Early studies demonstrated that radioactive gallium (⁶⁷Ga citrate) injected into tumor-bearing animals localized in malignant cells [9]. This observation led to the development of the ⁶⁷Ga scan for the detection of malignant tumors in patients [10]. Over the past two decades, ⁶⁷Ga scanning has been used most frequently in patients with Hodgkin’s and non-Hodgkin’s lymphomas to detect residual disease or disease that has relapsed following treatment with chemotherapy or radiotherapy [11–14]. The level of ⁶⁷Ga incorporation into lymphoma cells reflects the metabolic activity of the tumor and is indicative of the presence of viable malignant cells. Hence, a positive ⁶⁷Ga scan after treatment of lymphoma generally indicates the presence of residual malignancy and the need for further therapy. The intensity of ⁶⁷Ga uptake by malignancies such as lymphoma correlates with their proliferative rate; ⁶⁷Ga scanning thus provides a valuable insight into the aggressiveness of a tumor [12]. Gallium may also be taken up by macrophages and thus, areas of inflammation may show ⁶⁷Ga-positivity on the ⁶⁷Ga scan. Recently, the ⁶⁷Ga scan in lymphoma has been replaced by the Positron Emission Tomography (PET) scan which is based on the uptake of ¹⁸F-fluorodeoxyglucose (FDG) by tumors [15]. However, the ⁶⁷Ga scan remains an important diagnostic modality in geographical locations where the ¹⁸F-FDG PET scan may not be available.

Although tumor imaging with ¹⁸F-FDG PET is now being increasingly used for a variety of malignancies, it has its limitations. Tumors with a low growth rate such as hepatocellular carcinoma, neuroendocrine tumors, prostate cancer, and indolent (slow-growing) lymphomas may not take up FDG. Conversely, tumors surrounded by or adjacent to normal tissue with high metabolic activity (such as the brain) may be difficult to assess by ¹⁸F-FDG PET because of high background FDG uptake by the normal tissue (i.e., a low ratio of tumor/normal tissue FDG uptake). As a result of these limitations, there has been considerable interest in the development of ⁶⁸Ga-labeled radiopharmaceuticals for molecular tumor imaging (reviewed in references 16 and 17). This gallium isotope has a short half-life of approximately 68 minutes which is an advantage to its clinical use. Peptides labeled with radiometals are showing promising clinical activity as imaging agents for the diagnosis and management of neuroendocrine tumors, neural crest tumors (pheochromocytomas and paragangliomas), meningiomas, and prostate cancer. Somatostatin, which targets the somatostatin receptor subtypes present in increased numbers on these cells, has been the most developed model for synthetic peptides. The somatostatin analogue D-Phe¹-Tyr³-octreotide (TOC) coupled to the bifunctional chelate DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-teraacetic acid) labeled with ⁶⁸Ga (⁶⁸Ga-DOTATOC) has been used with great success for the imaging of neuroendocrine tumors in patients. In other studies, ⁶⁸Ga-DOTA-D-Phe¹-Tyr3-Thr8-octreotide (⁶⁸Ga-DOTA-TATE) PET was used to image the somatostatin receptor in recurrent pheochromocytoma and meningiomas in patients and in the latter disease led to changes in radiotherapy planning. Imaging of neuroendocrine tumors with another somatostatin analogue, ⁶⁸Ga-DOTANOC has also shown promise [18], while ⁶⁸Ga-DOTA-alpha melanocyte-stimulating hormone analog has been used for imaging metastases in melanoma [19]. In addition ⁶⁸Ga labeled-DOTA coupled to bombesin (⁶⁸Ga-DOTA-BOM) PET has been used to diagnose prostate cancer expressing the bombesin receptor, while ⁶⁷Ga- and ⁶⁸Ga-deferoxamine (DFO)-folate have been used to image tumors that express high levels of cell surface folate receptors in a tumor xenograft model [20,21].

