Drug-dependent and non-drug-dependent antiplatelet antibody in drug-induced immunologic thrombocytopenic purpura

The mechanism of drug-dependent immunologic thrombocytopenic purpura (DITP) was investigated by studying the sera of four patients with classic DITP (two with quinidine-, one with acetaminophen-, and one with phenazopyridine-dependent antiplatelet antibody) using a solid-phase radioimmunoassay with 125I-staphylococcal protein A. Two forms of antiplatelet antibody could be demonstrated: one that required drug to bind to platelets and one that bound to platelets in the absence of drug. Drug-dependent antiplatelet antibody required the simultaneous addition of drug and the Fc domain of the drug-dependent IgG molecule for binding to platelets. It did not require serum complement or factor VIII-related antigen for binding to platelets. Drug-dependent binding of antibody to platelets was saturation-dependent. Non-drug-dependent antiplatelet antibody of two patients (one with quinidine-induced thrombocytopenia and the other with acetaminophen-induced thrombocytopenia) reacted with autologous platelets as well as with homologous platelets, indicating that they were autoantibodies. Both autoantibodies had disappeared when their sera were tested 23 and 138 days, respectively, after withdrawal of their initial positive sera. Non-drug-dependent antiplatelet antibody binding could be demonstrated with the F(ab')2 fragment of the purified IgG of the serum of the second patient with quinidine DITP, who did not have detectable alloantibodies against HLA. None of the four patients with non-drug-dependent antiplatelet antibody had a past or present history of autoimmune thrombocytopenic purpura.     

Sera from patients with heparin-induced thrombocytopenia generate platelet-derived microparticles with procoagulant activity: an explanation for the thrombotic complications of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia is characterized by moderate thrombocytopenia and thrombotic complications, whereas quinine/quinidine-induced thrombocytopenia usually presents with severe thrombocytopenia and bleeding. Using flow cytometry and assays of procoagulant activity, we investigated whether sera from patients with these immune drug reactions could stimulate normal platelets to generate platelet-derived microparticles with procoagulant activity. Sera or purified IgG from patients with heparin-induced thrombocytopenia stimulated the formation of platelet-derived microparticles in a heparin-dependent fashion. Further studies showed that heparin-induced thrombocytopenia sera also produced a marked increase in procoagulant activity. In contrast, sera from patients with quinine- or quinidine-induced thrombocytopenia did not generate platelet-derived microparticles nor generate increased procoagulant activity. However, quinine/quinidine-induced thrombocytopenia sera produced a significant increase in the binding of IgG to platelets in a drug-dependent fashion, whereas sera from patients with heparin-induced thrombocytopenia demonstrated no drug-dependent binding of IgG to platelets. We also observed increased levels of circulating microparticles in patients with acute heparin-induced thrombocytopenia compared with control patients. Our observations indicate that the generation of procoagulant platelet-derived microparticles in vivo is a plausible explanation for the thrombotic complications observed in some patients with heparin-induced thrombocytopenia.     

Drug-induced Immune Thrombocytopenia

SUMMARY.

Immune thrombocytopenia is a relatively common problem associated with the clinical usage of drugs. Drugs frequently implicated include quinine, quinidine, heparin, penicillins, cephalosporins, co-trimoxazole, gold and D-penicillamine. Bleeding including bruising and purpura is the usual clinical manifestation except in immune heparin-induced thrombocytopenia in which thrombosis occurs more frequently than bleeding. Cessation of the offending drug is the important step in the treatment but other measures may also be required such as platelet transfusion and steroid therapy for patients with clinical bleeding or antithrombotic therapy with warfarin and dextran or low molecular weight heparin/heparinoid for patients with heparin-induced thrombocytopenia and thrombosis. Idiosyncratic drug-induced thrombocytopenia is mediated by an antibody which binds to platelets only in the presence of the drug resulting in the clearance of sensitised platelets by the reticuloendothelial system. In quinine/quinidine-induced thrombocytopenia, the antibodies recognise drug-dependent epitopes on platelet membrane glycoproteins Ib-IX and/or glycoproteins IIb-IIIa. In immune heparin-induced thrombocytopenia the current data suggest a mechanism which probably involves the binding of heparin-antibody complexes to the platelet Fc receptors but the precise mechanism is yet to be fully characterised. The associated thrombosis in this condition is likely to be due to platelet activation and possibly endothelial cell damage induced by the heparin-related antibody.     

