ctDNA dynamics and mechanisms of acquired resistance in patients treated with osimertinib with or without bevacizumab from the randomised phase II ETOP-BOOSTER trial.

BACKGROUND: ETOP 10-16 BOOSTER study was a randomised phase II trial of osimertinib and bevacizumab versus osimertinib in patients with an acquired EGFR T790M mutation. The mechanisms of acquired resistance to osimertinib and bevacizumab have not been described previously. METHODS: Next generation sequencing (Guardant360®) was conducted in serial plasma samples. The association between ctDNA and efficacy outcomes was explored and molecular alterations at progression were described. RESULTS: 136 patients (88% of 155 randomised) had plasma samples at baseline (68 per arm), 110 (71%) at week 9 and 65 (42%) at progression. In a multivariable model for progression-free survival (PFS), the treatment effect was found different by smoking status (interaction p=0.046), with the effect of smoking also different by baseline EGFR T790M (interaction p=0.033), while both TP53 at baseline and tissue EGFR Exon 21 L858R mutation were significantly associated with worse PFS outcome. Smokers (current/former) without baseline EGFR T790M showed a significant improvement in PFS under combination treatment, albeit with small numbers (p=0.015). Week-9 EGFR T790M clearance was associated with improved PFS in the osimertinib arm (p=0.0097). Acquired EGFR C797S mutations were detected in 22% and 13% of patients in the combination and osimertinib arm, respectively. CONCLUSIONS: The differential effect of treatment by smoking was not explained by TP53 mutation or other molecular alterations examined. Molecular mechanisms of acquired resistance were detected but no novel molecular alterations were identified in the combination arm.

Non-invasive imaging of tau-targeted probe uptake by whole brain multi-spectral optoacoustic tomography.

PURPOSE: Abnormal tau accumulation within the brain plays an important role in tauopathies such as Alzheimer's disease and frontotemporal dementia. High-resolution imaging of tau deposits at the whole-brain scale in animal disease models is highly desired. METHODS: We approached this challenge by non-invasively imaging the brains of P301L mice of 4-repeat tau with concurrent volumetric multi-spectral optoacoustic tomography (vMSOT) at ~ 115 µm spatial resolution using the tau-targeted pyridinyl-butadienyl-benzothiazole derivative PBB5 (i.v.). In vitro probe characterization, concurrent vMSOT and epi-fluorescence imaging of in vivo PBB5 targeting (i.v.) was performed in P301L and wild-type mice, followed by ex vivo validation using AT-8 antibody for phosphorylated tau. RESULTS: PBB5 showed specific binding to recombinant K18 tau fibrils by fluorescence assay, to post-mortem Alzheimer's disease brain tissue homogenate by competitive binding against [11C]PBB3 and to tau deposits (AT-8 positive) in post-mortem corticobasal degeneration and progressive supranuclear palsy brains. Dose-dependent optoacoustic and fluorescence signal intensities were observed in the mouse brains following i.v. administration of different concentrations of PBB5. In vivo vMSOT brain imaging of P301L mice showed higher retention of PBB5 in the tau-laden cortex and hippocampus compared to wild-type mice, as confirmed by ex vivo vMSOT, epi-fluorescence, multiphoton microscopy, and immunofluorescence staining. CONCLUSIONS: We demonstrated non-invasive whole-brain imaging of tau in P301L mice with vMSOT system using PBB5 at a previously unachieved ~ 115 µm spatial resolution. This platform provides a new tool to study tau spreading and clearance in a tauopathy mouse model, foreseeable in monitoring tau targeting putative therapeutics.