How Sunlight and Natural Compounds Are Joining Forces Against Cancer
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Introduction: When Light Meets Medicine
Imagine a drug that sits harmlessly in your body until you shine a specific color of light on a tumor. Turn the light on, and the drug activates—destroying a protein that keeps cancer cells in a dormant, treatment-resistant state. Turn the light off, and the drug becomes inert, leaving healthy tissues untouched.
This is not science fiction. In May 2026, researchers at ETH Zurich published a groundbreaking study in the Proceedings of the National Academy of Sciences demonstrating exactly this principle. They developed a molecule called KH-5-309—a light-activated “PROTAC” that degrades the glucocorticoid receptor (GR) in cancer cells.
Why the Glucocorticoid Receptor Matters in Cancer
The glucocorticoid receptor is a protein that responds to stress hormones like cortisol. When activated, it enters the nucleus of cells and changes gene expression. In cancer cells, GR activation triggers something dangerous: dormancy.
A dormant cancer cell:
- Stops dividing (cell cycle arrest)
- Becomes highly resistant to chemotherapy and radiation
- Can survive for years in this state
- Can reawaken later to cause metastatic relapse
This creates a cruel irony. Dexamethasone, a common anti-inflammatory drug given alongside chemotherapy to reduce side effects, activates GR. It may inadvertently promote dormancy in cancer cells, making treatment less effective.
But you cannot simply destroy GR everywhere in the body. GR has essential roles in immune function, metabolism, and stress response. Systemic GR destruction would cause devastating side effects.
How the PhotoPROTAC Works
The researchers solved this dilemma using photopharmacology—controlling drug activity with light.
KH-5-309 is a PROTAC (Proteolysis Targeting Chimera), a molecule with two ends:
- One end binds to the glucocorticoid receptor
- The other end recruits the cell’s protein-degradation machinery
When both ends bind simultaneously, the cell’s proteasome destroys GR.
The genius of KH-5-309 lies in its photoswitchable linker. The molecule changes shape when exposed to specific wavelengths:
- E-isomer (extended, active): Activated by 505-530 nm (green-yellow visible light). This form bridges GR and the degradation machinery, destroying the receptor.
- Z-isomer (bent, inactive): Activated by 365 nm (UVA light). This form is too short to bridge the two proteins, leaving GR intact.
By controlling the light, researchers control the drug. In lung cancer cells, the active E-isomer completely reversed GR-driven dormancy, restoring sensitivity to therapy. The inactive Z-isomer did nothing—providing a perfect control.
The Wavelengths Are in Sunlight
Here is where this connects to our series: 505-530 nm green-yellow light is naturally present in sunlight. So is 365 nm UVA.
This means that the principle of light-controlled cancer therapy is not purely artificial. The biological switches exist—and the light that flips them comes from the same spectrum as the sun.
But Here Is the Catch
KH-5-309 is not available to the public. It is an experimental synthetic molecule synthesized in a chemistry lab. It has never been tested in humans. It has no established safety profile. And you cannot get it from sunlight or any natural source.
This raises a critical question for our readers: Are there other substances—natural, accessible, and safe—that interact with sunlight to fight cancer?
The Answer Is Yes
The field of photodynamic therapy (PDT) has been using light-activated substances for decades. Many of these photosensitizers are derived from nature—from plants, fungi, and even vitamins.
In this article, we explore the remarkable world of natural photosensitizers. Some are already FDA-approved for cancer treatment. Some are found in your kitchen or supplement cabinet. Some are being studied in clinical trials.
While you cannot access KH-5-309 today, you can understand the principles behind light-activated medicine—and appreciate how nature has been providing us with light-sensitive molecules for millions of years.
I: What Is Photodynamic Therapy (PDT)?
Before we dive into specific substances, let us understand the basic mechanism.
The Three Components of PDT
Photodynamic therapy requires three elements:
- A photosensitizer (a light-activated substance)
- Light (specific wavelengths)
- Oxygen (present in all tissues)
When the photosensitizer is exposed to light of the correct wavelength, it absorbs the light energy and transfers it to oxygen molecules. This creates reactive oxygen species (ROS) —highly reactive molecules that damage cell membranes, proteins, and DNA.
