UPDATE ON THE NATURAL HISTORY AND SYSTEMIC TREATMENT OF PSORIASIS

Psoriasis is a common inflammatory disease of the skin and joints. Its etiology remains unknown, however, it has been linked to complex interactions between predisposing genes and the environment. The pathophysiology of psoriasis is characterized by epidermal hyperproliferation, enhanced antigen presentation, Th-1 cytokine production, T cell expansion, and angiogenesis. Tremendous advances in our understanding of this disorder has led to the development of novel therapeutics and the FDA approval of more systemic agents for its treatment in the last 5 years than in the previous 50 years combined. Our improved understanding of the pathogenesis of psoriasis has led to epidemiologic studies that have contributed towards further characterizing its natural history. In this review we focus on specific advances in our understanding of the pathogenesis, natural history, and systemic treatment of psoriasis which are of major clinical relevance to the clinician.

KEY ADVANCES IN THE PATHOGENESIS OF PSORIASIS

The biologic basis of psoriasis informs its natural history and treatment options. Here, we briefly review key discoveries in the pathogenesis of psoriasis relevant to the clinician and refer the reader to several comprehensive reviews for a more detailed discussion.^1–4 Psoriasis was initially believed to be a primary disorder of keratinocytes, however advances in molecular biology and immunology proved its etiology to be much more complex. In 1995, Gottleib et al demonstrated that psoriasis could be successfully treated with the lymphocyte-selective toxin DAB3891-2; a discovery which heralded a new era in our approach towards treating psoriasis, one focused on developing therapeutics to inhibit immunologic targets.^5–7 It is now believed that the clinical phenotype of psoriatic skin arises from the interplay between inflammatory cytokines and cells which make up the cutaneous microenvironment (i.e. lymphocytes, antigen presenting cells, endothelial cells, and keratinocytes).

Lymphocytes are believed to play a central role in the pathogenesis of psoriasis, as recent work has demonstrated how various lymphocyte subsets contribute towards this disorder. In particular, Th-1 lymphocytes have been identified as a primary source of inflammatory cytokine production in psoriatic skin; regulatory T cells, which normally suppress effector T cell activity, are dysfunctional in the blood and skin of patients with psoriasis; and recently identified Th17 cells produce the cytokine IL-17, which is critical to the establishment and maintenance of autoimmunity. Antigen presenting cells (i.e., plasmacytoid and myeloid dendritic cells) and endothelial cells lining the dermal microvasculature have also been shown to play a role in psoriatic disease. In particular, dermal dendritic cells have been shown to contribute towards the production of Th1 cytokines and the recruitment of inflammatory cells into psoriatic plaques. The production of IL-20 and IL-23 by myeloid dendritic cells has been reported to promote keratinocyte proliferation, upregulate inflammatory gene products, and stimulate T cell activation, all of which contribute towards psoriatic lesions.^8,9 Endothelial cells play a critical role in recruiting inflammatory cells through their expression of E-selectin, which enhances the homing of cutaneous lymphocyte associated antigen (CLA) positive T cells into the skin. Angiogenesis is stimulated by the inflammatory process and studies demonstrate that circulating levels of VEG-F correlate with psoriasis activity.¹⁰

Whether psoriasis reflects an abnormal response to an unidentified antigen or a reaction to the aberrant production of endogenous/exogenous immune cell activators remains uncertain. However, it is clear that the response of keratinocytes to locally produced cytokines underlies the formation of cutaneous lesions. In addition, keratinocytes have also been shown to produce their own cytokines, such as IL-6 and transforming growth factor-alpha, which may act in concert to promote their own proliferation in an autocrine fashion.¹¹

Underlying its immunopathogenesis is a complex role for genetics in promoting psoriasis disease susceptibility. Over 20 genetic loci containing varying numbers of genes, many of which have no known function, have been associated with psoriasis susceptibility. The strongest association was identified on a locus within the class I major histocompatibility complex (MHC I) on chromosome 6p21 known as PSORS1. This region is believed to account for 35–50% of psoriasis heritability. The PSORS1 locus contains fewer than 10 genes, three of which have been strongly implicated in psoriatic disease: HLA-C, CCHCR1, and CDSN. The HLA-Cw6 allele is present in up to 85% of individuals who develop psoriasis under the age of 40; these patients typically have more severe disease than individuals who develop psoriasis at a later time in life. Only 15% of individuals who develop psoriasis over the age of 40 express the HLA-Cw6 allele. Although much progress has been made towards dissecting the genetic components of this disease, few genes have been definitively implicated in its pathogenesis and genetic testing is not clinically useful. For example, only 10% of individuals who express the HLA-Cw6 allele go on to develop psoriasis.¹²

KEY ADVANCES IN THE NATURAL HISTORY OF PSORIASIS

Recent studies have broadened our knowledge of how genetics and environmental factors may lead to psoriasis and how the pathophysiology of psoriasis and/or its associated psychosocial behaviors and treatments may lead to adverse health outcomes (see Figure 1).

What are the risk factors for developing psoriasis?

