The two founders of Ribopharma AG, both former lecturers at the University of Bayreuth, Germany, Drs Roland Kreutzer and Stefan Limmer, discovered in 1999 that short, double-stranded RNA molecules can specifically inhibit gene expression in humans.

In 1998, Kreutzer and Limmer were inspired by Andrew Fire's and Craig Mello's groundbreaking publication (Nature 391, 806–811 (1998)) on RNA interference, work for which they won the Nobel Prize in 2006. Kreutzer and Limmer wanted to exploit this newly discovered biological principle to develop new medical agents. They were initially able to show that a certain cellular mRNA is reduced in a human in vitro transcription system if the corresponding double-stranded RNA is present. However, it was known that double-stranded RNA longer than about 30 base pairs leads to undesirable side reactions by stimulating the human innate immune system. Kreutzer and Limmer therefore tried to find ways to trigger RNA interference with shorter double strands that did not have this undesirable side effect. When studying the materials and methods section of the Fire and Mello publication in the journal Nature, Limmer noticed that the RNA strands used, which were several hundred nucleotides long, would not come together to form a homogeneous double strand under the annealing conditions applied. Rather, short double-stranded areas would arise, which would be interrupted by loops and bulges. From this, Kreutzer and Limmer concluded that complete double-strandedness could not have been a prerequisite for triggering RNA interference. The two scientists decided to experiment with short duplexes, in which the two 21 base pair single strands were linked by a non-nucleotide linker, which should therefore increase the stability of the double strand. The idea of ​​using 21-mers came from theoretical considerations: With this RNA length, a stable molecule with two helix turns is formed, which is often required for the interaction with proteins. In addition, statistical reasons dictate that a specific mRNA in the human genome can be uniquely addressed with this length. 

With these molecules, Kreutzer and Limmer obtained the first indications in experiments with human cell cultures that such short double strands can also trigger RNA interference in humans. Thus, they surmised that side effects of the long double strands should be avoidable. A big step was thereby taken towards the development of drugs based on short, double-stranded RNA. With this business idea, Ribopharma AG was founded in 2000 as the first company in the world that aimed to use RNA interference therapeutically. 

It was not until more than a year later that Thomas Tuschl and his working group discovered siRNAs as the trigger of RNA interference (Genes Dev 15(2), 188-200 (2001)), which happen to be exactly the same length as the molecules previously used by Kreutzer and Limmer. This scientifically confirmed their previous observations. 

The path from these first discoveries to mature drugs based on short, double-stranded RNA or siRNA was very long. In the meantime, however, the company Alnylam Pharmaceuticals in particular has developed some approved siRNA drugs, some of which can be used to treat diseases that were previously considered untreatable.

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