Immunotoxicotherapy: present status and future trends

Vaccines targeting drugs of abuse: is the glass half-empty or half-full?

The advent of vaccines targeting drugs of abuse heralded a fundamentally different approach to treating substance-related disorders. In contrast to traditional pharmacotherapies for drug abuse, vaccines act by sequestering circulating drugs and terminating the drug-induced 'high' without inducing unwanted neuromodulatory effects. Drug-targeting vaccines have entered clinical evaluation, and although these vaccines show promise from a biomedical viewpoint, the ethical and socioeconomic implications of vaccinating patients against drugs of abuse merit discussion within the scientific community.

Fab-bound Colchicine Appears to Adopt Fab Fragment Disposition in Rats

The disposition of colchicine-specific Fab fragments and the effect of Fab fragment administration on the disposition of colchicine were studied in anaesthetized bile duct-cannulated rats. One group of rats (n = 6) received a 125I-Fab dose of 38 mg kg−1 i.v. The plasma disposition was characterized by a volume of distribution of 179 ± 48 mL kg−1, total body clearance of 1·02 ±0·07 mL min−1 kg−1, t½α of 0·17 ± 0·03 h and t½α of 1·3 ± 0·3 h. Fab fragments were in part excreted by the renal route (15·6 ± 6% of the Fab dose), while biliary excretion was a minor route (< 2% of the Fab dose). Two other groups of rats received 15 μg kg−1 colchicine (n = 6) or 15 μg kg−1 colchicine plus 38 mg kg−1 colchicine-specific Fab fragments (n = 6) by intravenous infusion. Pharmacokinetics of colchicine was markedly altered in the Fab-colchicine-treated rats. In this group, distribution volume and total body clearance of colchicine were decreased by factors of 22 and 10, respectively, compared with the values in the colchicine-treated group and were very similar to those of Fab fragments. An 80% reduction of cumulative biliary excretion of colchicine was observed in Fab-colchicine-treated rats (P < 0·01). The fraction of colchicine dose excreted by the urinary route was 38 ± 6·9 and 9 ± 0·7% respectively in Fab-colchicine- and colchicine-treated groups (P < 0·01). These data show that during Fab treatment, colchicine followed the elimination kinetics of Fab fragments. This study supports the view that Fab fragments could be of benefit in acute colchicine poisoning by neutralizing colchicine in the vascular compartment and imposing its elimination kinetics on colchicine.

Immunopharmacotherapeutic manifolds and modulation of cocaine overdose

Cocaine achieves its psychostimulant, reinforcing properties through selectively blocking dopamine transporters, and this neurobiological mechanism impedes the use of classical receptor-antagonist pharmacotherapies to outcompete cocaine at CNS sites. Passive immunization with monoclonal antibodies (mAb) specific for cocaine circumvents this problem as drug is sequestered in the periphery prior to entry into the brain. To optimize an immunopharmacotherapeutic strategy for reversing severe cocaine toxicity, the therapeutic properties of mAb GNC92H2 IgG were compared to those of its engineered formats in a mouse overdose model. Whereas the extended half-life of an IgG justifies its application to the prophylactic treatment of addiction, the rapid, thorough biodistribution of mAb-based fragments, including F(ab')₂, Fab and scFv, may correlate to accelerated scavenging of cocaine and reversal of toxicity. To test this hypothesis, mice were administered the anti-cocaine IgG (180 mg/kg, i.v.) or GNC92H2-based agent after receiving an LD₅₀ cocaine dose (93 mg/kg, i.p.), and the timeline of overdose symptoms was recorded. All formats lowered the rate of lethality despite the >100-fold molar excess of drug to antibody binding capacity. However, only F(ab')₂-92H2 and Fab-92 H2 significantly attenuated the progression of premorbid behaviors, and Fab-92H2 prevented seizure generation in a percentage of mice. The calculation of serum half-life of each format demonstrated that the pharmacokinetic profile of Fab-92H2 (elimination half-life, t½~100 min) best approximated that of cocaine. These results not only confirm the importance of highly specific and tight drug binding by the mAb, but also highlight the benefit of aligning the pharmacokinetic and pharmacodynamic properties of the immunopharmacotherapeutic with the targeted drug.

