siRNA

siRNA is a drug in which 2 strands are paired. So the process starts by making the single strands that serve as its material, the sense strand and the antisense strand, separately.

The method is to link bases 1 at a time by chemical bonds on a foundation called a solid support, extending them to the target sequence as if climbing stairs. It is steady, step-by-step accumulation.

The key is the coupling efficiency per 1 cycle. If this drops even slightly, the full-length yield shrinks greatly after stacking 20 bases. Equipment and reagents that deliver high efficiency stably are the key.

siRNA is double-stranded. First each single strand is made by chemically linking bases 1 at a time on a solid support. Even a slight drop in coupling efficiency per 1 cycle greatly reduces the full-length yield for 20 bases. So equipment and reagents that deliver high coupling efficiency stably are key.

Alternatives in practice

• 2'-O-TOM-protected amidite synthesis · Cytiva ÄKTA oligopilot plus / OligoProcess (TOM-compatible) + ChemGenes/Hongene 2'-O-TOM amidites — Stable coupling efficiency for long RNA; deprotection is compatible with TBAF and fluoride systems
• Solution-phase / blockmer (liquid-phase) synthesis · Stirred reaction vessel (general-purpose GL reactor) + ultrafiltration. For the anchor method, liquid-phase platforms such as Ajinomoto AJIPHASE — Avoids the solid-phase packing limit at large scale. Purification load and anchor chemistry are challenges
• 5'-DMT-on synthesis · The same synthesizer + reversed-phase preparative purification (simplifies downstream IP-RP purification) — Uses DMT hydrophobicity as a purification handle to raise full-length selectivity

The strand that has finished elongating is still attached to its base. Moreover, during synthesis the bases and phosphates were covered and protected with protecting groups. This is the step that removes them.

Strong basic conditions such as aqueous ammonia or AMA carry out the cutting from the support and the removal of protecting groups in one go. The image is of returning to a chemically "bare strand."

However, if the conditions are too harsh the RNA itself is cleaved. It is a step of fine judgment, where the balance of temperature and time directly determines quality.

When elongation is finished, the synthesized strand is cut from the solid support and the protecting groups that guarded the bases and phosphates are removed. Strong ammonia-based conditions are used, but if the conditions are harsh here the RNA is cleaved, so moderation is important.

Alternatives in practice

• Aqueous ammonia alone, long time / low temperature · Temperature-controlled closed reaction vessel (extended at room temperature to 55 °C) — Mild, for when base-labile modifications are to be protected
• Mild cleavage with K2CO3/methanol · Stirred reaction vessel + filtration (Merck potassium carbonate/methanol) — Low damage to sensitive functional groups such as acyl-protected RNA and GalNAc

RNA has a 2'-hydroxyl that DNA lacks, and during synthesis this too was protected with a silyl protecting group (TBDMS/TOM). The last cap that remains is removed.

The remover is the fluoride ion derived from TEA·3HF. If the protecting group remains, the sequence and mass will not be correct, so it is important to remove it completely.

RNA immediately after the cap is removed is in a defenseless state prone to degradation. So the field rule is not to stop here but to send it promptly to the next step.

The silyl protecting group that guarded the 2' position unique to RNA is removed with fluoride ions. If it remains, the sequence and mass will not be correct. After removal the RNA is in a state prone to degradation, so the material is moved promptly to the next step.

Alternatives in practice

• TBAF/THF desilylation · Stirred reaction vessel + Sigma-Aldrich tetrabutylammonium fluoride (TBAF) 1 M/THF — Simple at lab scale, but salt removal and control of residual TBA are needed
• Hydrogen fluoride-pyridine / NMP system · HF-resistant reaction vessel (PTFE/Hastelloy) + HF-pyridine (Sigma-Aldrich Olah's reagent)/NMP — Fast reaction, but highly corrosive and requires handling equipment

In synthesis, strands 1 base short or long and strands not completely deprotected are always mixed in. Purification takes only the target full-length single strand (FLP) out of that mixture.

There are 2 ways of separating. Anion exchange, which separates by differences in charge, and ion-pair reversed phase, which separates by differences in hydrophobicity, are used selectively depending on the sequence and modifications.

How far impurities can be dropped here decides whether the later purity specification can be met. It is the crux step that forms the foundation of siRNA quality.

Strands 1 base short or 1 base too long, and strands not completely deprotected, are always mixed in during synthesis. Purification takes out only the target full-length strand. There are methods that separate by charge and methods that separate by hydrophobicity, and both are used selectively depending on the situation.

