Autoimmune diseases arise from dysregulated immune responses in which the body’s immune system mistakenly attacks its own tissues. Effective therapies must suppress pathological immune activity while preserving overall immune function. Cyclic peptides have emerged as promising therapeutic agents in this domain due to their unique structural and pharmacological properties.

Cyclic peptides are short peptide sequences whose ends are chemically linked to form a stable ring structure. This cyclic conformation offers several advantages for autoimmune disease treatment:

Targeting Protein–Protein Interactions (PPIs)
Many key immune signaling pathways, such as those involving cytokines, costimulatory molecules, and T-cell receptors, are governed by protein–protein interactions. These interfaces are often flat and difficult to target with small molecules. Cyclic peptides, due to their conformational rigidity and surface mimicry, are well suited to disrupt PPIs with high specificity.

Enhanced Stability and Bioavailability
Compared to linear peptides, cyclic peptides exhibit increased resistance to proteolytic degradation in biological fluids, resulting in prolonged circulation times. This makes them suitable for chronic conditions like rheumatoid arthritis, multiple sclerosis, and inflammatory bowel disease.

Immune Modulation
Cyclic peptides can be engineered to bind selectively to immune receptors such as integrins, interleukin receptors, or co-stimulatory molecules (e.g., CD28, CTLA-4), thereby modulating the activity of immune cells like T cells, B cells, and macrophages. For example, integrin-binding cyclic peptides have shown promise in preventing immune cell infiltration into inflamed tissues.

Tissue-Specific Delivery
Using cyclic peptides as targeting moieties or conjugates, drugs can be directed to specific organs or inflammatory sites. This approach enhances therapeutic efficacy while minimizing systemic immunosuppression and adverse effects.

Peptidomimetic Flexibility
The cyclic scaffold allows for the incorporation of non-natural amino acids or chemical modifications that fine-tune activity, stability, and immunogenicity. This design flexibility supports the development of selective immune modulators with favorable safety profiles.

In summary, cyclic peptides represent a highly adaptable and potent therapeutic platform for the treatment of autoimmune diseases. Their ability to specifically modulate immune responses, combined with improved pharmacokinetics and target selectivity, positions them as a next-generation class of immunomodulatory agents. 

Cyclic peptides are emerging as a powerful class of therapeutic molecules in oncology due to their unique structural features and versatile biological functions. Characterized by a covalently closed ring structure, cyclic peptides combine the high specificity of biologics with the stability and cell permeability advantages often associated with small molecules.

In the context of cancer therapy, cyclic peptides offer several key therapeutic advantages:

Targeting Protein–Protein Interactions (PPIs)
Many cancer-driving pathways involve protein–protein interactions that are difficult to inhibit with traditional small molecules. Cyclic peptides are particularly well-suited to disrupt these interfaces due to their conformational rigidity and ability to present functional residues in a spatially precise manner. They have shown promise in targeting oncogenic PPIs such as MDM2–p53, BCL2 family interactions, and YAP–TEAD complexes.

Tumor Targeting and Selectivity
Cyclic peptides can be engineered to bind tumor-specific receptors or integrins, enabling selective delivery of therapeutic payloads or imaging agents to cancer cells or tumor vasculature. This tumor-homing capability enhances therapeutic efficacy while minimizing off-target toxicity.

Improved Pharmacokinetic Properties
Compared to linear peptides, cyclic peptides exhibit increased protease resistance and metabolic stability, resulting in longer half-lives in vivo. This makes them attractive for systemic administration in solid tumors and hematologic malignancies.

Drug Conjugates and Payload Delivery
Cyclic peptides serve as modular carriers in peptide–drug conjugates (PDCs), where they facilitate targeted delivery of cytotoxic agents, radionuclides, or immune modulators directly to tumor cells. Their small size allows better tissue penetration compared to antibodies.

Immune Modulation and Checkpoint Targeting
Certain cyclic peptides can modulate immune responses by blocking immune checkpoints or activating co-stimulatory pathways. This opens opportunities in cancer immunotherapy, either as standalone agents or in combination with other immuno-oncology treatments.

In summary, cyclic peptides represent a promising new modality in cancer treatment. Their structural tunability, specificity for complex targets, and compatibility with targeted delivery systems position them as strong candidates for the next generation of precision oncology therapeutics. Ongoing clinical research and technological innovation continue to expand their therapeutic reach across multiple cancer types.