Genetics are believed to play a key role in the development of psoriasis. It is estimated that approximately 40% of individuals suffering from psoriasis or psoriatic arthritis have a first degree relative with the disease.¹³ In addition, concordance rates as high as 70% have been reported among identical twins.¹⁴ Given the strong genetic component of psoriasis, patients with psoriasis are often concerned about the heritability of the disease. Family studies indicate that if both parents have psoriasis then the offspring have a 50% chance of developing the disease; if only one parent has psoriasis then the risk of a child developing psoriasis is 16%. If neither parent is affected but a child develops psoriasis then their siblings have an 8% risk of developing the disease. Men have a higher risk of transmitting psoriasis to offspring than women, likely due to genomic imprinting, which is an epigenetic effect that causes differential expression of a gene depending on the gender of the transmitting parent.¹⁵

Since genetics are immutable, modifiable environmental risk factors for psoriasis are of special interest. Data from analytic epidemiologic studies (e.g. case-control and nested cohort studies) with appropriate control for confounding variables have recently identified smoking and obesity as risk factors for the development of psoriasis. A large cohort study of over 78,000 nurses from the United States demonstrated a “dose-response” relationship for obesity and smoking on the risk of developing incident psoriasis.^16,17 Similarly, a cohort study from the General Practice Research Database in the United Kingdom of almost 4000 incident cases of psoriasis confirmed that current smoking and obesity are independent risk factors for developing psoriasis. Finally, in a case-control study of 560 psoriasis patients seen by dermatologists, smoking and obesity were also found to be independent risk factors for the development of psoriasis.¹⁸ The consistency of the findings across different study populations and study designs, as well as the dose-response relationships observed, strongly support the validity of these associations. Both smoking and obesity trigger Th-1 mediated immunological pathways, suggesting a plausible biologic explanation for these associations.¹⁹

Clinical implications of risk factors for psoriasis

The importance of family history in the risk of developing psoriasis necessitates that clinicians be knowledgeable on counseling patients regarding the risk of their offspring developing the disease. Furthermore, the identification of obesity and smoking as consistent and reproducible risk factors for the development of psoriasis may provide an opportunity for prevention of this chronic disease through behavior modification. For example, in the Nurse’s cohort study, it was estimated that 30% of new psoriasis cases were due to being overweight (body-mass-index [BMI] >25).¹⁶ Future studies are necessary to determine if maintenance of ideal body weight and avoidance of smoking will truly lower one’s risk of developing psoriasis. Until such data are available, it is prudent to recommend weight management and smoking avoidance to individuals at greatest risk of developing the disease (e.g. those with a positive family history).

Which major comorbidities are patients with psoriasis at increased risk of developing?

Patients with psoriasis may be at increased risk of developing other diseases due to shared genetic pathways, common immune mechanisms, treatment related toxicities, psoriasis associated behaviors such as smoking and excess alcohol use, and the associated psychosocial burden of the disease (See Figure 1). For example, psoriasis patients have an increased prevalence of Crohn’s disease which may be due to shared genetic loci as the psoriasis susceptibility loci, PSORS8, has been shown to overlap with a Crohn’s disease locus on chromosome 16q.^20,21 Moreover, chronic Th-1 inflammation, central to the pathophysiology of psoriasis, can also lead to seemingly diverse conditions such as insulin resistance, atherosclerosis, and thrombosis.²² Therapies for psoriasis frequently are immunosuppressive, which could lead to a higher risk for infections and cancer. Similarly, psoriasis is associated with smoking, excess alcohol use, mood disorders, and decrements in income, all of which could lead to adverse health outcomes.^23,24 More recent epidemiological research has focused on determining which health outcomes in patients with psoriasis may be direct consequences of the disease itself.

Psoriatic arthritis

Patients with psoriasis are at significant risk of developing psoriatic arthritis. The frequency of psoriatic arthritis appears to be strongly related to the degree of skin severity. For example, a population-based study indicated that the prevalence of psoriatic arthritis in patients with <1% body surface area, 1–2% BSA, 3–10% BSA, and 10+% BSA was 6%, 14%, 18%, and 56% respectively.²⁵ Similarly, nail involvement is another clinical predictor of psoriatic arthritis. For example, nail lesions are seen in 80–90% of patients with concomitant PsA compared with 46% among those with psoriasis uncomplicated by arthritis.²⁶ Skin disease occurs concurrently or prior to the development of PsA in over 80% of cases. Studies of psoriatic arthritis from rheumatology referral centers indicate that the disease can be progressive and can be associated with permanent disability and excess mortality.^27,28 Studies of PsA in the general population and in the dermatology setting indicate that the disease may be less disabling and require fewer treatments for symptom management.^29,30 Having multiple joints involved at baseline as well as having elevated markers of systemic inflammation (e.g. ESR, CRP) predict a more aggressive course for psoriatic arthritis, whereas, severity of skin disease is a poor predictor of the severity of joint disease or its progression.^25,30

Cardiovascular and Metabolic Disease

Studies from early 1970s first identified that patients with psoriasis have higher frequencies of atherosclerotic disease and thrombotic complications.³¹ More recently, studies have indicated that patients with severe psoriasis have excess rates of cardiovascular disease that is not accounted for by traditional cardiovascular risk factors (obesity, hypertension, diabetes, hyperlipidemia, and smoking). For example, Gelfand et al demonstrated in a large observational cohort study that patients with mild and severe psoriasis (identified by treatment patterns) have an increased risk of MI and that the adjusted relative risk of MI is greatest in younger patients with severe disease.³² Similarly, Ludwig et al demonstrated that the prevalence and severity of coronary artery disease in 32 well characterized patients with severe psoriasis was greater than matched controls even when controlling for major cardiovascular risk factors.³³

The prevalence of obesity, diabetes, and metabolic syndrome has been shown to be increased in psoriasis patients in the general population and in referral centers.³⁴ At least one study has demonstrated a higher prevalence of diabetes in patients with psoriasis independent of traditional diabetes risk factors such as age, gender, obesity, hypertension, and hyperlipidemia; indicating that the disease itself, or possibly its chronic treatments, may predispose to the development of diabetes.¹⁹