Effects of antibodies induced by a conjugate vaccine on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone absorptive transport, metabolism, and proliferation of human lung cells

One of the most abundant and potent lung carcinogen is the nicotine-derived tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The monoclonal antibody P9D5 induced with a NNK-conjugate vaccine was used to investigate the ability of NNK-specific antibodies to modulate NNK-induced adverse effects as well as its absorptive transport and metabolism in two lung cancer cell lines (Calu-3 and NCI-H82). Transport experiments in Calu-3 cells with a 50-fold molar excess of apical P9D5 increased the recovery of coadministered apical NNK, with a concomitant decrease in NNK transepithelial transport of more than 50% compared to controls. In contrast, basolateral P9D5 did neither influence transepithelial transport of NNK nor its disappearance from the apical compartment. Calu-3 cells were also found to reduce NNK to NNAL and a 65-fold molar excess of NNK-specific antibody inhibited this metabolic conversion by 46 and 54% compared to irrelevant control antibody after 48 and 72 hr, respectively. The biological relevance of NNK redistribution by antibody was demonstrated by reversion of NNK-induced cell proliferation in NCI-H82 cells. Repartitioning of tobacco carcinogens by antibody may reduce their early effective peak concentrations in susceptible target organs and thus relieve overloaded local DNA repair mechanisms and diminish carcinogen-induced cell proliferation. These in vitro data therefore suggest that a prophylactic antibody response may be associated with a reduced risk of cancer.

A mechanism-based binding model for the population pharmacokinetics and pharmacodynamics of omalizumab

AIM

Omalizumab, a humanized IgG monoclonal antibody that binds to human immunoglobulin E (IgE), interrupts the allergic cascade in asthmatic patients. The aim was to compare simultaneously drug exposure and IgE biomarker responses in Japanese and White patient populations.

METHODS

An instantaneous equilibrium drug-ligand binding and turnover population model was built from 202 Japanese patients. A posterior predictive evaluation for the steady-state distributions of omalizumab and IgE was then carried out against 531 White patients.

RESULTS

The mean parameters estimated from the Japanese patients were as follows: omalizumab clearance 7.32 +/- 0.153 ml h(-1), IgE clearance 71.0 +/- 4.68 ml h(-1) and the difference between that for omalizumab and the complex 5.86 +/- 0.920 ml h(-1), the volume of distribution for omalizumab and IgE 5900 +/- 107 ml, and that for the complex 3630 +/- 223 ml, the rate of IgE production 30.3 +/- 2.04 microg h(-1). Half-lives of IgG (23 days) and IgE (2.4 days) were close to previous reports. The dissociation constant for binding, 1.07 nM, was similar to in vitro values. Clearance and volume of distribution for omalizumab varied with bodyweight, whereas the clearance and rate of production of IgE were predicted accurately by baseline IgE. Overall, these covariates explained much of the interindividual variability.

CONCLUSIONS

The predictiveness of the Japanese model was confirmed by Monte-Carlo simulations for a White population, also providing evidence that the pharmacokinetics of omalizumab and IgE were similar in these two populations. Furthermore, the model enabled the estimation of not only omalizumab disposition parameters, but also the binding with and the rate of production, distribution and elimination of its target, IgE.

Antibody pharmacokinetics and pharmacodynamics

The U.S. Food and Drug administration (FDA) has approved several polyclonal antibody preparations and at least 18 monoclonal antibody preparations (antibodies, antibody fragments, antibody fusion proteins, etc.). These drugs, which may be considered as a diverse group of therapeutic proteins, are associated with several interesting pharmacokinetic characteristics. Saturable binding with target antigen may influence antibody disposition, potentially leading to nonlinear distribution and elimination. Independent of antigen interaction, concentration-dependent elimination may be expected for IgG antibodies, due to the influence of the Brambell receptor, FcRn, which protects IgG from catabolism. Antibody administration may induce the development of an endogenous antibody response, which may alter the pharmacokinetics of the therapeutic antibody. Additionally, the pharmacodynamics of antibodies are also complex; these drugs may be used for a wide array of therapeutic applications, and effects may be achieved by a variety of mechanisms. This article provides an overview of many of the complexities associated with antibody pharmacokinetics and pharmacodynamics.

