Direct In Vivo Comparison of 99mTc-Labeled Scaffold Proteins, DARPin G3 and ADAPT6, for Visualization of HER2 Expression and Monitoring of Early Response for Trastuzumab Therapy

Abstract

Non-invasive radionuclide molecular visualization of human epidermal growth factor receptor type 2 (HER2) can provide stratification of patients for HER2-targeting therapy. This method can also enable monitoring of the response to such therapies, thereby making treatment personalized and more efficient. Clinical evaluation in a phase I study demonstrated that injections of two scaffold protein-based imaging probes, [99mTc]Tc-(HE)3-G3 and [99mTc]Tc-ADAPT6, are safe, well-tolerated and cause a low level of radioactivity in healthy tissue. The goal of this preclinical study was to select the best probe for stratification of patients and response monitoring. Biodistribution of both tracers was compared in mice bearing SKOV-3 xenografts with high HER2 expression or MDA-MB-468 xenografts with very low expression. Changes in accumulation of the probes in SKOV-3 tumors 24 h after injection of trastuzumab were evaluated. Both [99mTc]Tc-ADAPT6 and [99mTc]Tc-(HE)3-G3 permitted high contrast imaging of HER2-expressing tumors and a clear discrimination between tumors with high and low HER2 expression. However, [99mTc]Tc-ADAPT6 has better preconditions for higher sensitivity and specificity of stratification. On the other hand, [99mTc]Tc-(HE)3-G3 is capable of detecting the decrease of HER2 expression on response to trastuzumab therapy only 24 h after injection of the loading dose. This indicates that the [99mTc]Tc-(HE)3-G3 tracer would be better for monitoring early response to such treatment. The results of this study should be considered in planning of further clinical development of HER2 imaging probes.

Direct Intra-Patient Comparison of Scaffold Protein-Based Tracers, [99mTc]Tc-ADAPT6 and [99mTc]Tc-(HE)3-G3, for Imaging of HER2-Positive Breast Cancer

“…Understanding the differences in biodistribution between different scaffold proteins should facilitate the selection of the most appropriate molecules for further clinical development. A direct preclinical in vivo comparison of [ 99m Tc]Tc-ADAPT6 and [ 99m Tc]Tc-(HE) 3- G3 revealed a significantly higher uptake of both radiopharmaceuticals in HER2-positive compared to HER2-negative tumours [ 20 ]. However, [ 99m Tc]Tc-ADAPT6 provided better discrimination between HER2-positive and HER2-negative xenografts.…”

“…However, [ 99m Tc]Tc-ADAPT6 provided better discrimination between HER2-positive and HER2-negative xenografts. On the other hand, [ 99m Tc]Tc-(HE) 3- G3 was capable of sensing the decrease in HER2 expression in response to trastuzumab therapy, which would be suitable for monitoring the early response to such treatment [ 20 ]. Still, there are numerous differences in the physiology and biochemistry of animal models and humans, and animal studies do not reflect all the interactions of scaffold proteins with targets that are expressed in normal tissues or their off-target interactions.…”

“…ADAPT6 and trastuzumab bind to the same epitope of HER2 [ 16 ], while the binding epitope of DARPin G3 is different [ 32 ]. A preclinical evaluation demonstrated that trastuzumab blocks the binding of [ 99m Tc]Tc-ADAPT6 to HER2-expressing cells in vitro but does not prevent the binding of [ 99m Tc]Tc-(HE) 3 -G3 [ 20 ]. Therefore, [ 99m Tc]Tc-(HE) 3 -G3 might be suitable for detecting the downregulation of HER2 in tumours in response to treatments, which include trastuzumab or its conjugates with drugs.…”

Evaluation of Approaches for the Assessment of HER2 Expression in Breast Cancer by Radionuclide Imaging Using the Scaffold Protein [99mTc]Tc-ADAPT6

“…Albumin-binding domain-derived affinity proteins (ADAPTs) are engineered scaffold proteins, which are small (5–7 kDa), and display high affinity and specificity to selected targets [ 19 ]. In preclinical studies, an ADAPT labelled with 99m Tc ([ 99m Tc]Tc-ADAPT6) demonstrated efficient differentiation of HER2-positive and HER2-negative tumours, and low uptake in tissues, which frequently harbour breast cancer metastases [ 20 , 21 ].…”

The application of radionuclide therapy for breast cancer

“…During the process of an ideal TRT, internally administered radionuclides will migrate specifically to the tumor region to precisely exert their cytotoxicity without any significant detrimental effects on the surrounding normal tissues (29, 30), as exemplified by a famous case of administration of 131 I to patients with thyroid carcinoma (31)(32)(33)(34)(35). Specific targeting can be achieved through antigen-antibody recognition (36,37), ligand-receptor interaction (38), or the interaction between certain biomolecules and unique biomarkers on the surface of tumor cells based on their high affinity (22). As such, the required dosage of radionuclide used in the treatment can be much less, which will significantly minimize the unnecessary exposure of patients to radiation both temporally and spatially.…”