Abstract: Cyclosporin A (CsA) is a well-known macrocyclic immunosuppressant that exerts its primary biological effects by binding to cyclophilins and subsequently inhibiting calcineurin. Recently, its role in oncology and cancer therapeutics has garnered significant attention. Cyclophilins, the primary intracellular targets of CsA, are overexpressed in various malignancies—including liver, pancreatic, breast, prostate, and lung cancers—where they facilitate tumor proliferation, metastasis, and chemoresistance. Furthermore, CsA is utilized in the clinical management of hematological disorders such as myelodysplastic syndromes (MDS). While the direct application of CsA in cancer is limited by its inherent immunosuppressive properties, lack of isoform selectivity, and poor pharmacokinetic profile, the development of non-immunosuppressive CsA derivatives and isoform-selective cyclophilin inhibitors presents a promising frontier in targeted cancer therapy.
1. Introduction
Cyclosporin A (CsA) is a natural macrocyclic compound originally identified for its potent immunosuppressive properties. The primary intracellular receptors for CsA are cyclophilins (Cyps), a highly conserved family of enzymes possessing peptidyl-prolyl cis-trans isomerase (PPIase) activity [1]. To date, 17 human cyclophilin isoforms have been identified, characterized by a common cyclophilin-like domain (CLD) [1]. While traditionally utilized in solid organ transplantation and autoimmune diseases, CsA and its targets have increasingly become a focal point in oncology. Cyclophilins are implicated in diverse biological processes, including protein folding, intracellular trafficking, and signal transduction. In the context of cancer, various cyclophilin isoforms are overexpressed and contribute to tumor progression, making them emerging targets for drug development [1]. Consequently, CsA serves as a critical pharmacological tool and a structural scaffold for developing novel cancer therapeutics.
2. Pharmacological Activity
In the realm of oncology, the pharmacological activity of CsA is primarily linked to its ability to inhibit cyclophilins that drive tumorigenesis. Cyclophilin A (CypA), the most abundant isoform, is overexpressed in numerous cancers, including hepatocellular carcinoma, pancreatic, lung, esophageal, endometrial, breast, gastric, and melanoma cancers [1]. CypA participates in tumor proliferation, invasion, metastasis, and acquired chemoresistance (e.g., paclitaxel and cisplatin resistance) [1].
Other cyclophilin isoforms also exhibit pro-oncogenic activities that can be targeted by CsA. Cyclophilin 40 (Cyp40) is elevated in prostate cancer tissues, where it acts as a positive regulator of the androgen receptor (AR), driving androgen-dependent prostate cancer cell growth [1]. Cyclophilin B (CypB) overexpression is observed in breast, liver, colon, pancreatic, and stomach cancers, promoting malignant progression and protecting cancer cells against oxidative stress and apoptosis [1]. Cyclophilin J (CypJ) is upregulated in liver and stomach cancers, where it promotes cell cycle transition from the G1 to S phase, facilitating tumor growth [1].
Beyond solid tumors, CsA demonstrates pharmacological utility in hematological malignancies. It is employed as an immunosuppressive therapy (IST) in the management of myelodysplastic syndromes (MDS). Clinical studies have shown that CsA, often administered in combination with anti-thymocyte globulin (ATG), can induce hematologic improvements, complete or partial remissions, and red blood cell transfusion independence in lower-risk MDS patients [2].
3. Molecular Mechanism of Action
The molecular mechanism of CsA is dual-faceted, involving both the inhibition of calcineurin and the suppression of cyclophilin PPIase activity.
First, CsA acts as a "molecular glue." It binds to cyclophilins (such as CypA or CypB) to form a binary CsA/Cyp complex. This complex subsequently binds to and inhibits calcineurin, a calcium-dependent serine/threonine protein phosphatase [1] [3]. The inhibition of calcineurin prevents the dephosphorylation and nuclear translocation of the nuclear factor of activated T-cells (NF-AT). Consequently, this blocks the transcription of interleukin-2 (IL-2) and other cytokines, leading to potent immunosuppression [1] [3].
