Incorporation of a Triglutamyl Spacer Improves the Biodistribution of Synthetic Affibody Molecules Radiofluorinated at the N-Terminus via Oxime Formation with 18F-4-Fluorobenzaldehyde

Development of a Radiolabeled Cyclin-Dependent Kinases 4 and 6 (CDK4/6) Inhibitor for Brain and Cancer PET Imaging

The synthesis, biochemical evaluation and radiosynthesis of a cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor and radioligand was performed. NT431, a newly synthesized 4-fluorobenzyl-abemaciclib, exhibited high potency to CDK4/6 and against four cancer cell lines with IC50 similar to that of the parent abemaciclib. We performed a two-step one-pot radiosynthesis to produce [18F]NT431 with good radiochemical yield (9.6 ± 3%, n = 3, decay uncorrected), high radiochemical purity (>95%), and high molar activity (>370 GBq/µmol (>10.0 Ci/µmol). In vitro autoradiography confirmed the specific binding of [18F]NT431 to CDK4/6 in brain tissues. Dynamic PET imaging supports that both [18F]NT431 and the parent abemaciclib crossed the BBB albeit with modest brain uptake. Therefore, we conclude that it is unlikely that NT431 or abemaciclib (FDA approved drug) can accumulate in the brain in sufficient concentrations to be potentially effective against breast cancer brain metastases or brain cancers. However, despite the modest BBB penetration, [18F]NT431 represents an important step towards the development and evaluation of a new generation of CDK4/6 inhibitors with superior BBB penetration for the treatment and visualization of CDK4/6 positive tumors in the CNS. Also, [18F]NT431 may have potential application in peripheral tumors such as breast cancer and other CDK4/6 positive tumors.

Synthetic 18F labeled biomolecules that are selective and promising for PET imaging: major advances and applications

The concept of positron emission tomography (PET) based imaging was developed more than 40 years ago. It has been a widely adopted technique for detecting and staging numerous diseases in clinical settings, particularly cancer, neuro- and cardio-diseases. Here, we reviewed the evolution of PET and its advantages over other imaging modalities in clinical settings. Primarily, this review discusses recent advances in the synthesis of 18F radiolabeled biomolecules in light of the widely accepted performance for effective PET. The discussion particularly emphasizes the 18F-labeling chemistry of carbohydrates, lipids, amino acids, oligonucleotides, peptides, and protein molecules, which have shown promise for PET imaging in recent decades. In addition, we have deliberated on how 18F-labeled biomolecules enable the detection of metabolic changes at the cellular level and the selective imaging of gross anatomical localization via PET imaging. In the end, the review discusses the future perspective of PET imaging to control disease in clinical settings. We firmly believe that collaborative multidisciplinary research will further widen the comprehensive applications of PET approaches in the clinical management of cancer and other pathological outcomes.

N-Alkyl Carbamoylimidazoles as Versatile Synthons for the Synthesis of Urea-Based PSMA Inhibitors

We describe N-alkyl carbamoylimidazoles as readily available and highly versatile synthons for synthesizing urea-based prostate-specific membrane antigen (PSMA) inhibitors. Urea formation proceeded in high yields (>80%) at room temperature under aqueous conditions. All novel compounds were tested for their PSMA inhibitory potency in a cell-based radiometric binding assay. Compound 17 was identified as a novel high-affinity PSMA inhibitor (IC50 = 0.013 μM) suitable for developing an 18F-labeled radioligand for PET imaging of PSMA in prostate cancer.

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.

Imaging using radiolabelled targeted proteins: radioimmunodetection and beyond

The use of radiolabelled antibodies was proposed in 1970s for staging of malignant tumours. Intensive research established chemistry for radiolabelling of proteins and understanding of factors determining biodistribution and targeting properties. The use of radioimmunodetection for staging of cancer was not established as common practice due to approval and widespread use of [¹⁸F]-FDG, which provided a more general diagnostic use than antibodies or their fragments. Expanded application of antibody-based therapeutics renewed the interest in radiolabelled antibodies. RadioimmunoPET emerged as a powerful tool for evaluation of pharmacokinetics of and target engagement by biotherapeutics. In addition to monoclonal antibodies, new radiolabelled engineered proteins have recently appeared, offering high-contrast imaging of expression of therapeutic molecular targets in tumours shortly after injection. This creates preconditions for noninvasive determination of a target expression level and stratification of patients for targeted therapies. Radiolabelled proteins hold great promise to play an important role in development and implementation of personalised targeted treatment of malignant tumours. This article provides an overview of biodistribution and tumour-seeking features of major classes of targeting proteins currently utilized for molecular imaging. Such information might be useful for researchers entering the field of the protein-based radionuclide molecular imaging.

