Validated MALDI-TOF-MS method for anthrax lethal factor provides early diagnosis and evaluation of therapeutics

Anthrax lethal factor (LF) is a zinc-dependent endoprotease and a critical virulence factor for Bacillus anthracis, the causative agent of anthrax. The mass spectrometry (MS) method for total-LF quantification includes three steps; 1) LF specific antibody capture/concentration, 2) LF-specific hydrolysis of a peptide substrate, and 3) detection and quantification of LF-cleaved peptides by isotope-dilution MALDI-TOF/MS. Recombinant LF spiked plasma was used for calibration and quality control (QC) materials. Specificity was 100% from analysis of serum and plasma from 383 non-infected humans, 31 rabbits, and 24 rhesus macaques. Sensitivity was 100% from 32 human clinical anthrax cases including infections by inhalation, ingestion, cutaneous and injection exposures and experimental infections for 29 rabbits and 24 rhesus macaques with inhalation anthrax. Robustness evaluation included sample storage, serum and plasma, antimicrobial and antitoxin effects and long-term performance. Data from 100 independent runs gave detection limits 0.01 ng/mL (111 amol/mL) for the 4-h method and 0.0027 ng/mL (30 amol/mL) for an alternate 20-h method. QC precision ranged from 7.7 to 14.8% coefficient of variation and accuracy from 0.2 to 9.8% error. The validated LF MS method provides sensitive quantification of anthrax total-LF using a robust high throughput platform for early diagnosis and evaluation of therapeutics during an anthrax emergency.