A kit to prepare 111In-DTPA-trastuzumab (Herceptin) Fab fragments injection under GMP conditions for imaging or radioimmunoguided surgery of HER2-positive breast cancer

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.

Improved Characteristics of RANKL Immuno-PET Imaging Using Radiolabeled Antibody Fab Fragments

Purpose: RANKL expression in the tumor microenvironment has been identified as a biomarker of immune suppression, negating the effect of some cancer immunotherapies. Previously we had developed a radiotracer based on the FDA-approved RANKL-specific antibody denosumab, [⁸⁹Zr]Zr-DFO-denosumab, enabling successful immuno-PET imaging. Radiolabeled denosumab, however, showed long blood circulation and delayed tumor uptake, potentially limiting its applications. Here we aimed to develop a smaller radiolabeled denosumab fragment, [⁶⁴Cu]Cu-NOTA-denos-Fab, that would ideally show faster tumor accumulation and better diffusion into the tumor for the visualization of RANKL. Experimental design: Fab fragments were prepared from denosumab using papain and conjugated to a NOTA chelator for radiolabeling with ⁶⁴Cu. The bioconjugates were characterized in vitro using SDS-PAGE analysis, and the binding affinity was assessed using a radiotracer cell binding assay. Small animal PET imaging evaluated tumor targeting and biodistribution in transduced RANKL-ME-180 xenografts. Results: The radiolabeling yield of [⁶⁴Cu]Cu-NOTA-denos-Fab was 58 ± 9.2%, with a specific activity of 0.79 ± 0.11 MBq/µg (n = 3). A radiotracer binding assay proved specific targeting of RANKL in vitro. PET imaging showed fast blood clearance and high tumor accumulation as early as 1 h p.i. (2.14 ± 0.21% ID/mL), which peaked at 5 h p.i. (2.72 ± 0.61% ID/mL). In contrast, [⁶⁴Cu]Cu-NOTA-denosumab reached its highest tumor uptake at 24 h p.i. (6.88 ± 1.12% ID/mL). [⁶⁴Cu]Cu-NOTA-denos-Fab specifically targeted human RANKL in transduced ME-180 xenografts compared with the blocking group and negative ME-180 xenograft model. Histological analysis confirmed RANKL expression in RANKL-ME-180 xenografts. Conclusions: Here, we report on a novel RANKL PET imaging agent, [⁶⁴Cu]Cu-NOTA-denos-Fab, that allows for fast tumor imaging with improved imaging contrast when compared with its antibody counterpart, showing promise as a potential PET RANKL imaging tool for future clinical applications.

Comparative In Vitro Cytotoxicity Studies of 177Lu-CHX-A″-DTPA-Trastuzumab and 177Lu-CHX-A″-DTPA-F(ab')2-Trastuzumab in HER2-Positive Cancer Cell Lines

Background: Human epidermal growth factor receptor 2 (HER2) is found to be amplified in ∼15%-20% of breast cancers. In this study, the authors report the synthesis and comparative in vitro therapeutic efficacy of ¹⁷⁷Lu-CHX-A″-DTPA-trastuzumab and ¹⁷⁷Lu-CHX-A″-DTPA-F(ab')₂-trastuzumab to determine their potential as theranostic agents for patients with breast cancer. Materials and Methods: Bivalent F(ab')₂-trastuzumab was produced by enzymatic digestion of trastuzumab, conjugated with p-SCN-Bn-CHX-A″-DTPA and subsequently radiolabeled with ¹⁷⁷Lu. Cell viability, membrane toxicity assays, and apoptosis analysis were carried out with ¹⁷⁷Lu-CHX-A″-DTPA-trastuzumab and ¹⁷⁷Lu-CHX-A″-DTPA-F(ab')₂-trastuzumab in HER2-positive ovarian (SK-OV-3) and breast cancer (SK-BR-3 and MDA-MB-453) cells. Results: In vitro cell binding studies showed ∼20%-25% binding of ¹⁷⁷Lu-CHX-A″-DTPA-trastuzumab and ¹⁷⁷Lu-CHX-A″-DTPA-F(ab')₂-trastuzumab to SK-OV-3, SK-BR-3, and MDA-MB-453 cells. The cells exhibited similar degree of membrane integrity and cellular toxicity when treated with same amount (activity) of ¹⁷⁷Lu-CHX-A″-DTPA-F(ab')₂-trastuzumab and ¹⁷⁷Lu-CHX-A″-DTPA-trastuzumab, and the toxicity was dose dependent. The mode of cell death was predominantly by apoptosis and necrosis with both the radioimmunoconjugates. Conclusions: The results indicated that the efficacy of both the radioimmunoconjugates, in terms of inducing cell death, was similar thereby ascertaining their potential as good therapeutic agents for patients with breast cancer.

