1413
1. Robson KJH, Hall
JRS, Jennings MW, et al. A highly conserved amino-acid sequence in thrombospondin, properdin and in proteins from sporozoites and blood stages of a human malaria parasite. Nature 1988; 335: 79-82. 2. Lawler J, Hynes RO. The structure of human thrombospondin, an adhesive glycoprotein with multiple calcium-binding sites and homologies with several different proteins. J Cell Biol 1986, 103: 1635-48. 3. Goundis D, Reid KBN. Properdin, the terminal complement components, thrombospondin and the circumsporozoite protein of malana parasites contain similar sequence motifs. Nature 1988; 335: 82-85. 4. Robson KJH, Hall JRS, Davies LC, Crisanti A, Hill AVS, Wellems TE. Polymorphism of the TRAP gene of Plasmodium falciparum. Proc R Soc Lond [Biol] 1990; 242: 205-16. 5. Good MF, Berzofsky JA, Miller L. The T cell response to the malaria circumsporozoite protein: an immunological approach to vaccine development. Annu Rev Immunol 1988; 6: 663-88. 6. Khusmith S, Charoenvit Y, Kumar S, Sedegah M, Beaudoin RL, Hoffman SL. Protection against malaria by vaccination with sporozoite surface protein 2 plus CS protein. Science 1991; 252: 715-18. 7. Hedstrom RC, Campbell JR, Leef ML, et al. A malaria sporozoite surface antigen distinct from the circumsporozoite protein. Bull WHO 1990; 68: 152-57. 8. Malik A, Egan JE, Houghten RA, Sadoff JC, Hoffman SL. Human cytotoxic T lymphocytes against the Plasmodium falciparum circumsporozoite protein. Proc Natl Acad Sci USA 1991; 88: 3300-04.
Arsenic, selenium, and African
trypanosomiasis SIR,-Melarsoprol is used to treat African trypanosomiasis, despite an associated case fatality rate of 2-10%, because it is usually the only drug available. Eflornithine, a recently introduced alternative, costs about$200 for a 14-day course. Post-arsenical reactive encephalopathy (PARE) has restricted the use of melarsoprol to the final stages of the disease, by which time the patient is usually debilitated and severely malnourished. Dr Hunter and colleagues (May 18, p 956) demonstrate trypanosomal DNA in the brains of fatal cases of PARE and reproduce, in mice, its pathological features by using a drug that does not penetrate the central nervous system. They suggest that "removal of the parasites from all parts of the body except the brain induces a restored immune system to respond vigorously to parasites within the brain". They cite an unpublished observation that subcurative doses of arsenicals also induce PARE whereas
higher doses do not, and now recommend more aggressive treatment-presumably higher doses of melarsoprol. This presupposes that Hurst’s suggestion! that PARE is related to arsenic toxicity is incorrect. Although it is generally thought that PARE has an immunological basis, Hunter’s suggested mechanism not only requires that the patient’s immunosuppression is entirely due to the trypanosomiasis alone but also that immunocompetence is restored, within days, despite persistence of cerebral, trypanosomal infection. The demonstration that the dose of arsenical used is not toxic in normal uninfected animals is an unsafe basis for dismissing potential toxicity in chronically infected animals. The organic arsenicals chelate elements of the sulphur/selenium group. Trypanosomes are very vulnerable to depletion of glutathione in vivo.2Arsenicals are trypanocidal by depriving the protozoa of a unique low-molecular-weight thiol (trypanothione3) that has an even higher affmity for melarsoprol than does glutathione, the low-molecular-weight thiol of most organisms. Indeed, melarsoprol is a condensation product between melarsen oxide and dimercaprol and the latter moiety alleviates some of the basic compound’s effects on the thiols of normal cells. If an infected patient (or laboratory animal) is either sulphur or selenium deficient an organic arsenical is likely to be much more toxic than usual. Severe malnutrition, infection-associated immunosuppression, protozoal infection, and a low sulphur aminoacid intake can all lead to a depletion of tissue glutathione. These conditions, which are frequently present in African trypanosomiasis, will make patients much more vulnerable to arsenic toxicity. I hypothesise that the encephalopathy associated with arsenical treatment arises in patients who have an associated selenium deficiency. Arsenic toxicity can be alleviated by selenium and is In Chinese factory greatly enhanced by selenium deficiency.4- workers, DNA strand breaks caused by arsenic exposure were rapidly reversed by doses of selenium within the range of normal dietary intakes in the westSelenium deficiency is common in the
tropics. Soil chemistry studies and the low selenium/sulphur content of staple foods in the tropics point in this direction as does measurement of blood selenium in patients from Zaire8 and other countries. Furthermore, the clinical conditions associated with glutathione deficiency usually have an associated selenium deficiency. Selenium deficiency, with or without a combined sulphur deficiency, as a basis for individual variation in melarsoprol toxicity would provide an explanation for the higher prevalence of PARE inland and in rural areas than on the African coast (A. H. Fairlamb, personal communication). If my hypothesis is correct, more aggressive therapy, in the absence of nutritional supplementation, would not be appropriate. Melarsoprol is an effective and cheap drug; if the associated morbidity and mortality could be reduced by prior nutritional supplementation the use of this drug could be extended
to
the
earlier
stages
of the
disease.
