Molecular Imaging as a Tool for Personalized and Targeted Anticancer Therapy

Response Evaluation of Chemotherapy for Lung Cancer

Assessing response to therapy allows for prospective end point evaluation in clinical trials and serves as a guide to clinicians for making decisions. Recent prospective and randomized trials suggest the development of imaging techniques and introduction of new anti-cancer drugs. However, the revision of methods, or proposal of new methods to evaluate chemotherapeutic response, is not enough. This paper discusses the characteristics of the Response Evaluation Criteria In Solid Tumor (RECIST) version 1.1 suggested in 2009 and used widely by experts. It also contains information about possible dilemmas arising from the application of response assessment by the latest version of the response evaluation method, or recently introduced chemotherapeutic agents. Further data reveals the problems and limitations caused by applying the existing RECIST criteria to anti-cancer immune therapy, and the application of a new technique, immune related response criteria, for the response assessment of immune therapy. Lastly, the paper includes a newly developing response evaluation method and suggests its developmental direction.

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.

Advancing novel molecular imaging agents from preclinical studies to first-in-humans phase I clinical trials in academia--a roadmap for overcoming perceived barriers

There is a critical need to advance promising novel molecular imaging (MI) agents for cancer from preclinical studies to first-in-humans Phase I clinical trials in order to realize their full potential for cancer detection and for predicting or monitoring response to targeted ("personalized") cancer therapies. Steps to clinical translation include radiopharmaceutical formulation, preclinical pharmacology and toxicology studies, clinical trial design and human ethics approval, and regulatory agency submission. In this Topical Review, we provide a "roadmap" to advancing one class of novel MI agents to Phase I trials in academia and illustrate the processes that we have successfully applied for (111)In-labeled pertuzumab, a MI probe for monitoring response of HER2-positive breast cancer to treatment with trastuzumab (Herceptin). We hope that our experience will encourage other academic radiopharmaceutical scientists to embrace this challenge.

Visualization of molecular composition and functionality of cancer cells using nanoparticle-augmented ultrasound-guided photoacoustics

Assessment of molecular signatures of tumors in addition to their anatomy and morphology is desired for effective diagnostic and therapeutic procedures. Development of in vivo imaging techniques that can identify and monitor molecular composition of tumors remains an important challenge in pre-clinical research and medical practice. Here we present a molecular photoacoustic imaging technique that can visualize the presence and activity of an important cancer biomarker - epidermal growth factor receptor (EGFR), utilizing the effect of plasmon resonance coupling between molecular targeted gold nanoparticles. Specifically, spectral analysis of photoacoustic images revealed profound changes in the optical absorption of systemically delivered EGFR-targeted gold nanospheres due to their molecular interactions with tumor cells overexpressing EGFR. In contrast, no changes in optical properties and, therefore, photoacoustic signal, were observed after systemic delivery of non-targeted gold nanoparticles to the tumors. The results indicate that multi-wavelength photoacoustic imaging augmented with molecularly targeted gold nanoparticles has the ability to monitor molecular specific interactions between nanoparticles and cell-surface receptors, allowing visualization of the presence and functional activity of tumor cells. Furthermore, the approach can be used for other cancer cell-surface receptors such as human epidermal growth factor receptor 2 (HER2). Therefore, ultrasound-guided molecular photoacoustic imaging can potentially aid in tumor diagnosis, selection of customized patient-specific treatment, and monitor the therapeutic progression and outcome in vivo.

Improved dopamine transporter binding activity after bone marrow mesenchymal stem cell transplantation in a rat model of Parkinson's disease: small animal positron emission tomography study with F-18 FP-CIT

OBJECTIVES

We evaluated the effects of bone marrow-derived mesenchymal stem cells (BMSCs) in a model of Parkinson's disease (PD) using serial F-18 fluoropropylcarbomethoxyiodophenylnortropane (FP-CIT) PET.

METHODS

Hemiparkinsonian rats were treated with intravenously injected BMSCs, and animals without stem cell therapy were used as the controls. Serial FP-CIT PET was performed after therapy. The ratio of FP-CIT uptake in the lesion side to uptake in the normal side was measured. The changes in FP-CIT uptake were also analyzed using SPM. Behavioural and histological changes were observed using the rotational test and tyrosine hydroxylase (TH)-reactive cells.

