The use of compounds of formula (I), and salts thereof; and pharmaceutically acceptable in vivo cleavable prodrugs of said compound of formula (I); and pharmaceutically acceptable salts of said compound or said prodrugs; in formula (I), Ring C is phenyl or a carbon linked heteroaryl ring substituted as defined within; R¹ is an ortho substituent as defined within; n is 1 or 2; A-B is a linking group as defined within; R² and R³ are as defined within; R⁴ is hydroxy, hydrogen, halo, amino or methyl; in the manufacture of a medicament for use in the elevation of PDH activity in warm-blooded animals such as humans is described. Pharmaceutical compositions, methods and processes for preparation of compounds of formula (I) are also described ##STR1##