Validation of a Human Challenge Model Using an LT-Expressing Enterotoxigenic <i>E. coli</i> Strain (LSN03-016011) and Characterization of Potential Amelioration of Disease by an Investigational Oral Vaccine Candidate (VLA1701)

Controlled human infection models are important tools for the evaluation of vaccines against diseases where an appropriate correlate of protection has not been identified. Enterotoxigenic Escherichia coli (ETEC) strain LSN03-016011/A (LSN03) is an LT enterotoxin and CS17-expressing ETEC strain useful for evaluating vaccine candidates targeting LT-expressing strains. We sought to confirm the ability of the LSN03 strain to induce moderate-to-severe diarrhea in a healthy American adult population, as well as the impact of immunization with an investigational cholera/ETEC vaccine (VLA-1701) on disease outcomes. A randomized, double-blinded pilot study was conducted in which participants received two doses of VLA1701 or placebo orally, one week apart; eight days after the second vaccination, 30 participants (15 vaccinees and 15 placebo recipients) were challenged with approximately 5 × 109 colony-forming units of LSN03. The vaccine was well tolerated, with no significant adverse events. The vaccine also induced serum IgA and IgG responses to LT. After challenge, 11 of the placebo recipients (73.3%; 95%CI: 48.0–89.1) and 7 of the VLA1701 recipients (46.7%; 95%CI: 24.8–68.8) had moderate-to-severe diarrhea (p = 0.26), while 14 placebo recipients (93%) and 8 vaccine recipients (53.3%) experienced diarrhea of any severity, resulting in a protective efficacy of 42.9% (p = 0.035). In addition, the vaccine also appeared to provide protection against more severe diarrhea (p = 0.054). Vaccinees also tended to shed lower levels of the LSN03 challenge strain compared to placebo recipients (p = 0.056). In addition, the disease severity score was lower for the vaccinees than for the placebo recipients (p = 0.046). In summary, the LSN03 ETEC challenge strain induced moderate-to-severe diarrhea in 73.3% of placebo recipients. VLA1701 vaccination ameliorated disease severity, as observed by several parameters, including the percentage of participants experiencing diarrhea, as well as stool frequency and ETEC severity scores. These data highlight the potential value of LSN03 as a suitable ETEC challenge strain to evaluate LT-based vaccine targets (NCT03576183)

Clinical endpoints in the controlled human challenge model for Shigella: A call for standardization and the development of a disease severity score.

Since 1946 the controlled human infection model (CHIM) for Shigella has been used to improve understanding of disease pathogenesis, describe clinical and immunologic responses to infection and as a tool for vaccine development. As the frequency and intent for use in vaccine comparisons increases, standardization of the primary endpoint definition is necessary.Subject-level data were obtained from previously conducted experimental Shigella CHIM studies. Signs and symptoms severity were categorized consistently across all studies. Sign and symptom correlations were estimated and univariate models were utilized to describe the association between stool output and other Shigella-attributable signs and symptoms. Multiple correspondence and hierarchical clustering analyses were performed to describe the co-occurrence of signs and symptoms. A disease score is proposed based on the co-occurrence of these events.Data were obtained on 54 subjects receiving 800 to 2000 colony forming units (cfu) of S. flexneri. The median maximum 24 hour stool output was 514 ml (IQR: 300, 998 ml) with a median frequency of 6 (IQR: 4, 9). Subjects reported abdominal pain or cramps (81.5%), headache (66.7%) and anorexia (64.8%), 50.0% had a fever and 27.8% had gross blood in multiple loose stools. Multiple correspondence analyses highlighted co-occurrence of symptoms based on severity. A 3-parameter disease severity score predicted shigellosis endpoints and better differentiated disease spectrum.Dichotomous endpoints for Shigella CHIM fail to fully account for disease variability. An ordinal disease score characterizing the breadth of disease severity may enable a better characterization of shigellosis and can decrease sample size requirements. Furthermore, the disease severity score may be a useful tool for portfolio management by enabling prioritization across vaccine candidates with comparable efficacy estimates using dichotomous endpoints

Frequency (percent) of <i>Shigella</i>-attributed signs and symptoms in four CHIMs with <i>S</i>. <i>flexneri</i> 2a strain 2457T.

