D-Cysteine Shows Promise in Slowing Cancer Growth with Minimal Side Effects

Researchers have identified a unique approach to targeting cancer cells – a “mirror” molecule of the amino acid cysteine that selectively disrupts their metabolism whereas leaving healthy cells largely unharmed. This discovery, published in Nature Metabolism, offers a potential pathway toward more precise cancer therapies with fewer side effects, a long-standing goal in oncology.

The Challenge of Selective Cancer Treatment

Traditional cancer treatments, like chemotherapy and radiation, often work by targeting rapidly dividing cells. While effective at killing cancer cells, these methods as well damage healthy cells that divide quickly, such as those in the hair follicles, digestive system, and bone marrow. This non-selectivity leads to the debilitating side effects commonly associated with cancer treatment. The search for therapies that can distinguish between cancerous and healthy cells has been a central focus of cancer research for decades.

D-Cysteine: A Mirror Image with a Targeted Effect

Amino acids are the fundamental building blocks of proteins, essential for all life. There are 20 amino acids commonly used by living organisms, and each exists in two forms: L (levorotatory) and D (dextrorotatory). These forms are mirror images of each other, much like a left and right hand. While the human body almost exclusively utilizes L-amino acids for protein synthesis, the D-forms are rarely used. Researchers at the Universities of Geneva (UNIGE) and Marburg have discovered that this difference can be exploited to selectively target cancer cells.

The team, led by Jean-Claude Martinou, Honorary Professor in the Department of Molecular and Cellular Biology at the UNIGE Faculty of Science, focused on D-cysteine (D-Cys), the “mirror” form of cysteine. Their experiments demonstrated that D-Cys significantly slowed the growth of certain cancer cells in laboratory settings without affecting healthy cells. “This difference between cancer cells and healthy cells is easily explained: D-Cys is imported into cells via a specific transporter that is present only on the surface of certain cancer cells,” explains Joséphine Zangari, a PhD student in Professor Martinou’s laboratory and first author of the study. “In fact, we observed that if we express this transporter on the surface of healthy cells, those cells stop proliferating in the presence of D-Cys.”

How D-Cysteine Disrupts Cancer Cell Metabolism

The mechanism behind D-Cys’s selective toxicity lies in its ability to disrupt critical metabolic processes within cancer cells. Researchers, collaborating with Professor Roland Lill and his team at the University of Marburg, found that D-Cys inhibits an essential enzyme called NFS1, located within the mitochondria – the cell’s “powerhouses”. NFS1 is crucial for producing iron-sulfur clusters, little structures vital for numerous cellular functions, including cellular respiration, DNA and RNA production, and maintaining the integrity of the genome.

By blocking NFS1, D-Cys effectively halts the production of these iron-sulfur clusters, leading to a cascade of detrimental effects within the cancer cell. Respiration slows down, DNA damage accumulates, and the cell cycle comes to a standstill, ultimately preventing the cells from growing and dividing. This disruption of fundamental cellular processes is particularly impactful for cancer cells, which often rely heavily on efficient metabolism to fuel their rapid proliferation.

Promising Results in Animal Models

To assess the potential of this approach in a living organism, the researchers tested D-Cys in mice implanted with aggressive, triple-negative breast cancer tumors – a particularly challenging type of cancer to treat. The results were encouraging. Tumor growth slowed significantly in the mice treated with D-Cys, and the animals did not exhibit major side effects. This suggests that D-Cys could offer a more targeted and less toxic treatment option compared to conventional therapies.

The study, detailed in a publication in PubMed, also notes that the team has filed a patent application (EP 21 769 366.2 and US 18/022,824) on the effects of d-Cys in cancer, indicating a commitment to further development and potential commercialization.

The Role of xCT/CD98 Transporter

A key factor in D-Cys’s selectivity is the xCT/CD98 transporter. Many cancer cells overexpress this transporter, which facilitates the uptake of cystine, an amino acid precursor to cysteine. However, xCT/CD98 preferentially imports the D-enantiomer (D-Cys) over the L-enantiomer. This selective uptake allows D-Cys to accumulate within cancer cells, triggering the metabolic disruption described above. Cancers that overexpress xCT/CD98 are therefore most likely to be susceptible to D-Cys treatment.

What Comes Next: From Lab to Clinic

While the results are promising, significant hurdles remain before D-Cys can become a viable cancer therapy. “This is a very positive signal — we now know it’s possible to exploit this specificity to target certain cancer cells,” says Jean-Claude Martinou. “However, we still need to determine whether D-Cys could be administered at effective doses in humans without causing harm.”

Further research is needed to evaluate the safety and efficacy of D-Cys in human clinical trials. Researchers will need to determine the optimal dosage, administration route, and potential side effects. They will also need to identify which types of cancers are most likely to respond to D-Cys treatment based on their xCT/CD98 expression levels. The University of Geneva notes that the team is actively pursuing these investigations.

If successful, D-Cys could represent a relatively simple, selective, and innovative therapy for cancers that overexpress xCT/CD98. It may also offer a strategy to prevent metastasis, the spread of cancer to other parts of the body, a critical step in cancer progression. The potential for a targeted therapy with reduced side effects offers a beacon of hope for future cancer treatment strategies.