Imaging the pharmacodynamics of HER2 degradation in response to Hsp90 inhibitors

Enabling the formation of native mAb, Fab' and Fc-conjugates using a bis-disulfide bridging reagent to achieve tunable payload-to-antibody ratios (PARs)

Either as full IgGs or as fragments (Fabs, Fc, etc.), antibodies have received tremendous attention in the development of new therapeutics such as antibody-drug conjugates (ADCs). The production of ADCs involves the grafting of active payloads onto an antibody, which is generally enabled by the site-selective modification of native or engineered antibodies via chemical or enzymatic methods. Whatever method is employed, controlling the payload-antibody ratio (PAR) is a challenge in terms of multiple aspects including: (i) obtaining homogeneous protein conjugates; (ii) obtaining unusual PARs (PAR is rarely other than 2, 4 or 8); (iii) using a single method to access a range of different PARs; (iv) applicability to various antibody formats; and (v) flexibility for the production of heterofunctional antibody-conjugates (e.g. attachment of multiple types of payloads). In this article, we report a single pyridazinedione-based trifunctional dual bridging linker that enables, in a two-step procedure (re-bridging/click), the generation of either mAb-, Fab'-, or Fc-conjugates from native mAb, (Fab')2 or Fc formats, respectively. Fc and (Fab')2 formats were generated via enzymatic digestion of native mAbs. Whilst the same reduction and re-bridging protocols were applied to all three of the protein formats, the subsequent click reaction(s) employed to graft payload(s) drove the generation of a range of PARs, including heterofunctional PARs. As such, exploiting click reactivity and/or orthogonality afforded mAb-conjugates with PARs of 6, 4, 2 or 4 + 2, and Fab'- and Fc-conjugates with a PAR of 3, 2, 1 or 2 + 1 on-demand. We believe that the homogeneity, novelty and variety in accessible PARs, as well as the applicability to various antibody-conjugate formats enabled by our non-recombinant method could be a suitable tool for antibody-drug conjugates optimisation (optimal PAR value, optimal payloads combination) and boost the development of new antibody therapeutics (Fab'- and Fc-conjugates).

68Ga-Labeled Trastuzumab Fragments for ImmunoPET Imaging of Human Epidermal Growth Factor Receptor 2 Expression in Solid Cancers

Background: Trastuzumab, the first humanized antibody approved for therapeutic use has shown promising results for the treatment of patients with human epidermal growth factor receptor 2 (HER2) positive cancers. The aim of this study was to formulate immunoPET agents based on trastuzumab fragments and demonstrate their potential for early diagnosis of HER2-positive tumors. Materials and Methods: F(ab')₂ and F(ab') fragments of trastuzumab were prepared by enzymatic digestion and conjugated with chelator NOTA for labeling with ⁶⁸Ga. For comparison, intact trastuzumab was also radiolabeled. In vitro stability, immunoreactivity, and binding affinity of radio formulations toward HER2 receptors were evaluated by performing in vitro studies in cancer cell lines. Biodistribution and PET imaging studies were performed in animal model bearing tumors. Results: ⁶⁸Ga-NOTA-F(ab')-trastuzumab, ⁶⁸Ga-NOTA-F(ab')₂-trastuzumab, and ⁶⁸Ga-NOTA-trastuzumab could be prepared with >98% radiochemical purity (% RCP) and were found to be stable when studied up to 4 h. In vitro binding studies revealed high affinity and specificity of formulations toward HER2 receptors. Specific tumor uptake of ⁶⁸Ga-NOTA-F(ab')-trastuzumab and ⁶⁸Ga-NOTA-F(ab')₂-trastuzumab in HER2-positive tumors was observed in biodistribution and PET imaging studies. Conclusions: This study describes optimization of protocol for the formulation of ⁶⁸Ga-NOTA-F(ab')-trastuzumab and ⁶⁸Ga-NOTA-F(ab')₂-trastuzumab for targeting HER2-overexpressing tumors. Further studies with these radioformulations are warranted to confirm their potential as immunoPET agents for management of HER2-positive breast and other solid tumors.

Targeting HSP90 as a Novel Therapy for Cancer: Mechanistic Insights and Translational Relevance

Heat shock protein (HSP90), a highly conserved molecular chaperon, is indispensable for the maturation of newly synthesized poly-peptides and provides a shelter for the turnover of misfolded or denatured proteins. In cancers, the client proteins of HSP90 extend to the entire process of oncogenesis that are associated with all hallmarks of cancer. Accumulating evidence has demonstrated that the client proteins are guided for proteasomal degradation when their complexes with HSP90 are disrupted. Accordingly, HSP90 and its co-chaperones have emerged as viable targets for the development of cancer therapeutics. Consequently, a number of natural products and their analogs targeting HSP90 have been identified. They have shown a strong inhibitory effect on various cancer types through different mechanisms. The inhibitors act by directly binding to either HSP90 or its co-chaperones/client proteins. Several HSP90 inhibitors—such as geldanamycin and its derivatives, gamitrinib and shepherdin—are under clinical evaluation with promising results. Here, we review the subcellular localization of HSP90, its corresponding mechanism of action in the malignant phenotypes, and the recent progress on the development of HSP90 inhibitors. Hopefully, this comprehensive review will shed light on the translational potential of HSP90 inhibitors as novel cancer therapeutics.

Therapeutic Response Monitoring with 89Zr-DFO-Pertuzumab in HER2-Positive and Trastuzumab-Resistant Breast Cancer Models

Immuno-positron emission tomography (PET) has great potential to evaluate the target expression level and therapeutic response for targeted cancer therapy. Immuno-PET imaging with pertuzumab, due to specific recognition in different binding sites of HER2, could be useful for the determination of the therapeutic efficacy of HER2-targeted therapy, trastuzumab, and heat shock protein 90 (HSP90) inhibitor, in HER2-expressing breast cancer. The aim of this study is to evaluate the feasibility of monitoring therapeutic response with ⁸⁹Zr-DFO-pertuzumab for the treatment of HER2-targeted therapeutics, trastuzumab, or the HSP90 inhibitor 17-DMAG, in trastuzumab-resistant JIMT-1 breast cancer models. We prepared an immuno-PET imaging agent using desferoxamine (DFO)-pertuzumab labeled with ⁸⁹Zr and performed the biodistribution and PET imaging in breast cancer xenograft models for monitoring therapeutic response to HER2-targeted therapy. ⁸⁹Zr-DFO-pertuzumab was successfully prepared and showed specific binding to HER2 in vitro and clearly visualized HER2 expressing JIMT-1 tumors. ⁸⁹Zr-DFO-pertuzumab had prominent tumor uptake in HER2 expressing JIMT-1 tumors. JIMT-1 tumors showed trastuzumab-resistant and HSP90 inhibitor sensitive characterization. In immuno-PET imaging, isotype antibody-treated JIMT-1 tumors had similar uptake in trastuzumab-treated JIMT-1 tumors, but 17-DMAG-treated JIMT-1 tumors showed greatly reduced uptake compared to vehicle-treated tumors. Additionally, HER2 downregulation evaluated by immuno-PET imaging was verified by western blot analysis and immunofluorescence staining which resulted in a significant reduction in the tumor’s HER2 level in 17-DMAG-treated JIMT-1 tumors. ⁸⁹Zr-DFO-pertuzumab immuno-PET may be clinically translated to select pertinent patients for HER2-targeted therapy and to monitor the therapeutic response in HER2-positive cancer patients under various HER2-targeted therapeutics treatments.

Review: Radionuclide Molecular Imaging Targeting HER2 in Breast Cancer with a Focus on Molecular Probes into Clinical Trials and Small Peptides

As the most frequently occurring cancer worldwide, breast cancer (BC) is the leading cause of cancer-related death in women. The overexpression of HER2 (human epidermal growth factor receptor 2) is found in about 15% of BC patients, and it is often associated with a poor prognosis due to the effect on cell proliferation, migration, invasion, and survival. As a result of the heterogeneity of BC, molecular imaging with HER2 probes can non-invasively, in real time, and quantitatively reflect the expression status of HER2 in tumors. This will provide a new approach for patients to choose treatment options and monitor treatment response. Furthermore, radionuclide molecular imaging has the potential of repetitive measurements, and it can help solve the problem of heterogeneous expression and conversion of HER2 status during disease progression or treatment. Different imaging probes of targeting proteins, such as monoclonal antibodies, antibody fragments, nanobodies, and affibodies, are currently in preclinical and clinical development. Moreover, in recent years, HER2-specific peptides have been widely developed for molecular imaging techniques for HER2-positive cancers. This article summarized different types of molecular probes targeting HER2 used in current clinical applications and the developmental trend of some HER2-specific peptides.

