Good evening students Department of Pharmacology. SDL Topic (Museum visit) PH9.6 Describe drugs used in various skin disorders like acne vulgaris, scabies, pediculosis, psoriasis including sunscreens To be submitted on 20/07/2026.
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PH 9.6 — Drugs Used in Skin Disorders
SDL Topic | Department of Pharmacology | Submission: 20/07/2026
1. ACNE VULGARIS
Pathophysiology (Brief)
Acne vulgaris is a self-limited disorder primarily of teenagers and young adults. The key events are:
- Increased sebum production (triggered by androgens at puberty)
- Follicular hyperkeratosis leading to comedone formation
- Proliferation of Cutibacterium acnes (formerly Propionibacterium acnes) within comedones
- Release of free fatty acids - inflammation - rupture of cyst wall - inflammatory papules, pustules, nodules
The clinical hallmark is the comedone - closed (whitehead) or open (blackhead).
Drugs Used in Acne Vulgaris
A. TOPICAL AGENTS
| Drug | Mechanism | Use |
|---|---|---|
| Retinoic acid (Tretinoin) | Normalizes follicular desquamation, prevents comedone formation | Comedonal & mild inflammatory acne |
| Benzoyl Peroxide | Antibacterial (releases free radical oxygen), keratolytic | Mild to moderate acne; prevents antibiotic resistance |
| Salicylic Acid | Keratolytic, promotes comedone resolution | Mild comedonal acne |
| Clindamycin (topical) | Inhibits bacterial protein synthesis | Inflammatory acne - always combine with benzoyl peroxide |
| Erythromycin (topical) | Inhibits bacterial protein synthesis | Inflammatory acne - always combine with benzoyl peroxide |
| Azelaic Acid | Antibacterial + anti-inflammatory + reduces hyperpigmentation | Mild-moderate acne, post-inflammatory pigmentation |
| Dapsone (topical) | Anti-inflammatory, antibacterial | Inflammatory acne in adults |
| Clascoterone cream | Topical antiandrogen receptor antagonist (blocks DHT at sebaceous gland) | FDA approved; useful where systemic antiandrogens are not desired |
Important Note: Topical antibiotics (erythromycin, clindamycin) must ALWAYS be combined with benzoyl peroxide to prevent development of bacterial resistance.
B. SYSTEMIC AGENTS
Antibiotics (for moderate to severe inflammatory acne):
- Doxycycline 100 mg twice daily (or extended-release formulations) - tetracycline class; has anti-inflammatory effects independent of antibacterial action
- Minocycline 100 mg twice daily - similar profile
- Adequate response expected at 3 months
Hormonal Therapy (females only):
- Combined Oral Contraceptives (OCP) - Several are FDA-approved for acne (suppress androgen-driven sebum)
- Spironolactone - Antiandrogen; safe, effective, and durable treatment in women
Retinoids - Systemic:
- Isotretinoin (13-cis-retinoic acid) - Reserved for severe nodulocystic acne unresponsive to other treatments
- Mechanism: Reduces sebaceous gland activity, normalizes keratinization, anti-inflammatory
- Dosing: Weight-based and cumulative
- Adverse effects: Dry skin, cheilitis (very common); teratogenicity (most critical - category X)
- Monitoring: Two negative pregnancy tests before initiation, monthly pregnancy tests during therapy, liver function tests, lipid profile
- Prescribers must enroll in a risk management program (iPLEDGE in USA)
2. SCABIES
Causative Agent
Sarcoptes scabiei var. hominis - a pearl-like, translucent, oval mite (0.4 × 0.3 mm) belonging to class Arachnida. The female mite burrows into the stratum corneum, lives its entire life cycle within the epidermis.