The ability of ⁶⁷Ga to localize in tumor cells prompted investigators at the National Cancer Institute (NCI) to investigate the antineoplastic activities of salts of the group IIIa metals aluminum, gallium, indium, and thallium in tumor-bearing CDF1 mice and Sprague-Dawley rats [22]. Of these metal compounds, gallium nitrate proved to be both highly effective in suppressing the growth of subcutaneously implanted tumors and least toxic. As a result, it was advanced to NCI investigational drug status (NSC 15200) for assessment of toxicity and antitumor efficacy in Phase 1 and Phase 2 clinical trials [23]. In those clinical studies, gallium nitrate was administered by two different schedules: a brief (15–30 minutes) intravenous infusion or a continuous intravenous infusion given over 24 hours for 5 to 7 days.

While the clinical efficacy of continuous infusion gallium nitrate has been examined in a number of different malignancies, this drug has displayed its strongest antineoplastic activity in the treatment of non-Hodgkin’s lymphoma and bladder cancer. At least 4 separate clinical trials have demonstrated the anti-lymphoma activity of gallium nitrate administered as a single agent [24–27], while 3 additional trials have used it in combination with other antineoplastic agents to improve treatment outcome [28–30]. In all these studies, gallium nitrate was assessed in patients with relapsed non-Hodgkin’s lymphoma that failed to respond to many conventional chemotherapeutic drugs and also to stem cell transplantation. Overall, studies indicate that approximately 30% of patients with relapsed lymphomas will responded to treatment with gallium nitrate. Interestingly, aggressive large cell and mantle cell lymphomas appear to be more responsive to gallium nitrate than other lymphoma subtypes [27]. The activity of gallium nitrate in advanced bladder cancer has also been confirmed in several clinical studies. At least three studies have demonstrated an overall response in 17–63% of patients treated with single agent gallium nitrate [31–33], while three additional studies using gallium nitrate in combination with other drugs have shown overall response rates of 12–67% [34–36]. The 12% response rate was seen in a study that combined gallium nitrate with fluorouracil [36]. This low response is surprising given the higher responses seen in other studies; it raises the question as to whether this was the result of drug antagonism between fluorouracil and gallium nitrate. An advantage of gallium nitrate is that unlike many chemotherapeutic drugs, it does not produce myelosuppression and can therefore be used in patients with low white blood cell or platelet counts. In vitro, gallium has shown to be synergistic with hydroxyurea, fludarabine, interferon-alpha, gemcitabine, and paclitaxel, suggesting that combination therapy with these agents may be fruitful [37–41]. Moreover, gallium nitrate does not appear to share cross-resistance with conventional chemotherapeutic drugs since clinical responses have been noted in patients who have failed to respond to multiple other drugs.

Studies examining the antineoplastic activity of gallium nitrate revealed that a decrease in blood calcium levels occurred in a significant number of patients being treated with this agent [26]. Based on this observation, further investigations were pursued to examine the potential of gallium nitrate as a treatment for elevated blood calcium levels associated with cancer [42]. The bisphosphonate drugs have had an established role in the treatment of hypercalcemia and osteoporosis; randomized clinical trials were therefore conducted to compare the efficacy of gallium nitrate with the bisphosphonates etidronate and pamidronate in the treatment of hypercalcemia in cancer patients [43,44]. These studies demonstrated continuous infusion gallium nitrate (200 mg/m²/day for 5 days) to be superior to etidronate and at least as effective as pamidronate in acutely controlling elevated blood calcium levels. Gallium nitrate was also shown to be superior to calcitonin, a non-bisphosphonate drug used for treatment of hypercalcemia [45]. Based on its clinical efficacy, gallium nitrate (Ganite™) was approved by the Food and Drug Administration (FDA) for the treatment of cancer-associated hypercalcemia. Zoledronic acid, a newer generation bisphosphonate, has been shown to be more potent than pamidronate for treatment of hypercalcemia of malignancy [46]; however, randomized trials comparing gallium nitrate and zoledronic acid have not been conducted. Apart from its ability to control hypercalcemia, gallium nitrate has been shown to inhibit bone turnover and to decrease osteolysis in patients with multiple myeloma and in patients with bone metastases from a variety of different cancers [47,48]. Furthermore, administration of low, non-toxic doses of gallium nitrate to patients with Paget’s disease of bone, a disorder characterized by abnormal bone remodeling secondary to increased bone resorption by osteoclasts, led to a reduction in serum alkaline phosphatase, urinary hydroxyproline, and N-telopeptide, biochemical markers of bone turnover [49].