HER3 PET Imaging: 68Ga-Labeled Affibody Molecules Provide Superior HER3 Contrast to 89Zr-Labeled Antibody and Antibody-Fragment-Based Tracers

Simple Summary

HER3 is a known driver for oncogenesis and therapy resistance in solid cancers. PET imaging could be a useful tool to non-invasively detect and monitor HER3 expression and aid in the selection of patients for HER3-targeted therapy. PET tracers based on therapeutic antibodies have thus far shown limited success in reliably imaging HER3-expressing tumors in clinical trials. Smaller-sized tracers specifically designed for imaging might be needed for higher contrast imaging and sufficient sensitivity. Our group has previously studied the use of radiolabeled affibody molecules for imaging of HER3 expression. In the present study, we compared four different types of potential PET tracers for imaging of HER3 expression in a preclinical model. We demonstrated that the affibody-based tracer, [⁶⁸Ga]Ga-Z_HER3, could provide overall superior imaging contrast to antibody- and antibody-fragment-based tracers shortly after injection. Our results indicate that HER3-targeting affibody molecules are promising agents for PET imaging of HER3 expression.

Abstract

HER3 (human epidermal growth factor receptor type 3) is a challenging target for diagnostic radionuclide molecular imaging due to the relatively modest overexpression in tumors and substantial expression in healthy organs. In this study, we compared four HER3-targeting PET tracers based on different types of targeting molecules in a preclinical model: the ⁸⁹Zr-labeled therapeutic antibody seribantumab, a seribantumab-derived F(ab)₂-fragment labeled with ⁸⁹Zr and ⁶⁸Ga, and the ⁶⁸Ga-labeled affibody molecule [⁶⁸Ga]Ga-Z_HER3. The novel conjugates were radiolabeled and characterized in vitro using HER3-expressing BxPC-3 and DU145 human cancer cells. Biodistribution was studied using Balb/c nu/nu mice bearing BxPC-3 xenografts. HER3-negative RAMOS xenografts were used to demonstrate binding specificity in vivo. Autoradiography was conducted on the excised tumors. nanoPET/CT imaging was performed. New conjugates specifically bound to HER3 in vitro and in vivo. [⁶⁸Ga]Ga-DFO-seribantumab-F(ab’)₂ was considered unsuitable for imaging due to the low stability and high uptake in normal organs. The highest tumor-to-non-tumor contrast with [⁸⁹Zr]Zr-DFO-seribantumab and [⁸⁹Zr]Zr-DFO-seribantumab-F(ab’)₂ was achieved at 96 h and 48 h pi, respectively. Despite lower tumor uptake, [⁶⁸Ga]Ga-Z_HER3 provided the best imaging contrast due to the fastest clearance from blood and normal organs. The results of our study suggest that affibody-based tracers are more suitable for PET imaging of HER3 expression than antibody- and antibody-fragment-based tracers.

Quantitative Evaluation of the Effect of Antigen Expression Level on Antibody-Drug Conjugate Exposure in Solid Tumor

Antibody-drug conjugates (ADCs) rely on high expression of target antigens on cancer cells to effectively enter the cell and release a cytotoxic payload. Previous studies have shown that ADC efficacy is not always tied to antigen expression. However, our recent in vitro study suggests a linear relationship between antigen expression and the intracellular levels of the ADC payload. In this study, we have explored the relationship between antigen expression and intratumoral ADC exposure in vivo. Using trastuzumab-vc-MMAE (T-vc-MMAE) and four cell lines with varying expression of human epithelial growth factor receptor 2 (HER2), the pharmacokinetics of total trastuzumab, released ("free") MMAE, and total MMAE were evaluated in a tumor xenograft model. Nude mice were implanted with tumors originating from BT-474, MDA-MB-453, MCF-7, and MDA-MB-468 cell lines and dosed with 10 mg/kg or 1 mg/kg of ADC. Observed data were mathematically characterized using a mechanism-based PK model. A strong positive correlation was observed between antigen expression levels and free/total MMAE exposure (R² ≥ 0.91) (total MMAE being the sum of released and conjugated MMAE) within the tumor, but not for total trastuzumab exposure. The PK model was able to recapitulate plasma PK through simulation; however, the tumor PK was overpredicted or underpredicted in some cases potentially due to differences in tumor vasculature or extracellular matrix conditions. Our results indicate a linear relationship between antigen expression and tumor exposure of free/total ADC payload in vivo, validating our previous finding in vitro, while also revealing the need to understand complex physiology of the tumor to predict tumor PK of ADC and its components. Our findings also support the concept of antigen expression screening in patients for targeted therapies like ADCs to achieve the maximum therapeutic benefit of the treatment.

PET and SPECT Imaging of the EGFR Family (RTK Class I) in Oncology

The human epidermal growth factor receptor family (EGFR-family, other designations: HER family, RTK Class I) is strongly linked to oncogenic transformation. Its members are frequently overexpressed in cancer and have become attractive targets for cancer therapy. To ensure effective patient care, potential responders to HER-targeted therapy need to be identified. Radionuclide molecular imaging can be a key asset for the detection of overexpression of EGFR-family members. It meets the need for repeatable whole-body assessment of the molecular disease profile, solving problems of heterogeneity and expression alterations over time. Tracer development is a multifactorial process. The optimal tracer design depends on the application and the particular challenges of the molecular target (target expression in tumors, endogenous expression in healthy tissue, accessibility). We have herein summarized the recent preclinical and clinical data on agents for Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) imaging of EGFR-family receptors in oncology. Antibody-based tracers are still extensively investigated. However, their dominance starts to be challenged by a number of tracers based on different classes of targeting proteins. Among these, engineered scaffold proteins (ESP) and single domain antibodies (sdAb) show highly encouraging results in clinical studies marking a noticeable trend towards the use of smaller sized agents for HER imaging.

The emerging role of radionuclide molecular imaging of HER2 expression in breast cancer

Targeting of human epidermal growth factor type 2 (HER2) using monoclonal antibodies, antibody-drug conjugates and tyrosine kinase inhibitors extends survival of patients with HER2-expressing metastatic breast cancer. High expression of HER2 is a predictive biomarker for such specific treatment. Accurate determination of HER2 expression level is necessary for stratification of patients to targeted therapy. Non-invasive in vivo radionuclide molecular imaging of HER2 has a potential of repetitive measurements, addressing issues of heterogeneous expression and conversion of HER2 status during disease progression or in response to therapy. Imaging probes based of several classes of targeting proteins are currently in preclinical and early clinical development. Both preclinical and clinical data suggest that the most promising are imaging agents based on small proteins, such as single domain antibodies or engineered scaffold proteins. These agents permit a very specific high-contrast imaging at the day of injection.

Comparative evaluation of dimeric and monomeric forms of ADAPT scaffold protein for targeting of HER2-expressing tumours

ADAPTs are small engineered non-immunoglobulin scaffold proteins, which have demonstrated very promising features as vectors for radionuclide tumour targeting. Radionuclide imaging of human epidermal growth factor 2 (HER2) expression in vivo might be used for stratification of patients for HER2-targeting therapies. ADAPT6, which specifically binds to HER2, has earlier been shown to have very promising features for in vivo targeting of HER2 expressing tumours. In this study we tested the hypothesis that dimerization of ADAPT6 would increase the apparent affinity to HER2 and accordingly improve tumour targeting. To find an optimal molecular design of dimers, a series of ADAPT dimers with different linkers, -SSSG- (DiADAPT6L1), -(SSSG)₂- (DiADAPT6L2), and -(SSSG)₃- (DiADAPT6L3) was evaluated. Dimers in combination with optimal linker lengths demonstrated increased apparent affinity to HER2. The best variants, DiADAPT6L2 and DiADAPT6L3 were site-specifically labelled with ¹¹¹In and ¹²⁵I, and compared with a monomeric ADAPT6 in mice bearing HER2-expressing tumours. Despite higher affinity, both dimers had lower tumour uptake and lower tumour-to-organ ratios compared to the monomer. We conclude that improved affinity of a dimeric form of ADAPT does not compensate the disadvantage of increased size. Therefore, increase of affinity should be obtained by affinity maturation and not by dimerization.

PET imaging of EGFR expression using an 18F-labeled RNA aptamer

OBJECTIVE

Epidermal growth factor receptor (EGFR) is a theranostic biomarker for a variety of cancer types. The aim of the present study was to develop an ¹⁸F radiolabeled EGFR targeting RNA aptamer, and to investigate its ability to visualize and quantify EGFR in xenograft models.

