Assessment of the tolerance angle for pedicle screw insertion

Cannulation process intervenes before implantation of pedicle screw and depends on the surgeon's experience. A reliable experimental protocol has been developed for the characterization of the slipping behavior of the surgical tool on the cortical shell simulated by synthetic materials. Three types of synthetic foam samples with three different densities were tested using an MTS Acumen 3 A/T electrodynamic device with a tri-axis 3 kN Kistler load cell mounted on a surgical tool, moving at a constant rotational speed of 10° mm-1 and performing a three-step cannulation test. Cannulation angle varied between 10° and 30°. Synthetic samples were scanned after each tests, and cannulation coefficient associated to each perforation section was computed. Reproducibility tests resulted in an ICC for Sawbone samples of 0.979 (p < 0.001) and of 0.909 (p < 0.001) for Creaplast and Sawbone samples. Cannulation coefficient and maximum force in Z-axis are found the best descriptors of the perforation. Angular threshold for perforation prediction was found to be 17.5° with an area under the curve of the Receiver Operating Characteristic of 89.5%. This protocol characterizes the cannulation process before pedicle screw insertion and identifies the perforation tool angle until which the surgical tool slips on the cortical shell depending on bone quality.

Sliding on cortical shell: Biomechanical characterization of the vertebral cannulation for pedicle screw insertion

BACKGROUND

Pedicular screws pull-out has been well studied unlike their insertion. A need for characterizing cannulation before pedicle screw implantation is highlighted in literature and offers promising prospects for future intra-operation instrumentation. A reliable cannulation protocol for ex-vivo testing in swine and cadaver vertebrae is presented in this work to predict extra pedicular perforation.

METHODS

An MTS Acumen 3 A/T electrodynamic device, with a tri-axis 3 kN Kistler load cell mounted on a surgical tool was used to reproduce surgeon's gesture by moving at a constant rotational speed of 10°/mm and performing a three-section test. Perforation of the pedicle's cortical shell was planned through a design of experiment on the surgical tool angle at the entry point. Samples were scanned before and after mechanical tests and reproducibility of the protocol was tested on synthetic foam. Computation of the angle between cannulation tool and pedicle cortical shell was performed as well as cannulation coefficient of each perforation section.

FINDINGS

A total of 68 pedicles were tested: 19 perforated and 21 non-perforated human pedicles, 17 perforated and 16 non-perforated swine pedicles. The reproducibility of the protocol for cannulation coefficient computation resulted in an intraclass correlation coefficient of 0.979. Cannulation coefficients results presented variability within spinal levels as well as between swine and human model. Correlation between bone density and cannulation coefficient was found significant (p < 0.005). Torque measurement was found to be the best predictor of perforation. Threshold of angle for prediction of perforation was found to be 21.7°.

INTERPRETATION

Characterizing pedicle cannulation enables to predict extra pedicular perforation. Influence of bone mineral density and patient-specific morphology on pedicle cannulation has been highlighted together with a comparison of swine and cadaver models.

Traction mechanical characterization of porcine mitral valve annulus

The Mitral Annulus (MA) is an anisotropic, fibrous, flexible and dynamical structure. While MA dynamics are well documented, its passive mechanical properties remain poorly investigated to complete the design of adequate prostheses. Mechanical properties in traction on four sections of the MA (aortic, left, posterior and right segments) were assessed using a traction test system with a 30 N load cell and pulling jaws for sample fixation. Samples were submitted to a 1.5 N pre-load, 10 pre-conditioning cycles. Three strain rates were tested (5 %/min, 7 %/min and 13 %/min), the first two up to 10 % strain and the last until rupture. High-resolution diffusion-MRI provided microstructural mapping of fractional anisotropy and mean diffusion within muscle and collagen fibres. Ten MA from porcine hearts were excised resulting in 40 tested samples, out of which 28 were frozen prior to testing. Freezing samples significantly increased Young Moduli for all strain rates. No significant differences were found between Young Moduli at different strain rates (fresh samples 2.4 ± 1.1 MPa, 3.8 ± 2.2 MPa and 3.1 ± 1.8 MPa for increasing strain rates in fresh samples), while significant differences were found when comparing aortic with posterior and posterior with lateral (p < 0.012). Aortic segments deformed the most (24.1 ± 9.4 %) while lateral segments endured the highest stress (>0.3 MPa), corresponding to higher collagen fraction (0.46) and fractional anisotropy. Passive machinal properties differed between aortic and lateral segments of the MA. The process of freezing samples altered their mechanical properties. Underlying microstructural differences could be linked to changes in strain response.