Evidence that environmental gallium per se may be immunosuppressive was provided by the studies of Betoulle et al. in which an aquatic system was used to show that that fish (carp) exposed to sublethal concentrations of gallium nitrate in the water for up to 96 hours displayed a significant reduction in immune parameters including immunoglobulin production, phagocyte killing ability, and blood leukocytes [50]. Several studies using in vitro and in vivo animal systems have shown that gallium compounds have immunosuppressive activity in animal models of autoimmune disease. Gallium nitrate has been shown to suppress experimental autoimmune encephalomyelitis and prevent adjuvant inflammatory arthritis through suppression of macrophage function and T-cells in rat models [51,52]. Other studies showed that gallium nitrate can suppress lupus and prevent cardiac allograft rejection in murine models [53,54]. Transferrin-gallium and gallium nitrate were shown to inhibit the mixed lymphocyte culture response and prolong the survival of mice with severe graft-versus-host disease in a murine bone marrow transplant model [55]. Despite these interesting preclinical observations, the immunomodulatory and anti-inflammatory properties of gallium appear to have not been investigated in rigorous clinical studies. Further investigations appear warranted to establish whether the results of these in vitro and animal studies are relevant to patients with inflammatory or autoimmune diseases. The potential effects of gallium on inflammation and the immune system should be kept in mind when gallium compounds are being used for the treatment of other conditions.

Much of our understanding of gallium’s action on cells and its toxicity is derived from investigations conducted to evaluate its potential as a therapeutic agent. Insights into the pharmacology of gallium were provided by early studies which demonstrated that radioactive ⁶⁷Ga citrate injected into rabbits was transported in the circulation exclusively bound to transferrin, the transport protein for iron [56]. Harris and Sephton showed that the uptake of radiogallium by malignant cell lines in vitro could be significantly enhanced by the addition of transferrin to the culture medium [57]; the clinical relevance of this finding was underscored by the studies of Vallabhajosula et al. who showed that in the blood radiogallium binds to transferrin and is transported to tumor tissue [56]. In this respect, the initial transport of gallium in the blood resembles that of iron. However, whereas gallium shares certain properties with ferric iron [Fe(III)] with respect to ionic radius and bonding, trivalent gallium unlike Fe cannot be reduced to a divalent state. Nonradioactive gallium binds avidly to both metal-binding sites on transferrin, but it does so with a 300-fold less affinity than ferric iron [58]. Under physiologic conditions, only about one-third of circulating transferrin is occupied by iron; hence, the metal-binding sites of the remaining transferrin molecules are available to bind and transport gallium as transferrin-gallium complexes. With higher levels of gallium in the circulation, transferrin binding is exceeded, and gallium likely circulates as gallate, Ga(OH)⁴⁻ [3].

