Exposure, metabolism, and toxicity of rare earths and related compounds.

For the past three decades, most attention in heavy metal toxicology has been paid to cadmium, mercury, lead, chromium, nickel, vanadium, and tin because these metals widely polluted the environment. However, with the development of new materials in the last decade, the need for toxicological studies on those new materials has been increasing. A group of rare earths (RE) is a good example. Although some RE have been used for superconductors, plastic magnets, and ceramics, few toxicological data are available compared to other heavy metals described above. Because chemical properties of RE are very similar, it is plausible that their binding affinities to biomolecules, metabolism, and toxicity in the living system are also very similar. In this report, we present an overview of the metabolism and health hazards of RE and related compounds, including our recent studies.

be very similar to those of RE. The Clarke numbers (the ratio of the amount of a particular element mainly in the earth's crust) of RE are shown in Table 1.
Although RE are not abundant in the earth's crust, Ce, the most plentiful element of RE, is about 100 times more abundant than cadmium (Cd), one of the most well-known heavy metals in toxicology. The Clarke number of Ce is almost the same as those of cobalt, tin, zinc, and vanadium. Unlike all other RE, Pm, found as a decay product of uranium in 1947, has not been detected in the earth's crust (3). The global annual demand of RE is estimated to be about 30,000 tons (4,5). China has the world's largest reserve, which is sufficient to meet the global needs of RE for 1000 years (4,5).

Chemical Properties of Rare Earths
The chemical properties of RE and the detection limits of RE in atomic absorption, atomic emission, and mass spectrometry are summarized in Table 1 (1,2). Although +2, +4, and +5 valences are possible for some of the RE, their valences are usually +3 when they are dissolved. One of the most prominent features of lanthanoids is what is called lanthanoid contraction (6). From La to Lu, the radius of lanthanoid ions (+3) decreases as the atomic number increases. This phenomenon is due to attraction of electrons of 4f orbitals by increasing positive charge of the nucleus with the atomic number. Because the radius of Ca2+ (0.99 A) is very close to those of lanthanoids, lanthanoids have been used for Ca2+ probes in biochemical and physiological studies. The nitrates, chlorides, and sulfates of RE are soluble and their carbonates, phosphates, and hydroxides are insoluble (6). The differences in solubility among these ionic forms of RE seem to determine the metabolic fate of RE in the biological system. In general, the toxicity of lanthanoids decreases as the atomic number increases, probably due to greater solubility and ionic stability of heavier lanthanoids (7).

Exposure to Rare Earths
It was not until the nuclear era that attention was addressed to the health effects of RE. A fission product, 144Ce, was found in animal bones and clams (10) and in the lungs and lymph nodes obtained from deceased persons who had inhaled nuclear explosion aerosols (11).
Besides irradiation effects of radioactive nuclides, inhaled RE probably cause granulomatous lesions in the lung or pneumoconiosis (7). The concentration of La found in smelter's lungs was 2 to 16 times higher than in normal lungs (12); La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu were found in a deceased photoengraver's lungs (13). These workers are at risk of pneumoconiosis; one worker, who had been exposed for only 18 months to dust containing 60% of RE (mainly Ce, La, and Nd), was reported to have radiologic evidence of pneumoconiosis (14). Industrial uses of RE are shown in Table 2. They have been used for ceramics, fluorescent materials, abrasives, magnets, etc. (15). To our knowledge, however, RE concentration in the air of work places has not been reported. There is no evidence of pneumoconiosis or chronic pulmonary   (4) and Ohmachi (5). reactions in laboratory animals, even though YC13 and LaCd3 or oxides of Y, Nd, and Ce have been proven to cause bronchitis, pneumonitis, and granulomatous lesions (16)(17)(18).
Some radioactive RE nuclides have been used for cancer (19,20) and synovitis therapy (21,22). 90Y is a useful nuclide for clinical use because it has a moderate half-life (64 hr) and it is a pure n-emitter with high energy (2.28 MeV) (23). In addition, 90Y is easily separated by column chromatography from 90Sr, which has a very long half-life (28.8 years). It has been shown that Tm3+, Tb3+, and Yb3+ have a high affinity for tumor cells (24)(25)(26)(27). It is interesting to note that Tb3+ was temperature-dependently taken up by tumor cells (MCF-7), and cisplatin, a well-known anticancer drug, reduced the binding of Tb3+ to those tumor cells (25). However, there is a contradictory report that has shown that La concentration in malignant laryngeal tissue was lower than in nonmalignant adjacent tissues, although serum La concentrations of laryngeal cancer patients were significantly higher than those of normal subjects (28).
