Analysis of Botulinum Neurotoxin Serotype A Metalloprotease Inhibitors: Analogs of a Chemotype for Therapeutic Development in the Context of a Three-Zone Pharmacophore

Abstract

Botulinum neurotoxins (BoNTs), and in particular serotype A, are the most poisonous of known biological substances, and are responsible for the flaccid paralysis of the disease state botulism. Because of the extreme toxicity of these enzymes, BoNTs are considered highest priority biothreat agents. To counter BoNT serotype A (BoNT/A) poisoning, the discovery and development of small molecule, drug-like inhibitors as post-intoxication therapeutic agents has been/is being pursued. Specifically, we are focusing on inhibitors of the BoNT/A light chain (LC) (ie, a metalloprotease) subunit, since such compounds can enter neurons and provide post-intoxication protection of the enzyme target substrate. To aid/facilitate this drug development effort, a pharmacophore for inhibition of the BoNT/A LC subunit was previously developed, and is continually being refined via the incorporation of novel and diverse inhibitor chemotypes. Here, we describe several analogs of a promising therapeutic chemotype in the context of the pharmacophore for BoNT/A LC inhibition. Specifically, we describe: 1) the pharmacophoric 'fits' of the analogs and how these 'fits' rationalize the in vitro inhibitory potencies of the analogs and 2) pharmacophore refinement via the inclusion of new components from the most potent of the presented analogs.

Figure 1

The 3-Zone Pharmacophore for BoNT/A LC inhibition (in two-dimensions). Parent SMNPI 1 is shown as it ‘fits’ within pharmacophore Zone-1 and Zone-2.

Figure 2

The 3D superimposition of diverse SMNPIs in the context of the 3-Zone Pharmacophore for BoNT/A LC inhibition resulted in pharmacophore refinement (black arrows and text). Nitrogen atoms are blue, oxygen atoms are red, and chlorine atoms are light green. a) Overlay of SMNPIs 1, NSC 104999, and Q2-15. Carbon atoms are green for 1, magenta for NSC 104999, and cyan for Q2-15. b) Overlay of SMNPI 2, NSC 104999, and Q2-15. Carbon atoms are orange for SMNPI 2; all other atom colors are as indicated in (a).

Scheme 1

The synthesis of SMNPI 2.

Scheme 2

The syntheses of SPNPIs 3–5 and 7–8.