A targeted therapeutic agent comprising a compound of formula I: B-L-D  (I), wherein: B is a low molecular weight binding moiety for Carbonic Anhydrase IX (CAIX); D is a drug moiety; and L is a linker group that undergoes cleavage in vivo for releasing said drug moiety in an active form. The drug moiety is suitably a cytotoxic agent for targeted delivery to cancer cells expressing CAIX. The binding moiety B suitably comprises a sulfonamidothiadiazole moiety. The binding moiety B may comprise one, two or more groups capable of binding to CAIX. The linker group suitably comprises a disulfide bond and/or a triazole group and/or a cleavable peptide group.