Gaining ground: assays for therapeutics against botulinum neurotoxin.

Owing in part to recently heightened concern over bioterrorism, interest in the mechanism of action of botulinum neurotoxin (BoNT) and development of effective therapeutic strategies has dramatically increased. The emergence of BoNT as an effective treatment for a variety of neurological disorders and its growing use in the cosmetic industry have also increased interest in developing effective countermeasures. Although recent attempts to create effective vaccines appear promising, the multitude of clinical and cosmetic uses of BoNT make mass vaccination against the toxin undesirable and impractical, leading to intensified efforts to develop effective therapeutics to combat large-scale intoxications. In this review, we examine the relevant and available in vitro cell-based assays and in vivo assays for drug discovery and development, especially with regard to the potential for medium- to high-throughput automation and its use in identifying physiologically relevant inhibitors.

Primary cultures of embryonic chicken neurons for sensitive cell-based assay of botulinum neurotoxin: implications for therapeutic discovery.

Botulinum toxin is an exceedingly potent inhibitor of neurotransmission across the neuromuscular junction, causing flaccid paralysis and death. The potential for misuse of this deadly poison as a bioweapon has added a greater urgency to the search for effective therapeutics. The development of sensitive and efficient cell-based assays for the evaluation of toxin antagonists is crucial to the rapid and successful identification of therapeutic compounds. The authors evaluated the sensitivity of primary cultures from 4 distinct regions of the embryonic chick nervous system to botulinum neurotoxin A (BoNT/A) cleavage of synaptosomal-associated protein of 25 kD (SNAP-25). Although differences in sensitivity were apparent, SNAP-25 cleavage was detectable in neuronal cells from each of the 4 regions within 3 h at BoNT/A concentrations of 1 nM or lower. Co-incubation of chick neurons with BoNT/A and toxin-neutralizing antibodies inhibited SNAP-25 cleavage, demonstrating the utility of these cultures for the assay of BoNT/A antagonists.

Inhibition of metalloprotease botulinum serotype A from a pseudo-peptide binding mode to a small molecule that is active in primary neurons.

An efficient research strategy integrating empirically guided, structure-based modeling and chemoinformatics was used to discover potent small molecule inhibitors of the botulinum neurotoxin serotype A light chain. First, a modeled binding mode for inhibitor 2-mercapto-3-phenylpropionyl-RATKML (K(i) = 330 nM) was generated, and required the use of a molecular dynamic conformer of the enzyme displaying the reorientation of surface loops bordering the substrate binding cleft. These flexible loops are conformationally variable in x-ray crystal structures, and the model predicted that they were pivotal for providing complementary binding surfaces and solvent shielding for the pseudo-peptide. The docked conformation of 2-mercapto-3-phenylpropionyl-RATKML was then used to refine our pharmacophore for botulinum serotype A light chain inhibition. Data base search queries derived from the pharmacophore were employed to mine small molecule (non-peptidic) inhibitors from the National Cancer Institute's Open Repository. Four of the inhibitors possess K(i) values ranging from 3.0 to 10.0 microM. Of these, NSC 240898 is a promising lead for therapeutic development, as it readily enters neurons, exhibits no neuronal toxicity, and elicits dose-dependent protection of synaptosomal-associated protein (of 25 kDa) in a primary culture of embryonic chicken neurons. Isothermal titration calorimetry showed that the interaction between NSC 240898 and the botulinum A light chain is largely entropy-driven, and occurs with a 1:1 stoichiometry and a dissociation constant of 4.6 microM.

Reduced blood-to-tissue albumin movement after plasmapheresis.

We tested the hypothesis that a decrease in the blood-to-tissue movement of albumin contributes to the recovery of plasma albumin and plasma volume after acute plasma protein depletion (plasmapheresis). Awake and unrestrained male Sprague-Dawley rats (220-320 g) fitted with jugular catheters were plasmapheresed, and plasma volume, plasma albumin, and total plasma protein content were measured at 1, 5, 24, and 48 h postplasmapheresis. Plasma volume recovered to baseline within 1 h (4.6 +/- 0.42 vs. 4.7 +/- 0.46 mL/100 g body weight (bw), remained at baseline from 5 h to 24 h but increased to 5.5 + 0.57 mL/100 g bw at 48 h (P < 0.05). Plasma albumin and total protein content recovered rapidly but remained below baseline levels at 1 h (10.05 +/- 0.98 vs. 12.33 +/- 1.29 and 19.75 +/- 1.75 vs. 24.73 +/- 2.56 mg/100 g bw, respectively). Plasma protein content retumed to baseline by 5 h of recovery. Tissue uptake of I125-labeled albumin decreased in the heart, skin, skeletal muscle, and small Intestines of plasmapheresed rats (P < 0.05). These data support the hypothesis that a reduction in albumin efflux from the vascular space contrlbutes to the recovery of plasma albumin and total protein content during plasma volume recovery and eventual expansion after plasmapheresis.