Gallium pharmacokinetics have been examined in Phase-1 and -2 clinical trials in which gallium nitrate was administered to patients as a brief or a continuous intravenous infusion. When administered as an intravenous infusion over 30 minutes at doses ranging from 400–700 mg/m² gallium nitrate, a biphasic gallium urinary excretion pattern with an α-phase half-life of 8.3–26 minutes and a β-phase half-life of 6.3–196 hours was noted [59]. The latter phase reflects its binding to transferrin in the circulation. Sixty-nine percent and 91% of the gallium dose administered was excreted in the urine during the first 24 and 48 hours, respectively [59]. Similar serum gallium disappearance curves were noted when higher concentrations of gallium nitrate (500–900 mg/m²) were administered by brief infusion. Hydration and co-administration of mannitol with gallium nitrate increased the amount of gallium excreted in the urine over the first hour [60]. In contrast to the peak serum levels of gallium attained when gallium nitrate is administered by brief infusion, administration of gallium nitrate by continuous intravenous infusion at its currently recommended dose of 200 mg/m²/day for 7 days results in a steady-state plasma gallium level of 0.9–1.9 μg/ml which decreases to 0.45–0.7 μg/ml at 4 days after stopping the drug. The continuous intravenous infusion schedule for gallium nitrate has much less toxicity than the brief infusion schedule and allows patients to receive twice the amount of gallium.

Gallium nitrate is a simple metal salt that can be considered to belong to a first generation of gallium compounds. There is considerable interest in gallium-based metallodrugs and novel gallium agents are in development as illustrated in Table 1. These include the oral formulations gallium maltolate, G4544, and KP46, all of which are in clinical development [5,65,94]. Gallium maltolate inhibits hepatocellular carcinoma cell growth and induces apoptosis through action on the mitochondrial pathway in lymphoma cell lines [81,95]. On a mole-to-mole basis, this compound appears to be more cytotoxic to malignant cell lines in vitro than gallium nitrate. G4544 is being examined clinically for bone metastases and metabolic bone disease but may also prove to be active as an antineoplastic agent [65]. KP46 (Tris(8-quinolonato)Ga(III) is in clinical trials in Europe [96] and has been shown to induce apoptosis in a large panel of malignant cell lines [5,97]. In preclinical studies, gallium complexes of ligands containing pyridine and 4,6-substituted phenolate moieties with antitumor activity have been developed and have shown activity against prostate cancer xenografts in rats [88,98]. In addition, gallium thiosemicarbazones and novel organometallic gallium compounds are being aggressively advanced as therapeutic agents [99–102]. It is possible that these newer agents may have additional mechanisms of action that may be somewhat different from those of gallium nitrate. For example, gallium maltolate was recently shown to inhibit the growth of lymphoma cell lines with either acquired or inherent resistance to the cytotoxicity of gallium nitrate [81]. These data would suggest that the mechanisms of cell death induction by gallium maltolate occurs through pathways that are different than that for gallium nitrate [81]. The availability of lymphoma cell lines that are resistant to gallium nitrate-induced apoptosis provides the opportunity to examine whether the other newer gallium compounds share cross-resistance to gallium nitrate and whether these compounds may be more efficacious as therapeutic agents. The advancement of new gallium compounds to further preclinical and clinical testing is awaited with great anticipation.

The ability of gallium to interfere with the utilization of iron by certain microorganisms has led to some interesting investigations directed at exploring the potential of gallium as an antimicrobial drug. Gallium nitrate and transferrin-gallium were shown to block the iron-dependent growth of Mycobacterium tuberculosis and M. avium complex extracellularly as well as within human macrophages. Gallium interfered with iron acquisition by M. tuberculosis within the macrophage phagosome resulting in a bactericidal action which could be prevented by excess iron [108]. In another study, gallium nitrate inhibited Pseudomonas aeruginosa growth and biofilm formation, and killed planktonic and biofilm bacteria in vitro through mechanisms that included interference with iron uptake and iron signaling by the transcription regulator pvdS. In this study, gallium was effective against P. aruginosa pneumonia in mice and prolonged their survival [106]. In a mouse model of thermal injury inoculated with P. aeruginosa, gallium maltolate provided a 100% survival compared with 100% mortality in control mice not treated with gallium [107]. Patients with cystic fibrosis often develop Pseudomonas lung infections that may be resistant to standard antibiotics and difficult to eradicate; a clinical trial is in progress to assess the efficacy of gallium nitrate in the treatment of lung infections in this patient population (Genta Incorporated Inc; Press release December 2, 2009).

