of formula (Ia) or (Ib) and pharmaceutically acceptable salts thereof are disclosed, together with processes for their preparation. The compounds of formulae (Ia) and (Ib) are MPO inhibitors and are thereby particularly useful in the treatment or prophylaxis of neuroinflammatory disorders.