Peptide Drug Conjugates

With technology and science advancing at a faster pace, humans have been able to unlock new potential for treating diseases. In modern times, one of the innovative therapeutic approaches in biomedicine is peptide drug conjugates (PDCs). Creative Peptides specializes in offering services related to PDCs, ushering in the era of personalized medicine.

What are Peptide Drug Conjugates?

Peptide-drug conjugates (PDCs), a subset of drug conjugates, are composed of carrier peptides ranging from 5 to 30 amino acid residues, toxic payloads, and linkers that connect the payload to the peptide. PDCs are further broken down into cell-penetrating peptides (CPPs) and cell-targeting peptides (CTPs), each having their own differences in the delivery of cytotoxic payloads. The efficacity of PDCs lies in their ability to deliver drugs to targeted cells without affecting healthy cells. The peptide in the PDCs attaches to specific receptors or proteins on the disease-causing cells' surfaces. In this state, the medicine is released to the diseased cells alone, reducing the side effects commonly associated with traditional therapies. Generally, compared to antibody-drug conjugates (ADCs), PDCs have advantages in tumor penetration, ease of synthesis and cost, and reduced off-target effects. Further, as compared to traditional cancer treatments (e.g., chemotherapy and radiation), PDCs have higher specificity for the target cancer with generally less toxic side effects in smaller doses.

Fig. 1 A schematic of a peptide-drug conjugate construct consisting of a homing peptide, linker and payload. The structure of ¹⁷⁷Lu-dotatate an FDA approved peptide-drug conjugate. (Cooper, B. M., 2021)

Classification of Peptide Drug Conjugates

PDCs are classified as cell-penetrating peptides (CPPs) or cell-targeting peptides (CTPs).

• Peptides for Specific Organ Targeting

Directed targeting of specific organs has been considered a crucial step in limiting side effects associated with traditional anticancer therapy. Use of peptides to direct organ specific targeting has emerged as a distinct possibility. Currently there are two main ways to target a peptide 1) rely on natural protein sequences such as vascular endothelial growth factor (VEGF) or somatostatin. Alternatively libraries of peptides can be tested via phage display technique. However, these techniques often yield peptides that can target tumor microenvironments but are poorly directed to specific organs. Likewise certain organs are more easily targeted than others for example N-acetylgalactosamine (GalNAc) can be used to easily target the lungs in adenocarcinoma. However, some organs prove more difficult to effectively target when a strong physical barrier is in place as is the case with pancreatic cancers characterized by strong desmoplasia creating a mechanical barrier around the tumor cells or cancers of the brain that necessitate crossing the blood–brain barrier (BBB). That said, developments are underway to utilize phage-derived shuttle peptides which can select against BBB endocytic machinery and used in engineering novel PDCs for brain cancers.

• Cell-Penetrating Peptides (CPPs)

The cell membrane provides a physiological barrier that limits the transportation of various molecules, such as macromolecules, proteins, and nucleic acids, across the plasma membrane. However, the cell membrane can also limit drug penetration. Therefore, it is imperative to develop drugs that can cross the cell membrane of cancer cells to induce destruction. CPPs can transport drug payloads through cell membranes using specific amino acid sequences ranging from 5 to 30 residues. CPPs provide an effective method for transporting cell-impermeable compounds or drugs to reach their intracellular targets. Various mechanisms of action have been proposed regarding how CPPs penetrate the cell. Two generally accepted mechanisms are (1) direct penetration of the plasma membrane and (2) endocytosis. Direct penetration occurs when positively charged CPPs interact with negatively charged membrane components, destabilizing the membrane and forming a pore. Moreover, clathrin-mediated endocytosis and macropinocytosis have been observed to take up CPPs.

• Cell-Targeting Peptides (CTPs)

CTPs range from 3-14 amino acids long and utilize receptors that are overexpressed on cancer cell surfaces to target the delivery of the drug. Depending on the targeted receptor, CTPs can cause a localized build-up of the drug around the tumor or induce endocytosis upon CTP binding. CTPs exhibit similar characteristics to monoclonal antibodies (mAbs) by binding with high affinity to their respective receptor. However, unlike mAbs, CTPs can penetrate tumors better due to their small size.

