Determination of Related Substances in Chemical Pharmaceuticals: Methods, Standards, and Regulatory

Determination of related substances is a critical quality control and regulatory requirement for chemical pharmaceuticals, ensuring that organic impurities in active pharmaceutical ingredients (APIs) and finished drug products remain within safe, qualified limits. This article provides a comprehensive technical overview of related substance testing, covering impurity classification, internationally recognized standards such as ICH Q3A/Q3B/Q3C/Q3D and M7, analytical methodologies, method validation, and practical applications across the pharmaceutical product lifecycle.

Technical Overview

Related substances, also referred to as organic impurities, are chemical entities that are structurally related to the active pharmaceutical ingredient and arise from synthesis, degradation, or interactions with excipients and packaging. In chemical drug products, these impurities typically include starting materials, reagents, ligands, catalysts, by-products, intermediates, and degradation products. Because many related substances possess no therapeutic value and may exhibit toxicity or genotoxicity, their identification, quantification, and control constitute a fundamental element of pharmaceutical quality assurance and regulatory compliance.

The determination of related substances is governed by internationally harmonized guidelines, principally ICH Q3A(R2) for impurities in new drug substances, ICH Q3B(R2) for impurities in new drug products, ICH Q3C for residual solvents, ICH Q3D for elemental impurities, and ICH M7(R2) for mutagenic and nitrosamine impurities. Major pharmacopoeias, including the United States Pharmacopoeia (USP), the European Pharmacopoeia (Ph. Eur.), the British Pharmacopoeia (BP), the Japanese Pharmacopoeia (JP), and the Chinese Pharmacopoeia (ChP), establish monograph-specific limits and general chapters for impurity testing. Under these frameworks, impurity levels are evaluated against reporting, identification, and qualification thresholds that scale with the maximum daily dose (MDD) of the drug.

A scientifically sound related substance determination program integrates impurity profiling, forced degradation studies, validated chromatographic methods, and toxicological qualification, providing documented evidence that a drug substance or drug product remains safe, effective, and stable throughout its intended shelf life.

Testing Samples

Active Pharmaceutical Ingredients (APIs)

API batches from pilot-scale and commercial-scale production are tested to characterize process-related impurities such as unreacted starting materials, synthetic intermediates, reagents, catalysts, and by-products. Multi-batch data (typically a minimum of three representative batches) support the establishment of impurity profiles and specification limits.

Finished Drug Products

Tablets, capsules, injectable solutions, suspensions, ointments, and other dosage forms are analyzed to detect degradation products formed during manufacturing, storage, or interaction with excipients. Sample preparation strategies are adapted to the matrix, involving extraction, filtration, dilution, or solid-phase cleanup to recover impurities quantitatively.

Intermediates and Starting Materials

Process intermediates and key starting materials are monitored to trace impurity formation and carry-through, supporting impurity fate-and-purge studies and process control strategies.

Stability and Forced Degradation Samples

Samples stored under ICH long-term, intermediate, and accelerated conditions (per ICH Q1A(R2)) are periodically tested to monitor impurity growth. Stress-tested samples generated under hydrolytic, oxidative, thermal, and photolytic conditions demonstrate method specificity and stability-indicating capability.

Impurity Reference Standards

Qualified reference standards of known impurities, or well-characterized crude impurity fractions, are used as calibration materials. Where authentic impurity standards are unavailable, relative response factors (RRF) determined against the API standard or the diluted API method may be applied.

Testing Items

A complete related substance determination typically includes the following measurable attributes:

• Specified identified impurities: individually quantified against established acceptance limits
• Unspecified (unidentified) impurities: controlled using general limits, typically at the ICH reporting or identification threshold
• Total impurities: the sum of all detected impurities above the reporting threshold
• Degradation products: impurities formed from API breakdown or API–excipient interactions
• Residual solvents: Class 1, 2, and 3 solvents controlled per ICH Q3C using GC-based methods
• Elemental impurities: Class 1, 2A, 2B, and 3 elements per ICH Q3D, analyzed by ICP-MS or ICP-OES
• Mutagenic and nitrosamine impurities: controlled per ICH M7(R2), often at parts-per-million (ppm) or ppb levels relative to the TTC of 1.5 µg/day
• Enantiomeric impurities: evaluated for chiral APIs using chiral HPLC or capillary electrophoresis

For drug substances with a maximum daily dose of up to 2 g per day, ICH Q3A(R2) sets typical thresholds at 0.05% for reporting, 0.10% (or 1.0 mg/day, whichever is lower) for identification, and 0.15% (or 1.0 mg/day, whichever is lower) for qualification. Analogous thresholds in ICH Q3B(R2) apply to drug products, scaled by API content in the maximum daily dose.

Testing Methods

Reversed-Phase HPLC with UV/DAD Detection

Reversed-phase high-performance liquid chromatography (RP-HPLC) coupled with UV or photodiode-array (DAD) detection is the industry-standard technique for related substance determination. Method development involves systematic optimization of column chemistry (C18, C8, phenyl-hexyl), mobile phase pH and buffer strength, organic modifier gradients, temperature, and detection wavelength to achieve resolution of critical impurity pairs with target resolution values typically ≥ 1.5. DAD detection enables peak-purity assessment through spectral homogeneity analysis.