Clinical Features
- Intensely pruritic eruption (nocturnal pruritus is characteristic)
- Incubation period: 4-6 weeks
- Characteristic linear burrows in web spaces of fingers, wrists, genitalia
- Transmitted by close physical contact
Drugs for Scabies
| Drug | Dose/Regimen | Key Notes |
|---|---|---|
| Permethrin 5% cream | Apply to entire body (neck down) for 8-14 hours, then wash off; repeat in 7 days | Drug of choice (DOC); Pregnancy category B; synthetic pyrethroid - disrupts Na+ channel of nerve cell membrane of parasite |
| Ivermectin (oral) | 200 µg/kg on Days 1 and 8; for crusted scabies: Days 1, 2, 8, 9, and 15 | Highly effective; avermectin class; activates glutamate-gated Cl- channels causing hyperpolarization of nerve/muscle of parasite; not for <15 kg weight or pregnant/lactating |
| Crotamiton 10% cream | Apply for 8 hours on Days 1, 2, 3, and 8 | Has antipruritic qualities; effectiveness is marginal |
| Precipitated Sulfur 5-10% | Apply for 8 hours on Days 1, 2, and 3 | Considered safe in neonates and during pregnancy; limited efficacy data; inexpensive |
| Benzyl Benzoate 10% lotion | Apply for 24 hours | Not available in the United States |
| Lindane 1% lotion | Apply for 8 hours, repeat in 7 days | FDA "black box" warning - neurotoxicity; banned in California; avoid in children, elderly, <50 kg |
Crusted (Norwegian) scabies: Immunocompromised patients (HIV, leprosy) harbor millions of mites. Requires more intensive regimen - daily permethrin for 7 days, then twice weekly; plus oral ivermectin.
General Measures:
- All family members and close contacts must be treated simultaneously (even if asymptomatic)
- Wash clothing and bed sheets at 60°C
- Items that cannot be washed - seal in plastic bag for 2 weeks
- Pruritus may persist for weeks after successful treatment (hypersensitivity reaction to dead mites) - treat with oral antihistamines and emollients
3. PEDICULOSIS (Lice Infestation)
Types
- Pediculosis capitis (head lice) - Pediculus humanus capitis
- Pediculosis corporis (body lice) - Pediculus humanus humanus
- Pediculosis pubis (pubic/crab lice) - Phthirus pubis
Clinical Features
- Head lice: Pruritus of scalp; nits found in occipital and retroauricular regions; nits firmly attached to hair shaft (unlike dandruff)
- Body lice: Pruritus, linear excoriations on back, neck, shoulders; lice live in clothing seams; transmits typhus, trench fever, relapsing fever
- Pubic lice: Intense pruritus in pubic area; transmitted sexually; may also infest eyebrows, eyelashes
Drugs for Pediculosis
| Drug | Regimen | Notes |
|---|---|---|
| Permethrin 1% cream rinse | Apply to damp hair, leave 10 min, rinse; repeat at day 9 | DOC for head lice; ovicidal activity; safe in children >2 months |
| Malathion 0.5% lotion | Apply to dry hair for 8-12 hours, rinse; repeat in 7-9 days if needed | Organophosphate; cholinesterase inhibitor; effective against both lice and nits; flammable |
| Ivermectin (oral) | 200-400 µg/kg single dose, repeat in 7-10 days | Alternative for resistant cases |
| Ivermectin 0.5% lotion (topical) | Apply to dry hair, leave 10 min, rinse; single application | FDA approved for head lice |
| Spinosad 0.9% suspension | Apply to dry hair for 10 min, rinse; may repeat in 7 days | Derived from soil bacteria; pediculocidal + ovicidal |
| Benzyl Alcohol 5% lotion | Apply to dry hair for 10 min, rinse; repeat in 7 days | Kills lice by asphyxiation (blocks spiracles); NOT ovicidal - must repeat |
| Lindane 1% shampoo | Apply for 4 minutes, rinse | Second-line only; neurotoxicity risk |
| Pyrethrins + piperonyl butoxide (OTC) | Apply to dry hair for 10 min, rinse | Piperonyl butoxide inhibits louse enzymes that degrade pyrethrin |
For pubic lice: Permethrin 1% cream, malathion lotion, or pyrethrins applied to affected area. Sexual partners must be treated.
For eyelash involvement (phthiriasis palpebrarum): Petroleum jelly (Vaseline) applied twice daily for 8 days - physically suffocates lice; or 1% yellow mercuric oxide ophthalmic ointment.
Nit removal: Fine-toothed "nit comb" used after treatment to physically remove dead nits.
