1. Introduction
Currently, germanium is widely recognized as a vital trace element, in particular it is essential for the normal functioning of the immune system and playing a significant role in cancer prevention [
1,
2,
3,
4,
5,
6,
7]. Germanium is ubiquitously present in mammalian organs and tissues, with the highest concentration in the thymus. Germanium normalizes many physiological functions, particularly blood characteristics including pH, glucose, minerals, cholesterol, uric acid, hemoglobin and leukocytes [
8,
9]. Conversely, germanium deficiency can result in numerous diseases, primarily oncogenic conditions [
10]. Research has revealed that cancer patients exhibit anomalously low concentrations of germanium in their blood serum [
7,
11,
12]. Additionally, germanium levels in cancerous tissues are significantly lower than in adjacent healthy tissues [
13].
Germanium is primarily introduced into the body through the consumption of vegetable-based foods with an average daily human dose of only 0.4-1.5 mg [
14,
15]. Research on the determination of this element in plant raw materials unexpectedly revealed an elevated content in plants and mushrooms, traditionally used in ethnoscience, particularly, in China [
7,
16,
17,
18]. Germanium compounds in natural sources have long been considered a therapeutic agent with anticancer, antitumor, antiviral and anti-inflammatory effects [
19]. Thus, the highest germanium concentrations are contained in ginseng, saprophyte mushrooms, in particular, lacquered polypore (
Ganoderma lucidum) and chaga, as well as in garlic, aloe and echinacea [
20,
21,
22,
23,
24,
25]. Among these, ginseng and
Ganoderma lucidum are widely used in complex therapy of oncological diseases [
26,
27,
28,
29,
30]. Germanium compounds have been shown to normalize the oxygen respiration of cells, which can retard the growth of tumors [
26,
31,
32,
33]. Restoring cell oxygen respiration is key to treating Warburg-like cancers [
33]. The stimulating effect of germanium on oxidizing enzymes such as aldehyde reductase [
34], has also been established. Hence, germanium-containing drugs have long attracted the attention of researchers and medical practitioners.
Antitumor activity of inorganic germanium compounds was first detected in 1928 [
35]. However, the field only began to intensively develop in the 1970s, when the first water-soluble organic germanium compounds were synthesized, gaining attention due to their wide range of biological activities. This topic has been addressed in several reviews [
1,
5,
6,
8,
24,
36,
37,
38,
39,
40,
41,
42,
43,
44] as well as a monograph [
7].
This review specifically focuses on the latest research conducted within the past decade, during which inorganic and coordination compounds of germanium have been incorporated into medical practices alongside water-soluble organic germanium compounds [
3,
45,
46].
Moreover, the toxicity of germanium compounds has been the subject of much controversy and confusion, and the discovery history of stable water-soluble germanium compounds has been significantly distorted. Therefore, the initial focus of this review is to elucidate the tangle of errors, inaccuracies, and myths associated with germanium.
At the end of this review, the authors propose a putative mechanism for germanium-mediated cancer treatment and prevention based on the unique chemical properties of germanium.
2. Historical digression and toxicity of germanium compounds
The chemical element number 32 was predicted by D.I. Mendeleev in 1871, and later, in 1886, was discovered by C. Winkler, who named it after his homeland Germany.