Novel bicyclic lactam inhibitors of thrombin: potency and selectivity optimization through P1 residues

Peptidomimetic inhibitors of thrombin lacking the important Ser195-carbonyl interaction have been prepared. The binding energy lost after the removal of the activated carbonyl was recaptured through a series of modifications of the P1 residues of the bicyclic lactam inhibitors. Selected substituted compounds displayed useful pharmacological profiles both in vitro and in vivo.

Potent and selective bicyclic lactam inhibitors of thrombin. Part 4: transition state inhibitors

Bicyclic piperazinone based thrombin inhibitors of general structure 2 were prepared and evaluated in vitro and in vivo. These inhibitors, having in common an electrophilic basic trans-cyclohexylamine P1 residue, displayed high thrombin affinity, high selectivity against trypsin and good in vivo efficacy in the rat arterial thrombosis model.

A 5-arm maze enables parallel measures of sustained visuo-spatial attention and spatial working memory in mice

A 5-arm maze has been developed to provide parallel tests of sustained visuo-spatial attention and spatial working memory in mice. C57Bl/6 mice were trained to select, either by immediate response (attention) or by delayed-matching response (working memory), one target arm among the five open arms. For attention testing, mice were first trained to acquire the basic task in which one randomly selected baited arm remained lit until a choice was made. Criterion of >80% correct with a response latency <5 s was attained in 52-56 trials. Following this, attention was tested by using trials wherein light signal durations of 2, 1 or 0.5 s were intermixed to vary attentional load. In the working memory test, mice were submitted to a forced visit to a randomly selected baited arm during a presentation phase. Following a variable retention interval (R.I.), mice were replaced into the maze and rewarded for choosing this arm. Criterion of >80% correct was attained in 35-40 trials and mice exhibited high levels of retention for R.I.s up to 4 h. Results validate the 5-arm maze for evaluation of both sustained visuo-spatial attention and spatial working memory in mice. Both the tasks are rapidly acquired and the 20% chance level provides high resolution for evaluating performance. This comparative strategy allows to dissociate attention and memory and to reveal deficits in these processes during ageing or in knockout strains. The high level of retention performance over R.I.s of 4 h enables studies using pharmacological treatments differentially affecting the acquisition, encoding, retention or retrieval phases of working memory. Furthermore, functional brain imaging studies may be used to identify neuronal networks which are differentially activated during these distinct phases.

In vitro and in vivo properties of bicyclic lactam inhibitors: a novel class of low molecular weight peptidomimetic thrombin inhibitors

We have developed potent and selective thrombin inhibitors with a novel non-peptidic structure. A bicyclic lactam was used as the scaffold on which various P1 and P3 motifs were substituted. Herein, we report the in vitro and in vivo properties of four representatives of this novel class of inhibitors. Their Ki values were less than 10 nM, they inhibited equally both free and clot-bound thrombin, and they displayed high level of specificity for thrombin over other serine proteases (trypsin, factor Xa, activated Protein C, and plasmin). They prolonged the clotting time of human plasma to twice the control value in coagulation assays (TT, APTT, and PT) at a concentration below 3 microM. Their anticoagulant activities using rat plasma were similar to, although slightly weaker, than with human plasma. Furthermore, they inhibited thrombin-induced platelet aggregation (human and rat) at concentrations close to their Ki values for thrombin. These molecules demonstrated similar dose response antithrombotic efficacy in rat arterial and venous thrombosis models when given as i.v. bolus followed by infusion. Antithrombotic efficacy of 85% and greater was observed at a dose of 5-7 microM/kg/hour in each model. Bicyclic lactam inhibitor 3, at a dose which caused a complete inhibition of visible thrombus formation in the venous and arterial models of thrombosis, showed a 1.9-2.1 and a 4.0-4.8-fold shift in APTT and TT, respectively. Unfortunately, the bicyclic lactam inhibitors exhibited low oral bioavailability in rats. Therefore, this novel class of bicyclic lactam thrombin inhibitor has the potential to be promising intravenous antithrombotic agents for the treatment of arterial as well as venous thrombosis and warrants further investigation.

