- Email: [email protected]
Reply (contra)
Parasitology Today, voL 3, no. I I, 1987
353
Reply ( p r o )
Reply ( c o n t r a )
Sir- Thank you for giving me the opportunity to reply to the above letters. It is naive ofDrs Prasad and Virkto assume that Dr Mirelman and his group have demonstrated anything so far other than an unrepeatable experiment, which is the entirety of this group's publications, quoted by your correspondents. This sirgle experimental culture cannot be compared to the thousands of proven 'clinical isolates' demonstrating the various zymodemes of Entamoeba histolytica. Prasad and Virk should be reminded that Mirelman et al. have yet to publish the results of zymodeme characterization of their culture as originally defined. They have shown us a hexokinase photograph with a control drawn on the side, presumably as an afterthought, and a phosphoglucomutase photograph which defies interpretation. However, since they are using a different method of electorphoresis to that which is used here, comparisons are invalid until they have demonstrated a comprehensive range of zymodemes using adequate controls. The second paragraph of Prasad and Virk's letter is best left unanswer,~d since it demonstrates no understanding of the subject under discussion, particularly in relation to asymptomatic subjects. Dr Mehlotra assessesthe E. histolytica pathogenicity question in a chronological and logical fashion quoting very substantial references. I agree entirely with his recommendation regarding the use of cloned material, recently obliquely referred to in a Lancet editorial ~2.
Sir- We support the contentions of Prasad and Virk that we should refrain from using the zymodeme classification of pathogenic/ nonpathogenic amoebae and that any person found to harbour E. histolytica should be treated. Our conclusions are based not only on our demonstration that changes in isoenzyme electrophoretic pattern can occur during changes in culture conditions and presence or absence of bacterial flora 3,4, but also because of our recent finding that a number of xenically grown E.histolytica strains, which were isolated from asymptomatic carriers and possessed 'nonpathogenic' zymodemes, could induce the formation of hepatic abscesses in hamsters and caecal ulcerations in rats as efficiently as some of the most 'pathogenic' strains (G.D. Burchard and D. Mirelman, in preparation). There is no doubt that certain bacterial species can enhance the virulence of axenically grown amoebae. This was shown in laboratory animals 13as well as with tissue-cultured monolayers in vitro 14.The experiments mentioned in the letter of Mehlotra, in which no augmentation of amoebic virulence was noted 8~9,were all performed with bacteria-amoebae cultures so that the influence of the original and added bacteria could not be differentiated. We of course agree that factors related to the host, such as the intestinal bacterial flora and others which are still unknown, play a significant role in the pathogenesis of amoebiasis. We are convinced however, that only through further worldwide research with newer methods as well as with numerous strains and different hosts, will we ever be able to understand the original observations of Walker and Sellards15that an amoeba isolate can be pathogenic in some individuals and harmless in others, and that a
P.G.Sargeaunt Department of MedicalProtozoology, London School of Hygieneand Trap cal Medicine, Keppel Street, London WC I E7HT,UK
Primaquine: Pharmacokinetics, Metabolism, Toxicity and Activity edited by Walther H. Wernsdorfer and Peter I. Trigg UNDP/World Bank~WHO Special Programme for Research and 'Training in Tropical Diseases, John Wiley & Sons, 1987. £18.50 (x + 164 pages) ISBN0471 91182 8 The 8-aminoquinolines play a unique and vital role in malaria treatment because, in addition to a slight action on asexual blood stages at nontoxic dose.s, they are the only drugs capable of eliminating the
persistent liver forms (hypnozoites) responsible for relapses in Plasmodium vivax and P. ovale infections. They are also lethal to the mature gametocytes of P. falciporum in the blood, thus preventing mosquito infection. Pamaquine (plasmaquine), the first member of this series, also holds the distinction of being the first effective compound synthesized for antimalarial purposes, and was found to be active in finches by Roehl in 1926. Studies on P. gallinaceum in the 1950s showed that pamaquine itself had little activity on erythrocytic stages in vitro, but faeces from dosed chickens were active. A proposed scheme for metabolic alteration of primaquine, a less toxic analogue which replaced pamaquine in the 1950s, was based on earlier w o r k on pamaquine and pentaquine. The drug was believed to undergo demethylation at the 6-methoxy group, and oxidation at the adjacent 5 position, to give a 5,6 dihydroxy derivative (5-hydroxy de-
carrier can be responsible for the transmission of a parasite that is pathogenic to other subjects. We can only hope that such investigations will yield not only interesting scientific papers but also results which will enable us one day to control the virulence of this pathogen.