Dual-Targeted Delivery of Nanoparticles Encapsulating Paclitaxel and Everolimus: a Novel Strategy to Overcome Breast Cancer Receptor Heterogeneity

PURPOSE

The intratumoral heterogeneity observed in breast cancer (BC), in particular with regard to cell surface receptor expression, can hinder the success of many targeted cancer therapies. The development of novel therapeutic agents that target more than one receptor can overcome this inherent property of tumors and can facilitate their selective internalization in cancer cells. The goal of this study is to develop a drug combination-loaded nanoparticle (NP) formulation that is actively-targeted to HER2 and EGFR receptors on BC cells.

METHODS

A polymeric NP formulation was prepared which co-encapsulated a synergistic combination of the chemotherapeutic agent, paclitaxel (PTX), and the mTOR inhibitor, everolimus (EVER), and is targeted to HER2 and EGFR receptors on BC cells using antibody Fab fragments as the targeting moieties. The physicochemical characteristics of the dual-targeted formulation (Dual-NP) were evaluated, along with its cytotoxic profile (in both, monolayer and 3D BC models), as well as the degree of cellular uptake in HER2_high/EGFR_mod and HER2_neg/EGFR_low BC cells.

RESULTS

Dual-NPs were found to have significantly higher cytotoxicity relative to HER2 mono-targeted (T-NPs) and untargeted NPs (UT-NPs) in HER2_high/EGFR_mod monolayer BC cells after 72 h exposure, while no significant difference was observed in HER2_neg/EGFR_low cells. However, in the HER2_high/EGFR_mod spheroids, the cytotoxicity of Dual-NPs was comparable to that of T-NPs. This was thought to be attributed to the previously reported downregulation of EGFR in 3D in comparison to 2D BC models. Dual-NPs had significantly higher cellular uptake relative to UT-NPs and T-NPs in HER2_high/EGFR_mod BC cells after 24 h exposure, whereas in the HER2_neg/EGFR_low cells, the increase in cellular uptake of the Dual-NPs was not as high as the level achieved in the HER2_high/EGFR_mod cells. Blocking HER2 and EGFR significantly reduced the uptake of T-NPs and Dual-NPs in the HER2_high/EGFR_mod BC cells, demonstrating specific binding to both EGFR and HER2.

CONCLUSIONS

The dual-targeting strategy developed in this study in conjunction with a potentially promising delivery vector for a synergistic combination therapy can overcome receptor heterogeneity, yielding significant improvements in the cytotoxicity and cellular uptake in BC cells.

Evaluation of HER2-specific peptide ligand for its employment as radiolabeled imaging probe

HER2 transmembrane receptor is an important target in immunotherapy treatment of breast and gastroesophageal cancer. Molecular imaging of HER2 expression may provide essential prognostic and predictive information concerning disseminated cancer and aid in selection of an optimal therapy. Radiolabeled low molecular weight peptide ligands are particularly attractive as probes for molecular imaging, since they reach and bind to the target and clear from non-target organs and blood stream faster than bulky antibodies. In this study, we evaluated a potential HER2-imaging probe, an A9 nonapeptide, derived from the trastuzumab-Fab portion. Its cellular uptake was investigated by mass spectrometry analysis of the cytoplasmic cellular extracts. Moreover, based on in-silico modeling, DTPA chelator was conjugated to N-terminus of A9. ¹¹¹In-labeled A9 demonstrated nanomolar affinity to HER2-expressing BT474 cells and favorable biodistribution profile in NMRI mice. This study suggests that the peptide A9 represents a good lead candidate for development of molecular probe, to be used for imaging purposes and for the delivery of cytotoxic agents.

From preclinical development to clinical application: Kit formulation for radiolabelling the minigastrin analogue CP04 with In-111 for a first-in-human clinical trial

Introduction

A variety of radiolabelled minigastrin analogues targeting the cholecystokinin 2 (CCK2) receptor were developed and compared in a concerted preclinical testing to select the most promising radiotracer for diagnosis and treatment of medullary thyroid carcinoma (MTC). DOTA–DGlu–DGlu–DGlu–DGlu–DGlu–DGlu– Ala–Tyr–Gly–Trp–Met–Asp–Phe–NH₂ (CP04) after labelling with ¹¹¹In displayed excellent characteristics, such as high stability, receptor affinity, specific and persistent tumour uptake and low kidney retention in animal models. Therefore, it was selected for further clinical evaluation within the ERA-NET project GRAN-T-MTC. Here we report on the development of a pharmaceutical freeze-dried formulation of the precursor CP04 for a first multi-centre clinical trial with ¹¹¹In-CP04 in MTC patients.

Materials and methods

The kit formulation was optimised by adjustment of buffer, additives and radiolabelling conditions. Three clinical grade batches of a final kit formulation with two different amounts of peptide (10 or 50 μg) were prepared and radiolabelled with ¹¹¹In. Quality control and stability assays of both the kits and the resulting radiolabelled compound were performed by HPLC analysis.