RESULTS

FP-CIT uptake ratio was significantly different in the BMSCs treated group (n = 28) at each time point. In contrast, there was no difference in the ratio in control rats (n = 25) at any time point. SPM analysis also revealed that dopamine transporter binding activity was enhanced in the right basal ganglia area in only the BMSC therapy group. In addition, rats that received BMSC therapy also exhibited significantly improved rotational behaviour and preservation of TH-positive neurons compared to controls.

CONCLUSIONS

The therapeutic effect of intravenously injected BMSCs in a rat model of PD was confirmed by dopamine transporter PET imaging, rotational functional studies, and histopathological evaluation.

KEY POINTS

• Mesenchymal stem cells were intravenously injected to treat the PD rats • Dopamine transporter binding activity was improved after stem cell therapy • Stem cell therapy induced functional recovery and preservation of dopaminergic neurons • The effect of stem cells was confirmed by FP-CIT PET.

MicroPET/CT imaging of patient-derived pancreatic cancer xenografts implanted subcutaneously or orthotopically in NOD-scid mice using (64)Cu-NOTA-panitumumab F(ab')2 fragments

INTRODUCTION

Our objective was to study microPET/CT imaging of patient-derived pancreatic cancer xenografts in NOD-scid mice using F(ab')2 fragments of the fully-human anti-EGFR monoclonal antibody, panitumumab (Vectibix) labeled with (64)Cu. More than 90% of pancreatic cancers are EGFR-positive.

METHODS

F(ab')2 fragments were produced by proteolytic digestion of panitumumab IgG or non-specific human IgG, purified by ultrafiltration then modified with NOTA chelators for complexing (64)Cu. Panitumumab IgG and Fab fragments were similarly labeled with (64)Cu. EGFR immunoreactivity was determined in competition and direct (saturation) cell binding assays. The biodistribution of (64)Cu-labeled panitumumab IgG, F(ab')2 and Fab was compared in non-tumor-bearing Balb/c mice. MicroPET/CT and biodistribution studies were performed in NOD-scid mice engrafted subcutaneously (s.c.) or orthotopically with patient-derived OCIP23 pancreatic tumors, or in NOD-scid with s.c. PANC-1 human pancreatic cancer xenografts.

RESULTS

Panitumumab F(ab')2 fragments were produced in high purity (>90%), derivitized with 3.2±0.7 NOTA/F(ab')2, and labeled with (64)Cu (0.3-3.6MBq/μg). The binding of (64)Cu-NOTA-panitumumab F(ab')2 to OCIP23 or PANC-1 cells was decreased significantly by an excess of panitumumab IgG. The Kd for binding of (64)Cu-NOTA-panitumumab F(ab')2 to EGFR on PANC-1 cells was 0.14±0.05nmol/L. F(ab')2 fragments exhibited more suitable normal tissue distribution for tumor imaging with (64)Cu than panitumumab IgG or Fab. Tumor uptake at 48h post injection (p.i.) of (64)Cu-NOTA-panitumumab F(ab')2 was 12.0±0.9% injected dose/g (ID/g) in s.c. and 11.8±0.9% ID/g in orthotopic OCIP23 tumors vs. 6.1±1.1% ID/g in s.c. PANC-1 xenografts. Tumor/Blood (T/B) ratios were 5:1 to 9:1 for OCIP23 and 2.4:1 for PANC-1 tumors. Tumor uptake of (64)Cu-NOTA-non-specific F(ab')2 in OCIP23 xenografts was 5-fold lower than (64)Cu-panitumumab F(ab')2. All tumor xenografts were clearly imaged by microPET/CT at 24 or 48h p.i. of (64)Cu-NOTA-panitumumab F(ab')2.

CONCLUSIONS

(64)Cu-panitumumab F(ab')2 fragments bound with high affinity to EGFR on pancreatic cancer cells in vitro and localized specifically in patient-derived pancreatic cancer xenografts in mice in vivo, allowing tumor visualization by microPET/CT at 24 or 48h p.i.