Cancer

The immunologic nature of psoriasis, as well as therapies which are immunosuppressive or mutagenic, may predispose patients with psoriasis to an increased risk of cancer. A higher incidence of non-melanoma skin cancer (NMSC) has been reported in psoriasis patients and there are conflicting findings regarding internal cancers such as lung, breast, colon and prostate.^35–41 Lymphoma has been of special interest since inflammatory conditions may be associated with a higher risk of lymphoproliferative disease. Studies of the risk of internal lymphoma in psoriasis patients have yielded inconsistent results. The largest study to date found no increased risk of non Hodgkin’s lymphoma, but did observe an increased risk of Hodgkin’s lymphoma and a markedly increased relative risk for cutaneous T cell lymphoma (CTCL)⁴². This association of psoriasis with CTCL may be due to misdiagnosis of early CTCL as psoriasis or may be related to chronic lymphoproliferation leading to CTCL⁴². Recently, the results of 30 years of follow up of psoriasis patients treated with PUVA found that patients who received PUVA and were exposed to high levels of methotrexate (≥36 months) had an increased incidence of lymphoma compared to the general population (IRR 4.39, 95% CI 1.59–12.06)⁴³. Increased rates of lymphoma were also observed in other patient categories created by the author (e.g. PUVA patients who received >300 UVB treatments, patients with skin types 1 or 2, patients who received >200 PUVA treatments), but these were not statistically significant, possibly due to limitations of statistical power and/or incomplete capture of outcomes.

Psychiatric disease

Multiple studies, the majority of which are descriptive, have examined psychological characteristics of patients with psoriasis.^44–46 A wide range of problems have been described such as depression, anxiety, obsessive behavior, sexual dysfunction and suicidal ideation.^47–52 A study comparing 50 patients with psoriasis in outpatient clinics to 50 healthy controls found that patients with psoriasis had a higher average Beck Depression Inventory (BDI) score (16.96 vs. 5.48, respectively p<0.01).⁵³ Suicidal ideation was found to be present in 7.2% of patients hospitalized for psoriasis, 2.5% of psoriasis outpatients and 2.4–3.3% of general medical patients, suggesting that patients with more severe disease may suffer greater emotional impairment⁴⁴. Psychological distress may also impair response to psoriasis therapies. For example, in a cohort of psoriasis patients treated with PUVA, pathological or high-level worry was a significant predictor of time taken for PUVA to clear psoriasis, whereas clinical severity of psoriasis, skin phenotype, alcohol intake, anxiety, and depression were not⁵⁴.

Clinical Implications of Comorbidities in psoriasis

The emerging data on co-morbidities in psoriasis have important clinical implications for the care of these patients. First, the dermatologist must play a central role in the initial diagnosis of PsA given that the skin disease typically precedes the onset of joint disease. Early diagnosis of PsA may lead to improved joint function and a decreased risk for future disability. Second, given the high prevalence of concomitant PsA in patients with extensive psoriasis, dermatologists should consider joint symptoms when selecting a therapy to treat the skin disease. For example, methotrexate and TNF inhibitors are considered disease modifying anti-rheumatic drugs which may prevent joint destruction in patients with PsA. Data from controlled clinical trials indicate that etanercept decreases fatigue symptoms in psoriasis patients with concomitant PsA and also may improve symptoms suggestive of depression.⁵⁵ Furthermore, the broad evidence of elevated cardiovascular risk associated with psoriasis has resulted in new consensus statements suggesting that dermatologists play a role either screening for cardiovascular risk factors in patients with psoriasis or encouraging the patient to follow-up with their primary care physician for appropriate screenings and interventions aimed at lowering cardiovascular risk.⁵⁶ The importance of optimizing medical care for patients with psoriasis is emphasized by recent studies indicating that severe psoriasis patients die 3–4 years younger than patients without psoriasis, a finding similar to estimates of years of life lost due to severe hypertension.⁵⁷ Finally, the mental health status of patients with psoriasis should be assessed since mood disorders such as depression and anxiety are highly prevalent in this patient population and also may impair response to treatment.

KEY ADVANCES IN THE TREATMENT OF PSORIASIS

What are the current FDA approved systemic biologics available for the treatment of moderate/severe plaque psoriasis? What are their associated risks and benefits?

The therapeutic paradigm for treating psoriasis with systemic agents continues to evolve. Recent consensus statements suggest that indications for systemic psoriasis treatment or phototherapy are psoriasis affecting ≥5% body surface area, psoriasis affecting vulnerable areas (e.g. palmar-plantar), psoriasis with concomitant psoriatic arthritis, pustular, erythrodermic, and guttate psoriasis variants, and psoriasis unresponsive to topical medications in which there is significant physical, social, and/or emotional impairment(s).⁵⁸ Traditional therapies include methotrexate, acitretin, cyclosporine, phototherapy (including laser devices), and PUVA. In this update, we focus on recently approved biologic therapies for psoriasis. A biologic therapy is a compound engineered from living organisms. Biologics for psoriasis vary on structure (e.g. humanized vs. chimeric, antibody vs. fusion protein), target (e.g. cytokines vs. T cells), route of administration, safety, efficacy, and monitoring requirements. TNF inhibitors were first approved for use in 1998 and have been extensively studied in clinical trials across multiple diseases and in long term cohort studies in multiple populations involving tens of thousands of patients. By comparison, T cell inhibitors for psoriasis have been studied predominantly in psoriasis patients, and currently there are no published large, long-term, cohort studies of these drugs. A summary of relevant characteristics of biologics is shown in Tables 1 and 2.

TABLE 1.

