I now have comprehensive information from Fitzpatrick's Dermatology, Robbins Pathology, Goodman & Gilman's Pharmacology, and Family Medicine textbooks. Let me compose a well-structured B.Pharm-level explanation.
Plaque Psoriasis - B.Pharm Level Notes
1. Definition and Overview
Plaque psoriasis (psoriasis vulgaris) is a chronic, immune-mediated inflammatory skin disease characterized by discrete, well-demarcated erythematous (red) plaques covered with adherent silvery-white scales. It is the most common form of psoriasis, accounting for 80-90% of all psoriasis cases.
- Prevalence: 1-2% of the U.S. population
- Age of onset: Two peaks - 16 to 22 years and 57 to 60 years
- It is a systemic disease - skin inflammation is the most visible sign, but internal comorbidities exist
(Textbook of Family Medicine 9e; Fitzpatrick's Dermatology)
2. Etiology and Risk Factors
Psoriasis results from a combination of genetic and environmental factors:
| Factor | Details |
|---|
| Genetics | Polygenic inheritance; specific HLA gene variants (HLA-Cw6 is most associated). If both parents have psoriasis, risk ~50%; one parent affected, risk ~16% |
| Immune dysregulation | Aberrant activation of T lymphocytes, dendritic cells |
| Triggers | Skin trauma (Koebner phenomenon), infections (streptococcal pharyngitis triggers guttate psoriasis), stress, drugs (lithium, beta-blockers, antimalarials), smoking, alcohol |
| Withdrawal of systemic steroids | Can provoke pustular psoriasis |
(Robbins, Cotran & Kumar Pathologic Basis of Disease; Textbook of Family Medicine 9e)
3. Pathogenesis (Mechanism)
This is the most pharmacologically important section for B.Pharm students.
Step-by-step Immune Cascade:
Step 1 - Antigen Presentation:
An unknown trigger (possibly self-antigen or microbial antigen) activates antigen-presenting cells (APCs) - particularly dendritic cells in the skin.
Step 2 - T Cell Activation:
APCs activate CD4+ Th1 cells and Th17 cells (and CD8+ cytotoxic T cells accumulate in the epidermis). This is the central immune abnormality.
Step 3 - Cytokine Release (the "cytokine soup"):
Activated T cells and dendritic cells release a storm of pro-inflammatory cytokines:
| Cytokine | Source | Key Effect |
|---|
| TNF-α | Dendritic cells, T cells | Master pro-inflammatory cytokine; activates keratinocytes |
| IL-17 (key cytokine) | Th17 cells | Promotes neutrophil recruitment and keratinocyte activation; drives hyperproliferation |
| IL-23 | Dendritic cells | Required for Th17 cell development; composed of p19 + p40 subunits |
| IL-12 | APCs | Drives Th1 differentiation |
| IFN-γ | Th1 cells | Pro-inflammatory at keratinocyte level |
| IL-22 | T cells, innate lymphoid cells | Drives keratinocyte hyperplasia and migration |
Step 4 - Keratinocyte Hyperproliferation:
Cytokines drive markedly reduced basal keratinocyte transit times - normal skin cell turnover is ~28 days; in psoriasis it is reduced to 3-4 days. This results in thickened, poorly differentiated skin.
Step 5 - Vascular Remodeling:
Capillary loops in the dermal papillae elongate, dilate, and touch the epidermis ("kissing" phenomenon). Leukocytes exit through these remodeled vessels into the superficial epidermis ("squirting papillae").
The importance of this cytokine cascade is proven by treatment: TNF-α inhibitors and IL-17 inhibitors produce excellent clinical responses.
