Antisense oligonucleotides (ASO)

"Foundation building" for assembling the chain. The nucleoside that becomes the 3' terminus of the final sequence is attached in advance to porous beads like a fine-meshed sponge (preloaded support).

The procedure is simple: just pack the support with the 3' terminus already attached into the reaction vessel. But how uniformly and in what appropriate amount it is loaded per 1 bead affects every later step.

If the foundation is uneven, the yield and purity of the synthesis cycles stacked on top of it also vary. It is unglamorous, but this first loading is the reference point for quality.

This step builds the foundation for chain extension. How uniformly and in what appropriate amount the first nucleoside is attached to the support beads determines later yield and purity.

Alternatives in practice

• Universal linker support (independent of the terminal sequence) · Cytiva Primer Support 5G UnyLinker (formerly AM Chemicals UnyLinker) / Glen Research Universal Support III PS — 1 type of support accommodates any 3' terminus, reducing the number of stocked types. A phosphate terminus is generated on deprotection, so take care with the workup conditions.
• CPG (controlled pore glass) support · Prime Synthesis NittoPhase/CPG series or Merck/Sigma-Aldrich dT-CPG (500/1000 Å) — High quality, but loading is lower than polystyrene. Preferred for long chains, reversed-phase purification workflows, and lab to mid scale.

The heart of the synthesis, joining residues 1 at a time. Remove the 5' protection, attach the next nucleoside, block unreacted ends, fix the phosphorus linkage: these 4 moves are repeated as 1 cycle.

Fixing the phosphorus linkage by "oxidation" gives the ordinary phosphate backbone; making it "sulfurization" gives the phosphorothioate backbone (a sulfur-containing backbone) that is widely used in antisense oligonucleotides and resists degradation.

The key is the completeness of each reaction. If the efficiency of 1 cycle does not reach the high 99% range, deletion sequences missing 1 base slowly accumulate and directly lower the final purity.

This is the heart of the process, joining bases 1 at a time with precision. Unless the completeness of each reaction (coupling efficiency) is in the high 99% range, deletion sequences accumulate and purity falls.

Alternatives in practice

• Choice of sulfurizing reagent (DDTT vs PADS vs EDITH) · ChemGenes/Glen Research DDTT (Sulfurizing Reagent II), PADS (phenylacetyl disulfide), Link/Glen EDITH — DDTT has excellent solution stability and reactivity and is general-purpose. PADS is inexpensive and oriented to large scale, prepared at the time of use. EDITH gives fast sulfurization.
• Choice of activator (BTT/ETT/DCI) · Merck/ChemGenes BTT, Sigma-Aldrich ETT (5-ethylthiotetrazole), Sigma-Aldrich/Carbosynth DCI (4,5-dicyanoimidazole) — BTT balances solubility and reactivity well and is a large-scale staple. ETT is highly reactive. DCI is neutral and advantageous for base-sensitive sequences.
• Batch (stirred-tank) synthesis · AGC/formerly GE batch reactor-type synthesis, or in-house designed stirred-tank solid-phase reactors — Large-volume processing in a stirred tank, as opposed to flow-through column synthesis. Reagent contact uniformity and back-pressure management are the issues.

A step that detaches the finished chain from its foundation and removes all the "protective covers" that were on during synthesis, returning it to its native form. Treatment is with aqueous ammonia or AMA (a mixture of ammonia and methylamine).

The protecting groups that guarded the bases and phosphates are removed all at once, but if some are left on, or if side reactions such as deamination (in which a base is altered) occur, they become new impurities as they are.

In other words, quality is determined not only by whether the chain was built cleanly but also by whether it was stripped cleanly. Control of reagent, temperature and time conditions is what counts in this step.

This step releases the finished chain from the support and removes all the protecting groups that were on throughout synthesis, restoring the native structure. Incomplete removal and side reactions (such as deamination) become new impurities.