PET Imaging of HER2-Positive Tumors with Cu-64-Labeled Affibody Molecules

PURPOSE

Previous studies has demonstrated the utility of human epidermal growth factor receptor type 2 (HER2) as an attractive target for cancer molecular imaging and therapy. An affibody protein with strong binding affinity for HER2, Z_HER2:342, has been reported. Various methods of chelator conjugation for radiolabeling HER2 affibody molecules have been described in the literature including N-terminal conjugation, C-terminal conjugation, and other methods. Cu-64 has recently been extensively evaluated due to its half-life, decay properties, and availability. Our goal was to optimize the radiolabeling method of this affibody molecule with Cu-64, and translate a positron emission tomography (PET) probe with the best in vivo performance to clinical PET imaging of HER2-positive cancers.

PROCEDURES

In our study, three anti-HER2 affibody proteins-based PET probes were prepared, and their in vivo performance was evaluated in mice bearing HER2-positive subcutaneous SKOV3 tumors. The affibody analogues, Ac-Cys-Z_HER2:342, Ac-Z_HER2:342(Cys³⁹), and Ac-Z_HER2:342-Cys, were synthesized using the solid phase peptide synthesis method. The purified small proteins were site-specifically conjugated with the maleimide-functionalized chelator, 1,4,7,10-tetraazacyclododecane-1,4,7-tris- aceticacid-10-maleimidethylacetamide (maleimido-mono-amide-DOTA). The resulting DOTA-affibody conjugates were then radiolabeled with Cu-64. Cell uptake assay of the resulting PET probes, [⁶⁴Cu]DOTA-Cys-Z_HER2:342, [⁶⁴Cu]DOTA-Z_HER2:342(Cys³⁹), and [⁶⁴Cu]DOTA-Z_HER2:342-Cys, was performed in HER2-positive human ovarian SKOV3 carcinoma cells at 4 and 37 °C. The binding affinities of the radiolabeled peptides were tested by cell saturation assay using SKOV3 cells. PET imaging, biodistribution, and metabolic stability studies were performed in mice bearing SKOV3 tumors.

RESULTS

Cell uptake assays showed high and specific uptake by incubation of Cu-64-labeled affibodies with SKOV3 cells. The affinities (K_D) of the PET radio probes as tested by cell saturation analysis were in the low nanomolar range with the ranking of [⁶⁴Cu]DOTA-Cys-Z_HER2:342 (25.2 ± 9.2 nM) ≈ [⁶⁴Cu]DOTA-Z_HER2:342-Cys (32.6 ± 14.7 nM) > [⁶⁴Cu]DOTA-Z_HER2:342(Cys³⁹) (77.6 ± 22.2 nM). In vitro stability and in vivo metabolite analysis study revealed that all three probes were stable enough for in vivo imaging applications, while [⁶⁴Cu]DOTA-Cys-Z_HER2:342 showed the highest stability. In vivo small-animal PET further demonstrated fast tumor targeting, good tumor accumulation, and good tumor to normal tissue contrast of all three probes. For [⁶⁴Cu]DOTA-Cys-Z_HER2:342, [⁶⁴Cu]DOTA-Z_HER2:342(Cys³⁹), and [⁶⁴Cu]DOTA-Z_HER2:342-Cys, tumor uptake at 24 h are 4.0 ± 1.0 % ID/g, 4.0 ± 0.8 %ID/g, and 4.3 ± 0.7 %ID/g, respectively (mean ± SD, n = 4). Co-injection of the probes with non-labeled anti-HER2 affibody proteins confirmed in vivo specificities of the compounds by tumor uptake reduction.

CONCLUSIONS

The three Cu-64-labeled Z_HER2:342 analogues all display excellent HER2 targeting ability and tumor PET imaging quality. Although varied in the position of the radiometal labeling of these three Cu-64-labeled Z_HER2:342 analogues, there is no significant difference in tumor and normal tissue uptakes among the three probes. [⁶⁴Cu]DOTA-Cys-Z_HER2:342 stands out as the most superior PET probe because of its highest affinities and in vivo stability.

