Ricin is a toxalbumin produced by a shrub belonging to the Euphorbiaceae family, the Castor Oil Plant (Ricinus communis). Ricin is a highly toxic glycoprotein with a molecular weight of 66 kDa formed by two polypeptide chains A and B connected together by a disulphide bridge. Chain B allows the toxin to attach itself to the cell wall while Chain B, which is responsible for its toxic properties, is capable of inhibiting protein synthesis by inhibiting 28S ribosomal RNA, causing cell death. It is present in the castor oil seed in concentrations of between 1% and 10%.
It may be extracted from incompletely purified castor oil.
As a toxin, ricin is extremely toxic. However, its toxicity varies according to the means by which it penetrates the organism.
When ricin is absorbed through digestion, it is largely destroyed by proteolytic digestive enzymes but its perlingual absorption may increase the quantity absorbed.
In contrast, when ricin is inhaled (pulmonary route) or administered via the parenteral route its toxicity is multiplied 1000-fold.
Symptoms are fairly non-specific and vary according to the route by which the ricin is absorbed. They generally become evident within a period of 2 to 24 hours and rarely take longer than 2 days to appear. Absorption through ingestion causes vomiting, feelings of faintness, abdominal pain, bloody diarrhoea (stools resembling rice water), a painful need to defecate or urinate (anuria), dehydration, drowsiness, muscle weakness, cramps, vasomotor paralysis and tachycardia. Absorption via inhalation causes weakness, fever, dizziness, dyspnoea, coughing, pulmonary oedemas and pain in the limbs.
After an apparent improvement, infection may have a fatal outcome.
In humans, the dose of ricin estimated to be lethal is between 1 and 10 μg/kg.
In view of these varied symptoms and the danger caused by ricin at a very low dose, there is a real need for protection against ricin contamination, including in response to its potential use in the context of bioterrorist attacks.
A rapid diagnostic test for ricin poisoning via the pulmonary route has recently been developed (Guglielmo-Viret et al. 2007). After ricin exposure, the following antidotes may be used: sugar analogues to prevent the ricin from connecting to its target, or catalytic subunit inhibitors such as Azidothymidine.
Vaccination might constitute another ricin poisoning treatment strategy. For example, antibodies have been developed that are designed to interfer with the connection that the anthrax toxin makes with cell surface receptors or to inhibit the assembly of the toxin.
However, no ricin-specific therapy is currently available.
Wang et al. (Wang et al., 2007, Biotechnol Lett 29: 1811-1816) recently developed a human-mouse chimeric antibody against ricin. However, although this first-generation chimeric antibody is ricin-specific, it is capable of generating a Human Anti-Chimeric Antibody (HACA) immune response and of inducing low patient tolerance.
The international application WO 2009/053637 describes single-chain Fv (scFv) fragments from constant regions of macaque antibodies that are capable of effectively neutralising ricin, as well as humanized or super-humanized scFv fragments.
However, such fragments are small in size, have a very short half-life, are rapidly eliminated by the kidneys and are incapable of providing long-term protection. Furthermore, given the lack of a constant region, these fragments are relatively ineffective at stimulating the immune system (recruitment of immune effectors).
Thus, there is a real need to provide ricin-specific antibodies that are stable after administration and that possess a very high toxin neutralization rate.
It is also important to provide antibodies that do not contribute to a HACA-type immune response in the host when they are administered.