Pharmacokinetic Study In Rabbits Research Articles

In vitro and in vivo evaluation of self-microemulsifying drug delivery system of buparvaquone

Aim: The aim of this study was to prepare a lipid-based self-microemulsifying drug delivery system (SMEDDS) to increase the solubility and oral bioavailability of a poorly water-soluble compound, buparvaquone (BPQ). Methods: The solubility of BPQ was determined in various vehicles, and pseudo-ternary phase diagrams were constructed to determine the microemulsion region. A series of formulations with different compositions were selected in the microemulsion region for assessment of self-emulsification time and droplet size. The optimized SMEDDS formulation was used for in vitro dissolution and pharmacokinetic studies in rabbits. Results: The optimum formulation of SMEDDS consisted of Capryol 90 (9.82%), Cremophor EL (70.72%), Labrasol (17.68%), and BPQ (1.78%). Emulsification time and the mean droplet size were found to be 1 minute and 18.0 ± 0.25 nm, respectively, for the optimum formulation. The cumulative percentage of drug released in 90 minutes was 100% in both SGF and SIF. The calculated absolute oral bioavailability for BPQ was found to be 40.10%. Conclusions: The optimum SMEDDS formulation was increased the rate and extent of absorption of BPQ. The formulation is suitable for oral administration of BPQ. It would be useful to conduct efficacy studies of BPQ in diseased animal models and subsequently for toxicokinetics studies.

Tissue Distribution, Pharmacokinetics and Stability Studies of Zidovudine Delivered by Niosomes and Proniosomes

Niosomes containing zidovudine (ZDV), an anti-HIV drug for intravenous administration were formulated by a thin-film hydration technique. Proniosomes were prepared in the form of a slurry using beta-cyclodextrin as carrier. The effect of the surfactants Tween and Span and the negative charge-inducing agent dicetylphosphate (DCP) on tissue distribution of niosomes and proniosomes was studied. The distribution of ZDV in lungs, kidney, heart, liver and spleen of mice after intravenous bolus injection was higher in Tween 80 niosomes without DCP than either niosomes with DCP or Tween 80 proniosomes. The amount of ZDV in plasma was low in Tween 80 niosomes without DCP. The results of a pharmacokinetic study in rabbits confirmed that Tween 80 formulations with DCP were cleared from the circulation within five hours. An increased half-life of 202 minutes and mean residence time of 212.1 minutes was observed in Tween 80 formulation. A stability study showed that after 90 days of storage, the drug leakage from Tween 80 formulations stored at room temperature was significant (p < 0.001) compared to niosomes stored at 4 degrees C. Encapsulation ZDV in proniosomes reduced drug leakage from vesicles stored at room temperature. These results demonstrate that niosomes are a promising vehicle for targeted delivery of ZDV to macrophages in spleen and liver.

In situ gels of Metoclopramide Hydrochloride for intranasal delivery: In vitro evaluation and in vivo pharmacokinetic study in rabbits

Intranasal (IN) administration is a promising approach for rapid-onset delivery of medications and to circumvent their first-pass elimination when taken orally. Metoclopramide Hydrochloride (MET HCl) is a potent antiemetic, effective even for preventing emesis induced by cancer chemotherapy. The feasibility of developing an efficacious intranasal formulation of metoclopramide has been undertaken in this study. Formulations were modulated so as to have gelation at physiological ion content after intranasal administration. Gelation was determined by physical appearance. The mucoadhesive force in terms of detachment stress, determined using sheep nasal mucosal membrane, increased with increasing concentration of carbopol. The results of in vitro drug permeation studies across sheep nasal mucosa indicate that effective permeation could be significantly increased by using in situ gelling formulation with carbopol concentration 0.15% or greater. Histological examination did not detect any damage during in vitro permeation studies. Finally, the bioavailability study in rabbits revealed that the absolute bioavailability of MET HCl was significantly increased from 40.67% in the case of the oral drug solution to 54.61% in the case of the nasal in situ gel. This study points to the potential of mucoadhesive nasal in situ gel in terms of ease of administration, accuracy of dosing, prolonged nasal residence and improved drug bioavailability.

