Gold (Au) has long captivated human interest, but its interaction with the human body is complex. The body’s response varies dramatically depending on the element’s chemical state and reactivity. Elemental gold, gold compounds, and gold in nanoscale form each interact uniquely with biological systems. Understanding these distinct forms is crucial for grasping how gold can be used for everything from inert fillings to active medical treatments.
Elemental Gold and Its Inert Role
Pure, elemental gold, referred to as Au(0), is characterized by its remarkable chemical stability and resistance to oxidation. This form is largely non-reactive within the biological environment of the human body, meaning it does not readily dissolve or participate in biochemical reactions. Due to this inert nature, the body’s digestive system cannot break down or absorb solid elemental gold, and it will typically pass through unchanged if ingested.
This lack of reactivity makes elemental gold an ideal material for long-term placement inside the body, where it must coexist with tissues without causing irritation or degradation. Common medical and dental applications rely on this property, including the use of gold alloys for crowns, fillings, and certain surgical implants. When used in jewelry or as dental work, the gold remains a stable foreign body that does not release ions or trigger a systemic immune response. Gold is considered biocompatible, meaning it does not cause adverse biological effects when in direct contact with living tissue.
Therapeutic Uses of Gold Compounds
In contrast to the inert metal, specific gold compounds, or gold salts, are highly reactive and have been employed in medicine for decades, a practice historically known as chrysotherapy. These compounds, such as auranofin or gold sodium thiomalate, contain gold in an ionic state, typically Au(I), which makes them biologically active. The therapeutic effect stems from the gold ion’s ability to react with sulfur-containing groups (thiols) found in various proteins and enzymes within the body.
The primary historical use for these gold salts was as a disease-modifying anti-rheumatic drug (DMARD) for treating chronic inflammatory conditions, most notably Rheumatoid Arthritis (RA). The gold compounds interfere with the immune system’s inflammatory cascade by affecting immune cells like macrophages. Once taken up by these cells, gold is stored in compartments called aureosomes, where it inhibits the processing and presentation of antigens.
Gold compounds can also suppress the activation of key inflammatory pathways, such as the NF-kappa B pathway. This subsequently reduces the production of pro-inflammatory signaling molecules, including cytokines like tumor necrosis factor-alpha (TNF-alpha) and interleukins (IL-1 and IL-6). This action leads to a reduction in joint inflammation and long-term joint damage. Although newer biologic drugs have become prominent, gold compounds provided one of the first effective systemic treatments that actively modulated the disease process.
Modern Applications of Gold Nanoparticles
A third, distinct interaction involves gold in its nanoscale form, known as gold nanoparticles (AuNPs). These particles typically measure between 1 and 100 nanometers and exhibit unique optical and electronic properties due to their size, differing fundamentally from both bulk gold and gold compounds. This has made them promising tools in modern biomedical engineering for both diagnosis and therapy.
One significant application is in diagnostic imaging, where AuNPs are used as contrast agents. Their ability to strongly scatter light, a phenomenon called localized surface-plasmon resonance, allows them to be visualized with high contrast in imaging techniques. This makes them valuable for tracking biological processes or highlighting specific tissues, such as tumors, for a clearer view.
In therapeutic applications, AuNPs serve as sophisticated delivery vehicles for drugs, including chemotherapy agents. Drugs are attached to the nanoparticle surface, which is engineered to target specific cells, such as cancer cells, minimizing systemic exposure and side effects. The ability to tune the nanoparticle’s size and surface chemistry, often by bonding with amino acids like L-cysteine, is crucial for optimizing their performance and ensuring stability in the bloodstream.
Processing and Potential Toxicity
When absorbable forms of gold, such as therapeutic compounds, enter the body, they undergo specific processes of absorption and elimination. Gold salts administered orally, like auranofin, typically show poor absorption rates, while injectable forms are absorbed more readily. Once absorbed, gold ions travel through the bloodstream, often binding to plasma proteins like albumin.
The body eliminates gold primarily through the kidneys and, to a lesser extent, the liver. However, the clearance process for gold is notably slow, leading to the potential for accumulation in certain organs over time. For instance, gold nanoparticles have been observed to accumulate in the liver and spleen, where they can remain for a significant period after administration.
While elemental gold is generally safe, the biological activity that makes gold compounds effective drugs also makes them capable of causing toxicity. Side effects from chrysotherapy can be significant, ranging from skin rashes and dermatitis to more severe issues like kidney damage (nephrotoxicity) or blood disorders. This toxicity is directly related to the gold ion’s high reactivity with biological molecules.