Drug-mediated thrombocytopenia

Thrombocytopenia due to drug-dependent antibodies most frequently occurs with quinine/quinidine and with heparin. Considerable evidence has accumulated about the mechanism of action of quinine/quinidine-induced antibodies but less is known about the effect of heparin. Although there is controversy, it is likely that the action of quinine/quinidine-induced antibodies follows a loose association between drug and platelet with antibodies acting independently of the Fc receptor. There is strong evidence that the complex of glycoprotein Ib and glycoprotein IX, absent in the Bernard-Soulier syndrome, provides the binding site for quinine/quinidine-dependent antibodies. It also appears that the two glycoproteins must be present in complex form for antibody binding to occur. There is some heterogeneity of quinine/quinidine-dependent antibodies since there are reports of a proportion of patient antibodies reacting with other membrane determinants or acting independently of the drug. Heparin-induced thrombocytopenia may be the consequence of a direct effect, or a more serious condition associated with thrombosis may occur when heparin-dependent antibodies are formed. The mode of action of these antibodies and the nature of their antigenic determinants remain unclear. Recognition of heparin-associated thrombocytopenia is important so that serious bleeding or thrombotic sequelae can be forestalled.     

Molecular characterization of quinine/quinidine drug-dependent antibody platelet interaction using monoclonal antibodies

Two murine monoclonal antibodies, FMC 25 and AN 51, directed against distinct epitopes on the glycoprotein Ib complex, have been used to further define the mechanism of quinine/quinidine drug-dependent antibody interaction with platelets. FMC 25, directed against an epitope on glycoprotein IX, had no effect on platelet aggregation induced by collagen or adenosine diphosphate and little, if any, effect on ristocetin-induced platelet agglutination. FMC 25 and its (Fab)2 fragment, however, were potent inhibitors of drug-dependent antibody- induced platelet aggregation and blocked binding of drug-dependent antibody to platelets as assessed by indirect platelet immunofluorescence. In contrast, AN 51, directed against an epitope on the alpha-subunit of glycoprotein Ib, blocked ristocetin-induced, factor VIII/von Willebrand factor (FVIII/vWF)-dependent platelet agglutination but not drug-dependent antibody-induced platelet aggregation or binding of drug-dependent antibody to platelets. Selective proteolytic removal of the majority of the alpha-subunit of glycoprotein Ib (glycocalicin) from platelets by treatment with calcium- dependent protease did not affect binding of drug-dependent antibody. In addition, a quinidine-dependent antiplatelet antibody immunoprecipitated glycoprotein Ib complex from normal platelets and the membrane-associated proteolytic remnant of the glycoprotein Ib complex from calcium-dependent protease-treated platelets. Preincubation of drug-dependent antibody with purified glycoprotein Ib complex inhibited subsequent binding of antibody to platelets, but the separated components, glycoprotein Ib and glycoprotein IX, were both ineffective, suggesting that the normal interaction between glycoprotein Ib and glycoprotein IX in the intact complex was necessary for drug-dependent antibody recognition. The functional response of platelets to drug-dependent antibody was not mediated by way of platelet Fc receptor, since aggregation of washed platelets by acetone- aggregated IgG was not inhibited by FMC 25 (Fab)2. FVIII/vWF was not required for drug-dependent antibody-induced platelet aggregation. The combined evidence is consistent with quinine/quinidine-dependent antibody-platelet interaction occurring by way of a FVIII/vWF- independent, Fc receptor-independent mechanism that probably involves binding of antibody to glycoprotein IX or the beta-subunit of glycoprotein Ib or both.     