Affibody Molecules as Targeting Vectors for PET Imaging

Affibody molecules are small (58 amino acids) engineered scaffold proteins that can be selected to bind to a large variety of proteins with a high affinity. Their small size and high affinity make them attractive as targeting vectors for molecular imaging. High-affinity affibody binders have been selected for several cancer-associated molecular targets. Preclinical studies have shown that radiolabeled affibody molecules can provide highly specific and sensitive imaging on the day of injection; however, for a few targets, imaging on the next day further increased the imaging sensitivity. A phase I/II clinical trial showed that ⁶⁸Ga-labeled affibody molecules permit an accurate and specific measurement of HER2 expression in breast cancer metastases. This paper provides an overview of the factors influencing the biodistribution and targeting properties of affibody molecules and the chemistry of their labeling using positron emitters.

Influence of Residualizing Properties of the Radiolabel on Radionuclide Molecular Imaging of HER3 Using Affibody Molecules

Human epidermal growth factor receptor type 3 (HER3) is an emerging therapeutic target in several malignancies. To select potential responders to HER3-targeted therapy, radionuclide molecular imaging of HER3 expression using affibody molecules could be performed. Due to physiological expression of HER3 in normal organs, high imaging contrast remains challenging. Due to slow internalization of affibody molecules by cancer cells, we hypothesized that labeling (HE)₃-Z_HER3:08698-DOTAGA affibody molecule with non-residualizing [¹²⁵I]-N-succinimidyl-4-iodobenzoate (PIB) label would improve the tumor-to-normal organs ratios compared to previously reported residualizing radiometal labels. The [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA was compared side-by-side with [¹¹¹In]In-(HE)₃-Z_HER3:08698-DOTAGA. Both conjugates demonstrated specific high-affinity binding to HER3-expressing BxPC-3 and DU145 cancer cells. Biodistribution in mice bearing BxPC-3 xenografts at 4 and 24 h pi showed faster clearance of the [¹²⁵I]I-PIB label compared to the indium-111 label from most tissues, except blood. This resulted in higher tumor-to-organ ratios in HER3-expressing organs for [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA at 4 h, providing the tumor-to-liver ratio of 2.4 ± 0.3. The tumor uptake of both conjugates was specific, however, it was lower for the [¹²⁵I]I-PIB label. In conclusion, the use of non-residualizing [¹²⁵I]I-PIB label for HER3-targeting affibody molecule provided higher tumor-to-liver ratio than the indium-111 label, however, further improvement in tumor uptake and retention is needed.

FASTlab Radiosynthesis of the 18 F-labelled HER2-binding Affibody molecule [18 F]GE-226

An ¹⁸ F-labelled human epidermal growth factor receptor (HER2) receptor binding radiotracer is a potential tool to non-invasively identify HER2 positive tumour lesions in subjects with recurrent metastatic breast cancer. Having explored the manual radiochemistry to conjugate the Affibody molecule Z_HER2:2891 with [¹⁸ F]4-fluorobenzaldehyde, we have developed and optimised a full protocol for the automated GE FASTlab synthesiser. Our chemometric model predicted the best radiochemical purity for a short conjugation time (2.8 minutes), a low temperature (65°C), and a medium Affibody molecule precursor amount (5.5 mg). Under these optimised conditions, [¹⁸ F]GE-226 was produced after solid-phase extraction purification with activity yield of 30% ± 7 (n = 18) and a radiochemical purity of 94% ± 2 (n = 18). The synthesis and purification was complete after 43 minutes and provided apparent molar activities of 12 to 30 GBq/μmol (n = 12) at the end of synthesis.