D. Mirelman G.D. Burchard MacArthur Center for MolecularBiologyof ParasiticDiseases, The Weizmann Instituteof Science, Rehovot 76100, Israel
References I Mirelman, D., Sargeaunt, P. [Debate] (1987) Parasitol. Today2, 37~r3 2 Brumpt. E. (1925) Bull. Acad. Meal. (Paris) 94, 942-952 3 Mirelman, D. et al. (1986) Exp. Parasitol. 62, 142-148 4 Mirelman, D. et al. (1986) Infect. Irnmun. 54, 827-834 5 Durra, G.P. ( 1981) Experimental and Clinical Studies on Arnoebiasis; Tata McGraw Publications, New Delhi 6 Nanda, R., Baveja, U. and Anand, B.S.(1984) Lancet ii, 301-303 7 Neal, R.A.(1956)Parasitology46, 183-191 8 Phillips,B.P.et of. (] 955) Arn.J. Trap. Med. Hyg. 4, 675-692 9 Phillips,B.P.,Wolfe, P.A.and Bartgis,I.L (1958) Arn.j. Trap. Med. Hyg. 7, 392-99 I 0 Walliker, D. (1986) Parasital. Today 2, 43-44 I I Orozco, E. et al. (I 983)J.Exp.Med. 158, 151 I1521 12 [Anon](1986)Lancetii, 1133-1134 13 Wittner, M. and Rosenbaum,R.M. (1970)Am. J. Trap. Med. Hyg. 19,755-76 I 14 Bracha,R. and Mirelman,D. (I 984)J. Exp.MecL 160, 353-368 15 Walker, E.L.and'Sellards,A.W. ( 1913)Philipp. J. Sci.[B] Trap. Med. 8,253-330
methyl primaquine). Such a compound would be in oxidation-reduction equilibrium with the 5,6 diquinione (Fig. I, I) and might interfere in redox reactions in the malaria parasite, and in the red cell, giving rise to antiparasitic and toxic effects. The latter, notably methaemoglobinaemia and haemolysis, are common in people deficient in glucose 6-phosphate dehydrogenase I. The antimalarial activity of the 8aminoquinolines has been assumed to depend on the similarity of metabolites to the electron-transport intermediate ubiquinone, found in mitochondria, since malaria parasites have a form of this compound (ubiquinone 8) which is not seen in mammalian cells. The activity of the structurally similar hydroxynaphthoquinone antimalarials such as menoctone (Fig. 2) was shown conclusively in later studies to be linked to ubiquinone, but a clear confirmation of this proposed mode of action for 8-aminoquinoline metabolites has not yet been obtained.
353
Reply ( p r o )
Reply ( c o n t r a )
Sir- Thank you for giving me the opportunity to reply to the above letters. It is naive ofDrs Prasad and Virkto assume that Dr Mirelman and his group have demonstrated anything so far other than an unrepeatable experiment, which is the entirety of this group's publications, quoted by your correspondents. This sirgle experimental culture cannot be compared to the thousands of proven 'clinical isolates' demonstrating the various zymodemes of Entamoeba histolytica. Prasad and Virk should be reminded that Mirelman et al. have yet to publish the results of zymodeme characterization of their culture as originally defined. They have shown us a hexokinase photograph with a control drawn on the side, presumably as an afterthought, and a phosphoglucomutase photograph which defies interpretation. However, since they are using a different method of electorphoresis to that which is used here, comparisons are invalid until they have demonstrated a comprehensive range of zymodemes using adequate controls. The second paragraph of Prasad and Virk's letter is best left unanswer,~d since it demonstrates no understanding of the subject under discussion, particularly in relation to asymptomatic subjects. Dr Mehlotra assessesthe E. histolytica pathogenicity question in a chronological and logical fashion quoting very substantial references. I agree entirely with his recommendation regarding the use of cloned material, recently obliquely referred to in a Lancet editorial ~2.
