Residual Solvents ICH Q3C: Selection, Limits and Control

Residual Solvents ICH Q3C: Scope, Definitions and Exclusions

ICH Q3C defines residual solvents as organic volatile chemicals that are used or produced in making drug substances or excipients, or in preparing drug products. They are not completely removed by practical manufacturing techniques. Because residual solvents have no therapeutic benefit, the guideline says they should be removed to the extent possible. The current text is Q3C(R9), Step 4 dated 24 January 2024, and the European Medicines Agency lists 29 April 2024 as its legal effective date.

• In scope — drug substances, excipients and drug products, in all dosage forms and routes. Testing is needed when the process is known to use or produce such solvents.
• Out of scope — solvents used deliberately as excipients, and solvates. Their solvent content must still be evaluated and justified.
• Development stage — the guideline does not apply to new drug substances, excipients or products during clinical research, nor to existing marketed products.
• Flexibility — higher levels can be acceptable for short-term use of 30 days or less, or for topical application. Each case needs its own justification.

Revision R9 changed section 3.4 so that the volatility of the solvent is considered in the analytical method. Our guide to the pharmaceutical regulatory pathway shows where Q3C sits among the other ICH quality guidelines. The guideline scope sets who must comply, and the classes set what they must comply with.

Class 1, 2 and 3 Solvents: How the Risk Classes Work

Q3C places each solvent in one of three classes after a risk assessment. Known and strongly suspected human carcinogens and environmental hazards form Class 1, and these solvents are to be avoided. Class 2 holds non-genotoxic animal carcinogens, agents of irreversible toxicity such as neurotoxicity or teratogenicity, and solvents with other significant but reversible toxicity. Solvents of low toxic potential, for which the guideline needs no health-based exposure limit, form Class 3.

The lists are not exhaustive, and the guideline says classifications and limits for Class 1 and 2 solvents can change as new safety data arrive. Q3C also notes that many Class 3 solvents lack long-term toxicity or carcinogenicity studies. It accepts 50 mg per day or less without justification, and higher amounts if they are realistic for manufacturing capability and good manufacturing practice.

Class 2 limits come from a single calculation, which the next section unpacks.

How a PDE Is Derived and Turned Into a Concentration Limit

The permitted daily exposure, or PDE, is the maximum acceptable intake per day of a residual solvent in a pharmaceutical product. For Class 2 solvents it starts from the no-observed-effect level in the most relevant animal study. That level is scaled to a 50 kg adult and divided by five modifying factors. The guideline assumes 100 percent systemic exposure whatever the route of administration.

The guideline works through acetonitrile in a 13-week mouse study, with factors of 12, 10, 5, 1 and 1, and arrives at about 4.2 mg per day. Table 2 lists 4.1 mg per day. The 50 kg body weight is deliberately low, since it adds a safety factor against the 60 or 70 kg often used. A paediatric formulation would warrant a further adjustment. For Class 1 carcinogens, the guideline points to the Gaylor-Kodell method or a safety factor of 10,000 to 100,000 on the no-effect level.

The PDE then becomes a concentration for a given daily dose:

• Equation — concentration in ppm equals 1000 multiplied by the PDE in mg per day and divided by the dose in g per day.
• Acetonitrile example — a PDE of 4.1 mg per day and a dose of 10 g per day gives 410 ppm, which is the Option 1 limit in Table 2.
• Tight limits — solvents with PDEs of 0.5 to 1 mg per day, such as 2-methoxyethanol, give limits of 50 to 100 ppm at that dose.

Two ways of applying that equation give the guideline its flexibility.

Option 1 and Option 2: Setting Limits for Class 2 Solvents

Q3C offers two options for Class 2 solvents, and Option 1 uses the Table 2 concentration limits, which assume a product mass of 10 g taken daily. If the drug substance and every excipient meet those limits, they can be used in any proportion up to a daily dose of 10 g. Option 2 uses the PDE and the actual maximum daily dose, and it adds up the solvent from each component.

The guideline illustrates both options with acetonitrile and a 5.0 g daily product. In its first example, the drug substance and one excipient exceed the 410 ppm Option 1 limit, and the product sits at 728 ppm. The daily exposure is 3.64 mg, which is below the 4.1 mg PDE, so the product conforms under Option 2. In the second example one excipient carries more solvent, the daily exposure reaches 5.08 mg, and the product meets neither option.

When a product fails on summation, the guideline suggests testing the drug product, because the formulation process can reduce the solvent. If that fails, the manufacturer reduces the solvent at source. Only in exceptional cases is a summary of reduction efforts and a risk-benefit analysis offered.

Whichever option applies, the number is only as good as the choice of solvent upstream.

Solvent Selection for Drug Substance Synthesis and Crystallisation

Q3C recognises that the solvent can be a critical process parameter. A good choice can improve yield or decide crystal form, purity and solubility. The guideline therefore recommends the less toxic solvents of Class 3 where practical, and it limits Class 2 and avoids Class 1. The table below shows how widely the permitted levels vary.

The limits differ by a factor of 60 between methanol and 2-methoxyethanol, so the specific solvent matters as much as the class. Five questions guide selection:

1. Which class and what headroom? — prefer Class 3 where the chemistry allows. A Class 2 solvent with a PDE below 1 mg per day leaves very little room in the final steps.
2. Where in the process is it used? — the final step is the main concern. The guideline also counts earlier-step solvents that a validated process does not remove consistently.
3. What does it do to the solid form? — the solvent can determine crystal form, and a solvate falls outside Q3C but still needs evaluation and justification.
4. Can it be dried out? — switching to a more volatile solvent in which the product has low solubility aids drying, as the isolation study below explains.
5. Can the method resolve it? — choose solvents that the headspace procedure separates cleanly from the others in the process.