Quinidine-induced thrombocytopenic purpura: clinical presentation in relation to drug-dependent and drug-independent platelet antibodies

We have studied the clinical course of quinidine-induced thrombocytopenia in relation to the presence of drug-dependent (dd-ab:s) and drug-independent antibodies in 14 patients. Thrombocytopenia was reversible in 9 d after discontinuation of quinidine treatment in 10 patients. In four it lasted more than 1 month. Drug-dependent antibodies of IgG class were detectable in seven patients: in six by an immunofluorescence test applying flow cytometry and in one patient by a monoclonal antibody-immobilized platelet protein assay (MAIPA) only. The dd-ab:s of this patient had glycoprotein (GP) IIb/IIIa specificity. Five of the six patients with dd-ab:s by immunofluorescence test had GPIb/IX-specific dd-ab:s by MAIPA. They recovered within 5 d after discontinuation of the drug. All four patients with prolonged thrombocytopenia had elevated platelet-associated IgG (PAIgG) in the acute phase as studied by a direct platelet immunofluorescence test. The remaining five patients displayed a relatively rapid clinical recovery but less uniform pattern of immunological findings. The results suggest that patients with GPIb/IX-specific dd-ab:s recover promptly despite an acute and profound thrombocytopenia. Another sub-group with prolonged thrombocytopenia had persistently elevated PAIgG during the convalescent phase.     

Characterization of the binding domains on platelet glycoproteins Ib-IX and IIb/IIIa complexes for the quinine/quinidine-dependent antibodies.

Sera of 12 patients with quinine/quinidine-induced thrombocytopenia showed drug-dependent antibody binding to glycoprotein (GP) Ib-IX complex. The reaction with GPIb-IX complex of 11 of these 12 sera was strongly inhibited by the complex-specific monoclonal antibodies (MoAbs) AK1 and SZ1. The exception was a quinine-induced serum designated BU. The reaction of the six quinidine-induced sera was also partially blocked by an anti-GPIX MoAb, FMC25. Only 3 of the 12 patient sera showed drug-dependent antibody binding to GPIIb/IIIa, which was strongly inhibited by the anti-GPIIIa MoAb 22C4, and the anti-GPIIb alpha MoAb SZ22. With detergent-solubilized Serratia metalloprotease-treated platelets, quinine/quinidine-induced sera, except BU, immunoprecipitated a membrane-bound proteolytic fragment of GPIb-IX complex. In contrast, BU immunoprecipitated glycocalicin and a 40-Kd peptide tail fragment of GPIb alpha from the cell supernatant. Using purified GPIb-IX complex or its components as the target antigen, all the quinine-induced sera, except BU, immunoprecipitated GPIb-IX complex but failed to immunoprecipitate GPIb, GPIX, or the complex reformed from GPIb and GPIX. The quinidine-induced sera strongly immunoprecipitated purified GPIb-IX complex, weakly immunoprecipitated purified GPIX and the recombined complex, but did not immunoprecipitate purified GPIb. The combined data suggest that one quinine-dependent antibody (BU) recognizes an epitope in the peptide tail region of GPIb alpha and the other five quinine-dependent antibodies react with a complex-specific epitope on the membrane-associated region of GPIb-IX complex, whereas each of the six quinidine-induced sera contain two drug-dependent antibodies, one reactive with the GPIb-IX complex-specific epitope and the other reactive with GPIX. The binding domain(s) on GPIIb/IIIa for the quinine/quinidine-dependent antibodies appear to be sterically close to the epitopes for 22C4 and SZ22.     

