Determination of Purge Factors for Use in Oligonucleotide Control Strategies

The understanding, control, and removal of nonoligonucleotide process-related impurities (PRI) are of key importance for the manufacturing of therapeutic oligonucleotides as their presence in the final product is both a quality and safety concern. Regulatory agencies require manufacturers to demonstrate that PRI are under control or adequately purged during the manufacturing process. Purging depends on the physicochemical properties of the impurities and the unit operations of the manufacturing process but should be independent of oligonucleotide size or type. The purging power of unit operations relevant to oligonucleotide manufacturing (synthesis, cleavage and deprotection, chromatography, ultrafiltration/diafiltration) was measured using representative solvents and other small molecules typical for oligonucleotide synthesis. The results show that each unit operation has significant purging capability (synthesis >1000; cleavage and deprotection >100 (reactivity, when applicable); chromatography >1000; ultrafiltration/diafiltration >10) and that large overall purge factors can be obtained (≥1 × 107). Experimentally determined purge values are aligned with theoretical purge values; thus, the use of purge arguments in oligonucleotide control strategies is a sound scientific approach. Guidance on reasonable purge values for oligonucleotide unit operations is presented. Additionally, the data demonstrate that solvents and reagents typically used in oligonucleotide synthesis are robustly cleared by the process and should not require testing in the final product.