Subchronic Nasal Toxicity of Hexamethylphosphoramide Administered to Rats Orally for 90 Days

FUNDAMENTAL AND APPLIED TOXICOLOGY 40, 1 5 - 2 9 (1997) ARTICLE NO. FA972375 Subchronic Nasal Toxicity of Hexamethylphosphoramide Administered to Rats Orally for 90 Days Douglas A. Keller, Craig E. Marshall, and K. P. Lee E. I. du Pont de Nemours and Company, Haskell Laboratory for Toxicology and Industrial Medicine, 1090 Elkton Road, P.O. Box 50, Newark, Delaware 19714 Received September 30, 1996; accepted September 3, 1997 the nasal tissue via inhalation. Oral administration of HMPA is a less potent route for producing nasal lesions than is inhalation. Subchronic Nasal Toxicity of Hexamethylphosphoramide Administered to Rats Orally for 90 Days. Keller, D. A., Marshall, C. E., and Lee, K. P. (1997). Fundam. AppL ToxicoL 40, 15-29. C 1997 Sodety of Tmdcotogy. Rats were administered hexamethylphosphoramide (HMPA) at dosages of 10,100, 300, and 1000 ppm in drinking water or at 15, 40, or 120 mg/kg/day by gavage for approximately 90 days. Another group of rats was implanted subcutaneously with HMPAfilled osmotic minipumps, designed to deliver a dosage of 40 mg/ kg/day to prevent the possibility of direct contact of HMPA with the nasal epithelium. After 90 days at 10 ppm in the drinking water, some rats had tracheas lined with regenerated epithelium, but no HMPA-related lesions were present in any other organs and tissues. At 100 ppm, nasal lesions (epithelial denudation, regeneration, and squamous metaplasia) were mostly in the maxillo turbinates, tips of nasoturbinates, and the adjacent septum in the anterior nasal cavity (level I), but the lesions were confined to the ventral region of the mid-anterior nasal cavity (level II) and to recesses of the posterior nasal cavity (levels III and IV). At 300 ppm, nasal turbinates in level I were partially adhered to the nasal septum byfibroustissue. In level II the lesions were mainly confined to the ventral medial meatus, but were scattered diffusely in levels HI and IV. Denuded turbinates showed minimal bone proliferation. At 1000 ppm, the anterior nasal cavity was partially occluded by extensive adhesion of the turbinates to the nasal septum by granulation tissue and proliferating turbinate bone. The general architecture of the posterior nasal cavity was obliterated by the marked proliferation of turbinate bone and fibrous tissue in the interturbinate spaces. Tracheas showed regenerated epithelium and bronchi had focal epithelial denudation at 100, 300, and 1000 ppm. Foamy alveolar macrophages (histiocytosis) were increased in the lungs at 300 and 1000 ppm. Testicular atrophy occurred at 1000 ppm. No other tissues were affected by HMPA treatment Nasal lesions in rats given HMPA by gavage were identical in nature to, but sometimes slightly more severe than, the lesions in rats given HMPA in the drinking water. Rats given 40 mg/kg/day HMPA via an osmotic minipump had slightly less severe nasal lesions than did the rats given the same dosage of HMPA by gavage. Testicular atrophy was present in the rats given 120 mg/kg/day by gavage. The results of this study show that, with the exception of bone proliferation, systemic delivery of HMPA or its metabolites to the nasal tissue following oral administration causes tissue damage similar to that caused by direct exposure of 15 Hexamethylphosphoramide (HMPA) has unique solvent properties of being both highly water-soluble and highly organic soluble and was at one time widely used in organic and organo-metallic reactions in research laboratories. Industrially, it was used as a processing solvent for aromatic polyimide fiber, a polymerization catalyst, a stabilizer against thermal degradation in polystyrene, and an additive to polyvinyl and polyolefin resins. It also has been tested as a chemosterilant for insects, as an antistatic agent, a flame retardant, and a deicing additive for jet fuels (NIOSH, 1975). Acutely, HMPA has low oral toxicity with an LD50 in the range of 2500-5000 mg/kg, moderate dermal toxicity to rabbits with an LD50 of 2600 mg/kg, and moderate inhalation toxicity with a 4-h LC50 of 400 ppm. Subchronically, severe bronchiectasis and bronchopneumonia were observed in the lungs of rats fed a dietary concentration of 2000 ppm HMPA (127-106 mg/kg) over a period of 52-72 days (Kimbrough and Sedlak, 1968). Similarly, exacerbation of murine pneumonia was observed in rats fed HMPA at 10 and 50 mg/kg for 92 days (Short et ai, 1971). Rhinitis, murine pneumonia, and/or testicular atrophy were found in rats administered HMPA at 100 mg/kg for 62-100 days (Overcash etal, 1971). In a 2-year feeding study in rats at dosages of 6.25, 3.12, 1.56, or 0.78 mg/kg HMPA daily, there were no differences in the incidence of tumors between treated and control animals (Kimbrough and Gaines, 1973). Nasal tumors were detected after 6 months in rats exposed by inhalation to 400 and 4000 ppb and after 13 months of exposure to 50 ppb (Lee and Trochimowicz, 1982a). Several studies have shown that HMPA is extensively metabolized by rat and dog nasal epithelium (Dahl et ai, 1982; Hadley and Dahl, 1982; Rickard and Quarels, 1981; Kuykendall et al, 1995). The only metabolites of HMPA presently known are the demethylated derivatives penta- 0272-0590797 $25.00 Copyright © 1997 by the Society of Toxicology. All rights of reproduction in any form reserved. 16 KELLER, MARSHALL, AND LEE methylphosphoramide and tetramethylphosphoramide and the demethylation product formaldehyde (Jones and Jackson, 1968). The potency of the carcinogenic response produced by inhalation of HMPA in rats has generated much interest in the mutagenicity of HMPA and its metabolites. HMPA is mutagenic in several in vivo assays, such as the mouse bone marrow micronucleus test (Ashby et al., 1985a) and the Drosophila melanogaster sex-linked recessive lethal assay (Zijlstra et al., 1989). Results from in vitro tests show weak activity of HMPA with metabolic activation or, in some cases, lack of activity (Amacher and Turner, 1985; Ashby et al., 1985a,b). Most studies indicate a role for metabolism of HMPA to formaldehyde, a known rat nasal carcinogen, in the mutagenic process for HMPA (Kuykendall et al., 1995). HMPA has low volatility and is not uniquely an inhalation hazard. The high carcinogenic potency of HMPA by inhalation and the lack of overt nasal effects after chronic oral administration at low doses suggests that more information is needed about the dose-response relationships in the oral toxicity of HMPA. The purpose of the present study was to determine the potential oral toxicity of HMPA in rats, with respect to the dose-dependence and site-specificity of lesions, by administration in drinking water and by oral gavage. An osmotic minipump implant group was included in the oral gavage study to preclude definitively direct exposure of the nasal passages to HMPA (a potential confounding factor in the drinking water study), and to test the effect of delivery rate (continuous exposure versus bolus) of HMPA on the toxici (...truncated)