Alternatives in practice

• Concentrated aqueous ammonia alone (high temperature, long time) · Sealed pressure-resistant reaction vessel + temperature-controlled oven / water bath (e.g., 55 °C, 8–17 h) — Milder than AMA with fewer side reactions, suitable for sensitive modifications (some fluorescent dyes, specific base protections). Treatment time is long.
• Fluoride desilylation for RNA types (TEA·3HF) · Temperature-controlled reaction vessel (around 65 °C) + quench buffer (e.g., 1 M Tris-HCl / NaOAc) — An additional step that is essential for 2'-TBDMS/TOM-protected RNA. Complete deprotection and avoiding gel-like precipitation are the issues.

A purification step that picks out only the full-length chain as intended from the reaction solution. The target and the impurities are pulled apart by preparative anion exchange, which separates by charge difference, or by reversed-phase chromatography with a hydrophobic marker left at the 5' end.

The difficulty is that it separates "almost identical things." The opponents are closely related impurities with similar properties, such as the n-1 deletion that is one base shorter, the n+1 addition that is one base longer, and species with protecting groups left on.

Whether the closely related impurities can be removed completely here is what divides meeting the purity specification from failing it. This is the step in ASO manufacturing where separation skill is tested the most.

This step takes out only the desired full-length chain. Removing closely related impurities such as n-1 deletions, n+1 additions and not-yet-detritylated species completely here is the key to meeting the purity specification.

Alternatives in practice

• Membrane ion-exchange chromatography · Sartorius Sartobind Q / Cytiva Mustang Q membrane adsorbers — High flow rate, single-use, and no resin packing needed. Resolution is inferior to resin AEX, but capture and crude purification are faster.
• Orthogonal 2-step purification with reversed phase and AEX · Cytiva ÄKTA process + 2-stage purification: DMT-on reversed phase (Daisogel C18) → AEX (Source 30Q) — 2 modes with different principles remove n-1/n+1 and deeper impurities to a high degree. For high-purity API; a trade-off between yield and number of steps.

The fraction that comes out of purification is a thin aqueous solution rich in salt. If dried as it is, the salt remains in the drug substance, so this is the "desalting and solvent exchange" step that removes unneeded salt and low-molecular-weight by-products.

Gel filtration (size exclusion), which sieves by size difference, or tangential flow filtration (TFF, which filters through a membrane) removes elution salt, trityl-derived by-products and low-molecular-weight impurities and adjusts the composition to suit downstream steps.

Passing through here gives a solution in a "well-behaved state" that can be handed on to the next concentration or drying. It is a relay step that bridges the results of purification to the formulation.

The eluted fraction from preparative anion exchange is a dilute aqueous solution containing high salt concentrations, and if freeze-dried as it is, salt remains in the drug substance. So the elution salt, trityl by-products and low-molecular-weight impurities are removed by size exclusion (desalting) or TFF, and the buffer is exchanged.

Alternatives in practice

• Desalting and concentration by ultrafiltration/diafiltration (TFF) · Sartorius Sartocon Hydrosart / Cytiva Hollow Fiber or Merck Pellicon 3 (1–2 kDa MWCO) — Desalting and concentration can be done at the same time, which is advantageous for large-volume processing. Yield is higher than SEC, but membrane selection and gel layer management are the key points.
• Desalting and concentration by ethanol precipitation · Refrigerated centrifuge (e.g., Beckman Coulter Avanti J series) + addition of NaOAc/NaCl — Simple for the lab and small quantities. At large scale, solvent volume, waste liquid and handling are the bottleneck.

In reversed-phase purification, the dimethoxytrityl group (DMT, a hydrophobic handle) is deliberately left at the 5' end to sort out only the full-length product and raise purity. This is the step that removes that handle and returns the end to its native form.

DMT is an unneeded protecting group for the drug substance, so it is removed with a weak acid such as acetic acid or TCA, then neutralized immediately to return it to the native 5'-hydroxyl terminus. In anion-exchange DMT-off mode, the same treatment is done at the last stage of synthesis.

The marker attached temporarily to gain purity is cleanly removed at the end. It is the "finishing" of purification, the move that fixes the final sequence of the product.

In reversed-phase DMT-on mode, the 5'-DMT group is left on as a hydrophobic handle to selectively purify the full-length product and improve purity, but it is an unneeded protecting group for the drug substance, and after purification it must be detritylated with a weak acid such as acetic acid to return it to the native 5'-hydroxyl terminus. In anion-exchange DMT-off mode, the same treatment is performed in the final cycle of synthesis.

