Comment
Moving beyond a heat-labile enterotoxin-based vaccine against enterotoxigenic Escherichia coli After a decade of development, in The Lancet Infectious Diseases Ronald Behrens and colleagues1 report findings from a well executed, pivotal efficacy trial of a skin-patch vaccine against enterotoxigenic Escherichia coli (ETEC) in travellers from Europe to Latin America, addressing an important gap in global health.2 The reporting of this negative trial provides useful bearings for investigators working to develop an effective ETEC vaccine suitable for use in young children in poor countries as well as in travellers. The main finding is that 30 (3·7%, 95% CI 2·5–5·2) of 821 participants in the vaccine group and 46 (5·6%, 4·1–7·4) of 823 in the placebo group had moderate-tosevere ETEC diarrhoea, giving a vaccine efficacy 34·6% (95% CI –2·2 to 58·9, p=0·0621), meaning that the trial did not meet its primary endpoint. Arguably, however, the most important finding from this trial is that transcutaneous immunisation with ETEC heat-labile enterotoxin (LT) seemed to afford some protection (vaccine efficacy 61·0% (7·2–83·6, p=0·0417) against diarrhoea caused by ETEC that produce LT, but not against ETEC that produce the heat-stable enterotoxin (ST), or those that produce both LT and ST. This result substantiates the protective capacity of LT and the structurally similar cholera toxin B subunit, inasmuch as two previous field trials of an oral vaccine containing whole-cell, killed cholera plus cholera toxin B subunit showed a reduction in diarrhoea caused by LT-producing ETEC.3,4 The innovative aspect of Behrens and colleagues’ trial, however, is that this protection was achieved with the transcutaneous delivery of LT, the first clinical evidence that skin immunisation results in immunity against LT-producing ETEC—a pathogen confined to the intestinal tract. Advancement of an LT skin-patch vaccine to a phase 3 efficacy trial was based on the hypothesis that this singlecomponent vaccine would confer clinically relevant protection against all ETEC diarrhoea. In retrospect, was it a reasonable assumption that this one antigen would be enough? Evidence obtained in the lead-up to this trial included data from epidemiological studies, which showed that, worldwide, ST-only strains (for which antibodies against LT would be unlikely to confer
protective immunity) account for roughly 45% of all ETEC disease isolates.5 Furthermore, results from the earlier clinical trials by Clemens and colleagues3 and Peltola and colleagues4 suggested that cholera toxin B subunit affords partial protection that is largely directed against LT. In discussing their findings, Clemens and colleagues3 suggested that antigens in addition to cholera toxin B subunit might be necessary to augment efficacy against ETEC. However, the immune response against cholera toxin B subunit might not be optimally effective against LTproducing ETEC, and administration of these antigens by a non-oral route might have improved protection. With respect to these possibilities, the results of two studies6,7 with the LT skin-patch vaccine done before this phase 3 trial provide further insight. In a challenge study,6 volunteers who were given LT by transcutaneous immunisation were not significantly protected against the primary outcome of moderate-to-severe diarrhoea after oral challenge with a highly pathogenic LT/STpositive ETEC strain. In a small phase 2 clinical trial7 in US travellers to Mexico and Guatemala, LT skin-patch vaccination was associated with significant protection against moderate-to-severe all-cause diarrhoea, although protection against moderate-to-severe ETEC diarrhoea specifically was non-significant. Oddly, no apparent protection was seen against diarrhoea associated with LT-producing ETEC (LT-positive or LT/ ST-positive) in the phase 2 trial, which might have been expected to be the primary effect of LT vaccination. By contrast, in their trial of combined whole-cell and B subunit cholera vaccine in Finnish travellers to Morocco, Peltola and colleagues4 noted a gradient of protection against LT-producing ETEC (60%), all ETEC (52%), and all-cause diarrhoea (23%). Although Glenn and colleagues8 suggested a theoretical rationale for how an LT-based vaccine might result in broad protection against all ETEC and all-cause diarrhoea infection, we would suggest that the incongruity with previous results should have raised more scepticism about the alleged protection against diarrhoea associated with ST-only ETEC or non-ETEC pathogens. This sceptical view has been borne out by the failure to reproduce such
www.thelancet.com/infection Published online November 29, 2013 http://dx.doi.org/10.1016/S1473-3099(13)70355-4
Published Online November 29, 2013 http://dx.doi.org/10.1016/ S1473-3099(13)70355-4 See Online/Articles http://dx.doi.org/10.1016/ S1473-3099(13)70297-4
1
Comment