METHODS

Biolayer interferometry binding assay was used to detect the binding affinity of the alkyne-modified EGFR aptamer MinE07 (denoted as ME07) with recombinant human wild-type EGFR protein and the mutant EGFRvIII protein. Cy5-conjugated ME07 was used for flow cytometry and immunofluorescence staining, and an Alexa Fluor 488-labeled EGFR antibody (ab193244) was used as a control. ¹⁸F-Fluorobenzoyl (FB) azide was employed as a synthon to produce ¹⁸F-FB-ME07 via click chemistry, and the cellular uptake and internalization characteristics of ¹⁸F-FB-ME07 were investigated. Static PET scans, 60-min dynamic scans, and biodistribution study of ¹⁸F-FB-ME07 were performed in three types of tumor models.

RESULTS

The K_d values of ME07 to wtEGFR and EGFRvIII proteins were 0.3 nM and 271 nM respectively. The A431, U87MG, and HCT-116 cells showed strong, weak, and negative binding with Cy5-ME07, which is consistent with EGFR expression level in these cells. Peak cell uptake values of ¹⁸F-FB-ME07 in A431, U87MG and HCT-116 cells were 2.86%, 2.19% and 0.88% of the added dose respectively. The mean internalization of ¹⁸F-FB-ME07 in these cells were 60.02%, 53.1%, and 52.8% of the total accumulated radioactivity. In static PET imaging, despite high uptake in the liver and kidneys, ¹⁸F-FB-ME07 showed reasonable accumulation in A431 tumors (1.02 ± 0.13 %ID/g at 30 min after injection). Of note, the uptake of ¹⁸F-FB-ME07 in A431 xenografts was significantly higher than that in U87MG and HCT-116 xenografts. In A431 xenografted mice, the tumor/blood ratio was 3.89 and the tumor/muscle ratio reached 8.65.

CONCLUSIONS

We for the first time generated an aptamer-derived EGFR targeting PET tracer ¹⁸F-FB-ME07, which showed highly selective targeting ability in mouse tumor models expressing different levels of EGFR. Our results suggest that ¹⁸F-FB-ME07 is a potential EGFR targeting molecular imaging probe for future clinical translation.

ImmunoPET Imaging of Endogenous and Transfected Prolactin Receptor Tumor Xenografts

Antibodies labeled with positron-emitting isotopes have been used for tumor detection, predicting which patients may respond to tumor antigen-directed therapy, and assessing pharmacodynamic effects of drug interventions. Prolactin receptor (PRLR) is overexpressed in breast and prostate cancers and is a new target for cancer therapy. We evaluated REGN2878, an anti-PRLR monoclonal antibody, as an immunoPET reagent. REGN2878 was labeled with Zr-89 after conjugation with desferrioxamine B or labeled with I-131/I-124. In vitro determination of the half-maximal inhibitory concentration (IC50) of parental REGN2878, DFO-REGN2878, and iodinated REGN2878 was performed by examining the effect of the increasing amounts of these on uptake of trace-labeled I-131 REGN2878. REGN1932, a non-PRLR binding antibody, was used as a control. Imaging and biodistribution studies were performed in mice bearing tumor xenografts with various expression levels of PRLR, including MCF-7, transfected MCF-7/PRLR, PC3, and transfected PC3/PRLR and T4D7v11 cell lines. The specificity of uptake in tumors was evaluated by comparing Zr-89 REGN2878 and REGN1932, and in vivo competition compared Zr-89 REGN2878 uptake in tumor xenografts with and without prior injection of 2 mg of nonradioactive REGN2878. The competition binding assay of DFO-REGN2878 at ratios of 3.53-5.77 DFO per antibody showed IC50 values of 0.4917 and 0.7136 nM, respectively, compared to 0.3455 nM for parental REGN2878 and 0.3343 nM for I-124 REGN2878. Imaging and biodistribution studies showed excellent targeting of Zr-89 REGN2878 in PRLR-positive xenografts at delayed times of 189 h (presented as mean ± 1 SD, percent injected activity per mL (%IA/mL) 74.6 ± 33.8%IA/mL). In contrast, MCF-7/PRLR tumor xenografts showed a low uptake (7.0 ± 2.3%IA/mL) of control Zr-89 REGN1932 and a very low uptake and rapid clearance of I-124 REGN2878 (1.4 ± 0.6%IA/mL). Zr-89 REGN2878 has excellent antigen-specific targeting in various PRLR tumor xenograft models. We estimated, using image-based kinetic modeling, that PRLR antigen has a very rapid in vivo turnover half-life of ∼14 min from the cell membrane. Despite relatively modest estimated tumor PRLR expression numbers, PRLR-expressing cells have shown final retention of the Zr-89 REGN2878 antibody, with an uptake that appeared to be related to PRLR expression. This reagent has the potential to be used in clinical trials targeting PRLR.

I-124 codrituzumab imaging and biodistribution in patients with hepatocellular carcinoma

Background

I-124 codrituzumab (aka GC33), an antibody directed at Glypican 3, was evaluated in patients with hepatocellular carcinoma (HCC). Fourteen patients with HCC underwent baseline imaging with I-124 codrituzumab (~ 185 MBq, 10 mg). Seven of these patients undergoing sorafenib/immunotherapy with 2.5 or 5 mg/kg of cold codrituzumab had repeat imaging, with co-infusion of I-124 codrituzumab, as part of their immunotherapy treatment. Three patients who progressed while on sorafenib/immunotherapy were re-imaged after a 4-week washout period to assess for the presence of antigen. Serial positron emission tomography (PET) imaging and pharmacokinetics were performed following I-124 codrituzumab. An ELISA assay was used to determine “cold” codrituzumab serum pharmacokinetics and compare it to that of I-124 codrituzumab. Correlation of imaging results was performed with IHC. Short-term safety assessment was also evaluated.

Results

Thirteen patients had tumor localization on baseline I-124 codrituzumab; heterogeneity in tumor uptake was noted. In three patients undergoing repeat imaging while on immunotherapy/sorafenib, evidence of decreased I-124 codrituzumab uptake was noted. All three patients who underwent imaging after progression while on immunotherapy continued to have I-124 codrituzumab tumor uptake. Pharmacokinetics of I-124 codrituzumab was similar to that of other intact IgG. No significant adverse events were observed related to the I-124 codrituzumab.

Conclusions

I-124 codrituzumab detected tumor localization in most patients with HCC. Pharmacokinetics was similar to that of other intact iodinated humanized IgG. No visible cross-reactivity with normal organs was observed.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0374-8) contains supplementary material, which is available to authorized users.

JWA down-regulates HER2 expression via c-Cbl and induces lapatinib resistance in human gastric cancer cells

Human epidermal growth factor receptor 2 (HER2) targeted therapy is currently considered as the standard treatment for HER2-positive advanced gastric cancer (GC). However, unsatisfactory results of recent phase III clinical trials involving lapatinib suggested biomarkers for selection of patients. The aim of this study was to identify JWA as a biomarker for lapatinib resistance in GC cells and elucidate the underlying mechanisms. Lapatinib was effective to the intrinsic cisplatin-resistant GC cells. JWA activation conferred lapatinib unresponsiveness, but reversed cisplatin resistance in GC cells. Whereas, deletion of JWA significantly restored lapatinib suppression on proliferation and lapatinib-induced apoptosis. JWA-induced down-regulation of HER2 and activation of ERK phosphorylation led to lapatinib resistance. Furthermore, c-Cbl represented a novel mechanism for HER2 degradation enhanced by JWA in GC cells. Taken together, JWA is a potential predictive marker for lapatinib resistance, targeting the patients that may benefit from lapatinib treatment in human GC.

In Vivo Imaging in Pharmaceutical Development and Its Impact on the 3Rs

It is well understood that the biopharmaceutical industry must improve efficiency along the path from laboratory concept to commercial product. In vivo imaging is recognized as a useful method to provide biomarkers for target engagement, treatment response, safety, and mechanism of action. Imaging biomarkers have the potential to inform the selection of drugs that are more likely to be safe and effective. Most of the imaging modalities for biopharmaceutical research are translatable to the clinic. In vivo imaging does not require removal of tissue to provide biomarkers, thus reducing the number of valuable preclinical subjects required for a study. Longitudinal imaging allows for quantitative intra-subject comparisons, enhancing statistical power, and further reducing the number of subjects needed for the evaluation of treatment effects in animal models. The noninvasive nature of in vivo imaging also provides a valuable approach to alleviate or minimize potential pain, suffering or distress.

