5
EVALUATION OF THE ARMY'S INTERIM REFERENCE DOSE FOR GD

THE CHEMICAL-WARFARE agent GD (also known as soman) is an organophosphate nerve agent found at several stockpile and nonstockpile munition sites in the United States. At the request of the U.S. Army, Oak Ridge National Laboratory (ORNL) conducted a health risk assessment of GD. The assessment included a detailed analysis of GD's physical and chemical properties, environmental fate, mechanism of action, and animal and human toxicity data (see Appendix C, Health Risk Assessment of GD, ORNL 1996). On the basis of that assessment, ORNL proposed a reference dose (RfD) of 4 × 10-6 mg/kg of body weight per day for noncancer health effects of GD exposure. Because there was no evidence that GD is carcinogenic, a slope factor was not derived. The Army's Surgeon General accepted ORNL's proposed RfD as an interim exposure value until an independent evaluation of the proposed RfD was conducted by the National Research Council (NRC). This chapter contains the NRC's independent assessment of the scientific validity of the Army's interim RfD for GD.

DERIVATION OF THE ARMY'S INTERIM RFD

The Army's interim RfD for GD is 4 × 10-6 mg/kg per day. ORNL (1996) calculated that value on the basis of the lowest oral dose of GD that caused significant depression in plasma-cholinesterase (ChE) activity in



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents 5 EVALUATION OF THE ARMY'S INTERIM REFERENCE DOSE FOR GD THE CHEMICAL-WARFARE agent GD (also known as soman) is an organophosphate nerve agent found at several stockpile and nonstockpile munition sites in the United States. At the request of the U.S. Army, Oak Ridge National Laboratory (ORNL) conducted a health risk assessment of GD. The assessment included a detailed analysis of GD's physical and chemical properties, environmental fate, mechanism of action, and animal and human toxicity data (see Appendix C, Health Risk Assessment of GD, ORNL 1996). On the basis of that assessment, ORNL proposed a reference dose (RfD) of 4 × 10-6 mg/kg of body weight per day for noncancer health effects of GD exposure. Because there was no evidence that GD is carcinogenic, a slope factor was not derived. The Army's Surgeon General accepted ORNL's proposed RfD as an interim exposure value until an independent evaluation of the proposed RfD was conducted by the National Research Council (NRC). This chapter contains the NRC's independent assessment of the scientific validity of the Army's interim RfD for GD. DERIVATION OF THE ARMY'S INTERIM RFD The Army's interim RfD for GD is 4 × 10-6 mg/kg per day. ORNL (1996) calculated that value on the basis of the lowest oral dose of GD that caused significant depression in plasma-cholinesterase (ChE) activity in

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents rats. The lowest-observed-adverse-effect level (LOAEL) of GD was 0.0175 mg/kg per day in a subchronic toxicity study (Bucci et al. 1992). In that study, male and female rats were administered GD by gavage 5 days a week for 13 weeks. Because of the discontinuous exposure regimen, ORNL adjusted the LOAEL (LOAELadj) for continuous exposures by multiplying 0.0175 mg/kg per day by a factor of 5/7 (i.e., 5 days/7 days) to yield a LOAELadj of 0.0125 mg/kg per day. The RfD for GD was calculated to be 4 × 10-6 mg/kg per day by dividing the LOAELadj by 2,700, the product of the uncertainty factors and the modifying factor selected by ORNL. APPROPRIATENESS OF THE CRITICAL STUDY The critical study used by ORNL for deriving the RfD for GD was a subchronic toxicity study (Bucci et al. 1992) in which Caesarian-derived Sprague-Dawley rats (12 males and 12 females per group) were administered GD by gavage at doses of 0.0175, 0.035, and 0.07 mg/kg per day for 5 days a week for 13 weeks and then sacrificed and necropsied. Plasma-ChE and red-blood-cell (RBC)-acetylcholinesterase (AChE) measurements, as well as several other blood measurements, were taken before dosing and at the end of weeks 1, 3, 7, and 13. Significant depression in plasma ChE was observed in male and female rats of the high-dose group at weeks 1 and 7 and in mid-dose males at week 7 compared with control values. No significant effect on RBC-AChE concentrations were observed. ORNL reanalyzed the data with analysis of variance and Dunnett's and Scheffe's comparisons and reported that at week 3 in females and week 7 in males, RBC-AChE concentrations of all dose groups were significantly lower than those of controls, but no dose-response relationship was found. In females during week 1, RBC-AChE concentrations in the controls and in all dose groups were inexplicably increased relative to baseline values (measurements taken before exposure). With regard to plasma ChE, a dose-related significant decrease relative to controls was observed at weeks 1 and 7 for male and female rats. In comparison with baseline values, plasma-ChE concentrations of the mid- and high-dose groups were significantly reduced at weeks 1, 3, 7, and 13 for males and females (with the exception of high-dose females at week 3). The lowest dose of 0.0175 mg/kg per day was considered by ORNL to be the LOAEL for the study because the reduction in plasma

