MODELLING TITYUSSCORPION VENOM AND ANTIVENOMPHARMACOKINETICS.EVIDENCE OF ACTIVE IMMUNOGLOBULIN G's F(ab')2 EXTRUSIONMECHANISM  FROM BLOOD TO TISSUES.

C. Sevcik¹, G. D'Suze^{1, P. Díaz1}, V. Salazar², C. Hidalgo¹, H. Azpúrua³, N. Bracho³.

1 Laboratory of Cellular Neuropharmacology, 2 Histology Service from the Centro de Biofísica y Bioquímica and 3 Unit of Experimental Surgery, Instituto Venezolano de Investigaciones Científicas (IVIC) Venezuela.

We measured pharmacokinetic parameters for Tityusdiscrepans in rams. The venom was injected s.c. in the inner side of the thigh in doses of 40, 75 o 100 µg/kg. Plasma venom content (venenemia) was determined with a specific sandwich type ELISA sandwich enzyme-linked immunosorbent assay (ELISA) from 0 to 300 min after injecting venom. Venenemia was fit to a 3 compartment model (inoculation site, plasma and extra vascular space), it was assumed that the venom may also be irreversibly removed from plasma.

We calculated the time course of venom content in the 3 compartments and found that at any time no more that 30% of the venom is present in plasma. Venenemia peaks at 1h and decays afterwards. Where the symbols are: k_a,rate of diffusion of venom from the injection site to plasma; k_el, rate of elimination of venom from plasma; k_pe: diffusion rate constant of venom from plasma to the extra vascular extracellular space; k_ep: diffusion rate constant of venom from the extracellular compartment to plasma; M_inoc, mass of venom in the inoculation site; M_p: mass of venom in plasma; M_ev: mass of venom in the extracellular space; C_p, concentration of venom in plasma; V_p, distribution volume of venom in plasma. Fluorescently labeled antivenom [horse anti-Tityus F(ab')2 covalently bound to fluorescine or fluorescamine] pharmacokinetics was also determined.

Here the new symbols mean: k_fd.ep, diffusion rate constant for F(ab')2 from tissues to blood vessels; k_fd.pe: diffusion rate constant for F(ab')2 from blood vessels to tissues; M_fab.e, amount of F(ab')2 in the extracellular space. Although the molecular weight of F(ab')2 is 10 times bigger that toxin's, the rate of outflow of F(ab')2 from blood to tissues was 4 times faster than the venom's outflow. Venom content at the injection site decays exponentially for >6 h; this prediction was confirmed immunohistochemically. Only 5% of the venom is eliminated in up to 10 h;  80% of the venom is in the tissues after 2 h and remains there for >10 h when the venenemia is negligible. The plasma compartment is a transient pathway from the envenoming site to the target organs, therefore the venenemia underestimates the amount of venom received; this underestimation increases with time. Results where F(ab')2 therapy is introduced in the model, remark the importance of the early use of anti venom during the first hour of envenoming. Financed in part by Laboratorios Silanes SA de CV, Toluca, Monterrey, Mexico.