The epidermal growth factor receptor (EGFR) and its truncated autoactive form, EGFRvIII, are amplified/overexpressed in 60% and 30% of gliomas, respectively, and are integral to gliomagenesis. Several antibodies have been developed to neutralise the activity of these two receptors; however, promising pre-clinical data has not translated to clinical success for unknown reasons, with cetuximab (phase II) and nimotuzumab (phase III) failing to improve patient outcomes.
To investigate reasons for this, we analysed EGFRvIII-bearing primary neurosphere cell lines after antibody treatment using conventional and native western for EGFR/EGFRvIII status, proliferation assays, immunofluorescence assays and in vivo orthotopic tumour models.
We report that several clinical and pre-clinical antibodies (cetuximab, nimotuzumab, necitumumab and matuzumab) do not neutralise EGFRvIII auto-activity, leading to sustained cellular proliferation. The antibody ch806 neutralised EGFRvIII but not wtEGFR. Only panitumumab neutralised both EGFRvIII and wtEGFR, resulting in superior anti-proliferative effects and nullification of downstream signalling. Tracking of fluorescent panitumumab and western blotting demonstrated that anti-EGFR antibodies do not degrade either wtEGFR or EGFRvIII as previously thought – rather, the receptors are internalised and then recycled. Mechanistically, panitumumab’s unique avidity allows stable complex formation with EGFRvIII to prevent its activation whilst ch806 binds to EGFRvIII and retards the formation of the dimeric, active form of EGFRvIII. In orthotopic in vivo trials, panitumumab induced a significantly greater increase in survival compared to cetuximab, nimotuzumab, ch806 or a combination of ch806 and cetuximab.
Collectively, our research has discovered a previously unknown failure of most FDA-approved and pre-clinical anti-EGFR antibodies to neutralise EGFRvIII. Panitumumab as a single agent is vastly superior in vitro and in appropriate primary GBM in vivo orthograft models. These results suggest that panitumumab may offer an effective candidate for a further clinical trial in an EGFRvIII-positive patient population.
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T. Johns: Funding Amgen. All other authors have declared no conflicts of interest.