1. (a) Uhlmann, E.; Peyman, A. Chem. Rev. 1990, 90, 543;
2. (b) Szymkowski, D. E. Drug Discovery Today 1996, 1, 415;
3. (c) Englisch, U.; Gauss, D. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 613.
4. (a) Of the more recent modifications, the protein nucleic acids (PNAs) are worthy of special note. For a comprehensive review, see: Uhlmann, E.; Peyman, A.; Breipohl, G.; Will, D. W. Angew. Chem., Int. Ed. Engl. 1998, 37, 2797;
5. (b) The first ‘antisense oligonucleotide’ therapeutic (Vitravene™), based upon a phosphorothioate internucleotide linkage, was approved by the FDA in August 1998 for treatment of CMV retinitis (for more details please see the Isis pharmaceuticals web site at www.isip.com).