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents ChE (relative to controls) at that dose was statistically significant and because the plasma-ChE activity during week 1 was reduced to 39% of baseline in males and 57% of baseline in females. The critical study (Bucci et al. 1992) involved a relevant route of exposure (oral) for determining an RfD. Rats were administered GD by oral gavage, a route of administration that exaggerates the exposure that would normally occur from methods resulting in a slower rate of delivery (e.g., in feed or water). The study was subchronic in duration (13 weeks) rather than chronic (104 weeks), and ChE measurements varied and did not show a consistent dose-response relationship. Thus, the subcommittee believes that the study was too short in duration, and the results were too variable to form an ideal basis for determining a LOAEL. In addition, the methods used to analyze ChE were not ideal (see Appendix G). However, in the absence of other well-conducted studies, the sub-committee agrees with ORNL that the study by Bucci et al. (1992) is the most appropriate of the available studies for derivation of the RfD for GD. APPROPRIATENESS OF CRITICAL END POINT The LOAELadj (0.0125 mg/kg per day) used by ORNL for derivation of the RfD for GD was based on the lowest dose that caused a significant depression in plasma-ChE activity in rats (Bucci et al. 1992). The sub-committee notes that ChE inhibition is typically considered a biomarker of exposure to organophosphate agents rather than an adverse effect. However, it is generally agreed that inhibition of ChE contributes to the overall hazard identification of ChE-inhibiting agents. The U.S. Environmental Protection Agency (EPA) has used ChE inhibition to establish RfDs for several organophosphate pesticides, such as malathion (EPA 1992) and ethion (EPA 1989). The subcommittee considered other possible toxicity end points, notably neurotoxicity, associated with GD exposure. Organophosphate compounds like GD may act directly on nerve cell receptors or, by inhibiting neural AChE, interfere with neuromuscular transmission and produce delayed-onset subjunctional muscle damage. In addition, some organophosphate compounds cause a neurotoxic effect (organophosphate-induced delayed neuropathy, or OPIDN) that is not associated with ChE inhibition. Emerging research in this area might indicate alternative