Fig. 2 Theoretical summary of the steps that lead to the internalization of the specific (overexpressed receptor) within cancer cells. (T Heh, E., 2023)

Differences between PDCs and ADCs

• Immunogenicity

In the context of this review, immunogenicity can be defined as the unwanted ability for a drug or molecule to induce an immune response due to the body recognizing the infused drug as foreign and eliciting a response against it. PDCs generally are considered to have low immunogenicity characteristics since peptides have low intrinsic immunogenicity. ADCs on the other hand can exhibit increased risk for immunogenicity compared to PDCs. This is due to the various domains in the ADC, such as the epitopes, linker and cytotoxic agent that can result in the development of anti-drug antibodies (ADAs). ADAs could potentially inactivate the drug, causing decreased effectiveness and targeting and overall suboptimal exposure.

• Penetration

The weight of the PDC molecule is small (~2-20 kDa), allowing the PDC to penetrate the tumor stroma and enter the tumor cells. In comparison, ADCs have a large molecular weight (∼160 kDa) that results in the limitation of transport through solid tumor cell surface. In a study conducted by Chalouni and Doll, the internalization rate of an anti-CD30 mAb based ADC was determined using flow cytometry. It was demonstrated that 60% of the initial level of surface bound cAC10 antibody remained after 20 h.

• Elimination

PDCs generally have a short half-life and are rapidly eliminated by the kidneys. In addition, due to their small size, PDCs have the ability to reach tumor sites unattainable by larger molecules, such as ADCs. ADCs have a longer half-life than PDCs and are non-specifically taken up by the liver, resulting in potential dose-limiting toxicity to the liver.

Advantages of Peptide Drug Conjugates

Enhanced Selectivity: Peptide-drug conjugates (PDCs) enhance selectivity and specificity for targeted tissues, organs, or cells. This focus on a specific target reduces damage to non-targeted cells and tissue, reducing side effects. Due to their targeted delivery，PDCs can reduce the risk of drug resistance, which is a major concern in cancer chemotherapy.

Improved Pharmacokinetics: PDCs can potentially modify and improve the pharmacokinetics – absorption, distribution, metabolism and excretion of the drugs – resulting in better efficiency and fewer side effects.

Enhanced Drug Delivery: Certain peptides can cross biological barriers, such as the blood-brain barrier, to deliver drugs to otherwise inaccessible sites. Peptides can often be designed to be highly stable, resistant to metabolism, and prolonged circulating life.

Reduced Drug Dosage: Because of their high efficacy, PDCs often require a lower dosage, which minimizes the risk of drug overdoses and associated complications.

Versatility: Peptide is composed of 20 kinds of amino acids with different properties, so a wide variety of PDC with different properties can be constructed. This provides researchers with rich choices to design and optimize the structure and function of PDC according to different diseases and treatment needs. A variety of peptides and drugs can be combined, which provides potential for developing different treatment schemes.

Applications of Peptide Drug Conjugates

Cancer Treatment: The most promising application of PDCs is in the field of oncology. They can be used as a targeted therapy, where the peptide component of the conjugate is used to specifically target cancer cells while the drug component kills them. This is an improvement on traditional chemotherapy, which also affects healthy cells and leads to distressing side effects.

Antimicrobial Applications: PDCs have been studied for their potential use as antimicrobial agents. Conjugation with peptides can enhance the efficacy of antibiotic drugs and help overcome challenges related to drug resistance.

Neurodegenerative Diseases Treatments: There is hope that PDCs can be used in the treatment of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington’s disease. They could be used to deliver drugs across the blood-brain barrier, something that is notoriously difficult to accomplish.

Cardiac and Anti-inflammatory Treatments: PDCs could also be used in the management of heart disease or inflammatory conditions, improving the effectiveness of existing drugs through targeted delivery and helping to reduce the dosage required.

Diabetes Management: Some insulin-mimetic peptides have been developed that can be used in PDCs for the treatment of diabetes. These could potentially offer better control of blood glucose levels and reduce the frequency of injections required.

Immune Modulation: PDCs have been used to selectively modulate the immune system, for example, to prevent organ transplant rejection or manage autoimmune diseases.

Virology: PDCs can also be used in the prevention and treatment of viral infections. The peptide component can help to inhibit the entry of viruses into cells, or promote the immune response against them, while the drug component directly attacks the virus itself.

Our Peptide Drug Conjugates Services

Generally, structure of PDCs is composed of three functional parts, including peptide carrier, linker and payload drug. Creative Peptides provides a one-stop solution for all of your peptide-drug bioconjugation needs. Our custom peptide synthesis, peptide modification, and conjugation services involve chemical modification of peptide at highly specific sites. This allows for the elegant attachment of drugs or bioactive molecules though a wide variety of coupling techniques. Each peptide-drug conjugate is meticulously monitored during synthesis and controlled according to our quality assurance and quality control standards. The following is our successfully synthesized Mal-K-V-R-PABC small molecule-peptide conjugate.

