The concept of drug conjugates originated in the early 20th century. In 1910, Paul Ehrlich proposed the “Magic Bullet” theory, which aimed to describe the concept of a drug that could selectively kill malignant cells without harming normal tissue, that is, with the help of these “bullets”, the drug could be delivered to a specific site. In 1957, Mathé was the first to conjugate methotrexate to the anti-leukemia 1210 antigen immunoglobulin for the treatment of leukemia, thus opening the door to the study of drug conjugates.
As the first and most typical representative of drug conjugates, ADCs use antibodies as carriers to form antibody-drug couplings to deliver antibodies and cytotoxic drugs (payloads) to cancer cells. In 1967, the concept of antibody-drug conjugate (ADC) was formally proposed. The hybridoma technique of monoclonal antibodies, invented in 1975 by Georges J. F. Kohle and Cesar Milstein, played a crucial role in the development of antibodies. Subsequently, more and more monoclonal antibodies continued to be approved for marketing, such as trastuzumab, rituximab, cetuximab, etc., thus opening a new era of therapeutic antibody technology. However, although monoclonal antibodies can precisely target tumor surface antigens to treat cancer, their lethality to cancer cells is generally lower than that of chemotherapy drugs. In order to further combine the targeting and cytotoxic drug lethality advantages of monoclonal antibodies, ADC research has been continuously promoted since the 1980s. In 2000, the world’s first ADC drug gemtuzumab was successfully approved for marketing.
In fact, in addition to antibodies, the carriers of drug conjugations can also be peptides, proteins, small molecules of drugs and nucleic acid aptamers, etc. The basic structure is composed of carrier, linker and payload, as shown in the figure below.
Classification of Drug Conjugates
Drug conjugates can be divided into the following types by different carriers.
- Peptide-drug conjugate (PDC)
- Radionuclide drug conjugates (RDC)
- Antibody-oligonucleotide conjugates (AOC)
- Antibody degraducer conjugates (ADeC)
- Virus-like drug conjugates (VDC)
- Antibody-biologic conjugates (ABC)
- Antibody-cell conjugation (ACC)
- Aptamer drug conjugates (ApDC)
- Antibody fragment-drug conjugates (FDC)
- Immune-stimulating antibody conjugate (ISAC)
- Small molecule-drug conjugates (SMDC)
Mechanism of Action of PDC
Peptide-drug conjugate (PDC) is a peptide-conjugate drug, which consists of carrier peptides, linkers and cytotoxic drugs. Among them, targeted peptides can be generally divided into cell penetrating peptides and cell targeting peptides, cell penetrating peptides can transport drugs across cell membranes, and cell targeting peptides can specifically bind to the receptor on the target cell.
As a new type of drug conjugate, PDC not only retains the function and biological activity of peptides, but also has the cleavable property of Linker, which can release drugs responsively. In contrast to ADC, PDC enters the cell through direct penetration or mediated endocytosis of peptides, and then stimulates the cleavage of the cleavable linker, resulting in drug release, as shown in the figure below.
It can be seen from the figure above that the mechanism of action of PDC mainly depends on the targeting peptide and linker. As mentioned above, the cleavable linker can be cleaved under the stimulation of a specific pH or enzyme to release the cytotoxic drug. However, the cleavage of linker can occur inside and outside the cell. When cleaved outside the cell, the cytotoxic drug must diffuse into the cell before it can exert its biological activity. Therefore, when designing PDCs, extracellular cleavage needs to be considered to ensure that PDCs target tumor cells.
Key Advantages of PDC Technologies
As a new type of drug conjugate, PDC has the following advantages compared with ADC:
- Peptide drugs have small molecular weight and are easier to penetrate tumor matrix into tumor cells to play anti-tumor effects.
- Most of the peptide sequences are homologous to humans, with low immunogenicity and less likely to cause immune reactions.
- Compared with monoclonal antibodies, peptides are easier to design and produce, and the cost is lower.
- Due to the stronger tumor penetration of PDC, the selectivity of cytotoxic drugs is wider.