Pretargeted PET Imaging Using a Bioorthogonal 18F-Labeled trans-Cyclooctene in an Ovarian Carcinoma Model

In cancer research, pretargeted positron emission tomography (PET) imaging has emerged as an effective two-step approach that combines the excellent target affinity and selectivity of antibodies with the advantages of using short-lived radionuclides such as fluorine-18. One possible approach is based on the bioorthogonal inverse-electron-demand Diels-Alder (IEDDA) reaction between tetrazines and trans-cyclooctene (TCO) derivatives. Here, we report the first successful use of an ¹⁸F-labeled small TCO compound, [¹⁸F]1 recently developed in our laboratory, to perform pretargeted immuno-PET imaging. The study was performed in an ovarian carcinoma mouse model, using a trastuzumab-tetrazine conjugate.

Comparison of macrocyclic and acyclic chelators for gallium-68 radiolabelling

Gallium-68 (68Ga) is a positron-emitting isotope used for clinical PET imaging of peptide receptor expression. 68Ga radiopharmaceuticals used in molecular PET imaging consist of disease-targeting biomolecules tethered to chelators that complex 68Ga3+. Ideally, the chelator will rapidly, quantitatively and stably coordinate 68Ga3+ at room temperature, near neutral pH and low chelator concentration, allowing for simple routine radiopharmaceutical formulation. Identification of chelators that fulfil these requirements will facilitate development of kit-based 68Ga radiopharmaceuticals. Herein the reaction of a range of widely used macrocyclic and acyclic chelators with 68Ga3+ is reported. Radiochemical yields have been measured under conditions of varying chelator concentrations, pH (3.5 and 6.5) and temperature (25 and 90 °C). These chelators are: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), 1,4,7-triazacyclononane macrocycles substituted with phosphonic (NOTP) and phosphinic (TRAP) groups at the amine, bis(2-hydroxybenzyl)ethylenediaminediacetic acid (HBED), a tris(hydroxypyridinone) containing three 1,6-dimethyl-3-hydroxypyridin-4-one groups (THP) and the hexadentate tris(hydroxamate) siderophore desferrioxamine-B (DFO). Competition studies have also been undertaken to assess relative complexation efficiencies of each chelator for 68Ga3+ under different pH and temperature conditions. Performing radiolabelling reactions at pH 6.5, 25 °C and 5-50 μM chelator concentration resulted in near quantitative radiochemical yields for all chelators, except DOTA. Radiochemical yields either decreased or were not substantially improved when the reactions were undertaken at lower pH or at higher temperature, except in the case of DOTA. THP and DFO were the most effective 68Ga3+ chelators at near-neutral pH and 25 °C, rapidly providing near-quantitative radiochemical yields at very low chelator concentrations. NOTP and HBED were only slightly less effective under these conditions. In competition studies with all other chelators, THP demonstrated highest reactivity for 68Ga3+ complexation under all conditions. These data point to THP possessing ideal properties for rapid, one-step kit-based syntheses of 68Ga-biomolecules for molecular PET imaging. LC-MS and 1H, 13C{1H} and 71Ga NMR studies of HBED complexes of Ga3+ showed that under the analytical conditions employed in this study, multiple HBED-bound Ga complexes exist. X-ray diffraction data indicated that crystals isolated from these solutions contained octahedral [Ga(HBED)(H2O)], with HBED coordinated in a pentadentate N2O3 mode, with only one phenolic group coordinated to Ga3+, and the remaining coordination site occupied by a water molecule.

MicroPET/CT Imaging of AXL Downregulation by HSP90 Inhibition in Triple-Negative Breast Cancer

AXL receptor tyrosine kinase is overexpressed in a number of solid tumor types including triple-negative breast cancer (TNBC). AXL is considered an important regulator of epithelial-to-mesenchymal transition (EMT) and a potential therapeutic target for TNBC. In this work, we used microPET/CT with ⁶⁴Cu-labeled anti-human AXL antibody (⁶⁴Cu-anti-hAXL) to noninvasively interrogate the degradation of AXL in vivo in response to 17-allylamino-17-demethoxygeldanamycin (17-AAG), a potent inhibitor of HSP90. 17-AAG treatment caused significant decline in AXL expression in orthotopic TNBC MDA-MB-231 tumors, inhibited EMT, and delayed tumor growth in vivo, resulting in significant reduction in tumor uptake of ⁶⁴Cu-anti-hAXL as clearly visualized by microPET/CT. Our data indicate that ⁶⁴Cu-anti-hAXL can be useful for monitoring anti-AXL therapies and for assessing inhibition of HSP90 molecular chaperone using AXL as a molecular surrogate.

Development and preclinical studies of 64Cu-NOTA-pertuzumab F(ab')2 for imaging changes in tumor HER2 expression associated with response to trastuzumab by PET/CT

We previously reported that microSPECT/CT imaging with ¹¹¹In-labeled pertuzumab detected decreased HER2 expression in human breast cancer (BC) xenografts in athymic mice associated with response to treatment with trastuzumab (Herceptin). Our aim was to extend these results to PET/CT by constructing F(ab')₂ of pertuzumab modified with NOTA chelators for complexing ⁶⁴Cu. The effect of the administered mass (5-200 µg) of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ was studied in NOD/SCID mice engrafted with HER2-positive SK-OV-3 human ovarian cancer xenografts. Biodistribution studies were performed in non-tumor bearing Balb/c mice to predict radiation doses to normal organs in humans. Serial PET/CT imaging was conducted on mice engrafted with HER2-positive and trastuzumab-sensitive BT-474 or trastuzumab-insensitive SK-OV-3 xenografted mice treated with weekly doses of trastuzumab. There were no significant effects of the administered mass of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ on tumor or normal tissue uptake. The predicted total body dose in humans was 0.015 mSv/MBq, a 3.3-fold reduction compared to ¹¹¹In-labeled pertuzumab. MicroPET/CT images revealed specific tumor uptake of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ at 24 or 48 h post-injection in mice with SK-OV-3 tumors. Image analysis of mice treated with trastuzumab showed 2-fold reduced uptake of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ in BT-474 tumors after 1 week of trastuzumab normalized to baseline, and 1.9-fold increased uptake in SK-OV-3 tumors after 3 weeks of trastuzumab, consistent with tumor response and resistance, respectively. We conclude that PET/CT imaging with ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ detected changes in HER2 expression in response to trastuzumab while delivering a lower total body radiation dose compared to ¹¹¹In-labeled pertuzumab.

Development of Companion Diagnostics

The goal of individualized and targeted treatment and precision medicine requires the assessment of potential therapeutic targets to direct treatment selection. The biomarkers used to direct precision medicine, often termed companion diagnostics, for highly targeted drugs have thus far been almost entirely based on in vitro assay of biopsy material. Molecular imaging companion diagnostics offer a number of features complementary to those from in vitro assay, including the ability to measure the heterogeneity of each patient's cancer across the entire disease burden and to measure early changes in response to treatment. We discuss the use of molecular imaging methods as companion diagnostics for cancer therapy with the goal of predicting response to targeted therapy and measuring early (pharmacodynamic) response as an indication of whether the treatment has "hit" the target. We also discuss considerations for probe development for molecular imaging companion diagnostics, including both small-molecule probes and larger molecules such as labeled antibodies and related constructs. We then describe two examples where both predictive and pharmacodynamic molecular imaging markers have been tested in humans: endocrine therapy for breast cancer and human epidermal growth factor receptor type 2-targeted therapy. The review closes with a summary of the items needed to move molecular imaging companion diagnostics from early studies into multicenter trials and into the clinic.

PI3K-independent mTOR activation promotes lapatinib resistance and IAP expression that can be effectively reversed by mTOR and Hsp90 inhibition

Although HER2 targeted therapies have substantially improved outcomes in HER2 overexpressing (HER2+) breast cancer, resistance to these therapies remains a clinical challenge. To better understand the mechanisms of resistance to lapatinib, a HER2 and EGFR dual kinase inhibitor, we treated HER2+ breast cancer cells with lapatinib for an extended period to generate a lapatinib-resistant (LapR) cell line model and examined cancer-promoting signaling activation in LapR cells. We found that LapR cells possess enhanced mTOR activation, which was independent of PI3K and other known mTOR activators. Lapatinib resistance could be reversed by mTOR kinase inhibition. Intriguingly, LapR cells had constitutive cytosolic cytochrome C, indicating that LapR cells suppress lapatinib-induced apoptosis downstream of cytochrome C release from mitochondria into the cytosol rather than by preventing its release into the cytosol. Consistent with this notion, LapR cells possessed increased levels of 2 of the inhibitors of apoptosis (IAPs), survivin and c-IAP-2, which are reported to block caspase activation downstream of cytosolic cytochrome C release. Further, treatment with the mTOR kinase inhibitor AZD8055 or the Hsp90 inhibitor 17-AAG reversed expression of IAPs and overcame lapatinib resistance in LapR cells. Together, these data suggest that suppression of apoptosis downstream of cytosolic cytochrome C release, possibly through increased expression of IAPs or other caspase-suppressing proteins, may promote lapatinib resistance. Further, PI3K is thought to be the main driver of lapatinib resistance, but our findings indicate that PI3K inhibitors may be ineffective in some lapatinib-resistant HER2+ breast cancers with PI3K-independent activation of mTOR kinase, which may instead benefit from mTOR or Hsp90 inhibitors.