Intranasal Microemulsion of Sildenafil Citrate: In Vitro Evaluation and In Vivo Pharmacokinetic Study in Rabbits

The purpose of the present study was to prepare intranasal delivery system of sildenafil citrate and estimate its relative bioavailability after nasal administration in rabbits to attain rapid onset of action with good efficacy at lower doses. Sildenafil citrate saturated solubility was determined in different solvents, cosolvents, and microemulsion systems. For nasal application, sildenafil citrate was formulated in two different systems: the first was a cosolvent system (S3) of benzyl alcohol/ethanol/water/Transcutol/taurodeoxy cholate/Tween 20 (0.5:16.8:47.7:15.9:1:18.1% w/w). The second was a microemulsion system (ME6) containing Oleic acid: Labrasol/Transcutol/water (8.33:33.3:16.66:41.66% w/w). The prepared systems were characterized in relation to their clarity, particle size, viscosity, pH, and nasal ciliotoxicity. In vivo pharmacokinetic performance of the selected system ME6 (with no nasal ciliotoxicity) was evaluated in a group of six rabbits in a randomized crossover study and compared to the marketed oral tablets. The targeted solubility (>20 mg/ml) of sildenafil citrate was achieved with cosolvent systems S1, S3, and S5 and with microemulsion systems ME3-ME6. The saturated solubility of sildenafil citrate in cosolvent system S3 and microemulsion system ME6 were 22.98 +/- 1.26 and 23.79 +/- 1.16 mg/ml, respectively. Microemulsion formulation ME6 showed shorter t (max) (0.75 h) and higher AUC((0-infinity)) (1,412.42 ng h/ml) compared to the oral tablets which showed t (max) equals 1.25 h and AUC((0-infinity)) of 1,251.14 ng h/ml after administration to rabbits at dose level of 5 mg/kg. The relative bioavailability was 112.89%. In conclusion, the nasal absorption of sildenafil citrate microemulsion was found to be fast, indicating the potential of nasal delivery instead of the conventional oral administration of such drug.

Determination and pharmacokinetics of orientin in rabbit plasma by liquid chromatography after intravenous administration of orientin and Trollius chinensis Bunge extract

A high-performance liquid chromatography (HPLC) method was developed and validated for the determination of orientin in rabbit plasma using ultraviolet (UV) absorbance detection. Orientin is the active constituent of purified herbal extract (TRO PE) from the flower of Trollius chinensis Bunge. Protein precipitation was used as the sample preparation technique. A Diamonsil C18 column (150mm×4.6mm, 5μm) was equilibrated with a mobile phase composed of 0.1% acetic acid/methanol/acetonitrile (80/5/15, v/v/v). The calibration curve of orientin in rabbit plasma was linear in the concentration range of 0.530–53.0μg/mL. This validated method was successfully applied to a pharmacokinetic study in rabbits after the intravenous administrations of orientin and TRO PE at three different doses.

Stereoselective determination of benalaxyl in plasma by chiral high‐performance liquid chromatography with diode array detector and application to pharmacokinetic study in rabbits

A chiral high-performance liquid chromatography method with diode array detector was developed and validated for stereoselective determination of benalaxyl (BX) in rabbit plasma. Good separation was achieved at 20 degrees C using cellulose tris-(3,5-dimethylphenylcarbamate) as chiral stationary phase, a mixture of n-hexane and 2-propanol (97:3) as mobile phase at a flow rate of 1.0 ml/min. The assay method was linear over a range of concentrations (0.25-25 microg/ml) in plasma and the mean recovery was greater than 90% for both enantiomers. The limits of quantification and detection for both enantiomers in plasma were 0.25 and 0.1 microg/ml, respectively. Intra- and interday relative standard deviations (RSDs) did not exceed 10% for three-tested concentrations. The method was successfully applied to pharmacokinetic studies of BX enantiomers in rabbits. The result suggested that the pharmacokinetics of BX enantiomers was stereoselective in rabbits.

