65.2 Chronic Plaque Psoriasis & Biologic Therapies

Immunopathogenesis & Histopathology of Psoriasis

Psoriasis is a chronic, non-contagious, immune-mediated systemic inflammatory disorder with primary cutaneous and articular manifestations, affecting 2% to 3% of the global population. Far from being a localized epidermal disease, psoriasis is characterized by systemic vascular and metabolic inflammation, substantially conferring an increased risk of premature atherosclerotic cardiovascular disease, myocardial infarction, metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), and depression.

The IL-23 / Th17 / IL-17 Pathogenic Axis

The pathophysiology of psoriasis is driven by dysregulated innate and adaptive immune crosstalk within the skin:

1. Innate Immune Activation: Cutaneous trauma, infection, or environmental stress prompts stressed keratinocytes to release antimicrobial peptides, notably LL-37 (cathelicidin). LL-37 complexes with self-DNA or self-RNA, binding Toll-like receptors (TLR-7, TLR-8, TLR-9) on plasmacytoid dendritic cells (pDCs), stimulating robust production of interferon-alpha (IFN-α).
2. Myeloid Dendritic Cell Activation & IL-23 Secretion: IFN-α activates dermal myeloid dendritic cells, which migrate to regional lymph nodes and secrete high concentrations of interleukin-23 (IL-23) and interleukin-12 (IL-12).
3. T-Helper 17 (Th17) Polarization & Expansion: IL-23 binds its receptor on CD4+ T-helper 17 (Th17) cells and innate lymphoid cells, promoting their survival, proliferation, and activation.
4. Effector Cytokine Storm: Activated Th17 cells migrate into the skin and release potent effector cytokines: IL-17A, IL-17F, and IL-22:
   • IL-17A & IL-17F: Bind receptors on epidermal keratinocytes, inducing massive transcription of chemokines (CXCL1, CXCL8) that recruit circulating neutrophils and perpetuate feed-forward inflammatory loops.
   • IL-22: Acts synergistically with IL-17 to stimulate aberrant, uncontrolled keratinocyte hyperproliferation and inhibit terminal differentiation.

                      THE IMMUNOPATHOGENIC AXIS OF PSORIASIS

   Cutaneous Trauma / Stress ──► Keratinocytes release LL-37 + self-DNA/RNA
                                              │
                                              ▼
                              Plasmacytoid Dendritic Cells (pDCs)
                                              │  (Interferon-alpha [IFN-α])
                                              ▼
                                Dermal Myeloid Dendritic Cells
                                              │  (Interleukin-23 [IL-23])
                                              ▼
                                     Th17 Cell Expansion
                                              │  (IL-17A, IL-17F, IL-22)
                                              ▼
                          Epidermal Keratinocyte Hyperproliferation
                       & Neutrophilic Recruitment (Microabscesses)

Histopathologic Hallmarks & Epidermal Kinetics

• Accelerated Keratinocyte Transit: In normal epidermis, the transit time from basal layer stem cell to desquamation at the stratum corneum spans 28 days. In active psoriatic plaques, this transit is accelerated nearly tenfold to 3 to 5 days. Keratinocytes lack adequate time to mature, synthesize keratohyalin granules, or lose their nuclei.
• Parakeratosis: Retention of pyknotic nuclei within the stratum corneum, accompanied by complete absence or marked thinning of the stratum granulosum (agranulosis).
• Acanthosis & Rete Elongation: Marked, regular thickening of the epidermis with uniform, club-shaped elongation of the epidermal rete ridges into the dermis.
• Dermal Vascular Hyperplasia: The dermal papillae exhibit marked thinning of their suprapapillary epidermal plates, accompanied by prominent tortuous, dilated, hyperpermeable capillary loops driven by vascular endothelial growth factor (VEGF).
• Neutrophilic Infiltration: Neutrophils migrate into the spinous layer to form spongiform pustules of Kogoj, and into the parakeratotic stratum corneum to form Munro microabscesses.