Fab antibody fragments: some applications in clinical toxicology

This review provides current information on the use of antigen-binding fragments (Fab) from cleaved antibodies to treat poisoning with digoxin and other potent, low formula mass poisons, such as colchicine and tricyclic antidepressants. Anti-digoxin Fab fragments have been used successfully for many years in the management of severe poisoning with digoxin, digitoxin, and a range of other structurally related compounds, including cardiotoxins from Nerium and Thevetia sp. (oleander) and Bufo sp. (toads). However, their main use remains treating digoxin poisoning. Equimolar doses of anti-digoxin Fab fragments completely bind digoxin in vivo. The approximate dose of Fab fragments (mg) is 80 times the digoxin body burden (mg). If neither the dose ingested nor the plasma digoxin/digitoxin concentration is known, in an adult 380 mg of anti-digoxin Fab fragments should be given. The dose for elderly patients or those with renal impairment should be similar to that for those with normal renal function. Fab fragments have a plasma half-life of 12-20 hours, but this can be prolonged in patients with renal impairment. Analysis of serum ultrafiltrate using an immunoassay shown not to have matrix bias remains the most accurate approach to measuring free digoxin in the presence of anti-digoxin Fab fragments. The antibody fragments are given intravenously over 15-30 minutes after dilution to at least 250 mL with plasma protein solution, 0.9% (w/v) sodium chloride, or deionised water, except in infants where the volume infused can be reduced. Factors limiting the efficacy of Fab fragments are the dose, the duration of the infusion and any delay in administration. Guidelines for Fab fragment administration in children include (i) dilution to a final Fab concentration of 10 g/L in either 5% (w/v) dextrose or 0.9% (w/v) sodium chloride; (ii) infusion through a 0.22 microm filter; (iii) administration of the total dose over a minimum of 30 minutes; and (iv) avoiding coadministration of other drugs and/or electrolyte solutions. Fab fragments are generally well tolerated. Adverse effects attributable to Fab treatment include hypokalaemia and exacerbation of congestive cardiac failure; renal function could be impaired in some patients. Fab fragment preparations for treating acute colchicine and tricyclic antidepressant poisoning have been developed, but are not available commercially. Colchicine poisoning is rare in Western countries, and pharmacological management together with supportive care is usually effective even in severe tricyclic antidepressant overdosage. Attempts have been made to produce anti-paraquat antibodies capable of enhancing paraquat elimination from the lung, but thus far all such attempts have proved unsuccessful.

Newer antidotal therapies

Although the availability of an antidote for a toxic agent does not take away the primary responsibility of the clinician to manage the patient's clinical signs, the use of antidotes in appropriate situations can result in a more rapid recovery with potentially fewer long-term complications. Recent advances in pharmacology and molecular biology have resulted in the development of new and safer antidotal therapies for the management of toxicosis. The progress in immunotoxicotherapy over the last two decades continues and may ultimately lead to an era when the clinical toxicologist has a vast array of antibody fragments available for use with specific toxic agents. Development of specific pharmacologic antagonists for other agents should also enable the clinician to more reliably manage toxicoses. In spite of all these potential advances, the management of most toxicoses still relies on the application of sound veterinary medical principles.

Neutralization of dermonecrotic and lethal activities and differences among 32-35 kDa toxins of medically important Loxosceles spider venoms in Brazil revealed by monoclonal antibodies

Neutralization of dermonecrotic and lethal activities and differences among the principal toxic proteins (32-35 kDa) of medically important Loxosceles spider venoms in Brazil (Loxosceles gaucho, Loxosceles laeta and Loxosceles intermedia) were studied using monoclonal antibodies (MAbs) produced against the dermonecrotic component (35 kDa) of L. gaucho venom. MAb titers were 512,000 to homologous venom, between 2000 and 64,000 for L. intermedia venom and between 1000 and 64,000 for L. laeta venom. By Western blotting, MAbs could recognize mainly the 35 kDa protein of L. gaucho venom and with less intensity the 35 kDa protein of L. intermedia venom. These MAbs also recognized weakly or did not recognize the 32 kDa component of L. laeta venom. Only MoALg1 showed high affinity for L. gaucho venom and neutralized in vivo 90-97% of the dermonecrotic activity, besides delaying the lethality induced by homologous venom. MoALg1 maintained its capacity to neutralize the dermonecrotic activity, even when administered (i.v.) 6h after envenoming (i.d.). All MAbs obtained failed to neutralize the toxic activities of the heterologous venoms.These results suggest that different epitopes are present in the protein responsible for the dermonecrotic activity of Loxosceles venoms, and confirm the participation of other venom components during the local reaction process. This study also confirms the importance of antibodies for neutralization of dermonecrotic activity, even when administered some hours after envenoming, and emphasizes the differences of composition and toxicity of medically important Loxosceles venoms. These findings must be considered in order to improve loxoscelism immunotherapy.