Evaluation of Tumor-Targeting Properties of an Antagonistic Bombesin Analogue RM26 Conjugated with a Non-Residualizing Radioiodine Label Comparison with a Radiometal-Labelled Counterpart

Radiolabelled antagonistic bombesin analogues are successfully used for targeting of gastrin-releasing peptide receptors (GRPR) that are overexpressed in prostate cancer. Internalization of antagonistic bombesin analogues is slow. We hypothesized that the use of a non-residualizing radioiodine label might not affect the tumour uptake but would reduce the retention in normal organs, where radiopharmaceutical would be internalized. To test this hypothesis, tyrosine was conjugated via diethylene glycol linker to N-terminus of an antagonistic bombesin analogue RM26 to form Tyr-PEG₂-RM26. [¹¹¹In]In-DOTA-PEG₂-RM26 was used as a control with a residualizing label. Tyr-PEG₂-RM26 was labelled with ¹²⁵I with 95% radiochemical purity and retained binding specificity to GRPR. The IC₅₀ values for Tyr-PEG₂-RM26 and DOTA-PEG₂-RM26 were 1.7 ± 0.3 nM and 3.3 ± 0.5 nM, respectively. The cellular processing of [¹²⁵I]I-Tyr-PEG₂-RM26 by PC-3 cells showed unusually fast internalization. Biodistribution showed that uptake in pancreas and tumour was GRPR-specific for both radioconjugates. Blood clearance of [¹²⁵I]I-Tyr-PEG₂-RM26 was appreciably slower and activity accumulation in all organs was significantly higher than for [¹¹¹In]In-DOTA-PEG₂-RM26. Tumor uptake of [¹¹¹In]In-DOTA-PEG₂-RM26 was significantly higher than for [¹²⁵I]I-Tyr-PEG₂-RM26, resulting in higher tumour-to-organ ratio for [¹¹¹In]In-DOTA-PEG₂-RM26 at studied time points. Incorporation of amino acids with hydrophilic side-chains next to tyrosine might overcome the problems associated with the use of tyrosine as a prosthetic group for radioiodination.

Targeting HER2-positive gastric cancer with a novel 18F-labeled ZHER2:342 probe

To realize the diagnosis of HER2-positive gastric cancer via PET imaging, herein, a new kind of ¹⁸F-labeled HER2 affibody probe was created; the bifunctional maleimide derivative 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA-MAL) was first coupled to a polypeptide, and the resulting compound was subsequently labeled with the ¹⁸FAl complex. The binding characteristics of the probe were assessed using both in vitro studies and in vivo microPET imaging and biodistribution experiments. Immunohistochemical staining was performed to confirm the expression level of HER2 in the studied cell lines and tumors. The probe was successfully produced with the radiochemical purity of more than 95%. The NCI N87 cell-associated radioactivity was 19.31 ± 1.01% AD, and it decreased to 0.83 ± 0.04% AD per 10⁶ cells after blocking HER2 as early as 15 minutes post-incubation (p < 0.05). A competition binding assay between radiolabeled and non-radioactive affibody molecules with NCI N87 indicated that the IC₅₀ was 8.10 nM. The microPET imaging and biodistribution of human gastric cancer xenografts demonstrated that the probe could specifically accumulate in tumors at early time points. Protein detection confirmed a strong HER2 expression in NCIN87 and a weak HER2 expression in SGC7901. In conclusion, ¹⁸FAl-NOTA-MAL-Cys-GGGRDN(M⁰)-Z_HER2:342 was successfully prepared via a one-step method. The favorable preclinical data showed specific and effective tumor targeting capacity of the proposed probe; this revealed that the probe proposed herein might have potential application in gastric cancer imaging.

Tumor targeting of the novel ¹⁸F-labeled Z_HER2:342 probe in HER2-positive gastric cancer xenograft models.

Engineered Charge Redistribution of Gp2 Proteins through Guided Diversity for Improved PET Imaging of Epidermal Growth Factor Receptor

The Gp2 domain is a protein scaffold for synthetic ligand engineering. However, the native protein function results in a heterogeneous distribution of charge on the conserved surface, which may hinder further development and utility. We aim to modulate charge, without diminishing function, which is challenging in small proteins where each mutation is a significant fraction of protein structure. We constructed rationally guided combinatorial libraries with charge-neutralizing or charge-flipping mutations and sorted them, via yeast display and flow cytometry, for stability and target binding. Deep sequencing of functional variants revealed effective mutations both in clone-dependent contexts and broadly across binders to epidermal growth factor receptor (EGFR), insulin receptor, and immunoglobulin G. Functional mutants averaged 4.3 charge neutralizing mutations per domain while maintaining net negative charge. We evolved an EGFR-targeted Gp2 mutant that reduced charge density by 33%, maintained net charge, and improved charge distribution homogeneity while elevating thermal stability ( Tm = 87 ± 1 °C), improving binding specificity, and maintaining affinity ( Kd = 8.8 ± 0.6 nM). This molecule was conjugated with 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid for 64Cu chelation and evaluated for physiological distribution in mice with xenografted A431 (EGFRhigh) and MDA-MB-435 (EGFRlow) tumors. Excised tissue gamma counting and positron emission tomography/computed tomography imaging revealed good EGFRhigh tumor signal (4.7 ± 0.5%ID/g) at 2 h post-injection and molecular specificity evidenced by low uptake in EGFRlow tumors (0.6 ± 0.1%ID/g, significantly lower than for non-charge-modified Gp2, p = 0.01). These results provide charge mutations for an improved Gp2 framework, validate an effective approach to charge engineering, and advance performance of physiological EGFR targeting for molecular imaging.

