This diagram outlines the key mechanisms by which calcineurin (Cn) inhibitors and mammalian target of rapamycin (mTOR) inhibitors inhibit T-cell activation triggered by presentation of antigen via the T-cell receptor (TCR). The first step in antigen presentation involves presentation of a peptide that is bound within the peptide-binding groove of the major histocompatibility complex (MHC) class II molecule. This complex is then presented to the TCR. This causes transmembrane signaling that increases intracellular calcium concentrations. The liberated calcium, bound to calmodulin (calm), interacts with Cn, a calcium-dependent serine/threonine phosphatase that dephosphorylates nuclear factor of activated T cells (shown here as NFATc), and this causes NFAT to translocate to the nucleus. There it binds to other nuclear components of NFAT (shown here as NFATn). This complex regulates the transcription of many cytokine genes, shown in the diagram. Cyclosporine (CsA), tacrolimus (TCL), and pimecrolimus (PCL) diffuse freely into the cytoplasm of T cells and bind with their respective immunophilins. This drug/immunophilin complex binds to Cn and blocks its ability to dephosphorylate NFAT, and thereby inhibits the production of cytokines, chemokines, and growth factors [interleukin 2 (IL-2), IL-3, IL-4, tumor necrosis factor-α, interferon-γ, granulocyte-macrophage colony-stimulating factor, etc.] that would normally be induced after T-cell activation via the TCR. Sirolimus (SRL) has a more complex intracellular mechanism of action. Although SRL and everolimus (EVL) bind macrophilin-12 (also known as FKBP-12), the immunophilin of TCL and PCL, its mechanism of action does not appear to be mediated by Cn inhibition. mTOR inhibitors block cell kinases that would have a direct effect on the cell cycle by also an indirect effect in chemokine, cytokine, and growth factor synthesis. APC = antigen-presenting cell.
CnIs inhibit T-cell stimulation initiated by several pathways, of which the T-cell receptor (TCR) CD3 is the best understood.3 Mechanisms involving B-7 and CD28/cytotoxic T lymphocyte antigen-4, and other costimulatory pathways seem not to be affected by CsA or other CnIs. The high-affinity receptor for CsA is a 17-kd immunophilin called cyclophilin A (CyPA). The other CnIs (TCL and pimecrolimus) bind a structurally unrelated, different immunophilin called macrophilin-12 also known as FK-506 binding protein-12 (FKBP-12). The drug-immunophilin complex binds to calcineurin (Cn), a serine/threonine protease composed of two subunits: (1) CnA and (2) CnB. CnA constitutes the catalytic unit possessing binding sites for CnB and calmodulin. Cn activity strictly correlates with interleukin-2 (IL-2) production via CD3 activation. Cn is also involved in the induction of apoptosis and degranulation of cytotoxic T lymphocytes. Experimental data show that the CsA, TCL, and pimecrolimus/Cn complex inhibits the nuclear translocation of the nuclear factor of activated T cells c molecule by blocking its dephosphorylation. This is thought to be the key step by which these drugs uncouple TCR activation from IL-2 transcription. TCL is 10- to 100-fold more potent than CsA in vitro in the inhibition of Cn activity.
In summary, CsA inhibits the T-cell activation mediated by antigen, but it does not inhibit the early phases of lymphocyte signal transduction occurring after antigen-mediated activation. The immunomodulatory effects of these drugs are intricate and subject to constant discoveries. A simplified explanation of the mechanism of action of these drugs is presented in the following sections and is illustrated in Fig. 233-2.
CsA is a hydrophobic lipophilic undecapeptide extracted from fungi.
