Abstract B140: Positron emission tomography imaging of HER2 expression and pharmacodynamic response to HSP90 inhibition with the next-generation ZHER2:2891 Affibody molecule [18F]GE-226.

Accurate assessment of HER2 status remains a clinical challenge, with up to 20% of patients being potentially withdrawn from therapy or exposed to unnecessary toxicity. Non-invasive imaging is widely seen as a viable alternative to current methods, in particular in the setting of locoregional and distant recurrences not amenable to biopsy. A next-generation HER2-targeting Affibody-based radiotracer has been developed, [¹⁸F]GE-226, with enhanced pharmacokinetic characteristics and improved properties for large-scale and GMP grade synthesis. Kinetic modeling gave insights into Affibody-HER2 interactions.

Intrinsic affinity to HER2 (K_D = 76 pM) resulted in 11 to 67-fold higher [¹⁸F]GE-226 uptake in ten HER2 positive versus negative cell lines in vitro independent of lineage. Uptake correlated with HER2 protein expression but was independent of presence of other targets like EGFR. Blocking with [¹⁹F]GE-226 and HER2 siRNA treatment reduced uptake by 96.8 ± 2.6% and 81.7 ± 9.2%, respectively. Uptake in HER2 positive xenografts was rapid with steady state net irreversible binding kinetics making possible the distinction of HER2 negative (MCF7 (n = 6) and MCF7-p95HER2 (n = 3): NUV₆₀ (normalized uptake value at 60 min; %ID/mL) 6.1 ± 0.7; K_i (irreversible uptake rate; mL/cm³/min) 0.0069 ± 0.0014) from HER2 positive tumors (NUV₆₀ and K_i: MCF7-HER2, 10.9 ± 1.5 and 0.015 ± 0.0035; MDA-MB-361, 18.2 ± 3.4 and 0.025 ± 0.0052; SKOV-3, 18.7 ± 2.4 and 0.036 ± 0.0065; all n = 6) within 1 h. Tumor uptake correlated with HER2 expression determined by ELISA (r²=0.78). Specificity was further determined by comparing tumor localization of a fluorescently labeled tracer analogue with DAKO HercepTest. Affibody signal co-localized with HER2 expression at the cellular level independent of spatial heterogeneity. Tracer binding was not influenced by short-term or continuous exposure to trastuzumab in SKOV-3 xenografts (n=6) in keeping with differential epitope binding. Inhibition of the chaperone HSP90– of which HER2 is a client protein– by the therapeutic development candidate NVP-AUY922 caused dose-dependent HER2 degradation and consequently reduced tracer uptake in SKOV-3 cells in vitro and xenografts in vivo (area under the curve, AUC_0-60: 618.4±90.1 and 446.7±42.8 %ID/mL*min for vehicle (n=4) and drug (n=5), respectively; P=0.043).

In conclusion, [¹⁸F]GE-226 differentiates HER2 negative from HER2 expressing tumors. The tracer has potential utility for HER2 detection, irrespective of prior trastuzumab treatment and to monitor response to HSP90 inhibition. Lineage-independence of these results extends application beyond breast cancer. Due to the specific annotation to HER2, enhanced pharmacokinetic properties and completion of initial preclinical toxicology testing, [¹⁸F]GE-226 is now transitioning into clinical development.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B140.

Citation Format: Sebastian Trousil, Susan Hoppmann, Quang-Dé Nguyen, Maciej Kaliszczak, Giampaolo Tomasi, Peter Iveson, Duncan Hiscock, Eric O. Aboagye. Positron emission tomography imaging of HER2 expression and pharmacodynamic response to HSP90 inhibition with the next-generation ZHER2:2891 Affibody molecule [18F]GE-226. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B140.