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Off-target platelet activation in macaques unique to a therapeutic monoclonal antibody
Michael J Santostefano1, Jacqueline Kirchner, Christine Vissinga
1Comparative Biology and Safety Sciences, Toxicology Sciences, Amgen Inc., Seattle, Washington 98119-3105, USA.
Abstract:
AMG X, a human neutralizing monoclonal antibody (mAb) against a soluble human protein, caused thrombocytopenia, platelet activation, reduced mean arterial pressure, and transient loss of consciousness in cynomolgus monkeys after first intravenous administration. In vitro, AMG X induced activation in platelets from macaque species but not from humans or baboons. Other similar mAbs against the same pharmacological target failed to induce these in vivo and in vitro effects. In addition, the target protein was known to not be expressed on platelets, suggesting that platelet activation occurred through an off-target mechanism. AMG X bound directly to cynomolgus platelets and required both the Fab and Fc portion of the mAb for platelet activation. Binding to platelets was inhibited by preincubation of AMG X with its pharmacological target or with anti-human Fc antibodies or by preincubation of platelets with AMG X F(ab')(2) or human immunoglobulin (IVIG). AMG X F(ab')(2) did not activate platelets. Thus, platelet activation required both recognition/binding of a platelet ligand with the Fab domain and interaction of platelet Fc receptors (i.e., FcγRIIa) with the Fc domain. These findings reflect the complexity of the mechanism of action of mAbs and the increasing awareness of potential for unintended effects in preclinical species.
Insights
Monoclonal antibody AMG X caused severe side effects in monkeys, including thrombocytopenia and platelet activation, via an off-target mechanism involving Fc receptors. This highlights potential unintended consequences of antibody therapies in preclinical studies.
Area of Science:
- Immunology
- Pharmacology
- Toxicology
Background:
- Monoclonal antibodies (mAbs) are crucial therapeutics, but their mechanisms and potential off-target effects require thorough investigation.
- Understanding unintended consequences in preclinical models is vital for human safety.
Purpose of the Study:
- To investigate the mechanism of thrombocytopenia and platelet activation induced by the monoclonal antibody AMG X in cynomolgus monkeys.
- To elucidate the role of off-target binding and Fc receptor interactions in AMG X-mediated platelet activation.
Main Methods:
- In vivo studies in cynomolgus monkeys assessing hemodynamic parameters and platelet counts.
- In vitro platelet activation assays using platelets from various species (macaque, human, baboon).
- Binding studies using AMG X fragments (Fab, Fc, F(ab')(2)) and inhibitors (target protein, IVIG, anti-Fc antibodies).
Main Results:
- AMG X induced thrombocytopenia, platelet activation, and adverse hemodynamic effects in cynomolgus monkeys.
- In vitro, AMG X activated macaque platelets but not human or baboon platelets.
- Platelet activation required both Fab-mediated binding to a platelet ligand and Fc-mediated interaction with FcγRIIa.
Conclusions:
- AMG X-induced platelet activation in cynomolgus monkeys is an off-target effect mediated by both Fab and Fc domains of the antibody.
- The findings underscore the complexity of mAb mechanisms and the importance of considering species-specific Fc receptor interactions in preclinical safety assessments.
- This study highlights the potential for unintended immunomodulatory effects of therapeutic antibodies.
