Boron

Boron

Introduction

Boron has the atomic number 5 and the symbol B, and is a so-called metalloid (see Chapter 4). Boron compounds have been known for many centuries and especially used in the production of glass. Boric acid [B(OH)₃] is used in the large-scale production of glass. Borosilicate glasses (Pyrex^® glass), which are pro-duced by a fusion of B₂O₃ and silicate, are extremely heat resistant and often used in laboratories.

At the beginning of the nineteenth century, it was recognised that boron is an essential micronutrient for plants. A deficiency of boron can lead to deformation in the vegetable growth such as hollow stems and hearts. Furthermore, the plant growth is reduced and fertility can be affected. In general, boron deficiency leads to qualitative and quantitative reduction in the production of the crop. Boron is typically available to plants as boric acid [B(OH)₃] or borate [B(OH)₄]⁻. The exact role of boron in plants is not understood, but there is evidence that it is involved in pectin cross-linking in primary cell walls, which is essential for normal growth and development of higher plants.

Borax (Na₂[B₄O₅(OH)₄]⋅8H₂O) can be applied as a fertiliser and, together with kernite (Na₂[B₄O₅(OH)₄]⋅2H₂O), forms the two most commercially available borates. Borates find a wide range of practical applications such as in detergents, cosmetics, antifungal mixtures as well as components in fibreglass and others. 

The toxicity of borates in mammals is relatively low, but it exhibits a significantly higher risk to arthropods and can be used as an insecticide.

Boron-based compounds are used in a wide range of clinical applications including their use as antifun-gal and antimicrobial agent, as proteasome inhibitors and as a noninvasive treatment option for malignant tumours. The latter application will be discussed in the chapter on radiopharmaceuticals (Chapter 10).

Pharmaceutical applications of boric acid

Boric acid is a long-standing traditional remedy with mainly antifungal and antimicrobial effects. For medic-inal uses, it has become known as sal sedativum, which was discovered by Homberg, the Dutch natural philosopher, in 1702 . Diluted solutions were and sometimes still are used as antiseptics for the treatment of athletes’ foot and bacterial thrush, and in much diluted solutions as eyewash (Figure 4.3) .

Boric acid can be prepared by reacting borax with a mineral acid:

Na₂B₄O₇ ⋅ 10H₂O + 2HCl → 4B(OH)₃[or H₃BO₃] + 2NaCl + 5H₂O

In general, there are many other health claims around the clinical use of boric acid and boron-containing compounds, but many of those have no supporting clinical evidence.

Bortezomib

Bortezomib belong to the class of drugs called proteasome inhibitors and is licensed in the United States and the United Kingdom for the treatment of multiple myeloma. The drug has been licensed for patients in whom the myeloma has progressed despite prior treatment or where a bone marrow transplant is not possible or was not successful. It is marketed under the name Velcade^® or Cytomib^®. Velcade is administered via injection and is sold as powder for reconstitution (Figure 4.4) .

Bortezomib was the first drug approved in the new drug class of proteasome inhibitors and boron seems to be its active element. For the mode of action, it is believed that the boron atom binds with high affinity and specificity to the catalytic site of 26S proteasome and inhibits its action. Therapy with Bortezomib can lead to a variety of adverse reactions, including peripheral neuropathy, myelosuppression, renal impairment and gastrointestinal (GI) disturbances together with changes in taste. Nevertheless, the side effects are in most cases less severe than with alternative treatment options such as bone marrow transplantation .