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Germanium has had a tumultuous history since its discovery over half a century ago. Initially, it remained an inaccessible chemical element that did not garner much scientific attention. It was not until 1948 when the first semiconductor transistors and diodes were created using germanium that it gained significance in the field of microelectronics. However, this element as a semiconductor was soon replaced by silicon and was again forgotten. In the 1970s, biological activity of discovered stable organic germanium water-soluble compounds [
36] attracted the attention of scientists, among which bis(carboxyethylgermanium) sesquioxide (Ge-132) was most famous. However, in the late 1980s, interest in such compounds declined sharply as a result of an ongoing discussion about the allegedly anomalously high toxicity of organic germanium compounds (similar to organic mercury compounds). Unfortunately, the interest in such compounds declined sharply in the late 1980s due to a typo in an article published in 1987 in an inaccessible journal, which listed erroneous toxicity values for Ge-132 [
6,
32,
33,
47]. This mistake was not immediately noticed and led to erroneous criticism in subsequent publications issued in highly influential scientific journals. The correction was only published in 1988, but until recently, many authors quoted only secondary sources that cited the erroneous data about the high toxicity of organic germanium compounds. The situation was further aggravated by a barbaric experiment conducted in Japan to determine the lethal dose of Ge-132 for humans. The experiment involved the consumption of an astronomical dose of 328 g of germanium, which is not used in medical practice [
32,
48,
49,
50]. The result of this experiment showed that the toxicity of Ge-132 was due to the formation and precipitation of solid germanium dioxide (GeO
2) in the renal pelvis [
48,
49,
50]. The therapeutic doses of organic germanium derivatives are thousands of times less than this lethal dose. The situation was further exacerbated by cases of germanium poisoning in individuals suffering from severe diseases, who took Ge-132 for a long time with a huge excess of the recommended daily dose values without the recommendation of a doctor. These individuals consumed Ge-132 in total quantities from 15 to 300 grams over a period of up to three years or more (see review [
50]).
It is evident that in the instances mentioned, high doses of Ge-132 resulted in toxic effects due to its hydrolysis in the body to form solid GeO
2 [
15]. However, it is now known that such poisoning, even with extremely high doses of germanium, can be successfully treated with combined blood purification therapy [
51].
The occurrence of these tragic events led to various controversial political decisions concerning organic germanium. Specifically, Ge-132 was banned in several countries, despite being universally allowed as a dietary supplement as early as the 1980s. This resulted in long-term neglect of research on the biological activity of Ge-132, particularly its anticancer properties. Ultimately, this denial of the role of germanium in wildlife was based on erroneous toxicity data, published in influential journals. The combination of typographical errors and reliance on secondary sources of information led to the neglect of the potential clinical use of compounds of this unique microelement. These events have also delayed the study of biological activity of germanium compounds, as noted in reviews [
6,
47]. To date, many influential journals continue to reject work related to the physiological activity of germanium compounds. It is now time to rectify this situation, and restore justice by rehabilitating germanium and its biochemical role.
As of now, low toxicity Ge-132 has been established [
40,
52,
53,
54]. In fact, the toxicity of organic germanium compounds [
55,
56,
57,
58,
59,
60] is lower than that of table salt and inorganic germanium dioxide, for which the oral toxicity for mice (LD
50) is 5400 mg/kg [
55]. For example, for the best-known organic germanium sesquioxide Ge-132 oral toxicity for mice is LD
50 > 6300 mg/kg, oral for rats is >10000 mg/kg and intravenous toxicity for rats is >1000 mg/kg [
58]. Germatranol, another common germanium derivative, is also of low toxicity: oral toxicity (LD
50) is 8400 mg/kg for mice, intravenous toxicity is 300 mg/kg [
57]. Thus, both inorganic and organic compounds of germanium are perfectly safe in those doses in which they are usually used. It should be noted that all known chemical databases, such as PubChem, currently have correct toxicity values for these compounds.
Inorganic derivatives of germanium have also been involved in a number of incidents. Dietary supplements and elixirs containing cheap both inorganic GeO
2 and Ge (IV) coordination complexes (particularly germanium citrate and citrate-lactate) have been widely sold in Japan since the early 1970s. They were advertised primarily for cancer treatment [
51], wherein the recommended daily dose of 50-100 mg was completely safe. However, a number of precedents of poisoning by such germanium compounds in persons who have taken such elixirs for a long time have been described. In all cases, the daily dose of germanium was arbitrarily exceeded in tens and even hundreds of times (up to 5 g GeO
2 per day) for a long time (up to 18-24 months or more) [
48,
49,
61,
62]. As a result, the total dose of germanium in these people was between 100 and 500 grams! Some of the more common symptoms of inorganic germanium poisoning include weight loss, fatigue, gastrointestinal disorders, anemia, muscle weakness and, in all cases, kidney failure. [
48,
49,
50,
61,
62]. Moreover, several serious fatal cases were described (see also review [
50]). Because of such cases, these elixirs have also been banned in many countries [
60]. However, in each of the above-mentioned cases of poisoning with germanium it was necessary to understand fully and assess not only the harm from poisoning, but also the possible benefits. In view of the fact that cancer patients in the last stages took these drugs (both in the form of Ge-132 and in the form of GeO
2 and other derivative compounds) in such huge dose independently at their own risk. Taking germanium medications, even if in such toxic doses, the oncological sufferers, which usually live no more than 3-6 months with their diagnosis, have lived 1.5-3 years or more [
50,
63]. Moreover, during all this time, they lived a full life, contrary to the application of classical chemotherapy.