The microemulsion-based formulation of CsA has an improved bioavailability giving the drug more predictable absorption.3,4
CsA can also be administered intravenously as a 50-mg/mL solution made up in an ethanol-polyoxyethylated castor oil mixture. The accumulation of CsA in erythrocytes and leukocytes is the reason that whole-blood monitoring of CsA levels is much more accurate than measurement of plasma levels. Peak levels occur from 1.3–4.0 hours after oral administration. After gaining access to the circulation, CsA distributes widely and has a large apparent volume of distribution (13 L/kg). CsA is metabolized into greater than 30 cyclic, partly active metabolites by the liver cytochrome P450 3A enzyme (CYP 3A).5,6 Thus, drugs that compete for binding to CYP 3A will increase CsA levels, and drugs that induce P450 will accelerate metabolism and decrease blood levels (eTable 233-1.1). Certain foods rich in bioflavonoids, especially grapefruit, seem to increase the drug's bioavailability through an interaction with cytochromes. Its elimination half-life is 6–12 hours in the absence of severe hepatic disease, and biliary excretion accounts for more than 90% of its elimination. CsA solution does have some penetration of mucosal surfaces, but this is not true of cornified epithelium.
eTable 233-1.1 Medicationsa that Commonly Alter the Metabolism of Calcineurin and Mammalian Target of Rapamycin Inhibitors through the Cytochrome P450 3A Pathway ||Download (.pdf)
- Psoriasis: Cyclosporin is approved for use in patients with moderate-to-severe psoriasis, but the approved dosage is 2.5–3.0 mg/kg/d which in author's experience is too low a dosage to achieve adequate control of most patients. The dosage that appears adequate is between 4 and 5 mg/kg/d, but, at this level, the risk of nephrotoxicity occurs with continued use. There are some patients that are able to achieve response, whose response is held at lower doses that can be used for longer periods. The use of cyclosporin in combination with methotrexate seems synergistic, but combination with biologic therapies has not been well studied and should be avoided other than for short periods of time. Although this drug is approved, its use has lessened except for short-term usage.
- Pyoderma gangrenosum: It was one of the first diseases in dermatology in which cyclosporin was noted to be beneficial. Even now in the age of biologic therapy, some patients are effectively managed with this and other macrolactams.
- Paraneoplastic pemphigus: Patients have been treated with cyclosporin, but it should be used cautiously because of its potential to allow tumors to grow. With the advent of rituximab usage for lymphoma and pemphigus, cyclosporin would not in my opinion be a first-line therapy.
- Lichen planus: Multiple case reports and small case series have demonstrated that cyclosporin is effective in patients with lichen planus, particularly those with oral or esophageal erosive disease. This is not a first-line therapy in this condition.
- Atopic dermatitis: Severe atopic dermatitis can be one of the most challenging dermatologic conditions to manage. While cyclosporin might be useful, as will all chronic diseases, its indefinite use is associated with increasing frequency of toxicity. Therefore, most of the patients treated should only have relatively short-term use.
- Behçet disease: Ocular and mucocutaneous forms; moderate to severe cases.
- Dermatomyositis: Patients with inflammatory myopathies have responded to cyclosporin. However, in author's experience, it is the rare patient whose skin disease will be controlled with cyclosporin.
- Alopecia areata: Since this drug or others in its class have not been demonstrated to alter the long-term course of the disease, it is a rare patient who can be treated.
- Epidermolysis bullosa acquisita: This is often a difficult disorder to control. There are no therapies that have been proven to be effective in well designated studies. Cyclosporin has been used in individual patients and has been reported to be effective and potentially corticosteroid-sparing in some patients.
Table 233-2 Dosing of Macrolactam Immunosuppressive Agents ||Download (.pdf)
Table 233-2 Dosing of Macrolactam Immunosuppressive Agents
Children's Dosage mg/kg/day
3–10 mg/day for Patients of African or Asian descent
0.5–3 mg/d for Caucasian patients
0.75–1.5 mg twice daily
Doses higher than 5 mg/kg/day are not advisable other than for short periods of time. Some patients with acute uncontrolled disease might be treated with ‘loading’ doses in hopes of achieving more rapid control of their disease. The US Food and Drug Administration has approved CsA for psoriasis in doses up to 4 mg/kg/day, with a recommended starting dose of 2.5 mg/kg/day. Dosage increases should be performed after 4 weeks of therapy, and dose reductions are permitted at any time. Dosage increases should not exceed 0.5–1.0 mg/kg/day at 2- to 4-week intervals. Intravenous CsA formulation can be infused slowly over a period of 2–6 hours at about one-third of the usual oral dose, or about 2–3 mg/kg per day.7
When psoriatic patients improve, defined clinically, the CsA dose should be down-titrated. It is not clear with what rapidity this de-escalation of therapy should occur, but some have suggested that at least a 4-week interval should occur prior to further dose reduction. As therapy is decreased, there is a risk of relapse. Rebound is also possible, but is a relatively uncommon phenomenon.