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents end points to RBC-ChE inhibition that could be used to derive RfDs for nerve agents in the future. The subcommittee also notes that additional human data are available on anti-ChE agents, which were not included in ORNL's assessment. Data summaries from human experimentation conducted in the 1950s and 1960s were evaluated by the NRC in a series of reports titled Possible Long-Term Health Effects of Short-Term Exposure to Chemical Agents (NRC 1982, 1984, 1985). The reports include an evaluation of health records of volunteer soldiers who were exposed intravenously or intramuscularly to chemical-warfare agents, as well as a followup morbidity study conducted by the NRC in 1985. The NRC found no long-term health effects from short-term exposure to any specific chemical-warfare agent, but there was some evidence of an increase in malignant neoplasms among men exposed to anti-ChE agents. While these studies are not directly applicable to deriving RfDs, the studies add to the completeness of the data base on GD. Provided that appropriate assays were used, the subcommittee finds no reason at this time to alter the practice of using RBC-ChE or plasma-ChE inhibition as the critical toxicity end point, and agrees with ORNL that such inhibition is the best available critical noncancer end point on which to base the calculation of the RfD for GD. APPROPRIATENESS OF UNCERTAINTY FACTORS For GD, ORNL assigned values greater than 1 to five uncertainty factors and a value of 1 to the modifying factor. The product of those factors was 2,700. The subcommittee evaluated each of the uncertainty factors and the modifying factor below. EXTRAPOLATION FROM ANIMAL TO HUMAN ORNL assigned a factor of 10 for the uncertainty factor for the extrapolation of data from animals to humans (UFA) because no evidence suggests that humans are less susceptible than rats to GD. ORNL cited the evidence that rodents have much lower RBC-AChE activity than humans (Ellin 1981), suggesting that rats might be more susceptible than humans to anti-ChE compounds, but also noted that the lower RBC-AChE

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents activity might be offset by aliesterases (e.g., carbonyl esterase) present in the blood of rats. These enzymes, which are not found in humans (Cohen et al. 1971), are known to bind to and, therefore, reduce the toxicity of nerve agents. Rats have true ChE (AChE) in their plasma (Traina and Serpietri 1984), which might reduce the toxicity of GD in rats. Furthermore, studies (Grob and Harvey 1958; Bucci and Parker 1992) with a similar nerve agent (GB) suggest that depression of RBC or plasma ChE with repeated oral administrations of nerve agents is much more difficult to induce in rats than in humans. The available data on GD are insufficient to fully evaluate species differences with regard to ChE activity in humans and rats. Few human acute toxicity data can be compared with the available rat data. However, the subcommittee notes that acute toxicity data are available for comparisons with monkeys. For example, the intramuscular LD50 for GD is reported to be 9.5 µg/kg in monkeys (Lipp 1972) and 62 µg/kg in rats (Schoene et al. 1985), and the subcutaneous LD50 for GD is reported to be 13 µg/kg in monkeys (Fukuyama and Ashwick, unpublished material (1963), as cited in Dirnhubert et al. 1979) and 75 µg/kg in rats (Bošković et al. 1984). Those data suggest that monkeys are approximately six times more susceptible than rats to GD. The subcommittee acknowledges the limitations of drawing any conclusions from this comparison because the data from monkeys are not necessarily directly applicable to humans and because the studies did not use the oral route of administration and involved only single exposures. However, given the enzyme differences between humans and rats described above and the available data on GB (see Chapter 4), a similar nerve agent, the subcommittee agrees with ORNL that a factor of 10 is appropriate for interspecies extrapolation. The factor of 10 should be considered an estimate of the difference in sensitivity between humans and rats to GD and not a default value. PROTECTING SUSCEPTIBLE SUBPOPULATIONS ORNL used a factor of 10 for the uncertainty factor to protect susceptible subpopulations (UFH) because some individuals have a genetic polymorphism causing their serum-ChE activity to be abnormally low (Evans et al. 1952; Harris and Whittaker 1962). For homozygous individuals, the activity can be as low as 8–21% of the normal mean (Bonderman and Bonderman 1971). Genetic polymorphisms are also recognized for buty-