Glycoprotein Ib/IX complex is the target in rifampicin-induced immune thrombocytopenia

Thrombocytopenia is a major adverse effect of several drug treatments. Rifampicin has been recognized as a cause of immune thrombocytopenia during intermittent high-dose therapy. We characterized the antibody of a patient who presented with purpura and thrombocytopenia during treatment of tuberculosis with rifampicin. Drug-dependent binding of the antibody to platelets was demonstrated by flow cytometry. In a glycoprotein-specific immunoassay, the binding epitope of the IgG antibody was found in the glycoprotein Ib/IX complex, using four different monoclonal antibodies (mAbs) against various epitopes on the GPIb/IX complex, as well as mAbs against GPIIb/IIIa, GPIa/IIa and GPIV. By immunoprecipitation of biotin-labelled platelets, reactivity of the antibody with GPIb/IX was found only in the presence of the drug. These findings clearly demonstrate that rifampicin induces the formation of drug-dependent antibodies capable of causing thrombocytopenia. The binding site of the rifampicin-dependent antibody, located in the GPIb/IX complex, seems to be a favoured target for antibodies induced by different drugs.     

Vancomycin-dependent antibodies associated with thrombocytopenia and refractoriness to platelet transfusion in patients with leukemia

Two patients with leukemia experienced profound thrombocytopenia and refractoriness to platelet transfusion during vancomycin treatment. In one patient, withdrawal of drug and administration of platelet transfusions restored platelet counts to near normal levels (approximately 100 x 10(9)/L), however, subsequent challenge with vancomycin due to recurring infection again precipitated severe thrombocytopenia (platelets less than 10 x 10(9)/L) and life-threatening hemorrhagic symptoms. Potent vancomycin-dependent antiplatelet antibodies were detected in the serum of both patients during the refractory period using staphylococcal protein A rosette formation. Employing a monoclonal antibody-antigen capture enzyme-linked immunosorbent assay (ELISA), the patients were found to have vancomycin-dependent IgG antibodies that bound specifically to platelet glycoproteins (GP) IIb and/or IIIa. One of these antibodies failed to react with platelets deficient in GPIIb/IIIa obtained from an individual with Glanzmann's thrombasthenia. These findings provide the first major evidence for drug-dependent antibodies in association with severe thrombocytopenia and refractoriness to platelet transfusion in alloimmunized leukemia patients and, further, provide the first demonstration of vancomycin-dependent antibodies reactive with platelets.     

Detection of drug-dependent antibodies by the 51cr platelet lysis test: Documentation of immune thrombocytopenia induced by diphenylhydantoin,diazepam, and sulfisoxazole

Sudden, severe thrombocytopenia developed in each of three patients receiving diphenylhydantoin, diazepam, and sulfisoxazole, respectively. Recovery followed discontinuance of the drugs. An antiplatelet antibody requiring the presence of an appropriate drug for interaction with platelets was demonstrated in each case by the ⁵¹Cr platelet lysis test using normal, paroxysmal nocturnal hemoglobinuric, or enzyme-treated normal platelets as target cells. These antibodies could not be detected by techniques that depend on clot retraction inhibition, complement fixation, or platelet factor-3 activation. Quinine- and quinidine-dependent antiplatelet antibodies in the serum of 16 patients who developed acute thrombocytopenia while taking either quinine or quinidine could be demonstrated readily with the ⁵¹Cr platelet lysis test and could also be detected by other methods employed.     

Cumulative experience with a simplified solid-phase radioimmunoassay for the detection of bound antiplatelet IgG, serum auto-, allo-, and drug-dependent antibodies

