Raxibacumab: potential role in the treatment of inhalational anthrax

Figures & data

Figure 1 Pathophysiology of anthrax illustrated as a series of steps. 1) Bacillus anthracis spores germinate and release lethal factor and PA. Initially, PA is an 83 kDa monomer. 2) PA₈₃ binds to the ANTXR1/2 transmembrane receptors in the host cell. 3) Furin, a cell surface proprotein convertase, cleaves PA₈₃ into PA₂₀ and PA₆₃ fragments. The PA₂₀ fragment is cleaved off while PA₆₃ remains bound to the receptor. 4) Proteolytically processed PA₆₃ monomers assemble into a heptameric or octameric PA prepore. The PA prepore can bind up to three or four lethal factor or monomers. 5) Prepore clusters are internalized with or without the LRP6 coreceptor via receptor-mediated endocytosis, resulting in endosome formation. 6) Acidification of endosome results in prepore transformation into a transmembrane delivery pore. 7) Release of lethal factor and edema factor inside the cell. 8) Lethal factor, a zinc metalloproteinase, inactivates MAPKK, resulting in impaired lymphocyte activation, B cell proliferation, as well as macrophage apoptosis via activation of the cytosolic inflammasome pathway. 9) A calcium-dependent and calmodulin-dependent adenylate cyclase increases intracellular cAMP, resulting in activation of cAMP response genes. Migration of infected macrophages to lymph nodes is stimulated, as well as inhibition of T cell activation, impaired phagocytosis, oxidative burst, and cytokine dysregulation. cAMP induces vasodilation, leading to edema.

Abbreviations: LF, lethal factor; EF, edema factor; PA, protective antigen; LT, lethal toxin; ET, edema toxin; ANTXR1/2, low (ANTXR1, previously tumor endothelial marker) or high (ANTXR2, previously capillary morphogenesis protein) type 1 transmembrane receptors; LRP6, low-density lipoprotein receptor-related protein 6; MAPKK, mitogen-activated protein kinase kinases; CREB, cAMP response element binding protein; CRE, cAMP-response elements; cAMP, cyclic adenosine monophosphate.

Figure 2 Raxibacumab inhibits binding of PA to the ANTXR1/2 transmembrane receptors in the host cell.

Abbreviations: LF, lethal factor; EF, edema factor; PA, protective antigen; ANTXR1/2, low (ANTXR1, previously tumor endothelial marker) or high (ANTXR2, previously capillary morphogenesis protein) type 1 transmembrane receptors; LRP6, low-density lipoprotein receptor-related protein 6.

Table 1 Human substudies on raxibacumab

Study	Objective	Endpoint	Design	Results
Subramanian et al^Citation31	Safety	Pharmacokinetics, safety, and biological activity	105 subjects (80 raxibacumab, 25 placebo) IM (vastus lateralis or gluteus maximus) injection (0.3, 1, or 3 mg/kg) or placebo; IV injection (1, 3, 10, 20, or 40 mg/kg) or placebo	Half-life 15–19 days for IM and 16–19 days for IV injection; linear pharmacokinetics with both routes of administration. Higher bioavailability in IM vastus lateralis than gluteus maximus
Migone et al^Citation27	Safety	Safety and pharmacokinetics of concomitant administration with ciprofloxacin	88 subjects divided in three groups: raxibacumab plus ciprofloxacin (ciprofloxacin 500 mg orally every 12 hours for 6 days, then a single dose of raxibacumab on day 5); raxibacumab only (single dose on day 0), or ciprofloxacin only (400 mg IV every 12 hours on day 0, then 500 mg orally every 12 hours for 6 days)	No alterations in pharmacokinetics of raxibacumab or ciprofloxacin; safe to give both together
Migone et al^Citation27	Safety	14-day safety of additional dose and maintenance of drug serum levels	320 subjects (291 assigned to a single dose versus placebo; 29 assigned to a double dose at day 0 and day 14 versus placebo)	No significant difference in adverse reactions; no difference in serum levels of drug maintained at 28 days
Migone et al^Citation27	Safety	Four-month safety of additional booster dose	20 subjects who had already received raxibacumab given repeat “booster” at 4 months	Not published

Abbreviations: IM, intramuscular; IV, intravenous.