Fig. 3 Mal-K-V-R-PABC small molecule-peptide conjugate

• Successfully completed the orthogonal protection base design.
• Successful synthesis of small molecule-peptide conjugates.
• The side chain double coupling of lysine and 1,6- diaminohexane was successfully completed.
• Successfully synthesized in the presence of carbamate.

In addition to custom conjugation services, we also offer individual functional parts.

Peptide Carrier: we offer the common peptide carrier, including cell-penetrating peptides and cell-targeting peptides.

Fig. 4 Some cell-penetrating peptides and their source. (Zhou, J., 2021)

Fig. 5 Some cell-target peptides and their targets. (Zhou, J., 2021)

Linkers: As with conjugation techniques, since each drug molecule and targets have different chemical constraints, the choice of linker is target dependent and influenced by the drug molecule used, site of conjugation, number of attachment sites available as well as the cleavability and the polarity of the linker. We provide the following linkers:

• Cleavage linkers: they refer to those whose structure in part is degradable after penetrating the membrane and getting into the cells, for instance, linkers with structure of hydrazine (pH-sensitive), Glycine-Phenylalanine-Leucine-Glycine (enzyme-sensitive), disulfide bond (reduction-sensitive), etc.
• Non-cleavage linkers: they are relatively stable, carbon chains, amido bonds, ether bonds or other structures with physical and chemical properties of stability are all belonged to this category.

Payload Drugs: Anti-tumor cytotoxic agents such as Doxorubicin (DOX) and Paclitaxel (PTX). In addition to this, other payloads include peptides, siRNAs, antisense oligonucleotides and so on.

Our Procedures

• All custom synthesis of biomolecules, modification, or bioconjugation services are manufactured under strict quality control processes.
• Analytical HPLC and MS analyses are performed in every development cycle.
• Final target conjugates must first be isolated from excess or unreacted reagent.
• In many cases, simple dialysis may suffice to remove unreacted reagent from the reaction solution.
• The cross-linked target molecule may then be further characterized by biochemical or biophysical techniques.
• HPLC may also be used to either remove excess reagent or to isolate and characterize the cross-linked product.
• QC (quality control) and QA (quality assurance) procedures are also followed independently to ensure the highest quality possible of every delivered conjugate.
• The typical delivery consists of lyophilized sample in individual fully labeled vials. The shipment also contains COA, MS, HPLC and/or other analytical data. Additional analytical data is also available upon request.

Why Creative Peptides?

• Fully traceable documentation system
• Possessing the research and development technology of long-acting peptide drug formulations
• Perfect professional team for innovative drug development
• Innovative products
• High quality service
• Product quality is stable and reliable
• Low sales price

FAQ

1. How do peptide drug conjugates work?

Peptide drug conjugates work by using the peptide as a targeting agent to deliver the drug to specific cells or tissues in the body. The drug is released only when the conjugate reaches its target, maximizing therapeutic effectiveness while minimizing side effects.

2. What types of diseases can peptide drug conjugates treat?

PDCs have demonstrated potential in a range of diseases including cancers, infectious diseases, cardiovascular diseases, and disorders of the central nervous system among others.

3. Why should we choose your peptide drug conjugates service?

We offer comprehensive peptide drug conjugates services, from peptide synthesis, conjugation, purification to characterization, and biological evaluation. Our team of scientists are highly qualified with extensive experience in the field of peptide drug discovery, and we use advanced technology and proven methodologies to ensure the delivery of high-quality services.

4. Can you validate the stability of the peptide drug conjugates?

Yes, we perform comprehensive stability studies including physical, chemical, and biological stability tests to ensure the integrity and potency of our PDCs.

5. How do you ensure the safety of your peptide drug conjugates?

We have strict quality control and quality assurance systems in place to ensure the safety of our products. All our PDCs undergo rigorous testing including toxicity, immunogenicity, and pharmacologic studies before being advanced to clinical studies.

References

1. Zhou, J., et al. Source and exploration of the peptides used to construct peptide-drug conjugate. European Journal of Medicinal Chemistry. 2021, 224: 113712.
2. Cooper, B. M., et al. Peptides as a platform for targeted therapeutics for cancer: peptide-drug conjugates (PDCs). Chemical Society Reviews. 2021, 50(3): 1480-1494.
3. T Heh, E., et al. Peptide drug conjugates and their role in cancer therapy. International Journal of Molecular Sciences. 2023, 24(1): 829.