Polymeric nanoparticles for molecular imaging

Conventional imaging technologies (X-ray computed tomography, magnetic resonance, and optical) depend on contrast agents to visualize a target site or organ of interest. The imaging agents currently used in clinics for diagnosis suffer from disadvantages including poor target specificity and in vivo instability. Consequently, delivery of low concentrations of contrast agents to region of interest affects image quality. Therefore, it is important to selectively deliver high payload of contrast agent to obtain clinically useful images. Nanoparticles offer multifunctional capabilities to transport high concentrations of imaging probes selectively to diseased site inside the body. Polymeric nanoparticles, incorporated with contrast agents, have shown significant benefits in molecular imaging applications. These materials possess the ability to encapsulate different contrast agents within a single matrix enabling multimodal imaging possibilities. The materials can be surface conjugated to target-specific biomolecules for controlling the navigation under in vivo conditions. The versatility of this class of nanomaterials makes them an attractive platform for developing highly sensitive molecular imaging agents. The research community's progress in the area of synthesis of polymeric nanomaterials and their in vivo imaging applications has been noteworthy, but it is still in the pioneer stage of development. The challenges ahead should focus on the design and fabrication of these materials including burst release of contrasts agents, solubility, and stability issues of polymeric nanomaterials.

Influence of macrocyclic chelators on the targeting properties of (68)Ga-labeled synthetic affibody molecules: comparison with (111)In-labeled counterparts

Affibody molecules are a class of small (7 kDa) non-immunoglobulin scaffold-based affinity proteins, which have demonstrated substantial potential as probes for radionuclide molecular imaging. The use of positron emission tomography (PET) would further increase the resolution and quantification accuracy of Affibody-based imaging. The rapid in vivo kinetics of Affibody molecules permit the use of the generator-produced radionuclide ⁶⁸Ga (T_1/2 = 67.6 min). Earlier studies have demonstrated that the chemical nature of chelators has a substantial influence on the biodistribution properties of Affibody molecules. To determine an optimal labeling approach, the macrocyclic chelators 1,4,7,10-tetraazacylododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-N,N,N-triacetic acid (NOTA) and 1-(1,3-carboxypropyl)-1,4,7- triazacyclononane-4,7-diacetic acid (NODAGA) were conjugated to the N-terminus of the synthetic Affibody molecule Z_HER2:S1 targeting HER2. Affibody molecules were labeled with ⁶⁸Ga, and their binding specificity and cellular processing were evaluated. The biodistribution of ⁶⁸Ga-DOTA-Z_HER2:S1,⁶⁸Ga-NOTA-Z_HER2:S1 and ⁶⁸Ga-NODAGA-Z_HER2:S1, as well as that of their ¹¹¹In-labeled counterparts, was evaluated in BALB/C nu/nu mice bearing HER2-expressing SKOV3 xenografts. The tumor uptake for ⁶⁸Ga-DOTA-Z_HER2:S1 (17.9±0.7%IA/g) was significantly higher than for both ⁶⁸Ga-NODAGA-Z_HER2:S1(16.13±0.67%IA/g) and ⁶⁸Ga-NOTA-Z_HER2:S1 (13±3%IA/g) at 2 h after injection. ⁶⁸Ga-NODAGA-Z_HER2:S1 had the highest tumor-to-blood ratio (60±10) in comparison with both ⁶⁸Ga-DOTA-Z_HER2:S1 (28±4) and ⁶⁸Ga-NOTA-Z_HER2:S1 (42±11). The tumor-to-liver ratio was also higher for ⁶⁸Ga-NODAGA-Z_HER2:S1 (7±2) than the DOTA and NOTA conjugates (5.5±0.6 vs.3.3±0.6). The influence of chelator on the biodistribution and targeting properties was less pronounced for ⁶⁸Ga than for ¹¹¹In. The results of this study demonstrate that macrocyclic chelators conjugated to the N-terminus have a substantial influence on the biodistribution of HER2-targeting Affibody molecules labeled with ⁶⁸Ga.This can be utilized to enhance the imaging contrast of PET imaging using Affibody molecules and improve the sensitivity of molecular imaging. The study demonstrated an appreciable difference of chelator influence for ⁶⁸Ga and ¹¹¹In.

Personalized medicine: through the looking glass of functional imaging

Imaging techniques afford the opportunity to personalize chemotherapy delivery by prospectively determining how much of an agent is delivered to which tumor site. Drug distribution can be prescribed by altering the properties of the drug (nontechnology) or the physiology of the host (induction of alterations of blood flow).

