Polyethylene Glycol in Peptide-Drug Conjugates

Peptide-drug conjugates (PDCs) represent the next generation of targeted therapeutics, following antibody-drug conjugates (ADCs). PDCs' core advantages lie in their enhanced cellular permeability and improved drug selectivity. Both PDCs and ADCs have received approval from the U.S. Food and Drug Administration (FDA). Over the past two years, pharmaceutical companies have been actively exploring PDCs in targeted therapies for various fields, including oncology, COVID-19, and metabolic disorders. While PDCs demonstrate significant therapeutic efficacy, they face challenges such as poor stability, low bioactivity, long development timelines, and slow clinical development. Optimizing the design of PDCs is a crucial area of ongoing research.

Peptide Drug Conjugate

Peptide-drug conjugates are an emerging prodrug strategy in targeted drug delivery systems. They consist of three main components: a peptide, a linker, and a drug. Their mechanism of action varies depending on the type of peptide and linker. Initially, the peptide facilitates cellular entry by recognizing specific receptors on the cell surface. Subsequently, the linker undergoes cleavage upon specific stimulation, releasing the drug to exert its therapeutic effect. This prodrug strategy can potentially enhance drug targeting and reduce off-target toxicity. By targeting tumor cell receptors, PDCs enable the synergistic delivery of chemotherapeutic agents, extending their therapeutic effects in peptide-based treatments.

Fig. 1. Peptide drug conjugates (Acta Pharm Sin B. 2023, 13(9): 3659-3677).

Peptides

Peptides are an important component of PDCs, and they can be classified according to their functions into cell-penetrating peptides (CPPs), cell-targeting peptides (CPTs), self-assembling peptides (SAPs), and reactive peptides. CPPs are small, short peptide molecules that can enter cells without disrupting the integrity of their membranes, typically consisting of 5-30 amino acids. CPTs are defined as peptides that bind specifically to cells or tissues. There are generally two types of targeted drug delivery systems: passive targeting and active targeting. SAPs refer to peptides that spontaneously form one or more ordered structures from complex mixtures through non-covalent interactions (including van der Waals forces, electrostatic interactions, hydrogen bonding, and stacking interactions) without external stimulation. Amino acids, as the basic building blocks of peptides, provide the foundation for the self-assembly ability of peptides. Reactive peptides are peptides that undergo structural changes in response to external stimuli. This change occurs at the structural level, which is different from the self-assembly behavior of self-assembling peptides. Reactive peptides require external environmental stimuli to undergo structural changes. Typically, external environmental stimuli include temperature, pH, enzymes, and so on.

Payloads

The payloads in PDCs are cytotoxic or therapeutic drugs, which often suffer from poor water solubility, low selectivity, short half-life, and poor stability, limiting their clinical application. Drugs delivered through PDC strategies require feasible attachment sites and should remain pharmacologically inactive while conjugated. Upon release in the lesion tissue, they should exhibit therapeutic effects, clear mechanisms of action, and strong pharmacological activity. Conjugation with peptides can enhance drug solubility, promote selectivity, extend circulation time in vivo, optimize bioavailability, and prevent side effects and toxicity to other tissues. Drugs used in PDC construction can be divided into chemical drugs, protein drugs, and peptide drugs. Chemical drugs include doxorubicin, paclitaxel, camptothecin, methotrexate, and cisplatin. Protein drugs include interferons and tumor necrosis factors, which achieve effective antitumor therapeutic effects by inducing apoptosis and inhibiting intracellular protein synthesis. However, protein drugs face challenges such as poor stability in vivo, lack of targeting, and low bioavailability. The combination of peptides and protein drugs in PDCs has become a new approach to overcoming these issues. Peptide drugs have also become a research hotspot in new drug development in recent years.

Linkers

As the connecting bridge between drugs and peptides, linkers determine the circulation time and stability of PDCs in vivo. An ideal linker should remain stable during circulation to avoid premature drug release, while allowing rapid and effective release of the drug after reaching the lesion tissue. The linker should not affect the affinity of the peptide for its receptor or the drug's activity. Additionally, the hydrophobicity of the linker should not be too strong to prevent aggregation of PDCs, which could result in poor in vivo stability, reduced drug efficacy, and strong systemic toxicity and immune side effects. Linkers are mainly classified into non-cleavable linkers and cleavable linkers based on their drug-release mechanisms. Cleavable linkers can be further divided into pH-sensitive linkers, redox-sensitive linkers, and enzyme-sensitive linkers.