Colchicine poisoning by accidental ingestion of meadow saffron (Colchicum autumnale): pathological and medicolegal aspects

Although intoxications with colchicine, the alkaloid of Colchicum autumnale (meadow saffron), are well known, in most cases the intoxications are evoked by oral or parenteral preparations traditionally used as medication against gout. The accidental ingestion of Colchicum autumnale, on the other hand, is a rare event and has to our knowledge only twice been described in detail. We report a further case in which two persons confused this highly poisonous plant with wild garlic (Allium ursinum), a popular spice in the Central European cuisine. While one person merely complained about a 3-day episode of nausea, vomiting and watery diarrhea, the second person died of multi-organ system derangements 48 h after the ingestion of the colchicum leaves. At autopsy hemorrhagic lung oedema, hypocellular bonemarrow, centrilobular fatty necrosis of the liver and necrosis of the proximal convoluted tubuli of the kidneys were observed. A colchicine concentration of 7.5 micrograms/ml was found in the bile whereas no substance was detected in the postmortem blood.

Influence of various combinations of specific antibody dose and affinity on tissue imipramine redistribution

1. This study was designed to evaluate the distribution kinetics of imipramine (Imip) in the brain and the main peripheral organs (heart, kidney, liver and lung) of rats, and to establish the relationship between the redistribution of Imip from these tissues and the immunoreactive capacity (dose and affinity) of anti-TCA IgG. 2. [3H]-Imip (1 nmol kg(-1) body weight) was injected intravenously 6 min before the i.v. injection of antibodies. At this time, the concentrations of Imip and its main metabolites in plasma were determined. The radioactivity measured corresponded to 91.7% Imip, indicating that the pharmacokinetics reflected essentially Imip. Plasma and tissue Imip contents were measured over the interval 1 to 90 min in control and in treated rats. The antibodies used were a murine monoclonal IgG1 (Ka=3.8 10(7) M(-1)) at an IgG1/Imip molar ratio of 1000 (IgG1 1000), and a sheep polyclonal IgG (TAb, Ka=1.3 10(10) M(-1)) at IgG/ Imip molar ratios of 1, 10 and 100 (TAb1, TAb10 and TAb100). 3. The anti-TCA IgG increased the plasma [3H]-Imip concentrations: the AUC1-->60 min for [3H]-Imip were 4 (IgG1 1000), 9 (TAb1), 33.9 (TAb10) and 41.4 (TAb100) times higher in the treated groups than in the controls. The opposite effect occurred in the brain, heart and lungs, with large, rapid decreases in Imip. The increase in plasma Imip and the decrease in tissue Imip depended on the immunoreactive capacity (NKa) of the antibody, where N=molar concentration of IgG binding sites and Ka=IgG affinity constant. Maximal plasma and tissue redistribution occurred when NKa=33.8 x 10(4). 4. Imip redistribution can be controlled using various doses or affinities of specific antibodies, and the resulting rapid, extensive Imip redistribution from the main target organs could be very promising for TCA detoxification.

Interspecies scaling of clearance and volume of distribution for horse antivenom F(ab')2

F(ab')2 fragments are sometimes preferred to whole IgG for therapeutic or diagnostic uses. Preclinical pharmaceutical development studies are necessary before their use in humans. Here we propose an allometric approach among three mammalian species to predict F(ab')2 pharmacokinetic parameters in humans. Plasma disposition of horse antivenom F(ab')2 fragments labeled with iodine 125 was studied at a dose of 10 mg/kg i.v. in mice, rats, and rabbits. Using the allometric method, we demonstrate that the pharmacokinetic parameters that correlated with body weight were distribution volume (Vdc (ml) = 0.125 W0.87; Vdss (ml) = 0.251 W0.87; Vd beta (ml) = 0.290 W0.87, r2 = 1), total clearance (Cltot (ml/h) = 0.049 W0.53, r2 = 0.99), and terminal half-life (t1/2 beta (h) = 4.35 W0.33). The F(ab')2 plasma concentration-time data plotted as a complex Dedrick relationship were superimposable. Using these allometric techniques, Vdss, Vd beta, Cltot, and t1/2 beta were calculated as 4.12 liter, 4.78 liter, 19.07 ml/h, and 7.2 days, respectively, for a human subject of 70 kg body wt. Predicted human pharmacokinetic parameters were comparable for volume of distribution with the value reported by Hnatowich et al. (Cancer Res. 47, 6111-6117, 1987): 3.5 liter. However, the clearance was six-fold lower than values given by Hnatowich et al. (130 ml/h) and Ho et al.

