Peptide therapeutics have evolved from early biological curiosities into one of the most dynamic and commercially impactful drug modalities in modern medicine. Since the first clinical use of insulin in the 1920s, peptides have proven their value across a wide range of indications, including metabolic disease, oncology, cardiovascular disorders and infectious disease. Today, more than 110 peptide‑based drugs are approved worldwide, with market growth driven in particular by glucagon‑like peptide‑1 receptor agonists (GLP‑1 RAs, like semaglutide) and next‑generation multi‑agonists.

With a size generally ranging from 2 to 50 amino acids (500-5000 Da) peptides occupy a unique space between small molecules and biologics. They combine high specificity and potency with favorable safety profiles, lower immunogenicity, and comparatively efficient manufacturing. At the same time, advances in peptide chemistry, bioengineering and delivery have enabled increasingly sophisticated molecular designs. These innovations are central to clinical success, but they also redefine what “fit‑for‑purpose analytics” means across the product lifecycle.

Timeline of relevant milestones in the development of therapeutic peptides (only selected classes of FDA approved drugs from 2000 onwards), (b) Classification of the FDA approved molecules (from 1940 until October 2024) and their respective clinical use, (c) Distribution of peptide-based drugs and in-vivo diagnostics approval times from 2000 to 2024, compiled from the freely available database PepTherDia (https://peptherdia.herokuapp.com) with insulin-based drug data sourced from Walsh & Walsh (Biopharmaceutical benchmarks 2022, Nat Biotechnol 40 (2022) 1722–1760).

Designing stability: how peptide engineering shapes CQAs

Native peptides are inherently vulnerable to enzymatic degradation and rapid clearance, resulting in poor bioavailability and short half‑life. Modern peptide therapeutics therefore rely heavily on structural modification strategies to enhance stability, potency and exposure.

Key approaches include backbone and terminal modifications (e.g. incorporation of non‑natural amino acids such as α-aminoisobutyric acid (Aib), D‑amino acids, N‑methylation, N‑terminal acetylation or C‑terminal amidation), cyclization to stabilize secondary structure and conjugation strategies such as PEGylation, lipidation or fusion to albumin or Fc domains. In GLP‑1 RAs, lipidation combined with optimized spacers enables reversible albumin binding, reducing renal clearance and enabling once‑weekly dosing.

Each of these design choices introduces new critical quality attributes (CQAs). Beyond sequence identity, developers must control conjugation efficiency, positional isomers, over‑ or under‑modified species, free linker or lipid, aggregation, and degradation products. As peptide complexity increases, impurity profiles become richer, more subtle, and more difficult to resolve.

Selected engineered therapeutic peptide sequences used in the management of T2DM (ADO: 8-amino-3,6-dioxaoctanoic acid, AEEA: aminoethoxyethoxyacetic acid, Aib: α-aminoisobutyric acid).

Analytical reality: why peptides challenge conventional workflows

Compared to small molecules, peptides generate a broader spectrum of product‑ and process‑related impurities during synthesis, purification, formulation and storage. Impurities range from truncated and capped sequences to isomerization products, aggregates, residual reagents and variants arising from conjugation chemistry (e.g. under‑/over‑lipidation, free linker or PEG, unconjugated peptide). Many of these impurities differ from the main compound by only a single amino acid modification or small chemical change, resulting in near‑identical physicochemical behavior. This makes sensitive detection and confident identification particularly challenging.

Reversed‑phase liquid chromatography (RPLC), often with ion‑pairing, remains the gold standard for peptide purity and impurity analysis. When coupled to high‑resolution MS and MS/MS, it enables molecular mass confirmation, sequence verification and localization of modifications such as oxidation or deamidation. However, no single method can fully resolve the complexity of modern peptide products.

Orthogonal techniques are therefore essential. Size‑exclusion chromatography (SEC) is critical for assessing aggregation and monomeric purity, while ion‑exchange and hydrophilic interaction chromatography (HILIC) provide complementary selectivity for charged and highly polar species. Two‑dimensional LC (2D‑LC) approaches further extend separation power, enabling resolution of co‑eluting impurities and improved MS compatibility.

What GLP‑1 RAs teach us about lifecycle‑ready analytics

The analytical challenges surrounding GLP‑1 RAs illustrate a broader trend in peptide therapeutics. These molecules may be produced via chemical synthesis, recombinant expression or hybrid semisynthetic routes, each generating a distinct impurity fingerprint. Impurity profiles are therefore not only CQAs, but also critical indicators of process consistency, comparability and product authenticity.

In practice, advanced workflows increasingly combine RPLC, HILIC, SEC and MS (supported by low‑adsorption materials and MS‑compatible conditions) to ensure recovery, sensitivity and robustness. Dedicated methods for counter‑ions, metals, solvents and excipients (e.g. HILIC‑ELSD, ICP‑MS, GC‑FID) complete the analytical toolbox. For bioanalysis, LC‑MS/MS with isotopically labelled standards enables accurate PK/PD assessment even in complex biological matrices.

Importantly, hardware matters as much as chemistry. Many GLP 1 peptides and their impurities exhibit non-specific adsorption to metal surfaces, leading to peak tailing and loss of low-level impurities. Low adsorption column hardware significantly improves recovery and sensitivity, with clear differences observed between metal sensitive peptides such as exenatide and more shielded, lipidated molecules such as semaglutide.

From complexity to control

As peptide therapeutics continue to expand in molecular sophistication and clinical scope, analytical strategies must evolve accordingly. The future lies in integrated, orthogonal, and lifecycle‑oriented workflows that link molecular design choices directly to analytical control strategies. Such approaches support faster development, smoother regulatory transitions, and robust quality control, ensuring that innovative peptide medicines remain safe, effective and consistently manufactured from early development through commercial supply.

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