Recently, DTPA-chelated Gd (gadopentetate dimeglumine), tetraazacyclododecanetetraacetic acid (DOTA)-chelated Gd (gadoterate meglumine), and Gd-HP-D03A (gadoteriol) have been used as magnetic resonance imaging-contrast reagents (29,30). Although clearance of those intravenously (iv) injected imagingcontrast reagents have been reported to be rapid, it is possible that some ionic Gd is released from the complexes. Ionic RE are rapidly changed to colloidal RE (hydroxide and phosphates) in blood, and the colloidal RE are taken up by the reticuloendothelial system in the liver (30). Gd was found in the breast milk of a lactating patient who received an iv injection of gadopentetate dimeglumine (31).
It is also reported that Ce is a potent antiseptic drug for Gram-negative bacteria and fungi (32) and swabbing of La is effective in protecting teeth from caries (33,34). Thus, toxicological studies of RE are needed not only from the standpoint of environmental or industrial hygiene but also for medical treatment.
Interaction ofRare Earths with Cells or Biomolecules Tb3+ binds to Ca2+ binding sites of the intestinal brush-border membrane (35) and surfaces of platelets (36) and vascular smooth muscle (37). When bound to membranes, the fluorescence of Tb3+ is increased probably by energy transfer to aromatic residues such as tyrosine (35). Tb3+ and Pm3+ are removable from the Environmental Health Perspectives -Vol 104, Supplement * March 1996 surfaces of platelets and smooth musde by both Ca2+ and La3+ (36,37). Lanthanoids are also known to bind to Ca2+ or Mg2+ binding sites of calmodulin (38), ATPase of sarcoplasmic reticulum (39,40), cystatin (41), and phosphatidylserine (42). The binding mode to calmodulin, which has two high-affinity and two low-affinity Ca2+ binding sites, has been shown to be different among lanthanoids. Lu3+ and Er3+ bind like Ca2 , Eu3+ and Tb3+ bind in the opposite order from Ca2+, and La3+ and Nd3+ bind in a mode between them (38). La3+ has been shown to inhibit the Ca2+-dependent release of chemical mediators such as catecholamine from the adrenal medulla and histamine from mast cells (43).
It has been reported that Sc3+, Y3+, and La3+ bind to globulin and DNA (44), and transferrin is a major Sc3+or Y3+-binding protein in blood plasma (45,46). La3+ and Nd3+ have anticoagulant action (47,48); inhibition of prothrombin-thrombin transformation or blood coagulant factors such as VII, IX, and X may be responsible for the anticoagulant effect of those ions.
Deposion, Retntion, Metabolism, dClearance ofRare Earths Inhalation or Intratracheal Instilation. As shown in Figure 1, inhaled or intratracheally instilled RE chlorides have been shown to accumulate in alveolar and tissue macrophages and alveolar walls (16,17,49,50). In macrophages RE have been shown to localize in lysosomes; it is proposed that RE are changed to insoluble phosphates in lysosomes according to Gomori (phosphatase) reaction (49). The transmission electron microscopy and X-ray microanalysis revealed intratracheally instilled Y and La deposits in basement membranes of pneumocytes (16,17). Half-times of Y and La in the rat lung have been reported to be 168 (16) and 244 days (17), respectively, when these RE were instilled intratracheally as chlorides. Rhoads and Sanders (51) have reported a half-time of intratracheally instilled Yb2O3 in the rat lung of 21 days. In these intratracheal instillation studies, translocation of RE to extrapulmonary tissue was marginal or below the detection limit. On the other hand, it has been shown that significant amounts of inhaled CeCl3 (52), Ce(OH)3 (53), and Y (chemical form is unknown) (54) were translocated to the skeleton and liver in rats or hamsters. It has also been reported that a half-time of inhaled Ce(OH)3 was 140 days following initial rapid clearance in the rat lung (53). The differences in the extrapulmonary translocation of RE between the intratracheal instillion and inhalation studies may be due to absorption of RE from the upper airways or gastrointestinal tract after being transported through the esophagus. Another factor that influences the pulmonary retention and translocation of RE is the dose of RE in the lung because it has been shown that translocation of Y from the lung to the bone decreased as the deposition in the lung increased (54). The half-time of intratracheally instilled RE chlorides in the lung is relatively long (vide supra) compared to those of other soluble metal salts such as cadmium chloride (14 days) (55) and cupric sulfate (7.5 hr) (56). However, when rats were exposed to aerosols of gadopentate dimeglumine, a half-time of Gd in the lung was 2.16 hr (57). These results suggest that gadopentate dimeglumine was stable in the alveolar space and was hardly taken up by macrophages because of limited release of ionic Gd from the complex.
