Chemistry, Preparation, and Mechanism of Action
Antivenom for spiders is prepared in a similar manner as other antivenom products by first immunizing animals with non-toxic amounts of venom.5,25 Monkeys, horses, goats, sheep, chicken, camels, and rabbits have been used historically to source antivenom.28 The animals are placed on an inoculation schedule to allow gradual production of immunoglobulins, most importantly IgG. Sufficient antibody production usually requires up to 6 weeks. Animal choice for immune serum production is more often dictated by the species availability, financial considerations, and tradition rather than scientific modeling. Horses are used by the majority of antivenom producers, since they are relatively easy to maintain, and large volumes of serum can be obtained at one time without harm. Varying efforts are made during antivenom production to remove animal proteins such as albumin. Antivenoms target, bind, neutralize, and promote elimination or redistribution of toxins from body tissues. To date, no studies have compared immune sera of different animals for human compatibility or tolerance.
The antidotal fraction of an antivenom exists as either whole IgG, Fab, or F(ab′)2. The IgG molecule is composed of two antigen-binding fragments (Fab fragments) that are fused together and attached to the larger complement binding fragment (Fc fragment). It is the larger Fc portion that is generally considered to be the most responsible for immune mediated reactions. Digestion of the disulfide bonds of an IgG molecule with the enzyme pepsin will cleave the Fc fragment, allowing isolation of pure F(ab′)2 fragments (two fused Fab fragments). In contrast, digestion with papain cleaves the molecule more distally such that a larger Fc portion is removed from two separate Fab fragments. Both Fab and F(ab′)2 molecules can be isolated with affinity chromatography, and the highly antigenic Fc portion discarded. Although Fab and F(ab′)2 are more expensive to produce than their whole immunoglobin counterparts, they are generally regarded as less allergenic and therefore safer products.
Whole IgG antivenom is the easiest and least expensive to produce. It has a molecular weight of approximately 150 kDa, and is the largest of the three antivenom types. Because of its size, it is the least filterable at the glomerulus and has the smallest volume of distribution. IgG has a longer elimination half-life than either Fab or F(ab′)2.17
F(ab′)2 has an intermediate size (∼100 kDa) and elimination half-life. While lowering the risk of anaphylaxis compared to whole IgG, the F(ab)2 portion retains much of the allosteric configuration of the original IgG molecule that is lost in when Fab are formed. This configuration theoretically allows for tighter binding to venom. Fab is the smallest (∼50 kDa) antivenom molecule in size and is eliminated by the kidneys. It has the largest volume of distribution and a greater ability to reach extravascular compartments. Arachnid venoms that affect the central nervous system have low molecular weights and large volumes of distribution. Fab and F(ab)2 based antivenoms may therefore be best suited for this function.17
Immunoglobulin based antivenoms can be given by the intramuscular (IM), intravenous, or subcutaneous route. Intravenous administration achieves rapid peak serum concentrations, and the infusion can be stopped in the event of an allergic reaction.18 Intramuscular injection has been used when intravenous access is unobtainable.
Pharmacokinetics and Pharmacodynamics
Currently, there is no published pharmacokinetic or pharmacodynamic information available on Merck BW-AV in humans or animals. Graudins and colleagues demonstrated western blot binding of L. hasselti (Red Back Spider) Antivenom (RBS-AV) to purified α-latrotoxin and similar protein bands derived from multiple widow spiders (L. mactans, L. hesperus, L. lugubris, L. tredecimguttatus, and L. hasselti). When co-mixed with the venoms from these species prior to administration, RBS-AV prevented the development of a reproducible and rapid muscle contracture of an in vitro chicken nerve-muscle preparation. A dose-response relationship was observed with varying doses of RBS-AV administration.16 This confirms a direct in vitro binding effect of RBS-AV against widow spider venoms.
Previous animal studies have demonstrated that intramuscular administration of antivenom demonstrated very low serum venom concentrations.35,36 In these rabbit studies of intramuscular administration, F(ab′)2 and IgG had poor bioavailability (36%–42%) and delayed time to peak concentrations of 48 to 96 hours.
Pharmacokinetic and pharmacodynamic characteristics of equine derived antivenoms may differ between species studied. Equine derived antivenom maximum concentrations were greater in cows than in horses, and steady state distribution volumes were higher in cows than in horses in one study. Similar results were observed in rabbit models.37 Pharmacokinetic and pharmacodynamic parameters observed in animal models should therefore be interpreted with caution with regard to behavior of antivenoms in humans.
RBS-AV has been administered intramuscularly to treat human envenomations and clinical effectiveness equivalent to intravenous administration was previously reported.21 Subsequent studies, however, demonstrated the absence of RBS-AV in circulating serum up to 5 hours following intramuscular administration. Serum concentrations of RBS-AV were detected within 30 minutes of administration following intravenous administration.24 These results are consistent with animal studies demonstrating little if any effect on circulating venom when antivenoms are given intramuscularly. Additional studies on the pharmacokinetic and pharmacodynamic properties of RBS-AV, Merck Black Widow Antivenom, and F(ab′)2 black widow antivenom (in development) are needed.