Glycoprotein VI inhibitors: glenzocimab and EMA601 two inhibitory Fabs

This commentary refers to ‘The humanized platelet glycoprotein VI Fab inhibitor EMA601 protects from arterial thrombosis and ischaemic stroke in mice’, by S. Navarro et al., https://doi.org/10.1093/eurheartj/ehae482 and the discussion piece ‘EMA601 and glenzocimab: two glycoprotein VI inhibitory Fabs with different potency’, by B. Nieswandt et al., https://doi.org/10.1093/eurheartj/ehaf162. We read with great interest the description by Navarro et al.1 of new fragments of antibody Emf6.1 and its humanized version EMA601 targeting platelet glycoprotein VI (GPVI). The authors present an in-depth analysis showing that these Fabs efficiently inhibit GPVI function in vitro and ex vivo and prevent thrombosis in different murine models. Our concern relates to the comparison of EMA601 with a humanized anti-GPVI Fab ACT017, sold by ProteoGenix as a biosimilar of glenzocimab. Glenzocimab is the common international name of ACT017, defined as for other biologicals, on its sequence and biological properties that depend on its purity, formulation, and stability. A biosimilar should be analytically equivalent with the same potency, pharmacokinetics, and safety—in other words, changing the manufacturing process or cell line must have no effect on the type and level of activity. We would like to highlight that the properties of the commercial Fab reported by Navarro et al. are not reproducing those of glenzocimab. For instance, in the publication of Lebozec et al.,2 glenzocimab saturated human platelets and inhibited collagen-induced platelet aggregation since the concentration of 2 µg/mL. Moreover, the administration of a bolus dose of 1 mg/kg of glenzocimab to hGPVI mice resulted in a complete inhibition collagen-induced platelet aggregation ex vivo after 30 min whereas bolus doses of 4 and even 10 mg/kg of the commercial Fab had little effect on platelet activation.1 These discrepancies preclude to consider the commercial Fab ACT017 as biosimilar of glenzocimab. As such, the estimate that ‘EMA601 inhibits human GPVI function with >50-fold potency compared with ACT017’, a central claim of this manuscript, is overestimated. According to the better affinity of EMA601 for GPVI compared with published data on glenzocimab, EMA601 could be 10–20 more potent than glenzocimab to inhibit collagen-induced platelet aggregation. The epitopes of glenzocimab and EMA601 are distant from the collagen binding site, on D2 and membrane proximal domains, respectively.1,3 Glenzocimab inhibits GPVI dimerization and clustering, a feature apparently shared by EMA601, both Fabs preventing platelet activation at concentrations saturating GPVI. GPVI occupancy rate depends on Fab blood concentration and its affinity for GPVI. Glenzocimab and EMA601 have the same molecular weight and urinary clearance. Therefore, when considering a single administration for acute treatment, a greater amount of glenzocimab than EMA601 is required to saturate platelets, but ultimately both Fabs have the ability to effectively inhibit platelet activation by GPVI ligands. Pharmacokinetic (PK) and pharmacodynamic (PD) studies have shown effective inhibition of the GPVI biological activity after single or repeated administration of glenzocimab,4,5 and it would be interesting to have PK/PD data available for EMA601. Neither glenzocimab nor EMA601, because of their rapid urinary elimination and short half-life, appears suitable for secondary prevention of thrombosis or long-term treatment of chronic diseases in which GPVI plays a pathogenic role, such as the development of aortic aneurysms or metastases outgrowth. Modifying these Fabs to extend their circulation time would therefore be of interest. An alternative would be to identify low-molecular-weight antagonists, with the risk of less specificity than antibodies. M.J.-P. was cofounder and scientific advisor of Acticor Biotech. She received consulting fees and support for attending meetings and holds stocks from Acticor Biotech and is listed on patents WO EP US CN JP KR AU BR CA DK ES HK HR HU IL LT MX PL PT RS SI ZA CA2994629A1 and WO EP US CN JP KR AU BR CA IL MA MX NZ SG TW EP3577136A1. P.H.M. received study material from Acticor Biotech.