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents rylcholinesterase and paraoxonase, enzymes that might function in the sequestration and metabolism of organophosphate nerve agents (Loewenstein-Lichtenstein et al. 1995; Maekawa et al. 1997; Furlong et al. 1998). Individuals with those polymorphisms might be unusually susceptible to organophosphate anti-ChE compounds (Morgan 1989). The subcommittee agrees that a factor of 10 is appropriate for protecting this susceptible subpopulation. EXTRAPOLATION FROM LOAEL TO NOAEL ORNL assigned a factor of 3 rather than 10 to the uncertainty factor for extrapolation from a LOAEL to a NOAEL (UFL) because ChE inhibition is a biomarker of exposure rather than a toxic effect. Although it could be argued that a dose of GD that significantly induces ChE inhibition in the absence of toxic effects is indicative of a NOAEL rather than a LOAEL, the subcommittee agrees that a factor of 3 is a prudent choice for UFL. EXTRAPOLATION FROM SUBCHRONIC TO CHRONIC EXPOSURES ORNL noted that in the derivation of RfDs for other organophosphate compounds, EPA (1989, 1992) used NOAELs for ChE inhibition that were based on subchronic data without adjustment for chronic exposures, because ChE inhibition is unlikely to change over time. Hence, a factor of 1 was used for the uncertainty factor for extrapolation from subchronic to chronic exposures (UFS). For example, studies with the nerve agent VX indicate that maximal ChE inhibition occurs after 30–60 days of exposure and then levels off and sometimes shows signs of recovery (Goldman et al. 1988). However, because chronic exposure studies are not available to verify that additional effects would not occur from longer exposures to GD, ORNL assigned a factor of 3 to UFS. The subcommittee agrees that a factor of 3 is appropriate. DATA-BASE ADEQUACY As noted by ORNL, the data base for GD lacks chronic oral studies in two species and reproductive and developmental toxicity studies. However, because studies on other nerve agents, including a multigeneration

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents reproductive study on VX (Goldman et al. 1988) and two developmental toxicity studies of GB (Denk 1974; La Borde and Bates 1986), indicate that reproductive and developmental effects are unlikely, the subcommittee agrees that a factor of 3 for the uncertainty factor for data-base adequacy (UFD) is adequate to account for the incomplete data base. MODIFYING FACTOR FOR ADDITIONAL UNCERTAINTY The subcommittee considers the uncertainties of the data on GD to be represented adequately by the values assigned to the uncertainty factors above and agrees with ORNL that a modifying factor (MF) of 1 is appropriate. SUMMARY Table 5-1 presents the values assigned to the uncertainty factors and the modifying factor by ORNL and those recommended by the subcommittee. The subcommittee's recommendations are the same as those of ORNL. TABLE 5-1 Uncertainty Factors Used by ORNL and the NRC to Calculate the RfD for GD Uncertainty Factor Description ORNL NRC UFA For animal-to-human extrapolation 10 10 UFH To protect susceptible subpopulations 10 10 UFL For LOAEL-to-NOAEL extrapolation 3 3 UFS For subchronic-to-chronic extrapolation 3 3 UFD For data-base adequacy 3 3 MF Modifying factor for additional uncertainty 1 1 TOTAL UF   2,700 2,700 Abbreviations: LOAEL, lowest-observed-adverse-effect level; MF, modifying factor; NOAEL, no-observed-adverse-effect level; NRC, National Research Council; ORNL, Oak Ridge National Laboratory; RfD, reference dose; UF, uncertainty factor

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents WEIGHT AND STRENGTH OF EVIDENCE The subcommittee believes that the strength of evidence for the Army's interim RfD of 4 × 10-6 mg/kg per day is moderately good. There is a possibility that the LOAEL (0.0175 mg/kg per day) used to calculate the RfD for GD is not accurate, because lower doses were not tested and variability in the plasma-ChE values was considerable. The subcommittee believes that because ChE inhibition is a biomarker of exposure rather than a toxic effect, use of this end point overestimates the oral toxicity of GD. CONCLUSIONS The approach used by ORNL to calculate the RfD for GD is consistent with the guidelines of the EPA. On the basis of available toxicity and related data on GD, the subcommittee concludes that the Army's interim RfD for GD of 4 × 10-6 mg/kg per day is scientifically valid. DATA GAPS AND RESEARCH RECOMMENDATIONS The major gap in the available information on GD is the lack of an oral subchronic or chronic toxicity study that demonstrates a clear dose-response relationship between GD exposure and ChE inhibition. The absence of that type of data could be addressed by conducting a subchronic oral toxicity study that assesses anti-ChE activity in RBCs and plasma in one or preferably two species. At least one dose between 0 and 0.0175 mg/kg per day should be used. Range-finding studies focusing on ChE analytical methods offer the best possibility of filling the data gap. If further research reveals that significant toxic effects can be induced by any of the nerve agents evaluated (i.e., GA, GB, GD, or VX) at doses below those that cause significant ChE inhibition, new studies should be conducted to reassess the safety of the recommended RfD for GD. REFERENCES Bošković, B., V. Kovacević, and D. Jovanović. 1984. 2-PAM chloride, HI-6 and HGG 12 in soman and tabun poisoning. Fundam. Appl. Toxicol. 4:106–115.

