Biology and toxicology of lanthanoids

Biology and toxicology of lanthanoids

Mostly, lanthanoids are used in the production of batteries, lasers and other technological devices. Some lanthanoids salts, such as the salts of lanthanum, cerium and gadolinium (highlighted in Figure 9.1), are increasingly used in a clinical setting, for example, as a phosphate binder in the treatment of renal osteodys-trophy or as MRI (magnetic resonance imaging) contrast agents (CAs).

Lanthanoids (Ln) show a biological behaviour very similar to that of Ca²⁺, as they have similar ionic radii. Lanthanoids are mostly trivalent and therefore possess a higher charge than Ca²⁺. Lanthanoids display a high binding affinity to calcium-binding sites in biological molecules and to water molecules. The coordination number for lanthanoids varies from 6 to 12. Mostly, eight or nine water molecules are coordinated to the lanthanoid ion. This is a significantly lower coordination number compared to that of calcium, which is 6.

Within the human body, lanthanoid ions are known to block the receptor-operated calcium channels. Lanthanoids cannot cross the cell membrane, but they still block the Na⁺/Ca²⁺ synaptic plasma membrane exchange and therefore inhibit muscle contraction (e.g. in skeletal muscle or cardiac muscle). Lanthanoids can also displace calcium in proteins and enzymes, which can either lead to inhibition or activation of their catalytic activity. In general, lanthanoids mimic the biological behaviour of calcium ions and as a result lanthanoids can be used to study the mode of action of calcium ions in a variety of biological applications.

It is very interesting to look at the toxicity of lanthanoids. In general, lanthanoids are not regarded as toxic, as they cannot cross the cell membrane and therefore are not absorbed if administered orally. Lanthanoids are toxic if they are administered intravenously, as they can then interact with a variety of biological targets. A sudden decrease in blood pressure and sudden cardiovascular complications are signs of acute toxicity. Chronic toxicity manifests itself in liver damage and oedema. After intravenous administration, lanthanoids are often quickly distributed to the liver and the bones .