A simplified, sensitive, solid-phase radioimmunoassay employing 125I- staphylococcal protein A has been developed that is capable of detecting bound antiplatelet IgG as well as serum auto-, allo-, and drug-dependent antiplatelet antibodies. The simplified assay employs a ratio of test over control platelet counts per minute (cpm) for detection of positive results. All reagents are commercially available. The assay can be performed with as little as 10(6) washed platelets (10 microliters of whole blood) that have been stored for as long as 8 wk at 4 degrees C in microtiter plates. The assay time, employing stored platelets, is 4 hr. Bound platelet IgG is positive in 93% of 46 thrombocytopenic patients with autoimmune disease and correlates inversely with their platelet count, r = -0.65, p less than 0.001. The ability of this assay to detect serum antibody was studied with a rabbit anti-human platelet antibody capable of giving optimal immunoprecipitation with solubilized platelet membranes at a tier of 1:10. The present assay increases the sensitivity of antibody detection 256-fold to a titer of 1:2560. Human serum antiplatelet membrane antibody was positive in 2 of 2 patients with anti-PLA-1 antibody (titers of 1:256 and greater than 1:64); 7 of 12 multiply transfused patients who were refractory to platelet transfusion (2 had titers of greater than 1:256 and greater than 1:32); 5 of 19 patients with autoimmune thrombocytopenic purpura (2 had titers of 1:64 and 1:32); and 10 of 14 patients with clinical histories of drug-dependent antiplatelet antibody (2 had titers of 1:1280 for quinidine and 1:384 for phenazopyridine).     

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a target glycoprotein in drug-induced thrombocytopenia

Drug-induced immune thrombocytopenia (DITP) is a serious complication of drug treatment. Previous studies demonstrated that most drug-dependent antibodies (DDAbs) react with the platelet membrane glycoprotein (GP) complexes IIb/IIIa and Ib/IX/V. We analyzed the sera from 5 patients who presented with DITP after intake of carbimazole. Notably, thrombocytopenia induced by carbimazole was relatively mild in comparison to patients with DITP induced by quinidine. The sera reacted with platelets in an immunoassay on addition of the drug. In immunoprecipitation experiments with biotin-labeled platelets and endothelial cells, reactivity with the platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) could be demonstrated, whereas neither GPIIb/IIIa nor GPIb/IX was precipitated in the presence of the drug. These results could be confirmed by GP-specific immunoassay (MAIPA) using monoclonal antibodies (mabs) against PECAM-1. In addition, the binding of DDAbs could be abolished by preincubation with soluble recombinant PECAM-1. Carbimazole-dependent antibodies showed similar reactivity with platelets carrying the Leu(125) and Val(125) PECAM-1 isoforms, indicating that this polymorphic structure, which is located in the first extracellular domain, is not responsible for the epitope formation. Binding studies with biotin-labeled mutants of PECAM-1 and analysis of sera with mabs against different epitopes on PECAM-1 in MAIPA assay suggested that carbimazole-dependent antibodies prominently bound to the second immunoglobulin homology domain of the molecule. Analysis of 20 sera from patients with quinidine-induced thrombocytopenia by MAIPA assay revealed evidence that DDAbs against PECAM-1 are involved in addition to anti-GPIb/IX and anti-GPIIb/IIIa. We conclude that PECAM-1 is an important target GP in DITP. (Blood. 2000;96:1409-1414)     

