A Century of Progress: From Animal Pancreas Extracts to the GLP-1 Era

If small-molecule drugs are precision hammers and biologics are complex engineering vehicles, peptide therapeutics are multifunctional surgical tools that combine many of the strengths of both. With high target specificity, high potency, low toxicity, and favorable safety profiles, peptide drugs are widely used across metabolic and endocrine disorders, oncology, cardiovascular disease, and many other therapeutic areas. They have become one of the most dynamic segments of global innovative drug development.

Figure 1. Advantages of peptide therapeutics compared with small molecules and biologics [1].

The development of peptide therapeutics spans more than a century and has repeatedly reshaped biomedicine. In 1922, Frederick Banting and his colleagues isolated insulin and introduced it for the treatment of diabetes, transforming a once-fatal disease into a manageable chronic condition and inaugurating the peptide-drug era. In 1954, Vincent du Vigneaud achieved the total synthesis of oxytocin and vasopressin, work that later earned him the Nobel Prize in Chemistry and demonstrated the feasibility of chemical peptide synthesis. In 1963, R. Bruce Merrifield introduced solid-phase peptide synthesis (SPPS), enabling more efficient and scalable peptide production. The maturation of recombinant DNA technology in the 1980s further expanded the range of available manufacturing routes.

In the twenty-first century, peptide therapeutics entered a period of accelerated innovation, with research advancing from the replication of natural peptides to precise, purpose-built structural engineering. The 2005 launch of exenatide, the first GLP-1 receptor agonist, ushered in a wave of long-acting peptide development. In 2017, once-weekly semaglutide transformed dosing convenience, while the 2022 approval of the dual GIP/GLP-1 receptor agonist tirzepatide raised the efficacy benchmark once again. Over a century of progress, peptide drugs have evolved from natural extraction to chemical synthesis and precision structural modification, continually expanding the boundaries of clinical treatment.

Figure 2. Milestones in the development of peptide therapeutics [1].

The global peptide therapeutics market is now expanding at an unprecedented pace. By the end of 2025, more than 100 peptide drugs had been approved worldwide, and annual sales had surpassed USD 90 billion. Yet rapidly growing demand has also exposed major limitations in peptide synthesis. Long-chain and hydrophobic peptides are prone to aggregation and precipitation; sequence-dependent side reactions are common; and coupling efficiency often declines as the chain grows. These challenges can severely constrain manufacturing capacity. New chemical tools, led by pseudoproline technology, are emerging as important enablers of the industry's next stage of development.

Overcoming Synthetic Bottlenecks: Why Pseudoproline Chemistry Matters

The term pseudopeptide was formally introduced by American chemist Arno Spatola in 1981 to describe peptide analogues in which one or more backbone amide bonds are chemically modified or replaced. Among the many classes of pseudopeptide derivatives, pseudoproline dipeptides are among the most widely used in peptide synthesis.

Pseudoprolines can be formed through nucleophilic addition of the side-chain hydroxyl group of serine or threonine to an activated carbonyl carbon. The resulting hemiketal intermediate undergoes intramolecular cyclization and dehydration to form an oxazolidine ring. Because this cyclic scaffold resembles proline in its conformational behavior, it is known as a “pseudoproline.” Cysteine can similarly form a thiazolidine-ring pseudoproline analogue, extending this family of reversible cyclic modifications.

Figure 3. General chemical structure of a pseudoproline dipeptide [2].

Pseudoproline dipeptides are widely used in peptide drug synthesis because they offer three core advantages:

1. Reduced peptide-chain aggregation and higher synthesis efficiency. The rigid cyclic scaffold disrupts intermolecular hydrogen bonding, helps prevent aggregation of resin-bound peptide chains, and improves chain solvation. This can significantly enhance coupling efficiency and product purity in the solid-phase synthesis of long peptides.

2. Less racemization and better overall process performance. The cyclic structure helps restrict inversion at the α-carbon, reducing C-terminal racemization during fragment coupling or under strongly activating conditions. The conformational “kink” also increases chain flexibility, allowing the molecular termini to approach one another more readily during macrocyclization. This can promote ring closure, reduce deletion sequences and related impurities, and lower purification complexity and cost.

3. Reversible modification with high sequence fidelity. Pseudoproline rings tolerate the basic conditions used for Fmoc deprotection but open during the final TFA-mediated cleavage step. The native Ser, Thr, or Cys residue is thereby regenerated without changing the original sequence of the target peptide.

Figure 4. TFA-mediated deprotection of a cysteine-containing pseudoproline peptide [2].

