Correcting specificity in an antibody therapeutic
Off-target binding detected and rectified to counter toxicity risk for clinical stage antibody
A recent publication describes how an anti-PD1 antibody, which caused the unique side-effect of capillary hemangioma in early phase clinical studies, was screened using Retrogenix technology and found to bind to off-target receptors that are biologically relevant to the formation of hemangioma. Subsequent molecular remodelling of the antibody not only eradicated the in vitro off-target binding but also led to increased potency in PD1/PD-L1 blockade.
The anti-PD1 antibody SHR-1210 (also known as camrelizumab) has had the expected biological activity in early clinical studies for solid tumours; however, its toxicity profile is different to other anti-PD1 immunotherapies. The majority of patients receiving camrelizumab have developed capillary hemangioma, which is a benign tumour formed by a collection of excess blood vessels. This led the team at UltraHuman Eight Limited (based in Edinburgh, UK) to hypothesize that the target-binding domains of SHR-1210-IgG1 might interact with previously unidentified and unpredictable receptors that are associated with vascular development or tissue differentiation.
Retrogenix cell microarray technology was used to discover three human receptor interactions (in addition to PD1) that may explain the molecular mechanisms behind the observed toxicity. Notably, off-target binding to VEGFR2 and possibly frizzled class receptor 5 (FZD5), confirmed using multiple orthogonal approaches, may stimulate vascular neogenesis and lead to development of hemangioma. Importantly, the off-target binding of VEGFR2 actually led to potent activation of that receptor, which is known to be a key signalling event that stimulates capillary blood vessel proliferation. A third off-target, ULBP2, was also uncovered; however the consequence of modulating ULBP2 during cancer therapy with this anti-PD1 antibody is currently unknown.
Having proven that SHR-1210-IgG1 target binding domains exhibit relevant off-target reactivity, the UltraHuman Eight group investigated further and identified that the antibody’s polyspecificity existed prior to the humanisation process and was exacerbated during humanisation. Subsequent engineering of the antibody through mutagenesis and reselection generated novel antibodies that targeted PD1 and showed improved pharmacological properties, but which did not bind to the three off-targets originally identified.
The authors have highlighted the impact of this study in demonstrating that clinically-relevant polyspecificity is a phenomenon that can be identified and can be rapidly engineered out of a final therapeutic protein, potentially leading to improved potency and reduced toxicity. Commenting on the results, Dr Jonny Finlay, CEO of UltraHuman Eight said:
“UltraHuman is leading the way in making antibody therapeutics more specific, safer and more clinically effective. This unique study proves for the first time that antibody polyspecificity is not just an academic curiosity. It is a significant risk factor in the successful development of antibody-based drugs. Our collaboration with Retrogenix was an essential part of this study, allowing us to identify for the first time precisely what mechanisms might be driving the unique side effect profile of a clinical-stage antibody therapeutic.”
The paper, published in the journal ‘mAbs’, is entitled: “Anti-PD1 ‘SHR-1210ʹ aberrantly targets proangiogenic receptors and this polyspecificity can be ablated by paratope refinement.”