(Fitzpatrick's Dermatology Vol. 1, 2; Robbins Pathology; Goodman & Gilman's Pharmacology)
4. Histopathology (Microscopic Features)
Know these for exams:
Fig: Psoriasis - erythematous plaques (A/B) and histology showing epidermal hyperplasia with Munro microabscesses (C) - from Robbins, Cotran & Kumar Pathologic Basis of Disease
| Feature | Explanation |
|---|
| Acanthosis | Epidermal thickening; rete ridges elongated downward like "test tubes in a rack" |
| Parakeratosis | Retention of nuclei in stratum corneum (incomplete keratinization) |
| Absent/thin stratum granulosum | Lost due to rapid cell turnover |
| Munro microabscesses | Small neutrophil aggregates in the parakeratotic stratum corneum - pathognomonic |
| Spongiform pustules | Neutrophil aggregates in superficial epidermis |
| Thinned suprapapillary plates | Epidermis over dermal papillae tips is very thin |
| Dilated dermal papillary vessels | Tortuous, elongated capillary loops |
(Robbins, Cotran & Kumar Pathologic Basis of Disease)
5. Clinical Features
Lesion Description (Classic Morphology):
- Well-demarcated, raised erythematous plaques
- Covered with adherent silvery-white (micaceous) scales
- Plaques vary from small papules to large confluent areas
Sites of Predilection:
- Extensor surfaces - elbows and knees (most classic)
- Scalp, lower back (lumbosacral area)
- Umbilicus, intergluteal cleft
- Nails, genitalia, face
Clinical Signs to Remember:
| Sign | Description |
|---|
| Auspitz sign | Removal of scale reveals multiple minute bleeding points (dilated papillary vessels touch the thin suprapapillary epidermis) |
| Koebner phenomenon | New psoriatic plaques appear at sites of skin trauma (cuts, scratches, burns) |
| Candle sign (Grattage sign) | Scraping a plaque produces silvery scales resembling candle wax |
Types of Psoriasis (for context):
| Type | Key Feature |
|---|
| Plaque (vulgaris) | 80-90%, classic silvery plaques on extensor surfaces |
| Guttate | Small drop-like lesions; often post-streptococcal in children |
| Inverse | Flexural areas (axilla, groin); minimal scale due to moisture |
| Erythrodermic | Generalized erythema and scales; medical emergency |
| Pustular | Sterile pustules; can be generalized (von Zumbusch) |
| Nail psoriasis | Pitting, onycholysis, "oil spots" |
| Psoriatic arthritis | ~15% of patients; affects hands, feet, knees |
(Textbook of Family Medicine 9e; Goldman-Cecil Medicine)
Typical plaque psoriasis on legs: bilateral erythematous, well-demarcated plaques with silvery scales on extensor surfaces
6. Diagnosis
Diagnosis is primarily clinical (based on appearance and distribution). Investigations include:
- Skin biopsy - confirms histopathology (Munro microabscesses, acanthosis, parakeratosis)
- KOH preparation - to rule out fungal infection
- No specific blood test for diagnosis; ESR, CRP may be elevated
- PASI score (Psoriasis Area Severity Index) - used to grade severity for treatment decisions
7. Treatment (Pharmacology Focus)
Treatment follows a step-up approach based on severity:
Step 1 - Mild Psoriasis: Topical Agents
| Drug | Mechanism | Notes |
|---|
| Topical corticosteroids (e.g., clobetasol) | Reduce inflammation; suppress immune cells | First-line; 68-89% show clear improvement with ultrahigh-potency (clobetasol) |
| Calcipotriol/Calcipotriene (Vitamin D analog) | Inhibits keratinocyte proliferation; promotes differentiation | Comparable efficacy to potent steroids |
| Tazarotene (Retinoid) | Normalizes keratinocyte differentiation | Slightly more adverse effects |
| Combination (steroid + calcipotriol) | Additive anti-inflammatory + antiproliferative | Most promising topical approach; fewer side effects |