Overview of biologic dosing and efficacy^*

Biologic	Structure	Target	Dosing	Pharmacokinetics	PASI 75**
Adalimumab	Humanized monoclonal antibody	Soluble and membrane bound TNF-α	80mg SC followed by 40mg SC 1wk later, then 40mg SC qow	Half Life: 10–20 days	68% at wk 1297,134
Alefacept	Fusion protein	LFA-3	15mg IM qwk for 12 wks	Half Life: 11.25 days	33% during 14 week study period
Efalizumab	Humanized monoclonal antibody	LFA-1	0.7mg/kg SC followed by 1mg/kg qwk	Half Life: 6.21 days	27% at wk 12
Etanercept	Fusion protein	Soluble TNF-α, Lymphotoxin-α	50mg SC biw for 12wks, then 50mg SC qwk	Half Life: 4–12.5 days	34% at wk 12 (based on 25mg sc biw dosing) 49% at wk 12 (based on 50mg biw dosing)
Infliximab	Chimeric monoclonal antibody	Soluble and membrane bound TNF-α	5mg/kg IV at wks 0,2,6, then q8wks	Half Life: 8–9.5 days	80% at wk 10

^*Data from Physician’s Desk Reference. 61^st ed. 2007.

^**Data from Kurd SK, et al. Expert Rev Clin Immunol. 2007;3(2):171–185

Abbreviations: biw:2 times per week, pts:patients, qow:every other week, qwk:every week, SC:subcutaneous, sxs:symptoms, TB:tuberculosis, wk:week

Table 2.

Overview of biologic monitoring and safety

Biologic	FDA required screening and monitoring#	Common adverse effects (>5%)	Uncommon adverse effects (0.1%–5%)	Rare Adverse Effects (<0.1%)	Black Box warnings
Adalimumab	Screen for Latent TB Evaluate patients at risk for HBV or prior HBV infection	Injection site reaction; +ANA; upper respiratory infection; headache; nausea; elevated alk phos, cholesterol	Neutralizing antibodies; serious infections	Tuberculosis; malignancy; lupus-like syndrome; hypersensitivity; hep B reactivation; demyelination; congestive heart failure; pancytopenia	Infection (TB, sepsis, fungal and opportunistic)
Alefacept	CD4 count before + during treatment Evaluate pts for signs/symptoms of liver injury	Lymphopenia	LFT elevation; serious infection	Malignancy; hypersensitivity	None
Efalizumab	Platelet count before + during treatment (monthly with treatment initiation, then q 3 months with ongoing treatment) Follow for worsening psoriasis during or upon discontinuation of treatment	Flu-like symptoms, mild psoriasis flare, lymphocytosis	Severe psoriasis flare; serious infections; thrombocytopenia; arthritis; hypersensitivity; LFT elevation	Malignancies; hemolytic anemia; pancytopenia, interstitial pneumonia, toxic epidermal necrolysis, asceptic meningitis	None
Etanercept	Evaluate pts at risk for HBV or prior HBV infection Screen for Latent TB	Injection site reaction, +ANA	Serious infection	Tuberculosis; malignancy; lupus-like syndrome; hypersensitivity; hep B reactivation; demyelination; congestive heart failure; pancytopenia	Infection (bacterial sepsis and tuberculosis)
Infliximab	Screen for latent TB Evaluate patients at risk for HBV or prior HBV infection Evaluate pts for signs/symptoms of liver injury	Infusion reaction, +ANA, elevated liver function tests, neutralizing antibodies	Hypersensitivity; serious infection	Severe hepatic injury; tuberculosis; malignancy; lupus-like syndrome; hypersensitivity; hep B reactivation; demyelination; congestive heart failure; pancytopenia	Infection (TB, sepsis, fungal and opportunistic) Hepatosplenic T-cell lymphoma

^#Note that published guidelines suggest that latent TB be screened for prior to initiating therapy with all TNF inhibitors and many practitioners screen for latent TB prior to starting any biologic and then annually if the patient continues on the drug. Practitioners vary in their monitoring practices for biologics, but most monitor CBC + platelets and liver function tests periodically while on treatment.

The TNF Inhibitors

TNF-α is a 17-kD polypeptide that plays a central role in the regulation of innate immune responses. It is involved in stimulating the production of inflammatory cyokines, inducing the expression of cell surface adhesion molecules, enhancing the phagocytic/bactericidal properties of macrophages, and activating apoptotic pathways upon association with membrane-bound forms of its receptors, TNF-R1 (p55) and TNF-R2 (p75). These receptors also exist in soluble forms, which regulate TNF-α bioavailability in the circulation.

TNF-α is produced by a wide variety of cells, ranging from lymphocytes and monocytes, to keratinocytes, mast cells and antigen presenting cells in the skin. It is believed to contribute to the pathogenesis of psoriasis through its ability to both promote immune cell trafficking to the skin and induce keratinocyte proliferation.^59–61 At present, there are three anti-TNF therapeutics available for the treatment of autoimmune conditions; the fusion protein, etancercept, and two recombinant monoclonal antibodies (infliximab, adalimumab). Specific characteristics and safety issues of special interest with regard to these agents will now be discussed.

Adalimumab

Adalimumab is a fully humanized recombinant monoclonal antibody that blocks the interaction between TNF-α and its p55/p75 cell surface receptors. It differs from infliximab in that it is fully humanized, which may theoretically decrease the risk of auto-antibody formation against it. Nevertheless, neutralizing antibodies may develop to adalimumab in patients treated with this biologic.⁶² In January 2008, adalimumab received FDA approval for the treatment of adult patients with moderate to severe plaque psoriasis who are candidates for systemic therapy or phototherapy, or among whom other systemic therapies may not be appropriate. It is also approved for the treatment of psoriatic arthritis, rheumatoid arthritis, ankylosing spondylitis, and Crohn’s disease. The half-life of adalimumab ranges from 10–20 days, and it achieves a peak concentration approximately 130 hours after administration with an absolute bioavailability of 64%. For the treatment of psoriasis, adalimumab is administered as a subcutaneous injection with an initial 80mg dose, followed by a 40mg dose one week later. Subsequent maintenance doses of 40mg should be administered every other week thereafter.⁶³

