Compounds according to formula I in which formula R1 and R2, which may be the same or different, represent halogen, (C1-C6)hydrocarbyl, optionally substituted with one or two hydroxyl group or one or more fluorine atoms, or, together with the carbon atom to which they are both attached, R1 and R2 form a (C3-C6)carbocyclic ring, or one of R1 and R2 taken together with R3 forms a direct bond, such that a triple bond is constituted, or R1 and R2 represent both hydrogen; R3 when not forming a direct bond with one of R1 and R2 represents hydrogen or (C1-C3)hydrocarbyl; X represents (E)-ethylene, (Z)-ethylene, ethynylene, or a bond; Y and Z independently represent hydrogen or methyl; and prodrugs and stereo isomeric forms thereof are provided together with their use in therapy, and their use in the manufacture of medicaments.