Molecular design of radiocopper-labelled Affibody molecules

The use of long-lived positron emitters ⁶⁴Cu or ⁶¹Cu for labelling of Affibody molecules may improve breast cancer patients’ stratification for HER-targeted therapy. Previous animal studies have shown that the use of triaza chelators for ⁶⁴Cu labelling of synthetic Affibody molecules is suboptimal. In this study, we tested a hypothesis that the use of cross-bridged chelator, CB-TE2A, in combination with Gly-Glu-Glu-Glu spacer for labelling of Affibody molecules with radiocopper would improve imaging contrast. CB-TE2A was coupled to the N-terminus of synthetic Affibody molecules extended either with a glycine (designation CB-TE2A-G-ZHER2:342) or Gly-Glu-Glu-Glu spacer (CB-TE2A-GEEE-ZHER2:342). Biodistribution and targeting properties of ⁶⁴Cu-CB-TE2A-G-ZHER2:342 and ⁶⁴Cu-CB-TE2A-GEEE-ZHER2:342 were compared in tumor-bearing mice with the properties of ⁶⁴Cu-NODAGA-ZHER2:S1, which had the best targeting properties in the previous study. ⁶⁴Cu-CB-TE2A-GEEE-ZHER2:342 provided appreciably lower uptake in normal tissues and higher tumor-to-organ ratios than ⁶⁴Cu-CB-TE2A-G-ZHER2:342 and ⁶⁴Cu-NODAGA-ZHER2:S1. The most pronounced was a several-fold difference in the hepatic uptake. At the optimal time point, 6 h after injection, the tumor uptake of ⁶⁴Cu-CB-TE2A-GEEE-ZHER2:342 was 16 ± 6%ID/g and tumor-to-blood ratio was 181 ± 52. In conclusion, a combination of the cross-bridged CB-TE2A chelator and Gly-Glu-Glu-Glu spacer is preferable for radiocopper labelling of Affibody molecules and, possibly, other scaffold proteins having high renal re-absorption.

Evaluation of affibody charge modification identified by synthetic consensus design in molecular PET imaging of epidermal growth factor receptor

Tumor overexpression of epidermal growth factor receptor (EGFR) correlates to therapeutic response in select patient populations. Thus, molecular positron emission tomography (PET) imaging of EGFR could stratify responders versus non-responders. We previously demonstrated effectiveness of a "synthetic consensus" design principle to identify six neutralizing mutations within a 58-amino acid EGFR-targeted affibody domain. Herein, we extend the approach to identify additional neutralized variants that vary net charge from -2 to either -4 or +4 while retaining high affinity (1.6 ± 1.2 nM and 2.5 ± 0.7 nM), specific binding to EGFR, secondary structure, and stability (T_m = 68 °C and 59 °C). We radiolabeled the resultant collection of five charge variants with ⁶⁴Cu and evaluated PET imaging performance in murine models with subcutaneously xenografted EGFR^high and EGFR^low tumors. All variants exhibited good EGFR^high tumor imaging as early as 1 h, with EA35S (+3/-5) achieving 7.7 ± 1.4 %ID/g tumor at 4 h with 1.5 ± 0.3%ID/g EGFR^low tumor, 34 ± 5 tumor:muscle and 12 ± 3 tumor:blood ratios. The positively charged EA62S mutant (+6/-2) exhibited 2.2-3.3-fold higher liver signal than the other variants (p<0.01). The EA68 variant with higher charge density was more stable to human and mouse serum than neutralized variants. In a comparison of radiometal chelators, 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA) exhibited superior physiological specificity to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). In total, these studies comparatively evaluated a set of EGFR-targeted affibodies varying in net charge and charge density, which revealed functional variations that are useful in engineering an ideal probe for translational studies.