Why Cancer Cells Are Vulnerable
Cancer cells are particularly susceptible to PDT because:
- They absorb photosensitizers more readily than healthy cells
- They have higher metabolic rates and more fragile membranes
- They are often poorly oxygenated (which actually makes them more vulnerable to certain ROS)
The Clinical Reality
PDT is already FDA-approved for several cancers, including:
- Non-small cell lung cancer
- Esophageal cancer
- Skin cancer (basal cell carcinoma, squamous cell carcinoma)
- Bladder cancer
- Head and neck cancers
It is also used for non-cancerous conditions like acne and macular degeneration.
II: Natural Photosensitizers from Plants and Fungi
Many of the most effective photosensitizers come from nature. Here are the most important ones.
1. Hypericin (St. John’s Wort)
Source: Hypericum perforatum (St. John’s Wort)
Wavelength: 590-660 nm (visible red light)
Mechanism: Hypericin is a potent photosensitizer that generates singlet oxygen and other reactive species upon light activation. It induces apoptosis (programmed cell death) in cancer cells.
Research Highlights:
- Effective against glioblastoma (brain cancer) in preclinical studies
- Shows activity against breast, colon, and lung cancer cells
- Has been studied for photodiagnosis (detecting cancer) and phototherapy
Natural Source Connection: St. John’s Wort is a common herbal supplement used for depression. However, its photosensitizing properties mean that taking it orally can cause severe sunburn. This is actually the same mechanism being harnessed for cancer therapy—but at much higher concentrations and with controlled light exposure.
Current Status: Clinical trials have been conducted for hypericin-based PDT in bladder cancer and glioblastoma. It is not yet FDA-approved as a cancer therapy, but it is a promising investigational agent.
2. Curcumin (Turmeric)
Source: Curcuma longa (turmeric root)
Wavelength: 420-485 nm (blue visible light)
Mechanism: Curcumin has intrinsic photosensitizing properties. Upon light activation, it produces reactive oxygen species that kill cancer cells. It also has anti-inflammatory and antioxidant properties that may enhance its anti-cancer effects.
Research Highlights:
- Effective against breast, lung, prostate, and skin cancer cells in vitro
- Enhances the effects of conventional chemotherapy
- Has been studied for photodynamic therapy in oral cancer
Natural Source Connection: Turmeric is a common spice in Indian cuisine and a popular supplement. However, curcumin is poorly absorbed orally, and its photosensitizing effects are typically studied in combination with light delivery directly to the tumor.
Current Status: Curcumin is widely available as a supplement and spice. However, using it for PDT requires careful dosing and light delivery. It is not FDA-approved as a cancer therapy, but ongoing clinical trials are exploring its potential.
Fascinating Fact: Curcumin’s bright yellow color is itself a consequence of its light-absorbing properties. The same molecular structure that gives turmeric its color also makes it a photosensitizer.
3. Psoralens (Parsley, Celery, Citrus Fruits)
Source: Found in several plants, including Psoralea corylifolia (babchi), parsley, celery, and citrus fruits
Wavelength: 320-360 nm (UVA)
Mechanism: Psoralens intercalate into DNA. When activated by UVA light, they form covalent bonds with DNA, cross-linking strands and inhibiting cell division. This is the basis of PUVA therapy (Psoralen + UVA), which is used for skin conditions and some skin cancers.
Research Highlights:
- Effective against cutaneous T-cell lymphoma
- Used for precancerous skin conditions (actinic keratosis)
- Has been studied for psoriasis and vitiligo
Natural Source Connection: Psoralens are found in many plants. Interestingly, consuming celery, parsley, or citrus fruits, followed by sun exposure, can trigger phytophotodermatitis—a skin reaction caused by psoralens in their sap. This is the same mechanism as PUVA therapy, but unintended and uncontrolled.
Current Status: PUVA is FDA-approved for skin conditions and early-stage cutaneous T-cell lymphoma. It is one of the oldest photodynamic therapies in clinical use.
Clinical Reality: PUVA is typically performed with synthetic psoralen (methoxsalen), not dietary psoralens. The dosing must be precisely controlled to avoid severe burns and long-term skin cancer risk.