Intravenous Injection. Whole body retention and tissue distribution of ivinjected RE primarily depend on the stability of RE in blood. Urinary excretion of Ce during 14 days was less than 1% of the dose following injection of CeCl3 in mice (58), and a half-time of iv-injected CeCl3 was about 10 years in beagle dogs (59). On the other hand, chelated RE seems to be excreted rapidly; a whole body half-time of Tm3+-citrate was about 2.5 hr in rats (24), and approximately 50% of iv-injected Sm3 +-ethylenediaminetetramethylene phosphonic acid (EDTMP) was excreted in 8 hr in humans (60). Intravenously injected DTPA-chelated Gd was excreted rapidly via urine after transient accumulation in the kidney, and only 2% of the injected dose remained in the body at 2 hr postinjection; GdCl3 was taken up by reticuloendothelial cells, and 72% was accumulated in the liver and spleen in rats (61)(62)(63). It has also been shown that EDTA-chelated Sc was rapidly taken up by the kidney with subsequent elimination via the urine, while ScCl3 was extensively deposited in the liver and spleen in mice (64). We have reported that iv-injected YCl3 was taken up by phagocytes of the liver and spleen in rats and the half-time of Y in the liver was 144 days (46). Taken together, chelated RE are excreted mainly via urine after transient accumulation in the kidney and their whole body half-times are several hours; RE chlorides are taken up by the liver and spleen, and those RE are not easily excreted.
The whole body retention of iv-injected chelated RE fits to a three-phase model shown by the following equation: % Retention = Ae-(O.693/Ta)t + Be-(0.693/Tb)t + Ce-(0.693/Tc)t [1] where A, B, and C are constants (A+B+C=100), and Ta, Tb, and Tc denote half-times of fast, intermediate, and slow phases, respectively. Table 3 shows half-times of iv-injected RE in the threephase model (64)(65)(66)(67). Hiraki et al. (66) suggested that the fast, intermediate, and slow phases represent excretion via urine, from the soft tissues, and bone, respectively. These results indicate that although iv-injected chelated RE is excreted rapidly via urine, RE deposited in the bone is excreted very slowly.
It has been shown that accumulation of Sc3+-citrate (low stability) in the liver, spleen, and bone was much higher than that of Sc3+-EDTA (high stability) following iv injection in mice (68). Rosoff et al. (68) also have shown that when Sc3+-NTA (intermediate stability) was injected, a relatively high concentration of Sc was accumulated in the bone compared to Sc3+-citrate or Sc3+-EDTA. Yb accumulated in rat offspring through milk following iv injection of YbCl3, Yb3+-EDTA, and Yb3+-DTPA into rat mothers, and the transfer of Yb to new-born babies increased in this order (69).
From a detoxication point of view, it is interesting to note that injection of Ca2+or Zn2+-DTPA has been proven to be effective in removing Yb (70,71), Sc (72), and Ce (73) from the body. Liposome-encapsulated DTPA seems to be more effective than DTPA itself (70,71). Injection of either Na+2, Ca2+-EDTA or Na+3, Ca2+-DTPA into Yb-exposed rat mothers has been proven to be effective in reducing the transfer ofYb to their offspring (69).
Rossoff et al. (68) have suggested that RE chlorides are changed into colloidal forms of hydroxide, phosphate, and carbonate in blood. We have shown that Y was distributed to a high molecular weight fraction (colloidal material containing proteins and some minerals such as calcium, phosphorus, and iron), transferrin, and a low molecular weight fraction (probably citrate) in the blood plasma; the percent of colloidal fraction of injected Y increased with dose of YCl3 as shown in Figure 2 (46). Uptake of Y by the liver and spleen also increased with the dose of YCl3 (46).