Infliximab

Infliximab is a chimeric recombinant monoclonal antibody consisting of a human IgG1 constant region fused to a murine variable region that recognizes and binds to human TNF-α. It is FDA approved for the treatment of psoriatic arthritis, rheumatoid arthritis, ankylosing spondylitis, Crohn’s disease, ulcerative colitis, and chronic severe plaque psoriasis. Serum concentrations vary and are directly related to the administered dose. Infliximab concentrations of >0.1mg/kg are detected in the majority of patients up to 14 weeks after treatment.⁶⁴ The serum half-life of infliximab ranges from 8 to 9.5 days. It is administered intravenously over a 2–3 hour period at an infusion dose of 5mg/kg. Treatments are recommended at 2 weeks, 6 weeks and 8 weeks after the initial dose. It may then be dosed every 8 weeks thereafter. Among the biologics, infliximab provides the most rapid onset of clinical improvement among patients with moderate-to-severe psoriasis with approximately 80% of patients achieving a PASI 75 by treatment week 10.⁶⁵ When used as a monotherapy, however, treatment efficacy has been observed to decrease over time.⁶⁶ This loss of efficacy has been attributed to increased metabolism of the drug, possibly secondary to the generation of neutralizing autoantibodies. The development of neutralizing antibodies is associated with an increased risk for infusion reactions.⁶⁶ Such reactions may be quite serious, thus, strategies to reduce the incidence of neutralizing antibodies should be considered. It has been suggested that combining infliximab therapy with methotrexate may decrease the development of neutralizing antibodies and loss of efficacy, as was reported among patients with Crohn’s disease.⁶⁷ Furthermore, intermittent dosing of infliximab for psoriasis is similarly associated with an increased risk for neutralizing antibody formation and loss of efficacy over time; thus, intermittent dosing is discouraged.⁶⁸ While infliximab acts quickly, it requires IV administration in a medical setting, and thus is not as convenient as self-administered medications.

Etanercept

Etanercept is a recombinant dimer of human soluble TNF-R2 (p75) consisting of the extracellular ligand-binding portion of p75 fused to a human IgG1Fc region, it exhibits a higher affinity (~50 fold) for TNF-α than its endogenous soluble counterpart. Unique among the TNF inhibitors, etancercept also binds lymphotoxin-α (TNF-β), a member of the TNF family of cytokines. Etanercept reversibly binds to TNF-α in the circulation, thus, competitively inhibiting its ability to associate with its endogenous receptors. Etanercept is FDA approved for the treatment of psoriatic arthritis, rheumatoid arthritis, ankylosing spondylitis, and moderate-to-severe plaque psoriasis. It has a serum half-life between 4 and 25 days, achieving peak plasma concentrations approximately 48 hours after dosing.⁶⁹ The bioavailability of subcutaneous etanercept is approximately 58%.⁷⁰ Etanercept is self-administered by the patient as a 50mg subcutaneous injection - typically twice weekly for the first 12 weeks, then weekly thereafter. Etanercept efficacy, may be affected by patient weight particularly in those who are extremely obese (BMI > 40).⁷¹

TNF Inhibitors: Comparisons

While etanercept effectively neutralizes soluble forms of TNF-α, it exhibits minimal affinity for membrane bound forms and is incapable of inducing complement fixation and the apoptosis of cells. In contrast, the antibody based therapeutics (infliximab, adalimumab) inhibit both soluble and membrane bound forms of TNF-α. Their association with membrane bound forms accounts for their ability to induce apoptosis of targeted cells via complement fixation and antibody dependent cell mediated cytotoxicity.^72,73 Etanercept also differs from antibody based therapies with regard to its affinity for TNF. Unlike infliximab and adalimumab, etanercept sheds approximately 50% of soluble TNF within 10 minutes of binding;⁷³ whereas, the antibody based therapeutics exhibit irreversible high affinity binding to both soluble and membrane bound TNF. With regard to pharmacokinetics, the subcutaneous dosing of etanercept and adalimumab allow for more uniform serum concentration time profiles at steady state; whereas, the intravenous infusion of infliximab accounts for elevated peak to trough ratios.⁷⁴

Aside from their shared ability to inhibit TNF function, recent laboratory studies have revealed differences between etanercept and the antibody based therapeutics with regard to their effects on cytokine production by helper T cells. More specifically, results from cell culture experiments found that TNF inhibition by infliximab led to strong suppression of genes mediating Th1 cytokine production (e.g. IFN-g), whereas, a similar effect was not appreciated among etanercept treated cells. ⁷⁵

Given the importance of Th1 cytokines, such as IFN–γ, in promoting cell-mediated immune responses, their suppression by infliximab may account for the increased reactivation of M. tuberculosis reported among patients.⁷⁵ This characteristic, however, may prove beneficial in the setting of certain autoimmune disorders characterized by enhanced Th1 cytokine production, such as inflammatory bowel disease. In particular, antibody based TNF inhibitors have proven efficacious in the management of Crohn’s disease, a Th1 mediated disorder, whereas etanercept has not been proven effective in this setting.^76–78 Differences in the pharmacokinetic and pharmacodynamic profiles of the TNF inhibitors may also contribute towards differences in their clinical efficacy and safety in the management of specific disease states.

TNF Inhibitors: Adverse Effects/Safety

Given their immunosuppressive properties, patients should be screened for signs of infection or malignancy prior to the initiation of therapy, as well as during the course of treatment. The TNF inhibitors are contraindicated among patients with active, chronic, or localized infections. In addition, they should not be administered to patients receiving the IL-1 receptor antagonist, anakinra as this substantially increases the risk of infection.