Bench to bedside molecular functional imaging in translational cancer medicine: to image or to imagine?

Ongoing research on malignant and normal cell biology has substantially enhanced the understanding of the biology of cancer and carcinogenesis. This has led to the development of methods to image the evolution of cancer, target specific biological molecules, and study the anti-tumour effects of novel therapeutic agents. At the same time, there has been a paradigm shift in the field of oncological imaging from purely structural or functional imaging to combined multimodal structure-function approaches that enable the assessment of malignancy from all aspects (including molecular and functional level) in a single examination. The evolving molecular functional imaging using specific molecular targets (especially with combined positron-emission tomography [PET] computed tomography [CT] using 2- [(18)F]-fluoro-2-deoxy-D-glucose [FDG] and other novel PET tracers) has great potential in translational research, giving specific quantitative information with regard to tumour activity, and has been of pivotal importance in diagnoses and therapy tailoring. Furthermore, molecular functional imaging has taken a key place in the present era of translational cancer research, producing an important tool to study and evolve newer receptor-targeted therapies, gene therapies, and in cancer stem cell research, which could form the basis to translate these agents into clinical practice, popularly termed "theranostics". Targeted molecular imaging needs to be developed in close association with biotechnology, information technology, and basic translational scientists for its best utility. This article reviews the current role of molecular functional imaging as one of the main pillars of translational research.

High contrast tumor imaging with radio-labeled antibody Fab fragments tailored for optimized pharmacokinetics via PASylation

Although antigen-binding fragments (Fabs) of antibodies constitute established tracers for in vivo radiodiagnostics, their functionality is hampered by a very short circulation half-life. PASylation, the genetic fusion with a long, conformationally disordered amino acid chain comprising Pro, Ala and Ser, provides a convenient way to expand protein size and, consequently, retard renal filtration. Humanized αHER2 and αCD20 Fabs were systematically fused with 100 to 600 PAS residues and produced in E. coli. Cytofluorimetric titration analysis on tumor cell lines confirmed that antigen-binding activities of the parental antibodies were retained. The radio-iodinated PASylated Fabs were studied by positron emission tomography (PET) imaging and biodistribution analysis in mouse tumor xenograft models. While the unmodified αHER2 and αCD20 Fabs showed weak tumor uptake (0.8% and 0.2% ID/g, respectively; 24 h p.i.) tumor-associated radioactivity was boosted with increasing PAS length (up to 9 and 26-fold, respectively), approaching an optimum for Fab-PAS₄₀₀. Remarkably, 6- and 5-fold higher tumor-to-blood ratios compared with the unmodified Fabs were measured in the biodistribution analysis (48 h p.i.) for αHER2 Fab-PAS₁₀₀ and Fab-PAS₂₀₀, respectively. These findings were confirmed by PET studies, showing high imaging contrast in line with tumor-to-blood ratios of 12.2 and 5.7 (24 h p.i.) for αHER2 Fab-PAS₁₀₀ and Fab-PAS₂₀₀. Even stronger tumor signals were obtained with the corresponding αCD20 Fabs, both in PET imaging and biodistribution analysis, with an uptake of 2.8% ID/g for Fab-PAS₁₀₀vs. 0.24% ID/g for the unmodified Fab. Hence, by engineering Fabs via PASylation, plasma half-life can be tailored to significantly improve tracer uptake and tumor contrast, thus optimally matching reagent/target interactions.

Gallium-68-labeled anti-HER2 single-chain Fv fragment: development and in vivo monitoring of HER2 expression

PURPOSE

We aimed to develop a gallium-68 (Ga-68)-labeled single-chain variable fragment (scFv) targeting the human epidermal growth factor receptor 2 (HER2) to rapidly and noninvasively evaluate the status of HER2 expression.

PROCEDURES

Anti-HER2 scFv was labeled with Ga-68 by using deferoxamine (Df) as a bifunctional chelate. Biodistribution of [(68)Ga]Df-anti-HER2 scFv was examined with tumor-bearing mice and positron emission tomography (PET) imaging was performed. The changes in HER2 expression after anti-HER2 therapy were monitored by PET imaging.

RESULTS

[(68)Ga]Df-anti-HER2 scFv was obtained with high radiochemical yield after only a 5-min reaction at room temperature. The probe showed high accumulation in HER2-positive xenografts and the intratumoral distribution of radioactivity coincided with HER2-positive regions. Furthermore, [(68)Ga]Df-anti-HER2 scFv helped visualize HER2-positive xenografts and monitor the changes in HER2 expression after anti-HER2 therapy.

CONCLUSION

[(68)Ga]Df-anti-HER2 scFv could be a promising probe to evaluate HER2 status by in vivo PET imaging, unless trastuzumab is prescribed as part of the therapy.

⁸⁹Zr-Labeled Versus ¹²⁴I-Labeled αHER2 Fab with Optimized Plasma Half-Life for High-Contrast Tumor Imaging In Vivo

Immuno-PET imaging of the tumor antigen HER2/neu allows for the noninvasive detection and monitoring of oncogene expression; such detection and monitoring are of prognostic value in patients with breast cancer. Compared with the full-size antibody trastuzumab, smaller protein tracers with more rapid blood clearance permit higher imaging contrast at earlier time points. Antigen-binding fragments (Fabs) of antibodies with moderately prolonged circulation achieved through the genetic fusion with a long, conformationally disordered chain of the natural amino acids Pro, Ala, and Ser (PASylation)-a biologic alternative to chemical conjugation with polyethylene glycol, PEG-offer a promising tracer format with improved pharmacokinetics for in vivo imaging. Recently, the transition metal radionuclide (89)Zr has attracted increasing interest for immuno-PET studies, complementing the conventional halogen radionuclide (124)I.

METHODS

To allow direct comparison of these 2 radioactive labels for the same protein tracer, the recombinant αHER2 Fab fused with 200 Pro, Ala, and Ser (PAS200) residues was either conjugated with (124)I via an iodination reagent or coupled with deferoxamine (Df) and complexed with (89)Zr. After confirmation of the stability of both radioconjugates and quality control in vitro, immuno-PET and biodistribution studies were performed with CD1-Foxn1(nu) mice bearing HER2-positive human tumor xenografts.

RESULTS

(89)Zr⋅Df-Fab-PAS200 and (124)I-Fab-PAS200 showed specific tumor uptake of 11 and 2.3 percentage injected dose per gram 24 h after injection, respectively; both led to high tumor-to-blood (3.6 and 4.4, respectively) and tumor-to-muscle (20 and 43, respectively) ratios. With regard to off-target accumulation, overt (124)I activity was seen in the thyroid, as expected, whereas high kidney uptake was evident for (89)Zr; the latter was probably due to glomerular filtration and reabsorption of the protein tracer in proximal tubular cells.

CONCLUSION

Both (89)Zr- and (124)I-labeled versions of αHER2 Fab-PAS200 allowed PET tumor imaging with high contrast. With its residualizing radiometal, the tracer (89)Zr⋅Df-Fab-PAS200 showed better in vivo stability and higher tumor uptake.

Molecular imaging using PET for breast cancer

Molecular imaging can visualize the biological processes at the molecular and cellular levels in vivo using certain tracers for specific molecular targets. Molecular imaging of breast cancer can be performed with various imaging modalities, however, positron emission tomography (PET) is a sensitive and non-invasive molecular imaging technology and this review will focus on PET molecular imaging of breast cancer, such as FDG-PET, FLT-PET, hormone receptor PET, and anti-HER2 PET.