Feasibility evaluation of radioimmunoguided surgery of breast cancer

Breast-conserving surgery involves completely excising the tumour while limiting the amount of normal tissue removed, which is technically challenging to achieve, especially given the limited intraoperative guidance available to the surgeon. This study evaluates the feasibility of radioimmunoguided surgery (RIGS) to guide the detection and delineation of tumours intraoperatively. The 3D point-response function of a commercial gamma-ray-detecting probe (GDP) was determined as a function of radionuclide (¹³¹I, ¹¹¹In, ^99mTc), energy-window threshold, and collimator length (0.0–3.0-cm). This function was used to calculate the minimum detectable tumour volumes (MDTVs) and the minimum tumour-to-background activity concentration ratio (T:B) for effective delineation of a breast tumour model. The GDP had larger MDTVs and a higher minimum required T:B for tumour delineation with ¹³¹I than with ¹¹¹In or ^99mTc. It was shown that for ¹¹¹In there was a benefit to using a collimator length of 0.5-cm. For the model used, the minimum required T:B required for effective tumour delineation was 5.2 ± 0.4. RIGS has the potential to significantly improve the accuracy of breast-conserving surgery; however, before these benefits can be realized, novel radiopharmaceuticals need to be developed that have a higher specificity for cancerous tissue in vivo than what is currently available.

68Ga-DOTA-affibody molecule for in vivo assessment of HER2/neu expression with PET

PURPOSE

Overexpression of HER2/neu in breast cancer is correlated with a poor prognosis. It may vary between primary tumors and metastatic lesions and change during the treatment. Therefore, there is a need for a new means to assess HER2/neu expression in vivo. In this work, we used (68)Ga-labeled DOTA-Z(HER2:2891)-Affibody to monitor HER2/neu expression in a panel of breast cancer xenografts.

METHODS

DOTA-Z(HER2:2891)-Affibody molecules were labeled with (68)Ga. In vitro binding was characterized by a receptor saturation assay. Biodistribution and PET imaging studies were conducted in athymic nude mice bearing subcutaneous human breast cancer tumors with three different levels of HER2/neu expression. Nonspecific uptake was analyzed using non-HER2-specific Affibody molecules. Signal detected by PET was compared with ex vivo assessment of the tracer uptake and HER2/neu expression.

RESULTS

The (68)Ga-DOTA-Z(HER2:2891)-Affibody probe showed high binding affinity to MDA-MB-361 cells (K (D) = 1.4 ± 0.19 nM). In vivo biodistribution and PET imaging studies demonstrated high radioactivity uptake in HER2/neu-positive tumors. Tracer was eliminated quickly from the blood and normal tissues, resulting in high tumor-to-blood ratios. The highest concentration of radioactivity in normal tissue was seen in the kidneys (227 ± 14%ID/g). High-contrast PET images of HER2/neu-overexpressing tumors were recorded as soon as 1 h after tracer injection. A good correlation was observed between PET imaging, biodistribution estimates of tumor tracer concentration, and the receptor expression.

CONCLUSION

These results suggest that PET imaging using (68)Ga-DOTA-Z(HER2:2891)-Affibody is sensitive enough to detect different levels of HER2/neu expression in vivo.

A comparison of 111In- or 64Cu-DOTA-trastuzumab Fab fragments for imaging subcutaneous HER2-positive tumor xenografts in athymic mice using microSPECT/CT or microPET/CT

Background

Our objective was to compare ¹¹¹In- or ⁶⁴Cu-DOTA-trastuzumab Fab fragments for imaging small or large s.c. tumor xenografts in athymic mice that display a wide range of human epidermal growth factor receptor-2 (HER2) expression using microSPECT/CT or microPET/CT.

Methods

Trastuzumab Fab were labeled with ¹¹¹In or ⁶⁴Cu by conjugation to 1,4,7,10-tetraazacyclododecane N, N', N'', N'''-tetraacetic acid (DOTA). The purity of ¹¹¹In- and ⁶⁴Cu-DOTA-trastuzumab Fab was measured by SDS-PAGE and HPLC. HER2 binding affinity was determined in saturation radioligand binding assays using SKBR-3 cells (1.3 × 10⁶ HER2/cell). MicroSPECT/CT and microPET/CT were performed in athymic mice bearing s.c. BT-20 and MDA-MB-231 xenografts with low (0.5 to 1.6 × 10⁵ receptors/cell), MDA-MB-361 tumors with intermediate (5.1 × 10⁵ receptors/cell) or SKOV-3 xenografts with high HER2 expression (1.2 × 10⁶ receptors/cell) at 24 h p.i. of 70 MBq (10 μg) of ¹¹¹In-DOTA-trastuzumab Fab or 22 MBq (10 μg) of ⁶⁴Cu-DOTA-trastuzumab Fab or irrelevant ¹¹¹In- or ⁶⁴Cu-DOTA-rituximab Fab. Tumor and normal tissue uptake were quantified in biodistribution studies.