In
the
immunocompromised, anorexic, nutritionally deficient host there may be no therapeutic window between the dose of arsenical that will kill the parasite and the dose that will kill the host. Department of Medicine and Therapeutics, University of Aberdeen, Aberdeen AB9 2ZD, UK
MICHAEL H. N. GOLDEN
1. Hurst EW. The lesions
produced in the central nervous system by certain organic arsenical compounds. J Pathol Bacteriol 1959; 77: 523-34. 2. Arrick BA, Griffith OW, Cerami A. Inhibition of glutathione synthesis as a chemotherapeutic strategy for trypanosomiasis. J Exp Med 1981; 153: 720-25. 3. Hunter KJ, Strobos CA, Fairlamb AH. The interaction of trypanocidal drugs with polyamine and trypanothione metabolism. Biochem Soc Trans 1990; 18: 1094-96. 4. Sweins A. Protective effect of selenium against arsenic-induced chromosomal damage in cultured human lymphocytes. Hereditas 1983; 98: 249-52. 5. Levander OA. Metabolic interrelationships between arsenic and selenium. Environ Health Perspect 1977; 19: 159-64. 6. Muth OH, Whanger PD, Weswig PH, Oldfield JE. Occurrence of myopathy in lambs and ewes fed added arsenic in a selenium-deficient ration. Am J Vet Res 1971; 32: 1621-23 7. Hu GG. Investigation of protective effect of selenium on genetic materials among workers exposed to arsenic. Chung Hua Yu Fang I Hsueh Tsa Chih 1989; 23: 286-88. 8. Fondu P, Hariga Muller C, Mozes N, Neve J, Van Steirteghem A, Mandelbaum IM. Protein-energy malnutrition and anemia in Kivu. Am J Clin Nutr 1978; 31: 46-56.
Aflatoxin biomarkers SIR,-Professor Ross and colleagues (April 18, p 943) provide the first epidemiological evidence of an interaction at the individual level between hepatitis B persistence and aflatoxin exposure, by use of a urinary marker of exposure. The result is surprising, not because there is an interaction, but rather because the urinary aflatoxin method used can detect a risk of such magnitude. The urinary excretion of aflatoxin metabolites and adducts is very rapid and hence is dependent on the consumption of aflatoxin in the preceding 24 h.l Consequently a single measurement is unlikely to accurately indicate the long-term exposure of an individual to this carcinogen. The finding that it can demonstrate such a large statistical interaction with hepatitis B is therefore surprising. Perhaps the single measurement does indeed reflect this long-term exposure-ie, some individuals have consistently high exposure to aflatoxin and some the reverse. However, studies in the wet season in The Gambia have failed to demonstrate this happening in children.2 It seems more likely that there is considerable misclassification of individuals with the single urine sample for long-term exposure, and therefore the true interaction would be even larger than that described. As Ross et al point out, the use of the assay for aflatoxin/serum-albumin adducts would provide a longer term measure of exposure and should improve exposure classification. A second issue raised is the interval between exposure measurement and diagnosis of cancer. The major advantage of cohort studies is that the aflatoxin measurement is made before cancer develops, so that there is no question of the tumour affecting diet and/or hepatic metabolism. It would therefore be important to know the interval, at the individual level, between the urinary sampling and the diagnosis to allow some assessment of this potential bias. Chronic liver disease, including cirrhosis, often
1. Robson KJH, Hall
JRS, Jennings MW, et al. A highly conserved amino-acid sequence in thrombospondin, properdin and in proteins from sporozoites and blood stages of a human malaria parasite. Nature 1988; 335: 79-82. 2. Lawler J, Hynes RO. The structure of human thrombospondin, an adhesive glycoprotein with multiple calcium-binding sites and homologies with several different proteins. J Cell Biol 1986, 103: 1635-48. 3. Goundis D, Reid KBN. Properdin, the terminal complement components, thrombospondin and the circumsporozoite protein of malana parasites contain similar sequence motifs. Nature 1988; 335: 82-85. 4. Robson KJH, Hall JRS, Davies LC, Crisanti A, Hill AVS, Wellems TE. Polymorphism of the TRAP gene of Plasmodium falciparum. Proc R Soc Lond [Biol] 1990; 242: 205-16. 5. Good MF, Berzofsky JA, Miller L. The T cell response to the malaria circumsporozoite protein: an immunological approach to vaccine development. Annu Rev Immunol 1988; 6: 663-88. 6. Khusmith S, Charoenvit Y, Kumar S, Sedegah M, Beaudoin RL, Hoffman SL. Protection against malaria by vaccination with sporozoite surface protein 2 plus CS protein. Science 1991; 252: 715-18. 7. Hedstrom RC, Campbell JR, Leef ML, et al. A malaria sporozoite surface antigen distinct from the circumsporozoite protein. Bull WHO 1990; 68: 152-57. 8. Malik A, Egan JE, Houghten RA, Sadoff JC, Hoffman SL. Human cytotoxic T lymphocytes against the Plasmodium falciparum circumsporozoite protein. Proc Natl Acad Sci USA 1991; 88: 3300-04.
