Oligonucleotide therapeutics

Oligonucleotide based gene silencing, discovered in the late seventies by Stephenson and Zamecnik, has evolved over the years into oligonucleotide based therapeutics, the two most important directions being antisense and RNA interference. In the therapeutic context it is believed that chemically modified oligonucleotides have superior properties compared to natural DNA or RNA as they allow to address and improve on critical parameters such as RNA target affinity, nuclease resistance and cellular uptake and distribution. As a consequence, a large variety of chemically modified oligonucleotide analogues have been synthesized in the past and their biological properties evaluated.
In the early nineties we introduced the concept of conformational restriction with the example of bicyclo-DNA (bc-DNA)[1], resulting in oligonucleotide structures the show reduced entropic penalty upon duplex formation. This concept has been adapted by others and has produced excellent oligonucleotide drug candidates such as locked-nucleic acids (LNA) and hexitol nucleic acids (HNA).These analogues typically exhibit increased affinity to RNA and feature higher nuclease resistance.
While in this way the challenges linked to target affinity and nuclease resistance have largely been met, other requirements such as improving cellular uptake and distribution are yet elusive. In the context of oligonucleotide therapeutics our laboratory has developed over the years the bicyclo-DNA molecular platform (Figure 1). The 5-membered carbocyclic ring in bicyclo-DNA offers unique possibilities for further chemical modification, specifically at C(6’) and C(7’), to introduce additional functional groups for various purposes. In recent work we reported on the DNA and RNA affinity of 6-alkyl or -oxyalkyl modified bc-DNA [2], [3], and particularly on C(6’)-oxime modified bc-DNA (bcox-DNA)[4], where the uncharged lipophilic benzyl group was shown to improve cellular uptake relative to unmodified oligonucleotides.
The hitherto most interesting candidate, however, is tc-DNA showing improved binding properties to complementary RNA by increases in thermal melting by 2-4°C per modification.[5] We have investigated its antisense properties as steric block agents as well as splice modulators and have found it to be superior to second generation oligonucleotide therapeutics. We are currently developing a tc-oligonucleotide 15-mer for exon skipping in the context of Duchenne muscular dystrophy and expect it to enter clinical phase I in the next year.


Figure 1: The bicyclo-DNA molecular platform


Research Group of Prof. Christian Leumann © 2017