4. PSORIASIS
Pathophysiology (Brief)
Psoriasis is an immune-mediated chronic inflammatory skin disease. The key cytokines are TNF-α, IL-17A, IL-12, IL-23. There is excessive keratinocyte proliferation (turnover reduced from 28 days to 3-4 days), resulting in the hallmark silvery-scaled plaques over an erythematous base (Auspitz sign - pinpoint bleeding on scale removal).
Drugs Used in Psoriasis
A. TOPICAL AGENTS (for mild-moderate psoriasis)
| Drug | Mechanism | Notes |
|---|---|---|
| Topical Corticosteroids | Anti-inflammatory, antiproliferative | Mainstay of topical therapy; high-potency on body, low-potency on face/flexures |
| Vitamin D Analogues (Calcipotriol/Calcipotriene, Calcitriol) | Inhibit keratinocyte proliferation; promote differentiation; immunomodulation | Effective alone or in combination with steroids |
| Coal Tar | Inhibits excessive skin cell proliferation; anti-inflammatory | Keratolytic; cosmetically unappealing so low compliance; largely supplanted by newer agents |
| Salicylic Acid | Keratolytic - removes scales | Often combined with steroids or coal tar; improves penetration |
| Tazarotene (topical retinoid) | Normalizes keratinocyte differentiation | May cause irritation |
| Dithranol (Anthralin) | Inhibits DNA synthesis in keratinocytes, anti-inflammatory | Stains clothing; short contact therapy used |
B. PHOTOTHERAPY (for moderate-severe psoriasis)
| Modality | Details |
|---|---|
| Narrowband UVB (NB-UVB) | 311 nm wavelength; now preferred over broadband UVB; suppresses T-cell activity in skin |
| PUVA (Psoralen + UVA) | Oral/topical methoxsalen (psoralen) + UVA irradiation; psoralen intercalates with DNA, forms cyclobutane adducts with pyrimidine bases after UVA activation, inhibiting DNA synthesis; risks: cataracts, skin cancer |
C. SYSTEMIC AGENTS (for moderate-severe psoriasis)
Methotrexate:
- Most commonly used systemic agent for psoriasis
- Mechanism: Immunosuppressive - reduces DNA synthesis in T lymphocytes (dihydrofolate reductase inhibitor)
- Available in oral and injectable forms
- Adverse effects: Nausea, diarrhea, mouth ulcers, hair loss, hepatotoxicity (periodic LFTs required), bone marrow suppression, teratogenicity
- Folic acid supplementation reduces side effects
Acitretin (Systemic Retinoid):
- Oral retinoid used for severe psoriasis including pustular and erythrodermic psoriasis
- Dose: 25-50 mg/day for 2-4 months
- Adverse effects: Mucocutaneous dryness (cheilitis, dry eyes), hyperlipidemia, hepatotoxicity, teratogenicity (contraception required for 3 years after stopping)
Cyclosporin:
- Calcineurin inhibitor; suppresses T-cell activation
- Used for rapid response in severe psoriasis
- Adverse effects: Nephrotoxicity, hypertension, increased infection risk
Apremilast:
- Oral phosphodiesterase-4 (PDE-4) inhibitor
- Increases intracellular cAMP → reduces production of TNF-α, IL-17, IL-23, and other inflammatory mediators
- Common adverse effects: Diarrhea, nausea, headache, depression
- Avoid with strong CYP3A4 inducers (carbamazepine, phenytoin)
D. BIOLOGIC AGENTS (for moderate-severe psoriasis - injectable antibody-based proteins)
| Class | Drugs | Target |
|---|---|---|
| Anti-TNF-α | Etanercept, Infliximab, Adalimumab, Certolizumab pegol, Golimumab | TNF-α |
| Anti-IL-12/IL-23 | Ustekinumab | IL-12 and IL-23 (p40 subunit) |
| Anti-IL-23 | Guselkumab, Risankizumab | IL-23 (p19 subunit) |
| Anti-IL-17A | Secukinumab, Ixekizumab, Brodalumab | IL-17A pathway |
Shared adverse effects of biologics: Injection site reactions, infusion reactions, increased susceptibility to infections (especially TB - screen before initiating), risk of anti-drug antibody development.
