A compound of formula in which X 1 is C or N; R 1, R 2, and R 3, which may be the same or different, are selected from the group consisting of H, NH 2, OH, optionally substituted C 1 -C 6 alkyl, and optionally substituted C 1 -C 6 heteroalkyl wherein the heteroatoms are one or more N, O or S; and R 4 ' is H; and R 4 is selected from H, F, optionally substituted C 1 -C 6 alkyl or C 2 -C 6 alkenyl, optionally substituted phenyl or a 5- or 6-membered heteroaryl group; or R 3 and R 4 are fused to form a 5- or 6-membered ring, preferably a 6-membered ring, and thereby a compound of formula Ila; or R 1 and R 4 are fused to form a 5- or 6-membered ring, preferably a 6-membered ring, and thereby a compound of formula IIb in which X 2 is NR 10, O or S, wherein R 10 is H or C 1-3 alkyl which may be partially or fully fluorinated; and X 3 is a single or double bond, -CO, -SO 2 or -CH 2 -; R 5, R 6, R 7, R 8 and R 9, which may be the same or different, are selected from the group consisting of H, OH, SH, halogen, CN, NO 2, NH 2, optionally substituted C 1 -C 6 alkyl, optionally substituted C 2 -C 6 alkenyl, optionally substituted C 1 -C 6 aminoalkyl, optionally substituted C 1 -C 6 heteroalkyl wherein the hetero atom is N, O or S, optionally substituted C 1 -C 6 alkoxy, optionally substituted benzyloxy, optionally substituted C 1 -C 6 thiol, optionally oxidized to -S - or -S 2 -, and an aromatic group, optionally containing one or more heteroatoms selected from O, N and S, said aromatic group being optionally substituted, two of R 5, R 6, R 7, R 8 and R 9 may together form an optionally substituted carbocyclic or heterocyclic group; or a pharmaceutically acceptable salt or solvate thereof or a tautomer thereof, for use in the treatment of hyperphenylalaninemia, wherein the treated condition is mild HPA, mild PKU (phenylketonuria) or classic PKU in a subject exhibiting ≥ 10% of wild-type PAH (phenylalanine hydroxylase) activity. X 1 is C or N; R 1, R 2, and R 3, which may be the same or different, are selected from the group consisting of H, NH 2, OH, optionally substituted C 1 -C 6 alkyl, and optionally substituted C 1 -C 6 heteroalkyl wherein the heteroatoms are one or more N, O or S; and R 4 ' is H; and R 4 is selected from H, F, optionally substituted C 1 -C 6 alkyl or C 2 -C 6 alkenyl, optionally substituted phenyl or a 5- or 6-membered heteroaryl group; or R 3 and R 4 are fused to form a 5- or 6-membered ring, preferably a 6-membered ring, and thereby a compound of formula Ila; or R 1 and R 4 are fused to form a 5- or 6-membered ring, preferably a 6-membered ring, and thereby a compound of formula IIb in which X 2 is NR 10, O or S, wherein R 10 is H or C 1-3 alkyl which may be partially or fully fluorinated; and X 3 is a single or double bond, -CO, -SO 2 or -CH 2 -; R 5, R 6, R 7, R 8 and R 9, which may be the same or different, are selected from the group consisting of H, OH, SH, halogen, CN, NO 2, NH 2, optionally substituted C 1 -C 6 alkyl, optionally substituted C 2 -C 6 alkenyl, optionally substituted C 1 -C 6 aminoalkyl, optionally substituted C 1 -C 6 heteroalkyl wherein the hetero atom is N, O or S, optionally substituted C 1 -C 6 alkoxy, optionally substituted benzyloxy, optionally substituted C 1 -C 6 thiol, optionally oxidized to -S - or -S 2 -, and an aromatic group, optionally containing one or more heteroatoms selected from O, N and S, said aromatic group being optionally substituted, two of R 5, R 6, R 7, R 8 and R 9 may together form an optionally substituted carbocyclic or heterocyclic group; or a pharmaceutically acceptable salt or solvate thereof or a tautomer thereof, for use in the treatment of hyperphenylalaninemia, wherein the treated condition is mild HPA, mild PKU (phenylketonuria) or classic PKU in a subject exhibiting ≥ 10% of wild-type PAH (phenylalanine hydroxylase) activity. X 2 is NR 10, O or S, wherein R 10 is H or C 1-3 alkyl which may be partially or fully fluorinated; and X 3 is a single or double bond, -CO, -SO 2 or -CH 2 -; R 5, R 6, R 7, R 8 and R 9, which may be the same or different, are selected from the group consisting of H, OH, SH, halogen, CN, NO 2, NH 2, optionally substituted C 1 -C 6 alkyl, optionally substituted C 2 -C 6 alkenyl, optionally substituted C 1 -C 6 aminoalkyl, optionally substituted C 1 -C 6 heteroalkyl wherein the hetero atom is N, O or S, optionally substituted C 1 -C 6 alkoxy, optionally substituted benzyloxy, optionally substituted C 1 -C 6 thiol, optionally oxidized to -S - or -S 2 -, and an aromatic group, optionally containing one or more heteroatoms selected from O, N and S, said aromatic group being optionally substituted, two of R 5, R 6, R 7, R 8 and R 9 may together form an optionally substituted carbocyclic or heterocyclic group;