Residual Solvents in UDCA: An ICH Q3C Guide for Buyers

Residual solvents in ursodeoxycholic acid are not set by the UDCA monograph. The Ph. Eur. monograph for ursodeoxycholic acid (1275) defines identity, purity and the lettered impurities; it does not fix solvent limits. Those come from a separate framework — ICH Q3C, Impurities: Guideline for Residual Solvents — which sorts every solvent used in pharmaceutical manufacture into one of three classes and assigns a limit to each.

That division is the reason solvent results are so often the weakest part of a UDCA specification. A certificate can be perfectly compliant with the monograph and still say nothing useful about solvents, because the monograph was never going to ask.

This guide sets out what ICH Q3C actually requires, which limits apply, how the numbers are derived, and what a solvent list tells you about how a material was made. Limits are from ICH Q3C(R9), the version in force since January 2024.

The three classes, and what each one commits you to

ICH Q3C does not rank solvents by how well they work. It ranks them by toxicological risk, and the class determines what you are allowed to do with the solvent and how much may remain.

The class is what determines whether a solvent choice becomes a lifelong control problem. A Class 3 solvent costs a routine gas-chromatography test against a generous limit. A Class 2 solvent costs a tighter limit, a purge argument, and more scrutiny at every process change. That difference is why solvent selection is a compliance decision, not only a chemistry one.

Class 1 — solvents to be avoided

Class 1 solvents are the ones a specification should not contain at all. Their limits are set at trace levels, and their presence must be specifically identified and quantified if it is plausible.

The practical check is not whether the numbers are met — they almost always are, because these solvents are avoided by design. The check is whether the certificate states that Class 1 solvents were considered at all. The guideline expects them to be identified and quantified if they are likely to be present, so a specification should be able to show that the question was asked.

Class 2 — the ones that carry a limit

Class 2 is where UDCA specifications actually have something to report. These are the solvents that appear in real synthesis and purification steps, and each carries a solvent-specific limit derived from a permitted daily exposure.

Methanol is the one most UDCA buyers will see. It sits at the top of the Class 2 table with the most generous limit of the group — which is exactly why its presence in a process is not, by itself, a concern. What matters is the number against the limit.

Class 3 — the 5,000 ppm default

Class 3 solvents are those with low toxic potential, and ICH Q3C does not require a health-based limit for them below a default ceiling.

Common Class 3 solvents include ethanol, isopropanol, acetone, ethyl acetate, acetic acid, heptane and methyl tert-butyl ether. The default acceptable concentration is 5,000 ppm (0.5%), corresponding to a permitted daily exposure of 50 mg/day.

A common misreading is that Class 3 means “unregulated”. It does not. A validated method and a specification are still expected, and exceeding the default requires justification. What is missing is a health-based limit at normal use levels — not the paperwork.

What the solvent list tells you about the route

Solvents are chosen by the process, so the residues a certificate reports are a fingerprint of how the material was made. That makes the solvent block one of the more informative parts of a UDCA specification, even though it is usually read last.

The main UDCA route runs from chenodeoxycholic acid: oxidation to the 7-oxo intermediate, then stereoselective reduction to the 7β configuration.

Two of the solvent families that appear in this chemistry leave visible traces. Methanolic steps, such as esterification or workup in methanol, explain a methanol entry — and the same chemistry shows up on the impurity side as the methyl ester, which is impurity G in the EP lettering. Ester hydrolysis back to the free acid is what removes it. Crystallisation and purification solvents such as isopropanol and ethyl acetate, or reduction solvents such as tetrahydrofuran, explain the rest.

The useful reading is therefore not “are there solvents” but “do the solvents reported match the route implied by the impurities”. A specification that lists reduction solvents but no methyl-ester-related impurity, or vice versa, is at least worth a question.

Where the ppm figure comes from: Option 1 and Option 2

The Class 2 ppm values are not primary numbers. They are derived from the permitted daily exposure, and the derivation assumes a maximum daily dose.

Option 2 works from the actual maximum daily dose of the finished product instead. Where the daily dose is lower than 10 g — as it is for a typical UDCA tablet — Option 2 can justify a higher permitted concentration, because what the PDE governs is the patient’s total exposure, not the concentration in one material.

For a buyer this matters in one specific way: a limit quoted without saying which option produced it is incomplete. The same solvent can be compliant under Option 1 and non-compliant under Option 2, or the reverse, depending on the dose. Asking which assumption sits behind the number is a precise question that most suppliers can answer immediately.

What changed in the 2024 revision

ICH Q3C reached revision R9 at Step 4 on 24 January 2024. The change was a minor revision to section 3.4, concerning how solvent volatility is taken into account in analytical methods. In substance, the guideline confirms that where only Class 3 solvents are present, a non-specific method such as loss on drying may be used, provided the method is properly validated and the effect of volatility on that method has been considered.

Implementation is not simultaneous across regulators, and this is worth knowing if you sell into more than one market. The European Commission implemented Q3C(R9) on 29 April 2024, and China’s NMPA implemented it on 20 June 2024. Some major authorities were still listed as not yet implemented at the time of writing. A specification that cites “ICH Q3C” without a revision number may therefore be referencing an older edition.