findings in the much larger phase 3 trial1 and in a smaller field trial9 in European travellers to India, wherein no protection against diarrhoea associated with ST-only ETEC or all-cause diarrhoea was seen. Supplemented by Behrens and colleagues’ findings,1 the weight of evidence suggests the need to include other antigens in addition to an LT-based component to achieve enough protection against ETEC diarrhoea to have a positive effect on public health. Our understanding of ETEC pathogenesis continues to develop, and novel target antigens have been identified and need to be fully assessed in relevant animal models and moved rapidly to the field for testing.10 We encourage Behrens and colleagues to present more complete data from this trial. For example, more might be learned from advanced cellular and humoral immunological study of available samples to identify a correlate of protection. Furthermore, the potential of an ETEC vaccine to prevent both the acute disease and the chronic consequences such as postinfectious irritable bowel syndrome, which some studies have shown to be associated with travellers’ diarrhoea,11 has yet to be fully appreciated from the phase 3 data. A preliminary positive signal was noted from the phase 2 LT-patch study,12 when at the 6 month follow-up none of the eight assessable participants in the LT-vaccine group who had diarrhoea developed incident irritable bowel syndrome, whereas three of 17 participants in the placebo group did develop the disorder (although this difference was nonsignificant). If such an association were to hold true in the phase 3 results, the potential value of an ETEC vaccine in a traveller target population could rise substantially. We believe that the future of ETEC vaccine research has brightened with coalescence of a broad interest across the global health community, industry, and academia. The synergy between these partners will
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hopefully continue to push basic and applied research forward, and advance vaccine candidates expeditiously through clinical assessment. *Mark S Riddle, Stephen J Savarino Naval Medical Research Center, Silver Spring, MD 20910, USA [email protected] We declare that we have no conflicts of interest. Both authors are employees of the US Government or military service members. The views expressed are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, or the US Government. Copyright protection is not available for any work of the US Government. 1
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Behrens RH, Cramer JP, Jelinek T, et al. Efficacy and safety of a patch vaccine containing heat-labile toxin from Escherichia coli against travellers’ diarrhoea: a phase 3, randomised, double-blind, placebo-controlled field trial in travellers from Europe to Mexico and Guatemala. Lancet Infect Dis 2013; published online Nov 29. http://dx.doi.org/10.1016/S14733099(13)70355-4. PATH, BIO Ventures for Global Health. The case for investment in enterotoxigenic Escherichia coli vaccines. PATH, BIO Ventures for Global Health, 2011. http://www.path.org/publications/files/VAC-etecinvestment-rpt.pdf (accessed Nov 6, 2013). Clemens JD, Sack DA, Harris JR, et al. Cross-protection by B subunit-whole cell cholera vaccine against diarrhea associated with heat-labile toxinproducing enterotoxigenic Escherichia coli: results of a large-scale field trial. J Infect Dis 1988; 158: 372–77. Peltola H, Siitonen A, Kyrönseppä H, et al. Prevention of travellers’ diarrhoea by oral B-subunit/whole-cell cholera vaccine. Lancet 1991; 338: 1285–89. Isidean SD, Riddle MS, Savarino SJ, Porter CK. A systematic review of ETEC epidemiology focusing on colonization factor and toxin expression. Vaccine 2011; 29: 6167–78. McKenzie R, Bourgeois AL, Frech SA, et al. Transcutaneous immunization with the heat-labile toxin (LT) of enterotoxigenic Escherichia coli (ETEC): protective efficacy in a double-blind, placebo-controlled challenge study. Vaccine 2007; 25: 3684–91. Frech SA, Dupont HL, Bourgeois AL, et al. Use of a patch containing heat-labile toxin from Escherichia coli against travellers’ diarrhoea: a phase II, randomised, double-blind, placebo-controlled field trial. Lancet 2008; 371: 2019–25. Glenn GM, Francis DH, Danielsen EM. Toxin-mediated effects on the innate mucosal defenses: implications for enteric vaccines. Infect Immun 2009; 77: 5206–15. Steffen R, Cramer JP, Burchard G, et al. Efficacy of a travelers’ diarrhea vaccine system in travelers to India. J Travel Med 2013; 20: 374–79. Fleckenstein JM, Munson GM, Rasko D. Enterotoxigenic Escherichia coli: orchestrated host engagement. Gut Microbes 2013; 4: 392–96. Connor BA, Riddle MS. Post-infectious sequelae of travelers’ diarrhea. J Travel Med 2013; 20: 303–12. Frech S. Use of a patch containing heat-labile toxin from Escherichia coli against travellers’ diarrhoea: a phase II, randomised, double-blind, placebo-controlled field trial. 42nd US–Japan Conference on Cholera; Austin, TX, USA; Dec 4–7, 2007.