Patients who seemingly do not respond to CsA should be assessed for compliance and absorption by measuring a trough level. The two most common assays for trough levels—high-performance liquid chromatography (HPLC) and radioimmunoassay—are performed in EDTA-containing whole blood. The recommended range is 200–400 ng/mL. Routine use of CsA levels in dermatology patients is not necessary.
Children have comparable bioavailability of orally ingested CsA, but have a higher renal drug clearance rate (11.8 mL/min per kg vs. 5.7 mL/min per kg in adults), and a correspondingly shorter blood level half-life (7.3 hours vs. 10.7 hours in adults). Children may therefore require somewhat higher dosages and more frequent administration to achieve comparable trough levels to adults.
Serum creatinine levels should be carefully monitored during CsA therapy. If creatinine levels increase more than 30% above baseline, the dosage should be reduced for 1–2 weeks. If, after that time, the creatinine levels decrease below the 30% elevation mark, continuation at the lower dose is advisable. In cases where the creatinine remains elevated by more than 30%, discontinuation of CsA is recommended until the creatinine returns to levels within 10% of pretreatment levels. More accurate studies monitoring the glomerular filtration rate clearance are used in individualized cases. From a practical standpoint, adequate monitoring of CsA dosage can be achieved by the avoidance of CsA doses higher than 5 mg/kg/day, evaluation of clinical response, and vigilance for signs of toxicity. This requires a detailed physical examination, including blood-pressure monitoring with threshold concern triggered by a persistent diastolic blood pressure above 90 mm Hg, a complete history with emphasis on concomitant drug ingestion and medical conditions that may potentiate CsA toxicity, and laboratory evaluations for complete blood count, creatinine and blood urea nitrogen, uric acid, liver enzymes, serum electrolytes and magnesium, and a urinalysis. In patients receiving long-term CsA therapy (>6 months), the serum creatinine and its clearance may not be a reliable predictor of altered renal function, and potentially irreversible chronic cyclosporine nephrotoxicity may ensue. In such circumstances, more reliable studies for the evaluation of renal function may be indicated.
Renal Function, Liver, and Neurologic
Overall, one in four patients taking CsA develops clinical and laboratory evidence of altered renal function, including hypertension. Two types of CsA-induced nephrotoxicity are encountered in dermatologic patients. The first type usually starts within 2–3 weeks after drug initiation, and it is usually associated with high CsA blood levels. In this toxicity, there is an insidious decrease in glomerular filtration rate along with hypertension, and tubular dysfunction in association with a complete recovery of renal function upon dose-lowering or discontinuing therapy. The second type is likely a result of cumulative subclinical chronic renal toxicity. This may occur in the absence of any detectable elevation of creatinine or blood pressure. Histologic changes include interstitial fibrosis, tubular atrophy, and some degree of vasculopathy. Although renal function can improve somewhat after discontinuation of the drug, this type of toxicity is generally irreversible. In psoriatic patients receiving 5 mg/kg/day CsA, elevation of serum creatinine may persist for more than 4 months after discontinuation of the drug. Several mediators, including endothelin-1, angiotensin II, osteopontin, and transforming growth factor-β1, have been implicated in the pathogenesis of CsA-associated nephrotoxicity and hypertension. Calcium channel antagonists exert beneficial effects on CsA-induced hypertension and nephrotoxicity, presumably through the inhibition of endothelin-1. Other drugs, including angiotensin converting inhibitors (enalapril, lisinopril, etc.) and angiotensin receptor blockers (losartan, valsartan, etc.), are also effective mainly when used in combination with other antihypertensive drugs. New drugs, such as endothelin A receptor blockers (bosentan, darusentan, etc.) and renin inhibitors (aliskiren), appear to be promising for CsA-treated patients.8–10 Diuretics and nephrotoxic drugs should be used with caution in CsA-treated patients. Another uncommon adverse renal complication is the development of thrombotic microangiopathy/hemolytic uremic syndrome,11,12 especially in allogenic bone marrow transplant patients receiving CsA for acute graft-versus-host disease. Hypomagnesemia and hyperkalemia or hypokalemia are not uncommonly encountered.