Most of these poisoning cases occurred more than 25 years ago. However, they somehow worsened the already bad reputation of the germanium compounds.
In natural compounds, germanium forms very weak chemical bonds with organic molecules, primarily with oxygen atoms. At present, there are no methods of isolation, separation and purification of such substances, so the natural germanium compounds and/or its complexes have not yet been isolated and characterized. For the moment, scientists have drawn attention to the water-soluble synthetic germanium derivatives that makes them bioavailable and enables them to be used in safe milligram doses.
The development of water-soluble organic derivatives of germanium (i.e. containing at least one Ge-C bond) is inextricably connected with the N.D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences (ZIOC RAS) and its scientists.
Although the germanium sesquioxides were known long ago, they were insoluble in water. The first water-soluble derivatives were discovered in 1965 by Prof. S.P. Kolesnikov [
64,
65,
66], at that time a graduate student in the laboratory of Prof. O.M. Nefedov [
67]. These water-soluble compounds were produced by hydrolysis of HGeCl
3 adducts with cyclohexanone or methyl methacrylate. Later, in 1967 Prof. V.F. Mironov, a former employee of the same laboratory, similarly synthesized another stable water-soluble germanium sesquioxide - bis(carboxyethylgermanium) sesquioxide (Ge-132, CEGS), which is now best known [
68,
69].
(O1.5GeCH2CH2COOH)n
In the 1960s, the synthesis of such compounds seemed simple only on paper, and in reality required highly qualified chemists and specialized equipment, which was available only in a few laboratories in the USSR and the USA. However, there is often a misconception in the literature that K.Asai, a well-known popularizer and author of several books about germanium, was the first to synthesize Ge-132. In 1967, at the international scientific conference K. Asai learned about the discovery of water-soluble germanium compounds from Soviet scientists, who later gave him samples for testing. Kazuhiko Asai graduated from Law School at Tokyo Imperial University (currently the University of Tokyo) [
70,
71]. He may have been the first to become aware of the pharmaceutical potential of the Ge-132 [
24]. However, in his writings he is completely silent about the works from the USSR on the synthesis of this compound that misled the scientific community. Thus, the method of obtaining Ge-132 in patents of K. Asai [
72,
73] exactly repeats the method of synthesis published in the article by V.F. Mironov [
68] without reference to the original source. Furthermore, Asai was included as a co-author to the paper on Ge-132 crystal structure published in a very influential journal [
74]. This article also does not mention the work of Russian chemists, but is cited by many authors as the first publication on Ge-132 synthesis, although the true history of Ge-132 is now well known (see e.g. [
6,
7,
24,
75,
76]).
It was Ge-132 that led to the active study of biological activity of germanium compounds and their application in medical practice, especially in complex cancer therapy [
7,
19,
31,
36,
77]. There are clinically proven cases of successful use of these compounds in cancer treatment, for example, the complete remission of lung cancer was achieved when taking Ge-132 [
78]. The spectrum of biological activity of Ge-132 turned out to be very extensive, with the most pronounced being antitumor activity [
40,
52,
53,
54].
Microbiological methods are another direction for synthesis of organic germanium compounds. Thus, the yeast fermentation method produces Bio-Germanium, a medicine that acts as an effective immunostimulant, increasing the cytotoxicity of NK cells and activating immunoglobulin, B-cells and tumor necrosis factor. [
19]. However, such drugs will remain outside the scope of this review.
4. Inorganic and coordination germanium compounds
The inorganic and coordination germanium compounds are now well established in medical practice (see reviews [
3,
149,
150] and monograph [
46]). The structure of such compounds is discussed in detail in the review [
151].