Quinine- and quinidine platelet antibodies can react with GPIIb/IIIa

Quinine- and quinidine-dependent antiplatelet antibodies are generally believed to bind to the membrane glycoprotein complex, GPIb/IX. However, we and others have found that some drug-dependent antibodies bind to platelets from patients with the Bernard-Soulier syndrome which lack these glycoproteins. We therefore studied the reactions of a group of these antibodies with normal and Bernard-Soulier platelets and their membrane proteins. As indicated by rosette formation of the sensitized platelets around protein A-Sepharose beads, two quinine- and two quinidine-dependent antibodies reacted with both normal and Bernard-Soulier syndrome platelets at a high (300 microM) concentration of drug. At a pharmacologic drug concentration (10 microM), all four antibodies reacted with normal platelets but only the two quinine-induced antibodies reacted with Bernard-Soulier platelets. Immunoprecipitation studies with solubilized, tritium-labelled normal platelets, at both high and low drug concentrations, showed that each of the four antibodies precipitated proteins corresponding to GPIb and GPIX. Fainter bands corresponding to glycoproteins IIb and IIIa, which do not label well with tritium, were also detected. With radioiodinated normal platelets, it was found that each of the four antibodies was capable of precipitating GPIIb/IIIa, but only in the presence of drug. The four antibodies also promoted drug-dependent precipitation of GPIIb and GPIIIa from lysates of radioiodinated Bernard-Soulier platelets. The two quinine-dependent antibodies precipitated these glycoproteins at both high and low drug concentrations, while the quinidine-dependent antibodies reacted much more strongly at the higher drug concentration. Precipitation of GPIb/IX was not observed with BSS platelets. Absorption of a quinine-induced antibody with Bernard-Soulier platelets in the presence of drug eliminated its ability to precipitate GPIIb and GPIIIa. However, the absorbed antibody retained the ability to precipitate GPIb from solubilized normal platelets. Thus, at least two drug-dependent antibodies were present, one specific for GPIb/IX and the other for GPIIb/IIIa. These findings indicate that glycoproteins IIb and/or IIIa, in addition to the GPIb/IX complex, can serve as targets for drug-dependent antibodies in both intact and detergent-solubilized platelet preparations.     

Detection of Quinidine-Specific Antibodies with Platelet 125J-Labeled Staphylococcal Protein A Test

Abstract. Remarkably high titers (80–160) of quinidine-specific antibodies were detected in the serum of 5 patients with suspected quinidine purpura, when ¹²⁶I-labeled staphylococcal protein A was used as the marker of IgG bound onto platelets. During a follow-up of 3 of the patients, the antibody titers declined.
The antibodies could be detected even when the final drug concentration was reduced from 0.3 to 0.003 mmol/I (i.e. therapeutic levels). The reaction between serum and platelets was independent of the type of quinidine salt used. No cross-reactivity occurred with quinine.
Heat inactivation of the serum did not decrease the quantity of specific antibodies detected on the platelets. The sedimentation behavior of antiplatelet activity in density gradient centri-fugation resembled that of 7 S antibodies. When sedimentation was, however, effected on the drug-serum mixture, the reactivity of 7 S decreased.     

Acute thrombocytopenia after treatment with tirofiban or eptifibatide is associated with antibodies specific for ligand-occupied GPIIb/IIIa

Acute thrombocytopenia is a recognized complication of treatment with GPIIb/IIIa inhibitors whose cause is not yet known. We studied 9 patients who developed severe thrombocytopenia (platelets less than 25 x 10(9)/L) within several hours of treatment with the GPIIb/IIIa inhibitors tirofiban (4 patients) and eptifibatide (5 patients). In each patient, acute-phase serum contained a high titer (range, 1:80-1:20 000) IgG antibody that reacted with the glycoprotein IIb/IIIa complex only in the presence of the drug used in treatment. Four patients had been previously treated with the same drug, but 5 had no known prior exposure. Pretreatment serum samples from 2 of the latter patients contained drug-dependent antibodies similar to those identified after treatment. No tirofiban- or eptifibatide-dependent antibodies were found in any of 100 randomly selected healthy blood donors, and only 2 of 23 patients receiving tirofiban or eptifibatide who did not experience significant thrombocytopenia had extremely weak (titer, 1:2) tirofiban-dependent antibodies. In preliminary studies, evidence was obtained that the 9 antibodies recognize multiple target epitopes on GPIIb/IIIa complexed with the inhibitor to which the patient was sensitive, indicating that they cannot all be specific for the drug-binding site. The findings indicate that acute thrombocytopenia after the administration of tirofiban or eptifibatide can be caused by drug-dependent antibodies that are "naturally occurring" or are induced by prior exposure to drug. These antibodies may be human analogs of mouse monoclonal antibodies that recognize ligand-induced binding sites (LIBS) induced in the GPIIb/IIIa heterodimer when it reacts with a ligand-mimetic drug.     