Table 2 Animal studies of raxibacumab

Study	Species	Objective	Endpoint	Design	Results
Rat
Cui et al^Citation32	Rat (Sprague-Dawley)	Post-exposure prophylaxis	Survival rates at different time points	324 rats exposed to 24-hour infusion of anthrax lethal toxin; raxibacumab was administered at time of, or 3, 6, 9 or 12 hours after initiation of infusion; raxibacumab given at different concentrations at 6 hours	Odds of survival greater for rats that received raxibacumab early; no difference from placebo when received at 9 and 12 hours; decrease in survival with lower concentrations of raxibacumab given
Migone et al^Citation27	Rat (Fisher 344)	Pre-exposure	Survival at 24 hours	30 rats divided into 6 groups (IV, IM, or SC administration of raxibacumab or IgG placebo)	100% survival rate versus 0% for all three groups given raxibacumab, regardless of route of administration, when compared with placebo
Rabbit
Mazumdar^Citation29	Rabbit (New Zealand white)	Pre-exposure and post-exposure	14-day survival	72 rabbits divided into 6 groups (placebo or raxibacumab SC at 1, 5, 10, or 20 mg/kg; IV 40 mg/kg placebo or raxibacumab immediately post-exposure) exposed to anthrax spores	Higher survival rate in rabbits that received a higher dose
Migone et al^Citation27	Rabbit (New Zealand white)	Post-exposure treatment	14-day survival	Time-course experiment: 60 rabbits exposed to anthrax spores, then administered IV raxibacumab at time 0, 12, 24, or 36 hours post-exposure; controls given placebo at time 0. Dose-response experiment: 60 rabbits exposed to anthrax spores and then administered IV raxibacumab 24 hours later (0, 5, 10, 20, and 40 mg/kg); another group given raxibacumab 20 mg/kg 36 hours post-exposure	Higher survival with earlier administration; higher survival at higher doses
Migone et al^Citation27	Rabbit (New Zealand white)	Post-exposure treatment	14-day survival	54 rabbits in three equal groups exposed to anthrax spores on day 0; if increase in temperature or detection of PA in blood, then given single dose of IV raxibacumab (20 mg/kg or 40 mg/kg) or placebo	Significantly greater survival for both raxibacumab groups than for placebo
Corey et al^Citation33	Rabbit (New Zealand white)	Post-exposure combined treatment	28-day survival	180 rabbits challenged with anthrax spores and then given intragastric levofloxacin (50 mg/kg daily) or levofloxacin plus IV raxibacumab (40 mg/kg) 84 hours post-exposure	24 survivors (65%) in levofloxacin group and 32 (82%) in levofloxacin plus raxibacumab (P=0.0874)
Monkey
Migone et al^Citation27	Monkey (Cynomolgus macaque)	Pre-exposure	28-day survival	40 monkeys randomized to single SC dose of placebo or raxibacumab (10, 20, or 40 mg/kg) 48 hours prior to anthrax spore exposure	22 monkeys that received raxibacumab survived (60% for 10 mg/kg, 70% for 20 mg/kg, and 90% for 40 mg/kg); all monkeys in the placebo group died before day 7
Migone et al^Citation27	Monkey (Cynomolgus macaque)	Post-exposure treatment	28-day survival	40 monkeys randomized to single IV raxibacumab (20 mg/kg or 40 mg/kg) or placebo after anthrax spore exposure given upon detection of PA in serum	Greater survival for raxibacumab than placebo

Abbreviations: IgG, immunoglobulin; IM, intramuscular; IV, intravenous; SC, subcutaneous; PA, protective antigen.

Table 3 Monoclonal antibodies against PA³⁰

Antibody	Origin	Binding epitope	Mechanism of action	Efficacy animal model
Abthrax™ (GlaxoSmithKline)	Human	IV	Receptor binding inhibition	Rat, rabbit, and monkey
AVP-21D9	Human	III	Blocks PA heptamer formation	Rat and rabbit
ETI-204 (Anthim™; Elusys Therapeutics, Inc., Pine Brook, NJ, USA)	Humanized	IV	Receptor binding inhibition	Rabbit
MDX 1303 (Valortim; PharmAthene, Inc., Annapolis, MD, USA)	Human	III	Disrupts preformed PA heptamers	Rabbit and monkey
IQNPA	Human	IV	Receptor binding inhibition	Mouse
W1	Chimpanzee	IV	Receptor binding inhibition	Rat and mouse

Abbreviation: PA, protective antigen.

SubramanianGMCroninPWPoleyGA phase 1 study of PAmAb, a fully human monoclonal antibody against Bacillus anthracis protective antigen, in healthy volunteersClin Infect Dis2005411122015937757

 PubMed Web of Science ®Google Scholar

MigoneTSSubramanianGMZhongJRaxibacumab for the treatment of inhalational anthraxN Engl J Med2009361213514419587338

 PubMed Web of Science ®Google Scholar

CuiXLiYMoayeriMLate treatment with a protective antigen-directed monoclonal antibody improves hemodynamic function and survival in a lethal toxin-infused rat model of anthrax sepsisJ Infect Dis2005191342243415633102

 PubMed Web of Science ®Google Scholar

MazumdarSRaxibacumabMAbs20091653153820068396

 PubMed Web of Science ®Google Scholar

CoreyAMigoneTSBolmerSBacillus anthracis protective antigen kinetics in inhalation spore-challenged untreated or levofloxacin/raxibacumab-treated New Zealand white rabbitsToxins (Basel)20135112013823344456

 PubMed Web of Science ®Google Scholar

ChenZMoayeriMPurcellRMonoclonal antibody therapies against anthraxToxins (Basel)2011381004101922069754

 PubMed Web of Science ®Google Scholar