Tacrolimus (FK506) and cyclosporine (cyclosporin A, CsA) are cornerstone immunosuppressive agents given to solid organ transplant recipients to prevent and treat allograft rejection. The discovery of cyclosporine in the 1970s, and its entry into the collection of immunosuppressants in the early 1980s, was a major breakthrough in medicine. Cyclosporine was the most successful anti-rejection drug to date, and it radically improved the chance of survival for transplant recipients. In 1994, the Food and Drug Administration (FDA) approved tacrolimus, an effective alternative to cyclosporine [Article:15041303]. Since then, tacrolimus and cyclosporine have become the principal immunosuppressive drugs for solid organ transplantation. The United States Organ Procurement and Transplantation Network and the Scientific Registry of Transplant Recipients show that in 2011, 86% of the 16,055 patients who received a kidney transplant were prescribed tacrolimus upon discharge, and 2.4% were prescribed cyclosporine. One year after transplant, 84% and 4% of patients were receiving tacrolimus and cyclosporine therapy, respectively. Global differences exist in the usage of tacrolimus and cyclosporine: 2008 figures from the Australia & New Zealand Dialysis and Transplant Registry show that 61% of the 391 Australian patients who received a deceased kidney donor graft were prescribed tacrolimus, and 35% were prescribed cyclosporine. At one-year post transplant, these numbers changed to 55% and 33% for tacrolimus and cyclosporine, respectively. Tacrolimus and cyclosporine are also prescribed for liver, intestinal, lung and heart transplant recipients, and can be used to manage severe autoimmune conditions, such as atopic dermatitis [Articles:15175770, 14522634] and rheumatoid arthritis [Articles:15187241, 8448639].
Tacrolimus and cyclosporine differ in their chemical structure: cyclosporine is a cyclic endecapeptide [Article:8513650], while tacrolimus is a macrocyclic lactone [Article:8588225]. However, they act in a similar manner. Both are calcineurin inhibitors; their main mechanism of action involves inhibition of this important phosphatase [Article:15041303]. Tacrolimus exhibits similar effects to cyclosporine, but at concentrations 100 times lower [Article:2445722]. Despite these differences in potency, tacrolimus and cyclosporine both show excellent survival rates for grafts across many comparative studies (summarized in Maes et al. [Article:15041305]). However, several studies have shown that use of tacrolimus is associated with a lower allograft rejection rate compared to cyclosporine [Articles:16686762, 15741208, 16157605].
Within lymphocytes, tacrolimus and cyclosporine exert immunosuppression by several pathways, including inhibiting the calcineurin and the c-Jun N-terminal kinase (JNK) and p38 pathways, and inducing the increased expression of transforming growth factor-ß1 (TGF-ß1). The majority of studies on the pharmacodynamic effects of tacrolimus and cyclosporine have focused on their role in T cells. The involvement of natural killer (NK) cells in transplant rejection is not very well defined, however, both drugs have been found to inhibit natural killer cell degranulation in a dose-dependent manner [Article:20967261].
Action on calcineurin and NFAT
Upon entering T cells, both cyclosporine and tacrolimus bind with high affinity to proteins known as immunophilins. Cyclosporine binds mainly to cyclophilin A (encoded by the PPIA gene), the predominant cyclophilin found within T cells, and tacrolimus binds to FK-binding proteins, in particular FKBP12 (encoded by the FKBP1A gene). Both immunophilins interact with calcineurin in the absence of any ligands. However, the affinity of the immunophilin for calcineurin is enhanced upon binding of the drugs, resulting in the inhibition of the protein's activity [Article:7529175]. Calcineurin is a calmodulin-dependent phosphatase, which is stimulated during T cell activation by a chain of events involving calcium and calmodulin [Articles:11015619, 1377362]. Once activated, it associates with and dephosphorylates members of the nuclear factor of activated T cells (NFAT) family, thereby activating these proteins [Articles:10878286, 7479966]. Upon activation, NFAT proteins translocate to the nucleus [Article:7479966], where it associates with other transcription factors, such as members of the activator protein-1 (AP-1) family, and binds to DNA to promote the transcription of interleukin-2 (IL-2) [Article:10970869]. It also binds to promoter sites on a large variety of other cytokine genes, including those for interleukin-4 (IL-4), interleukin-10 (IL-10) and interleukin-17 (IL-17) [Article:20103781]. Inhibition of calcineurin, therefore, prevents its ability to dephosphorylate and activate NFAT, affecting the transcription of cytokines important in the immune response. The impact of the drugs on the transcription of IL-2 is probably the best addressed mechanism, and this particular cytokine plays a large role in the immune response, including the maintenance of regulatory T cells and the differentiation and survival of CD4+ and CD8+ T cells [Article:22343569].
In addition to NFAT and AP-1 family members, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-¿B) is also involved in the induction of IL-2 transcription [Articles:2497518, 7546397, 20103781]. NF-¿B is the name given to a group of dimeric transcription factors that bind as homo or heterodimers, and exert both positive and negative effects on gene transcription [Article:22302935]. In general, NF-¿B has a large impact on the development, homeostasis, survival, and function of T cells [Article:21199863]. It has a huge variety of target genes within T cells, and in addition to IL-2 is also involved in the regulation of cytokines such as tumor necrosis factor-beta (TNFß) [Article:2104232] and interferon-gamma (IFN¿) [Article:9374532]. Calcineurin is also involved in the activation of NF-¿B. It indirectly induces the degradation of a compound known as I¿B, which is bound to inactive NF-¿B and acts as an inhibitory protein, preventing NF-¿B from associating with its nuclear target genes. The blockade of calcineurin activity by both drugs thereby affects the ability of NF-¿B to exert its action on the genes of the immune system [Articles:8112299, 21199863].