Incorporation of an Asp-Ser sequence to form an RGDS-like motif in hirutonin: the effect on in vitro platelet function

We investigated the effect on in vitro platelet function of hirutonin, a modified hirutonin with an RGD-like motif, a pseudo-RGDS peptide and a linear RGDS peptide. Inhibition of expression of surface fibrinogen on ADP-activated platelets with 40 microM of the peptide was as follows: hirutonin 10+/-3%, modified chimeric peptide 26+/-5%, pseudo-RGDS 66+/-11% and linear RGDS 93+/-13%. Both hirutonin and the chimeric peptide significantly inhibited ADP-induced platelet activation as detected by CD62 expression. Unlike the RGDS and pseudo-RGDS controls, neither the chimeric peptide nor the parent hirutonin inhibited ADP-induced platelet aggregation even at 140 microM. The chimeric hirutonin peptide reduced ATP release from ADP-stimulated platelets by 40+/-4%. This inhibition was stronger than that caused by hirutonin (23+/-13%), but less than the RGDS (90+/-2%) and pseudo RGDS-peptides (59+/-11%). Primary platelet haemostasis was slightly but not significantly affected by the peptide at 40 and 80 microM. However, shear-induced platelet adhesion to vWF and especially subsequent aggregate formation was interrupted after the addition of the chimeric peptide. Similar results were obtained with hirutonin. This inhibition was not as marked as with the RGDS- and pseudo-RGDS peptides. Both the parent hirutonin and the chimeric peptide caused prolongation of the clinical coagulation assays aPTT and TT. In conclusion, the chimeric hirutonin peptide with introduction of the RGD motif retained its anticoagulant effect but had little formal disintegrin activity. Instead, it appeared to have novel anti-platelet effects that may be of therapeutic use.

A retrospective evaluation of the causes of death of 448 insured French horses in 1995

Epidemiological studies should allow comparisons to be made of the prevalence of disease in populations from different countries, but the population characteristics and health problems in French horses are not well established. We have conducted a retrospective evaluation of the causes of death and vital characteristics of insured horses in France for the year 1995, with a view to comparison with published data from other countries. Files on 448 deceased horses were provided by nine insurance companies. Most of the animals were used for breeding (60%), followed by leisure (20%), eventing and racing (10% each). Physical characteristics were associated significantly with occupational categories. The overall mortality rate was 2.47%, and was due, in decreasing order, to foaling (24%), colic (21%) or locomotor (21%), cardiovascular (9%), neurological (8%), respiratory (5%) or infectious (4%) disease. Infectious disease was more frequent in younger animals (p < 0.05) and locomotor disease in racehorses (p < 0.01). Horses aged over 15 years had a lower incidence of colic (p <0.05). The cause of death was not significantly linked to breed, insurance value or season. Despite some selection bias, the study provides useful information about mortality in the French equine population.

The design of potent and selective inhibitors of thrombin utilizing a piperazinedione template: part 2

Potent and selective thrombin inhibitors have been prepared with a piperazinedione template and L-amino acids. Likewise, incorporation of D-amino acids led to potent inhibitors with a novel mode of binding. Herein, the structure activity relationships and structural aspects of these compounds will be described.

Insertion of the Asp-Ser/Phe sequence in the P' position of hirutonin provides molecules having both antithrombin and disintegrin activity

We have developed novel synthetic peptides that display both antithrombin and disintegrin activity. These peptides were derived from hirutonins, a class of potent proteolytically resistant thrombin inhibitors, in which a dipeptidyl sequence, Asp-Phe or Asp-Ser, was introduced after the proteolytically resistant ketomethylene arginyl glycine isostere. These modified hirutonins inhibited the amidolytic activity of alpha-thrombin (Ki approximately 35 nM), prevented fibrinogen clotting (dTT approximately 100 nM) and inhibited human platelet aggregation and 5-hydroxytryptamine secretion induced by alpha-thrombin (IC50 approximately 600 nM). Unlike their parent hirutonins, they inhibited SFLLR-NH2-induced human platelet aggregation (IC50 approximately 45 microM) without inhibition of 5-HT secretion. These peptides also competed for fibrinogen binding to purified GpIIbIIIa integrin (IC50 approximately microM) and prevented attachment of B16-F10 mouse melanoma cells to vitronectin. We conclude that addition of the dipeptidyl sequence, Asp-Phe or Asp-Ser, in hirutonin molecules confers disintegrin activity. However, this activity was not superior to the activity observed with the linear RGDS peptide and was achieved at the expense of direct antithrombin activity. Additional modifications around the RGD-like adhesion sequence may permit identification of the appropriate conformation for optimal binding to thrombin and to specific integrin receptors.