Monitoring therapeutic response of human ovarian cancer to trastuzumab by SPECT imaging with (99m)Tc-peptide-Z(HER2:342)

INTRODUCTION

Patients with human epidermal growth factor receptor 2 (HER2)-positive cancer are candidates for treatment with the anti-HER2 antibody trastuzumab. How to systemically assess tumor HER2 expression and identifying appropriate use of anti-HER2 therapies by noninvasive imaging in vivo is an urgent issue. The purpose of this study was to evaluate SPECT imaging of (99m)Tc-Gly-(D)Ala-Gly-Gly-Z(HER2:342) ((99m)Tc-peptide-Z(HER2:342)) for monitoring therapeutic response to trastuzumab in nude mice bearing HER2-positive SKOV-3 xenografts.

METHODS

Nude mice bearing HER2-positive SKOV-3 xenografts were treated with trastuzumab (treatment group) or saline (control) with ten mice in each group. Mice in trastuzumab-treated group were given trastuzumab intraperiotoneally 4 mg/kg on day 1 and 2 mg/kg on day 8; Mice in control group were given physiological saline on day 1 and 8. Mice body weights and tumour volume were monitored every three days during treatment. In vivo SPECT imaging was performed in mice of the two groups using (99m)Tc-peptide-Z(HER2:342) before treatment, on day 8 and 15 after treatment. Radiolabeled probe uptake in tumours was measured as the ratio of radioactive counts in the tumour to that in the contralateral equivalent region (T/NT). After SPECT imaging on day 15, all the mice were euthanized, biodistribution studies of the SKOV-3 xenografts were carried out to validate the imaging results and HER2 expression of the transplanted tumours was analyzed by immunohistochemistry (IHC). Correlation analysis was performed between T/NT ratios acquired by in vivo SPECT imaging on day 15 and the HER2 level of tumours. In vitro cell binding capacity of (99m)Tc-Z(HER2:342) with SKOV-3 cells in the absence and presence of varying amount of trastuzumab were also conducted in the study.

RESULTS

Twenty mice body weight in the two groups gradually increased during treatment, but there was no statistical difference (p > 0.05). Though volumes of SKOV-3 xenografts gradually increased in each group during the treatment, the transplanted tumours in trastuzumab-treated group had a slower growth than those in control group (p < 0.05). Compared with the baseline, the results of in vivo imaging showed that radionuclide accumulation in transplanted tumours reduced significantly in trastuzumab-treated group after treatment (p < 0.05), whereas the tumour accumulation in control group increased after treatment. Biodistribution studies demonstrated that the results corresponded well with in vivo imaging data. Immunohistochemical staining confirmed the significant reduction in tumor HER2 level upon trastuzumab treatment, and there was an obviously positive correlation between T/NT ratios and HER2 level of tumours with correlation coefficient rs = 0.919, p < 0.05. There was no significant significance in cell binding ratios between varying amount of trastuzumab and the absence of trastuzumab (p > 0.05).

CONCLUSIONS

The early response to trastuzumab in mice bearing SKOV-3 xenografts was successfully monitored by SPECT imaging using (99m)Tc-peptide-Z(HER2:342). This approach may be valuable in monitoring the therapeutic response in HER 2-positive tumours under HER2-targeted therapy.

Development of the designed ankyrin repeat protein (DARPin) G3 for HER2 molecular imaging

PURPOSE

Human epidermal growth factor receptor-2 (HER2) overexpression is a predictor of response to anti-HER2 therapy in breast and gastric cancer. Currently, HER2 status is assessed by tumour biopsy, but this may not be representative of the larger tumour mass or other metastatic sites, risking misclassification and selection of suboptimal therapy. The designed ankyrin repeat protein (DARPin) G3 binds HER2 with high affinity at an epitope that does not overlap with trastuzumab and is biologically inert. We hypothesized that radiolabelled DARPin G3 would be capable of selectively imaging HER2-positive tumours, and aimed to identify a suitable format for clinical application.

METHODS

G3 DARPins tagged with hexahistidine (His6) or with histidine glutamate (HE)3 and untagged G3 DARPins were manufactured using a GMP-compatible Pichia pastoris protocol and radiolabelled with (125)I, or with (111)In via DOTA linked to a C-terminal cysteine. BALB/c mice were injected with radiolabelled G3 and tissue biodistribution was evaluated by gamma counting. The lead construct ((HE)3-G3) was assessed in mice bearing HER2-positive human breast tumour (BT474) xenografts.

RESULTS

For both isotopes, (HE)3-G3 had significantly lower liver uptake than His6-G3 and untagged G3 counterparts in non-tumour-bearing mice, and there was no significantly different liver uptake between His6-G3 and untagged G3. (HE)3-G3 was taken forward for evaluation in mice bearing HER2-positive tumour xenografts. The results demonstrated that radioactivity from (111)In-(HE)3-G3 was better maintained in tumours and cleared faster from serum than radioactivity from (125)I-(HE)3-G3, achieving superior tumour-to-blood ratios (343.7 ± 161.3 vs. 22.0 ± 11.3 at 24 h, respectively). On microSPECT/CT, (111)In-labelled and (125)I-labelled (HE)3-G3 could image HER2-positive tumours at 4 h after administration, but there was less normal tissue uptake of radioactivity with (111)In-(HE)3-G3. Preadministration of trastuzumab did not affect the uptake of (HE)3-G3 by HER2-positive tumours.

CONCLUSION

Radiolabelled DARPin (HE)3-G3 is a versatile radioligand with potential to allow the acquisition of whole-body HER2 scans on the day of administration.

Molecular imaging in the era of personalized medicine

Clinical imaging creates visual representations of the body interior for disease assessment. The role of clinical imaging significantly overlaps with that of pathology, and diagnostic workflows largely depend on both fields. The field of clinical imaging is presently undergoing a radical change through the emergence of a new field called molecular imaging. This new technology, which lies at the intersection between imaging and molecular biology, enables noninvasive visualization of biochemical processes at the molecular level within living bodies. Molecular imaging differs from traditional anatomical imaging in that biomarkers known as imaging probes are used to visualize target molecules-of-interest. This ability opens up exciting new possibilities for applications in oncologic, neurological and cardiovascular diseases. Molecular imaging is expected to make major contributions to personalized medicine by allowing earlier diagnosis and predicting treatment response. The technique is also making a huge impact on pharmaceutical development by optimizing preclinical and clinical tests for new drug candidates. This review will describe the basic principles of molecular imaging and will briefly touch on three examples (from an immense list of new techniques) that may contribute to personalized medicine: receptor imaging, angiogenesis imaging, and apoptosis imaging.

[Molecular breast imaging. An update]

CLINICAL/METHODICAL ISSUE

The aim of molecular imaging is to visualize and quantify biological, physiological and pathological processes at cellular and molecular levels. Molecular imaging using various techniques has recently become established in breast imaging.

STANDARD RADIOLOGICAL METHODS

Currently molecular imaging techniques comprise multiparametric magnetic resonance imaging (MRI) using dynamic contrast-enhanced MRI (DCE-MRI), diffusion-weighted imaging (DWI), proton MR spectroscopy ((1)H-MRSI), nuclear imaging by breast-specific gamma imaging (BSGI), positron emission tomography (PET) and positron emission mammography (PEM) and combinations of techniques (e.g. PET-CT and multiparametric PET-MRI).

METHODICAL INNOVATIONS

Recently, novel techniques for molecular imaging of breast tumors, such as sodium imaging ((23)Na-MRI), phosphorus spectroscopy ((31)P-MRSI) and hyperpolarized MRI as well as specific radiotracers have been developed and are currently under investigation.

PRACTICAL RECOMMENDATIONS

It can be expected that molecular imaging of breast tumors will enable a simultaneous assessment of the multiple metabolic and molecular processes involved in cancer development and thus an improved detection, characterization, staging and monitoring of response to treatment will become possible.

Molecular imaging of targeted therapies with positron emission tomography: the visualization of personalized cancer care

INTRODUCTION

Molecular imaging has been defined as the visualization, characterization and measurement of biological processes at the molecular and cellular level in humans and other living systems. In oncology it enables to visualize (part of) the functional behaviour of tumour cells, in contrast to anatomical imaging that focuses on the size and location of malignant lesions. Available molecular imaging techniques include single photon emission computed tomography (SPECT), positron emission tomography (PET) and optical imaging. In PET, a radiotracer consisting of a positron emitting radionuclide attached to the biologically active molecule of interest is administrated to the patient. Several approaches have been undertaken to use PET for the improvement of personalized cancer care. For example, a variety of radiolabelled ligands have been investigated for intratumoural target identification and radiolabelled drugs have been developed for direct visualization of the biodistibution in vivo, including intratumoural therapy uptake. First indications of the clinical value of PET for target identification and response prediction in oncology have been reported. This new imaging approach is rapidly developing, but uniformity of scanning processes, standardized methods for outcome evaluation and implementation in daily clinical practice are still in progress. In this review we discuss the available literature on molecular imaging with PET for personalized targeted treatment strategies.