Relative contraindications to treatment include a personal history of congestive heart failure or a family history of demyelinating disease (e.g., multiple sclerosis). Injection site and infusion reactions are the most common side effects reported among patients receiving anti-TNF-α therapy. Other adverse effects include a potentially increased risk for infection, lymphoma, demyelinating disease, congestive heart failure, and autoantibody formation. In particular, infliximab at doses >5mg/kg is contraindicated in patients with moderate to severe CHF as studies suggest that it may increase the risk of mortality in this patient population^79,80. Adalimumab, infliximab, and etanercept are all pregnancy category B medications and are metabolized by proteolysis.

Consideration should be given to vaccination against common serious infections, such as pneumonia and influenza, prior to the initiation of therapy whenever possible; as treatment during therapy, although likely efficacious, may result in decreased antibody titer responses against vaccination antigens.^81,82 Live vaccines are generally contraindicated during treatment with TNF inhibitors. We will now discuss additional safety issues related to the use of these medications which are of special interest.

Safety issues related to malignancy

There is theoretical concern that TNF inhibitors may increase the risk of malignancy, such as lymphoma. Observational cohort studies of TNF inhibitors for the treatment of RA have reported an increased risk of lymphoma amongst treated patients.^83,84 Whether this finding represents a disease associated predisposition for lymphoma, and/or a drug/treatment associated phenomenon remains to be determined. Of note, studies which have controlled for the severity of rheumatoid arthritis have not found and increased risk of lymphoma in RA patients treated with TNF inhibitors.⁸⁵ A meta-analysis of randomized, controlled trials involving the use of infliximab and adalimumab for the treatment of RA reported an increased risk for solid organ malignancies, of which 35% were skin cancers, among patients.⁸⁶ Observational studies in patients with rheumatoid arthritis have also found a modest increased risk of non-melanoma skin cancer in patients treated with TNF inhibitors.⁸⁷ An increased risk for solid organ cancer was also reported among patients receiving etancercept and cyclophosphamide concurrently for the treatment of Wegener’s granulomatosis in a randomized, controlled trial.⁸⁸ Thus, one should consider avoiding the concurrent use of TNF inhibitor(s) with cyclophosphamide.⁸⁸ The applicability of malignancy risk associated with TNF inhibitors in the psoriasis population is not clear as psoriasis patients are generally treated with monotherapy whereas the patients treated with TNF inhibitors for RA are commonly treated with concomitant immunosuppressives which could alter the safety profile of these drugs.

Safety issues related to viral hepatitis and tuberculosis

Among psoriasis and rheumatoid arthritis patients with concomitant HCV infection, no exacerbation of liver disease was reported in the setting of anti-TNF-α therapy (i.e. etanercept, adalimumab, infliximab).^89–91 In fact, TNF-α inhibition may be beneficial in the management of HCV infection, as excess TNF is believed to contribute towards the hepatic inflammation and fibrosis characteristic of this condition. A recent retrospective survey and prospective trial reported no substantial change in liver transaminases or hepatitis C viral load among rheumatoid arthritis patients who underwent treatment with etanercept or infliximab for their arthritis.⁹² Furthermore, a randomized controlled trial demonstrated that etanercept may be useful as adjuvant therapy (e.g. in addition to interferon alfa-2b and ribavirin) for hepatitis C infection.⁹³

Patients who are chronically infected with hepatitis B virus (HBV) require special attention when being treated with anti-TNF-α therapy. More specifically, HBV reactivation and associated fatalities have been reported among patients treated with TNF–α inhibitors (i.e. infliximab).⁹⁴ Thus, consideration should be given towards screening prospective treatment subjects for HBV if they are at risk for chronic HBV infection [www.cdc.gov/ncidod/diseases/hepatitis/b/Bserology.htm]. If the patient is chronically infected with hepatitis B then therapies which are not immunosuppressive or hepatotoxic are preferred. In situations where TNF inhibitors must be used in a patient with chronic hepatitis B infection, close monitoring should occur. Additionally, concomitant treatment for hepatitis B may be considered. For example, a small case series reported no changes in serum transaminases or viral load among chronic HBV patients receiving concomitant TNF inhibitor therapy and lamivudine.⁹⁵

As mentioned previously, the risk for tuberculosis among patients on TNF-α inhibitor therapy appears to be greatest among those receiving antibody based therapeutics (i.e. infliximab and adalimumab), as opposed to receptor-based therapy (i.e. etanercept). Infliximab has been associated with a 4 to 20 fold increased risk for tuberculosis infection.⁹⁶ Checking a PPD is recommended based on consensus guidelines prior to initiation of treatment with all TNF inhibitors as all three may increase the risk of reactivation of latent TB.⁹⁷ Patients should also be monitored for signs/symptoms of active TB during treatment. If active disease is detected, further anti-TNF therapy should be withheld until the infection is effectively treated or resolved. If a patient tests positive for latent disease, treatment for latent TB should be initiated according to standard guidelines and TNF inhibitor therapy may be considered.^98,99

Patients who have had the BCG vaccine may have a positive PPD even in the absence of infection, thus, alternative screening methods would be appropriate in this setting such as QuantiFERON-TB Gold (www.quantiferon.com) or T-Spot.TB assays. These whole blood tests have higher specificity than the traditional TST, ranging from 96–100% among BCG-vaccinated subjects, while having comparable sensitivity.⁹⁶ A PPD should not be performed prior to QuantiFERON-TB Gold or T-Spot.TB assays as it can theoretically result in a higher risk of false positive results.¹⁰⁰

