Quinine- and quinidine-dependent antiplatelet antibodies. Requirement of factor VIII-related antigen for platelet damage and for in vitro transformation of lymphocytes from patients with drug-induced thrombocytopenia

Thrombocytopenia and drugs

Thrombocytopenia is a relatively common side effect of drug therapy. The pathogenetic mechanisms of drug-induced thrombocytopenia are varied and sometimes poorly understood. Immunologic and other cytotoxic effects may result in decreased production and/or increased destruction of platelets. The pathogenesis of drug-induced thrombocytopenia is reviewed. Diagnosis and treatment are also discussed.

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Prolonged thrombocytopenia in post-transfusion purpura (PTP) associated with changes in the crossed immunoelectrophoretic pattern of von Willebrand factor (vWF), circulating immune complexes and endothelial cell cytotoxicity

A 39-year-old multigravida presented with a platelet count of 2 X 10(9)/112 d after a blood transfusion. The clinical presentation was typical of post-transfusional purpura (PTP). She was successfully managed with intensive plasma exchange with plasma replacement. Replacement with albumin saline was unsuccessful. Elevated levels of immune complexes were detected on presentation but were reduced to normal levels by plasma exchange but were elevated again during relapse. Raised levels of circulating platelet aggregates and platelet-aggregating activity were found during relapse but were absent during remission. Crossed immunoelectrophoresis of vWF showed a left shift suggesting the presence of abnormally high molecular weight multimeric forms. In contrast, multimeric assay of the same samples were within normal limits suggesting that the shift noted on crossed immunoelectrophoresis may have resulted from complexing of vWF with some other components. Immunological studies demonstrated the presence of endothelial cell cytotoxicity and lymphocytotoxicity in the patient's serum. It is concluded that the thrombocytopenia observed in PTP may not occur as a direct result of the immunological event initiating the disorder but rather indirectly via the release of vWF consequent upon endothelial cell damage which, either alone or in combination with another factor, possible immune complexes, induces intravascular platelet aggregation and sequestration. There were significant similarities between the case reported and recent observations in thrombotic thrombocytopenic purpura suggesting that these syndromes may have aetiological factors in common.

Drug-induced thrombocytopenia

A variety of drugs may cause thrombocytopenia. Although it occurs more often than drug-induced anemia it is less well understood because techniques for studying drug-platelet-immune interactions have been unavailable until recently. The mechanisms by which drugs cause thrombocytopenia are varied. Bone marrow suppression or increased peripheral destruction of platelets could be involved. Nonimmunologic as well as immunologic mechanisms may also occur. These different mechanisms of drug-induced thrombocytopenia are reviewed. Diagnostic methods and treatment are also summarized.

Technical considerations for platelet aggregation and related problems

Platelet aggregation generally is ordered by the physician to evaluate platelet function in hemorrhagic or thrombotic disorders. Malfunction of the platelet may be the result of an intrinsic congenital defect or an acquired problem induced by drugs or certain circulating plasma factors. It is necessary to obtain information from the patient with respect to family history, drug ingestion, physical or mental stress. In addition, other laboratory studies should be obtained to rule out general coagulation disorders affecting the plasma factors. A bleeding time will be helpful in establishing the severity of any platelet dysfunction. Technical considerations with regard to the preparation of the samples are of primary importance in determining platelet aggregation. Aggregating studies require the use of a variety of binding agents. (Studies on shape change, adhesion of platelets, release of platelet granule substance, and or lysis with extrusion of cytoplasmic constituents may be helpful in certain cases.) Instrumentation for platelet aggregation presently is available in many hospitals. The technical factors to be considered for routine aggregation studies include the type and strength of anticoagulant, centrifugation technique used in preparing the platelet-rich and platelet-poor plasma, platelet concentration, time of storage of the sample after venipuncture and after centrifugation, temperature, and the mixing of the sample. In general, critical concentrations of each reagent should be employed to improve the discrimination capability of the assay. Small differences in response may be obliterated by using excessive concentrations of a given reagent. Comparison in response to the test platelets with control platelets is best done at the same time by performing the aggregation in a dual instrument so that handling procedures will be identical and artifactual differences eliminated.

Drug-antibody-platelet interaction in quinine- and quinidine-induced thrombocytopenia

Binding of quinine- and quinidine-dependent antibodies to platelets was studied using an electroimmunoassay to measure platelet-bound IgG. Antibodies from four patients with drug-induced thrombocytopenia differed significantly in their interaction with platelets: association constants for binding to platelets at high drug concentrations ranged from 0.29 to 2.6 x 10(7) M(-1), the maximum number of antibody molecules bound ranged from 36,000 to 161,000/platelet, the amount of drug necessary to achieve half-maximum binding of antibodies to platelets ranged from 2 to 60 muM, and only one of the antibodies cross-reacted with the stereoisomer of the drug to which the patient was sensitized. Binding of the antibodies to platelets was enhanced at the highest achievable molar ratio of drug:antibody, 10,000:1, rather than being inhibited, as would be expected in a conventional, hapten-dependent reaction. The drug-antibody-platelet reaction was unaffected by Factor VIII/von Willebrand protein, nonspecifically aggregated IgG, or heat-labile complement components. After pretreatment with tritiated quinine, platelets retained several hundred thousand molecules of drug each, but failed to bind detectable amounts of antibody. However, platelets treated simultaneously with quinine-dependent antibody and tritiated quinine retained significantly more drug after repeated washes than platelets treated with drug and normal serum. These findings support the proposition that in quinine- and quinidine-induced thrombocytopenia, drug and antibody combine first in the soluble phase to form a complex, which then binds with high affinity to a receptor on the platelet surface (innocent bystander reaction), and demonstrate that these antibodies are heterogeneous in respect to the amount of drug required to promote their binding to platelets, the number of platelet receptors they recognize, and their binding affinities.