www.thelancet.com/infection Published online November 29, 2013 http://dx.doi.org/10.1016/S1473-3099(13)70355-4
Moving beyond a heat-labile enterotoxin-based vaccine against enterotoxigenic Escherichia coli After a decade of development, in The Lancet Infectious Diseases Ronald Behrens and colleagues1 report findings from a well executed, pivotal efficacy trial of a skin-patch vaccine against enterotoxigenic Escherichia coli (ETEC) in travellers from Europe to Latin America, addressing an important gap in global health.2 The reporting of this negative trial provides useful bearings for investigators working to develop an effective ETEC vaccine suitable for use in young children in poor countries as well as in travellers. The main finding is that 30 (3·7%, 95% CI 2·5–5·2) of 821 participants in the vaccine group and 46 (5·6%, 4·1–7·4) of 823 in the placebo group had moderate-tosevere ETEC diarrhoea, giving a vaccine efficacy 34·6% (95% CI –2·2 to 58·9, p=0·0621), meaning that the trial did not meet its primary endpoint. Arguably, however, the most important finding from this trial is that transcutaneous immunisation with ETEC heat-labile enterotoxin (LT) seemed to afford some protection (vaccine efficacy 61·0% (7·2–83·6, p=0·0417) against diarrhoea caused by ETEC that produce LT, but not against ETEC that produce the heat-stable enterotoxin (ST), or those that produce both LT and ST. This result substantiates the protective capacity of LT and the structurally similar cholera toxin B subunit, inasmuch as two previous field trials of an oral vaccine containing whole-cell, killed cholera plus cholera toxin B subunit showed a reduction in diarrhoea caused by LT-producing ETEC.3,4 The innovative aspect of Behrens and colleagues’ trial, however, is that this protection was achieved with the transcutaneous delivery of LT, the first clinical evidence that skin immunisation results in immunity against LT-producing ETEC—a pathogen confined to the intestinal tract. Advancement of an LT skin-patch vaccine to a phase 3 efficacy trial was based on the hypothesis that this singlecomponent vaccine would confer clinically relevant protection against all ETEC diarrhoea. In retrospect, was it a reasonable assumption that this one antigen would be enough? Evidence obtained in the lead-up to this trial included data from epidemiological studies, which showed that, worldwide, ST-only strains (for which antibodies against LT would be unlikely to confer
protective immunity) account for roughly 45% of all ETEC disease isolates.5 Furthermore, results from the earlier clinical trials by Clemens and colleagues3 and Peltola and colleagues4 suggested that cholera toxin B subunit affords partial protection that is largely directed against LT. In discussing their findings, Clemens and colleagues3 suggested that antigens in addition to cholera toxin B subunit might be necessary to augment efficacy against ETEC. However, the immune response against cholera toxin B subunit might not be optimally effective against LTproducing ETEC, and administration of these antigens by a non-oral route might have improved protection. With respect to these possibilities, the results of two studies6,7 with the LT skin-patch vaccine done before this phase 3 trial provide further insight. In a challenge study,6 volunteers who were given LT by transcutaneous immunisation were not significantly protected against the primary outcome of moderate-to-severe diarrhoea after oral challenge with a highly pathogenic LT/STpositive ETEC strain. In a small phase 2 clinical trial7 in US travellers to Mexico and Guatemala, LT skin-patch vaccination was associated with significant protection against moderate-to-severe all-cause diarrhoea, although protection against moderate-to-severe ETEC diarrhoea specifically was non-significant. Oddly, no apparent protection was seen against diarrhoea associated with LT-producing ETEC (LT-positive or LT/ ST-positive) in the phase 2 trial, which might have been expected to be the primary effect of LT vaccination. By contrast, in their trial of combined whole-cell and B subunit cholera vaccine in Finnish travellers to Morocco, Peltola and colleagues4 noted a gradient of protection against LT-producing ETEC (60%), all ETEC (52%), and all-cause diarrhoea (23%). Although Glenn and colleagues8 suggested a theoretical rationale for how an LT-based vaccine might result in broad protection against all ETEC and all-cause diarrhoea infection, we would suggest that the incongruity with previous results should have raised more scepticism about the alleged protection against diarrhoea associated with ST-only ETEC or non-ETEC pathogens. This sceptical view has been borne out by the failure to reproduce such
www.thelancet.com/infection Published online November 29, 2013 http://dx.doi.org/10.1016/S1473-3099(13)70355-4
Published Online November 29, 2013 http://dx.doi.org/10.1016/ S1473-3099(13)70355-4 See Online/Articles http://dx.doi.org/10.1016/ S1473-3099(13)70297-4
1
Comment