| Coal tar | Anti-inflammatory; reduces keratinocyte proliferation | Old but still used; smelly |
| Tacrolimus (calcineurin inhibitor) | Suppresses T-cell activation | Used for facial and inverse psoriasis |
Step 2 - Moderate-to-Severe: Systemic (Non-biologic)
| Drug | Mechanism | Key Notes |
|---|
| Methotrexate (MTX) | Antimetabolite (inhibits dihydrofolate reductase); antiproliferative and immunosuppressive | Oral, 5-15 mg/week; effective for widespread disease; hepatotoxic with long-term use |
| Acitretin (Oral retinoid) | Normalizes keratinocyte differentiation | Used for generalized/palmoplantar psoriasis not responding to topicals; teratogenic |
| Cyclosporine | Calcineurin inhibitor; selectively suppresses T lymphocytes (blocks IL-2 production) | Remarkably effective; nephrotoxic with long-term use |
| Narrow-band UVB phototherapy | Induces apoptosis of T cells; immunomodulatory | Safer and more effective than broadband UVB |
| PUVA (Psoralen + UVA) | Psoralen intercalates DNA; UVA activation inhibits cell proliferation | Effective but risk of photodamage and skin cancer |
Step 3 - Moderate-to-Severe: Biologic Agents (Targeted)
These are the most pharmacologically interesting drugs - they are monoclonal antibodies or fusion proteins that target specific cytokines in the pathogenic cascade:
Anti-TNF-α Agents:
| Drug | Class | Route |
|---|
| Etanercept | Human p75 TNF receptor-Fc fusion protein | SC |
| Adalimumab | Human IgG1 monoclonal antibody | SC |
| Infliximab | Chimeric IgG1 monoclonal antibody | IV |
| Certolizumab pegol | PEGylated humanized Fab fragment | SC |
Anti-IL-17 Agents (highly effective in psoriasis):
| Drug | Target | Route |
|---|
| Secukinumab | IL-17A | SC |
| Ixekizumab | IL-17A | SC |
| Brodalumab | IL-17 receptor A | SC |
| Bimekizumab | IL-17A and IL-17F | SC |
Anti-IL-12/23 and Anti-IL-23 Agents:
| Drug | Target | Notes |
|---|
| Ustekinumab | IL-12 and IL-23 (p40 subunit) | PASI75 achieved in >66% at 45 mg and >75% at 90 mg by week 12 |
| Guselkumab, Risankizumab, Tildrakizumab | IL-23 (p19 subunit) | Newer IL-23 specific agents |
(Goodman & Gilman's Pharmacology; Fitzpatrick's Dermatology)
8. Quick Summary Diagram
TRIGGER (trauma, infection, stress, drugs)
↓
Dendritic cell activation
↓
T cell activation → Th1 + Th17 cells
↓
Cytokines: TNF-α, IL-17, IL-23, IL-22, IFN-γ
↓
Keratinocyte hyperproliferation + Neutrophil recruitment
↓
PLAQUE: Thickened epidermis + Parakeratosis + Silvery scale + Erythema
↓
Clinical Signs: Auspitz sign, Koebner phenomenon
9. Key Points to Remember for Exams
- Plaque psoriasis = most common type (80-90%); silvery-white scales on erythematous plaques on extensor surfaces
- Key pathogenic cytokines: TNF-α, IL-17, IL-23 (these are the drug targets!)
- Histology hallmark: Munro microabscesses (neutrophils in stratum corneum) + acanthosis + parakeratosis
- Auspitz sign = pin-point bleeding on scale removal (due to papillary vessel remodeling)
- Koebner phenomenon = new lesions at trauma sites
- Keratinocyte turnover reduced from 28 days → 3-4 days
- Cyclosporine works by suppressing T cells - proved T cells are the central pathogenic cell
- Biologics work by blocking the cytokine cascade - each targets a specific step in pathogenesis
- Methotrexate 5-15 mg/week is the key systemic non-biologic for moderate-severe disease
- Teratogenic drugs in psoriasis: Acitretin (retinoid) - contraindicated in pregnancy
Sources: Fitzpatrick's Dermatology Vols. 1 & 2; Robbins, Cotran & Kumar Pathologic Basis of Disease; Goodman & Gilman's Pharmacological Basis of Therapeutics; Textbook of Family Medicine 9e; Goldman-Cecil Medicine