In Vivo Imaging of the Programmed Death Ligand 1 by 18F PET

Programmed death ligand 1 (PD-L1) is an immune regulatory ligand that binds to the T-cell immune check point programmed death 1. Tumor expression of PD-L1 is correlated with immune suppression and poor prognosis. It is also correlated with therapeutic efficacy of programmed death 1 and PD-L1 inhibitors. In vivo imaging may enable real-time follow-up of changing PD-L1 expression and heterogeneity evaluation of PD-L1 expression across tumors in the same subject. We have radiolabeled the PD-L1-binding Affibody molecule NOTA-Z_{PD-L1_1} with ¹⁸F and evaluated its in vitro and in vivo binding affinity, targeting, and specificity. Methods: The affinity of the PD-L1-binding Affibody ligand Z_{PD-L1_1} was evaluated by surface plasmon resonance. Labeling was accomplished by maleimide coupling of NOTA to a unique cysteine residue and chelation of ¹⁸F-AlF. In vivo studies were performed in PD-L1-positive, PD-L1-negative, and mixed tumor-bearing severe combined immunodeficiency mice. Tracer was injected via the tail vein, and dynamic PET scans were acquired for 90 min, followed by γ-counting biodistribution. Immunohistochemical staining with an antibody specific for anti-PD-L1 (22C3) was used to evaluate the tumor distribution of PD-L1. Immunohistochemistry results were then compared with ex vivo autoradiographic images obtained from adjacent tissue sections. Results: NOTA-Z_{PD-L1_1} was labeled, with a radiochemical yield of 15.1% ± 5.6%, radiochemical purity of 96.7% ± 2.0%, and specific activity of 14.6 ± 6.5 GBq/μmol. Surface plasmon resonance showed a NOTA-conjugated ligand binding affinity of 1 nM. PET imaging demonstrated rapid uptake of tracer in the PD-L1-positive tumor, whereas the PD-L1-negative control tumor showed little tracer retention. Tracer clearance from most organs and blood was quick, with biodistribution showing prominent kidney retention, low liver uptake, and a significant difference between PD-L1-positive (percentage injected dose per gram [%ID/g] = 2.56 ± 0.33) and -negative (%ID/g = 0.32 ± 0.05) tumors (P = 0.0006). Ex vivo autoradiography showed excellent spatial correlation with immunohistochemistry in mixed tumors. Conclusion: Our results show that Affibody ligands can be effective at targeting tumor PD-L1 in vivo, with good specificity and rapid clearance. Future studies will explore methods to reduce kidney activity retention and further increase tumor uptake.

Preparation and biological evaluation of 99mTc-HYNIC-(Ser)3-D4 peptide for targeting and imaging of non-small-cell lung cancer

Aim: In this study, radiolabeled D4 peptide conjugate was studied as a radiotracer for imaging of non-small-cell lung cancer with overexpression of EGFR. Methods: HYNIC-(Ser)3-D4 peptide was labeled with 99mTc using tricine as a co-ligand. Cellular specific binding and internalization as well as in vivo tumor targeting were assessed. Results: The in vitro experiments showed good cellular specific binding. Tumor uptake values as %ID/g were 7.55 and 6.82% at 1 and 4 h after injection, respectively. The presaturation of EGFR in xenografted nude mice reduced 36% tumor uptake of radioactivity at 1 h after injection that confirmed in vivo specificity. Conclusion: Findings showed this radiolabeled peptide is a promising candidate for tumor targeting and molecular imaging of non-small-cell lung cancer.

Development & automation of a novel [(18)F]F prosthetic group, 2-[(18)F]-fluoro-3-pyridinecarboxaldehyde, and its application to an amino(oxy)-functionalised Aβ peptide

2-[(18)F]-Fluoro-3-pyridinecarboxaldehyde ([(18)F]FPCA) is a novel, water-soluble prosthetic group. It's radiochemistry has been developed and fully-automated for application in chemoselective radiolabelling of amino(oxy)-derivatised RI-OR2-TAT peptide, (Aoa-k)-RI-OR2-TAT, using a GE TRACERlab FX-FN. RI-OR2-TAT is a brain-penetrant, retro-inverso peptide that binds to amyloid species associated with Alzheimer's Disease. Radiolabelled (Aoa-k)-RI-OR2-TAT was reproducibly synthesised and the product of the reaction with FPCA has been fully characterised. In-vivo biodistribution of [(18)F]RI-OR2-TAT has been measured in Wistar rats.