findings in the much larger phase 3 trial1 and in a smaller field trial9 in European travellers to India, wherein no protection against diarrhoea associated with ST-only ETEC or all-cause diarrhoea was seen. Supplemented by Behrens and colleagues’ findings,1 the weight of evidence suggests the need to include other antigens in addition to an LT-based component to achieve enough protection against ETEC diarrhoea to have a positive effect on public health. Our understanding of ETEC pathogenesis continues to develop, and novel target antigens have been identified and need to be fully assessed in relevant animal models and moved rapidly to the field for testing.10 We encourage Behrens and colleagues to present more complete data from this trial. For example, more might be learned from advanced cellular and humoral immunological study of available samples to identify a correlate of protection. Furthermore, the potential of an ETEC vaccine to prevent both the acute disease and the chronic consequences such as postinfectious irritable bowel syndrome, which some studies have shown to be associated with travellers’ diarrhoea,11 has yet to be fully appreciated from the phase 3 data. A preliminary positive signal was noted from the phase 2 LT-patch study,12 when at the 6 month follow-up none of the eight assessable participants in the LT-vaccine group who had diarrhoea developed incident irritable bowel syndrome, whereas three of 17 participants in the placebo group did develop the disorder (although this difference was nonsignificant). If such an association were to hold true in the phase 3 results, the potential value of an ETEC vaccine in a traveller target population could rise substantially. We believe that the future of ETEC vaccine research has brightened with coalescence of a broad interest across the global health community, industry, and academia. The synergy between these partners will
2
hopefully continue to push basic and applied research forward, and advance vaccine candidates expeditiously through clinical assessment. *Mark S Riddle, Stephen J Savarino Naval Medical Research Center, Silver Spring, MD 20910, USA [email protected] We declare that we have no conflicts of interest. Both authors are employees of the US Government or military service members. The views expressed are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, or the US Government. Copyright protection is not available for any work of the US Government. 1
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Behrens RH, Cramer JP, Jelinek T, et al. Efficacy and safety of a patch vaccine containing heat-labile toxin from Escherichia coli against travellers’ diarrhoea: a phase 3, randomised, double-blind, placebo-controlled field trial in travellers from Europe to Mexico and Guatemala. Lancet Infect Dis 2013; published online Nov 29. http://dx.doi.org/10.1016/S14733099(13)70355-4. PATH, BIO Ventures for Global Health. The case for investment in enterotoxigenic Escherichia coli vaccines. PATH, BIO Ventures for Global Health, 2011. http://www.path.org/publications/files/VAC-etecinvestment-rpt.pdf (accessed Nov 6, 2013). Clemens JD, Sack DA, Harris JR, et al. Cross-protection by B subunit-whole cell cholera vaccine against diarrhea associated with heat-labile toxinproducing enterotoxigenic Escherichia coli: results of a large-scale field trial. J Infect Dis 1988; 158: 372–77. Peltola H, Siitonen A, Kyrönseppä H, et al. Prevention of travellers’ diarrhoea by oral B-subunit/whole-cell cholera vaccine. Lancet 1991; 338: 1285–89. Isidean SD, Riddle MS, Savarino SJ, Porter CK. A systematic review of ETEC epidemiology focusing on colonization factor and toxin expression. Vaccine 2011; 29: 6167–78. McKenzie R, Bourgeois AL, Frech SA, et al. Transcutaneous immunization with the heat-labile toxin (LT) of enterotoxigenic Escherichia coli (ETEC): protective efficacy in a double-blind, placebo-controlled challenge study. Vaccine 2007; 25: 3684–91. Frech SA, Dupont HL, Bourgeois AL, et al. Use of a patch containing heat-labile toxin from Escherichia coli against travellers’ diarrhoea: a phase II, randomised, double-blind, placebo-controlled field trial. Lancet 2008; 371: 2019–25. Glenn GM, Francis DH, Danielsen EM. Toxin-mediated effects on the innate mucosal defenses: implications for enteric vaccines. Infect Immun 2009; 77: 5206–15. Steffen R, Cramer JP, Burchard G, et al. Efficacy of a travelers’ diarrhea vaccine system in travelers to India. J Travel Med 2013; 20: 374–79. Fleckenstein JM, Munson GM, Rasko D. Enterotoxigenic Escherichia coli: orchestrated host engagement. Gut Microbes 2013; 4: 392–96. Connor BA, Riddle MS. Post-infectious sequelae of travelers’ diarrhea. J Travel Med 2013; 20: 303–12. Frech S. Use of a patch containing heat-labile toxin from Escherichia coli against travellers’ diarrhoea: a phase II, randomised, double-blind, placebo-controlled field trial. 42nd US–Japan Conference on Cholera; Austin, TX, USA; Dec 4–7, 2007.
www.thelancet.com/infection Published online November 29, 2013 http://dx.doi.org/10.1016/S1473-3099(13)70355-4
