A single dose of raxibacumab improved survival in rabbits and monkeys with symptomatic inhalational anthrax. (ClinicalTrials.gov number, NCT00639678.)

Inhaled Bacillus anthracis spores germinate and the subsequent vegetative growth results in bacteremia and toxin production. Anthrax toxin is tripartite: the lethal factor and edema factor are enzymatic moieties, while the protective antigen (PA) binds to cell receptors and the enzymatic moieties. Antibiotics can control B. anthracis bacteremia, whereas raxibacumab binds PA and blocks lethal toxin effects. This study assessed plasma PA kinetics in rabbits following an inhaled B. anthracis spore challenge. Additionally, at 84 h post-challenge, 42% of challenged rabbits that had survived were treated with either levofloxacin/placebo or levofloxacin/raxibacumab. The profiles were modeled using a modified Gompertz/second exponential growth phase model in untreated rabbits, with added monoexponential PA elimination in treated rabbits. Shorter survival times were related to a higher plateau and a faster increase in PA levels. PA elimination half-lives were 10 and 19 h for the levofloxacin/placebo and levofloxacin/raxibacumab groups, respectively, with the difference attributable to persistent circulating PA-raxibacumab complex. PA kinetics were similar between untreated and treated rabbits, with one exception: treated rabbits had a plateau phase nearly twice as long as that for untreated rabbits. Treated rabbits that succumbed to disease had higher plateau PA levels and shorter plateau duration than surviving treated rabbits.

PA-mAb may augment conventional hemodynamic support during anthrax LT-associated shock.

bAlthough antibiotics treat bacteremia in inhalational anthrax, pathogenesis is mainly driven by bacterial exotoxins. Raxibacumab, an IgG1 monoclonal antibody, binds the protective antigen (PA) of Bacillus anthracis, thus blocking toxin effects and leading to improved survival in the rabbit and monkey models of inhalational anthrax. To assess raxibacumab's added benefit over levofloxacin (LVX) alone, rabbits surviving to 84 h after a challenge with 200 times the median (50%) lethal dose of B. anthracis spores were randomized to receive 3 daily intragastric LVX doses of 50 mg/kg of body weight, with the first LVX dose administered just prior to administration of a single intravenous dose of placebo or 40 mg/kg raxibacumab. The percentages of animals alive at 28 days following the last LVX dose were compared between the 2 treatment groups using a two-sided likelihood-ratio chi-square test. The 82% survival rate for the LVX-raxibacumab combination was higher than the 65% survival rate for LVX alone (P ‫؍‬ 0.0874). There were nearly 2-fold fewer deaths for the combination (7 deaths; n ‫؍‬ 39) than for LVX alone (13 deaths; n ‫؍‬ 37), and the survival time was prolonged for the combination (P ‫؍‬ 0.1016). Toxin-neutralizing-activity titers were similar for both treatment groups, suggesting that survivors in both groups were able to mount a toxin-neutralizing immune response. Microscopic findings considered consistent with anthrax were present in animals that died or became moribund on study in both treatment groups, and there were no anthrax-related findings in animals that survived. Overall, raxibacumab provided a meaningful benefit over antibiotic alone when administered late in the disease course.

Addition of 12-tetradecanoylphorbol 13-acetate (TPA) to cultures of intact Swiss mouse 3T3 fibroblasts induced a dose-dependent increase in ornithine decarboxylase (OrnDCase) activity. Over the same concentration range, 10-9 to 10-6 M, TPA induced the release of radioactively labeled fibronectin (FN) from the cells into the culture medium. Retinoic acid, a derivative of vitamin A, inhibited in a dose-dependent manner both the increase in OrnDCase activity and the release of FN induced by TPA. To examine the effects of TPA and retinoic acid in enucleated cells, the cells were treated with 7.5 ,.g of cytochalasin B per ml of medium during centrifugation at 10,000 X g for 35 min at 370C. In a series offive experiments, the treated cells were 94.7 ± 4.8% (± SEM) enucleated as measured by [3H]thymidine incorporation and verified by Giesma staining for nuclei. In the enucleated cells, TPA did not induce increased OrnDCase activity but did induce FN release in a dose-dependent fashion over the same concentration range effective for FN release from intact cells. Moreover, addition of retinoic acid to the enucleated cells inhibited the phorbol ester-induced release of FN in a dose-dependent manner. A series of phorbol ester derivatives showed the same order of activity in causing FN release from the enucleated cells as reported for inducing OrnDcase activity in intact cells or in promoting mouse skin tumors in vivo. Similarly, several retinoids were tested for their ability to inhibit the phorbol ester-induced release of FN from enucleated cells. The efficacy of the retinoids in preventing FN release paralleled their activity in inhibiting phorbol ester-induced OrnDCase activity and skin tumor promotion, as reported in the literature. We conclude that at least one aspect of tumor promotion induced by phorbol esters-the loss of FN-does not require the cell nucleus, and further, that retinoids can inhibit this aspect of tumor promotion without nuclear involvement.From the recent work of Liau et al. demonstrating the binding of retinol to chromatin (1) and that of Fuchs and Green on the regulation of specific messenger RNA by retinol (2), one can safely conclude that in its normal physiological function, vitamin