5. SUNSCREENS
Why Sunscreens are Needed
UV radiation causes direct DNA damage (UVB - 280-320 nm) and indirect damage via reactive oxygen species (UVA - 320-400 nm). UVB causes sunburn and skin cancer; UVA penetrates deeper, causing photoaging, pigmentation, and contributes to skin cancer. UVA also worsens conditions like polymorphous light eruption, cutaneous lupus, and drug-induced photosensitivity.
Classification of Sunscreens
I. Chemical (Organic) Sunscreens - absorb UV radiation
| Class | Examples | Spectrum |
|---|---|---|
| PABA and esters | p-Aminobenzoic acid, padimate O | UVB (290-320 nm) |
| Benzophenones | Oxybenzone, dioxybenzone, sulisobenzone | UVB + shorter UVA (250-360 nm) |
| Dibenzoylmethanes | Parsol 1789 (avobenzone), Eusolex | Longer UVA (up to 360-400 nm) |
| Cinnamates | Octyl methoxycinnamate (octinoxate) | UVB |
| Salicylates | Octyl salicylate, homosalate | UVB |
| Ecamsule (Mexoryl) | Ecamsule | UVA - superior to dibenzoylmethanes; more photostable |
II. Physical (Inorganic) Sunscreens - reflect/scatter UV radiation
- Zinc Oxide and Titanium Dioxide - broad-spectrum; reflect both UVA and UVB; considered safe for infants, children, and pregnant women; "reef-safe" (unlike oxybenzone/octinoxate which damage coral reefs)
Sun Protection Factor (SPF)
SPF = Minimum Erythema Dose (MED) WITH sunscreen / MED WITHOUT sunscreen
- SPF primarily measures UVB protection
- FDA limits label claims to SPF 50+ (insufficient data beyond this)
- "Broad spectrum" label requires passing standardized UVA/UVB ratio test
- Products with SPF ≥ 15 labeled broad-spectrum can claim protection against skin cancer and early skin aging
- SPF 2-14: can only claim protection against sunburn
Practical Guidelines for Sunscreen Use (from Dermatology textbooks)
- Use SPF ≥ 30 broad-spectrum sunscreen
- Apply 15 minutes before sun exposure
- Use 1-2 ounces to cover exposed skin in an adult
- Reapply every 2 hours or after swimming/sweating
- Use water-resistant formulation if swimming or perspiring
- Avoid direct sun exposure between 10 AM and 4 PM
- Not approved for infants < 6 months of age
Special Notes
- Reef protection: Oxybenzone and octinoxate are harmful to coral reefs; zinc oxide and titanium dioxide are "reef-safe"
- Percutaneous absorption: Recent FDA studies have shown systemic absorption of several chemical sunscreen ingredients - ongoing safety evaluation
- Sunscreen is the "last line of defense" - physical measures (hats, clothing, shade) come first
SUMMARY TABLE
| Condition | First-Line Drug | Alternatives |
|---|---|---|
| Acne - mild (comedonal) | Topical tretinoin ± benzoyl peroxide | Salicylic acid |
| Acne - mild-moderate (inflammatory) | Topical clindamycin/erythromycin + benzoyl peroxide | Azelaic acid, dapsone |
| Acne - moderate-severe | Doxycycline/minocycline (oral) | Spironolactone (females), OCP |
| Acne - severe nodulocystic | Oral isotretinoin | - |
| Scabies | Permethrin 5% cream | Oral ivermectin, crotamiton |
| Scabies - neonates/pregnancy | Precipitated sulfur 5-10% | - |
| Pediculosis capitis | Permethrin 1% cream rinse | Malathion, spinosad, ivermectin |
| Psoriasis - mild | Topical steroids + vitamin D analogue | Coal tar, salicylic acid |
| Psoriasis - moderate-severe | NB-UVB / PUVA + methotrexate | Cyclosporin, acitretin |
| Psoriasis - severe/refractory | Biologics (anti-TNF, anti-IL-17, anti-IL-23) | Apremilast |
| Sunscreen | SPF ≥ 30 broad-spectrum (zinc oxide / titanium dioxide for children/pregnancy) | - |
Sources: Harrison's Principles of Internal Medicine 22E (2025); Katzung's Basic and Clinical Pharmacology 16th Ed.; Lippincott Illustrated Reviews: Pharmacology; Fitzpatrick's Dermatology 9th Ed.; Dermatology 2-Volume Set 5e (Elsevier).
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