Arsenic, selenium, and African
trypanosomiasis SIR,-Melarsoprol is used to treat African trypanosomiasis, despite an associated case fatality rate of 2-10%, because it is usually the only drug available. Eflornithine, a recently introduced alternative, costs about$200 for a 14-day course. Post-arsenical reactive encephalopathy (PARE) has restricted the use of melarsoprol to the final stages of the disease, by which time the patient is usually debilitated and severely malnourished. Dr Hunter and colleagues (May 18, p 956) demonstrate trypanosomal DNA in the brains of fatal cases of PARE and reproduce, in mice, its pathological features by using a drug that does not penetrate the central nervous system. They suggest that "removal of the parasites from all parts of the body except the brain induces a restored immune system to respond vigorously to parasites within the brain". They cite an unpublished observation that subcurative doses of arsenicals also induce PARE whereas
higher doses do not, and now recommend more aggressive treatment-presumably higher doses of melarsoprol. This presupposes that Hurst’s suggestion! that PARE is related to arsenic toxicity is incorrect. Although it is generally thought that PARE has an immunological basis, Hunter’s suggested mechanism not only requires that the patient’s immunosuppression is entirely due to the trypanosomiasis alone but also that immunocompetence is restored, within days, despite persistence of cerebral, trypanosomal infection. The demonstration that the dose of arsenical used is not toxic in normal uninfected animals is an unsafe basis for dismissing potential toxicity in chronically infected animals. The organic arsenicals chelate elements of the sulphur/selenium group. Trypanosomes are very vulnerable to depletion of glutathione in vivo.2Arsenicals are trypanocidal by depriving the protozoa of a unique low-molecular-weight thiol (trypanothione3) that has an even higher affmity for melarsoprol than does glutathione, the low-molecular-weight thiol of most organisms. Indeed, melarsoprol is a condensation product between melarsen oxide and dimercaprol and the latter moiety alleviates some of the basic compound’s effects on the thiols of normal cells. If an infected patient (or laboratory animal) is either sulphur or selenium deficient an organic arsenical is likely to be much more toxic than usual. Severe malnutrition, infection-associated immunosuppression, protozoal infection, and a low sulphur aminoacid intake can all lead to a depletion of tissue glutathione. These conditions, which are frequently present in African trypanosomiasis, will make patients much more vulnerable to arsenic toxicity. I hypothesise that the encephalopathy associated with arsenical treatment arises in patients who have an associated selenium deficiency. Arsenic toxicity can be alleviated by selenium and is In Chinese factory greatly enhanced by selenium deficiency.4- workers, DNA strand breaks caused by arsenic exposure were rapidly reversed by doses of selenium within the range of normal dietary intakes in the westSelenium deficiency is common in the
tropics. Soil chemistry studies and the low selenium/sulphur content of staple foods in the tropics point in this direction as does measurement of blood selenium in patients from Zaire8 and other countries. Furthermore, the clinical conditions associated with glutathione deficiency usually have an associated selenium deficiency. Selenium deficiency, with or without a combined sulphur deficiency, as a basis for individual variation in melarsoprol toxicity would provide an explanation for the higher prevalence of PARE inland and in rural areas than on the African coast (A. H. Fairlamb, personal communication). If my hypothesis is correct, more aggressive therapy, in the absence of nutritional supplementation, would not be appropriate. Melarsoprol is an effective and cheap drug; if the associated morbidity and mortality could be reduced by prior nutritional supplementation the use of this drug could be extended
to
the
earlier
stages
of the
disease.