4. Chlorophyll Derivatives (Green Plants and Spirulina)
Source: Chlorophyll from green plants, algae, and cyanobacteria (spirulina)
Wavelength: 660-690 nm (red light)
Mechanism: Chlorophyll derivatives, such as chlorin e6 and pheophorbide, are highly effective photosensitizers. They have strong absorption in the red/near-infrared region, which penetrates deeply into tissues.
Research Highlights:
- Effective against lung, liver, breast, and bladder cancers
- High selectivity for cancer cells
- Rapid clearance from the body, reducing side effects
Natural Source Connection: Chlorophyll is the pigment that gives plants their green color. When extracted and modified, it becomes a powerful photosensitizer. Green algae like spirulina are rich sources of chlorophyll.
Current Status: Several chlorophyll-based photosensitizers are FDA-approved or in clinical trials:
- Photofrin (porfimer sodium) – FDA-approved for esophageal and lung cancer
- ALA (aminolevulinic acid) – A precursor to a photosensitizer, FDA-approved for actinic keratosis and skin cancer
- Temoporfin (Foscan) – Approved in Europe for head and neck cancer
Fascinating Fact: The molecular structure of chlorophyll is remarkably similar to hemoglobin. The difference is that chlorophyll contains magnesium at its core, while hemoglobin contains iron. This similarity allows chlorophyll derivatives to be taken up by rapidly dividing cells, including cancer cells.
5. Riboflavin (Vitamin B2)
Source: Found in many foods (eggs, dairy, lean meats, green vegetables) and available as a supplement
Wavelength: 370-450 nm (UVA/blue light)
Mechanism: Riboflavin is a photosensitizer that generates reactive oxygen species upon light activation. It also has antioxidant properties that may enhance its selectivity for cancer cells.
Research Highlights:
- Effective against breast, cervical, and skin cancer cells in vitro
- May enhance the effectiveness of chemotherapy
- Has been studied for photodynamic therapy in oral cancer
Natural Source Connection: Riboflavin is a vitamin that everyone needs. It is already present in the body, making it a potentially safer photosensitizer than synthetic agents.
Current Status: Riboflavin has been studied as a photosensitizer for PDT, particularly for skin cancers and superficial tumors. It is not FDA-approved as a cancer therapy, but it is widely available as a dietary supplement.
Interesting Note: Riboflavin is also used in corneal cross-linking for keratoconus—a non-cancerous application of light-activated riboflavin. This demonstrates the broader utility of light-activated compounds beyond oncology.
6. Berberine (Chinese Herbal Medicine)
Source: Berberis species (barberry), Coptis chinensis (Chinese goldthread), Hydrastis canadensis (goldenseal)
Wavelength: 405-480 nm (blue light)
Mechanism: Berberine is an alkaloid with photosensitizing properties. It induces apoptosis through the mitochondrial pathway and may have synergistic effects with other anti-cancer agents.
Research Highlights:
- Effective against colorectal, liver, breast, and lung cancer cells
- Has antimicrobial and anti-inflammatory properties
- May enhance the effects of conventional PDT agents
Natural Source Connection: Berberine has been used in traditional Chinese medicine for centuries. It is a dietary supplement available in health food stores.
Current Status: Berberine is being studied as an adjunct to PDT and as a photosensitizer itself. It is not FDA-approved as a cancer therapy, but it is an active area of research.
