The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is involved in multiple physiological pathways including targeting of lysosomal enzymes, degradation of IGF2, and cicatrization through TGFbeta activation. To target potential therapeutics to this membrane receptor, four carboxylate analogues of mannose 6-phosphate (M6P) were synthesized. Three of them, two isosteric carboxylate analogues and a malonate derivative, showed a binding affinity for the M6P/IGF2R equivalent to or higher than that of M6P. Contrary to M6P, all these analogues were particularly stable in human serum. Moreover, these derivatives did not present any cytotoxic activity against two human cell lines. These analogues represent a new potential for the lysosomal targeting of enzyme replacement therapy in lysosomal diseases or to prevent the membrane-associated activities of the M6P/IGF2R.