- PDC can be quickly eliminated by the kidney, higher safety.
PDC Design
PDC carrier (peptide)
In recent years, with the rapid development of proteomics, peptide solid-phase synthesis and phage display technology, more and more novel peptides have been designed, which has greatly promoted the development of peptide carriers in PDC. Currently, according to the function of peptides, they can be divided into Cell-penetrating peptides (CPP), Cell-targeting peptides (CTP), and self-assembling peptides (SAP) and response peptide. Among them, the peptides used to construct PDC are mainly cell penetrating peptides and cell targeting peptides.
1.CPP is usually a short peptide composed of 5-30 amino acids, which is a cationic peptide, a hydrophobic peptide or an amphiphilic peptide. According to its structure, CPP can be divided into linear cell penetrating peptides and cyclic cell penetrating peptides. Among them, the linear cell penetrating peptide structure has a simpler structure and easier to synthesize. CPP mainly penetrates the cell membrane through endocytosis or direct translocation, which is mediated by the interaction of positively charged CPP with negatively charged membrane components and phospholipids to promote direct penetration of the cell membrane. This process is a non-energy-consuming process. Endocytosis, on the other hand, is an energy-dependent pathway that penetrates the cell membrane. There are three pathways: macroendocytosis, caveolae-mediated endocytosis (CvME) and clathrin-mediated endocytosis (CME).
2.CTPs, defined as peptides showing cell- or tissue-specific binding activity, are the second class of peptides used in PDCs. CTP has high specific affinity for antigens or receptors of tumor cells, and can bind to antigens or receptors overexpressed on tumor cells to mediate the entry of cytotoxic drugs into tumor cells.
3.SAP is defined as a peptide that spontaneously or triggered the formation of one or more ordered structures from a complex mixture by intermolecular interactions, including hydrogen bonding, electrostatic interactions, π-π stacking, van der Waals forces, dispersion forces, entropy forces, etc. Compared with common peptides, SAP has the advantages of biocompatibility, biodegradability and versatility.
4.Responsive peptides are defined as peptides that undergo structural changes in response to external environmental stimuli, including temperature, pH, enzymes, etc. Therefore, according to the different external environmental stimuli, it can be divided into temperature-response peptide, pH-response peptide, enzyme-response peptide.
PDC Linker
The linker in PDC is an important part of linking small molecule drugs to peptides and maintaining the structural integrity of PDC. The Linker should be reasonably selected according to its stability in the circulation to avoid premature release of the drug and ensure that the drug reaches the target. Cells can quickly and effectively release drugs. The ideal Linker should have three key characteristics: First, it should have high water solubility to facilitate bioconjugation without affecting the affinity of the carrier to its receptor. Second, the in vivo transport diffusion is sufficiently stable to avoid premature drug release, and third, there is sufficient elimination activity to efficiently release the payload at the target site.
PDC Payload
The payload of PDCs is a drug with cytotoxic or therapeutic effects, and the ideal payload should have the following characteristics:
1.Ultra-high cytotoxicity, usually the selected payload should have a low IC50 and be able to kill target cells at subnanomolar concentrations.
2.The immunogenic effect is low, , as far as possible to avoid the occurrence of immunogenic reaction.
3.High stability, so that the drug has enough time to reach the target cells.
4.Appropriate hydrophobicity, the combination of appropriate hydrophobic linker and hydrophobic payload often promotes the accumulation of PDC complexes.
5.Good solubility, soluble in water buffer for easier coupling.
References
- Chavda VP, Solanki HK, Davidson M, Apostolopoulos V, Bojarska J. Peptide-Drug Conjugates: A New Hope for Cancer Management. Molecules. 2022 Oct 25;27(21):7232. doi: 10.3390/molecules27217232. PMID: 36364057; PMCID: PMC9658517.
- Gong L, Zhao H, Liu Y, et al. Research advances in peptide‒drug conjugates[J]. Acta Pharmaceutica Sinica B, 2023.