---

Clinical Phenotypes & Physical Diagnostic Signs

                       CLINICAL SUBTYPES OF PSORIASIS VULGARIS

   Phenotype               Clinical Morphology & Distribution               Triggers & Clinical Pearls
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════════════
   Chronic Plaque          Sharp, raised erythematous plaques covered by     Predilection for extensor surfaces (elbows,
   (Psoriasis Vulgaris)    thick, micaceous silvery-white scales;            knees), scalp, lumbosacral region, umbilicus;
   [80%–90% of cases]      symmetrical distribution; pruritus variable.      displays Auspitz sign and Koebner phenomenon.
   ───────────────────────────────────────────────────────────────────────────────────────────────────────────
   Guttate Psoriasis       Sudden eruption of crops of small (0.5–1.5 cm),   Classic onset 1–3 weeks following Streptococcus
                           discrete, drop-like salmon-pink papules on        pyogenes (GAS) pharyngitis or perianal strep;
                           the trunk and proximal extremities.               check ASO titer / throat swab; phototherapy.
   ───────────────────────────────────────────────────────────────────────────────────────────────────────────
   Inverse /               Smooth, red, glistening, sharply demarcated       Affects warm, moist body folds (axillae, groin,
   Intertriginous          plaques COMPLETELY LACKING SILVERY SCALE          inframammary, gluteal cleft); easily misdiagnosed
                           (scale is macerated and sloughed by friction).    as Candida (Candida has satellite pustules!).
   ───────────────────────────────────────────────────────────────────────────────────────────────────────────
   Pustular Psoriasis      Sheets of sterile, pinhead-sized yellow pustules  CLASSICALLY PRECIPITATED BY ABRUPT WITHDRAWAL
   (von Zumbusch)          arising on fiery, tender erythematous skin;       OF SYSTEMIC ORAL CORTICOSTEROIDS! Inpatient
                           fever, leukocytosis, hypocalcemia, toxicity.      medical emergency; high mortality.
   ───────────────────────────────────────────────────────────────────────────────────────────────────────────
   Erythrodermic           Generalized, inflammatory erythema and scaling    Compromised barrier: hypothermia, high-output
   Psoriasis               involving > 90% of Total Body Surface Area;       cardiac failure, protein loss, sepsis; requires
                           life-threatening dermatologic emergency.          burn-unit / ICU-level fluid resuscitation.
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════════

Cardinal Physical Signs

1. Auspitz Sign: When the thick, micaceous, silvery scale of a chronic plaque is gently scraped or peeled away, tiny punctate bleeding droplets appear beneath. This bleeding occurs because the suprapapillary epidermal plates are severely thinned, leaving fragile, engorged, tortuous papillary capillary loops vulnerable to immediate rupture upon minimal mechanical friction.
2. Koebner Phenomenon (Isomorphic Response): The emergence of characteristic psoriatic plaques in previously unaffected skin along the precise lines or sites of cutaneous mechanical trauma (e.g., surgical incisions, physical scratches, thermal burns, tape application, or sunburn). Typically manifests 7 to 14 days following cutaneous injury.

Psoriatic Nail Disease: Predictor of Arthritis

Nail involvement occurs in up to 50% of cutaneous psoriasis patients and in >80% of individuals with psoriatic arthritis. Anatomical location dictates morphology:

• Nail Matrix Involvement: Produces nail pitting (small, thimble-like, punctate surface depressions caused by focal parakeratosis within the dorsal nail matrix) and leukonychia.
• Nail Bed Involvement: Produces pathognomonic "oil-drop" or "salmon-patch" dyschromia (translucent, yellowish-red discoloration beneath the nail plate due to localized capillary pooling and exudate), subungual hyperkeratosis (accumulation of flaky scale under the free edge), splinter hemorrhages, and distal onycholysis (separation of the nail plate from the bed).
• Clinical Pearl: Severe nail dystrophy is a powerful independent predictor of concurrent or impending Psoriatic Arthritis (PsA), particularly involving the distal interphalangeal (DIP) joints, due to the close anatomical entheseal continuity between the extensor tendon, nail bed, and periosteum.

---

Psoriatic Arthritis (PsA) & The CASPAR Criteria

Psoriatic arthritis develops in 20% to 30% of patients with plaque psoriasis, typically arising 5 to 10 years after skin manifestations, though in 15% of cases arthritis precedes or coincides with cutaneous disease. It is a seronegative spondyloarthropathy (rheumatoid factor [RF] and anti-CCP antibodies are typically negative).