Alternatives in practice

• Detritylation/treatment with cation-exchange resin · Cytiva ÄKTA process + strong cation-exchange resin (e.g., Cytiva SP Sepharose series) — Reduces the amount of acid added and avoids local excess acid. Adds a resin step but improves process controllability.
• On-column detritylation (on-column acid wash) · Dilute acid (TEAA/acetic acid) wash on a DMT-on reversed-phase column (Cytiva ÄKTA process + Daisogel C18) — Links purification and DMT removal and omits a separate step. Control of acid contact time and resin tolerance are the key points.

A step that removes the salt used in purification and concentrates while exchanging into the target solvent (mostly water for injection). With tangential flow filtration (TFF, filtering through a membrane), ultrafiltration (concentration) and diafiltration (exchanging the contents) are carried out.

The final amount of sodium counterion and the salt concentration are built here. An oligo is a negatively charged salt, so the composition of the drug substance is determined including the amount of the paired cation.

In other words, this step fixes "in what salt form and at what concentration the drug substance will be." It prepares the final form to hand on to the next solidification and filling.

This step removes the salt used in purification, exchanges into the target solvent (mostly water for injection) and concentrates. The final sodium counterion content and salt concentration are built here.

Alternatives in practice

• Size exclusion (desalting) chromatography · Cytiva ÄKTA process + Sephadex G-25 — No concentration, but simple and highly reproducible if dilution is acceptable. Buffer consumption increases at large volumes.
• Ethanol precipitation · Refrigerated centrifuge + NaOAc/ethanol — For small quantities and the lab. At large scale, solvent volume and securing sterility are the issues.

"Sterile filtration" removes microorganisms by passing the drug solution through a 0.22 µm filter immediately before final filling. It is the terminal step that captures bacteria larger than the pores and ensures the sterility of the product.

When the process moves from an organic-solvent system to an aqueous system downstream, the environment approaches one in which microorganisms grow readily. That is why bioburden (viable count) is cut off with a filter at the end.

After filtration, an integrity test is also performed to check the filter for holes or tears. For drug substances and drug products administered by injection, this step cannot be omitted.

In the downstream transition from an organic-solvent system to an aqueous system, the risk of microbial contamination increases, so the drug solution is filtered through a 0.22 µm sterilizing filter immediately before final filling to ensure sterility. It is a mandatory terminal step for injectable drug substances.

Alternatives in practice

• Sterile filtration with a 0.22 µm PES capsule filter · Sartorius Sartopore 2 capsule / Merck Opticap XL (PES) — Single-use capsules need no cleaning validation. Higher flow rate and low adsorption compared with PVDF, well suited to protein-free solutions.
• Redundant 2-in-series (0.45 + 0.22 µm) sterile filtration · Sartorius Sartopore 2 XLG (0.45 + 0.2 µm double layer) or Merck Millipore Express SHC (double layer) — The upstream layer reduces the bioburden load and prevents clogging of the main filter. Improves reliability for high-load solutions.

A step that sets the drug substance into a form that can be stored and shipped stably, and determines the final form of the product. Many antisense drug substances have the water removed by freeze-drying and are finished as an easy-to-handle solid powder.

When formulated as an injection, the solution undergoes 0.2 µm sterile filtration and is then aseptically filled into vials. Whether to make a dry powder or a liquid is chosen according to the stability of the product and operational needs.

For freeze-dried products, whether the product dissolves easily at the time of use (reconstitution) and the residual moisture are the quality points. Once past this, it becomes a product that can finally be released.

This step puts the material in a stable form so it can be stored and shipped. Many antisense drug substances are freeze-dried powders, and the drug product is sterile-filtered and filled as an injection.

Alternatives in practice

• Liquid (no freeze-drying) formulation · Aseptic filling line (Bosch/Syntegon) + 0.2 µm sterilizing filtration (Sartopore 2) — Omits the freeze-drying step to reduce cost and time. Adopted for formulations that stability allows (many ASO aqueous solutions).
• Adding a delivery conjugate step such as GalNAc · Conjugation reaction tank + AEX/reversed-phase purification (Cytiva ÄKTA process) — Targets hepatocyte ASGPR, enabling subcutaneous administration and lower doses. GalNAc is introduced after synthesis or on the solid phase, and additional purification is needed.