Alternatives in practice

• DMT-on reversed-phase preparative purification · Cytiva ÄKTA process + Daicel DAISOGEL ODS / Waters XBridge OST C18 (full-length product captured by the hydrophobic DMT) — High full-length selectivity and good compatibility with the subsequent detritylation
• Mixed-mode (RP+IEX) HPLC · Process HPLC + mixed-mode stationary phase (e.g., Tosoh/Daiso types) — Combines hydrophobicity and charge in 1 step to separate closely related impurities
• Preparative CGE / gel filtration · SCIEX PA 800 Plus (analytical confirmation) + Cytiva Sephadex G-25 (gel filtration desalting) — Used as a supplement for high-resolution purity confirmation and final desalting

To separate the full-length product cleanly in purification, there is a strategy of temporarily attaching a hydrophobic handle called DMT to the 5' end of the strand (DMT-on). This step removes that handle at the end.

Cleavage is done by making it acidic at around pH 3.5 with acetic acid. The DMT-OH by-product that comes off is properly removed by subsequent filtration, precipitation and UF/DF.

Without this, the correct 5'-OH drug substance structure cannot be reached. It is unglamorous, but an essential finishing step that fixes the identity of the product.

In the "DMT-on" strategy, in which a hydrophobic DMT group is temporarily added to raise the resolving power of IP-RP/reversed-phase purification, detritylation (usually acetic acid treatment at around pH 3.5) is always performed after purification, and the DMT-OH by-product released is removed by subsequent filtration, precipitation or UF/DF. Without it the correct 5'-OH drug substance structure is not reached; a standard essential step.

Alternatives in practice

• Detritylation using cation exchange resin · CEX resin columns such as Cytiva Capto SP ImpRes / Tosoh TSKgel SP-5PW — DMT-oligo is retained on the resin and acid-treated, minimizing downstream removal of salts and acid
• One-step on-column detritylation integrated with IEX purification · Cytiva ÄKTA process + DMT removal built in on Source 30Q — Integrates purification and detritylation to reduce the number of steps

In purification, ion-pair reagents and various salts are used to aid separation. This step washes out those that have served their purpose.

Ultrafiltration/diafiltration (UF/DF) uses a membrane to remove only small molecules while adjusting the material to an easy-to-handle sodium salt form and concentration.

It is, so to speak, groundwork for the next step. Aligning composition and concentration accurately here lets downstream lyophilization and quality testing proceed without scatter.

Ion-pair reagents and salts used in purification are removed, and the material is adjusted to an easy-to-handle salt form and concentration. This is the groundwork for accurately carrying out downstream lyophilization and quality testing.

Alternatives in practice

• Desalting by ethanol precipitation · Cooled stirred tank + centrifugation (NaOAc/EtOH precipitation) — Simple at small to medium scale; a trade-off between yield and residual salt
• Size exclusion/reversed-phase cartridge desalting · Cytiva Sephadex G-25 (NAP/PD) columns / Waters Sep-Pak C18 — Rapid desalting for small samples and process checks

The oligo drug substance eventually becomes the starting material of a sterile drug product, the injectable. So the microbial and particle load is lowered before lyophilization or isolation.

The method is bioburden reduction filtration through a 0.22 µm sterilizing-grade membrane at the aqueous solution stage after UF/DF. Bioburden and endotoxin are held down to the control level.

Only after passing through this can the material proceed to drying and isolation in a clean state. It is positioned like a checkpoint that cuts off microbial risk in the later steps.

Oligonucleotide drug substance is usually the starting material of a sterile drug product, so at the aqueous solution stage after chromatographic purification/UF-DF, control of bioburden and endotoxin and bioburden reduction filtration through a 0.2/0.22 µm sterilizing-grade membrane are performed as standard. Without it, microbial load before lyophilization cannot be controlled, and the specification (bioburden, endotoxin) cannot be assured.

Alternatives in practice

• Sartopore 2 (dual-layer PES) 0.2 µm sterilizing-grade filtration · Sartorius Sartopore 2 capsule (0.45+0.2 µm dual-layer PES) — Dual-layer structure gives high throughput and is advantageous for liquids prone to clogging
• Supor/Fluorodyne 0.2 µm sterilizing-grade membranes · Pall Supor EKV / Fluorodyne II 0.2 µm capsule — Wide chemical compatibility; a different membrane material secures redundancy

Here at last, the 2 single strands made separately are united. The sense strand and antisense strand are mixed at equimolar ratio and assembled into the target duplex siRNA.

The procedure is to unwind with heat once and then cool slowly. During the slow cooling, the correct pairs fit together snugly and form a stable duplex.

