Drug-induced thrombocytopenia: MIBS trumps LIBS

Drug-induced immune thrombocytopenia is a common hematologic problem. More than 200 drugs have been reported to cause immune thrombocytopenia² ; these include commonly used drugs such as antibiotics, anticonvulsants, and RGD mimetic agents such as eptifibatide and tirofiban, which are used to prevent in-stent thrombosis in patients undergoing percutaneous transluminal coronary angioplasty and stenting. Thrombocytopenia induced by RGD mimetic drugs occurs in 0.1% to 2.0% of patients receiving the drug.³  Given the widespread use of these drugs and the relatively high incidence, the disease burden because of this adverse effect is potentially huge. Furthermore, the thrombocytopenia is characteristically severe with patient platelet counts dropping frequently below 10 × 10⁹/L exposing the patient to high risk of bleeding, particularly at the femoral artery puncture site through which the coronary artery catheter is inserted. Serious bleeding including fatal cerebral hemorrhage has been reported.⁴  Although the thrombocytopenia usually resolved within a week or 2 after drug withdrawal, at present there is no drug to rapidly increase patient platelet count or to stop serious bleeding if it occurs during the thrombocytopenic phase.

Immune thrombocytopenia can occur on first exposure to an RGD mimetic agent and the platelet count usually drops abruptly within hours of commencement of drug administration,³  suggesting the presence of a naturally occurring antiplatelet antibody. The antibody is known to recognize and bind in a drug-dependent manner to the platelet integrin, α_IIbβ₃, but the precise antibody binding site remains unclear.^3,–5  It is also known that binding of fibrinogen, RGD peptides, or RGD mimetic drugs to the RGD recognition site of α_IIbβ₃ induces conformational changes and emergence of cryptic epitopes previously “unseen” by the immune system.³  These epitopes are popularly termed LIBS (ligand-induced binding sites) and are recognized by several well-characterized murine monoclonal antibodies such as PMI-1, AP5, and D3.⁶  Understandably, many investigators in the field believe that these LIBS probably represent the epitopes of the antibodies in patients with RGD mimetic–induced thrombocytopenia, even though there is no experimental evidence to support this. In contrast, different mechanisms have been described for other immune drug–induced thrombocytopenias.^3,7,8 

Here, Bougie and colleagues directly investigated this issue and found that patient antibodies did not recognize LIB determinants as previously expected but instead recognized conformational changes in α_IIbβ₃ induced by the drug.¹  In the majority of patients with eptifibatide- and tirofiban-induced thrombocytopenia (30 of 43 patients), antibody binding was drug specific and only occurred when the integrin complexed with the drug that caused the thrombocytopenia. They called these changes in the integrin mimetic-induced binding sites, or MIBs. Cross-reactivity existed with the antibodies of some patients (13 of 43 patients). The cross-reacting antibodies bound α_IIbβ₃ when platelets were pretreated with any one RGD mimetic drug or RGD peptide. These MIBS were located in the head region of α_IIbβ_3, near the RGD recognition site, probably on the β-propeller domain of α_IIb or the βA domain of β₃ (see figure). There appeared to be 3 eptifibatide-dependent and 3 tirofiban-dependent antibody binding sites, each with an individual footprint. MIBS in this report were identified by cross-blocking studies using monoclonal antibodies with known epitopes. More precise epitope mapping, particularly identification of α_IIb or β₃ sequences that mediate antibody binding, may have to await further in-depth investigations using site-directed mutagenesis or structural studies such as x-ray crystallography or nuclear magnetic resonance.

The findings of Bougie et al provide important insights into the mechanism whereby patient antibodies bind integrin α_IIbβ₃ and the results have significant clinical implications. Before this study it was assumed that eptifibatide- and tirofiban-dependent antibodies do not cross-react as the 2 drugs have different chemical structures. However, Bougie and colleagues found that the 2 drug-dependent antibodies did cross-react in approximately 30% of patients.¹  Their findings suggest that it might not be safe to administer tirofiban to a patient with eptifibatide-induced thrombocytopenia and vice versa. One approach is to assess antibody cross-reactivity before administration of an alternative RGD mimetic as some clinical laboratories may be able to detect eptifibatide- and tirofiban-dependent antibodies⁴  and assess antibody cross-reactivity using flow cytometry. However, the clinical usefulness of such laboratory investigations will have to await confirmation by future studies.

Understanding the pathogenesis of drug-induced thrombocytopenia may lead to development of safer RDG mimetic inhibitor drugs. Recently, Zhu et al described an RDG mimetic inhibitor (designated RUC-1) that blocks fibrinogen binding to activated α_IIbβ₃ but does not induce conformational changes in the integrin nor emergence of LIBS and presumably MIBS.⁹  Consequently, RUC-1 and RUC-1–like drugs are unlikely to be antigenic and may not cause immune thrombocytopenia. Currently, if a patient with RGD mimetic-induced thrombocytopenia has a serious or potentially fatal bleed, there is no treatment to rapidly increase the patient's platelet count. Identification of α_IIb or β₃ sequences that mediate antibody binding by more precise epitope mapping studies may allow development of effective drugs such as inhibitory small molecules, peptides, or antibody fragments that block antibody binding to platelets and promote platelet recovery. Such novel drugs could be very helpful in this clinical setting.