Protection against Clostridium difficile infection with broadly neutralizing antitoxin monoclonal antibodies.

The spore-forming bacterium Clostridium difficile represents the principal cause of hospital-acquired diarrhea and pseudomembranous colitis worldwide. C. difficile infection (CDI) is mediated by 2 bacterial toxins, A and B; neutralizing these toxins with monoclonal antibodies (mAbs) provides a potential nonantibiotic strategy for combating the rising prevalence, severity, and recurrence of CDI. Novel antitoxin mAbs were generated in mice and were humanized. The humanized antitoxin A mAb PA-50 and antitoxin B mAb PA-41 have picomolar potencies in vitro and bind to novel regions of the respective toxins. In a hamster model for CDI, 95% of animals treated with a combination of humanized PA-50 and PA-41 showed long-term survival relative to 0% survival of animals treated with standard antibiotics or comparator mAbs. These humanized mAbs provide insight into C. difficile intoxication and hold promise as potential nonantibiotic agents for improving clinical management of CDI.

Multiple CCR5 conformations on the cell surface are used differentially by human immunodeficiency viruses resistant or sensitive to CCR5 inhibitors.

Resistance to small-molecule CCR5 inhibitors arises when HIV-1 variants acquire the ability to use inhibitor-bound CCR5 while still recognizing free CCR5. Two isolates, CC101.19 and D1/85.16, became resistant via four substitutions in the gp120 V3 region and three in the gp41 fusion peptide (FP), respectively. The binding characteristics of a panel of monoclonal antibodies (MAbs) imply that several antigenic forms of CCR5 are expressed at different levels on the surfaces of U87-CD4-CCR5 cells and primary CD4(+) T cells, in a cell-type-dependent manner. CCR5 binding and HIV-1 infection inhibition experiments suggest that the two CCR5 inhibitor-resistant viruses altered their interactions with CCR5 in different ways. As a result, both mutants became generally more sensitive to inhibition by CCR5 MAbs, and the FP mutant is specifically sensitive to a MAb that stains discrete cell surface clusters of CCR5 that may correspond to lipid rafts. We conclude that some MAbs detect different antigenic forms of CCR5 and that inhibitor-sensitive and -resistant viruses can use these CCR5 forms differently for entry in the presence or absence of CCR5 inhibitors.

Treatment of advanced human immunodeficiency virus type 1 disease with the viral entry inhibitor PRO 542.

Viral entry inhibitors represent an emerging mode of therapy for human immunodeficiency virus type 1 (HIV-1) infection. PRO 542 (CD4-immunoglobulin G2) is a tetravalent CD4-immunoglobulin fusion protein that broadly neutralizes primary HIV-1 isolates. PRO 542 binds to the viral surface glycoprotein gp120 and blocks attachment and entry of virus into CD4(+) cells. Previously, PRO 542 demonstrated antiviral activity without significant toxicity when tested at single doses ranging to 10 mg/kg. In this study, 12 HIV-infected individuals were treated with 25-mg/kg single-dose PRO 542 and then monitored for safety, antiviral effects, and PRO 542 pharmacokinetics for 6 weeks. The study examined two treatment cohorts that differed in the extent of HIV-1 disease progression. PRO 542 at 25 mg/kg was well tolerated and demonstrated a serum half-life of 3 days. Statistically significant acute reductions in HIV-1 RNA levels were observed across all study patients, and greater antiviral effects were observed in the cohort of patients with more advanced HIV-1 disease. In advanced disease (HIV-1 RNA > 100,000 copies/ml; CD4 lymphocytes < 200 cells/mm(3)), PRO 542 mediated an 80% response rate and statistically significant approximately 0.5 log(10) mean reductions in viral load for 4 to 6 weeks posttreatment. Similar findings were obtained in an analysis of all (n = 11) advanced disease patients treated to date with single doses of PRO 542 ranging from 1 to 25 mg/kg. In addition, a significant correlation was observed between antiviral effects observed in vivo and viral susceptibility to PRO 542 in vitro. The findings support continued development of PRO 542 for salvage therapy of advanced HIV-1 disease.