In Japanese quails, iv-injected LaCl3 and CeCl3 were deposited mainly in the liver and oocytes (74,75), and vitellogenin is a major lanthanoid-binding protein in these birds (75). At a dose of 15 pmol Gd/100 g body weight (bw), 80% of the dose was deposited in the liver; at doses below 0.15 pmol Gd/100 g bw, 80% of the dose was deposited in the oocytes (75). Deposition of iv-injected GdCI3 in the liver, oocytes, and ova decreased as blood vitelloginin concentration was increased by intramuscular injection of estradiol in male Japanese quails (76).
Intraperitoneal Injection. It is reported that intraperitoneally (ip) injected CeCl3 or Ce3+-citrate was deposited mainly in the liver and skeleton in hamsters (52) and rats (77). Electron microprobe and ionic microanalysis revealed that ipinjected CeCI3 was localized in lysosomes of hepatocytes and Kupffer cells, in lysosomes of bone marrow macrophages, and basement membranes of proximal convoluted tubules in the kidney of rats (50). Although Tb content in the liver was the largest among organs tested, tissue concentrations of Tb (1.g Tb/g tissue) were higher in the seminal vesicles, pancreas, and spleen than in the liver of mice (78).
Following ip injection of Lu3+-citrate in mice, Lu was deposited in the skeleton, liver, kidney, spleen, and lung, in this order (79). However, the percent of deposition in the liver was increased as the dose of Lu3+-citrate increased, and the percent of deposition in the skeleton was decreased as the dose increased (79). As described above, Ca 2+ or Zn2+-DTPA has been effective in removing RE deposited in the tissues following ip injection (77,80,81).
Per Oral Administration. By oral intake through drinking water or per oral  (82)(83)(84) and deposited in the skeleton, teeth, and soft tissues such as the lung, liver, and kidney (33,(85)(86)(87). Although swabbing of teeth with La(NO3)3 is known to replace Ca with La in the enamel in rats (33) and hamsters (88), La absorbed from the small intestine has also been shown to deposit in the teeth (33). It has been shown that 13.3% of po-administered CeCI3 was excreted via bile during the first 4 hr in rats (89), suggesting that a significant amount of Ce was absorbed from the intestine. However, the intestinal absorption of RE seems to depend on the diet. Retention of Pm in the soft tissues in neonatal rats was two orders of magnitude higher than that in adult rats (82), probably because the neonates were on milk diet (84,90). Fasting significantly increased the absorption of RE from the gastrointestinal tract (90,91). This phenomenon is not hard to understand; it has been demonstrated that about 45% of po-administered CeCI3 was present in the gastrointestinal content even 1 day after the administration in pigs (86).
The po administration of Zn2+-DTPA reduced the whole body retention of Ce to 1/20 to 1/30 of that in the untreated group by chelating Ce present in the gut and intestinal content (83,92).
Exposure to RE via Other Routes. Absorption of RE from the skin is known to be negligible (93); however, when the skin was stripped or wounded, RE seem to be absorbed into the body to some extent (93,94). Inaba and Yasumoto (93) reported that 4% of applied CeCl3 was absorbed from the stripped guinea pig's skin while 89% of CsCl and 79% of CoCl2 were absorbed from the skin under the same experimental conditions. It has been shown that Ce3+ was deposited in the liver, spleen, and bone following subcutaneous (sc) injection of Ce3+-citrate (95,96). Intramuscularly injected CeCI3 has been reported to accumulate in the lysosomes of the liver in rats and hamsters (97). Allard et al. (98) reported that 6% of intracisternally injected Gd3+-DOTA was found in the brain at 0.5 hr postinjection, and 58% of the brain Gd was located in the soluble fraction, suggesting that even chelated Gd with high stability is taken up by the brain to some extent.
Because RE is known to deposit in the skeleton, it is interesting to know what cells in the bone marrow take up RE. Only macrophages take up ip-injected CeCl3 in the bone marrow of rats (50); however, La was found in nuclear pores of marrow cells (especially erythroid cells) and the cell sap of light stromal cells when the rat bone marrow cells were exposed to La(NO3)3 in vitro under fixing conditions (99,100).