Results

¹¹¹In- and ⁶⁴Cu-DOTA-trastuzumab were > 98% radiochemically pure and bound HER2 with high affinity (K_d = 20.4 ± 2.5 nM and 40.8 ± 3.5 nM, respectively). MDA-MB-361 and SKOV-3 tumors were most clearly imaged using ¹¹¹In- and ⁶⁴Cu-DOTA-trastuzumab Fab. Significantly higher tumor/blood (T/B) ratios were found for ¹¹¹In-DOTA-trastuzumab Fab than ¹¹¹In-DOTA-rituximab Fab for BT-20, MDA-MB-231 and MDA-MB-361 xenografts, and there was a direct association between T/B ratios and HER2 expression. In contrast, tumor uptake of ⁶⁴Cu-DOTA-trastuzumab Fab was significantly higher than ⁶⁴Cu-DOTA-rituximab Fab in MDA-MB-361 tumors but no direct association with HER2 expression was found. Both ¹¹¹In- and ⁶⁴Cu-DOTA-trastuzumab Fab imaged small (5 to 10 mm) or larger (10 to 15 mm) MDA-MB-361 tumors. Higher blood, liver, and spleen radioactivity were observed for ⁶⁴Cu-DOTA-trastuzumab Fab than ¹¹¹In-DOTA-trastuzumab Fab.

Conclusions

We conclude that ¹¹¹In-DOTA-trastuzumab Fab was more specific than ⁶⁴Cu-DOTA-trastuzumab Fab for imaging HER2-positive tumors, especially those with low receptor density. This was due to higher levels of circulating radioactivity for ⁶⁴Cu-DOTA-trastuzumab Fab which disrupted the relationship between HER2 density and T/B ratios. Use of alternative chelators that more stably bind ⁶⁴Cu may improve the association between T/B ratios and HER2 density for ⁶⁴Cu-labeled trastuzumab Fab.

Use of a HEHEHE purification tag instead of a hexahistidine tag improves biodistribution of affibody molecules site-specifically labeled with (99m)Tc, (111)In, and (125)I

Affibody molecules are a class of small (∼7 kDa) robust scaffold proteins suitable for radionuclide molecular imaging in vivo. The attachment of a hexahistidine (His(6))-tag to the Affibody molecule allows facile purification by immobilized metal ion affinity chromatography (IMAC) but leads to high accumulation of radioactivity in the liver. Earlier, we have demonstrated that replacement of the His(6)-tag with the negatively charged histidine-glutamate-histidine-glutamate-histidine-glutamate (HEHEHE)-tag permits purification of Affibody molecules by IMAC, enables labeling with [(99m)Tc(CO)(3)](+), and provides low hepatic accumulation of radioactivity. In this study, we compared the biodistribution of cysteine-containing Affibody molecules site-specifically labeled with (111)In, (99m)Tc, and (125)I at the C-terminus, having a His(6)-tag at the N- or C-terminus or a HEHEHE-tag at the N-terminus. We show that the use of a HEHEHE-tag provides appreciable reduction of hepatic radioactivity, especially for radiometal labels. We hope that this information can also be useful for development of other scaffold protein-based imaging agents.

