Photodynamic Therapy

Pros and Cons of PDT

Studies have shown that PDT can work as well as surgery or radiation therapy in treating certain kinds of cancers and pre-cancers. It has several advantages over traditional treatments:

• No long-term side effects when used properly.
• Less invasive than surgery.
• Short procedure, most often done outpatient.
• Can be targeted very precisely.
• Unlike radiation, PDT can be repeated many times on the same site if needed.
• Little to no scarring after the site heals.
• Costs less than traditional cancer treatments.

But PDT has limits too:

• PDT can only treat areas where light can reach. This means it’s mainly used to treat problems on or just under the skin, or in the lining of organs that can be reached with a light source. Because light can’t travel very far through body tissues, PDT can’t be used to treat large cancers or cancers that have grown deeply into the skin or other organs.
• PDT can’t be used to treat cancers that have spread to multiple places in the body.
• The drugs used for PDT leave people very sensitive to light for some time, so special precautions must be taken after the drugs are applied to the body.
• PDT can’t be used on people who have certain blood diseases, such as any of the porphyrias (a rare group of diseases that affect the skin or nervous system) or people who are allergic to porphyrins. This allergy is rare, but it may happen in those who have gotten porphyrins in the past.

Photosensitizer Properties

The ideal PDT photosensitizer has the following characteristics:

1. available in pure form, of known chemical composition;
2. synthesizable from available precursors and easily reproduced;
3. high singlet oxygen quantum yield (ΦΔ);
4. strong absorption in the red region of the visible spectrum (λ_max = 680–800 nm) with a high extinction coefficient (ε_max = 50,000–100,000 M⁻¹ cm⁻¹);
5. effective accumulation in tumor tissue and possession of low dark toxicity for both photosensitizer and its metabolites;
6. stable and soluble in the body’s tissue fluids, and easy delivery to the body via injection or other methods;
7. excreted from the body upon completion of treatment.

Properties of some photosensitizer dyes approved for PDT treatment and used in PDT-related clinical trials:

Examples of non-porphyrin PDT candidates. None have received FDA approval for their application areas: