A pharmaceutically-acceptable salt of a heterocyclic compound according to Formula 1: wherein R 1 or R 2 comprise at least one carboxylate functional group; E 1 and E 2 are either N or O; said salt further comprises at least one cation; and wherein said salt is not a lithium salt of 2,5-diaspartyl-3,6-diketopiperazine or 2,5-diglutamyl-3,6-diketopiperazine and said salt is in a solid form.