Treatment with continuous intravenous infusion gallium nitrate at the present recommended dose for hypercalcemia (200 mg/m²/day for 5 days) is generally well-tolerated, even by elderly patients. Higher continuous infusion doses (300 mg/m²/day for 5–7 days) are used for treatment of cancer. In phase 1 and 2 studies, renal toxicity was dose-limiting when gallium nitrate was administered as a brief intravenous infusion over 30 minutes [60]; this side-effect was seen in ∼12.5% of patients treated for hypercalcemia by continuous intravenous infusion. With the continuous infusion schedule for treatment of lymphoma, diarrhea rather than renal toxicity was dose-limiting [26]. Renal toxicity can be minimized by ensuring adequate fluid intake and avoiding of coadministration of nephrotoxic drugs. Microcytic anemia may develop in patients treated with gallium nitrate; however, platelet counts and white blood cell counts are not suppressed [23]. The latter is a major advantage as it means that unlike other chemotherapeutic drugs, gallium nitrate can be administered to patients with thrombocytopenia or neutropenia. Visual and auditory toxicities may occur but have been reported in <1% of patients. Hypocalcemia may develop in patients with normal blood calcium levels; this can be managed by oral supplementation with calcium carbonate.

Gallium arsenide is a valuable compound that is employed widely in the electronics industry. It possesses several advantages over silicon in that it has several-fold greater electron mobility and only a fraction of the electrical capacitance of silicon [110]. This property, along with its light-emitting, electromagnetic, and photovoltaic properties allow it to be used extensively in high-speed semi-conductor devices, in high power microwave and millimeter-wave devices, and in optoelectronic devices [1]. The latter include light-emitting diodes (LEDs), laser diodes, photodetectors and solar cells that are used in numerous applications.

Workers in the microelectronics industry may be exposed to gallium arsenide during the process of sandblasting gallium arsenide ingots, in the slicing, grinding, and polishing of gallium arsenide-silicon wafers, and in clean-up of work areas [1]. For these individuals, the primary portals of entry of gallium arsenide into the body would be via the respiratory tract and, to a much lesser extent, the gastrointestinal tract. Gallium and arsenic have been shown to be elevated in ground water obtained from semiconductor manufacturing industrial areas, thus suggesting that contaminated drinking water may also be a source of environmental exposure to gallium [111]. While it is felt that the toxicities of gallium arsenide are related primarily to the arsenic component of this compound, the contribution of gallium to some of these toxicities cannot be discounted especially in light of the fact that other gallium compounds display antineoplastic and antimicrobial activities. Therefore, a discussion of the toxicities of gallium arsenide and the relative contribution of gallium to these toxicities is warranted.

The National Toxicology Program reported the effects inhalation of gallium arsenide particles by F344/N rats and B6C3F1 mice produced over a period of 16 days, 14 weeks, or 2 years [112]. In the 16-day study, animals were exposed to 0, 1, 10, 37, 75, or 150 mg/m³ gallium arsenide inhalation, 6 hours per day, 5 days per week for the duration of each period. At the end of the study, animal weights were similar to controls, and there were no deaths. The liver and lung weights were increased in male rats exposed to >1 mg/m³ and in female rats and mice of both sexes exposed to >10 mg/m³, while thymus weights were decreased in male rats exposed to 10 mg/m³. Gallium arsenide particles were detected in the alveolar spaces and within alveolar macrophages at higher concentrations in both animals. Minimal histiocytic cellular infiltrate and moderate proteinosis in the lung were noted in animals exposed to >10 mg/m³, while laryngeal epithelial hyperplasia and squamous metaplasia were noted in mice and rats exposed to >10 mg/m³ and 150 mg/m³ gallium arsenide, respectively. Mild chronic inflammation was noted in mice exposed to 75 or 150 mg/m³.