Toxicity
Mortality. As shown in Table 4, iv-, ip-, and po-administered ionic or chelated forms of RE are not highly toxic as far as the median lethal dose (LD50) is concerned. However, is it really possible to determine LD50 values for iv-injected RE? It has been shown that the percent mortality peaked at 20 to 40 mg Pr(NO3)3 /kg bw following iv injection in both mice and rats of both sexes; however, the lethality then decreased  as the dose increased. Even the lethality was abolished at 80 to 100 mg Pr(NO3)3/kg bw (105). In this bell-shaped dose-response mortality curve, mortality did not exceed 50% in male mice. Although more extensive study is required to answer the question about why the dose-response curve of the percent mortality is bell-shaped, the colloid formation of ionic RE in blood at higher doses of RE chlorides or nitrates might be resposible for the unusual dose-response curve in lethality. A marked increase in death due to pneumonia was found in mice when they were subacutely exposed to 30 mg/m3 of Gd2O3 dust (6 hr/day, 5 days/week, and up to 120 days) (106). Effects ofRare Earths on the Lung. As we described earlier, chronic exposure to RE dust probably causes pneumoconiosis in humans (14). It has been shown that intratracheal instillation of YC13 caused granulomatous changes in the rat lung (16). Inhalational exposure to high concentrations of Gd2O3 (106) and intratracheal instillation of YCI3 (16), LaCl3 (17), and GdCl3 (107) have been shown to cause pneumonitis and acute inflammation in the lung, e.g., infiltration of neutrophils and leakage of enzymes and proteins into the alveolar space, in mice and rats. The acute toxicity of YC13 in the rat lung was between those of ZnO and Cd compounds, judging from dose-related changes in lactate dehydrogenase activity in the bronchoalveolar lavage fluid (16).
Effeets ofRare Earths on the Liver. Intravenously injected RE chlorides increase vascular permeability for low molecular weight substances (108) and cause necrosis in the liver (109). Subcutaneous administration of Ce(NO3)3 has also been found to cause hepatic necrosis (96). Hepatic endoplasmic reticulum (ER) has been shown to be the primary target of ivinjected CeCl3 in the rat liver, and dilation, disorganization, and degranulation of rough ER and proliferation of smooth ER occurred following,the iv injection (110).
Pretreatment of rats with pregnenolone-16a-carbonitrile, spironolactone, and phenobarbital, which are known to proliferate smooth ER, and estradiol, a putative stabilizer of smooth ER, have been shown to reduce hepatic damage caused by CeCI3 in rats (101). It has also been demonstrated that pretreatment with pregnenolone 16acarbonitrile or nefolopin increased the relative liver weight and significantly reduced mortality caused by iv injection of CeCl3 in mice (58), suggesting that the liver is the primary target organ of iv-injected CeCl3.
It has been shown that iv injection of CeCl3 caused fatty liver in female rats (110,111) but not in male rats (111). Intravenous injection of YCl3, TbCl3, HoCl3 and YbCl3 caused focal necrosis with Ca deposition in rats but CeCl3 did not (111). We have shown that patchy Ca deposition occurred in the focal necrotic area of the rat liver following iv injection of YCl3 (-50 jig Y/g liver) (46). However, the reason that fatty liver was limited to female rats that received CeCI3 remained unknown. It seems that iv injection of CeCl3 produces lipid droplets in the liver of male mice (109).
There is a battery of reports about hepatic biochemical changes following iv injection of ionic RE; these reports are summarized in Table 5. There are differences in changes of RNA polymerase II activity among nitrates of Pr, Nd, Sm, Gd, Dy, and Er (120). The first three RE decreased RNA polymerase II activity while the latter three RE increased the activity; only Pr and Nd nitrates decreased RNA polymerase I activity while the other four did not change the RNA polymerase I activity. Otherwise, the biochemical changes are consistent among RE; those biochemical changes are increase in triglyceride in the liver (105,110,113,117) and increases in leakage of hepatic enzymes into blood (46,105,(111)(112)(113)(114)(115)(116). RE-induced hepatic injury seems to reduce P450 content and P450-related enzyme activities in rat (113) and mouse (109,119); however, the decreases in P450 activities (coumarin 7-hydroxylase and 7-ethoxyresorufin O-deethylase) at 3 to 4 days after iv injection of CeCI3 were preceded by increases in these enzyme activities at 1 to 2 days postinjection in DBA/2 mice (109,119). Serum very low density lipoprotein (VLDL) and high density lipoprotein (HDL) have been shown to be decreased following iv injection of Pr(NO3)3 in rats; the decrease is probably due to a decrease in hepatic secretion of these lipoproteins (118). It has also been reported that ip injection of CeCl3 causes lipid peroxidation and a decease in glutathione reductase activity in the chick liver (121).