Nausea, vomiting, anorexia, and diarrhea commonly occur with the use of CsA. Elevations of liver enzymes greater than 100% over baseline should be managed by reduction of the CsA dose by 25% weekly, until enzyme levels normalize.
Headache is a common complaint, especially in patients with a history of migraines. It tends to resolve spontaneously as therapy continues. CsA-induced seizures appear to be associated with hypomagnesemia and concomitant use of high doses of systemic corticosteroids. Thus, serum magnesium levels should be always monitored and kept above the lower normal range in CsA-treated patients. Cortical blindness, lethargy, confusion, seizures, hemiplegia, tremors, and paresthesias have also been described in patients receiving CsA.8
There is evidence of impaired fibrinolysis and endothelial damage and proliferation associated with CsA. Hypercoagulability seems to contribute to the progression of atherosclerosis and glomerular damage in CsA-treated patients. Significant cytopenias have rarely been reported with the use of CsA.
Hypercholesterolemia, elevation of low-density lipoproteins, and hypertriglyceridemia may be seen with CsA.
CsA-treated transplant recipients have a relative risk for all skin cancers of 6.8 versus 2.2–5.5 in patients receiving other immunosuppressive therapies. CsA-treated dermatologic patients also have a higher risk of skin cancers, including squamous cell carcinomas, basal cell carcinomas, human papilloma virus-associated anogenital carcinoma, and Kaposi sarcoma. The high incidence of squamous cell carcinoma in psoriatic patients treated with CsA could be biased by previous exposures to psoralen and ultraviolet A light or ultraviolet B. Approximately 25% of nonvisceral Kaposi sarcomas can be expected to undergo complete or partial remission following cessation or reduction of immunosuppressive therapy. Epstein-Barr virus-associated post-transplant lymphoproliferative disorder (PTLD) is quite rare in dermatologic patients. PTLD frequently usually fails to respond to chemotherapy, but it may regress spontaneously after reduction and/or cessation of immunosuppression. The incidence of lymphoma in CsA-treated dermatologic patients appears to be less than 0.2%. The incidence of lymphoma in transplant patients receiving CsA alone or in combination with corticosteroids is less than 1%, whereas for those receiving CsA in conjunction with other immunosuppressive drugs is as high as 8%. In contrast to the high mortality rate attributed to lymphomas arising in immunosuppressed patients, lymphomas developing in CsA-treated patients seem to carry a better prognosis despite of having a shorter latency period seems.13–15 In rheumatoid arthritis patients treated with CsA, there was no increase in malignancies in comparison with a matched control group of patients with rheumatoid arthritis who were not treated with CsA. Other neoplasms, including melanoma, are also reported in CsA-treated patients, although their true incidence is unknown.
Hypertrichosis occurs in virtually all patients on long-term CsA therapy. It is not limited to androgen-dependent, hair-bearing areas, and shows no tendency to spontaneous remission. Gingival hypertrophy is reported in up to 70% of CsA-treated patients; it is more common in children, individuals with poor oral hygiene and concomitant use of calcium channel drugs. Improvement of this gingival complication with topical or systemic azithromycin and metronidazole can be seen. An acneiform eruption, indistinguishable from that seen in steroid-induced acne, is frequently reported. A disseminated comedonal or cystic acneiform eruption can also occur. These side effects can appear at any time during CsA therapy, although they are more commonly described at the time of initiation of CsA therapy. Keratosis pilaris, sebaceous hyperplasia, warts, and epidermal inclusion cysts occur in up to one-third of CsA-treated patients.