III: A Comprehensive Table of Light-Activated Substances
| Substance | Source | Wavelength | Cancer Types | Status |
|---|---|---|---|---|
| Photofrin (Porfimer) | Synthetic (hematoporphyrin derivative) | 630 nm | Lung, esophageal, bladder | FDA-Approved |
| ALA (Aminolevulinic acid) | Natural precursor in heme synthesis | 635 nm | Skin, bladder, brain | FDA-Approved |
| Hypericin | St. John’s Wort | 590-660 nm | Bladder, brain, breast | Clinical Trials |
| Curcumin | Turmeric | 420-485 nm | Breast, lung, oral, skin | Preclinical / Clinical |
| Psoralens | Parsley, celery, citrus | 320-360 nm | Skin (lymphoma) | FDA-Approved (PUVA) |
| Chlorophyll Derivatives | Green plants, spirulina | 660-690 nm | Lung, liver, breast | FDA-Approved |
| Riboflavin (Vitamin B2) | Eggs, dairy, greens | 370-450 nm | Skin, cervical, breast | Preclinical |
| Berberine | Barberry, goldenseal | 405-480 nm | Colorectal, liver | Preclinical |
| Temoporfin (Foscan) | Synthetic | 652 nm | Head and neck | Approved (Europe) |
| Verteporfin (Visudyne) | Synthetic (benzoporphyrin) | 690 nm | Macular degeneration, cancer off-label | FDA-Approved |
| Hypocrellin | Hypocrella bambusae (fungus) | 490-570 nm | Various | Preclinical |
| Pheophorbide | Chlorophyll derivative | 660-690 nm | Lung, liver, breast | Clinical Trials |
IV: How Natural Photosensitizers Compare to Synthetic PROTACs
The Scientific Distinction
It is important to distinguish between the photoPROTAC (E-KH-5-309) discussed in the previous article and the photosensitizers discussed here.
| Feature | PhotoPROTAC (KH-5-309) | Photosensitizers (PDT) |
|---|---|---|
| Mechanism | Destroys a specific protein (GR) | Produces reactive oxygen species that kill cells |
| Target | Molecular (protein degradation) | Cellular (membrane damage, apoptosis) |
| Specificity | High (targets a specific protein) | Moderate (targets cancer cells but can affect healthy tissue) |
| Availability | Not available to the public | Some are FDA-approved or available as supplements |
| Status | Preclinical | Clinical (decades of use) |
The Takeaway
The key distinction is this:
- Photosensitizers (PDT) are clinically available for certain cancers. They are used in hospitals with medical supervision.
- PhotoPROTACs are experimental. They are a glimpse into the future, not a present option.
- Dietary photosensitizers (curcumin, riboflavin, hypericin) are available as supplements but should NOT be used for self-treatment of cancer. Their photosensitizing effects can cause severe burns if combined with sunlight in an uncontrolled way.
V: Practical Advice for Readers
What You Should NOT Do
Do NOT:
- Take hypericin (St. John’s Wort) and sunbathe thinking you are treating cancer
- Apply turmeric to your skin and stand in the sun
- Use any photosensitizer without medical supervision
- Assume that natural equals safe—phytophotodermatitis is real and painful
What You CAN Do
DO:
- Discuss PDT with your oncologist if you have a cancer that may be treated this way
- Learn about the science—understanding these mechanisms is fascinating and empowering
- Consider the role of sensible sun exposure in general health, as discussed in our series
- Be aware that dietary sources of photosensitizers (like curcumin, riboflavin, and hypericin) are safe at normal dietary levels—it is concentrated therapeutic use that requires caution
The Medical Reality
PDT is a hospital-based therapy. It involves:
- Administering the photosensitizer (usually intravenously or topically)
- Waiting for the agent to concentrate in the tumor (hours to days)
- Delivering light to the tumor (via laser, endoscope, or external light source)
- Monitoring for side effects (pain, swelling, photosensitivity)
This is not something you can do at home, and it is not something you should attempt.
VI: The Bigger Picture—Sunlight as a Therapeutic Partner
A Unified Theory of Light and Health
Across our entire series, we have seen that sunlight works through multiple, independent pathways to promote health:
| Mechanism | Substance/Process | Wavelength | Status |
|---|---|---|---|
| Vitamin D synthesis | Endogenous (in skin) | UVB | Clinically established |
| Nitric oxide release | Endogenous (in skin) | UVA | Clinically established |
| Circadian regulation | Endogenous (via eyes) | Blue light | Clinically established |
| Photosensitizer activation | Natural or synthetic drugs | UVA, visible, red | Clinically established (PDT) |
| Protein degradation | PhotoPROTACs | Visible | Preclinical |
Why This Matters
The reason this is so exciting—and the reason we have dedicated an entire series to it—is that light is not a single, simple thing. It is a spectrum of wavelengths, each with unique biological effects. And these effects can be harnessed with or without the help of substances.