Sir- We support the contentions of Prasad and Virk that we should refrain from using the zymodeme classification of pathogenic/ nonpathogenic amoebae and that any person found to harbour E. histolytica should be treated. Our conclusions are based not only on our demonstration that changes in isoenzyme electrophoretic pattern can occur during changes in culture conditions and presence or absence of bacterial flora 3,4, but also because of our recent finding that a number of xenically grown E.histolytica strains, which were isolated from asymptomatic carriers and possessed 'nonpathogenic' zymodemes, could induce the formation of hepatic abscesses in hamsters and caecal ulcerations in rats as efficiently as some of the most 'pathogenic' strains (G.D. Burchard and D. Mirelman, in preparation). There is no doubt that certain bacterial species can enhance the virulence of axenically grown amoebae. This was shown in laboratory animals 13as well as with tissue-cultured monolayers in vitro 14.The experiments mentioned in the letter of Mehlotra, in which no augmentation of amoebic virulence was noted 8~9,were all performed with bacteria-amoebae cultures so that the influence of the original and added bacteria could not be differentiated. We of course agree that factors related to the host, such as the intestinal bacterial flora and others which are still unknown, play a significant role in the pathogenesis of amoebiasis. We are convinced however, that only through further worldwide research with newer methods as well as with numerous strains and different hosts, will we ever be able to understand the original observations of Walker and Sellards15that an amoeba isolate can be pathogenic in some individuals and harmless in others, and that a
P.G.Sargeaunt Department of MedicalProtozoology, London School of Hygieneand Trap cal Medicine, Keppel Street, London WC I E7HT,UK
Primaquine: Pharmacokinetics, Metabolism, Toxicity and Activity edited by Walther H. Wernsdorfer and Peter I. Trigg UNDP/World Bank~WHO Special Programme for Research and 'Training in Tropical Diseases, John Wiley & Sons, 1987. £18.50 (x + 164 pages) ISBN0471 91182 8 The 8-aminoquinolines play a unique and vital role in malaria treatment because, in addition to a slight action on asexual blood stages at nontoxic dose.s, they are the only drugs capable of eliminating the
persistent liver forms (hypnozoites) responsible for relapses in Plasmodium vivax and P. ovale infections. They are also lethal to the mature gametocytes of P. falciporum in the blood, thus preventing mosquito infection. Pamaquine (plasmaquine), the first member of this series, also holds the distinction of being the first effective compound synthesized for antimalarial purposes, and was found to be active in finches by Roehl in 1926. Studies on P. gallinaceum in the 1950s showed that pamaquine itself had little activity on erythrocytic stages in vitro, but faeces from dosed chickens were active. A proposed scheme for metabolic alteration of primaquine, a less toxic analogue which replaced pamaquine in the 1950s, was based on earlier w o r k on pamaquine and pentaquine. The drug was believed to undergo demethylation at the 6-methoxy group, and oxidation at the adjacent 5 position, to give a 5,6 dihydroxy derivative (5-hydroxy de-
carrier can be responsible for the transmission of a parasite that is pathogenic to other subjects. We can only hope that such investigations will yield not only interesting scientific papers but also results which will enable us one day to control the virulence of this pathogen.
D. Mirelman G.D. Burchard MacArthur Center for MolecularBiologyof ParasiticDiseases, The Weizmann Instituteof Science, Rehovot 76100, Israel
References I Mirelman, D., Sargeaunt, P. [Debate] (1987) Parasitol. Today2, 37~r3 2 Brumpt. E. (1925) Bull. Acad. Meal. (Paris) 94, 942-952 3 Mirelman, D. et al. (1986) Exp. Parasitol. 62, 142-148 4 Mirelman, D. et al. (1986) Infect. Irnmun. 54, 827-834 5 Durra, G.P. ( 1981) Experimental and Clinical Studies on Arnoebiasis; Tata McGraw Publications, New Delhi 6 Nanda, R., Baveja, U. and Anand, B.S.(1984) Lancet ii, 301-303 7 Neal, R.A.(1956)Parasitology46, 183-191 8 Phillips,B.P.et of. (] 955) Arn.J. Trap. Med. Hyg. 4, 675-692 9 Phillips,B.P.,Wolfe, P.A.and Bartgis,I.L (1958) Arn.j. Trap. Med. Hyg. 7, 392-99 I 0 Walliker, D. (1986) Parasital. Today 2, 43-44 I I Orozco, E. et al. (I 983)J.Exp.Med. 158, 151 I1521 12 [Anon](1986)Lancetii, 1133-1134 13 Wittner, M. and Rosenbaum,R.M. (1970)Am. J. Trap. Med. Hyg. 19,755-76 I 14 Bracha,R. and Mirelman,D. (I 984)J. Exp.MecL 160, 353-368 15 Walker, E.L.and'Sellards,A.W. ( 1913)Philipp. J. Sci.[B] Trap. Med. 8,253-330
methyl primaquine). Such a compound would be in oxidation-reduction equilibrium with the 5,6 diquinione (Fig. I, I) and might interfere in redox reactions in the malaria parasite, and in the red cell, giving rise to antiparasitic and toxic effects. The latter, notably methaemoglobinaemia and haemolysis, are common in people deficient in glucose 6-phosphate dehydrogenase I. The antimalarial activity of the 8aminoquinolines has been assumed to depend on the similarity of metabolites to the electron-transport intermediate ubiquinone, found in mitochondria, since malaria parasites have a form of this compound (ubiquinone 8) which is not seen in mammalian cells. The activity of the structurally similar hydroxynaphthoquinone antimalarials such as menoctone (Fig. 2) was shown conclusively in later studies to be linked to ubiquinone, but a clear confirmation of this proposed mode of action for 8-aminoquinoline metabolites has not yet been obtained.