Osteoporosis seems to result from the CsA's action on osteoblasts and osteoclasts, and by altering lymphokine release. CsA can induce hyperuricemia in up to 15% of patients, and it is also an indicator of early CsA-induced nephrotoxicity. Myopathy has been reported in transplant patients receiving high doses of CsA, and thus, cation should be used with the concomitant use of statins.
CsA is categorized as a pregnancy category C and as such should only be used when the benefits outweigh the risks.8 It does not seem to be mutagenic or teratogenic, although there is a higher than expected incidence of preterm newborns, fetal growth retardation, abortions, preeclampsia, and hypertension in mothers taking CsA, and these occurrences are magnified in transplant patients. There are no reports of neonatal complications in children born to fathers receiving CsA. Adequate contraceptive measures are recommended in women of childbearing potential. CsA crosses the placenta and is excreted in breast milk. CsA-treated transplant patients seem to have a relative risk for infectious, life-threatening complications that is much lower than that seen in patients receiving azathioprine and prednisolone. However, increased vigilance for infectious complications is recommended in CsA-treated patients.
TCL, a CnI formerly known as FK506, is a macrolactam drug first isolated from Streptomyces tsukubaensis.
The postulated mechanisms of action of TCL are similar, if not identical, to the ones described for CsA (see Section “Macrolactam Drugs”).16
TCL can be administered orally, topically, and intravenously. The pharmacokinetics of systemic TCL are characterized by a two-compartment model with a rapid initial drop and a long elimination half-life of 12–21 hours. TCL is metabolized in the liver with less than 1% of the drug excreted intact. TCL has not proved to be more effective than CsA, but it appears to have a better bioavailability. The topical formulations have a concentration of 0.3% and 0.1% in an ointment vehicle, and for more detailed information, see Chapter 221.
The dosage of systemic TCL should be reduced in the presence of hepatic dysfunction by 10%–30% of the standard dosing.
There is a good correlation between enzyme-linked immunosorbent assay and HPLC mass spectrometry measurements in monitoring of whole blood concentrations of TCL. Enzyme-linked immunosorbent assays may also detect metabolites of the drug, and thus, caution should be taken in patients with liver diseases or with concomitant use of CyP4503A-binding drugs. Blood levels of 5–15 ng/mL are recommended.
The risks and precautions are quite similar to the ones applied for CsA. However, neurotoxicities and glucose intolerance are somewhat higher than with CsA use, and thus, close monitoring of magnesium and glucose serum levels and, if indicated based on personal or family history, glucose tolerance tests are strongly recommended. The risk for PTLD seems to be higher in children with hepatic transplants who received both TCL and anti-OKT3, but it has not been definitively proved in dermatologic patients or in different clinical scenarios. Dose reduction and close monitoring of TCL levels by HPLC mass spectrometry in patients with liver diseases are strongly recommended. A lesser graft-versus-leukemia effect in comparison with the one seen with CsA is observed in bone marrow transplant patients. A great deal of concern has been raised regarding the potential risk of lymphoproliferative diseases with this drug, which led to a highly controversial placement of a black-box warning by the US Food and Drug Administration with the topical formulation (see Chapter 221).
Mucocutaneous and metabolic side effects seem to be less frequent with the use of TCL. However, nephrotoxicity seems to be comparable to that seen with the use of CsA. The rest of the side effects are comparable with those seen with CsA use.
There are no approved indications for tacrolimus in dermatology. Its mechanism of action suggests that any patient who might be responsive to CsA would likely respond to tacrolimus. Therefore it might be useful in patients with psoriasis, pyoderma gangrenosum, dermatomyositis, lupus erythematosus, atopic dermatitis, and graft-versus-host disease, among other potential indications.17–21
Systemic TCL can be used virtually in any inflammatory and autoimmune skin disease in which systemic CsA has proved effective. For topical use of TCL, see Chapter 221.