Evaluation of proteinase-activated receptor-1 (PAR1) agonists and antagonists using a cultured cell receptor desensitization assay: activation of PAR2 by PAR1-targeted ligands

We developed a calcium signaling-based assay, using cultured human embryonic kidney cells (HEK), that evaluates simultaneously, the activation/desensitization or blockade of the proteinase-activated receptors, PAR1 and PAR2. Using this assay, we analyzed the actions of a number of previously described putative PAR1-targeted peptide agonists and antagonists. We found that most of the previously described PAR1-targeted agents can also activate/desensitize PAR2, and most of these peptides can also activate a calcium signaling pathway in a target cell that possesses PAR2 along with PAR1. Furthermore, we used this assay to develop a PAR1 receptor-activating probe [Ala-parafluoroPhe-Arg-Cha-Cit-Tyr-NH2 (Cit-NH2)], which displays a high degree of specificity for PAR1 over PAR2, and we used the assay to quantitate the ability of trypsin to disarm the activation of PAR1 by thrombin. The abilities of the PAR1-targeted agents to desensitize or block PAR1 in the HEK cell assay were compared with their activities in a human platelet aggregation assay. Our data illustrate the usefulness of the HEK cell assay for evaluating the PAR1/PAR2 selectivity of PAR-activating agonists. The PAR1-selective agonist that we developed using the assay should prove useful for studying the effects of selectively activating PAR1 in vivo.

Potent bicyclic lactam inhibitors of thrombin: Part I: P3 modifications

Peptidomimetic inhibitors of general structure 1 have been prepared. Optimization of the binding affinities of these compounds through variation of the P3 hydrophobic residue is described. Selected substituted bicylic lactams displayed interesting pharmacological profiles both in vitro and in vivo.

BCH-2763, a novel potent parenteral thrombin inhibitor, is an effective antithrombotic agent in rodent models of arterial and venous thrombosis--comparisons with heparin, r-hirudin, hirulog, inogatran and argatroban

Current clinical use of heparin as an antithrombotic agent is limited by suboptimal efficacy and safety considerations. Thrombin's central role in thrombosis makes it an attractive target to develop more effective and safer antithrombotic agents. BCH-2763 is a novel, potent (Ki: 0.11 nM), low molecular weight (1.51 kDa), bivalent direct thrombin inhibitor. The antithrombotic efficacy of BCH-2763 in vivo following i.v. bolus plus infusion in rats was compared in arterial and venous thrombosis models with two other bivalent direct thrombin inhibitors, r-hirudin and hirulog, with two catalytic site-directed thrombin inhibitors, inogatran and argatroban, and with heparin. In vivo efficacy was related to inhibition in vitro of fibrin clot formation, thrombin-induced aggregation of rat or human washed platelets and activity of free and plasma clot-bound thrombin. All the direct thrombin inhibitors were effective on both arterial and venous thrombosis at markedly lower fold aPTT increases than heparin. The antithrombotic doses of all inhibitors against venous thrombosis were less than against arterial thrombosis. The rank order of potency based on doses (mg/kg/h) required for full efficacy against arterial thrombosis was BCH-2763 (1.2) > inogatran (1.5) > r-hirudin (1.8) > hirulog (3.3) > argatroban (> 3.0); heparin required a markedly higher dose (5.7). In venous thrombosis the doses required for full efficacy were substantially lower for the bivalent (BCH-2763: 0.12; r-hirudin: 0.12; hirulog: 0.18) than for the catalytic site-directed (inogatran: 0.48; argatroban: 0.90) thrombin inhibitors; the dose required for heparin was 0.19. All the direct thrombin inhibitors caused similar shifts in aPTT at doses required to inhibit arterial thrombosis, but BCH-2763 inhibited venous thrombosis at lower aPTT fold increases. In vivo antithrombotic efficacy of direct thrombin inhibitors correlated with their inhibitory activity in vitro against fibrin clot formation and platelet aggregation. In contrast to heparin, all the direct thrombin inhibitors inhibited plasma clot-bound thrombin, but the relative IC50s did not correlate with their antithrombotic efficacy. In summary, direct thrombin inhibitors are more effective than heparin in inhibiting arterial and venous thrombosis in rats with less aPTT increases. BCH-2763 is effective at lower doses than the other direct thrombin inhibitors and for venous thrombosis at a smaller aPTT increase. BCH-2763 may offer an improved therapeutic index in the treatment of thromboembolic complications over heparin and other direct thrombin inhibitors.