Preclinical assessment of immunoreactivity of a new purified equine F(ab')2 against European viper venom

The immunological and pharmacokinetic properties of a new, further purified, pasteurized preparation of equine F(ab')2 (VIPERFAV) against Vipera aspis, Vipera berus, and Vipera ammodytes venom were compared with the current equine F(ab')2 preparation (IPSER Europe). Affinity constants of the V. aspis-specific F(ab')2 were determined using biosensor technology and found to be in the range of 10(8) M-1 for the four antigenic fractions of V. aspis toxins and for both F(ab')2 preparations. The improvement of 51% in the specific activity (LD50 mg-1) of the new F(ab')2 was in close agreement with the 1.8-fold increase in the immunoreactive fraction of the new preparation. In vivo investigations of venom immunocomplexation by F(ab')2 in rabbits confirmed the ability of F(ab')2 to neutralize and redistribute toxin venom. Infusion of a stoichiometric molar ratio (i.e., 1 mg kg-1) of the new antivenom induced a 2.3-fold elevation of the plasma venom concentration with a Tmax observed 8 h after F(ab')2 administration and a decline in the terminal half-life from 31.92 +/- 4.49 h to 16.73 +/- 4.34 h, in contrast, for the venom alone. The area under the curve was 1.4-fold greater in the VIPERFAV group than in the IPSER Europe group during the post-F(ab')2 infusion period. Increasing the F(ab')2 dose to 3 mg kg-1 increased by 27% the percent of venom bound to F(ab')2. Finally, the greater the venom distribution, the smaller and less pronounced the plasma redistribution. These results demonstrate that the purification and pasteurization steps involved in the preparation of the new F(ab')2 have no deleterious influence on F(ab')2 affinity but, on the contrary, improve the protective efficacy. Alteration of viper venom kinetics by specific F(ab')2 antivenom was also shown to be dependent on the interval between of F(ab')2 administration and venom bite and on the specific F(ab')2 dose administered.

Immunoreactivity of a new generation of horse F(ab')2 preparations against European viper venoms and the tetanus toxin

The immunoreactivity of the current and the new more purified, pasteurized preparations of horse F(ab')2 against the tetanus toxin and Vipera aspis venom was investigated with a biosensor based on technology using the optical phenomenon of surface plasmon resonance. Immunoreactivity data were compared with seroneutralization titres to investigate immunoreactivity-immunoprotection efficacy relationships. The association-dissociation rate and affinity constants of the current and the new tetanus toxin-specific F(ab')2 preparations were similar, at about 10(4) M-1 sec-1, 10(-4) sec-1 and 10(8) M-1, respectively. Similar values were found using a solid immunoradiometric assay. To assess the immunoreactivity of V. aspis venom-specific horse F(ab')2, the mol. wt and percentage of the antigenic fractions of V. aspis venom were determined. Western blotting of electrophoresis gels showed four antigenic fractions of V. aspis venom (mol. wts 17,500, 28,500, 32,000 and 60,000), which represented 6, 3.4, 17.7 and 5% of total venom, respectively. Association and dissociation rate constants were in the same range as those of the tetanus toxin-F(ab')2 interactions for each of the four antigenic fractions. Seroneutralization of both tetanus toxin and V. aspis by the corresponding specific F(ab')2 showed that the LD50 mg-1 protein was 1.76-fold and 1.51-fold higher with the new than with the current preparations, respectively. These improvements in efficacy were in close agreement with the higher immunoreactive fraction ratios, which were 2-fold and 1.8-fold higher with the new preparations. These results demonstrate that the removal of non-IgGT immunoglobulins and the pasteurization treatment have no overall influence on F(ab')2 affinity but improve the specific activity of these new antitoxin horse F(ab')2.