Although serum glutamic-oxaloacetic and glutamic-pyruvic transaminase activities, well-known markers for acute hepatic injury, were increased with doses of  iv-injected Pr(NO3)3 up to 20 mg/kg bw, their activities were remarkably decreased at doses higher than 20 mg/kg bw in rats (105). Because formation of colloidal RE in blood significantly increased with doses of YCI3 (46), it is reasonable to suppose that iv-injected RE was taken up by Kupffer cells rather than by hepatocytes at doses higher than a maximum lethality. The uptake of colloidal RE by Kupffer cells may have reduced the uptake of RE by hepatocytes, resulting in the reduced hepatic injury.
Effects ofRare Earths on the Kidney, Spleen, and Gastrointestinal Tract. When the rat kidney was perfused with Krebs-Henseleit bicarbonate buffer containing 3 to 5.5 mM of chelated Dy (tripolyphosphate or triethylenetriaminehexaacetic acid) for 30 min, urinary concentrating ability was decreased and renal vascular resistance was increased (122).
Ethoxyresorufin O-deethylase activity in the kidney was decreased following iv injection of CeCl3 in mice (109). Lipid peroxidation was increased and glutathione content and antioxidant enzymes were decreased in the renal cortex following ip injection of LaCl3 in chicks (123).

Intravenous injection of LaCd3 or CeCl3
increased vascular permeability of the spleen in mice (108), and both sc and po administration of Ce3+-citrate caused hypertrophy, reticuloendothelial hyperplasia, and hyperactive lymphoid follicles in mice (96). Significant Ca deposition occurred in the spleen following ip injection of YCl3 (46). Oral administration of Ce3+-citrate has been shown to cause focal hemorrhage, necrosis of mucosa, and neutrophil infiltration in the stomach and duodenum (96). Effects ofRare Earths on the Eye and Skin. Exposure to EuCl3, DyCl3, HoC13, and ErCI3 caused conjunctivitis in rabbits when these RE chlorides were applied directly to their eyes (103,104). These RE chlorides have also been demonstrated to cause severe irritation when they are applied to abraded skin in rabbits and cause epilation and nodule formation when injected intradermally in guinea pigs (103,104). It has also been shown that sc injection of RE chlorides caused local calcification with mild fibrosis and accumulation of multinucleated giant cells, and the calcification area was increased with dose (up to 2 mg of RE chlorides) in mice (124).
Effects ofRare Earths on the Blood, Bone Marrow and Other Cells/Tissues. Intraperitoneal injection of LaC13 or NdCl3 significantly decreased the contents of sulfhydryl groups, cholesterol, phospholipid and lipid peroxides, and activities of galactosidase, glucuronidase, acetylcholinesterase, NADH dehydrogenase, ATPase, and p-nitrophenyl phosphatase in the red blood cell membrane in chicks. (125). It has also been shown that ip injection of LaCl3 decreased contents of sulfhydryl groups and lipid peroxides and increased activities of glutathione peroxidase, glutathione reductase, glutathione-S-transferase, and catalase in the bone marrow of chicks (126). Slight but significant aberration of bone marrow cells has been found following po administration of 1/10 of LD50 dose of RE nitrates in mice (102); however, no aberration was observed in spermatogoriia, spermatocytes, and sperm in those mice.
Basu et al. (127) have shown that the ip injection of LaCl3 caused a marked depression in the activities of neural Ca2+-ATPase, Mg2+-ATPase, and cholinesterase in chicks. The depression of these enzyme activities may be related to inhibitory effects of La3+ on binding of Ca2+ to brain synaptosomal membrane.
The median lethal concentration (LC50) for rat alveolar macrophages of CdO, CdCl2, LaC13, CeCl3, and Nd2O3 were 15, 28, 52, 29, and 101 pM, respectively, in vitro, and although La2O3 and Ce2O3 were less toxic than LaCl3 and CeCl3, respectively, Nd2O3 was more toxic than NdCl3 (128). Cytotoxicity of superconducting partides (YBa2Cu3067) has been shown to be almost the same as that of quartz (DQ12) using bovine alveolar macrophages (129). These in vitro studies using macrophages have been carried out in culture medium without serum. Thus, it remains unanswered as to how addition of serum (fetal bovine serum) in the macrophage culture system affected the cytotoxicity of RE.