Solving Difficult Sequences: Pseudoproline Dipeptides in Liraglutide Synthesis

Liraglutide is commonly prepared through a convergent process that combines solid-phase and solution-phase synthesis. Multiple peptide fragments are first produced by SPPS and then condensed in solution. The Cabri group initially used the assembly strategy fragment 1–8 + [fragment 9–16 + fragment 17–31]. However, fragment 1–8 is highly hydrophobic, poorly soluble, and particularly prone to intermolecular aggregation, resulting in low overall process efficiency.

In the improved process, the pseudoproline dipeptide Fmoc-Thr(tBu)-Ser(Psi(Me,Me)pro)-OH was introduced at the Thr7–Ser8 fragment-coupling site. This strategy provided two complementary benefits. First, it markedly suppressed formation of the C-terminal oxazolone intermediate and reduced racemization during fragment condensation. Second, it disrupted β-sheet formation and limited peptide-chain aggregation.

Cabri and colleagues demonstrated that replacing Ser8 with a pseudoproline residue between Thr7 and Asp9 substantially improved reaction selectivity. The fully protected liraglutide 1–8 fragment was obtained at 93.6% purity and 80% yield. The final crude liraglutide reached 64% overall purity and 75% overall yield.

Figure 5. Application of a pseudoproline dipeptide in liraglutide synthesis [3].

Turning the “Impossible” into a 10-Hour Synthesis: Pseudoproline-Enabled hAmylin1–37

Human amylin (hAmylin) is a hydrophobic, structurally complex peptide composed of 37 amino acids. It readily forms β-sheet-rich aggregates, leading to poor coupling efficiency, incomplete cyclization, and low product purity during solid-phase synthesis. As a result, it is widely regarded as a classic “difficult sequence.”

To address this challenge, the Park group introduced three pseudoproline dipeptides at key positions: Fmoc-Ala-Thr(Psi(Me,Me)pro)-OH at Ala8–Thr9, Fmoc-Ser(tBu)-Ser(Psi(Me,Me)pro)-OH at Ser19–Ser20, and Fmoc-Leu-Ser(Psi(Me,Me)pro)-OH at Leu27–Ser28.

These pseudoproline dipeptides act like flexible joints within the peptide chain. They disrupt β-sheet aggregation, enhance chain flexibility and solvation, and create a favorable conformation for formation of the Cys2–Cys7 disulfide bond. The approach also substantially streamlined coupling, deprotection, and washing operations, reducing the synthesis time for linear hAmylin1–37 to 8.5 hours. Oxidative cyclization followed by TFA cleavage and deprotection required only another 1.5 hours. Using a Prelude automated peptide synthesizer, the complete preparation of hAmylin1–37 was therefore accomplished in under 10 hours.

Figure 6. Application of pseudoproline dipeptides in the synthesis of hAmylin1–37 [4].

TideChem: Pseudoproline Building Blocks for Peptide R&D

A century of innovation has taken peptide therapeutics from natural extraction to precision molecular engineering. Yet the synthesis of long-chain and hydrophobic peptides remains challenging: interchain aggregation, low reactivity, incomplete coupling, and by-product formation continue to impede scale-up and commercialization. Pseudoproline technology functions as a molecular scalpel for addressing these bottlenecks, while high-quality pseudoproline building blocks form the foundation of this efficiency-driven transformation.

TideChem has established pseudoproline dipeptides as a key specialty within its peptide-synthesis portfolio. Drawing on years of experience in peptide chemistry and a mature quality-control system, TideChem offers a broad product portfolio designed to support the synthesis of diverse peptide therapeutics:

• Broad portfolio. More than 30 serine- and threonine-based pseudoproline dipeptides have been developed, covering multiple protecting-group combinations and modification patterns to meet the needs of diverse peptide sequences.

• Reliable quality. Standard specifications include purity of at least 99.0% and no more than 0.2% for any single impurity. An end-to-end quality system—from raw-material sourcing through final batch release—supports consistent and reliable performance from batch to batch.

• Flexible customization. An experienced R&D team provides custom pseudoproline solutions for sequence-specific requirements, with one-stop support spanning molecular design and process optimization. Rapid response and integrated technical support help shorten customer development timelines.

Serine-Based Pseudoproline Dipeptides

Threonine-Based Pseudoproline Dipeptides

Figure 7. TideChem's pseudoproline dipeptide building-block portfolio.

Looking ahead, TideChem will continue to optimize pseudoproline manufacturing processes and expand its product portfolio. With a broad selection, dependable quality, and flexible customization, TideChem's pseudoproline building blocks help accelerate peptide drug discovery, development, scale-up, and commercialization worldwide.

References

[1] Signal Transduction and Targeted Therapy. 2025;10(1):74.

[2] Organic Letters. 2014;16(6):1772–1775.

[3] Organic Process Research & Development. 2021;25(7):1598–1611.

[4] Journal of Peptide Science. 2007;13(12):833–838.