Severe thrombocytopenia in an acquired immunodeficiency syndrome patient associated with pentamidine-dependent antibodies specific for glycoprotein IIb/IIIa

Severe thrombocytopenia developed in a patient with acquired immunodeficiency syndrome during treatment with intravenous pentamidine for Pneumocystis carinii pneumonia. The patient's bone marrow contained adequate numbers of megakaryocytes, suggesting peripheral platelet destruction. Platelet counts ranged between less than 3 and 20 x 10(9)/L for 2 weeks despite cessation of pentamidine, platelet transfusions, high-dose intravenous IgG, and 2 mg/kg/d prednisone. Thereafter, the platelet count increased to prepentamidine levels (95 x 10(9)/L0, permitting rapid withdrawal of steroids. Testing by immunofluorescence disclosed a high-titer, pentamidine-dependent IgG antibody in the patient's acute-phase serum that almost entirely disappeared by the time the patient's platelet count returned to baseline levels. This antibody reacted only with platelet glycoprotein (GP) IIb/IIIa as shown by antigen-capture enzyme-linked immunosorbent assay using monoclonal antibodies specific for various GPs, and was absorbable by normal, but not by GPIIb/IIIa-deficient platelets (from a patient with Glanzmann's thrombasthenia). The pentamidine-dependent antibody could not be demonstrated by immunoprecipitation using the patient's serum and 125I-labeled normal platelets, although a separate pentamidine-independent antibody was detected by this method. This latter antibody reacted with two GPs having molecular weights consistent with GPIIb/IIIa, and was present in postrecovery as well as acute-phase sera. However, only the pentamidine-dependent antibody was temporally associated with the severe thrombocytopenia. Therefore, we believe that these studies demonstrate, for the first time, that intravenous pentamidine therapy can provoke formation of drug-dependent antibodies that induce immunologic thrombocytopenia.     

Acute vancomycin-dependent immune thrombocytopenia as an anamnestic response

Drug-related thrombocytopenia is a well-described but relatively rare complication of antibiotic therapy. In this entity, platelet destruction is immune-mediated, often resulting in a precipitous drop in platelet count over a short period of time. Most of these cases of thrombocytopenia are drug-dependent, as discontinuation of the offending agent frequently results in a timely return to baseline, pre-exposure platelet levels. We report the case of a 61-year-old male patient receiving vancomycin and ceftazidime for lower extremity wet gangrene who experienced a marked, acute reduction in platelet count 12 to 15 hours after starting antibiotic therapy. There was no readily apparent clinical or laboratory explanation for his thrombocytopenia. Pre- and post- antibiotic serum samples were preserved and sent for drug-dependent platelet antibody analysis. The pre-exposure specimen revealed the presence of IgG vancomycin-dependent platelet antibodies, while the post-exposure specimen demonstrated both IgG and IgM vancomycin-dependent platelet antibodies. Ceftazidime-dependent platelet antibodies were not identified in either sample. These findings indicate prior sensitization to vancomycin, with subsequent acute production of IgM anti-platelet antibodies after re-exposure to the antibiotic. The patient's antibiotics were held after the acute-onset of thrombocytopenia with subsequent restoration of normal platelet counts within 4 days of drug withdrawal, and the patient at no time experienced significant adverse bleeding events. Antibiotic therapy with vancomycin is a rare and perhaps overlooked cause for new-onset thrombocytopenia in hospitalized patients. This case illustrates that the development of severe thrombocytopenia within hours of vancomycin administration does not rule out drug-related immune clearance, as the rapid platelet destruction may indicate an anamnestic antibody response to the drug after previous exposure. In such a scenario, immediate discontinuation of vancomycin is recommended to improve platelet counts. From a laboratory perspective, retrieval of serum both pre- and post-administration of vancomycin is most helpful in determining a patient's drug-immunization status and can help guide safe drug use during future infections.     