Increasing the Net Negative Charge by Replacement of DOTA Chelator with DOTAGA Improves the Biodistribution of Radiolabeled Second-Generation Synthetic Affibody Molecules

A promising strategy to enable patient stratification for targeted therapies is to monitor the target expression in a tumor by radionuclide molecular imaging. Affibody molecules (7 kDa) are nonimmunoglobulin scaffold proteins with a 25-fold smaller size than intact antibodies. They have shown an apparent potential as molecular imaging probes both in preclinical and clinical studies. Earlier, we found that hepatic uptake can be reduced by the incorporation of negatively charged purification tags at the N-terminus of Affibody molecules. We hypothesized that liver uptake might similarly be reduced by positioning the chelator at the N-terminus, where the chelator-radionuclide complex will provide negative charges. To test this hypothesis, a second generation synthetic anti-HER2 ZHER2:2891 Affibody molecule was synthesized and labeled with (111)In and (68)Ga using DOTAGA and DOTA chelators. The chelators were manually coupled to the N-terminus of ZHER2:2891 forming an amide bond. Labeling DOTAGA-ZHER2:2891 and DOTA-ZHER2:2891 with (68)Ga and (111)In resulted in stable radioconjugates. The tumor-targeting and biodistribution properties of the (111)In- and (68)Ga-labeled conjugates were compared in SKOV-3 tumor-bearing nude mice at 2 h postinjection. The HER2-specific binding of the radioconjugates was verified both in vitro and in vivo. Using the DOTAGA chelator gave significantly lower radioactivity in liver and blood for both radionuclides. The (111)In-labeled conjugates showed more rapid blood clearance than the (68)Ga-labeled conjugates. The most pronounced influence of the chelators was found when they were labeled with (68)Ga. The DOTAGA chelator gave significantly higher tumor-to-blood (61 ± 6 vs 23 ± 5, p < 0.05) and tumor-to-liver (10.4 ± 0.6 vs 4.5 ± 0.5, p < 0.05) ratios than the DOTA chelator. This study demonstrated that chelators may be used to alter the uptake of Affibody molecules, and most likely other scaffold-based imaging probes, for improvement of imaging contrast.

Novel chemoselective (18)F-radiolabeling of thiol-containing biomolecules under mild aqueous conditions

We report a novel prosthetic group based on a heterocyclic methylsulfone derivative for the rapid, stable, and chemoselective (18)F-labeling of thiol-containing (bio)molecules under mild aqueous reaction conditions. Compared to established maleimide approaches, the new methodology displays some clear advantages for imaging probe development.

PET Imaging of Macrophage Mannose Receptor-Expressing Macrophages in Tumor Stroma Using 18F-Radiolabeled Camelid Single-Domain Antibody Fragments

Tumor-associated macrophages constitute a major component of the stroma of solid tumors, encompassing distinct subpopulations with different characteristics and functions. We aimed to identify M2-oriented tumor-supporting macrophages within the tumor microenvironment as indicators of cancer progression and prognosis, using PET imaging. This can be realized by designing (18)F-labeled camelid single-domain antibody fragments (sdAbs) specifically targeting the macrophage mannose receptor (MMR), which has been identified as an important biomarker on this cell population.

METHODS

Cross-reactive anti-MMR sdAbs were generated after immunization of an alpaca with the extracellular domains of both human and mouse MMR. The lead binder was chosen on the basis of comparisons of binding affinity and in vivo pharmacokinetics. The PET tracer (18)F-fluorobenzoate (FB)-anti-MMR sdAb was developed using the prosthetic group N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB), and its biodistribution, tumor-targeting potential, and specificity in terms of macrophage and MMR targeting were evaluated in mouse tumor models.