Sustained-release tablets of indomethacin-loaded microcapsules: Preparation, in vitro and in vivo characterization

Indomethacin (IDM) was encapsulated in gelatin-cellulose acetate phthalate (CAP) microcapsules (A) by complex coacervation method and in CAP microcapsules (B) by simple coacervation method. Microcapsules A and B, having mean diameters of 38.24 and 35.74 μm, respectively, were used to prepare sustained-release tablets A and B. The activation energy of thermal degradation for tablets A and B was calculated based on differential scanning calorimetry (DSC) to be 258.9 and 284.8 kcal/mol, respectively. In vitro release profiles showed no burst effect and release t 1/2 of the two sustained-release tablets were found to be 41.30 ± 1.86 and 33.25 ± 2.84 min, respectively, while that of IDM plain tablets C was 6.30 ± 0.39 min ( P < 0.01). In vitro release of IDM from tablets A and B could be described by Higuchi equation and zero-order kinetics, respectively. After per os (po) administration with physiological saline, their irritation to rat stomach was obviously reduced in comparison with tablets C. Pharmacokinetic study in rabbits showed that t max was delayed and C max lowered compared with tablets C and the values of AUC 0–24 h of the three tablets were very close.

Quantification of antihistamine acrivastine in plasma by solid-phase extraction and high-performance liquid chromatography

An automated solid-phase extraction method was developed for the determination of the H 1-antihistamine acrivastine in plasma samples. Acrivastine was analyzed at the wavelength of 254 nm using a reversed-phase HPLC assay. Both extraction procedure and analytical condition were optimized and validated for maximum recovery and resolution. The developed method was further applied to plasma samples collected from an in vivo pharmacokinetic study in rabbits. The assay was found to be simple, specific, accurate and reproducible.

Intraocular injection of an aptamer that binds PDGF‐B: A potential treatment for proliferative retinopathies

Platelet-derived growth factor-B (PDGF-B) has been implicated in the pathogenesis of proliferative retinopathies and other scarring disorders in the eye. In this study, we sought to test the therapeutic potential of an aptamer that selectively binds PDGF-B, ARC126, and its PEGylated derivative, ARC127. Both ARC126 and ARC127 blocked PDGF-B-induced proliferation of cultured fibroblasts with an IC50 of 4 nM. Pharmacokinetic studies in rabbits showed similar peak vitreous concentrations of approximately 110 microM after intravitreous injection of 1 mg of either ARC126 or ARC127, but the terminal half-life was longer for ARC127 (98 versus 43 h). Efficacy was tested in rho/PDGF-B transgenic mice that express PDGF-B in photoreceptors and develop severe proliferative retinopathy resulting in retinal detachment. Compared to eyes injected with 20 microg of scrambled aptamer in which five of six developed detachments (three total and two partial), eyes injected with ARC126 (no detachment in five of six and one partial detachment), or ARC127 (no detachment in six of six) had significantly fewer retinal detachments. They also showed a significant reduction in epiretinal membrane formation. These data demonstrate that a single intravitreous injection of an aptamer that specifically binds PDGF-B is able to significantly reduce epiretinal membrane formation and retinal detachment in rho/PDGF-B mice. These striking effects in an aggressive model of proliferative retinopathy suggest that ARC126 and ARC127 should be considered for treatment of diseases in which PDGF-B has been implicated, including ischemic retinopathies such as proliferative diabetic retinopathy, proliferative vitreoretinopathy (PVR), and choroidal neovascularization.

