Photodynamic Therapy

Photodynamic Therapy - Light, Drug, Action!

Photodynamic therapy (PDT) is a non-invasive treatment using a photosensitizer, specific wavelength light, and oxygen to selectively destroy target cells.

• Key Components (📌 POL):

  • Photosensitizer (PS): Drug that absorbs light.
  • Light: Activates the PS, typically laser or LED.
  • Oxygen ($O_2$): Essential for cytotoxic reactions.

• Mechanism:

  • PS absorbs light, enters an excited state.
  • Energy transfer to $O_2$ generates Reactive Oxygen Species (ROS).
  • Type I reaction: Produces superoxide, hydroxyl radicals.
  • Type II reaction: Predominantly generates cytotoxic singlet oxygen ($^1O_2$). The key reaction is $O_2 + h\nu \rightarrow ^1O_2$.
  • ROS cause cellular damage and apoptosis/necrosis.

⭐ PDT's efficacy largely stems from Type II photoreactions generating cytotoxic singlet oxygen.

Photodynamic Therapy - Sensitizers & Spectra

• Photosensitizers: Absorb light, produce cytotoxic reactive oxygen species.
  • 1st Generation: e.g., Porfimer sodium.
  • 2nd Generation: Improved properties (e.g., ALA, MAL, Verteporfin).

    ⭐ 5-ALA is a prodrug converted intracellularly to Protoporphyrin IX (PpIX), the active photosensitizer.

-   **Lasers:** Monochromatic (e.g., Diode **630-690 nm**).
-   **LEDs:** Narrowband (Blue **~417 nm** for superficial ALA; Red **~633 nm** for ALA/MAL).
-   **IPL/Broadband Lamps:** Filtered to specific wavelengths. 

Photodynamic Therapy - Skin Savers

PDT: Photosensitizer (ALA, MAL) + Light + $O_2$ $\rightarrow$ Reactive Oxygen Species ($O_2^-$) $\rightarrow$ Selective cell kill.

📌 Key Indication Mnemonic: PDT Acts on Bad Cells (Actinic Keratosis, Basal Cell Carcinoma, Bowen's Disease).

Key Indications:

• Acne vulgaris (ALA; Blue/Red light)
• Photorejuvenation (ALA; IPL, Red/Yellow light)
• Warts (various agents)
• Cutaneous leishmaniasis

⭐ PDT is FDA-approved for actinic keratoses and often preferred for widespread lesions due to field treatment capability.

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Photodynamic Therapy - Zap & Aftercare

• Mechanism: Photosensitizer + Light + $O_2$ $\rightarrow$ Reactive Oxygen Species (ROS) $\rightarrow$ Selective cell kill.
• Advantages: Non-invasive, targeted therapy, excellent cosmesis, repeatable, effective for field cancerization (multiple lesions).
• Disadvantages: Significant pain/burning during illumination, post-procedure photosensitivity (sun avoidance 24-48 hours), higher cost vs. some alternatives, limited tissue penetration depth (superficial lesions).

  ⭐ Pain during PDT illumination is a significant limiting factor, often managed with cooling, analgesia, or interruptions.

• Common Side Effects & Aftercare:
  • Strict sun/bright light avoidance for 24-48 hours is crucial.
  • Expected: Erythema, edema, crusting, peeling, pustules, localized pain/itching. Manage with cool compresses, emollients.
• Contraindications: Porphyria (absolute), known allergy to photosensitizer or its components, pregnancy (relative).

High‑Yield Points - ⚡ Biggest Takeaways

• PDT mechanism: Photosensitizer + light + oxygen → singlet oxygen → selective cell death.
• Photosensitizers: ALA & MAL (prodrugs) convert to Protoporphyrin IX (PpIX), the active compound.
• Light: Blue light (superficial lesions like AKs), Red light (deeper lesions like sBCC).
• Primary uses: Actinic keratosis, Bowen's disease, superficial BCC; also for acne.
• Benefits: Non-invasive, good cosmesis, treats field cancerization.
• Adverse effects: Pain during procedure, photosensitivity, erythema, edema post-treatment.
• Avoid in: Porphyrias.