Facilitation of imipramine efflux from the brain by systemic specific antibodies

1. This study investigated the capacity of circulating anti-tricyclic antidepressant (TCA) IgG to increase the efflux of imipramine (Imip) from the rat brain. 2. A tracer amount of [3H]-Imip (40 pmol) was injected into the cerebral lateral ventricle and its efflux was determined in control rats and in rats given anti-TCA antibody. The monoclonal anti-TCA IgG1 was injected i.v. 48 h before Imip at 4 IgG:Imip molar ratios (10, 100, 1000 and 10,000). The [3H]-Imip in arterial and venous plasma was measured for up to 60 min, and in the brain and peripheral organs (heart, liver, lung, kidney) 5 and 60 min after Imip injection. 3. The arterial plasma concentration of Imip in control rats was significantly higher (26.7 +/- 2.1 pM) than the venous one (17.7 +/- 2.0 pM) at 5 min, indicating that Imip released from brain becomes distributed in peripheral tissues. These concentrations were not significantly different at 60 min suggesting that Imip was, at this time, redistributing from extravascular tissues to the blood. In rats given anti-TCA IgG, any Imip leaving the brain was immediately bound by the circulating antibody at 5 min. This greatly reduced the Imip in the heart (63.9%) and lung (61.3%) at the highest IgG:Imip ratio. The brain Imip was markedly lower at 60 min (31.5% with an IgG Imip ratio of 1000 and 57.5% at a ratio of 10,000). The two lowest IgG:Imip ratios had less effect on the plasma Imip because of the relative low affinity of the anti-TCA IgG (3.8 x 10(7) M-1). 4. These data indicate that the anti-TCA IgG facilitated the efflux of Imip from the brain, even though these antibodies cannot cross the blood-brain barrier. This may be an efficient system for increasing drug organ clearance, as more than half the Imip in the brain was actively removed by the antibody in 1 h.

Monoclonal digoxin-specific antibodies induce dose- and affinity-dependent plasma digoxin redistribution in rats

The effect of three monoclonal digoxin-specific antibodies on total and free digoxin plasma disposition was studied in rats in order to determine the role of affinity constant (Ka) and dose. Thirty minutes after digoxin infusion, administration of a stoichiometrical dose of the ICIO, 6C9 and 9F5 IgG (Ka = 6 10(9), 3.1 10(8) and 2.5 10(7) M-1, respectively) resulted in a plasma digoxin increase linearly related to Ka. The mean free plasma digoxin was 0.6 +/- 0.4, 7.8 +/- 3.3 and 43 +/- 22% respectively after 1C10, 6C9, and 9F5 IgG infusion in comparison to 70 +/- 9% in the control group. When the IgG:digoxin ratio increased from 1 to 5, plasma digoxin Cmax and AUCT also increased as a function of both affinity (Ka) and dose (N), but not linearly. The product of NKa defined an immunoreactivity factor that was well fitted to the digoxin redistribution parameters (Cmax and AUCT) by a Hill equation.

Antibody treatment of toxin poisoning--recent advances

The major responses to the administration of specific antibody or toxin-specific fragment are described. Toxin sequestration depends on the extent and rate of antibody distribution, the antibody affinity and its ability to form a non-active immune complex. Toxin redistribution is mainly influenced by the reversible binding and efflux kinetics of the toxin from the receptor. Finally, toxin elimination adopts the antibody elimination properties for low molecular weight compounds. These three basic mechanisms of the immuno-detoxification process could be optimized by designing the ideal antibody, in terms of size and origin, to inactivate the toxic properties. Calculation of the amount of infused antibody should be derived from the slope of the dose-effect curve rather than stoichiometrically.

Analytical control procedures of immunoreactivity for IgG and Fab fragments specific to haptens

This study investigates immunoreactivity control procedures, i.e., specificity, affinity constant (Ka), and specific active binding sites (SABS), for polyclonal anticolchicine, monoclonal antidigitoxin IgG and Fab fragments, and antidigoxin Fab fragments (Digidot). Preliminary control procedures for IgG and Fab fragment purity indicated that all reagents were immunologically pure. All IgG and Fab fragments exhibited similar cross-reactivity and Ka. No decrease in percentage of Fab fragment SABS was observed after papain cleavage of anticolchicine and antidigitoxin IgG. Nevertheless, only 4.3 +/- 1.2% of nonimmunopurified anticolchicine polyclonal Fab fragments and 76.2 +/- 2.3 to 88.7 +/- 2.5% of different batches of immunopurified anti-digoxin Fab (Digidot) were active, the latter percentage being in the range of the 85% specified by the manufacturer. Only 58 +/- 3% of digitoxin-specific monoclonal IgG was active and 67 +/- 7% of its Fab fragments. Results show the importance of determining the ratio of SABS to presumed total specific binding sites for pharmaceutical monoclonal and polyclonal antibody preparations against haptens.

