The physics behind killing cancer with light (photodynamic therapy) | with Stephen Bown

Overview

Stephen Bown details the development and applications of photodynamic therapy (PDT), a cancer treatment combining a photosensitizing drug with light. Originating from 19th-century heliotherapy and early experiments with acridine orange and paramecia, PDT evolved through the work of Thomas Dougherty with hematoporphyrin derivative (HPD) in the 1970s. Bown highlights PDT's selective destruction of cancerous cells while preserving tissue scaffolding, its use in treating skin cancers, internal organs like the bladder and esophagus, and its potential in immunotherapy and infection control.

Key takeaways

Photodynamic therapy (PDT) relies on the synergistic effect of a photosensitizing drug and specific light wavelengths to generate cytotoxic reactive oxygen species, primarily singlet oxygen.
PDT's selectivity allows it to target diseased tissue (e.g., cancer cells) while largely sparing surrounding healthy tissue, leading to improved outcomes and reduced side effects compared to traditional treatments.
The development of flexible endoscopes and precise laser delivery systems has enabled PDT to be applied to internal organs, including the bladder, esophagus, and lungs, for treating precancerous lesions and recurrent tumors.
PDT is not limited to cancer treatment; it shows promise in areas like age-related macular degeneration (AMD), infection control (e.g., MRSA), and potentially even metabolic disorders like type 2 diabetes.
The evolution of photosensitizers, from early compounds like HPD to more targeted agents like ALA, has improved PDT's efficacy and safety by controlling drug distribution and light activation depth.
Future directions for PDT include its integration with immunotherapy to harness the body's immune response against cancer and the development of simpler, more accessible light delivery systems for remote or resource-limited settings.

Chapters

0:00 Historical Roots of Light Therapy

Heliotherapy, or sun treatment, has been used for centuries for health and well-being.
19th-century sanatoriums in Davos promoted light and air therapy.
Niels Ryberg Finsen received a Nobel Prize for using ultraviolet light to treat tuberculosis.

3:50 The Discovery of Photodynamic Therapy's Principle

A medical student in Munich observed that acridine orange dye combined with light killed paramecia.
This effect required the combination of a chemical (dye) and light, with oxygen present.
This marked the birth of the concept of photodynamic therapy (PDT).

8:36 Thomas Dougherty and the Development of HPD

Thomas Dougherty at Roswell Park Memorial Cancer Institute developed hematoporphyrin derivative (HPD) as a photosensitizer.
HPD was selectively taken up by cancer cells in mice.
PDT utilizes drug, light, and oxygen to kill cells, with singlet oxygen as the cytotoxic agent.

12:22 Challenges and Applications in Normal Tissues

Early research focused on the mechanism of PDT, with less consideration for normal tissue response.
A key challenge was understanding PDT's effect on normal tissues to ensure selective cancer treatment.
PDT's ability to destroy cancer cells while preserving tissue scaffolding leads to good cosmetic results.

13:28 PDT for Skin Cancers

PDT is effectively used for skin cancers like actinic keratosis.
Photosensitizer is applied as a cream, followed by light exposure.
PDT offers excellent cosmetic results by preserving the underlying tissue structure.

17:30 Endoscopic Delivery of PDT for Internal Organs

Delivering PDT internally requires getting the photosensitizer to the target area and applying the correct light wavelength.
Early endoscopes were rigid and limited to straight passages like the bladder.
Modern flexible endoscopes with integrated light sources and fiber optics enable internal PDT.

22:15 The Role of Lasers in PDT

Lasers provide intense, single-color light beams that can be precisely matched to photosensitizer absorption peaks.
Laser light can be delivered through thin, flexible fibers, even through endoscope channels.
For solid tissues, light delivery can be achieved by inserting laser fibers via needles under image guidance.

27:17 PDT Applications in Hollow Organs: Bladder and Lungs

PDT can treat lesions in the mucosa (top layer) of hollow organs like the bladder and esophagus, preserving underlying layers.
Photodiagnosis using blue light can highlight abnormal areas (red fluorescence) in the bladder.
PDT is used for recurrent lung tumors in patients unfit for surgery, avoiding cumulative toxicity.

42:25 PDT in the Mouth and Esophagus

PDT effectively treats small lip lesions, with the laser light (red) activating the photosensitizer.
In India, PDT is explored for mouth cancer, a common issue due to chewing carcinogenic agents.
PDT is approved for precancerous conditions in the esophagus, treating dysplasia caused by acid reflux.

1:01:50 Advanced Photosensitizers and Diabetes Research

Newer photosensitizers like ALA concentrate in the mucosa, allowing for selective superficial treatment.
Research explores PDT for type 2 diabetes by ablating the duodenal wall's surface layer to affect sugar metabolism.
This approach aims to achieve metabolic effects non-surgically, with early experimental success in rats.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, The Royal Institution.