Effects ofRare Earths on Behavior, Pregnaney, and Offspring. Ce-exposed mice exhibited significantly reduced open field behavior; ambulations were depressed after 10 sc injections (at 3-day intervals) of Ce3+-citrate at 20 mg Ce/kg body weight (95), and ambulation and rearing were depressed following sc injection of Ce3+citrate at doses of 136 to 173 mg Ce/kg body weight (96).
A single sc injection of Ce3+-citrate at a dose of 80 mg Ce/kg bw during either pregnancy or the lactating period significantly reduced the body weight of offspring in mice (130). It has also been shown that ip injection of LaCl3 (44 mg La/kg bw) increased the cessation of pregnancy and decreased the average litter size in pregnant mice (131). No malformation was observed in fetuses, even when the dams were administered po with a high dose of RE(NO3)3 (331 mg/ RE(NO3)3/kg bw) starting from the 16th day of gestation in rats (102).
Effects ofRare Earths on Growth, Longevity, and Carcinogenicity. The aortic contents of cholesterol, collagen, elastin, and Ca and urinary hydroxyproline excretion were increased in rabbits when they were kept on an atherogenic diet; intake of La (40 mg LaCl3/kg bw/day) significantly reduced the increases of these atherosclerotic parameters (132). The growth of mice was depressed when they were given 5 ppm of Sc3+ or Y3+ in drinking water, and the longevity was increased in Y3+-fed mice (133). However, no effect on growth was found in rats that had been fed a diet containing 0.1 to 1% of DyCl3, HoCl3, or ErCl3 for 12 weeks (104).
No carcinogenicity of RE has been found in animals (102,113,133). In addition, at 0.5 to 50 mg/ml of RE(NO3)3 (a mixture of Ce, La, Nd, Pr, and Sm), Ames mutagenicity tests were negative (133).
Rare Earths as Ca" Antagonists. The tonus and contractility of the rabbit ileum in response to acetylcholine or nicotine was decreased dose dependently by EuCl3 (103), DyCl3, HoCl3, and ErCI3 (104) in vitro. In the guinea pig, Tm3+, La3+, and Ce3+ inhibited contractile responses to K+ of longitudinal ileal muscle and the inhibitory effects increased in this order (134). The inhibitory effects of La3+ and Tm3+ on K+-or noradrenaline-induced contractile responses have also been demonstrated using the vas deferens of rats (135). The inhibitory effects of RE3+ on the contractility are due to displacement of membrane-bound Ca2+ with RE3+ (134) or modulation of the membrane stability by RE3+ (135).

Summary and Implications
The chemical forms of RE compounds primarily determine deposition and retention of RE following iv, po, sc, intratracheal, and inhalational exposure. The clearance of chelated RE from the body depends on the stability of the complexes. The chelated RE are excreted rapidly via urine, while unchelated ionic RE easily form colloid in blood, and the colloidal material is taken up by phagocytic cells of the liver and spleen.
Although the bone is one of the target organs of RE, it is not clear what cells in the bone take up the most REmacrophages, erythroid cells, or light reticular cells. It is important to investigate effects of RE on bone marrow cells because the clearance of RE from the bone is known to be very slow.
Inhalational or intratracheal exposure of animals to RE has been proven to cause acute pneumonitis with neutrophil infiltration in the lung; long-term exposure to RE dust seems to cause pneumoconiosis in human. However, the mechanism of neutrophil recruitment or interaction of RE with lung cells has not been fully investigated, except that intratracheally injected YC13 and LaCl3 were deposited in the lysosomes of macrophages and basement membranes of pneumocytes.
Mortality studies reveal that RE are not highly toxic (LD50 values for iv-injected RE are 10 to 100 mg/kg/bw and those of ip-injected RE are 150 to 700 mg/kg bw); cytotoxicity of RE to macrophages is comparable to that of Cd or silica in vitro. These discrepancies in lethal toxicity between in vivo and in vitro studies seem to be due to chemical forms of RE in the experimental system because those cytotoxicity studies were carried out in culture medium without serum. It is of interest to study the toxicity of RE using macrophages and other cells in various culture conditions.
There is much evidence that lanthanoid ions function as Ca2+ antagonists in vitro; however, there are few in vivo studies that relate the toxicity of RE to Ca2+-displacement from cells or biomolecules.
Because RE have been used directly in humans for therapy of cancer and synovitis and for diagnosis by magnetic resonance imaging, more extensive studies, including chronic exposure experiments, are required.