How residual solvents are measured

Residual solvents are volatile, so the standard approach is gas chromatography with headspace sampling — the sample is sealed and heated, and the vapour phase is analysed. Headspace GC with flame-ionisation detection is the common configuration, and it is the technique behind the solvent numbers on most certificates.

The method choice matters most for the Class 3 case. Because the 2024 revision explicitly allows a non-specific method when only Class 3 solvents are present, a certificate showing residual solvents as loss on drying rather than as named solvent peaks is not automatically deficient — but it is only defensible if the material genuinely contains no Class 1 or Class 2 solvent. Where a Class 2 solvent is plausible, a named, specific method is what the number requires.

Reading the residual solvents block on a COA

1. Check a block exists. If the certificate is silent on solvents, the question is unanswered rather than answered as zero.
2. Check the method is named. Gas chromatography is expected; a loss-on-drying figure is only acceptable under the Class 3 condition above.
3. Read each solvent against its own class. A Class 2 solvent is judged against its specific limit, not a common figure.
4. Check Class 1 is addressed. Either the certificate rules them out, or it reports them explicitly.
5. Match the solvents to the route. The solvent list and the impurity list should tell the same story about the process.
6. Ask which option underlies the limit. Option 1’s 10 g/day assumption is not the same as your product’s actual dose.

Questions buyers ask

Does the UDCA monograph set residual solvent limits?
No. The Ph. Eur. monograph defines identity, purity and impurities. Residual solvents are governed separately by ICH Q3C.

What is the ICH Q3C limit for methanol in UDCA?
Methanol is a Class 2 solvent with a permitted daily exposure of 30 mg/day, corresponding to a concentration limit of 3,000 ppm.

Is 5,000 ppm a universal limit?
No. It is the default ceiling for Class 3 solvents. Class 1 and Class 2 solvents have their own, generally lower, limits.

Why is the limit in ppm rather than a percentage?
The ppm figures are derived from a permitted daily exposure, assuming a maximum daily dose. The concentration is what a certificate can measure directly.

Can loss on drying be used instead of gas chromatography?
Only where Class 3 solvents alone are present, and only if the method is validated with volatility taken into account. That is the point of the 2024 revision.

Which version of ICH Q3C applies?
Q3C(R9), in force since 24 January 2024. Its implementation date differs by regulator, so a specification citing “Q3C” should also be able to name the revision.

What to ask a supplier about solvents

The solvent limits are set by the guideline; a buyer cannot move them. What a buyer can establish is whether the certificate answers the question or avoids it. Three asks cover most of it.

Ask for the residual solvent block with the method named and each solvent reported against its own class limit. Ask whether the limit reflects Option 1’s 10 g/day assumption or the actual daily dose. And ask for Class 1 solvents to be explicitly ruled out rather than omitted.

The solvent block reads best next to the rest of the specification. If you want the surrounding parameters, they sit in the full UDCA specification, and the impurity side — including the methyl ester that methanolic processing produces — is mapped in the A–I impurity reference. Our ursodeoxycholic acid grade is described with those parameters, and you can request a quotation against your own acceptance criteria.

The classification and limits quoted here are those of ICH Q3C, published alongside the other quality guidelines by the International Council for Harmonisation. Pharmacopoeias reproduce the same framework in their general chapters on residual solvents — the European Pharmacopoeia and the United States Pharmacopeia among them — so a certificate can be checked against either the guideline or the general chapter.

This article describes pharmacopoeial and regulatory requirements, and analytical interpretation, only. It is not a statement of therapeutic use, and it does not set out commercial guarantees. Always work from the guideline edition in force at the time of your release.

Frequently asked questions

Which residual solvents are typical for UDCA?

Whichever the crystallisation and washing steps use, commonly class 3 solvents such as ethanol, acetone or ethyl acetate. If the route uses class 2 solvents anywhere, those carry tighter limits and deserve their own lines. The solvent list mirrors the process, which is why a change of crystallisation solvent is a change the buyer should hear about.

Why does a COA sometimes show no residual solvent lines at all?

Either the framework the certificate was written against does not require them, or the supplier controls them in-house without printing the results. Those are different situations with different risk. Ask which it is. Absence of a line is not absence of solvent, and it should never be read that way.

Is "residual solvents: conform" enough?

It is thin. A conform line without solvent names and limits cannot be compared between suppliers and contributes little to a regulatory file. Ask for names, limits, method and result. Suppliers who control this area well already have that page printed; the request costs them nothing.

Do class 3 solvents always need individual testing?

The ICH Q3C framework allows some class 3 control through loss on drying in defined situations, but the destination market may still want data, and the buyer decides which risk to carry. The decision should be made deliberately, with the framework in view, not inherited from whatever the supplier's template happens to print.

How do residual solvents interact with the assay figure?

They barely appear in it. Material carrying half a percent of solvent can still read full assay, because the assay measures the molecule, not the drum. That gap is exactly what the mass-balance check across assay, water and solvents is for. A buyer who never adds those lines together never sees the gap.

What should change in the buyer's file when the supplier changes crystallisation solvent?

The residual-solvent panel and part of the impurity profile move with it, and the buyer's own regulatory file may name the solvent. This is why solvent changes belong in the change-notification list, not in a footnote on the next COA. Ask for solvent changes to be covered in the quality agreement.