Metabolic stability of 6,7-dialkoxy-4-(2-, 3- and 4-[18F]fluoroanilino)quinazolines, potential EGFR imaging probes

Epidermal growth factor receptors (EGFR), upregulated in many tumor types, have been a target for therapeutic development and molecular imaging. The objective of this study was to evaluate the distribution and metabolic characteristics of fluorine-18 labeled anilinoquinazolines as potential imaging agents for EGFR tyrosine kinase expression. Fluorine-18 labeled fluoronitrobenzenes were prepared by reaction of potassium cryptand [(18)F]fluoride with 1,2- and 1,4-dinitrobenzenes, and 3-nitro-N,N,N-trimethylanilinium triflate in 5min. Decay-corrected radiochemical yields of [(18)F]fluoride incorporation into the nitro-aromatic compounds were 81±2%, 44±4% and 77±5% (n=3-5) for the 2-, 3- and 4-fluoro isomers, respectively. Sodium borohydride reduction to the corresponding [(18)F]fluoroanilines was achieved with greater than 80% conversion in 5min. Coupling of [(18)F]fluoroaniline-hydrochlorides to 6,7-dimethoxy-4-chloro-quinazoline gave the corresponding 6,7-dimethoxy-4-(2-, 3- and 4-[(18)F]fluoroanilino)quinazolines in 31±5%, 17±2% and 55±2% radiochemical yield, respectively, while coupling to the 6,7-diethoxy-4-chloro-quinazoline produced 6,7-diethoxy-4-(2-, 3- and 4-[(18)F]fluoroanilino)quinazolines in 19±6%, 9±3% and 36±6% radiochemical yield, respectively, in 90min to end of synthesis from [(18)F]fluoride. Biodistribution of 2- and 4-[(18)F]fluoroanilinoquinazolines was conducted in tumor-bearing mice (MDA-MB-435 and MDA-MB-468 xenografts). Low tumor uptake (<1% injected dose per gram (ID/g) of tissue up to 3h postinjection of the radiotracers) was observed. High bone uptake (5-15% ID/g) was noted with the 4-[(18)F]fluoroanilinoquinazolines. The metabolic stabilities of radiolabeled quinazolines were further evaluated by incubation with human female cryopreserved isolated hepatocytes. Rapid degeneration of the 4-fluoro-substituted compounds to baseline polar metabolites was observed by radio-TLC, whereas, the 2- and 3-[(18)F]fluoroaniline derivatives were significantly more stable, up to 2h, corroborating the in vivo biodistribution studies. para-Substituted [(18)F]fluoroanilines, a common structural motif in radiopharmaceuticals, are highly susceptible to metabolic degradation.

Radiolabeled small molecule protein kinase inhibitors for imaging with PET or SPECT

Imaging protein kinase expression with radiolabeled small molecule inhibitors has been actively pursued to monitor the clinical potential of targeted therapeutics and treatments as well as to determine kinase receptor density changes related to disease progression. The goal of the present review is to provide an overview of the breadth of radiolabeled small molecules that have been synthesized to target intracellular protein kinases, not only for imaging in oncology, but also for other areas of interest, particularly the central nervous system. Considerable radiotracer development has focused on imaging receptor tyrosine kinases of growth factors, protein kinases A, B and C, and glycogen synthase kinase-3ß. Design considerations, structural attributes and relevant biological results are summarized.

HEHEHE-tagged affibody molecule may be purified by IMAC, is conveniently labeled with [⁹⁹(m)Tc(CO)₃](+), and shows improved biodistribution with reduced hepatic radioactivity accumulation

Affibody molecules are a class of small (ca. 7 kDa) robust scaffold proteins suitable for radionuclide molecular imaging of therapeutic targets in vivo. A hexahistidine tag at the N-terminus streamlines development of new imaging probes by enabling facile purification using immobilized metal ion affinity chromatography (IMAC), as well as convenient [⁹⁹(m)Tc(CO)₃](+)-labeling. However, previous studies in mice have demonstrated that Affibody molecules labeled by this method yield higher liver accumulation of radioactivity, compared to the same tracer lacking the hexahistidine tag and labeled by an alternative method. Two variants of the HER2-binding Affibody molecule Z(HER)₂(:)₃₄₂ were made in an attempt to create a tagged tracer that could be purified by immobilized metal affinity chromatography, yet would not result in anomalous hepatic radioactivity accumulation following labeling with [⁹⁹(m)Tc(CO)₃](+). In one construct, the hexahistidine tag was moved to the C-terminus. In the other construct, every second histidine residue in the hexahistidine tag was replaced by the more hydrophilic glutamate, resulting in a HEHEHE-tag. Both variants, denoted Z(HER)₂(:)₃₄₂-H₆ and (HE)₃-Z(HER)₂(:)₃₄₂, respectively, could be efficiently purified using IMAC and stably labeled with [⁹⁹(m)Tc(CO)₃](+) and were subsequently compared with the parental H₆-Z(HER)₂(:)₃₄₂ having an N-terminal hexahistidine tag. All three variants were demonstrated to specifically bind to HER2-expressing cells in vitro. The hepatic accumulation of radioactivity in a murine model was 2-fold lower with [⁹⁹(m)Tc(CO)₃](+)-Z(HER2:342)-H₆ compared to [⁹⁹(m)Tc(CO)₃](+)-H₆-Z(HER)₂(:)₃₄₂, and more than 10-fold lower with [⁹⁹(m)Tc(CO)₃](+)-(HE)₃-Z(HER)₂(:)₃₄₂. These differences translated into appreciably superior tumor-to-liver ratio for [⁹⁹(m)Tc(CO)₃](+)-(HE)₃-Z(HER)₂(:)₃₄₂ compared to the alternative conjugates. This information might be useful for development of other scaffold-based molecular imaging probes.