<p>Frequency (percent) of <i>Shigella</i>-attributed signs and symptoms in four CHIMs with <i>S</i>. <i>flexneri</i> 2a strain 2457T.</p

Box and whisker plot of developed <i>Shigella</i> CHIM disease severity score by previously utilized dichotomous endpoints for shigellosis.

<p>Footnote: Endpoint 1 (solid line): diarrhea (≥2 loose stools ≥200 grams over 48 hours or a single loose stool ≥300 grams) OR fever (oral temperature ≥100.0°F) OR gross blood confirmed by hemoccult in ≥1 loose stool; Endpoint 2 (dashed line): severe diarrhea (≥6 loose stools in 24 hours or >800 grams of loose stool in 24 hours) OR moderate diarrhea (4–5 loose stools in 24 hours or 401–800 grams of loose stool in 24 hours) AND [fever (oral temperature >100.4°F) or with moderate enteric/constitutional symptoms (nausea, vomiting, abdominal cramps/pain, myalgia, arthralgia, rigors, tenesmus, fecal urgency)] OR gross blood confirmed by hemoccult in ≥2 loose stools in 24 hours and enteric/constitutional symptoms (nausea, vomiting, abdominal cramps/pain, myalgia, arthralgia, rigors, tenesmus, fecal urgency).</p

Stool output information in four CHIMs with <i>S</i>. <i>flexneri</i> 2a strain 2457T.

<p>1A. Time to first loose stool from time of inoculation. The line indicates the proportion of subjects with at least 1 loose stool over time (in hours) following inoculation with <i>S</i>. <i>flexneri</i> 2a strain 2457T. 1B. Correlation between maximum number and volume of loose stools in any 24 hour period post-inoculation. Each dot represents each subject’s maximum 24 hour loose stool output by frequency and volume. The line represents the linear correlation between the maximum 24 hour frequency and volume.</p

Uses of the <i>Shigella</i> CHIM to assess the efficacy of investigational products or prior challenge.

<p>Uses of the <i>Shigella</i> CHIM to assess the efficacy of investigational products or prior challenge.</p

Results of hierarchical cluster analysis of subject signs and symptoms and correlation with developed <i>Shigella</i> CHIM disease severity score.

<p>Footnote: Signs and symptoms were used to identify clusters of disease profiles (numbered 1–6). Those clusters were then assessed as an ordinal predictor of the <i>Shigella</i> CHIM disease score. Box and whisker plots are utilized to demonstrate the distribution of the disease score across each sign/symptom cluster. The receiver operator curves demonstrate the ability of the disease score to predict disease cluster.</p

Studies utilized to characterize <i>Shigella</i> CHIM endpoints and to develop a disease severity score.

<p>Studies utilized to characterize <i>Shigella</i> CHIM endpoints and to develop a disease severity score.</p

Receiver operator curves using the developed <i>Shigella</i> CHIM disease severity score to predict two separate dichotomous endpoints of shigellosis.

<p>Footnote: Receiver operator curves using a <i>Shigella</i> disease score to predict two separate dichotomous endpoints as follows. Endpoint 1 (solid line): diarrhea (≥2 loose stools ≥200 grams over 48 hours or a single loose stool ≥300 grams) OR fever (oral temperature ≥100.0°F) OR gross blood confirmed by hemoccult in ≥1 loose stool; Endpoint 2 (dashed line): severe diarrhea (≥6 loose stools in 24 hours or >800 grams of loose stool in 24 hours) OR moderate diarrhea (4–5 loose stools in 24 hours or 401–800 grams of loose stool in 24 hours) AND [fever (oral temperature >100.4°F) or with moderate enteric/constitutional symptoms (nausea, vomiting, abdominal cramps/pain, myalgia, arthralgia, rigors, tenesmus, fecal urgency)] OR gross blood confirmed by hemoccult in ≥2 loose stools in 24 hours and enteric/constitutional symptoms (nausea, vomiting, abdominal cramps/pain, myalgia, arthralgia, rigors, tenesmus, fecal urgency).</p

Multiple correspondence analysis of signs and symptoms observed during four CHIMs with <i>S</i>. <i>flexneri</i> 2a strain 2457T.

<p>Multiple correspondence analysis of signs and symptoms observed during four CHIMs with <i>S</i>. <i>flexneri</i> 2a strain 2457T.</p