Categorization of nano-structured titanium dioxide according to physicochemical characteristics and pulmonary toxicity

Abstract

A potentially useful means of predicting the pulmonary risk posed by new forms of nano-structured titanium dioxide (nano-TiO₂) is to use the associations between the physicochemical properties and pulmonary toxicity of characterized forms of TiO₂. In the present study, we conducted intratracheal administration studies in rats to clarify the associations between the physicochemical characteristics of seven characterized forms of TiO₂ and their acute or subacute pulmonary inflammatory toxicity. Examination of the associations between the physicochemical characteristics of the TiO₂ and the pulmonary inflammatory responses they induced revealed (1) that differences in the crystallinity or shape of the TiO₂ particles were not associated with the acute pulmonary inflammatory response; (2) that particle size was associated with the acute pulmonary inflammatory response; and (3) that TiO₂ particles coated with Al(OH)₃ induced a greater pulmonary inflammatory response than did non-coated particles. We separated the seven TiO₂ into two groups: a group containing the six TiO₂ with no surface coating and a group containing the one TiO₂ with a surface coating. Intratracheal administration to rats of TiO₂ from the first group (i.e., non-coated TiO₂) induced only acute pulmonary inflammatory responses, and within this group, the acute pulmonary inflammatory response was equivalent when the particle size was the same, regardless of crystallinity or shape. In contrast, intratracheal administration to rats of the TiO₂ from the second group (i.e., the coated TiO₂) induced a more severe, subacute pulmonary inflammatory response compared with that produced by the non-coated TiO₂. Since alteration of the pulmonary inflammatory response by surface treatment may depend on the coating material used, the pulmonary toxicities of coated TiO₂ need to be further evaluated. Overall, the present results demonstrate that physicochemical properties may be useful for predicting the pulmonary risk posed by new nano-TiO₂ materials.

Keywords: Intratracheal administration; Nano materials; Pulmonary toxicity; Risk assessment; Titanium dioxide.

Fig. 1

Differential neutrophil ratio in bronchoalveolar lavage fluid at three days (A) and at four weeks (B) after intratracheal administration of various forms of TiO₂. Values are presented as average ± SD. * significant difference from control group (P < 0.05).

Fig. 2

Total protein content in bronchoalveolar lavage fluid at three days (A) and at four weeks (B) after intratracheal administration of various forms of TiO₂. Values are presented as average ± SD. * significant difference from control group (P < 0.05).

Fig. 3

Lactate dehydrogenase (LDH) activity in bronchoalveolar lavage fluid at three days (A) and at four weeks (B) after intratracheal administration of various forms of TiO₂. Values are presented as average ± SD. * significant difference from control group (P < 0.05).

Fig. 4

Relative lung weight at three days (A) and at four weeks (B) after intratracheal administration of various forms of TiO₂. Values are presented as average ± SD. * significant difference from control group (P < 0.05).

Fig. 5

Representative light micrographs of lung tissue from rats after intratracheal administration of 6 mg/kg of the indicated form of TiO₂. At three days after intratracheal administration: (A) Control, (B) MT-150AW, (C) TTO-S-3, (D) TTO-S-3 (Coated), (E) MP-100. At four weeks after intratracheal administration: (F) Control, (G) MT-150AW, (H) TTO-S-3, (I) TTO-S-3 (Coated), (J) MP-100.

Fig. 6

Associations between crystallinity (percentage of TiO₂ in rutile form) and bronchoalveolar lavage fluid parameter values at three days after intratracheal administration of indicated form of TiO₂. (A) Differential neutrophil ratio, (B) Total protein content, (C) Lactate dehydrogenase (LDH). Values are presented as the average of the difference between the value for the 2-mg/kg group and that of the control group. Percentage of TiO₂ in rutile form: AMT-100, 0%; MT-150AW, 100%; TTO-S-3, 100%; TTO-S-3 (Coated), 100%; P25, 20%; MP-100, 100%; FTL-100, 100%.

Fig. 7

Associations between aspect ratio and bronchoalveolar lavage fluid parameter values three days after intratracheal administration of various forms of TiO₂. (A) Differential neutrophil ratio, (B) Total protein content, (C) Lactate dehydrogenase (LDH). Values are presented as the average of the difference between the value for the 2-mg/kg group and that of the control group. Aspect ratios: AMT-100, 1.00; MT-150AW, 3.79; TTO-S-3, 5.00; TTO-S-3 (Coated), 5.00; P25, 1.00; MP-100, 1.00; FTL-100, 12.9.

Fig. 8

Associations between particle size (nm) and bronchoalveolar lavage fluid parameter values three days after intratracheal administration of various forms of TiO₂. Volume average diameter: (A) Differential neutrophil ratio, (B) Total protein content, (C) Lactate dehydrogenase (LDH). Number average diameter: (D) Differential neutrophil ratio, (E) Total protein content, (F) LDH. Solid line is the linear regression line derived from five test materials (excluding the data for TTO-S-3 (Coated) and FTL-100). Values are presented as the average of the difference between the value for the 2-mg/kg group and that of the control group.

Fig. 9

Dose—response curves for bronchoalveolar lavage fluid parameters three days after intratracheal administration of various forms of TiO₂. Administration doses expressed as mass (mg/kg): (A) Differential neutrophil ratio, (B) Total protein content, (C) Lactate dehydrogenase (LDH). Administration doses normalized by particle size (mg/kg/nm): (D) Differential neutrophil ratio, (E) Total protein content, (F) LDH. Solid lines are the sigmoidal dose—response curve derived from five test materials (excluding the data for TTO-S-3 (Coated) and FTL-100). Values were taken from the 2-mg/kg group and are presented as average ± SD.