Abstract: The invention provides compounds of formula (I), their pharmaceutically acceptable salts and prodrugs thereof for use in preventing, inhibiting or treating a disease caused by a mutation in the gene coding for hydroxymethylbilane synthase, in particular for preventing, inhibiting or treating acute intermittent porphyria: (I) wherein: A is selected from N and CR10 (wherein R10 is H, —NO2, C1-6 haloalkyl or —C(O)R17 in which R17 is H or C1-6 alkyl); Z is selected from N and CR9 (wherein R9 is H, halogen (e.g. F, Cl, Br or I) or —OR16 in which R16 is H, C1-6 haloalkyl, or optionally substituted C1-6 alkyl); L is selected from —CH2—, —C(O)—, —CH(OH)—, —C(O)—NR?—, and —NR?—C(O)— (wherein R? is H or C1-3 alkyl, e.g. —CH3); R1 is H; R2 is selected from H, halogen (e.g.