It has been estimated that screening for latent TB (risk assessment, tuberculin skin testing, and chest X-ray) prior to anti-TNF therapy may decrease the rate of tuberculosis infection by as much as 90%.¹⁰¹ Although tuberculosis screening, and subsequent treatment in positive cases, reduces the incidence of disease reactivation among patients who then receive TNF-α inhibitor therapy, one study reported the development of TB among 19% of patients who received adequate chemoprophylaxis prior to anti-TNF therapy.¹⁰²

Safety issues related to demyelinating diseases

There have been rare reports of new onset demyelinating events among patients treated with TNF inhibitor therapy. Furthermore, a randomized controlled trial of the TNF inhibitor lenercept for treatment of multiple sclerosis demonstrated that TNF inhibition may lead to a higher rate of disease exacerbations compared to placebo. ¹⁰³ A definitive link between TNF inhibitor treatment and new onset demyelinating disease has been raised but remains uncertain. Whether TNF inhibitor therapy unmasks an underlying predisposition for autoimmune neurologic disease, promotes it, or has no direct association with it remains unclear. A recommendation to cease TNF inhibitor use upon development of neurologic symptoms, and to avoid use among patients with pre-existing demyelinating disease has been made given rare reports of new onset demyelinating events among treated patients. It has also been recommended that TNF inhibitor therapy be avoided patients who have a first degree relative affected by demyelinating disease.¹⁰⁴ The impact of the T cell inhibitors used for psoriasis on the risk of demyelination events has not been well characterized; however, such events have been reported to occur in individuals treated with efalizumab.¹⁰⁵

Inhibitors of T-cell Activation

Alefacept

Alefacept is a fully humanized recombinant dimeric fusion protein consisting of the terminal end of leukocyte function-associated antigen-3 (LFA-3) bound to the Fc portion of human IgG1. Under normal circumstances, endogenous LFA-3, which is expressed on the surface of antigen presenting cells (APC), is recognized by CD2, which is preferentially expressed at high levels by natural killer (NK) cells and effector/memory CD4 and CD8 T cells. The interaction of APC LFA-3 with CD2 on the T-cell surface plays an essential costimulatory role in T cell activation. By binding to CD2, alefacept inhibits T-cell costimulation and activation.¹⁰⁶ In addition, alefacept may induce the selective apoptosis of effector/memory T-cells through its simultaneous binding of CD2 on the T cell surface and the FcγRIII receptor expressed by NK cells, which recognizes the Fc portion of alefacept.¹⁰⁷ Recent data suggest that alefacept may exhibit both agonistic and antagonistic properties with regard to the expression of specific cytokines (i.e., the suppression of inflammatory cytokines and induction of IL-8, STAT1, and Mig).¹⁰⁸ Gene expression patterns specific to responders and non-responders to alefacept have also been identified.¹⁰⁹

Alefacept is FDA approved for the treatment of adults with chronic moderate-to-severe plaque psoriasis. After drug administration, peak plasma concentrations of alefacept are achieved between 24 and 192 hours; its elimination half-life is approximately 12 days.¹¹⁰ Alefacept is administered as a weekly intramuscular injection of 15mg for 12 consecutive weeks. In a recent international phase 3 trial, a PASI 75 was achieved by 33% of patients receiving alefacept 15mg IM weekly and by 13% of placebo treated patients during the 14week study period.¹¹¹ Recent studies suggest that longer courses of treatment (16 weeks) or repeated courses may lead to enhanced efficacy.^112–114 Although not FDA approved for psoriatic arthritis (PsA), recent trials indicate that alefacept may also improve PsA symptoms.¹¹⁵ It has been utilized both as monotherapy and in combination with methotrexate in this setting with promising results.^115,116

In comparison to the TNF inhibitors, alefacept appears to exhibit lower treatment efficacy with a delayed onset of action. Clinical improvement usually occurs late during the treatment course with maximal responses often noted weeks after the final dose. The potential for long periods of disease remission upon cessation of therapy exists; however, this occurs in only a minority of patients.¹¹⁷ It is recommended that a CD4+ T cell count be established at baseline, with subsequent re-evaluation every two weeks throughout the 12 week treatment course (therapy should be discontinued if the count falls below 250/uL). Approximately 10% of patients require temporary discontinuation of therapy secondary to dose-dependent lymphopenia. ¹¹⁸

The most common side effects associated with alefacept therapy include injection site reactions, headaches, chills, nausea and upper respiratory symptoms. A limited number of patients experience lymphopenia, serious infections, malignancies, and elevated serum transaminases. The FDA recommends that alefacept not be administered to patients with HIV or CD4+ T cell counts below normal given the risk for lymphopenia.¹¹⁸ Alefacept should be used with caution among patients with a history of systemic malignancy or at increased risk for infection. Studies addressing the efficacy of influenza and pneumococcal vaccines among treated patients have not been published at this time, however, a study of psoriasis patients exposed to ΦX174 neoantigen and recall antigen tetanus toxoid immuniation, after a 12 week treatment course with alefacept, revealed intact CD4+ T-cell-mediated antibody titer responses which were comparable to controls.¹¹⁹ Large, long term follow-up studies of alefacept treatment of psoriasis will be necessary to further define its safety profile.

Efalizumab

Efalizumab is a humanized, recombinant, IgG1 monoclonal antibody against the CD11a subunit of leukocyte function associated antigen-1 (LFA-1). LFA-1 is endogenously expressed on the surface of T-cells. Its ligand, intercellular adhesion molecule-1 (ICAM-1), is expressed on the surface of dermal endothelial cells and APCs. The interaction of LFA-1 with ICAM-1 on the surface of APCs promotes T-cell activation and cytotoxicity.¹²⁰ In addition, its recognition of ICAM-1 expressed on the dermal microvasculature promotes the firm adhesion and subsequent migration of lymphocytes into the cutaneous micronenvironment. Thus, inhibition of LFA-1 decreases lymphocyte migration to the skin and activation by APCs.