Second, CsA directly inhibits the intrinsic PPIase activity of cyclophilins, which is crucial for their role in cancer. For example, the PPIase activity of CypJ is critical for regulating the cell cycle through the upregulation of cyclin D1; inhibiting this activity represses tumor growth [1]. Additionally, cyclophilins act as chaperones; Cyp40 binds to Heat Shock Protein 90 (Hsp90) via its tetratricopeptide repeat (TPR) domain to regulate steroid receptor activity, including the androgen receptor in prostate cancer [1]. By binding to these cyclophilins, CsA disrupts these vital oncogenic signaling and chaperone pathways.
4. Structure-Activity Relationship (SAR)
CsA is a macrocyclic peptide that binds to the highly conserved cyclophilin-like domain (CLD) present across the cyclophilin family [1]. The active site of cyclophilins contains a catalytic S1' pocket (which binds proline residues in substrates) and an S2 pocket (which determines substrate specificity via "gatekeeper" residues) [1]. CsA interacts with these pockets; for instance, the (4R)-4[(E)-2-butenyl]-4,N-dimethyl-L-threonine (Bmt) residue of CsA targets the S1 pocket [1].
SAR studies have revealed that the immunosuppressive effects of CsA are independent of its PPIase inhibitory effects. Modifications to the CsA macrocycle have yielded non-immunosuppressive derivatives—such as NIM-811, SCY-635, alisporivir (Debio025), and rencofilstat (CRV-431)—which retain the ability to bind cyclophilins and inhibit PPIase activity without forming the ternary complex with calcineurin [1]. Furthermore, structural differences in the S2 gatekeeper residues among cyclophilin isoforms (e.g., Lys82 in CypA versus Thr116 in CypC) dictate the binding affinity and tolerance for specific structural modifications on the CsA scaffold [1].
5. Current Limitations
The clinical translation of CsA as an anti-cancer agent faces several significant hurdles:
1. Immunosuppression: The primary limitation of native CsA in oncology is its potent immunosuppressive effect via the calcineurin-NFAT pathway. Suppressing the host's immune system is generally counterproductive in cancer therapy, as it impairs anti-tumor immunity and increases the risk of opportunistic infections [1] [3].
2. Lack of Selectivity: CsA is a pan-cyclophilin inhibitor. Because the CLD active site is highly conserved across the 17 human cyclophilin isoforms, CsA lacks selectivity. This indiscriminate inhibition can lead to off-target effects and disrupt normal physiological processes mediated by other cyclophilins [1].
3. Poor Pharmacokinetic Properties: As a macrocyclic peptide, CsA possesses troublesome drug-like properties. It suffers from low oral bioavailability, poor aqueous solubility, and is associated with severe adverse events, including nephrotoxicity, hepatotoxicity, and significant drug-drug interactions [1].
6. Future Perspectives
To overcome the limitations of CsA, future research in cyclophilin-targeted cancer therapeutics is moving in two main directions. First is the continued clinical development of non-immunosuppressive CsA derivatives (e.g., alisporivir, rencofilstat). These compounds decouple PPIase inhibition from calcineurin inhibition, allowing for the targeting of tumor-promoting cyclophilins without compromising the patient's immune system [1].
Second is the rational design of highly selective, non-peptidic small-molecule inhibitors. Structural biology has identified distinct diversity regions within cyclophilins, such as the S2 gatekeeper residues and the less conserved "three o'clock" pocket [1]. Exploiting these subtle structural differences allows for the creation of isoform-specific inhibitors. For example, compounds utilizing hydrogen bonding with specific S2 gatekeeper residues have shown up to 100-fold selectivity for specific isoforms [1]. Developing such selective inhibitors will be crucial for targeting specific cancer-driving cyclophilins (like CypA, Cyp40, or CypJ) while minimizing systemic toxicity and off-target effects.