In
the
immunocompromised, anorexic, nutritionally deficient host there may be no therapeutic window between the dose of arsenical that will kill the parasite and the dose that will kill the host. Department of Medicine and Therapeutics, University of Aberdeen, Aberdeen AB9 2ZD, UK
MICHAEL H. N. GOLDEN
1. Hurst EW. The lesions
produced in the central nervous system by certain organic arsenical compounds. J Pathol Bacteriol 1959; 77: 523-34. 2. Arrick BA, Griffith OW, Cerami A. Inhibition of glutathione synthesis as a chemotherapeutic strategy for trypanosomiasis. J Exp Med 1981; 153: 720-25. 3. Hunter KJ, Strobos CA, Fairlamb AH. The interaction of trypanocidal drugs with polyamine and trypanothione metabolism. Biochem Soc Trans 1990; 18: 1094-96. 4. Sweins A. Protective effect of selenium against arsenic-induced chromosomal damage in cultured human lymphocytes. Hereditas 1983; 98: 249-52. 5. Levander OA. Metabolic interrelationships between arsenic and selenium. Environ Health Perspect 1977; 19: 159-64. 6. Muth OH, Whanger PD, Weswig PH, Oldfield JE. Occurrence of myopathy in lambs and ewes fed added arsenic in a selenium-deficient ration. Am J Vet Res 1971; 32: 1621-23 7. Hu GG. Investigation of protective effect of selenium on genetic materials among workers exposed to arsenic. Chung Hua Yu Fang I Hsueh Tsa Chih 1989; 23: 286-88. 8. Fondu P, Hariga Muller C, Mozes N, Neve J, Van Steirteghem A, Mandelbaum IM. Protein-energy malnutrition and anemia in Kivu. Am J Clin Nutr 1978; 31: 46-56.
Aflatoxin biomarkers SIR,-Professor Ross and colleagues (April 18, p 943) provide the first epidemiological evidence of an interaction at the individual level between hepatitis B persistence and aflatoxin exposure, by use of a urinary marker of exposure. The result is surprising, not because there is an interaction, but rather because the urinary aflatoxin method used can detect a risk of such magnitude. The urinary excretion of aflatoxin metabolites and adducts is very rapid and hence is dependent on the consumption of aflatoxin in the preceding 24 h.l Consequently a single measurement is unlikely to accurately indicate the long-term exposure of an individual to this carcinogen. The finding that it can demonstrate such a large statistical interaction with hepatitis B is therefore surprising. Perhaps the single measurement does indeed reflect this long-term exposure-ie, some individuals have consistently high exposure to aflatoxin and some the reverse. However, studies in the wet season in The Gambia have failed to demonstrate this happening in children.2 It seems more likely that there is considerable misclassification of individuals with the single urine sample for long-term exposure, and therefore the true interaction would be even larger than that described. As Ross et al point out, the use of the assay for aflatoxin/serum-albumin adducts would provide a longer term measure of exposure and should improve exposure classification. A second issue raised is the interval between exposure measurement and diagnosis of cancer. The major advantage of cohort studies is that the aflatoxin measurement is made before cancer develops, so that there is no question of the tumour affecting diet and/or hepatic metabolism. It would therefore be important to know the interval, at the individual level, between the urinary sampling and the diagnosis to allow some assessment of this potential bias. Chronic liver disease, including cirrhosis, often
