Section Review: Cardiovascular & Renal: Novel antiplatelet agents: The glycoprotein IIb/IIIa inhibitors

A perspective on the toxicological mechanisms possibly contributing to the failure of oral glycoprotein IIb/IIIa antagonists in the clinic

In clinical trials in patients with acute or unstable coronary syndromes and/or undergoing percutaneous coronary intervention, oral glycoprotein (GP) IIb/IIIa antagonists did not show therapeutic benefit over aspirin during long-term administration. Moreover, high-dose oral administration of these agents was associated with greater fatality risk compared with that of lower doses. This article postulates that continuous exposure of the GP IIb/IIIa receptor (integrin alpha(IIb)beta(3)) to these agents may result in some form of resistance or activation of other biological systems. These toxicological mechanisms may help explain some factors that could potentially contribute to the failure of these agents in clinical trials. Several hypotheses are presented: (i) modulation of platelet response because of long-term exposure to GP IIb/IIIa antagonists; (ii) role of related integrins and associated proteins to compensate for the loss of platelet activity because of dysfunctional GP IIb/IIIa receptors occupied by inhibitors; (iii) effects of the GP IIb/IIIa antagonists on other cellular systems such as the caspase and procaspase enzymes in apoptosis and possibly the ryanodine receptor involved in sarcoplasmic reticulum calcium release. These toxicological mechanisms could potentially limit the utility of these oral agents in long-term administration.

Ethics and equipoise: rationale for a placebo-controlled study design of platelet glycoprotein IIb/IIIa inhibition in coronary intervention

Rarely is it straightforward to specify the design and parameters of a clinical trial investigating an alternative therapy where effective therapies already exist. If existing therapeutic interventions are highly efficacious, safe, inexpensive, and firmly entrenched, an active-control design becomes the logical first choice. Short of this absolute condition, however, the merits and realities of the scientific, clinical, corporate, and regulatory environments need to be weighed before determining the appropriate approach. A state of clinical equipoise with regard to the use of glycoprotein (GP) IIb/IIIa therapy in percutaneous coronary intervention (PCI) provided the unique opportunity to address the complexities in selecting a placebo-controlled design for the Enhanced Suppression of the Platelet IIb/IIIa Receptor with Integrilin Trial (ESPRIT). ESPRIT investigators assessed whether a high dose of eptifibatide would improve the outcomes of patients undergoing coronary stenting. By using the example of the ESPRIT trial, we examine factors warranting the need for this trial and evaluate the process whereby the United States Food and Drug Administration (FDA) gave approval for a placebo-controlled design. Although the focus of this trial is GP IIb/IIIa inhibition therapy, the issues pertaining to the trial and how they were resolved are general enough to be applied to the design and conduct of clinical trials across a broad spectrum of illnesses and therapeutic modalities.