A sensitive and selective HPLC/ESI-MS/MS assay for the simultaneous quantification of 16-dehydropregnenolone and its major metabolites in rabbit plasma☆

A sensitive, selective and rapid liquid chromatographic/electrospray ionization tandem mass spectrometric assay was developed and validated for the simultaneous quantification of 16-dehydropregnenolone (DHP) and its five metabolites 4,16-pregnadien-3, 20-dione (M(1)), 5-pregnene-3beta-ol-20-one (M(2)), 5-pregnene-3beta, 20-diol (M(3)), 5-pregnene-3beta-ol-16, 17-epoxi-20-one (M(4)) and 5,16-pregnadien-3beta, 11-diol-20-one (M(5)) in rabbit plasma using dexamethasone as internal standard (IS). The analytes were chromatographed on Spheri-5 RP-18 column (5 microm, 100 mm x 4.6 mm i.d.) coupled with guard column using acetonitrile:ammonium acetate buffer (90:10, v/v) as mobile phase at a flow rate of 0.65 ml/min. The quantitation of the analytes was carried out using API 4000 LC-MS-MS system in the multiple reaction monitoring (MRM) mode. The method was validated in terms of linearity, specificity, sensitivity, recovery, accuracy, precision (intra- and inter-assay variation), freeze-thaw, long-term, auto injector and dry residue stability. Linearity in plasma was observed over a concentration range of 1.56-400 ng/ml with a limit of detection (LOD) of 0.78 ng/ml for all analytes except M(3) and M(5) where linearity was over the 3.13-400 ng/ml with LOD of 1.56 ng/ml. The absolute recoveries from plasma were consistent and reproducible over the linearity range for all analytes. The intra- and inter-day accuracy and precision method were within the acceptable limits and the analytes were stable after three freeze-thaw cycles and their dry residues were stable at -60 degrees C for 15 days. The method was successfully applied to determine concentrations of DHP and its putative metabolites in plasma during a pilot pharmacokinetic study in rabbits.

Simultaneous determination of buprenorphine and its prodrug, buprenorphine propionate, by high-performance liquid chromatography with fluorescence detection: application to pharmacokinetic studies in rabbits

A rapid, sensitive, precise and accurate high-performance liquid chromatographic assay with fluorescence detection was developed for the simultaneous determination of buprenorphine and buprenorphine propionate in human and animal blood. Buprenorphine propionate was also proven to be a prodrug of buprenorphine. It was comprised of only a one-step extraction procedure with ethyl acetate and normal-phase chromatography on a Betasil Silica column. The recoveries of buprenorphine and buprenorphine propionate were above 84%. Calibration graphs were linear for buprenorphine over the concentration range 2–1500 ng/ml and for buprenorphine propionate over the concentration range 20–1500 ng/ml with a coefficient of variation, both within- and between-day, or less than 10% at any level. The limits of quantitation of buprenorphine and buprenorphine propionate in human or animal blood were 2.0 and 20 ng/ml, respectively, based on a single-to-noise ratio of 3. The method has been successfully applied to pharmacokinetic studies of buprenorphine and buprenorphine propionate in rabbits. The results demonstrated that buprenorphine propionate was rapidly and totally converted to its parent drug, buprenorphine, following intravenous administration. Buprenorphine propionate is a prodrug of buprenorphine.

Preparation and Pharmacokinetic Evaluation of Tashinone IIA Solid Lipid Nanoparticles

Tashinone IIA loaded solid lipid nanoparticles (TA-SLN) coated with poloxamer 188 was prepared by emulsification/evaporation. The TA-SLN was characterized by transmission electron microscope and dynamic light scattering (DLS). The results showed that the TA-SLN had an average diameter of 98.7 nm with a zeta potential of − 31.6 mv and the drug loading of 4.6% and entrapment efficiency of 87.7%. In vitro release experiment showed that the release of Tashinone IIA from TA-SLN was in accordance with the Weibull equation. The best model fitting experimental data was a two-compartment open model with first-order. The area under curve of plasma concentration–time (AUC) and mean residence time (MRT) of TA-SLN were much higher than those of Tashinone IIA control solution (TA-SOL). The results of pharmacokinetic studies in rabbits indicated that the formulation of TA-SLN was successful in providing a delivery of slow release of Tashinone IIA.

