Treating Arthritis with Rapamycin-Releasing Nanoparticles

This approach reverses macrophage polarization.

Researchers have described how specialized nanoparticles, which deliver rapamycin and the antioxidant astaxanthin, restore macrophage balance and reduce inflammation in a mouse model of osteoarthritis.

A matter of polarization

Inflammation is a critical part of arthritis. Previous studies have found that inflammation of the membranes in joints (synovitis) strongly contributes to osteoarthritis; in fact, even without mechanical joint damage, synovitis can lead to this crippling disease.

Macrophage polarization has a profound effect on inflammation. M1-polarized macrophages are responsible for generating inflammation, such as interleukins and TNF-α, in response to threats; the M2 polarization is responsible for the healing and repair afterwards, and it can do so in joints. However, in arthritis and many other age-related diseases, the inflammatory state sticks around. This is known as inflammaging.

Delivering the anti-inflammatories

Astaxanthin is a naturally occurring antioxidant and anti-inflammatory compound that is commonly used for treating inflammatory diseases. The Interventions Testing Program (ITP) has found that astaxanthin extends the lives of healthy mice.

Rapamycin needs little introduction, as it is one of the most well-known longevity-related compounds and has also been found to extend the lives of mice in the ITP. Among its many metabolic effects, rapamycin has been reported to spur the cellular maintenance process of autophagy and subsequently reduce inflammation.

We have previously reported on a study in which extracellular vesicles are only released from a hydrogel in the presence of reactive oxygen species (ROS). These researchers have followed a somewhat similar approach, creating nanoparticles that enter cells and burst open only in the presence of ROS to deliver a payload containing astaxathin and rapamycin. The exterior filaments of the nanoparticle themselves neutralize ROS, the astaxathin is attached to those filaments, and the rapamycin is clustered inside.

Effects in macrophages

Further testing revealed that this nanoparticle is taken up by macrophages and is effective inside them. The researchers used lipopolysaccharide to irritate macrophages into the M1 state, then applied their new compound and its payload. Althouth this nanoparticle is toxic to macrophages in higher doses, the combined payload of nanoparticle and rapamycin was found to reduce ROS to a small fraction of that of an M1 control group.

Similarly, inflammatory interleukins were reduced by this novel nanoparticle. The inflammation in macrophages also affects their own function. Levels of naturally occurring antioxidants were increased by the treatment, markers of autophagy were improved, and the membrane potential of mitochondria was restored. Overall, biomarkers of inflammation were significantly ameliorated in the targeted macrophages.

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