Pretreatment with drug-specific antibody reduces desipramine cardiotoxicity in rats

The effect of a drug-specific antibody on desipramine (DMI) cardiotoxicity was studied in rats. Animals were pretreated i.v. with 4.2 g/kg of a monoclonal antibody (anti-TCA) followed by DMI HCl 30 mg/kg i.p. (molar ratio of anti-TCA binding sites to DMI = 0.56). Peak QRS complex prolongation was substantially lower after pretreatment with anti-TCA than after control antibody (70 +/- 14 v. 21 +/- 4%, p less than 0.001). Time to peak toxicity was the same in both groups. Binding of DMI by anti-TCA was demonstrated by a higher serum total DMI concentration and increased DMI binding in serum after anti-TCA compared to controls. The DMI concentration in anti-TCA treated animals was lower in some organs (brain, lung, liver, spleen), but not in others (heart, muscle, kidney, fat). The calculated fraction of the DMI dose bound by anti-TCA was 19.9%. The steepness of the DMI dose-response curve was examined by administering DMI alone (without antibody) at various doses to rats. Compared to 30 mg/kg DMI, a dose reduction of 30-50% was needed to reduce QRS duration to the same extent as anti-TCA pretreatment. We conclude that DMI cardiotoxicity was markedly reduced by the binding of a relative small fraction of the DMI body burden to anti-TCA. This disproportionate effect of DMI binding was not due to the steepness of the DMI dose-response curve, nor to slowing of the rate of DMI distribution to tissues.

Drug-specific F(ab')2 fragment reduces desipramine cardiotoxicity in rats

Current therapy for cardiotoxicity due to tricyclic antidepressant (TCA) overdose is often ineffective in seriously poisoned patients. We studied the effect of a drug-specific antibody fragment on TCA cardiotoxicity in rats. Animals received anti-TCA F(ab')2 2.0 g/kg i.v. over 10 min starting 15 min after administration of a toxic dose of desipramine (DMI). This anti-TCA F(ab')2 dose was 36.9% of the molar DMI dose in terms of binding sites. Anti-TCA F(ab')2 infusion had no adverse effects and rapidly reduced DMI induced QRS prolongation compared with control F(ab')2 (23 +/- 14 vs 71 +/- 11% QRS prolongation at the end of infusion, P less than 0.001). This beneficial effect lasted for the 45 min duration of the study. Markedly enhanced DMI binding in serum after anti-TCA F(ab')2 was demonstrated by a 48-fold increase in the total DMI concentration over controls and a reduction in the fraction of unbound DMI (44.5 +/- 19.4 vs 0.7 +/- 0.2%). Anti-TCA F(ab')2 reduced the DMI concentration in brain but not in other organs. We conclude that anti-TCA F(ab')2 substantially reduces DMI cardiotoxicity in rats, and does so rapidly enough to be of potential clinical benefit for patients with DMI overdose. Because only a small fraction of the DMI dose was bound by antibody, these data suggest that antibody fragment doses considerably less than equimolar to the DMI dose may be effective in treating DMI cardiotoxicity.

The effect of nortriptyline-specific active immunization on amitriptyline toxicity and disposition in the rabbit

Rabbits were actively immunized by a conjugate of nortriptyline (NT) to study the effect of specific anti-NT antibodies on toxicity and disposition of amitriptyline (AT). Control and immunized rabbits received 115 mg/kg AT intraperitoneally (i.p.). The lethality dose (LD) profile exhibited a gentle slope; LD100 and LD0 were separated by 100 mg/kg. Mortality was significantly reduced from LD67 to LD43 (P less than 0.05). Total plasma concentrations of the toxin were increased in the immunized group compared to the control group. AUC0.5-24 h value was 5-fold higher in the immunized group than in the control group. Moreover, a smaller fraction of unbound toxin in plasma was observed in the immunized group than in the control group. These observations indicate that AT was actively sequestered by antibodies. The intensity of this phenomenon was a function of both the antibody affinity constant (10(9) M-1) and the neutralizing capacity (varying from 0.005 to 0.2 mg/kg) of the circulating antibodies in each immunized rabbit. Results clearly show that anti-NT antibodies are able to effectively sequestrate AT.