Clinical Patterns of Psoriatic Arthritis

• Distal Interphalangeal (DIP) Predominant: Classic form; affects DIP joints with concurrent nail dystrophy; spared in rheumatoid arthritis.
• Asymmetric Oligoarthritis: Involves fewer than 5 small or large joints (e.g., one knee, one PIP, one DIP).
• Symmetric Polyarthritis: Clinically mimics rheumatoid arthritis involving MCP and PIP joints, but distinguished by negative RF and DIP involvement.
• Axial Spondyloarthritis: Asymmetric sacroiliitis and spondylitis (inflammatory back pain, morning stiffness improving with activity).
• Arthritis Mutilans: Severe, osteolytic, deforming joint destruction producing "telescoping" or "opera-glass" digits (main en lorgnette).

Diagnostic Hallmarks

• Dactylitis ("Sausage Digits"): Diffuse, uniform, painful soft-tissue swelling of an entire finger or toe extending beyond the joint capsule, driven by concurrent flexor tenosynovitis, joint synovitis, and periarticular enthesitis.
• Enthesitis: Severe inflammation at the anatomic insertion sites of tendons, ligaments, and fascia into bone. The most common clinical targets are the insertion of the Achilles tendon onto the calcaneus and the plantar fascia.
• The CASPAR Classification Criteria (Score ≥3 indicates PsA):
  • Established inflammatory articular disease (joint, spine, or entheseal) PLUS ≥3 points from:
    1. Current psoriasis (2 points) OR Personal history of psoriasis (1 point) OR Family history of psoriasis (1 point).
    2. Characteristic psoriatic nail dystrophy (1 point).
    3. Negative rheumatoid factor (1 point).
    4. Current or historical dactylitis (1 point).
    5. Radiographic evidence of new bone formation (ill-defined ossification near joint margins, excluding osteophytes; 1 point).

---

Stepwise Management Ladder (AAD/NPF Guidelines)

Therapeutic selection depends on disease severity, quantified by Total Body Surface Area (BSA) involvement (using the patient's palm with closed fingers as equivalent to ~1% BSA), anatomic location, and the presence of joint disease.

• Mild Disease: <3% to 5% BSA, with no involvement of sensitive sites (face, palms, soles, genitalia) and no psoriatic arthritis.
• Moderate-to-Severe Disease: >5% to 10% BSA, or involvement of high-impact/sensitive sites causing severe functional/psychosocial disability, or concurrent Psoriatic Arthritis.

Step 1: Topical Therapies for Mild-to-Moderate Disease

1. High-Potency Topical Corticosteroids (Class 1 & 2):
   • Agents: Clobetasol propionate 0.05% (Class 1 superpotent) or Betamethasone dipropionate 0.05% (Class 2 potent) ointment/cream applied twice daily.
   • Indication: Thick, stubborn plaques on extensor surfaces (elbows, knees, trunk).
   • Dosing Protocol: Applied BID for up to 2 to 4 consecutive weeks to induce clearance, followed by a transition to intermittent "pulse dosing" (e.g., weekend-only application) to prevent tachyphylaxis, epidermal atrophy, striae distensae, and hypothalamic-pituitary-adrenal (HPA) axis suppression.
2. Topical Vitamin D3 Analogues (Calcipotriene, Calcitriol):
   • Mechanism: Bind intracellular vitamin D receptors (VDR) in keratinocytes, directly inhibiting keratinocyte proliferation, promoting normal terminal differentiation, and suppressing IL-17 and IL-22 release.
   • The Calcipotriene-Steroid Synergy: Combining a topical vitamin D analogue with a high-potency topical corticosteroid (available as fixed-dose combination calcipotriene 0.005%/betamethasone dipropionate 0.064% suspension, ointment, or foam) yields superior clearance rates compared to either agent alone. Crucially, calcipotriene counteracts corticosteroid-induced epidermal atrophy, while the steroid mitigates calcipotriene-induced local skin irritation.
3. Topical Calcineurin Inhibitors (Tacrolimus 0.1% ointment, Pimecrolimus 1% cream) & Topical PDE-4 Inhibitors (Roflumilast 0.3% cream):
   • Highly effective, non-steroidal therapies specifically indicated for sensitive, skin-thinning-prone areas: facial psoriasis and intertriginous/inverse psoriasis.
4. Topical Tazarotene (0.05%–0.1%): Topical retinoid; used synergistically with topical corticosteroids to reduce plaque thickness.