HPLC determination and pharmacokinetics of chlorogenic acid in rabbit plasma after an oral dose of Flos Lonicerae extract.

A simple and sensitive high-performance liquid chromatography (HPLC) method has been developed for the determination of chlorogenic acid (3-O-caffeoyl-D-quinic acid) in plasma and applied to its pharmacokinetic study in rabbits after administration of Flos Lonicerae extract. Plasma samples are extracted with methanol. HPLC analysis of the extracts is performed on a C(18) reversed-phase column using acetonitrile-0.2% phosphate buffer (11:89, v/v) as the mobile phase. The UV detector is set at 327 nm. The standard curves are linear in the range 0.0500-1.00 microg/mL (r = 0.9987). The mean extraction recovery of 85.1% is obtained for chlorogenic acid. The interday precision (relative standard deviation) ranges from 5.0% to 7.5%, and the intraday precision is better than 9.0%. The limit of quantitation is 0.0500 microg/mL. The plasma concentration of chlorogenic acid shows a C(max) of 0.839 +/- 0.35 microg/mL at 34.7 +/- 1.1 min and a second one of 0.367 +/- 0.16 microg/mL at 273.4 +/- 39.6 min.

Evaluation of the potential of ion pair formation to improve the oral absorption of two potent antiviral compounds, AMD3100 and PMPA

9-(2-phosphonyomethoxypropyl)adenine (PMPA) and AMD3100 are highly potent and selective antiretroviral agents. Since PMPA is negatively charged and AMD3100 positively charged at physiological pH, their transepithelial transport and their potential for oral drug delivery is very low. In this study, ion pair formation was evaluated as a possible strategy to enhance transepithelial transport of PMPA and AMD3100. Positively charged counter ions such as t-hexyl-, t-heptyl-, t-octylammonium bromide and dodecyl-, tetradecyl-, hexadecyltrimethylammonium bromide were used to form ion pairs with PMPA, while sodium taurodeoxycholate (in vitro experiments) and sodium taurocholate (in vivo experiments) were used as counter ions for AMD3100. The effect of counter ions on transepithelial transport of PMPA (1 mM) and AMD3100 (1 mM) was investigated by measuring the flux across Caco-2 monolayers. An enhancement in drug transport could be observed at a concentration of 2 mM of hexadecyltrimethylammonium bromide (counter ion for PMPA) and 10 mM of sodium taurodeoxycholate (counter ion for AMD3100), but at the concentrations used, the absorption enhancing effect could be attributed to a reduction of the integrity of the monolayers. When AMD3100 transport was tested at a concentration of 200 μM, no flux was observed, even in the presence of relatively high concentrations of counter ion (20 times the concentration of AMD3100). Results obtained from partitioning studies of the drugs in the presence or absence of counter ion revealed that competition by other ions was responsible for the absence of an effect: when pure water was used as the aqueous phase, a reduction up to 24.4±1.4% and 17.0±1.3% of the initial aqueous concentration was observed for PMPA and AMD3100, respectively; however, as soon as other ions were present in the aqueous phase, the effect of the counter ion was diminished (25–50 mOsm) or completely abolished (270–305 mOsm). The absorption enhancing effect of counter ions was also studied in vivo: pharmacokinetic studies in rabbits showed that the oral bioavailability of AMD3100 in the presence of 4 equivalents of taurocholic acid remained very low and was only 3.2-fold better (i.e. 3.6%) in comparison to pure AMD3100. In view of the results obtained in the Caco-2 system, this absorption enhancement can be attributed to an effect on monolayer integrity rather than to the potential to form ion pairs. We can conclude that the formation of ion pairs may not be very efficient as a strategy to enhance transepithelial transport of charged hydrophilic compounds, as competition by other ions may abolish the beneficial effect of counter ions.