Some comments on misspecification of priors in Bayesian modelling of measurement error problems

In this paper we discuss some aspects of misspecification of prior distributions in the context of Bayesian modelling of measurement error problems. A Bayesian approach to the treatment of common measurement error situations encountered in epidemiology has been recently proposed. Its implementation involves, first, the structural specification, through conditional independence relationships, of three submodels-a measurement model, an exposure model and a disease model- and secondly, the choice of functional forms for the distributions involved in the submodels. We present some results indicating how the estimation of the regression parameters of interest, which is carried out using Gibbs sampling, can be influenced by a misspecification of the parametric shape of the prior distribution of exposure.

Synergistic actions of a thrombin-derived synthetic peptide and a thrombin receptor-activating peptide in stimulating fibroblast mitogenesis

We measured the ability of the thrombin receptor activating peptide, SFLLR-NH2 (P5A) to stimulate 3H-thymidine incorporation in hamster CCL-39 fibroblasts either alone or in combination with the thrombin-derived polypeptides, YPPWNKNFTENDLL (TDP-1) and AGYKPDEGKRGDACEGDSGGPFV (TDP-2). In the presence (but not absence) of the amino peptidase inhibitor amastatin (10 microM), P5A alone (7.5 to 100 microM) caused a 1.5- to 2-fold stimulation of thymidine incorporation above basal, even though this inhibitor did not abrogate the degradation of P5A by other peptidases present in the assay medium. Neither TDP-1 nor TDP-2 alone had any effect on thymidine incorporation. However, TDP-1 (30 to 90 microM) considerably augmented P5A-mediated thymidine incorporation at low P5A concentrations (7.5 to 30 microM), shifting the P5A concentration-effect curve to the left. TDP-2 was inactive in this regard. The EC50 for this potentiating action of TDP-1 was approximately 40 microM. Further, thrombin, rendered proteolytically inactive by a low-molecular-weight bifunctional inhibitor, hirutonin-6, also acted synergistically with P5A to stimulate CCL-39 cell thymidine incorporation. We hypothesize that thrombin can cause its cellular effects, such as thymidine incorporation, not only via the proteolytic activation of its G-protein-coupled receptor, but also via the concurrent and synergistic interaction of its TDP-1 peptide domain with a separate cell surface docking site.

Uptake of high density lipoprotein cholesterol ester by HepG2 cells involves apolipoprotein E localized on the cell surface

High density lipoprotein (HDL) cholesterol ester (CE) is taken up by many cells without simultaneous uptake of HDL apoprotein. The studies described herein demonstrate that the selective uptake of cholesterol ester by HepG2 cells is reduced by antibody directed against the receptor-binding domain of apoE (monoclonal antibody (mAb) 1D7) but not by antibody directed against the NH2-terminal portion of the molecule. The reduction, by 1D7, of HDL cholesteryl ester uptake is not due to apoE acquisition by the labeled HDL preparation or by the transfer of [3H]CE of HDL to apoE-containing lipoproteins and uptake by the apoB/E or apoE receptors. Rather, it appears that mAb 1D7 recognizes apoE localized at the cell surface of HepG2 cells. This conclusion is supported by the fact that: 1) reduction of HDL-CE uptake by HepG2 cells is observed within 15 min after the addition of the antibody-ligand mixture; 2) 1D7 is similarly effective in reducing the selective uptake of HDL-CE when added to the ligand or to the cells; 3) three different anti-apoE mAbs (1D7, 3B7, and 3H1) bind specifically to the surface of the cells. We have also demonstrated that heparin (5 mg/ml) does not reduce the amount of apoE-immunoreactive material bound at the cell surface when added before or after the binding period. 1D7, but not 3B7 or 3H1, binds less in the presence of heparin. The observations are consistent with a localization of apoE on the cell membrane rather than on lipoproteins bound to apoB/E or apoE receptors.

