Melanin injections found to highlight and hinder tumors

The pigment melanin is a natural defense against skin cancer – and now it could help fight other cancers too. Researchers at the Technical University of Munich and Helmholtz Zentrum München have found that melanin-loaded nanoparticles can help diagnose tumors and slow their growth.

It’s a basic rule of physics that dark colors absorb more light, and as a result heat up. Melanin uses that to our advantage, preventing the Sun’s UV light from burning our skin by dissipating the energy as heat.

Being such a great absorber of light and producer of heat, melanin is perfect for use in optoacoustics. As the name suggests, this technique is a mix of light and sound – first, weak laser pulses are shone onto tissue to gently heat it and make it expand slightly. As the tissue cools and contracts again, it produces ultrasound waves. Specialized sensors can read those waves and figure out what type of tissue is there – including, importantly, whether or not it’s cancerous. At the same time, the heat given off makes tumors extra visible in infrared.

To test how well melanin performed in this kind of situation, the team loaded the pigment into nanoparticles known as outer membrane vesicles (OMVs). These were then injected directly into tumors in mice, and heated gently with infrared laser pulses.

Because these nanoparticles tend to gather in cancer cells, they can help scientists image tumors, and may even play a role in treating them. And sure enough, the researchers found that the tumors stood out much more clearly in mice that had been injected with melanin, compared to those that had just received plain OMVs.

“Melanin absorbs light very readily – even in the infrared spectrum,” says Vipul Gujrati, first author of the study. “We use precisely this light in our optoacoustic imaging technique for tumor diagnosis. It simultaneously converts this absorbed energy into heat, which is then emitted. Heat is also a way to combat tumors – other researchers are currently exploring this method in clinical trials.”

The team also tested the technique in photothermal therapy, where stronger laser pulses are used to heat the target more intensely, with the aim of killing them. OMVs loaded with melanin raised the tumor temperature from the usual 37° C (98.6° F) to a sweltering 56° C (132.8° F), which is far more impressive than regular OMVs could manage, topping out at 39° C (102.2° F). As an added bonus, the treatment also triggered a slight inflammation, which alerted the immune system to help out.

Other approaches have had similar results using gold nanoparticles or nanotubes, but they’re not the cheapest materials out there. But for the new study, the researchers showed that bacteria can be engineered to not only produce both OMVs and melanin, but even package the latter inside the former, ready for use.