Peptide Drug Conjugate Linker

Polyethylene glycol (PEG) linkers play a critical role in PDCs by providing molecular flexibility and spatial shielding, which enhance the stability and solubility of the conjugates. Additionally, PEG modification can extend the drug's half-life in vivo, reduce immune responses, and optimize the pharmacokinetic properties of the therapeutic, ultimately improving its efficacy. As a leading chemical supplier, BOC Sciences offers a wide range of PEG linkers in various specifications and functionalities, including linear PEG, branched PEG, and PEG derivatives with reactive functional groups. These options cater to the diverse needs of PDC development. With advanced manufacturing capabilities and rigorous quality control, BOC Sciences ensures the supply of high-purity, high-quality PEG linkers. The company provides customized solutions to accelerate the research, development, and commercialization of PDCs.

What is the Difference Between ADC and PDC?

PDCs and ADCs are conceptually similar but have very different structures and properties. In ADCs, a specific monoclonal antibody (mAb) targeting antigens expressed on tumor cells is used to deliver cytotoxins, while minimizing damage to normal cells, resulting in better therapeutic effects. However, monoclonal antibodies can induce immunogenicity. The targeting mechanism of PDCs is related to many factors—the function of the receptor will determine the mechanism of action. Compared to ADCs with similar construction strategies, PDCs have unique advantages: (1) peptides have a small molecular size, high drug-loading capacity, and can more easily penetrate the tumor stroma to enter tumor cells; (2) peptides are highly biodegradable and do not trigger any immunogenic response in the body; (3) certain targeting peptides can eliminate tumor cell resistance by altering the drug's cell entry mechanism, allowing effective killing of resistant tumors; (4) the short peptide nature of PDCs makes their structure more flexible, easier to modify, and conjugate with various drugs, including chemicals, proteins, and peptide drugs, to create targeted therapeutics, significantly reducing off-target toxicity; (5) peptide fragments are easy to produce and scale up. Therefore, PDCs have great potential in developing targeted drug delivery systems.

Table 1. Main characteristics of PDCs and ADCs.

Stability of Peptide-Drug Conjugates

Similar to peptides, the main drawback of PDCs is poor circulation stability and rapid renal clearance. PDCs should remain stable in circulation to prevent systemic exposure prior to drug release. It has been confirmed that nanoparticles can enhance the stability of PDCs. One method to overcome poor circulation stability is to conjugate PDCs with gold nanoparticles (AuNPs). Due to their ideal physicochemical properties, safety, relative ease of synthesis, and longer circulation half-life, the overall stability of PDCs can be improved. When PDCs for treating mouse lymphoma cells were conjugated with PEG-coated AuNPs, a selective PEG-AuNP-PDC was formed, and the circulation half-life of the PDC was increased by 90 times. Nanoparticles enhance the stability of PDCs through a bifunctional approach. This design principle is based on near-infrared non-invasive photothermal therapy (PTT), where the nanoparticle-enhanced pharmacokinetics (PK) characteristics of PDCs is an ongoing research area, offering great potential for clinical trials.

What are Peptide-Drug Conjugates Used For?

Currently, PDCs are widely used in the treatment of cancer, infectious diseases, and inflammation. Peptides offer high specificity and low immunogenicity, targeting specific cells or tissues, improving drug delivery efficiency, and reducing off-target toxicity. For example, PDCs can precisely deliver anticancer drugs, significantly reducing side effects. In addition, peptides can also be used as cell-penetrating carriers or receptor ligands, enhancing the tissue permeability and targeting of drugs. With their flexible design and broad application prospects, PDCs show great potential in the field of precision medicine.

Anti-inflammatory Drugs

PDCs can be applied in the treatment of inflammation. For example, naproxen is a non-steroidal anti-inflammatory drug that exerts its anti-inflammatory effect by inhibiting prostaglandin synthesis. However, due to a lack of selectivity, it has gastrointestinal side effects. To improve naproxen's selectivity, Moreira et al. constructed a naproxen-dehydropeptide complex and found that they easily formed nanostructured fibrous supramolecular hydrogels, which could be the best strategy for treating inflammation.

Antibacterial Drugs

PDCs are widely used in bacterial infections. Effective and safe treatment of bacterial infections is a major challenge in modern medicine, largely due to the limitations of current antibiotics in host toxicity, effective drug delivery, and the increasing antimicrobial resistance. To address these issues, Brankiewicz et al. covalently linked fluconazole with cell-penetrating peptides (CPTs) to form PDCs. Compared to free fluconazole, the PDC exhibited a higher efficacy in killing Candida.