The amphipathic alpha-helical repeats of apolipoprotein A-I are responsible for binding of high density lipoproteins to HepG2 cells

Nine monoclonal antibodies (mAbs) against apoA-I reacting with distinct but overlapping epitopes covering more than 90% of the sequence have been used to block the interaction of 125I-labeled high density lipoprotein (125I-HDL) with HepG2 cells in order to delineate the cell binding domain of apolipoprotein A-I (apoA-I). While 2 mAbs reacting with epitopes exclusively localized in the N-terminal region (residues 1 to 86) enhanced slightly association of 125I-HDL, all other mAbs, which react with epitopes localized in the regions of amphipathic alpha-helical repeats, inhibited that association by 9 to 15%. Although this inhibition is not significant compared to the effect of an irrelevant mAb, combination of these mAbs could significantly inhibit the association of 125I-HDL (32 to 43%) as could polyclonal antibodies (up to 95%). These results are compatible with the concept of HDL binding to these cells via the nonexclusive interaction of each of the amphipathic alpha-helical repeats of apoA-I. When the same approach was applied to block the association of 3H-cholesteryl ether (CE)-labeled HDL to HepG2 cells, each anti-apoA-I could inhibit by 15 to 25% the cellular association of cholesteryl ether while mAbs in combination or polyclonal antibodies could inhibit this association up to 45% or 60%, respectively. The cholesteryl ether radioactivity that remained associated with the cells (40%) in the presence of polyclonal antibodies could be effectively blocked by addition of an antibody against the receptor binding domain of apoE (1D7). Therefore, the differential cellular association of cholesteryl ether compared to apolipoprotein can be explained by the presence of apoE secreted by HepG2 and apoE or apoB/E receptors. Thus, we conclude that the optimum uptake of both cholesteryl ether and apoA-I of HDL by cells requires the accessibility of the entire apoA-I and the cooperative binding of the amphipathic alpha-helical repeats to HepG2 cell membranes. This type of interaction would explain the competitive binding observed for apoA-I, -A-II, and -A-IV by others.

Lipid peroxidation changes the expression of specific epitopes of apolipoprotein A-I

Incubation of human serum or high density lipoprotein (HDL) at 37 degrees C in the presence of Fe2+, Fe2+/Fe3+, or Mn2+ results in the increased immunoreactivity (up to 12-, 40-, and 80-fold, respectively) of specific apoA-I epitopes identified as 3D4 and 6B8, while Mg2+, Ca2+, or Cu2+ have minimal or nonsignificant effects. The effect of Mn2+ on the 3D4 epitope requires a specific association with lipids since it can be observed with HDL but not with apoHDL, even in the presence of other lipoproteins. The increase in immunoreactivity noted with Fe2+/Fe3+ or Mn2+ can be blocked with either EDTA or antioxidants (GSH and ascorbic acid), suggesting that it takes place during a peroxidative reaction of the lipids. The peroxidation of lipids which accompanies the increase in immunoreactivity does cross-link apoA-I both with itself and with apoA-II but does not cleave the molecule. The apoA-I-containing lipoproteins which float between 1.18 and 1.22 g/ml and have a pre B-electrophoretic migration are characterized by a very low immunoreactivity with monoclonal antibody 3D4 but are 10-fold or more responsive to Mn2+ treatment than other lipoprotein subfractions, thus demonstrating heterogeneity under oxidative conditions. Proteoliposomes containing apoA-I, cholesterol, and dilinoleyl-lecithin are sensitive to Mn2+ treatment, but not those made with dioleyl- or dimyristoyl-lecithins. However, the increase in 3D4 immunoreactivity is weak and transient and is followed by the disappearance of the epitope caused by cross-linking. We conclude that lipid peroxidation can specifically cross-link apoA-I and change its conformation and antigenicity.

Screening of monoclonal antibodies for hCG for affinity and specificity

Using human chorionic gonadotropin (hCG) as a model polypeptide, we have developed a strategy that allows the direct screening of supernatant fluids from hybridomas for the presence of monoclonal antibodies of high affinity and predefined specificities. The assay evaluates the competition between 125I-labeled and unlabeled homologous or heterologous antigens in a solid-phase two-site immunoradiometric assay. This assay is fast and accurate, and is of general use provided the antigen of interest can be purified in nanomolar quantities. This strategy led to the isolation of nine new monoclonal antibodies for hCG, two of which could be used for elaborating a sensitive two-site immunoradiometric assay for this hormone.