The most important test of quality, determining how pure the target is and what impurities are mixed in and in what amounts. Ion-pair reversed-phase HPLC separates closely related species with similar chain length and sequence.

Detection uses UV (ultraviolet absorption) and mass spectrometry (MS) together, and species that are 1 base short (deletions), 1 base longer (additions) or oxidized are told apart by identity and quantitated.

This result decides pass or fail against the purity specification. It is also a mirror of what happened where in manufacturing, and hints for process improvement can be read from it.

This is the most important test, measuring how pure the target is and what impurities are mixed in and in what amounts. Deletions, additions and oxidized species are quantitated to judge conformance to the specification.

Alternatives in practice

• Anion-exchange HPLC (UV detection) · Thermo Dionex DNAPac PA200 / PA200 RS column + Vanquish / UltiMate 3000 UHPLC — Separates impurities differing in length/charge with high resolution without ion-pair reagents or MS. MS identification is needed separately.
• Capillary gel electrophoresis (CGE) · SCIEX PA 800 Plus / Agilent 7100 CE + ssDNA gel kit — High-resolution quantitation of length-type impurities such as n-1. Fast with small sample amounts; optimization for salt and sequence dependence is the key point.
• IP-HILIC (ion-pair hydrophilic interaction) · Waters ACQUITY Premier + BEH Amide / HALO HILIC series columns — Effective for polar and short-chain species that are difficult to separate by reversed phase. Complementary for separating metabolites and 5'-phosphate species.

An "identification" test to confirm that what was made really has the intended sequence and molecular weight. High-resolution LC-MS measures the accurate mass and first confirms that the molecular weight matches.

MS/MS then breaks the molecule into fragments, and the sequence is read 1 position at a time from the fragment pattern. This supports, on both mass and sequence, that the product is identical to the target.

Whereas the purity test looks at "how clean it is," this one guarantees "whether it is the intended product in the first place." It is the step equivalent to an ID card for the product.

A test to confirm that what was made really has the intended sequence and molecular weight. Identity is demonstrated on both mass and sequence.

Alternatives in practice

• Identification support by UV absorption spectrum and molar extinction coefficient · Agilent Cary 60 / Thermo NanoDrop One UV-Vis spectrophotometer — Simple supporting identification. Not sequence-specific, so it is used together with MS/CGE.
• Sequence confirmation by hybridization / enzymatic digestion + MS · Nuclease digestion (e.g., SVPD/phosphodiesterase) + Waters BioAccord LC-MS — Confirms the sequence from the masses of ladder fragments. Useful for identifying modification positions and detecting sequence variants.

A content and potency test that determines how much active ingredient is in 1 vial. It uses the property that oligonucleotides absorb ultraviolet light strongly, and calculates concentration and amount from UV absorbance.

The content determined here bears directly on the dose that reaches the patient. Too much or too little is a problem, so accuracy is strictly demanded.

The measurement principle is simple, but it is an essential basic test that guarantees the product's "amount of efficacy."

A test that determines how much active ingredient is in 1 vial. It bears directly on the accuracy of the dose.

Alternatives in practice

• qNMR (quantitative NMR) · Bruker Avance NEO 400/600 MHz or JEOL ECZ 400 / internal standard (e.g., maleic acid) — Absolute quantitation is possible and no extinction coefficient is needed. Equipment and time cost and sensitivity are the issues.
• Calculation of content = purity × total amount · Combination of IP-RP-LC-UV (purity) + UV-Vis (total amount) — Calculates the drug substance content as the product of separate measurements. Note that the uncertainty of each measurement propagates.

Because an oligonucleotide is a negatively charged salt, a partner cation (mostly sodium) is always bound. This test measures how much of that counterion is attached.

The counterion is quantitated by ICP-MS or ICP-OES (analyses that measure metals with high sensitivity). This confirms what salt form the product is in.