Antiviral Drugs

Antiviral PDCs, particularly for HIV, mainly target cell surface receptors, with the aim of blocking HIV entry into cells. After dengue virus invades cells, it requires a protease system to decompress the packaged viral protein (DENV). Therefore, PDCs are primarily developed to act on this protease system. In the development of such drugs, researchers have also experimentally tested the length of the PEG arms between peptides and drugs. When the polymerization degree was 4 and 8, drug doses of 1.11 and 1.52 nM were required to block HIV. With polymerization degrees of 12 and 24, only 0.35 and 0.12 nM were needed to achieve the block.

Analgesic Drugs

Furthermore, PDCs can be applied as delivery systems for therapeutic drugs, such as analgesics, antimalarial drugs, and diabetes medications. Among analgesics, PDCs mainly regulate the G protein-coupled receptor (GPCR), μ- and κ-opioid receptor (MOR, KOR) pathways. Neurodegenerative disease drugs mainly include PDCs conjugating acetylcholinesterase inhibitor galantamine, used for treating Alzheimer's disease.

Peptide Drug Conjugate FDA Approved

Although PDCs have significant application prospects, the field is still in a developmental lull. So far, only two PDC drugs have been approved globally: one is Lutathera, developed by a Novartis subsidiary, which was approved by the FDA in January 2018 for the treatment of gastroenteropancreatic neuroendocrine tumors that express somatostatin receptors. The other is Pepaxto (Melflufen), developed by Oncopeptides, which received accelerated approval from the FDA in February 2020 for the treatment of multiple myeloma. However, due to a series of issues, Pepaxto was rapidly withdrawn after its swift approval by the FDA.

• Lutathera (Lu-177-DOTATATE) is an emerging peptide receptor radionuclide therapy (PRRT). It is a Lu-177-labeled somatostatin analog, which is a radioactive isotope-labeled octreotide. As a radioactive isotope-labeled drug, Lutathera contains both anticancer properties and the radioactive isotope, which emits radiation that can physically destroy cancer cells. Lutathera is constructed by coupling a peptide molecule called DOTA-TATE (similar to octreotide) with Lu-177 using a chelator to form a novel complex drug. Once injected into the patient's body, Lutathera targets tumor cells through receptor-ligand recognition, delivering the radioactive isotope to the tumor tissue, where Lu-177 releases high-energy beta radiation, ultimately killing the tumor cells.
• Melflufen is a First-in-Class PDC drug targeting aminopeptidase. The toxic molecule used is an old drug for treating multiple myeloma—melphalan. By conjugating melphalan with a peptide targeting aminopeptidase, it is specifically delivered to tumor cells, enhancing the efficacy of melphalen in myeloma cells. In preclinical studies, melflufen has shown cytotoxic effects on myeloma cells that are resistant to other therapies, including alkylating agents, and has shown to inhibit DNA repair induction and angiogenesis.

Peptide Drug Conjugates in Clinical Trials

In addition to the two approved drugs, Table 2 lists the PDC drugs currently undergoing clinical trials. Clinical trials of PDCs are gradually progressing to evaluate their efficacy and safety in various diseases. In the oncology field, peptides are used to deliver anticancer drugs by targeting specific receptors (such as HER2 or EGFR), and have shown favorable pharmacokinetic properties and significant anticancer activity in multiple clinical stages. Furthermore, PDCs targeting infectious diseases and autoimmune diseases are also under investigation, showing the potential to reduce systemic toxicity and enhance therapeutic effects. Current clinical trials are focused on optimizing the selectivity and stability of peptides, while exploring novel drug payloads and linkage strategies.

Table 2. Clinical research progress of PDCs.

In Conclusion

From the number of PDC drugs available on the market, it is clear that despite the many unique advantages of PDCs, there are still numerous challenges and obstacles in the clinical translation process. For example, from a safety perspective, peptides derived from natural amino acids exist in a water-soluble form, are fully biodegradable, and are typically biocompatible without adverse reactions. However, it is currently unclear whether the biocompatibility and biodegradability of peptides are preserved after conjugation with drugs. The development of efficient PDCs requires the design and synthesis of multifunctional peptides and linkers. We believe that with advancements and maturation in peptide screening technology and linker synthesis techniques, more PDC drugs will be explored in the future.

** Recommended Products **

Reference

1. Gong, L. et al. Research advances in peptide‒drug conjugates. Acta Pharm Sin B. 2023, 13(9): 3659-3677.