Synthesis and preliminary biological evaluations of [18F]-1-deoxy-1-fluoro-scyllo-inositol

A novel PET radiotracer, [18F]-1-deoxy-1-fluoro-scyllo-inositol, was synthesized via a one-pot reaction in 16 +/- 3% uncorrected radiochemical yield within 80 minutes; although this compound revealed low brain penetration it shows promise in rodent tumour models for breast cancer imaging.

Micro-SPECT/CT with 111In-DTPA-pertuzumab sensitively detects trastuzumab-mediated HER2 downregulation and tumor response in athymic mice bearing MDA-MB-361 human breast cancer xenografts

Pertuzumab is a HER2 dimerization inhibitor that binds to an epitope unique from that of trastuzumab. Our objective was to determine whether SPECT with (111)In-diethylenetriaminepentaacetic acid-pertuzumab ((111)In-DTPA-pertuzumab) could sensitively detect an early molecular response to trastuzumab manifested by HER2 downregulation and a later tumor response revealed by a decreased number of HER2-positive viable tumor cells.

METHODS

Changes in HER2 density in SKBr-3 and MDA-MB-361 BC cells exposed to trastuzumab (14 microg/mL) in vitro were measured by saturation binding assays using (111)In-DTPA-pertuzumab and by confocal immunofluorescence microscopy and flow cytometry with fluorescein isothiocyanate-labeled HER2/neu antibodies. Imaging of HER2 downregulation was studied in vivo in athymic mice with subcutaneous MDA-MB-361 tumors treated for 3 d with trastuzumab (4 mg/kg) or nonspecific human IgG (hIgG) or phosphate-buffered saline (PBS). Imaging of tumor response to trastuzumab was studied in mice bearing subcutaneous MDA-MB-361 xenografts treated with trastuzumab (4 mg/kg), followed by weekly doses of nonspecific hIgG or rituximab or PBS (2 mg/kg). Mice were imaged on a micro-SPECT/CT system at 72 h after injection of (111)In-DTPA-pertuzumab. Tumor and normal-tissue biodistribution was determined.

RESULTS

(111)In-DTPA-pertuzumab saturation binding to SKBr-3 and MDA-MB-361 cells was significantly decreased at 72 h after exposure in vitro to trastuzumab (14 microg/mL), compared with untreated controls (62% +/- 2%, P < 0.0001; 32% +/- 9%, P < 0.0002, respectively). After 3 d of trastuzumab, in vivo tumor uptake of (111)In-DTPA-pertuzumab decreased 2-fold in trastuzumab- versus PBS-treated mice (13.5 +/- 2.6 percentage injected dose per gram [%ID/g] vs. 28.5 +/- 9.1 %ID/g, respectively; P < 0.05). There was also a 2-fold decreased tumor uptake in trastuzumab- versus PBS-treated mice by image volume-of-interest analysis (P = 0.05), suggesting trastuzumab-mediated HER2 downregulation. After 3 wk of trastuzumab, tumor uptake of (111)In-DTPA-pertuzumab decreased 4.5-fold, compared with PBS-treated mice (7.6 +/- 0.4 vs. 34.6 +/- 9.9 %ID/g, respectively; P < 0.001); this decrease was associated with an almost-completed eradication of HER2-positive tumor cells determined immunohistochemically.

CONCLUSION

(111)In-DTPA-pertuzumab sensitively imaged HER2 downregulation after 3 d of treatment with trastuzumab and detected a reduction in viable HER2-positive tumor cells after 3 wk of therapy in MDA-MB-361 human breast cancer xenografts.

