PDT Basics - Light, Drug, Action!
- Core principle: A non-toxic photosensitizer (PS) drug, activated by light of a specific wavelength, interacts with molecular oxygen ($O_2$) to produce cytotoxic Reactive Oxygen Species (ROS).
- 📌 P.L.O.T. Mnemonic: Photosensitizer, Light, Oxygen, Target tissue.
- Mechanism:
- PS administration & accumulation in target tissue.
- Illumination with specific wavelength light → PS activation.
- Energy transfer to $O_2$ → generates ROS (e.g., singlet oxygen $^1O_2$).
- ROS induce cellular necrosis/apoptosis & vascular damage.
⭐ The efficacy of PDT relies on the triad of a photosensitizer, light of a specific wavelength, and molecular oxygen.
The Magic Bullets - Photosensitizers Deep Dive
Photosensitizers: The 'magic bullets' in PDT, activated by specific light wavelengths to induce cell death.
⭐ Aminolevulinic acid (ALA) and Methyl aminolevulinate (MAL) are pro-drugs metabolized intracellularly to Protoporphyrin IX (PpIX), the active photosensitizer.
| Photosensitizer | Route | Pro-drug | Active Form | λ (nm) | Indications | Incubation |
|---|---|---|---|---|---|---|
| ALA | Topical | Y | PpIX | 400-450, 630-635 | AK, sBCC, Bowen's | 3-6h |
| MAL | Topical | Y | PpIX | 630-635 | AK, sBCC, Bowen's | 3h |
| Verteporfin | IV | N | Verteporfin | 680-690 | AMD, Ocular tumors | 15 min |
| Porfimer Sodium | IV | N | Porfimer | 630 | Esophageal Ca, Lung Ca | 24-72h |
- 📌 Mnemonic: ALA & MAL are Pro-drugs to PpIX. Think "A Lousy Mole Proliferates Patiently" (Active Lesions Metabolized to Protoporphyrin IX).
Shining a Light - Sources & Techniques
Light source choice is critical: wavelength dictates penetration; fluence rate impacts treatment time.
| Source | λ (nm) | Coherence | Fluence Rate | Cost | PDT Uses |
|---|---|---|---|---|---|
| Lasers | Specific (e.g., 630) | High | High | High | Precise, deep (BCC) |
| LEDs | Narrowband (e.g., 417, 630) | Low | Moderate | Mod | Superficial (AK, acne), large areas |
| IPL | Broadband (500-1200) filt. | Low | High pulses | High | Less common for PDT, some rejuvenation |
| Lamps | Broad spectrum | Low | Low-Mod | Low | Superficial, large (historical) |
⭐ Red light (approx. 630-700 nm) penetrates deeper, used for deeper lesions. Blue light (approx. 400-450 nm) is superficial.
Skin Savers - PDT in Derma Action
- PDT: Photosensitizer + Light + Oxygen → Selective cell kill. (📌 POL: Photosensitizer, Oxygen, Light)
- Photosensitizers: 5-ALA (Aminolevulinic acid), MAL (Methyl aminolevulinate).
- Mechanism: Light activates photosensitizer → Type I (free radicals) & Type II (singlet oxygen $^1O_2$) reactions → apoptosis, necrosis.
Key Dermatological Indications for PDT:
| Condition | Photosensitizer | Light Parameters (nm, J/cm²) | Efficacy/Clearance Rates (%) |
|---|---|---|---|
| Actinic Keratosis (AK) | ALA, MAL | Blue (417 nm), Red (630-635 nm); 10-20 J/cm² | 70-90% |
| Superficial BCC (sBCC) | MAL | Red (630-635 nm); 37 J/cm² (2 sessions) | 70-90% |
| Bowen's Disease | ALA, MAL | Red (630-635 nm) | 80-90% |
| Acne Vulgaris | ALA | Blue (415 nm), Red (630 nm) | Variable, moderate improvement |
| Photoaging | ALA, MAL | Various (IPL, Red/Blue light) | Improves texture, fine lines |
The PDT Journey - Care & Caveats
- Patient Care:
- Pre-treatment: Counselling, ensure no photosensitizing drugs.
- During: Manage burning/stinging; cooling methods.
- Post-treatment: Strict sun protection (📌 S.P.F. - Sun Protection Factor), cool compresses, analgesics. Photosensitivity lasts 24-72 hours depending on sensitizer.
- Advantages:
- Non-invasive, excellent cosmesis.
- Selective tissue destruction.
- Repeatable; often outpatient.
- Caveats (Disadvantages):
- Significant pain/burning during illumination.
- Prolonged photosensitivity.
- Localized erythema, edema, pustules, crusting.
- Not for deep, large, or metastatic lesions.
⭐ Strict photoprotection for 24-72 hours post-PDT is crucial to prevent severe phototoxic reactions, depending on the photosensitizer used.
High‑Yield Points - ⚡ Biggest Takeaways
- PDT Triad: Photosensitizer, light, and oxygen interact to produce cytotoxic ROS.
- Pro-drugs: ALA and MAL are converted to protoporphyrin IX (PpIX), the active photosensitizer.
- Primary uses: Actinic keratoses (AKs), superficial BCCs, and Bowen's disease.
- Light activation: Specific wavelengths of light (often red light from LEDs) activate the sensitizer.
- Key adverse effect: Burning pain during illumination; transient photosensitivity post-procedure.
- Process: Sensitizer application, incubation period (drug uptake), then illumination.
- Advantages: Non-invasive, excellent cosmetic outcome, and lesion-specific targeting.
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