Open-label, dose escalation phase I study in healthy volunteers to evaluate the safety and pharmacokinetics of a human monoclonal antibody to Clostridium difficile toxin A
Introduction
Clostridium difficile is an important cause of antibiotic-associated diarrhea, and results in a wide spectrum of disease, ranging from asymptomatic colonization to fulminant pseudomembranous colitis [1]. Pathogenic strains produce two protein exotoxins: toxin A and toxin B [1]. Experiments in rabbits, hamsters, mice, and rats have shown that toxin A causes a severe inflammatory response, epithelial damage, and hemorrhagic fluid accumulation in intestinal loops [2], [3], [4]. Toxin B induces cytotoxic effects on cultured intestinal epithelial cells, but lacks enterotoxicity in rodents in vivo [5], [6], [7]. Active immunization of hamsters with toxin A was sufficient to prevent fatal clindamycin-induced C. difficile-associated ileocecitis [8]. However, immunization with toxin B alone was not protective in this model [8]. Passive immunization with monoclonal antibodies against toxin A protected axenic mice from pseudomembranous cecitis [9]. Therefore, toxin A has been considered to be the principal mediator of C. difficile-associated disease in animals.
Conventional management of C. difficile-associated diarrhea consists of discontinuing the precipitating antimicrobial agent(s) if possible, and providing supportive care [1]. If improvement does not occur, therapy with oral metronidazole or vancomycin is recommended [1], [10], [11], [12], [13]. Although the majority of cases respond to such therapy, 15–30% of patients experience recurrent diarrhea after treatment is discontinued [12], [13]. Treating recurrent disease is often problematic, and an optimal therapeutic approach remains to be elucidated [12], [13].
Humoral immune responses to C. difficile toxins play an important role in determining the severity of disease induced by the organism [14], [15], [16], [17]. Approximately 60% of healthy individuals produce serum immunoglobulin (Ig) G antitoxin A antibodies and intestinal secretory antitoxin A antibodies [14]. Patients with low antibody levels against toxin A are at risk of experiencing severe, prolonged, or recurrent C. difficile-associated disease [15], [16], [17]. A prospective study of hospitalized patients who received antibiotics and were colonized by toxigenic C. difficile showed that those with low serum IgG antitoxin A antibody levels were 48 times more likely to develop C. difficile-associated diarrhea compared to individuals with high antibody levels [15]. Small case series have demonstrated that administering pooled human immunoglobulin to children and adults with severe or protracted C. difficile-associated disease can lead to clinical improvement [18], [19], [20], [21]. Therefore, passive immunization may be a promising adjunct to standard therapies for severe or recurrent disease.
Given the above observations, a novel human monoclonal antibody against C. difficile toxin A designated CDA1 was developed by the Massachusetts Biologic Laboratories in partnership with Medarex, Inc. CDA1 is an IgG1κ molecule that was generated against C. difficile toxin A using genetically altered mice (Hco7 HuMAB mice) which produce human antibodies. CDA1 binds to the receptor-binding domain of toxin A with high affinity and has demonstrated efficacy in reducing mortality in the established hamster model for C. difficile associated disease [19]. An open-label, dose escalation study was therefore undertaken to assess the safety and pharmacokinetics of single infusions of CDA1 in healthy adult volunteers.
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Subject population
Thirty subjects were enrolled at 2 centers (Beth Israel Deaconess Medical Center, Boston, MA and Tufts-New England Medical Center, Boston, MA). Inclusion criteria were as follows: age between 18 and 55 years, inclusive; good general health without a history of any of the conditions listed in the exclusion criteria; and screening laboratory values that met the following criteria: (1) WBC ≥ 4500 and ≤ 13,000 cells/μl; (2)Platelet count ≥ 150,000 cells/μl; (3) hemoglobin level ≥ 12 gm/dl; (4)creatinine level
Study population demographics
Thirty subjects were enrolled at 2 centers between September 2004 and April 2005. Overall, 33% of the subjects were men, and the median age was 27.5 years (range, 20–53 years). Seventy-three percent of subjects were white, 3% were African American, 13% were Asian, 3% were American Indian/Alaskan Native, and 7% were of other ethnicities. All subjects completed the study.
The majority of AE were mild to moderate in severity and assessed as unlikely related or unrelated to receipt of CDA1 (Table 1
Discussion
C. difficile-associated diarrhea usually responds to conventional treatment with metronidazole or vancomycin [1], [10], [11], [12], [13]. A significant proportion of patients, however, develop recurrent disease that is associated with significant morbidity [12], [13]. Humoral immune responses are thought to play an important role in determining the severity of C. difficile-associated disease [14], [15], [16], [17] which can range from asymptomatic carriage to severe colitis with
Acknowledgments
This study was sponsored by Massachusetts Biologic Laboratories, University of Massachusetts medical school, Jamaica Plain, MA and Medarex, Inc., Bloomsbury, NJ.
Study investigators were supported in part by grants from the National Institutes of Health (RO-1 AI053069 to CPK, K30-HL04095 to the Scholars in Clinical Science Program at Harvard Medical School, in which CPT was enrolled, T32-DK0776 to the Division of Gastroenterology, Beth Israel Deaconess Medical Center, RR 01032 to BIDMC-General
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