UHPLC and High-Resolution Separations

Ultra-high-performance liquid chromatography using sub-2 µm particles provides superior resolution, shorter run times, and improved sensitivity, and is widely adopted for complex impurity profiles and high-throughput quality control laboratories.

LC-MS and LC-MS/MS for Impurity Identification

When impurities exceed identification thresholds, liquid chromatography–mass spectrometry (single quadrupole, triple quadrupole, or high-resolution MS) is used to determine molecular weights, propose structures, and confirm assignments through fragmentation analysis and comparison with synthesized or isolated reference materials.

Gas Chromatography for Volatile Impurities and Residual Solvents

GC-FID and headspace GC-FID are applied for residual solvent determination per ICH Q3C and pharmacopoeial general chapters, while GC-MS and GC-MS/MS are used for trace-level nitrosamines and volatile mutagenic impurities at sub-ppm sensitivities.

Orthogonal and Complementary Techniques

Ion chromatography (IC), capillary electrophoresis (CE), thin-layer chromatography (TLC), supercritical fluid chromatography (SFC), and normal-phase HPLC serve as orthogonal methods to demonstrate method specificity or to resolve impurities not adequately separated by the primary RP-HPLC method.

Quantification Strategies

Common quantification approaches include external standard calibration using impurity reference standards, the diluted-API comparison method, correction using experimentally determined relative response factors (RRF), and, where justified, area normalization. The selected approach must be justified analytically and validated.

Method Validation

Related substance methods are validated in accordance with ICH Q2(R2), demonstrating specificity (including forced degradation and peak purity), linearity across the reporting threshold to 120–150% of the specification level, accuracy by recovery studies, precision (repeatability and intermediate precision), detection and quantitation limits (typically LOD ≤ reporting threshold and LOQ ≤ 50% of the reporting threshold), range, and robustness. System suitability criteria—resolution, tailing factor, theoretical plates, and replicate injection precision—are established in line with chromatographic general chapters such as USP <621> and corresponding Ph. Eur. and ChP chapters.

Applications

Determination of related substances supports multiple functions across the pharmaceutical lifecycle:

• Regulatory submissions: impurity profiles, analytical procedures, and validation data are documented in CTD format (Sections 3.2.S.3, 3.2.S.5, 3.2.P.5, and 3.2.P.6) for new drug applications, abbreviated applications, and generics (including ANDA and drug master file packages)
• Stability programs: trending of specified and total impurities under ICH stability conditions to justify shelf life and storage conditions
• Process development and control: impurity fate mapping, route scouting, crystallization optimization, and establishment of in-process controls
• Routine GMP release testing: batch conformity assessment against registered or pharmacopoeial specifications
• Post-approval change management: comparability testing following process, site, or supplier changes, including variation submissions
• Out-of-specification (OOS) and out-of-trend (OOT) investigations: root-cause analysis and batch disposition decisions
• Nitrosamine risk assessment and confirmatory testing programs responding to evolving regulatory expectations
• Supplier and excipient qualification: evaluation of impurity carryover and compatibility in formulation development

Accredited third-party laboratories routinely perform these analyses in compliance with current Good Manufacturing Practice (cGMP), ISO/IEC 17025 quality systems, and data integrity expectations (ALCOA+ principles), providing independent analytical support to sponsors worldwide.

FAQ

What is the difference between related substances, residual solvents, and elemental impurities?

Related substances are organic impurities structurally related to the API, arising from synthesis or degradation. Residual solvents are inorganic or organic volatile chemicals remaining from manufacturing (controlled per ICH Q3C), while elemental impurities are trace catalysts or heavy metal contaminants controlled per ICH Q3D. All three categories together define the overall impurity control strategy.

How are impurity acceptance limits established?

Limits are established by comparing batch impurity data with ICH qualification thresholds, conducting toxicological or literature-based safety assessments, applying the Threshold of Toxicological Concern (TTC, 1.5 µg/day) for mutagenic impurities under ICH M7, and considering pharmacopoeial monograph requirements. Qualified levels observed in pivotal clinical and registration batches typically anchor the final specification.

Why is a stability-indicating method required?

Regulators require the analytical method to resolve the API from all potential degradation products so that any impurity growth during storage can be accurately measured. Specificity is demonstrated through forced degradation studies and peak purity evaluation, confirming that co-elution does not mask impurity increases.

What is a relative response factor (RRF) and when is it used?

RRF corrects for differences in detector response between an impurity and the reference standard (often the API or a diluted API solution). It is experimentally determined during validation and applied when authentic impurity standards are unavailable in sufficient quantity or when the impurity response differs significantly from the standard.

How are nitrosamine impurities controlled?

Nitrosamines are assessed as cohort-of-concern compounds under ICH M7, with acceptable intake limits as low as 18–96 ng/day depending on the specific nitrosamine. Confirmatory testing uses highly sensitive GC-MS/MS or LC-MS/MS methods, supported by risk evaluation of amines, nitrites, and process conditions in the manufacturing chain.

How often should related substance methods be revalidated?

Validated methods are maintained under a lifecycle approach per ICH Q14 and USP General Chapter <1220>. Full or partial revalidation is triggered by changes to the synthetic route, formulation, analytical system components, or site transfers, and is assessed through risk analysis and comparability data.