The amount of salt also affects physical properties and the dose calculation from content. It is unglamorous, but it is an essential item for correctly pinning down the nature of the drug substance.

Oligos are negatively charged salts, so this measures how much of the paired cation (mostly sodium) is bound. It bears on the salt form, physical properties and dose calculation.

Alternatives in practice

• Ion chromatography (IC) · Thermo Dionex Integrion / ICS-6000 + IonPac CS / AS separation columns + conductivity detection — Directly quantitates counterions such as sodium, with no acid pretreatment. Coverage of metal species is inferior to ICP-based methods.
• Atomic absorption spectrometry (AAS) · Agilent 240/280 series AA (flame / graphite furnace) — Single element, low cost. Lower throughput than multi-element simultaneous ICP-MS/OES.

A test that measures how much of the organic solvents used in synthesis and purification, such as acetonitrile and triethylamine, remains in the product. Volatile solvents are the target.

Headspace GC (a technique that analyzes the gas above the solution) or GC-MS quantitates the remaining solvents 1 type at a time. The key is to capture low-boiling components without losing them.

The yardstick for judgment is the safety guideline ICH Q3C. It is confirmed that levels fall within the set thresholds, assuring patient safety.

This measures how much of the organic solvents used in synthesis and purification, such as acetonitrile and triethylamine, remains in the product. It confirms that the levels are within the safety (ICH Q3C) thresholds.

Alternatives in practice

• Direct-injection GC · Agilent 8890 GC + split/splitless injection + FID — No headspace unit needed, advantageous for high-boiling solvents. Watch for matrix contamination and liner degradation.
• Residual solvent screening by NMR · Bruker Avance NEO / JEOL ECZ (1H-NMR) — Little pretreatment and grasps multiple solvents at once. Inferior to GC in sensitivity and quantitation, suited to confirmation tests.

Because the product goes directly into the body by injection, this is a test bearing directly on patient safety that guarantees the absence of fever-causing endotoxin and microbial contamination.

Endotoxin is quantitated by LAL (the Limulus test, which uses blood cell components of the horseshoe crab) and evaluated according to the standards of USP <85>. Bioburden (viable count) and sterility are also confirmed.

This is a bastion that looks not at efficacy but at "whether it can be administered safely." It is an absolutely essential item: a product that falls outside the criteria cannot be released.

Because the product is administered by injection, this test guarantees the absence of pyrogens (endotoxin) and microbial contamination. It is a mandatory item bearing directly on patient safety.

Alternatives in practice

• Recombinant Factor C (rFC) method · Lonza PyroGene rFC assay + fluorescence plate reader (e.g., Molecular Devices SpectraMax / BMG LABTECH FLUOstar) — Does not use horseshoe crab-derived LAL, so it does not depend on animal resources. Less β-glucan interference and high specificity.
• Membrane filter sterility test · Merck Milliflex / Steritest Symbio or Sartorius Sterisart system — The antimicrobial components of the drug can be washed away, reducing the influence of inhibitory substances. The standard method in accordance with USP <71>.

A group of tests that confirm the appearance and basic physical properties of the product across the board. Appearance is checked visually, pH and osmolality are measured, and water content is determined by the Karl Fischer method.

The aim is to make sure there is no precipitation, discoloration, or pH or osmolality outside the specification. For injectables, control of osmolality and water content is especially important.

They are not flashy, but these tests support the quality of the drug product from the ground up. Only when these are in order can it be called a product that can be used with confidence.

A group of tests that confirm the basic appearance and physical properties of the product. They guarantee that there is no precipitation or discoloration and no out-of-specification pH or osmolality.

Alternatives in practice

• Automated particulate (insoluble foreign matter) inspection · HIAC 9703+ (light obscuration, Beckman Coulter) or Bosch/Syntegon AIM automated visual inspection machine — Objectively quantitates particulates in accordance with USP <788>. Improves reproducibility and throughput compared with visual inspection.
• Water content alternative by loss on drying (LOD) · Mettler Toledo HX204 halogen moisture analyzer or vacuum drying oven + weighing — Simple, but volatile components are also counted, so specificity is lower than KF. For a rough grasp of water content in freeze-dried products.