Pimecrolimus, also known as ASM 981, is a derivative of the parent compound ascomycin originally isolated from the fermentation products of S. hygroscopicus var. ascomyceticus. Pimecrolimus has a similar mechanism of action as that of other CnIs. Pimecrolimus is formulated in vehicle cream at concentrations of 0.2%, 0.6%, and 1.0% (see Chapter 221).
Mammalian Target of Rapamycin Inhibitors
SRL and its derivatives EVL and temsirolimus are most widely known as mTORIs. Although mTORIs bind to FKBP-12 (macrophilin-12), this drug-immunophilin complex inhibits gene transcription through a complex mechanism that is independent from the one involving Cn.
African and Asian descent patients require a higher initial and maintenance dose of SRL than Caucasians. EVL bears a stable 2-hydroxyethyl chain substitution at position 40 on the SRL structure, which has given a greater polarity than SRL, improving the pharmacokinetic and particularly its oral bioavailability. Oral EVL is absorbed rapidly and reaches peak concentration after 1.5 hours. In adults, EVL pharmacokinetic characteristics do not differ according to age, weight, or sex, but body weight-adjusted dosages are necessary in children.25–26 The target trough concentration of EVL should range between 3 and 15 ng/dL. SRL topical formulation (2.2% and 8.0%) has been used in psoriasis patients (see Chapter 221).
There are no approved indications for everolimus in dermatology. Its mechanism of action suggests that any patient who might be responsive to CsA or tacrolimus would likely respond to everolimus. Therefore, it might be useful in patients with psoriasis, pyoderma gangrenosum, dermatomyositis, lupus erythematosus, atopic dermatitis, and graft-versus-host disease, among other potential indications.
As mentioned in Table 233-2, due to ethnic differences, the SRL recommended oral loading dose is 6 mg and oral maintenance dose is 2 mg per day for Caucasian patients, and the oral loading dose is 10 mg and oral maintenance dose is 5 mg per day for African Americans. Asians also appear to require higher doses of this drug. EVL starting dose are the same regardless of sex and race. The usual dosing for adults is 0.75–1.5 mg bid and for children 0.8–1.2 mg/m2.
Due to significant intra- and interindividual variability, measuring trough levels is critical with the use of mTORIs. Trough levels for SRL and EVL are 5–20 ng/dL and 3–15 ng/mL, respectively. Complete blood cell count, electrolytes, liver function test, and a lipid profile, along with a comprehensive clinical examination with periodic blood pressure measurement, are mandatory.
Concomitant use of antimetabolite drugs, including mycophenolate mofetil (MMF) and azathioprine in patients with mTORIs of upper normal or higher trough blood levels (e.g., SRL: equal or higher than 20 ng/dL or EVL: equal or higher than 5 ng/mL), raises the risk of cytopenias, namely thrombocytopenia. Concomitant use of CnIs, namely TCL, may increase the risk of thrombotic microangiopathy/hemolytic uremic syndrome, and thus, monitoring of blood cell count, dyslipidemia, electrolyte abnormalities, edema, and increase of liver function tests are also associated with high trough levels of these drugs. Patients with extensive psoriasis treated with SRL should be monitored closely for capillary leak syndrome, a rare complication reported only in one series.27
Cytopenias, namely thrombocytopenia, hypertriglyceridemia, hypercholesterolemia, arthralgias, edema, and impaired wound healing, have been frequently associated with the use of mTORIs. Occurrence of cytomegalovirus appears to be lower than that observed with the use of CnIs. No gingival hyperplasia has been observed with SRL. Tremor or any other neurologic complications are not frequent. Hypertension and malignancies are significantly reduced in comparison with the use of CnIs. This is most likely related to the endothelial, mesangial antiproliferative and antineoplastic properties of mTORIs.