Introduction

The German chemist Winkler,1,2 working at the Freiberg Academy of Mines, was able to isolate during the investigation of the then-discovered mineral argyrodite (Ag6GeS5, containing 5 to 7% germanium) a new element in its free state. He called the new element germanium in honour of his motherland.

Germanium (atomic number 32, atomic weight 72.59, melting point 937.4, boiling point 2830 °C), a greyish-white lustrous crystalline metal, belongs to family 14 of periodic table (formerly called group IVA), so that its physical and chemical properties resemble those of the non-metal silicon and, to a lesser extent, tin. Germanium is usually classified as semi-metal and has semiconductor properties. In nature, germanium is widely, albeit sparsely, distributed and constitutes approximately 7 ppm of the earth's crust. It is associated with sulfide ores of other elements, particularly with those of copper, zinc, lead, tin and antimony.3 Germanium is highly concentrated in some coals at about 500 ppm. The highest reservoirs, worldwide, of germanium are found in Zaire with concentrations reaching 1000 ppm. The concentration of germanium in

Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine Edited by Gielen and Tiekink © 2005 John Wiley & Sons, Ltd sea water was estimated at 0.05 mg l_1. Germanium is generally found in the +4 oxidation state as the oxide or in solution as germanic acid, Ge(OH)4.

Up to the middle of the twentieth century, organogermanium derivatives were the least understood among the analogous compounds of the silicon subgroup elements. The first organogermanium compound, i.e. tetraethylger-mane, was synthesized for the first time by Winkler in 1887 by the reaction of tetrachlorogermane and diethylzinc.4,5 Its properties were consistent with those predicted by D. Mendeleev for 'ekasilicon'.

The major use of germanium is as an optical material (60% infrared systems, 8% fibre optics, 9% 7-ray, X-ray and infrared detectors), 10% semiconductors, transistors, diodes and rectifiers, 13% other applications (catalysts, metallurgy and chemotherapy).6

Edible plant foods usually contain less than 1 mgg1 although some plants used for Chinese herbs, medicines and vegetables such as ginseng, garlic, oats, soya beans, Shiitake mushroom, aloe, litchi contain appreciably higher amounts of germanium.7,8 Germanium is very well absorbed after ingestion (>90%). Germanium in plants is present in organic forms with Ge—O bonds, but the actual chemical structure of these germanium compounds is not known.9 It has been hypothesized that germanium plays an important role in the photo-electrochemical process of photosynthesis, the metabolism and self-defence (protection from invading viruses) processes of these germanium-bearing plants.7,8 These questions regarding the role of germanium in plant metabolism and protection are undoubtedly an important research topic requiring investigation.

The mean concentrations of germanium in normal human tissues are: lymph node: 0.9mgkg-1; skeletal muscle: 3.0mgkg-1; liver: 0.04 mgkg-1; lung: 0.09mgkg_1; brain: 0.1mgkg_1; blood: 0.2mgkg_1; testes: 0.5mgkg_1; kidney: 9.0mgkg_1 wet weight.9

In the 1970s, dietary germanium supplements became very popular due to the alleged therapeutic value of germanium (stimulation of iron consumption and haemoglobin production). In 1994, the first organogermanium pharmaceutical propagermanium was launched in Japan under the trade name Serocion® (Sanwa Kagaku Kenkyusho Co. Ltd). Its biological spectrum of activity includes protection against viruses, immunostimulation and hepatoprotection. Propagerma-nium, belonging to the class of germsesquioxanes, has low toxicity.

This achievement has stimulated further investigations of the biological activity not only of germsesquioxanes but also of other classes of low-toxic organogermanium compounds.

Today, numerous organogermanium compounds possessing anti-tumour, immunomodulating, interferon-inducing, radioprotective, hypotensive and neurotropic properties have been synthesized. The most intensively investigated organogermanium compounds are germsesquioxanes of general formula [(GeCHR1CHR2COOH)2O3]n (R1, R2 = H, alkyl, aryl, hetaryl); 1, R1 = R2 = H 2-carboxyethylgermanium sesquioxide, repargermanium, rexagermanium, proxigermanium, propagermanium, Serocion®; 2 2-(3-dimethylaminopropyl)-

8,8-diethyl-2-aza-8-germaspiro[4,5]decane, spirogermanium; 3, germatranes; 4, germylporphyrines; 5, monomeric and air-stable decaphenylgermanocene; 6, germanium-modified triazoles and many of germyl-substituted heterocycles (mainly derivatives of furan, thiophene, isoxazoline and uracil).

(GeCH2CH2COOH)A

2-Carboxyethylgermanium sesquioxide (anti-viral, immunomodulation, anti-tumour, anti-arthritic)

Spirogermanium (anti-tumour, anti-malarial, hepatoprotection)

2-Carboxyethylgermanium sesquioxide (anti-viral, immunomodulation, anti-tumour, anti-arthritic)

Spirogermanium (anti-tumour, anti-malarial, hepatoprotection)

Several reviews covering the biological properties of organogermanium compounds have appeared owing to the numerous biological investigations on these compounds.7,8,10 22

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