The key to the future of light-based medicine lies in matching:
- The right wavelength to
- The right substance to
- The right disease at
- The right time
Conclusion: The Future Is Bright—But Requires Patience
The landscape of light-activated cancer therapies is vast and growing rapidly. From ancient herbal remedies to cutting-edge synthetic molecules, researchers are discovering that the intersection of light and chemistry holds immense therapeutic promise.
But for the average reader, the message remains:
- Do not self-treat with photosensitizers. Sunlight + herbal supplements can cause severe burns and skin damage.
- Do talk to your doctor if you are interested in PDT—it is a real, established therapy for certain cancers.
- Do appreciate the science. Understanding these mechanisms helps us appreciate why sensible sun exposure is beneficial and why light-based therapies are an exciting frontier in medicine.
- Do stay curious. The photoPROTACs of today will become the medicines of tomorrow. And many of those medicines will be based on molecules that nature has been perfecting for millions of years.
Key Takeaways
- The ETH Zurich photoPROTAC study demonstrated that light-activated molecules can destroy the glucocorticoid receptor in cancer cells, reversing dormancy and restoring treatment sensitivity
- KH-5-309 is not available to the public—it is an experimental synthetic compound, not a supplement or natural substance
- Natural photosensitizers include hypericin (St. John’s Wort), curcumin (turmeric), psoralens (parsley, celery, citrus), chlorophyll derivatives, riboflavin (vitamin B2), and berberine
- FDA-approved photosensitizers include Photofrin (lung, esophageal), ALA (skin), and Verteporfin (non-cancer uses, with off-label cancer applications)
- The same natural substances that heal can also harm—uncontrolled light activation can cause severe burns, as seen in phytophotodermatitis
- Do not self-treat with photosensitizers—PDT is a hospital-based therapy requiring precise dosing, controlled light delivery, and medical supervision
- The principles of photomedicine are expanding—from established PDT to emerging photoPROTACs, the future is bright
Quick Reference: Natural Photosensitizers in Everyday Life
| Substance | Found In | Light Activation | Potential Use |
|---|---|---|---|
| Hypericin | St. John’s Wort (herbal supplement) | Sunlight (UV/visible) | Experimental anti-cancer |
| Curcumin | Turmeric (spice and supplement) | Blue light | Investigational |
| Psoralens | Celery, parsley, citrus (eating or sap contact) | Sunlight (UVA) | Can cause burns (phytophotodermatitis) |
| Chlorophyll derivatives | Green plants, spirulina | Red light | Clinically used in PDT |
| Riboflavin | Eggs, dairy, greens (dietary) | UV/blue light | Investigational |
| Berberine | Barberry, goldenseal (supplement) | Blue light | Investigational |
A Word of Caution: Phytophotodermatitis
One of the most common ways people unintentionally experience photosensitization is through phytophotodermatitis—a chemical reaction that occurs when plant sap containing psoralens is exposed to sunlight.
Common culprits:
- Wild parsnip
- Giant hogweed
- Celery
- Parsley
- Citrus fruits (especially lime)
- Fig tree sap
Symptoms: Redness, blistering, and hyperpigmentation (darkening) of the skin—sometimes lasting for months.
Takeaway: Natural photosensitizers are powerful. They can heal (in controlled medical settings) or harm (in uncontrolled exposure). Respect the power of light-activated substances, and never use them for self-treatment.
A Final Thought
“Nature is the master chemist. Some of her most powerful molecules are those that dance with light. Our task is not to improve on nature, but to understand her language—so that we can use light and molecules together, precisely and safely, to heal what nature has injured.”
This article is informed by research in photodynamic therapy, photopharmacology, and natural product chemistry. Specific sources include Freitag et al. (2026) on photoPROTACs, clinical guidelines on PDT from the American Cancer Society and the National Cancer Institute, and research on natural photosensitizers from the Journal of Photochemistry and Photobiology.
Don’t Get Sick!
About Dr. Jesse Santiano, MD
Dr. Santiano is a retired internist and emergency physician with extensive clinical experience in metabolic health, cardiovascular prevention, and lifestyle medicine. He reviews all medical content on this site to ensure accuracy, clarity, and safe application for readers. This article is for educational purposes and is not a substitute for personal medical care.
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