Our guides to crystallisation fundamentals and cooling, antisolvent and reactive crystallisation cover how the solvent shapes the crystal. Once the solvent is chosen, the process has to remove it, which is the next step.

Removing Residual Solvents: Washing, Deliquoring and Drying

Solvent is removed from the crystallised product in the isolation train. An open Strathclyde study of filtration and washing divides isolation into cake formation, filtration, washing, deliquoring and drying. Deliquoring displaces as much as possible of the liquor trapped in the pore structure of the cake. Drying performance then depends on the residual solvent content and composition at its start, the gas flow, the heat applied and the dryer design.

• Washing — the study cites typical industrial practice of washing the cake with at least three cake volumes of solvent, roughly 5 to 7 mL per gram of product.
• Solvent switch — moving to a more volatile wash solvent in which the product has low solubility aids drying. The switch must be designed to limit aggregates.
• Agglomeration — product dissolved in the residual solvent can deposit solid bridges between particles as the solvent evaporates. Higher solubility in the wash solvent raises the risk of bridging.
• Wash choice — in the paracetamol case study, low-boiling aliphatic hydrocarbons were desirable final wash solvents because they gave weaker, more readily disrupted agglomerates.
• Lattice-held solvent — one patent reports that some recrystallised drug salts keep around 1 percent of ethanol or isopropanol even after days of vacuum drying. It proposes displacing the solvent with water vapour.

Drug product processes add their own sources. Granulation, coating and spray drying can all use organic solvents, and Q3C says the drug product should be tested if a solvent is used during its manufacture. Our guide to spray drying amorphous solid dispersions shows a route where solvent removal is built into the process design. Removal strategy and analysis go together, because a process that cannot be measured cannot be controlled.

Residual Solvent GC Headspace Testing and USP 467 Procedures

Q3C says residual solvents are typically determined by chromatographic techniques such as gas chromatography, and that harmonised pharmacopoeial procedures should be used if feasible. If only Class 3 solvents are present, a non-specific method such as loss on drying can be used if it is validated. Validation follows ICH Q2, and the impact of solvent volatility on the method must be considered. Our article on analytical method development and validation under ICH Q2 and Q14 covers that framework.

A study by USP authors describes the procedures of General Chapter 467 as static headspace gas chromatography with flame ionisation detection. The solubility of the sample decides which section applies: water for water-soluble articles, and dimethylformamide or dimethyl sulfoxide for water-insoluble ones.

Procedures A and B complement each other, because peaks that co-elute on one phase resolve on the other. The authors chose flame ionisation for its sensitivity, linear range and low noise. Static headspace keeps less volatile matrix components off the column, which simplifies the analysis and protects the instrument. The diluent matters: the more polar hydroxyzine pamoate gave greater recoveries in dimethyl sulfoxide, while the less polar prednisone did better in dimethylformamide.

The study dates from 2010, and the chapter has been revised since, so the current official text must be checked before a method is written into a specification. A method that suits one matrix needs verification for the next.

Supplier Statements, Cumulative Calculation and Specifications

A formulator rarely tests every excipient for every solvent. Q3C allows a cumulative calculation of the solvent in the drug product from the levels in its ingredients. If the result is at or below the recommended level, no product testing needs to be considered. If it is above, the product is tested to see whether the formulation process reduced the solvent.

• Class 3 only — the supplier can state that only Class 3 solvents are likely to be present and that loss on drying is below 0.5 percent.
• Class 2 named — the supplier names the Class 2 solvents likely to be present and states that all are below the Option 1 limit.
• Mixed — the supplier names the Class 2 solvents, states that they are below Option 1, and states that Class 3 solvents are below 0.5 percent.
• Class 1 likely — these solvents are identified and quantified.
• Above the thresholds — Class 2 or Class 3 solvents present above their Option 1 limit or 0.5 percent are identified and quantified.

A good specification names each solvent and its class, states whether Option 1 or Option 2 applies, defines the method and sets the frequency of testing. It also records how the excipient supplier's statement is verified. Our guide to drug excipient compatibility studies covers the preformulation side of excipient selection, where supplier data first enter the file.

Working With a Pharmaceutical Formulation Consultant

Residual solvent control spans chemistry, process design and analysis, and gaps between them show up late. A late-stage solvent change, an excipient lot that breaks the summation or a method that cannot resolve a co-eluting solvent all trace back to early choices. A pharmaceutical formulation consultant tests those choices against Q3C before the filing. Pharmaceutical product development services then carry the work through methods, documentation and scale-up.

• Solvent screen — a class, limit and headroom review of every solvent in the route, with replacement options.
• Cumulative calculation — an Option 1 and Option 2 model of drug substance, excipients and drug product at the highest dose.
• Isolation and drying design — wash, deliquoring and drying plans aimed at the specification.
• Analytical support — headspace method selection, verification and a validation plan under ICH Q2.
• Supplier control — statement templates and verification testing for excipients.
• Technology transfer — handover to a contract formulation CDMO support partner with a clear specification.

The full range of services sits under pharmaceuticals and health care. Before committing to a route, collect the solvent list, the maximum daily dose and the excipient suppliers' statements. Those three inputs shape every decision in this article.