Simplified solid-phase extraction method for determination of dihydroergotamine in rabbit and human serum using high-performance liquid chromatography with fluorescence detection

A rapid, selective and sensitive method for the determination of dihydroergotamine (DHE) in serum was developed. Dihydroergocristine (DHEC) was used as an internal standard. Human and rabbit serum samples were extracted using commercial solid-phase cyano (CN) columns. Proteins were washed from these columns with pure acetonitrile, resulting in clean extracts. Extracts were subsequently separated by HPLC in an isocratic way, using a reversed-phase C 18 analytical column. Fluorometric detection was performed at excitation and emission wavelengths of 277 and 348 nm, respectively. Calibration curves with amounts of DHE ranging from 2 to 32 ng, were linear. The limit of detection found for DHE was 0.2 ng, extracted from 0.5 ml rabbit or from 2.5 ml human serum. The limit of quantification in serum of both species was 0.7 ng. The method has been shown to be suitable for monitoring DHE in serum during pharmacokinetic studies in rabbits.

Expression and Characterization of CD4-IgG2, a Novel Heterotetramer That Neutralizes Primary HIV Type 1 Isolates

CD4-IgG2 is a novel fusion protein comprising human IgG2 in which the Fv portions of both heavy and light chains have been replaced by the V1 and V2 domains of human CD4. This tetrameric protein is being developed as an immunoprophylactic agent to reduce the probability of infection following HIV-1 exposure, in settings such as occupational or perinatal exposure to the virus. CD4-IgG2 has been expressed in Chinese hamster ovary cells and is secreted as a fully assembled heterotetramer. The protein binds with nanomolar affinity to purified gp120 from both a laboratory-adapted strain and a primary isolate of HIV-1. Pharmacokinetic studies in rabbits demonstrated that CD4-IgG2 has a plasma terminal half-life greater than 1 day, compared with 15 min for soluble CD4 (sCD4). CD4-IgG2 does not bind to Fc receptors on the surface of U937 monocyte/macrophage cells. Compared to molecules that incorporate the Fc portion of IgG1, CD4-IgG2 has less potential to mediate functions such as antibody-dependent enhancement of infection or transplacental transmission of HIV-1. When tested in a virus-free HIV-1 envelope glycoprotein-mediated cell fusion assay, the tetrameric CD4-IgG2 molecule inhibited syncytium formation more effectively than monomeric sCD4 or a dimeric CD4-gamma 2 fusion protein. This suggests the protein will block cell-to-cell transmission of HIV-1. Moreover, CD4-IgG2 effectively neutralized a panel of laboratory-adapted strains and primary isolates of HIV-1, including strains with different tropisms and isolated from different stages of the disease, at concentrations that should be readily achieved in vivo.

High-performance liquid chromatographic assay for sematilide in plasma using solid-phase extraction microcolumn technology.

A simple and sensitive high-performance liquid chromatographic assay for quantification of sematilide in rabbit plasma was developed. After extraction of samples via solid-phase extraction on C8 microcolumns, baseline resolution was achieved on a reversed-phase 5 microns Inertsil ODS-2 column using isocratic conditions with mobile phase consisting of water-glacial acetic acid-acetonitrile-methanol-triethylamine (93.5:4.0:1.5:0.5:0.5) and UV detection at 254 nm. The assay did not require evaporation or reconstitution steps. The injection interval was 8 minutes. The inter-day coefficient of variation for replicate analysis of spiked samples was less than 7.6% and the accuracy was more than 97% over the standard curve range (0.128 to 3.191 microM) using 0.5 ml of plasma. The assay has been successfully applied to pharmacokinetic studies in rabbits.

High-performance liquid chromatographic assay for ibuprofen in whole blood using solid-phase extraction

A precise, accurate, reproducible one-step method for the high-performance chromatographic determination of ibuprofen in whole blood is described. Samples were, after haemolysis, prepared by solid-phase extraction. Analyses were performed using reversed-phase chromatography on a Separon SGX C 18 column with a mobile phase of methanol—water (pH 3) and ultraviolet detection at 222 nm. The method was used for pharmacokinetic studies in rabbits.