Problems with the use of GeO
2 in medical practice in the 1980s were related to its low solubility, which required a substantial increase in the dose. It has recently been possible to synthesize highly soluble forms of GeO
2 [
152]. This opens up new avenues for its use, including in medicine.
Among coordination germanium compounds, the most studied are germanium (IV) citrate and germanium (IV) citrate-lactate, which, like GeO
2, are of low toxicity but exhibit nephrotoxicity in high doses [
6,
47,
58]. These compounds activate the immune system and are recommended for the treatment of a wide range of diseases, primarily oncological [
3,
43,
46,
149,
150,
153].
There are also known complexes of germanium (IV) with acetylacetone ligand [Ge(acac)
3)]
+ with different anions (
16) [
154]. The obtained complexes exhibit high activity against different cancer cell lines with high selectivity in cancer cells over normal epithelial cells. Furthermore, the compounds induce significant apoptosis [
154].
A number of Ge (IV) complexes with natural polyphenols have also been synthesized and have been shown to be promising pharmacologically active substances for cancer treatment. Thus, the quercetin-germanium complex (
17) showed high cytotoxicity against four tumor cell lines (PC-3, Hela, EC9706 and SPC-A-1) [
155,
156].
Among other polyphenolic compounds that were used in the synthesis of complexes with Ge (IV), we note a natural coumarin daphnetin (
18) and glucosylxanthone mangiferin (
19) [
157].]. The resulting Ge (IV) complexes with the above compounds exhibit high antioxidant activity and also demonstrates a strong intercalating ability in calf thymus DNA molecules. In addition, these two complexes had a strong inhibitory proliferative effect on cancer cells HepG2 [
157].
Last not the least, the germanium (IV) complex with hesperidin, a flavanon glycoside, has been synthesized, the structure of which, however, has not been established [
158]. This complex showed high activity in hepatocellular carcinoma of rats.
5. A possible mechanism of anticancer action of germanium compounds
A century ago, the Nobel Prize winner Otto Warburg observed that tumors produce excess lactate in the presence of oxygen. He proposed that the cancer origin lays in the replacement of oxidation phosphorylation by glucose fermentation, which he interpreted as mitochondrial dysfunction [
159,
160,
161,
162,
163]. This phenomenon was called aerobic glycolysis or the “Warburg effect”. Later, the concept of mitochondrial oxidative stress was developed [
164,
165,
166,
167,
168]. The mitochondrial oxidative stress leads over-production of ROS that, in cellular level, causes aerobic glycolysis, DNA damage, autophagy/mitophagy, protection again apoptosis [
168]. In the oxidative stress, the most reactive and damaging ROS is hydroxyl radical (HO
•), which produced from hydrogen peroxide by the Fenton reaction [
169]. To protect/prevent the oxidative stress the antioxidants should be applied. Antioxidants
stoichiometrically react with ROS. They are required in large amounts to suppress oxidative stress, and can have side effects [
170,
171,
172,
173].
Germanium compounds have been found to be effective against oxidative stress [
43,
76,
101]. Old publications describe unique properties of germanium derivatives, which led us to suggest a putative mechanism of the oxidative stress suppression/prevention. In 1930 R. Schwarz and H. Giese studied the reaction of alkali germanates with hydrogen peroxide and obtained peroxyhygrates [
174]. Later in 1935, R. Schwarz and F. Heinrich proved that these peroxyhygrates are coordination germanium compounds (not peroxides) with H
2O and H
2O
2 as ligands [
175]: K
2Ge
2O
5·2H
2O
2·2H
2O, Na
2Ge
2O
5·2H
2O
2·2H
2O, Na
2GeO
3··2H
2O
2·2H
2O. Such complexes
do not oxidize iodides and evolve oxygen. By this means, germanium derivatives
catalytically decompose hydrogen peroxide, and germanium trace quantities can keep hydrogen peroxide at low level, thus dramatically reducing formation of the HO
•, the most damaging ROS, by the Fenton reaction. Therefore, germanium derivatives can dramatically reduce hydrogen peroxide level in cells suppressing/preventing the oxidative stress. This explains the important role of germanium in restoration of the oxygen respiration (oxidative phosphorylation) in Warburg-like cancers.