Production of anti-horse antibodies induced by IgG, F(ab’) 2 and Fab applied repeatedly to rabbits. Effect on antivenom pharmacokinetics. Hilda Vázquez a , Felipe Olvera a , Alejandro Alagón a , Carlos Sevcik b,∗ a Instituto de Biotecnología (IBt), Universidad Nacional Autónoma de México, Cuernavaca, México b Laboratory on Cellular Neuropharmacology, Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas (IVIC), Caracas, Venezuela. Abstract We separated whole IgG, Fab and F(ab’) 2 fragments from horse plasma. We previously studied the pharmacokinetics of these immunoglobulins and fragments in rabbits and shown that Fab and F(ab’) 2 pharmacokinetics was well described by a three-exponential kinetics, while IgG and IgG(T) pharmacokinetics, however, deviated from the three-exponetial kinetics 120 h after injecting a bolus of the immunotherapeutics; this departure was shown to be due to a surge of anti-horse antibodies occurring after 120 h, peaking at ≈260 h and decaying slowly afterwards (Vázquez et al., 2010). We now describe antivenom pharmacokinetics and anti-horse IgG production in rabbits receiving three boluses (300 μg/kg, I.V.) of Fab, F(ab’) 2 or IgG separated by 21 days. Keywords: Pharmacokinetics, Antivenoms, IgG, Fab, F(ab’) 2 , Aquired immunity 1. Introduction Treatment of envenomation has depended almost entirely on the individual clinician’s expe- rience in assessing the severity of envenomation and involves the use of antivenom and other maintenance therapy directed at the consequence of envenoming; since the latter are particular for each envenoming, considered they are outside from this article scope. The efficacy of treat- ment is related to the neutralization potency of the antivenom used, the route by which it is administered, the dose and its pharmacokinetics (PK). The introduction of enzyme immunoas- says (Theakston et al., 1977) has permitted a more scientific approach, allowing the estimation of circulating venom and antivenom concentrations at any time after envenomation in the patient’s samples (mostly blood). The main objective of this study arised serendipitously during a PK study of anti-Loxosceles antivenoms; and in this sense it may be considered a basic science study. A point hard to document, mostly because data are hard to get, is the incidence of multiple envenomings in humans. For scorpionism, in areas of high incidence such as Cuernavaca (State of Morelos, Mexico) people stung and treated as may as four times are not uncommon, a 2 or 3 sting incidences are common. From 1990 to 1993 the Instituto Mexicano del Seguro Social carried out a survey on the 215,815 scorpion sting cases treated on their clinics and hospitals; 65.3% were patients stung by the first * Corresponding Author: Prof. Carlos Sevcik, IVIC CBB, Apartado 20632, Caracas 1020A, Venezuela. Email: csevcik@ivic.gob.ve. Private Email: carlos.sevcik.s@gmail.com. Phone: +58 212 504 1399. Fax: +58 212 504 1093. Mobile: +58 412 931 9162 Preprint submitted to Toxicon June 21, 2014