Step 2: Phototherapy

• Narrowband Ultraviolet B (NB-UVB: 311–313 nm): The phototherapy standard of care. Administered in outpatient light booths 2 to 3 times weekly. NB-UVB penetrates the epidermis and superficial papillary dermis, inducing apoptosis of pathogenic infiltrating T-lymphocytes, suppressing Langerhans cell antigen presentation, and stimulating anti-inflammatory cytokines (IL-10). Highly effective, cost-efficient, and exceptionally safe during pregnancy.
• Psoralen + UVA (PUVA): Combines oral methoxsalen with long-wave UVA; now rarely utilized due to a substantial cumulative, dose-dependent risk of non-melanoma skin cancer (squamous cell carcinoma) and cutaneous melanoma.

Step 3: Traditional Oral Systemic Therapies

Used for moderate-to-severe disease when phototherapy is inaccessible or inadequate:

• Methotrexate (MTX):
  • Dosing: 10 to 25 mg once weekly (orally or subcutaneous) + folic acid 1 mg daily (to reduce GI and hematologic toxicity).
  • Mechanism: Dihydrofolate reductase inhibitor; suppresses lymphocyte DNA synthesis and enhances adenosine-mediated anti-inflammatory signaling. Treats both cutaneous plaques and peripheral psoriatic arthritis.
  • Toxicity & Monitoring: Baseline CBC, CMP (liver transaminases, creatinine), and hepatitis serologies. Serial monitoring every 8–12 weeks. High risk of hepatotoxicity (liver fibrosis), bone marrow suppression (macrocytosis, leukopenia), and hypersensitivity pneumonitis. Strictly teratogenic (Pregnancy Category X; requires reliable contraception in men and women).
• Apremilast (Otezla):
  • Dosing: 30 mg orally twice daily (titrated over 5 days to reduce GI side effects).
  • Mechanism: Small-molecule oral phosphodiesterase-4 (PDE-4) inhibitor; raises intracellular cyclic AMP (cAMP) levels, downregulating TNF-α, IL-23, and IL-17 while upregulating IL-10.
  • Advantages: Requires zero routine laboratory blood monitoring; safe in active infections where biologics are contraindicated.
  • Adverse Effects: Diarrhea and nausea (usually transient during weeks 1–2), tension headaches, weight loss (screen weight), and new or worsening depression/suicidal ideation.
• Cyclosporine:
  • Dosing: 2.5 to 5.0 mg/kg/day divided twice daily.
  • Mechanism: Calcineurin inhibitor; suppresses IL-2 transcription and T-cell activation. Rapid onset of action (clears disease within 2–4 weeks).
  • Clinical Role & Hard Rule: Reserved strictly for rapid rescue therapy in acute, catastrophic flares (e.g., severe erythrodermic or pustular psoriasis). Treatment duration must be strictly capped at <1 year due to cumulative, irreversible nephrotoxicity, malignant hypertension, and increased risk of cutaneous malignancies. Frequent monitoring of blood pressure and serum creatinine (every 2 weeks initially) is mandatory.

Step 4: Targeted Biologic Therapies (PASI-90 to PASI-100 Clearance)

Biologics are recombinant monoclonal antibodies or fusion proteins that target specific cytokines driving the immunopathogenic cascade. They achieve unprecedented clinical clearance rates (often achieving PASI-90 or PASI-100, representing 90% to 100% plaque clearance).