If the ratio deviates, one strand is left over, which is directly tied to quality and potency. Control of the mixing ratio and of heating and slow cooling is where it counts.

The 2 purified single strands are mixed at equimolar ratio, heated and then cooled slowly, and assembled into the target duplex siRNA. If the ratio deviates, single strand is left over, which is directly tied to quality and potency.

Alternatives in practice

• On-column annealing · Cytiva ÄKTA process + association of complementary strands on a preparative column — Integrates purification and duplex formation, allowing simultaneous removal of excess strand
• Scale-optimized batch annealing · Jacketed vessel with temperature profile control (in-process slow cooling) — Secures a uniform thermal history at large scale and optimizes duplex content

For siRNA aimed at the liver, a sugar chain called GalNAc that fits snugly into the receptor ASGPR on the surface of hepatocytes is attached, serving, so to speak, as an address label.

In practice, this ligand is often synthesized in one piece at the solid-phase synthesis stage and incorporated from the start. It is less a step of attaching afterward than a check of what is already built in.

So the main aim at this stage is to determine whether the conjugate has the intended purity and structure, because it is the heart that carries deliverability.

For siRNA aimed at the liver, GalNAc sugar chains that bind the receptor on the hepatocyte surface (ASGPR) are attached. In many cases they are already incorporated during synthesis, but the purity and structure as a conjugate are confirmed.

Alternatives in practice

• Post-conjugation in solution phase · Stirred reaction vessel + GalNAc-NHS/activated linker attached to the purified strand, TFF purification — Separates the strand from conjugation, making quality control easier
• Modular conjugation by click chemistry · CuAAC/SPAAC reagents (azide-alkyne) + reaction vessel — Multivalent ligands are added modularly with azide/DBCO etc.

RNA has the property of degrading readily in water. So this step takes the drug substance out in the form of a stable powder by removing water at the end.

In lyophilization, water is gently driven off while quality is preserved, through the stages of freezing, primary drying and secondary drying. The trick is not to dry it roughly.

The sterile powder obtained in this way is the final drug substance form that can withstand storage and shipping. It is the endpoint that marks a milestone in siRNA manufacturing.

RNA degrades readily in water, so water is finally removed to give a stable powder. This is the form of drug substance that can be stored and shipped.

Alternatives in practice

• Spray drying · GEA Niro / BÜCHI Mini Spray Dryer B-290 (scale-appropriate machines) — Continuous processing and particle design are possible; note heat and shear sensitivity
• Precipitation + vacuum drying · Stirred crystallization tank + vacuum dryer (after ethanol precipitation) — Alternative isolation when no lyophilization facility is available

Is what has been produced truly the siRNA of the intended sequence? Identity confirmation checks that with the yardstick of molecular weight.

The intact mass is measured by LC-ESI-MS, and the actual molecular weight is read by deconvolution. A difference of just 1 base, or the presence or absence of modifications, shows up clearly in the mass.

That is exactly why this is the central item of identity testing. It is a check that, so to speak, serves as the product's identification card.

Confirms by molecular weight that what was made is truly the target siRNA. Even a 1-base difference and the presence or absence of modifications show up in the mass, making this the center of identity testing.

Alternatives in practice

• MALDI-TOF MS · Bruker rapifleX / autoflex + 3-HPA/THAP matrix — Rapid molecular weight confirmation; low resolution for quantitation and closely related impurities
• Oligo mapping (partial digestion + MS/MS) · Waters BioAccord/Thermo Orbitrap + nuclease digestion — Used for confirmation of sequence and modification positions

How pure the target strand is. Conversely, how much impurities such as strands 1 base shorter/longer (n±1) are mixed in. This step pins down those proportions in numbers.

Ion-pair reversed-phase HPLC and anion exchange HPLC are used for measurement, giving a clear ratio of full-length product (% full-length product).

This purity is the most important quality indicator, directly tied to potency and safety. It is a number that cannot be compromised and is central to release decisions as well.

Measures how pure the target strand is and how much of impurities such as strands 1 base shorter or longer there is. The most important quality indicator, directly tied to potency and safety.

Alternatives in practice

• Capillary gel electrophoresis (CGE) · SCIEX PA 800 Plus / BioPhase 8800 + ssDNA/RNA separation kit — High length-separation resolution and identifies impurities under denaturing conditions
• Mixed-mode HPLC (RP+IEX combined) · Thermo DNAPac / mixed-mode columns + Agilent/Waters UHPLC — Combines hydrophobicity and charge in 1 method to secure orthogonality
• Duplex purity by non-denaturing IP-RP · Agilent 1290 Bio LC + non-denaturing conditions C18 — Evaluates the purity of the associated species without dissociating the duplex

Did the annealing go well? It is checked by the ratio of "how much has become duplex and how much single strand is left over."

