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

Abstract

Background: 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.

Methods: 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.

Results: 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.

Conclusion: 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.

Fig 1. Stool output information in four CHIMs with S. flexneri 2a strain 2457T.

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 S. flexneri 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.

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

Fig 3. Results of hierarchical cluster analysis of subject signs and symptoms and correlation with developed Shigella CHIM disease severity score.

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 Shigella 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.

Fig 4. Receiver operator curves using the developed Shigella CHIM disease severity score to predict two separate dichotomous endpoints of shigellosis.

Footnote: Receiver operator curves using a Shigella 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).

Fig 5. Box and whisker plot of developed Shigella CHIM disease severity score by previously utilized dichotomous endpoints for shigellosis.

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).