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Title: Process development using organic solvent nanofiltration (OSN) for oligonucleotide synthesis
Author: Kim, Jeong
ISNI:       0000 0001 2437 7513
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Following the discovery of RNA interference (RNAi) and its exquisite control of protein expression, oligonucleotide (oligos) based drugs have been considered as the next generation of therapeutics, as they can deliver the treatment at the protein expression level. Oligos are short chains of nucleotides, typically 15-28 base-pairs long, synthesized in an iterative manner by sequential addition of nucleotide monomers. As of year 2014, there are currently 3 FDA approved oligo-based drugs, and over 100 candidates are going through clinical trials. The current mainstream synthesis platform is solid-phase oligo synthesis (SPOS) which is fast and efficient, but difficult to scale-up. This thesis discusses work to develop a versatile liquid-phase oligo synthesis (LPOS) platform to manufacture oligos at large scale using emerging Organic Solvent Nanofiltration (OSN) technology. By attaching the growing oligos chain to a soluble, monodisperse, and branched polyethyleneglycol (PEG) support and operating entirely in liquid-phase, excess reagents and impurities can be filtered out using polybenzimidazole (PBI) OSN membranes. This process neatly solves cumbersome purification problems while allowing high reaction rates and a large reaction volume. With the proposed process, 5-mer and 9-mer 2’-OMe oligoribonucleotides have been successfully synthesized. The synthesis chemistry and membrane purification procedures have been modified to suit the LPOS platform. The transport of solutes through the PBI membranes has been studied in detail which revealed both size-exclusion and ion-exchange behavior. An economic analysis showed the competitiveness of the LPOS-OSN process, but also the necessity for a solvent recovery unit. Several key challenges, both expected and unexpected, have been identified, and appropriate solutions have been implemented. One of the main challenges was the low process yield, which was overcome through the use of membrane cascades. An adsorptive solvent recovery unit was also investigated to reduce the overall solvent consumption. Notably, employing a membrane reactor combined with a smaller protecting group (methoxyisopropyl ether, i.e. MIP, MW 74 Da) significantly simplified the overall process (9 to 3 unit operations per chain extension cycle). It is envisaged that the proposed LPOS platform can be used synergistically with the current SPOS platform to stimulate further growth in the field of oligo therapeutics. Furthermore, through the process development work using OSN technology, an effort was made to understand the gap between membrane science in academia and membrane technology in industry.
Supervisor: Livingston, Andrew Sponsor: European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral