The pyruvate-derived compound also may be a ???modified pyruvate analogue??? which refers to a compound comprising a modified pyruvate skeleton such as (???C???C(N)???C(???O)O) or (???C???C(???N)???C(???O)O) which may be in a substituted form such as (???CrxRy???C(NRsRt)???C(???O)ORz) or (???CRxRy???C(???NRs)???C(???O)ORz), wherein Rs, Rt, Rx, Ry and Rz, if present, are substituents such as H, alk