An agent which is a modulator of the activitiy of the PRKD1 gene product, protein kinase D1, for use in the treatment of Friedreich's ataxia, wherein said agent is a protein kinase D1 inhibitor and is a [2,6] Naphthyridine of formula: wherein R 1 and R 2 are independently hydrogen, alkyl, cycloalkyl, heterocyclyl, each of which is optionally substituted by one to two R 8, wherein R 8 is hydrogen, halogen, alkyl, R 9 -O--, (R 10)(R 11)N--, (R 12)(R 13)N-C --, aryl, or heterocyclyl or heteroaryl, said heterocyclyl and heteroaryl are optionally substituted by one or two alkyl groups; R 1 and R 2 taken together with the nitrogen atom to which they are attached to optionally form a 4-7 membered ring; R 3 is (R 14)(R 15)N--, or halogen; R 4, R 5, R 6 and R 7 are independently hydrogen, halogen, alkyl, (C 3 -C 7) cycloalkyl, aryl-alkyl, aryl, or alkoxy; R 9, R 10, R 11, R 12 and R 13 are independently hydrogen, alkyl-O-C --, alkyl-NH-C --, alkyl-C -NH-C --, cycloalkyl, cycloalkyl-alkyl--, R 16 -SO 2 --, R 17 -C --, heterocyclyl or alkyl, said heterocyclyl is further optionally substituted by one or two cycloalkyl-alkyl-- groups, and said alkyl is further optionally substituted by one or two groups selected from hydroxy, alkoxy, alkylamine, dialkylamine, or heteroaryl; R 10 and R 11 taken together with the nitrogen atom to which they are attached to optionally form a 5-7 membered ring; R 12 and R 13 taken together with the nitrogen atom to which they are attached to optionally form a 5-7 membered ring; R 14 and R 15 are independently hydrogen, alkyl, aryl, cycloalkyl, aryl-alkyl--, heterocyclyl or heteroaryl, said alkyl, cycloalkyl, aryl and heteroaryl are further optionally substituted by one or two groups selected from alkyl, alkoxy, hydroxy, halogen, haloalkyl, cyano, or R 18 -NH-C --; R 16 is aryl or heteroaryl; R 17 is heterocyclyl, or alkyl optionally substituted by one or two groups selected from H 2 N--, aryl-alkyl--, or alkyl-C -NH--; R 18 is heterocyclyl-alkyl--; or a pharmaceutically acceptable salt thereof; or an optical isomer thereof; or a mixture of optical isomers. R 1 and R 2 are independently hydrogen, alkyl, cycloalkyl, heterocyclyl, each of which is optionally substituted by one to two R 8, wherein R 8 is hydrogen, halogen, alkyl, R 9 -O--, (R 10)(R 11)N--, (R 12)(R 13)N-C --, aryl, or heterocyclyl or heteroaryl, said heterocyclyl and heteroaryl are optionally substituted by one or two alkyl groups; R 1 and R 2 taken together with the nitrogen atom to which they are attached to optionally form a 4-7 membered ring; R 3 is (R 14)(R 15)N--, or halogen; R 4, R 5, R 6 and R 7 are independently hydrogen, halogen, alkyl, (C 3 -C 7) cycloalkyl, aryl-alkyl, aryl, or alkoxy; R 9, R 10, R 11, R 12 and R 13 are independently hydrogen, alkyl-O-C --, alkyl-NH-C --, alkyl-C -NH-C --, cycloalkyl, cycloalkyl-alkyl--, R 16 -SO 2 --, R 17 -C --, heterocyclyl or alkyl, said heterocyclyl is further optionally substituted by one or two cycloalkyl-alkyl-- groups, and said alkyl is further optionally substituted by one or two groups selected from hydroxy, alkoxy, alkylamine, dialkylamine, or heteroaryl; R 10 and R 11 taken together with the nitrogen atom to which they are attached to optionally form a 5-7 membered ring; R 12 and R 13 taken together with the nitrogen atom to which they are attached to optionally form a 5-7 membered ring; R 14 and R 15 are independently hydrogen, alkyl, aryl, cycloalkyl, aryl-alkyl--, heterocyclyl or heteroaryl, said alkyl, cycloalkyl, aryl and heteroaryl are further optionally substituted by one or two groups selected from alkyl, alkoxy, hydroxy, halogen, haloalkyl, cyano, or R 18 -NH-C --; R 16 is aryl or heteroaryl; R 17 is heterocyclyl, or alkyl optionally substituted by one or two groups selected from H 2 N--, aryl-alkyl--, or alkyl-C -NH--; R 18 is heterocyclyl-alkyl--; or a pharmaceutically acceptable salt thereof; or an optical isomer thereof; or a mixture of optical isomers.