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents Bonderman, R.P., and D.P. Bonderman. 1971. Atypical and inhibited human serum pseudocholinesterase. A titrimetric method for differentiation. Arch. Environ. Health 22:578–581. Bucci, T.J., and R.M. Parker. 1992. Toxicity Studies on Agents GB and GD (Phase II): 90 Day Subchronic Study of GB (Sarin, Type II) in CD Rats. Final Report. FDA 224-85-0007. DTIC AD-A248618. Prepared by the National Center for Toxicological Research, Jefferson, Ark., for the U.S. Army Biomedical Research and Development Laboratory, Fort Detrick, Frederick, Md. Bucci, T.J., R.M. Parker, J.A. Crowell, J.D. Thurman, and P.A. Gosnell. 1992. Toxicity Studies on Agents GB and GD (Phase II): 90 Day Subchronic Study of GD (Soman) in CD Rats. Final Report. FDA 224-85-0007. DTIC ADA257182. Prepared by the National Center for Toxicological Research, Jefferson, Ark., for the U.S. Army Biomedical Research and Development Laboratory, Fort Detrick, Frederick, Md. Cohen, E.M., P.J. Christen, and E. Mobach. 1971. The Inactivation of Oximes of Sarin and Soman in Plasma from Various Species. I. The Influence of Diacetylmonoxime on the Hydrolysis of Sarin. Pp. 113–131 in Proceedings of the Koninklijke Nederlandse Akademie Van Wetenschappen, Series C, Biological and Medical Sciences, Vol. 74. J.A. Cohen Memorial Issue. Amsterdam: North-Holland. Denk, J.R. 1975. Effects of GB on Mammalian Germ Cells and Reproductive Parameters. EB-TR-74087. AD-A006503. Edgewood Arsenal, Aberdeen Proving Ground, Edgewood, Md. Dirnhuber, P., M.C. French, D.M. Green, L. Leadbeater, and J.A. Stratton. 1979. The protection of primates against soman poisoning by pretreatment with pyridostigmine. J. Pharm. Pharmacol. 31:295–299. Ellin, R.I. 1981. Anomalies in Theories and Therapy of Intoxication by Potent Organophosphorus Anticholinesterase Compounds. Special Publication USA-BML-SP-81-003. DTIC AD-A101364. U.S. Army Medical Research and Development Command, Biomedical Laboratory, Aberdeen Proving Ground, Edgewood, Md. EPA (U.S. Environmental Protection Agency). 1989. Ethion Reference Dose for Chronic Oral Exposure. Integrated Risk Information System (IRIS). Online file. http://www.epa.gov/iris/ (Accessed July 22, 1998). EPA (U.S. Environmental Protection Agency). 1992. Malathion Reference Dose for Chronic Oral Exposure. Integrated Risk Information System (IRIS). Online file. http://www.epa.gov/iris/ (Accessed July 22, 1998). Evans, F.T., P.W.S. Gray, H. Lehmann, and E. Silk. 1952. Sensitivity to succinylcholine in relation to serum cholinesterase. Lancet i:1129–1230. Furlong, C.D., W.F. Li, L.G. Costa, R.J. Richter, D.M. Shih, and A.J. Lusis. 1998. Genetically determined susceptibility to organophosphorus insecticides and nerve agents: Developing a mouse model for the human PON1 polymorphism. Neurotoxicology 19:645–650.