CONCLUSION

Molecular imaging with radiolabelled targeted anticancer drugs has great potential for the improvement of personalized cancer care. The non-invasive quantification of drug accumulation in tumours and normal tissues provides understanding of the biodistribution in relation to therapeutic and toxic effects.

Visualising dual downregulation of insulin-like growth factor receptor-1 and vascular endothelial growth factor-A by heat shock protein 90 inhibition effect in triple negative breast cancer

PURPOSE

Triple negative breast cancer (TNBC) is biologically characterised by heterogeneous presence of molecular pathways underlying it. Insulin-like growth factor receptor-1 (IGF-1R) expression and vascular endothelial growth factor-A (VEGF-A) have been identified as key factors in these pathways in TNBC. In this study, we aimed at in vivo PET imaging the effect of heat shock protein (Hsp) 90 inhibition by means of NVP-AUY922 on these pathways, with zirconium-89 ((89)Zr) labelled antibodies targeting IGF-1R and VEGF-A.

MATERIALS AND METHODS

In vitro NVP-AUY922 effects on cellular IGF-1R expression and VEGF-A secretion were determined in MCF-7 and MDA-MB-231 cell lines. Moreover human TNBC bearing MDA-MB-231 mice received 50mg/kg NVP-AUY922 or vehicle q3d intraperitoneally for 21days. PET scans with (89)Zr-MAB391 and (89)Zr-bevacizumab for visualisation of IGF-1R and VEGF-A were performed before and during treatment. Ex vivo biodistribution and correlative tissue analyses were performed.

RESULTS

NVP-AUY922 treatment reduced IGF-1R expression and VEGF-A excretion in both cell lines. Hsp90 inhibition lowered tumour uptake on (89)Zr-MAB391-PET by 37.3% (P<0.01) and on (89)Zr-bevacizumab-PET by 44.4% (P<0.01). This was confirmed by ex vivo biodistribution with a reduction of 41.3% injected dose (ID)/g for (89)Zr-MAB391 and 37.8% ID/g for (89)Zr-bevacizumab, while no differences were observed for other tissues. This coincided with reduced IGF-1R expression and mean vessel density in the NVP-AUY922 treated tumours.

CONCLUSION

(89)Zr-MAB391 and (89)Zr-bevacizumab PET reflect effect of Hsp90 inhibitors and can therefore potentially be used to monitor therapeutic effects of Hsp90 inhibitor therapy in TNBC.

Pilot study of 68Ga-DOTA-F(ab')2-trastuzumab in patients with breast cancer

Objective

⁶⁸Ga-1,4,7,10-Tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetic acid (DOTA)-F(ab′)₂-trastuzumab [⁶⁸Ga-DOTA-F(ab′)₂-trastuzumab] has been developed at our institution as a positron imaging reagent for assessing human epidermal growth factor receptor 2 (HER2) expression status by in-vivo imaging. Initial studies on animals demonstrated promising results in the monitoring of treatment response to heat shock protein 90-targeted drugs that inhibit the client protein HER2. We report here our initial clinical experience in the assessment of the toxicity, pharmacokinetics, biodistribution, and dosimetry profile of ⁶⁸Ga-DOTA-F(ab′)₂-trastuzumab with PET/computed tomography using a mean of 236 MBq/5 mg administered intravenously.

Materials and methods

A group of 16 women with breast cancer were enrolled in this study. The one patient who did not receive ⁶⁸Ga-DOTA-F(ab′)₂-trastuzumab was excluded from analysis. Both HER2-negative (n=7) and HER2-positive (n=8) cases were studied. Among the latter, seven had undergone trastuzumab treatment previously and one had not.

Results

It was determined that ⁶⁸Ga-DOTA-F(ab′)₂-trastuzumab was well tolerated, with a T_½ of ∼3.6±0.9 h; the critical organ was the kidney, with a mean dose of 0.383 cGy/37 MBq; and tumor targeting was seen in 4/8 patients with HER2-positive disease.

Conclusion

The reagent is safe, and assessments through additional studies in a better-defined group of patients, using larger administered masses of antibodies, with a better immunoreactive fraction are needed.

In Vitro and In Vivo Pre-Clinical Analysis of a F(ab')(2) Fragment of Panitumumab for Molecular Imaging and Therapy of HER1 Positive Cancers

BACKGROUND

The objective of this study was to characterize the in vitro and in vivo properties of the F(ab')(2) fragment of panitumumab and to investigate its potential for imaging and radioimmunotherapy.

METHODS

The panitumumab F(ab')(2) was generated by enzymatic pepsin digestion. After the integrity and immunoreactivity of the F(ab')(2) was evaluated, the fragment was radiolabeled. In vivo studies included direct quantitation of tumor targeting and normal organ distribution of the radiolabeled panitumumab F(ab')(2) as well as planar γ-scintigraphy and PET imaging.

RESULTS

The panitumumab F(ab')(2) was successfully produced by peptic digest. The F(ab')(2) was modified with the CHX-A"-DTPA chelate and efficiently radiolabeled with either (111)In or (86)Y. In vivo tumor targeting was achieved with acceptable uptake of radioactivity in the normal organs. The tumor targeting was validated by both imaging modalities with good visualization of the tumor at 24 h.

CONCLUSIONS

The panitumumab F(ab')(2) fragment is a promising candidate for imaging of HER1 positive cancers.

Positron emission tomography imaging with 18F-labeled ZHER2:2891 affibody for detection of HER2 expression and pharmacodynamic response to HER2-modulating therapies

PURPOSE

Expression of HER2 has profound implications on treatment strategies in various types of cancer. We investigated the specificity of radiolabeled HER2-targeting ZHER2:2891 Affibody, [(18)F]GE-226, for positron emission tomography (PET) imaging.

EXPERIMENTAL DESIGN

Intrinsic cellular [(18)F]GE-226 uptake and tumor-specific tracer binding were assessed in cells and xenografts with and without drug treatment. Specificity was further determined by comparing tumor localization of a fluorescently labeled analogue with DAKO HercepTest.

RESULTS

[(18)F]GE-226 uptake was 11- to 67-fold higher in 10 HER2-positive versus HER2-negative cell lines in vitro independent of lineage. Uptake in HER2-positive xenografts was rapid with net irreversible binding kinetics making possible the distinction of HER2-negative [MCF7 and MCF7-p95HER2: NUV60 (%ID/mL) 6.1 ± 0.7; Ki (mL/cm(3)/min) 0.0069 ± 0.0014] from HER2-positive tumors (NUV60 and Ki: 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) within 1 hour. Tumor uptake correlated with HER2 expression determined by ELISA (r(2) = 0.78), and a fluorophore-labeled tracer analogue colocalized with HER2 expression. Tracer uptake was not influenced by short-term or continuous treatment with trastuzumab in keeping with differential epitope binding, but reflected HER2 degradation by short-term NVP-AUY922 treatment in SKOV-3 xenografts (NUV60: 13.5 ± 2.1 %ID/mL vs. 9.0 ± 0.9 %ID/mL for vehicle or drug, respectively).

CONCLUSIONS

[(18)F]GE-226 binds with high specificity to HER2 independent of cell lineage. The tracer has potential utility for HER2 detection, irrespective of prior trastuzumab treatment, and to discern HSP90 inhibitor-mediated HER2 degradation.