Action on the JNK and p38 pathways
Though the effect of tacrolimus and cyclosporine on calcineurin is the probably the best-studied mechanism, both drugs are also thought to be involved in the inhibition of the mitogen-activated protein kinase (MAPK) pathway. The MAPK pathway is a signaling cascade involved in a wide variety of processes, particularly within immune system cells [Article:17473844]. It consists of three protein kinases: MAPK, MAPKK and MAPKK-K. MAPKK-K phosphorylates and activates MAPKK, which in turn phosphorylates and activates MAPK [Article:11274345]. There are three distinct MAPK subgroups - ERK, JNK and p38 [Article:17473844]. Both cyclosporine and tacrolimus (in complex with their immunophilins) have been shown to inhibit the JNK (MAPK8) and p38 (MAPK14) pathways, but not the ERK pathway. A study in Jurkat T lymphocytes showed reduced levels of both the JNK and p38 proteins under the administration of cyclosporine or tacrolimus [Article:11258483]. JNK and p38 are activated through the MAPK signaling cascade by T cell and CD28 co-stimulatory receptors [Article:9575191], and upon activation, translocate to the nucleus where they can fulfill their various roles [Article:12077368]. This includes the regulating the activity of AP-1 members [Article:7622446], which are involved in promoting the transcription of IL-2 [Article:10970869] and other cytokines [Article:9468273]. Indeed, the blockade of the p38 and JNK pathways was shown to prevent the expression of the IL-2 gene [Article:9575191]. The pathway of JNK and p38 activation through various kinases can be seen in the figure, and the two drugs are thought to inhibit the pathways upstream of the MAPKK-K level, as cyclosporine and tacrolimus have both been seen to inhibit the activation of an MAPKK-K known as MAP3K1 [Article:11258483]. It is not believed that calcineurin is involved in this mechanism, since inhibitors of calcineurin have been seen to block the activation of NFAT, but not JNK or p38 pathways within T cells [Article:11258483].
Action on TGF-ß1
TGF-ß1 is a cytokine critical for the regulation of immune system cells. It is a member of the TGF-ß family, which also includes TGF-ß2 and TGF-ß3. TGF-ß has been shown to inhibit IL-2-dependent T cell proliferation [Article:8423782], as well as exerting a variety of other immunosuppressive effects within T cells [Article:11905837]. In vivo studies in patients with end-stage renal disease undergoing transplantation have shown an increase in TGF-beta1 mRNA and protein expression after treatment with cyclosporine [Article:10071036], and in vitro studies of tacrolimus in T cells also showed a significant increase in mRNA and protein levels after administration of the drug [Article:9484745]. However, the mechanism by which these drugs affect levels of TGF-ß1 remains to be elucidated. It is also important to note that some studies have found evidence showing that neither tacrolimus nor cyclosporine are capable of affecting TGF-ß1 protein or mRNA levels at concentrations in which IL-2 production is successfully inhibited [Articles:12588317, 15716327]. Therefore, at this stage, it is not possible to state definitively whether the two drugs affect TGF-ß1 levels. However, it is important to note that along with being involved in the immune system, TGF-ß1 also has fibrogenic properties that can lead to the development of nephrotoxicity [Article:19218475]. A study in renal transplant patients found that expression of TGF-ß1 mRNA within kidney biopsies was increased in patients with either tacrolimus or cyclosporine nephrotoxicity, compared to those who exhibited acute rejection. This suggests that increased levels of the protein may lead to the nephrotoxicity often associated with these drugs [Article:12427154].
The majority of pharmacogenetic studies on tacrolimus and cyclosporine have focused on the effects of variants in the CYP3A4, CYP3A5 and ABCB1 genes because of the central role the enzymes and transporters they code for play in tacrolimus and cyclosporine disposition. However, a few studies have examined the influence of single nucleotide polymorphisms (SNPs) within the gene encoding the pregnane X receptor (NR1I2), which regulates the expression of multiple genes including CYP3A and ABCB1 [Article:23095803]. Additionally, a couple of studies have examined SNPs in the POR gene, which encodes for CYP450 oxidoreductase, a protein responsible for transferring electrons from NADPH to CYP450 enzymes, enabling their activity [Article:11371558]. Several studies have also looked at variations in the TGF-ß1 gene (TGFB1), the cyclophilin A gene (PPIA), and the CYP2C8 gene; CYP2C8 is involved in the metabolism of arachidonic acids (AAs) into epoxyeicosatrienoic acids (EETs), metabolites implicated in maintaining normal renal function. Despite these numerous studies, only the *3 allele (rs776746) in the CYP3A5 gene has shown strong associations with tacrolimus pharmacokinetics. Very little consistent evidence has emerged for factors affecting tacrolimus pharmacodynamics or cyclosporine pharmacokinetics and pharmacodynamics. The overall inconsistency of these studies may be related to ethnic variability, small numbers of patients, non-specific pharmacokinetic assays, variation in when outcomes are measured, and the impact of donor genotype - particularly in nephrotoxicity studies in kidney transplant patients or pharmacokinetic studies in liver transplant patients. Larger studies and meta-analyses that take ethnicity and donor genotype into account may help resolve some of this variability.
Barbarino Julia M, Staatz Christine E, Venkataramanan Raman, Klein Teri E, Altman Russ B. "PharmGKB summary: cyclosporine and tacrolimus pathways" Pharmacogenetics and genomics (2013).
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