Heterocyclic compounds

Compounds of the formula (I): or a pharmaceutically acceptable salt or ester thereof, wherein X , is nitrogen or a group CH and X is nitrogen or a group CR wherein R is hydrogen, hydroxy, halo or a group NHR wherein R is hydrogen or a C1-4 alkyl or C1-4 alkylcarbonyl group, R is amino or hydrogen; R is hydrogen, halo, SH, a group NHR wherein R is hydrogen or a C1-4 alkyl or C1-4 alkylcarbonyl group or R is a group OR wherein R is hydrogen, C1-4 alkyl or benzyl, R and R are the same or different and each is hydrogen or halo, X is a chain containing m CH2 groups and n oxygen atoms, m being 1, 2, 3, 4, or 5 and n being 0, 1 or 2 the sum of m and n being 2, 3, 4, 5, or 6; are disclosed as inhibitors of the enzyme, purine nucleoside phosphorylase, pharmaceutical compositions containing the compounds, their use in medicine and processes for their preparation are also described.