Oligonucleotide therapeutics. A long and windy road towards future medicines

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INTRODUCTION
The research of new drugs to treat diseases considered “undruggable”, such as genetic disorders, has led to the use of complex molecules, such as, antibodies, nucleic acids and others. Although the idea of taking advantage of the base pairing properties of oligonucleotides to design specific drugs to inhibit gene expression is present in the bibliography since the early 70’s (1), Zamecnik and Stephenson in 1978 demonstrated for the first time the inhibition of Rous sarcoma virus replication (2). The development of the phosphoramidites (3) and the outbreak of the HIV (4) triggered the interest for the first antisense studies, mainly focused on the development of antiviral oligonucleotides (Chart 1). At that time, one of us was doing his postdoctoral training at the Beckman Research Institute of City of Hope and, later on, at the University of Colorado at Boulder. At City of Hope we had the chance of preparing oligonucleotides carrying methylphosphonates linkages (Figure 1), following the methodology described by Dr. Miller’s group (5). A series of short oligonucleotides complementary to the first splice acceptor site of the tat-3 gene of HIV were found active against HIV infection (6). Then, at the University of Colorado I was studying the preparation and application of O-phenylphosphoramidites to combine the advantages of phosphoramidites and H-phosphonate methods in the preparation of phosphate modified oligonucleotides (7). Within the 80’s, a large number of important discoveries generate the future trends in nucleic acid chemistry: 1) The discovery of the Polymerase Chain Reaction (PCR) (8) that led to modern molecular biology techniques, to the use of oligonucleotides in diagnostic purposes and to the beginning of the human genome project (9). 2) The discovery of nucleic acids with binding properties to defined targets known as “Aptamers” by the systematic evolution of ligands by exponential enrichment (SELEX, 10). 3) The phosphoramidite methodology (11) and, in less extent, the H-phosphonate method (12, 13) led to the development of hundreds of modified oligonucleotides that were critical for the advance of the nucleic acids as drugs avoiding the degradation of oligonucleotides to nucleases present is sera (Figure 1). The main discoveries will be described below.