Drug-induced Immune Thrombocytopenia

SUMMARY. Immune thrombocytopenia is a relatively common problem associated with the clinical usage of drugs. Drugs frequently implicated include quinine, quinidine, heparin, penicillins, cephalosporins, co-trimoxazole, gold and D-penicillamine. Bleeding including bruising and purpura is the usual clinical manifestation except in immune heparin-induced thrombocytopenia in which thrombosis occurs more frequently than bleeding. Cessation of the offending drug is the important step in the treatment but other measures may also be required such as platelet transfusion and steroid therapy for patients with clinical bleeding or antithrombotic therapy with warfarin and dextran or low molecular weight heparin/heparinoid for patients with heparin-induced thrombocytopenia and thrombosis. Idiosyncratic drug-induced thrombocytopenia is mediated by an antibody which binds to platelets only in the presence of the drug resulting in the clearance of sensitised platelets by the reticuloendothelial system. In quinine/quinidine-induced thrombocytopenia, the antibodies recognise drug-dependent epitopes on platelet membrane glycoproteins Ib-IX and/or glycoproteins IIb-IIIa. In immune heparin-induced thrombocytopenia the current data suggest a mechanism which probably involves the binding of heparin-antibody complexes to the platelet Fc receptors but the precise mechanism is yet to be fully characterised. The associated thrombosis in this condition is likely to be due to platelet activation and possibly endothelial cell damage induced by the heparin-related antibody.     

Drug-induced thrombocytopenia.

Many drugs can induce thrombocytopenia mediated by drug-dependent antiplatelet antibodies. Recent studies have documented specific epitopes for drug-dependent antibody binding on glycoprotein Ib-IX, glycoprotein IIb-IIIa, and platelet-endothelial cell adhesion molecule-1. Molecular identification of antibody binding sites may help to identify susceptible individuals. Management of patients with unexpected thrombocytopenia who are taking multiple drugs remains a difficult clinical problem. A recent systematic review of all published case reports of drug-induced thrombocytopenia ranks drugs according to the strength of clinical evidence for a causal relation to thrombocytopenia. This database is available online at http://moon.ouhsc.edu/jgeorge.     

Precise quantitation of PAIgG: a new radiometric microtechnique

We report the development of a radiometric assay for platelet-bound IgG that is both sensitive and quantitative. The assay utilized 96-well millititer plates incorporating a 0.2 microns filter membrane in the bottom. A 125I-labeled monoclonal antihuman IgG, as a secondary antibody, detected the platelet-bound human IgG. Since 5 x 10(6) platelets were used for each assay, tests for platelet-bound IgG can be performed on persons with severe thrombocytopenia. For the detection of circulating antiplatelet alloantibodies, as little as 10 microliters of platelet-free plasma per assay is required. Antiplatelet IgG was quantitated by using anti-PIA1 antibody that was purified with affinity and elution and DEAE chromatography. This purified antiplatelet antibody was labeled with 125I and was used to determine the binding ratio of secondary antibody to primary antibody. Under our standard conditions, this ratio was found to be stable at approximately 0.35 over the sensitivity range of the assay. The assay can detect approximately 200 molecules of human IgG per platelet (0.1 ng of secondary antibody bound per 5 x 10(6) platelets). It has a linear range from 0 to 7,000 molecules per platelet. Quantitation of anti-PIA1 binding for platelets stored for up to 6 months under refrigeration showed no change in number of PIA1 binding sites. Clinical studies showed that 18 of 19 ITP patients had an increased number of IgG molecules per platelet as did patients with malignancy and drug-induced immune thrombocytopenia. Patients who had received multiple platelet transfusions had antiplatelet antibody in their plasma. Normal amounts of PAIgG were observed in platelets and plasma of patients with nonimmune thrombocytopenia. The advantages of this method are that it is: 1) a more precise quantitation of PAIgG via direct measurement of binding ratio with PIA1 antibody; 2) performed with small amounts of platelets and plasma; 3) both sensitive and specific; and 4) reliably reproducible with both fresh and stored platelets.