RESULTS

Four sdAbs were selected after affinity screening, but only 2 were found to be cross-reactive for human and mouse MMR. The lead anti-MMR 3.49 sdAb, bearing an affinity of 12 and 1.8 nM for mouse and human MMR, respectively, was chosen for its favorable in vivo biodistribution profile and tumor-targeting capacity. (18)F-FB-anti-MMR 3.49 sdAb was synthesized with a 5%-10% radiochemical yield using an automated and optimized protocol. In vivo biodistribution analyses showed fast clearance via the kidneys and retention in MMR-expressing organs and tumor. The kidney retention of the fluorinated sdAb was 20-fold lower than a (99m)Tc-labeled counterpart. Compared with MMR- and C-C chemokine receptor 2-deficient mice, significantly higher uptake was observed in tumors grown in wild-type mice, demonstrating the specificity of the (18)F tracer for MMR and macrophages, respectively.

CONCLUSION

Anti-MMR 3.49 was denoted as the lead cross-reactive MMR-targeting sdAb. (18)F radiosynthesis was optimized, providing an optimal probe for PET imaging of the tumor-promoting macrophage subpopulation in the tumor stroma.

Site-Specific Radioiodination of HER2-Targeting Affibody Molecules using 4-Iodophenethylmaleimide Decreases Renal Uptake of Radioactivity

Affibody molecules are small scaffold-based affinity proteins with promising properties as probes for radionuclide-based molecular imaging. However, a high reabsorption of radiolabeled Affibody molecules in kidneys is an issue. We have shown that the use of ¹²⁵I-3-iodo-((4-hydroxyphenyl)ethyl)maleimide (IHPEM) for site-specific labeling of cysteine-containing Affibody molecules provides high tumor uptake but low radioactivity retention in kidneys. We hypothesized that the use of 4-iodophenethylmaleimide (IPEM) would further reduce renal retention of radioactivity because of higher lipophilicity of radiometabolites. An anti-human epidermal growth factor receptor type 2 (HER2) Affibody molecule (Z_HER2:2395) was labeled using ¹²⁵I-IPEM with an overall yield of 45±3 %. ¹²⁵I-IPEM-Z_HER2:2395 bound specifically to HER2-expressing human ovarian carcinoma cells (SKOV-3 cell line). In NMRI mice, the renal uptake of ¹²⁵I-IPEM-Z_HER2:2395 (24±2 and 5.7±0.3 % IA g⁻¹at 1 and 4 h after injection, respectively) was significantly lower than uptake of ¹²⁵I-IHPEM-Z_HER2:2395 (50±8 and 12±2 % IA g⁻¹at 1 and 4 h after injection, respectively). In conclusion, the use of a more lipophilic linker for the radioiodination of Affibody molecules reduces renal radioactivity.

Gallium-68-labeled affibody molecule for PET imaging of PDGFRβ expression in vivo

Platelet-derived growth factor receptor β (PDGFRβ) is a transmembrane tyrosine kinase receptor involved, for example, in angiogenesis. Overexpression and excessive signaling of PDGFRβ has been observed in multiple malignant tumors and fibrotic diseases, making this receptor a pharmaceutical target for monoclonal antibodies and tyrosine kinase inhibitors. Successful targeted therapy requires identification of responding patients. Radionuclide molecular imaging would enable determination of the PDGFRβ status in all lesions using a single noninvasive repeatable procedure. Recently, we have demonstrated that the affibody molecule Z09591 labeled with (111)In can specifically target PDGFRβ-expressing tumors in vivo. The use of positron emission tomography (PET) as an imaging technique would provide superior resolution, sensitivity, and quantitation accuracy. In this study, a DOTA-conjugated Z09591 was labeled with the generator-produced positron emitting radionuclide (68)Ga (T1/2 = 67.6 min, Eβ + max = 1899 keV, 89% β(+)). (68)Ga-DOTA-Z09591 retained the capacity to specifically bind to PDGFRβ-expressing U-87 MG glioma cells. The half-maximum inhibition concentration (IC50) of (68)Ga-DOTA-Z09591 (6.6 ± 1.4 nM) was somewhat higher than that of (111)In-DOTA-Z09591 (1.4 ± 1.2 nM). (68)Ga-DOTA-Z09591 demonstrated specific (saturable) targeting of U-87 MG xenografts in immunodeficient mice. The tumor uptake at 2 h after injection was 3.7 ± 1.7% IA/g, which provided a tumor-to-blood ratio of 8.0 ± 3.1. The only organ with higher accumulation of radioactivity was the kidney. MicroPET imaging provided high-contrast imaging of U-87 MG xenografts. In conclusion, the (68)Ga-labeled affibody molecule Z09591 is a promising candidate for further development as a probe for imaging PDGFRβ expression in vivo using PET.