With a 14-week inhalation exposure, rats and mice were exposed to gallium arsenide particulate at concentrations of 0, 0.1, 1, 10, 37, or 75 mg/m³, 6 hours per day, 5 days per week for 14 weeks. No animal deaths were observed; however, final mean body weight and weight gain were significantly diminished in male animals compared with controls. A microcytic anemia with increased zinc protoporphyrin/heme ratios, increased platelet and neutrophil counts, and increased serum alanine aminotransferase and sorbitol dehydrogenase was noted. In males, testicular toxicity was noted at the higher concentrations of gallium arsenide that was characterized by a decrease in weights of the testes, cauda epididymis, and epididymis. Testicular atrophy was seen at the highest concentration of gallium arsenide in male rats, while total spermatid heads, spermatid counts, and spermatic motility were significantly decreased at lower concentrations. Increases in the incidence of plasma cell hyperplasia of mandibular lymph notes, bone marrow hyperplasia, and hemosiderosis of the liver and spleen were observed with the higher concentrations of gallium arsenide. The pulmonary toxicity noted with gallium arsenide inhalation for 14 weeks was an extension of that seen with a 16-day exposure. Lung weights were increased; proteinosis, histiocytic infiltration, and epithelial hyperplasia in the lung accompanied by inflammation and squamous metaplasia in the larynx were observed with the higher concentrations of gallium arsenide.

In a 2-year study of gallium arsenide inhalation exposure, B6C3F1 mice were exposed to 0, 0.1, 0.5, or 1.0 mg/m³ gallium arsenide for 6 hours a day, 5 days per week for 105 weeks (males) or 106 weeks (females). This resulted in a spectrum of inflammatory and proliferative lesions in the respiratory tract without the development of cancer. In another study, male and female Fischer 344/N rats were exposed to inhalation of 0–1 mg/m³ of gallium arsenide particles for 6 hours per day, 5 days a week for 105 weeks. Although there was no adverse impact on survival, inflammatory lesions, atypical hyperplasia, and metaplasia in the respiratory tract were noted. Particles of gallium arsenide were detected in the alveolar spaces and macrophages in animals exposed to the higher concentrations of this compound [112]. In addition to displaying a spectrum of such changes in the lung, female but not male rats had a gallium arsenide concentration-dependent increase in benign and malignant lung lesions (alveolar/bronchiolar, adeno, and squamous cell carcinomas). Females also had a gallium arsenide concentration-dependent increase in adrenal neoplasms, and an increased incidence of large granular lymphocytic leukemia with exposure to 1.0 mg/m³ of this compound.

The toxicities of gallium arsenide administered by intratracheal installation to rodents using various schedules ranging from a single dose to repetitive exposure are summarized in the studies in Table 2. Details of these and related studies have been presented in recent reviews [1,113]. As with inhalation exposure, intratracheal installation of gallium arsenide produces various levels of damage to the respiratory tract. In addition, as shown in Table 2, this mode of gallium arsenide delivery may also produce testicular toxicity, inhibition of the heme biosynthetic pathway, decreases in hemoglobin levels, renal damage, and suppression of immunological functions.

Two representative studies of oral gallium arsenide in animals are summarized in Table 3. With oral administration, the majority of gallium and arsenide appears in the feces thus indicating that this compound has limited bioavailability. Not surprisingly, pulmonary toxicity is not encountered with oral gallium arsenide; however, Flora et al. (Table 3) found a dose-dependent inhibitory effect of oral gallium arsenide on heme biosynthesis and primary immune response along with evidence of hepatic oxidative stress [125]. Collectively, animal studies show that gallium arsenide administered by inhalation or by intratracheal instillation is considerably more toxic than oral gallium arsenide.

In aqueous medium, gallium arsenide rapidly dissociates to form gallium and arsenic oxides which may be further hydrolyzed [114]. The toxicities of arsenic are well recognized, and it has been suggested that the toxicities of gallium arsenide may be largely caused by the arsenic component of this compound [129]. However, this may be dependent on the target organ examined and on the conditions under which animal experiments were conducted. The following studies shed some light on this matter.