Efalizumab is FDA approved for the treatment of moderate-to-severe plaque psoriasis. Peak plasma concentrations are achieved approximately 2–3 days after subcutaneous injection of the drug, with an elimination half-life of approximately 6 days.¹²¹ Treatment is initiated with a 0.7mg/kg subcutaneous conditioning dose, followed by weekly 1.0 mg/kg subcutaneous injections for an indefinite period of time depending upon patient response to therapy.

In a 2003 phase 3 study, 27% of patients with moderate to severe plaque psoriasis achieved a PASI 75 after 12 weeks of treatment with efalizumab at a weekly subcutaneous dose of 1mg/kg, whereas, only 4% of the placebo group achieved a similar result.¹²⁰ The most common side effects associated with treatment include flu-like symptoms (upon initial dosing), leukocytosis/lymphocytosis, and non-serious infections. There is a small risk for hemolytic anemia and thrombocytopenia among treated patients that is not well understood. It is therefore recommended that treated patients undergo monthly evaluation of their platelet count for the first three months, and every three months thereafter.

A small number of patients treated with efalizumab have experienced a flare of their disease or a change in the nature of their psoriasis during treatment.¹²² Additionally, some patients may experience worsening of their disease upon discontinuation of therapy.¹⁰⁵ It has been estimated that approximately 5% of patients will experience a rebound flare upon cessation of therapy.¹²³ In clinical trials, serious disease flares characterized by inflammatory, pustular and erythrodermic psoriasis affected 0.7% of patients.¹²⁴ Based on a small trial, inflammatory flares associated with efalizumab appear to respond best to cyclosporine or methotrexate, as compared to oral steroids or retinoids.¹²⁵

Efalizumab must be used with caution among patients with a history of systemic malignancy or at increased risk for infection. The package insert recommends against the administration of acellular, live, and live attenuated vaccines during treatment. Although a decreased response to tetanus booster has been reported among treated patients (with titers still in the protective range), no studies addressing the safety or efficacy of the pneumococcal and influenza vaccine in this setting have been reported to date.¹²⁶ Large, long term follow-up studies of efalizumab treatment of psoriasis will be necessary to further define its safety profile.

What novel biologic agents are currently under investigation?

The discovery of the proinflammatory cytokines, IL-12 and IL-23, in the past two decades has led to increasing interest in their potential roles as mediators of psoriatic disease. Both are involved in the regulation of cell-mediated immune responses. IL-12 is involved in the activation of natural killer (NK) cells and has been shown to promote the differentiation of naïve CD4 T-cells into effector/memory cells that secrete Th1 cytokines.¹²⁷ IL-23 stimulates the production of TNF, IL-6 and IL-17, by a unique subset of helper T-cells called Th17 cells.¹²⁸ The production of IL-17 has been shown to induce the production of inflammatory cytokines by multiple cell types¹²⁹ (including macrophages, fibroblasts and endothelial cells) and is believed to play a pivotal role in sustaining inflammatory responses in multiple autoimmune disorders.

IL-12 and IL-23 share structural similarity in that both possess an IL-12p40 subunit. Their receptors are also similar, in that they share the IL-12Rβ1 subunit, which recognizes IL-12p40. Elevated levels of IL-12p40 mRNA have been reported in the skin lesions of psoriasis patients.¹³⁰ Several studies have correlated clinical improvement in psoriasis lesions with marked reductions in both IL-12 and IL-23 expression levels in affected skin,¹³⁰ further supporting a pathogenic role for these cytokines in psoriasis. To evaluate the therapeutic efficacy of IL-12/23 blockade on psoriasis, a recombinant human monoclonal antibody against IL-12p40 (CNTO 1275) was recently developed (Centocor, Corp.).

A phase II study of CNTO 1275 among patients with moderate-to-severe plaque psoriasis reported a PASI 75 by week 12 in 52% of patients who received one 45mg intravenous dose, 59% of patients who received one 90mg dose, and 81% of patients who received four weekly 90mg doses; compared to 2% who received placebo.¹³¹ Twenty-three percent to 52% of patients achieved a PASI 90 depending upon the dose.

Another monoclonal antibody against IL-12/23 has been developed (ABT-874; Abbot Laboratories) and studied in the setting of Crohn’s disease, and more recently, psoriasis. A phase II trial among Crohn’s disease patients receiving subcutaneous ABT-784 for 7 weeks resulted in marked improvements in patient symptom scores with no serious adverse effects.¹³² A phase II trial to assess its safety and efficacy in the treatment of moderate to severe chronic plaque psoriasis was recently conducted.¹³³ Over 90% of patients receiving repeated doses of this agent, in varying amounts and duration, achieved a PASI 75 after 12 weeks.

These studies provide early evidence that IL-12/23 monoclonal antibody therapy may offer a safe and efficacious treatment alternative for patients with moderate-to-severe plaque psoriasis.

CONCLUSION

The onset of psoriatic disease and its associated comorbidities involves the interplay between a myriad of genetic and environmental risk factors. As we gain further insight into the immunopathogenesis of psoriasis, we hope it will provide the basis for the development of safer, more efficacious, and more durable therapeutics in the future. Given its enormous toll on patient health and quality of life, steps should be taken to prevent or decrease the risk of psoriasis associated co-morbidities through behavior modification and use of preventative health screenings and treatments. Future studies will need to be performed to determine if successful treatment of psoriasis will lead to a decreased risk of developing psoriasis associated co-morbidities over time.