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents Goldman, M., B.W. Wilson, T.G. Kawakami, L.S. Rosenblatt, M.R. Culbertson, J.P. Schreider, J.F. Remsen, and M. Shifrine. 1988. Toxicity Studies on Agent VX. Final Report. DTIC AD-A201397. Prepared by the Laboratory for Energy-Related Health Research, University of California, Davis, Calif., for the U.S. Army Medical Research and Development Command, Fort Detrick, Frederick, Md. Grob, D., and J.C. Harvey. 1958. Effects in man of the anticholinesterase compound Sarin (isopropyl methyl phosphonofluoridate). J. Clin. Invest. 37:350–368. Harris, H., and M. Whittaker. 1962. The serum cholinesterase variants. A study of twenty-two families selected via the ''intermediate" phenotype. Ann. Hum. Genet. 26:59–72. La Borde, J.B., and H.K. Bates. 1986. Developmental Toxicity Study of Agent GB—DCSM Types I and II in CD Rats and NZW Rabbits. Final Report. Prepared by the National Center for Toxicological Research, U.S. Food and Drug Administration, Jefferson, Ar., for the U.S. Army Medical Research and Development Command, Fort Detrick, Md. Lipp, J.A. 1972. Effect of diazepam upon soman-induced seizure activities and convulsions. Electroencephalogr. Clin. Neurophysiol. 32:557–560. Lowenstein-Lichtenstein, Y., M. Schwarz, D. Glick, B. Norgaard-Pederson, H. Zakut, and H. Soreq. 1995. Genetic predisposition to adverse consequences of anti-cholinesterases in "atypical" BCHE carriers. Nat. Med. 1:1082–1085. Maekawa, M., K. Sudo, D.C. Dey, J. Ishikawa, M. Izumi, K. Kotani, and T. Kanno. 1997. Genetic mutations of butyrylcholine esterase identified from phenotypic abnormalities in Japan. Clin. Chem. 43:924–929. Morgan, D.P. 1989. Recognition and Management of Pesticide Poisonings, 4th Ed. EPA-540/9-88-001. Health Effects Division, Office of Pesticide Programs, U.S. Environmental Protection Agency, Washington, D.C. NRC (National Research Council). 1982. Possible Long-Term Health Effects of Short-Term Exposure to Chemical Agents, Vol. 1. Anticholinesterases and Anticholinergics. Washington, D.C.: National Academy Press. NRC (National Research Council). 1984. Possible Long-Term Health Effects of Short-Term Exposure to Chemical Agents, Vol. 2. Cholinesterase Reactivators, Psychochemicals, and Irritants and Vesicants . Washington, D.C.: National Academy Press. NRC (National Research Council). 1985. Possible Long-Term Health Effects of Short-Term Exposure to Chemical Agents, Vol. 3. Final Report. Current Health Status of Test Subjects. Washington, D.C.: National Academy Press. ORNL (Oak Ridge National Laboratory). 1996. Health Risk Assessment for the Nerve Agent GD (Soman). Draft Report. Interagency Agreement No. 1769-1769-A1. Prepared by Oak Ridge National Laboratory, Life Sciences Division, Oak Ridge, Tenn., for the U.S. Department of the Army, Army Environmental Center, Aberdeen Proving Ground, Edgewood, Md.

OCR for page 48
Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents Schoene, K., D. Hochrainer, H. Oldiges, M. Krugel, N. Franzes, and H.J. Bruckert. 1985. The protective effect of oxime pretreatment upon the inhalative toxicity of sarin and soman in rats. Fundam. Appl. Toxicol. 5:S84–S88. Traina, M.E. and L.A. Serpietri. 1984. Changes in the levels and forms of rat plasma cholinesterases during chronic diisopropylphosphorofluoridate intoxication. Biochem. Pharmacol. 33:645–653.