Beyond RECIST: molecular and functional imaging techniques for evaluation of response to targeted therapy

The development of targeted therapies is a major breakthrough in the treatment of cancer. By evoking necrosis and cavitation, evaluation based on tumour size alone, as is done in the RECIST criteria, is no longer an adequate method. New molecular and functional imaging techniques are developed. This review focuses on the use of new imaging modalities for the evaluation of treatment response of pathway based targeted therapies. First, the basic principles of functional and molecular imaging modalities are briefly discussed. Thereafter, their clinical application in targeted therapies is correlated to the underlying biological mechanism. In this way, the best method for response evaluation for a new agent can be identified.

Optical contrast agents and imaging systems for detection and diagnosis of cancer

Molecular imaging has rapidly emerged as a discipline with the potential to impact fundamental biomedical research and clinical practice. Within this field, optical imaging offers several unique capabilities, based on the ability of cells and tissues to effect quantifiable changes in the properties of visible and near-infrared light. Beyond endogenous optical properties, the development of molecularly targeted contrast agents enables disease-specific morphologic and biochemical processes to be labeled with unique optical signatures. Optical imaging systems can then provide real-time visualization of pathophysiology at spatial scales from the subcellular to whole organ levels. In this article, we review fundamental techniques and recent developments in optical molecular imaging, emphasizing laboratory and clinical systems that aim to visualize the microscopic and macroscopic hallmarks of cancer.

Associations between the uptake of 111In-DTPA-trastuzumab, HER2 density and response to trastuzumab (Herceptin) in athymic mice bearing subcutaneous human tumour xenografts

PURPOSE

The purpose of the study was to investigate the associations between uptake of (111)In-DTPA-trastuzumab, tumour HER2 density and response to trastuzumab (Herceptin) of human breast cancer (BC) xenografts in athymic mice.

MATERIALS AND METHODS

The tumour uptake of (111)In-DTPA-trastuzumab in athymic mice bearing BC xenografts with increasing HER2 density (0 to 3+) was evaluated. Specific uptake ratios were established in biodistribution (SUR) and imaging studies (ROI-SUR) using (111)In-labeled mouse IgG ((111)In-DTPA-mIgG). Further corrections were made for circulating radioactivity using tumour-to-blood ratios defined as a localization index (LI) and region-of-interest localization index (ROI-LI), respectively. Mice were treated with trastuzumab (Herceptin). A tumour growth inhibition index (TGI) was calculated and relative TGIs calculated by dividing the TGI of control by that of trastuzumab-treated mice.

RESULTS

Strong, nonlinear associations with HER2 density were obtained if the uptake of (111)In-DTPA-trastuzumab was corrected for nonspecific IgG localization (i.e., SUR; r (2) = 0.99) and circulating radioactivity (i.e., LI; r (2) = 0.87), but without these corrections, the association between HER2 density and tumour uptake was poor (r (2) = 0.22). There was a strong association between ROI-SUR and ROI-LI values and HER2 expression (r (2) = 0.90 and r (2) = 0.95, respectively. All tumours were imaged. Relative TGI values were associated with increasing uncorrected tumour uptake of (111)In-DTPA-trastuzumab but not always with HER2 density (i.e., MCF-HER2-18 cells with trastuzumab-resistance).

CONCLUSION

HER2 expression (0 to 3+) can be differentiated using (111)In-DTPA-trastuzumab, but requires correction of tumour uptake for nonspecific IgG localization and circulating radioactivity. The uncorrected uptake of (111)In-DTPA-trastuzumab was associated with tumour response to trastuzumab.

In Vivo Molecular Imaging Biomarkers: Clinical Pharmacology’s new “PET”?

Medicine, including the pharmaceutical and biotechnology industries as well as many clinical practitioners, has recognized the importance of using molecular imaging biomarkers, including those labeled in such a way as to be imaged by positron emission tomography (PET), as tools for predicting outcomes in drug development and creating opportunities for "personalized" medicine, for diagnosing early-stage disease, and for the follow-up of the effectiveness of treatment.(1) However, only one important and widely used PET biomarker is currently approved by the Food and Drug Administration (FDA). If the technology is so important, we can ask why there is such a limitation to the availability of these biomarkers.