New Polyacetal Polysulphate Active against Human Immunodeficiency Virus and other Enveloped Viruses

A new polyacetal polysulphate, termed PAPS, was synthesized starting from dextran through oxidation, reduction, and subsequent sulphation. PAPS inhibited HIV-1- and HIV-2-induced cytopathicity in MT-4 cells at concentrations comparable to those required for dextran sulphate (MW5000) to inhibit the cytopathicity of these viruses (50% inhibitory concentration: 0.4–0.04 μg ml−1). At these concentrations PAPS had no anticoagulant activity. PAPS suppressed syncytium formation between MOLT-4 cells and persistently HIV-1- or HIV-2-infected HUT-78 cells at a concentration of 1 μg ml−1, that is 25- to 30-fold lower than that required for dextran sulphate to inhibit syncytium formation. Like dextran sulphate, PAPS inhibited HIV-1 binding to the cells and anti-gp120 mAb binding to HIV-1 gp120. Also, PAPS proved equally active as dextran sulphate against herpes simplex virus, cytomegalovirus and the arenaviruses Junin and Tacaribe, and 10-fold more active than dextran sulphate against vaccinia, Sindbis, influenza A, and vesicular stomatitis virus. Neither PAPS nor dextran sulphate proved inhibitory to the non-enveloped viruses polio, Coxsackie and reovirus. Pharmacokinetic studies in rabbits revealed that after intravenous bolus injection the serum concentrations of PAPS decayed biphasically, with an initial half-life of approximately 45–60 min. Twenty-four hours following their intraperitoneal administration to mice, PAPS as well as dextran sulphate generated low titres of an antiviral principle that was at least partially interferon-like.

Experimental treatment protocols for scorpion envenomation: A review of common therapies and an effect of kallikrein-kinin inhibitors

Nine fatal cases from the sting of the scorpion Leiurus quinquestriatus are presented. All victims showed association of CNS and cardiovascular manifestations. Either the CNS or the cardiovascular effects could occur first in the early phases of the scorpion envenoming syndrome; the CNS manifestations, however, always preceded the terminal hypotension and cardiac arrest. Pharmacokinetic studies in rabbits following s.c. injection of the labelled venom showed that rapid absorption took place with about 70% of the maximum blood concentration reached within 15 min. Intramuscular injection of antivenom did not significantly affect the absorption of the venom or the other pharmacokinetic parameters. The total area under concentration time curve was not significantly different from that following i.v. injection, showing that nearly complete absorption of the venom from the s.c. site would occur in 7–8 hr. The i.v. infusion of venom into anaesthetized rats, at a rate comparable to the absorption rate from s.c. sites, allowed the determination of the minumum lethal dose (MLD) with reasonable accuracy. In rescue experiments, anaesthetized rats were injected s.c. with multiple MLD of venom and infused i.v. with drugs commonly used in the treatment of scorpion envenomation. The prepared potent specific antivenoms, but not the commercial polyvalent antivenom, rescued all animals from the lethal effect of the venom, even when injected late. Atropine, atropine + phentolamine, chlorpromazine, hydrocortisone and indomethacin were able, in varying degrees, to rescue some rats injected with 2 MLD of venom. Phentolamine, propranolol, hydralazine and calcium gluconate significantly prolonged the survival time, but did not rescue any animals. Chlorpheniramine, saline and 1 4 saline + 5% dextrose were without any effect. Aprotinin, the kallikrein-kinin inhibitor, was able to rescue half of the animals from the lethal action of the venom. Electrocardiographic studies showed that L. quinquestriatus venom, irrespective of the route of administration, causes myocardial ischaemia and either inferior or anterior wall infarction. This was associated with an initial moderate and a terminal severe bradycardia together with a variety of rhythm and conduction defects. Except for minor and transient electrocardiographic changes, either the prepared antivenoms or aprotinin protected rabbits and rats from the cardiac effects of the venom.