Non-denaturing IP-RP, SEC-HPLC and melting temperature (Tm) analysis are used to see whether the material has been assembled correctly as duplex siRNA.

Performance drops when much single strand remains. In other words, this is an indicator like a report card that reflects the success or failure of the assembly step.

Confirms whether the material has been assembled correctly as duplex siRNA and how much leftover single strand there is. Because a lot of single strand lowers potency, this is an indicator of whether assembly succeeded.

Alternatives in practice

• Size exclusion chromatography (SEC) · Agilent 1260 Bio-inert + Tosoh TSKgel UP-SW3000 — Separates duplex from single strand/associated species by size
• DSC / circular dichroism (CD) · Malvern MicroCal PEAQ-DSC / Applied Photophysics Chirascan — Directly evaluates thermodynamic stability and higher-order structure

How much of the target actually is in the drug substance. The content/assay test measures that net amount.

UV absorbance at 260 nm is measured and converted with the extinction coefficient for quantitation. RNA absorbs UV strongly, so this wavelength is the yardstick of quantity.

This value is the starting point of dose design and a number that shows yield and concentration. It is indispensable basic data that is central to release decisions.

Quantifies how much of the target is contained in the drug substance. The basis for dose design and the key value in release decisions.

Alternatives in practice

• Quantitative NMR (qNMR) / phosphorus determination · Bruker AVANCE NEO 400/500 MHz + internal standard (maleic acid, etc.) — Absolute quantitation, independent of counterion; avoids sequence ε uncertainty
• Gravimetric method (dry solids) · Mettler Toledo microbalance + Karl Fischer moisture meter — Used together with salt and moisture correction to take a mass balance

Drug substance carries counterions such as sodium, and slight amounts of the metals and solvents used in manufacturing may remain. This step measures the amount of each.

Counterions and elemental impurities are quantified by ICP-MS/ICP-OES, and residual solvents by headspace GC. They are methods that do not miss even trace amounts.

These bear directly on patient safety and on accurately expressing the net amount of drug substance. It is steady confirmation that supports the reliability of the numbers from behind.

Measures how much counterion such as sodium is attached, and how much of the metals and organic solvents used in manufacturing remain. Bears on safety and on the accuracy of the net amount of drug substance.

Alternatives in practice

• Counterion quantitation by ion chromatography · Thermo Dionex ICS-6000 + IonPac columns — Directly quantifies counterions such as Na+/acetic acid; no ICP needed
• Residual solvent screening by NMR · Bruker AVANCE NEO + 1H-qNMR — Screens multiple solvents at once; rapid confirmation before GC

siRNA ultimately becomes an injectable. That is exactly why it is strictly checked at the end whether it is clean enough to be put into the body.

Endotoxin, which causes fever, is checked by the bacterial endotoxins test, the amount of microorganisms by bioburden, and, where applicable, by the sterility test.

This is an essential item for release, directly tied to patient safety. A product cannot go out into the world without passing it.

Because it becomes an injectable, it is confirmed that endotoxin, which causes fever, and microorganisms are below the standard. An essential release item directly tied to patient safety.

Alternatives in practice

• Recombinant factor C (rFC) method · Lonza PyroGene Recombinant Factor C Assay + fluorescence microplate reader (Ex/Em 380/440 nm) — Independent of horseshoe crabs and consistent with animal welfare; accepted by FDA/EP
• Rapid microbial methods (RMM) · bioMérieux BacT/ALERT / Merck Milliflex Rapid — Shortens the time to a sterility/bioburden determination

Drug substance is not finished once made. This step watches over time whether RNA is cleaved or modifications fall off while it is stored.

It is stored under long-term and accelerated conditions per ICH Q1A, and purity, content and degradation products are measured over time. The image is of observing the actual course.

The results provide the basis for deciding shelf life and storage conditions. It is a test that scientifically backs up how long and how the product can be kept.

Checks over time whether the RNA is cleaved or modifications fall off during storage. Provides the basis for deciding shelf life and storage conditions.

Alternatives in practice

• Forced degradation studies (acid/base/oxidation/light/heat) · Photostability chambers (Atlas Suntest / Binder KBF-LQC ICH Q1B) + HClO/H2O2 reagents — Used to identify degradation pathways and to verify the stability-indicating (SI) nature of analytical methods
• Stability monitoring by CGE · SCIEX PA 800 Plus / BioPhase 8800 + ssDNA/RNA kit — Tracks changes over time in strand cleavage and length heterogeneity at high resolution