                   TARGETED BIOLOGIC THERAPIES FOR MODERATE-TO-SEVERE PSORIASIS

   Biologic Class       Targeted Molecular Axis        Specific Agents               Key Indications & Clinical Warnings
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════════════
   TNF-alpha            Inhibits soluble &             • Adalimumab (Humira)         First-line for severe coexisting PsA
   Inhibitors           membrane-bound TNF-α;          • Infliximab (Remicade)       (especially axial/spine involvement);
                        dampens broad systemic         • Etanercept (Enbrel)         Black box warning: Latent TB reactivation,
                        inflammatory cascades          • Certolizumab (safe in preg) demyelinating disease (MS), worsens CHF.
   ───────────────────────────────────────────────────────────────────────────────────────────────────────────
   IL-17                Neutralizes IL-17A or          • Secukinumab (Cosentyx)      Ultra-rapid cutaneous clearance;
   Inhibitors           blocks IL-17 receptor          • Ixekizumab (Taltz)          Side effects: Mucocutaneous candidiasis;
                        (IL-17RA)                      • Brodalumab (Siliq)          CRITICAL CONTRAINDICATION: INFLAMMATORY
                                                                                     BOWEL DISEASE (triggers Crohn / UC flares!).
   ───────────────────────────────────────────────────────────────────────────────────────────────────────────
   IL-23                Binds p19 subunit of IL-23     • Guselkumab (Tremfya)        Exceptional long-term durability;
   Inhibitors           (or p40 subunit shared by      • Risankizumab (Skyrizi)      Infrequent dosing (every 8 to 12 weeks);
                        IL-12/23: Ustekinumab)         • Tildrakizumab (Ilumya)      Superior safety profile; does not trigger
                                                       • Ustekinumab (Stelara)       candidiasis or inflammatory bowel disease.
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════════

Mandatory Pre-Biologic Screening Protocol

Prior to administering the first dose of any biologic therapy, the family physician must execute a strict, standardized pre-immunosuppression screening protocol:

1. Latent Tuberculosis Screening:
   • Interferon-Gamma Release Assay (IGRA): QuantiFERON-TB Gold or T-SPOT (preferred over tuberculin skin test [PPD] in patients vaccinated with BCG).
   • Baseline Chest Radiograph: Mandatory to rule out active pulmonary tuberculosis.
   • Management of Positive Screen: If latent TB is diagnosed (positive IGRA without active disease on radiograph), the patient must initiate preventive chemotherapy (e.g., daily isoniazid for 9 months or rifampin daily for 4 months) for at least 1 full month prior to initiating the biologic.
2. Viral Hepatitis Serological Panel:
   • Hepatitis B: Must obtain HBsAg (surface antigen), anti-HBs (surface antibody), and total anti-HBc (core antibody).
   • Why anti-HBc is essential: Even if HBsAg is negative, an isolated positive anti-HBc indicates past resolved infection where HBV cccDNA persists in hepatocytes. Under potent biologic immunosuppression (especially anti-TNF), HBV can fulminantly reactivate, precipitating acute liver failure and death. Prophylactic entecavir or tenofovir is required under hepatology guidance.
   • Hepatitis C: Anti-HCV antibody (with reflex HCV RNA if positive).
3. Complete Blood Count & Comprehensive Metabolic Panel: Establish baseline bone marrow reserve and hepatic/renal clearance.
4. Immunization Review:
   • All indicated age-appropriate inactivated vaccines (Pneumococcal conjugate/polysaccharide, Recombinant Zoster [Shingrix], COVID-19, annual Influenza) should ideally be completed at least 2 to 4 weeks before immunosuppressive initiation.

   • [!CRITICAL] Strict Prohibition of Live Vaccines on Biologics LIVE-ATTENUATED VACCINES ARE STRICTLY CONTRAINDICATED while a patient is receiving biologic therapies. These include MMR, Varicella, Yellow Fever, Oral Typhoid, and Live-Attenuated Intranasal Influenza. Live vaccines risk uninhibited viral replication, disseminated infection, and death in the immunosuppressed host.

The Cardinal Board Exam Contraindication

[!CRITICAL] Never Prescribe Systemic Oral Corticosteroids for Plaque Psoriasis Systemic oral corticosteroids (e.g., prednisone, dexamethasone, methylprednisolone "dose packs") should NEVER be administered for the treatment of chronic plaque psoriasis. While oral steroids may produce dramatic, rapid, temporary skin clearance, tapering or abruptly discontinuing the steroid triggers a catastrophic rebound phenomenon. This rebound routinely precipitates life-threatening Generalized Pustular Psoriasis of von Zumbusch or Erythrodermic Psoriasis, resulting in high-output heart failure, septic shock, hypothermia, and death. Systemic steroids have no role in psoriasis management.