Anti-CEA antibody fragments labeled with [(18)F]AlF for PET imaging of CEA-expressing tumors

A facile and rapid method to label peptides with (18)F based on chelation of [(18)F]AlF has been developed recently. Since this method requires heating to 100 °C, it cannot be used to label heat-sensitive proteins. Here, we used a two-step procedure to prepare (18)F-labeled heat-labile proteins using the [(18)F]AlF method based on hot maleimide conjugation. 1,4,7-Triazacyclononae-1,4-diacetate (NODA) containing a methyl phenylacetic acid group (MPA) functionalized with N-(2-aminoethyl)maleimide (EM) was used as a ligand which was labeled with [(18)F]AlF and then conjugated to the humanized anti-CEA antibody derivatives hMN-14-Fab', hMN-14-(scFv)2 (diabody), and a Dock-and-Lock engineered dimeric fragment hMN-14 Fab-AD2 at room temperature. The in vivo tumor targeting characteristics of the (18)F-labeled antibody derivatives were determined by PET imaging of mice with s.c. xenografts. NODA-MPAEM was radiolabeled with [(18)F]AlF at a specific activity of 29-39 MBq/nmol and a labeling efficiency of 94 ± 2%. The labeling efficiencies of the maleimide conjugation ranged from 70% to 77%, resulting in [(18)F]AlF-labeled hMN14-Fab', hMN14-Fab-AD2, or hMN14-diabody with a specific activity of 15-17 MBq/nmol. The radiolabeled conjugates were purified by gel filtration. For biodistribution and microPET imaging, antibody fragments were injected intravenously into BALB/c nude mice with s.c. CEA-expressing LS174T xenografts (right flank) and CEA-negative SK-RC-52 xenografts (left flank). All [(18)F]AlF-labeled conjugates showed specific uptake in the LS174T xenografts with a maximal tumor uptake of 4.73% ID/g at 4 h after injection. Uptake in CEA-negative SK-RC-52 xenografts was significantly lower. Tumors were clearly visualized on microPET images. Using a [(18)F]AlF-labeled maleimide functionalized chelator, antibody fragments could be radiofluorinated within 4 h at high specific activity. Here, we translated this method to preclinical PET imaging studies and showed feasibility of [(18)F]AlF-fluorinated hMN-14-Fab', [(18)F]AlF-hMN-14-Fab-AD2, and [(18)F]AlF-hMN-14-diabody for microPET imaging of CEA-expressing colonic cancer.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Tumour targeting with radiometals for diagnosis and therapy

Use of radiometals in nuclear oncology is a rapidly growing field and encompasses a broad spectrum of radiotracers for imaging via PET (positron emission tomography) or SPECT (single-photon emission computed tomography) and therapy via α, β(-), or Auger electron emission. This feature article opens with a brief introduction to the imaging and therapy modalities exploited in nuclear medicine, followed by a discussion of the multi-component strategy used in radiopharmaceutical development, known as the bifunctional chelate (BFC) method. The modular assembly is dissected into its individual components and each is discussed separately. The concepts and knowledge unique to metal-based designs are outlined, giving insight into how these radiopharmaceuticals are evaluated for use in vivo. Imaging nuclides (64)Cu, (68)Ga, (86)Y, (89)Zr, and (111)In, and therapeutic nuclides (90)Y, (177)Lu, (225)Ac, (213)Bi, (188)Re, and (212)Pb will be the focus herein. Finally, key examples have been extracted from the literature to give the reader a sense of breadth of the field.

Engineered antibodies for molecular imaging of cancer

Antibody technology has transformed drug development, providing robust approaches to producing highly targeted and active therapeutics that can routinely be advanced through clinical evaluation and registration. In parallel, there is an emerging need to access similarly targeted agents for diagnostic purposes, including non-invasive imaging in preclinical models and patients. Antibody engineering enables modification of key properties (immunogenicity, valency, biological inertness, pharmacokinetics, clearance route, site-specific conjugation) in order to produce targeting agents optimized for molecular imaging. Expanded availability of positron-emitting radionuclides has led to a resurgence of interest and applications of immunoPET (immuno-positron emission tomography). Molecular imaging using engineered antibodies and fragments provides a general approach for assessing cell surface phenotype in vivo and stands to play an increasingly important role in cancer diagnosis, treatment selection, and monitoring of molecularly targeted therapeutics.

Tumor-intrinsic and tumor-extrinsic factors impacting hsp90- targeted therapy

In 1994 the first heat shock protein 90 (Hsp90) inhibitor was identified and Hsp90 was reported to be a target for anticancer therapeutics. In the past 18 years there have been 17 distinct Hsp90 inhibitors entered into clinical trial, and the small molecule Hsp90 inhibitors have been highly valuable as probes of the role of Hsp90 and its client proteins in cancer. Although no Hsp90 inhibitor has achieved regulatory approval, recently there has been significant progress in Hsp90 inhibitor clinical development, and in the past year RECIST responses have been documented in HER2-positive breast cancer and EML4-ALK-positive non-small cell lung cancer. All of the clinical Hsp90 inhibitors studied to date are specific in their target, i.e. they bind exclusively to Hsp90 and two related heat shock proteins. However, Hsp90 inhibitors are markedly pleiotropic, causing degradation of over 200 client proteins and impacting critical multiprotein complexes. Furthermore, it has only recently been appreciated that Hsp90 inhibitors can, paradoxically, cause transient activation of the protein kinase clients they are chaperoning, resulting in initiation of signal transduction and significant physiological events in both tumor and tumor microenvironment. An additional area of recent progress in Hsp90 research is in studies of the posttranslational modifications of Hsp90 itself and Hsp90 co-chaperone proteins. Together, a picture is emerging in which the impact of Hsp90 inhibitors is shaped by the tumor intracellular and extracellular milieu, and in which Hsp90 inhibitors impact tumor and host on a microenvironmental and systems level. Here we review the tumor intrinsic and extrinsic factors that impact the efficacy of small molecules engaging the Hsp90 chaperone machine.

Camelid single-domain antibody-fragment engineering for (pre)clinical in vivo molecular imaging applications: adjusting the bullet to its target

INTRODUCTION

Molecular imaging is a fast developing field and there is a growing need for specific imaging tracers in the clinic. Camelid single-domain antibody-fragments (sdAbs) recently emerged as a new class of molecular imaging tracers.

AREAS COVERED

We review the importance of molecular imaging in the clinic and the use of camelid sdAbs as in vivo molecular imaging tracers. Interest in imaging tracers based on antibody fragments or man-made protein scaffolds expanded over the last years. Camelid sdAbs are small, monomeric binding fragments that are derived from unique heavy-chain-only antibodies. In vivo imaging studies with sdAbs targeting various cell membrane receptors in different disease models have been reported and more sdAb imaging tracers are under development. The first clinical trial with a camelid sdAb as a molecular imaging tracer targeting the breast cancer marker Human Epidermal growth factor Receptor 2 is currently ongoing.

EXPERT OPINION

We expect that the development and use of sdAbs as tracers for both preclinical and clinical molecular imaging applications will become widespread.

Molecular pathways and molecular imaging in breast cancer: an update

Breast cancer is a heterogenic cancer being characterized by a variability of somatic mutations and in particular by different receptor expressions, such as estrogen, progesterone and human epidermal receptor. These phenotype characteristics play a crucial role in determining tumour response to various chemotherapies and other treatments and in the development of resistance to therapies. Positron emission tomography (PET) as a nuclear medicine technique, has recently demonstrated the advantages in determining the severity of disease and in evaluating the efficacy of treatments in a variety of neoplasm, including breast cancer. Because this procedure is able to pinpoint molecular activity within the body, it offers the potential to identify disease in its earliest stages as well as a patient's immediate response to therapeutic interventions in a non-invasive way. In this paper we performed an extended view about the correlation between molecular factors of breast cancer and PET tracers; in particular, we focalized our attention on their possible advantages in terms of 1) early detection of primary or recurrent cancer; 2) as a guide for target therapies and 3) for the evaluation of response to specific and now-available molecular treatments.

Monitoring afatinib treatment in HER2-positive gastric cancer with 18F-FDG and 89Zr-trastuzumab PET

We evaluated the ability of the PET imaging agent (89)Zr-trastuzumab to delineate HER2-positive gastric cancer and to monitor the pharmacodynamic effects of the epidermal growth factor receptor (EGFR)/human epidermal growth factor receptor 2 (HER2) tyrosine kinase inhibitor afatinib.

METHODS

Using (89)Zr-trastuzumab, (18)F-FDG, or 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT PET), we imaged HER2-positive NCI-N87 and HER2-negative MKN74 gastric cancer xenografts in mice. Next, we examined the pharmacodynamic effects of afatinib in NCI-N87 xenografts using (89)Zr-trastuzumab and (18)F-FDG PET and comparing imaging results to changes in tumor size and in protein expression as monitored by Western blot and histologic studies.

RESULTS

Although (18)F-FDG uptake in NCI-N87 tumors did not change, a decrease in (89)Zr-trastuzumab uptake was observed in the afatinib-treated versus control groups (3.0 ± 0.0 percentage injected dose per gram (%ID/g) vs. 21.0 ± 3.4 %ID/g, respectively; P < 0.05). (89)Zr-trastuzumab PET results corresponded with tumor reduction, apoptosis, and downregulation of HER2 observed on treatment with afatinib. Downregulation of total HER2, phosphorylated (p)-HER2, and p-EGFR occurred within 24 h of the first dose of afatinib, with a sustained effect over 21 d of treatment.

CONCLUSION

Afatinib demonstrated antitumor activity in HER2-positive gastric cancer in vivo. (89)Zr-trastuzumab PET specifically delineated HER2-positive gastric cancer and can be used to measure the pharmacodynamic effects of afatinib.