Results

Use of ascorbic acid buffer (pH 4.5) allowed freeze-drying of the kit formulation with satisfactory pellet-formation. Addition of methionine and gentisic acid as well as careful selection of radiolabelling temperature was required to avoid extensive oxidation of the Met¹¹-residue. Trace metal contamination, in particular Zn, was found to be a major challenge during the pharmaceutical filling process in particular for the 10 μg formulation. The final formulations contained 10 or 50 μg CP04, 25 mg ascorbic acid, 0.5 mg gentisic acid and 5 mg l-methionine. The radiolabelling performed by incubation of 200–250 MBq ¹¹¹InCl₃ at 90 °C for 15 min resulted in reproducible radiochemical purity (RCP) >94%. Kit-stability was proven for >6 months at +5 °C and at +25 °C. The radiolabelled product was stable for >4 h at +25 °C.

Conclusion

A kit formulation to prepare ¹¹¹In-CP04 for clinical application was developed, showing high stability of the kit as well as high RCP of the final product.

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.

Kit for the preparation of 111In-labeled pertuzumab injection for imaging response of HER2-positive breast cancer to trastuzumab (Herceptin)

We previously reported that ¹¹¹In-labeled pertuzumab imaged trastuzumab (Herceptin)-mediated changes in HER2 expression preclinically in breast cancer tumors. To advance ¹¹¹In-labeled pertuzumab to a Phase I/II clinical trial, a kit was designed for preparing this agent in a form suitable for human administration. Unit-dose kits containing pertuzumab modified with 2-(4-isothiocyanatobenzyl)-diethylenetriaminepentaacetic acid (BzDTPA) were prepared that labeled to high efficiency (>90%) with ¹¹¹In and met specifications for pharmaceutical quality. The kits were stable for 4 months and the final radiopharmaceutical was stable for 24h. Imaging studies demonstrated high and specific uptake in HER2-positive tumors in mice using this clinical kit formulation.

Labelling Herceptin with a novel oxaliplatin derivative: a computational approach towards the selective drug delivery

The clinical use of platinum(II)-based drugs has serious side effects due to the non-specific reactions with both malignant and normal cells. To circumvent such major drawback, novel metallodrugs might be combined with suitable carrier molecules, as antibodies, to ensure selective attacks on tumours while sparing healthy tissues. In this contribution, we investigate the stability of a novel oxaliplatin derivate drug embedded in Herceptin (trastuzumab), an antibody which is able to recognise breast cancer cells, by using a wide panel of theoretical tools: docking, molecular dynamics and quantum calculations. Our calculations reveal the binding mechanism: the drug initially interacts non-covalently with the Pro40A and Asp167A residues, and the nitrogen of His171B subsequently replaces one of the water molecules coordinated to the platinum center, where the latter step reversibly fixes the drug into the antibody. These data might be used to further rationalise the synthesis of improved drugs beyond classical platinum(II) derivatives by improving the ligand-protein coupling mode.

Phase I trial of intraoperative detection of tumor margins in patients with HER2-positive carcinoma of the breast following administration of 111In-DTPA-trastuzumab Fab fragments

INTRODUCTION

Our aim was to conduct a Phase I clinical trial to determine the feasibility of intraoperative detection of tumor margins in HER2 positive breast carcinoma using a hand-held γ-probe following administration of (111)In-DTPA-trastuzumab Fab fragments. Accurate delineation of tumor margins is important for preventing local recurrence.

METHODS

Six patients with HER2-positive in situ or invasive ductal carcinoma were administered 74MBq (0.5mg) of (111)In-DTPA-trastuzumab Fab fragments and counts in the tumor, surgical cavity wall and en face margins were measured intraoperatively at 72h post-injection using the Navigator or C-Trak γ-probes. Margins were evaluated histologically. Quantitative whole body planar imaging was performed to estimate radiation absorbed doses using OLINDA/EXM software. SPECT imaging of the thorax was performed to evaluate tumor uptake. The pharmacokinetics of elimination from the blood and plasma were determined over 72h.

RESULTS

There were no acute adverse reactions from (111)In-DTPA-trastuzumab Fab fragments and no changes in hematological or biochemical indices were found over a 3month period. (111)In-DTPA-trastuzumab Fab fragments exhibited a biphasic elimination from the blood and plasma with t1/2α=11.9h and 7.5h, respectively, and t1/2β=26.6 and 20.7h, respectively. The radiopharmaceutical accumulated in the liver, spleen and kidneys. SPECT imaging did not reveal tumor in any patient. The mean effective dose was 0.146mSv/MBq (10.8mSv for 74MBq). Counts in excised tumors were low but were higher than in margins. Margins in two patients harboured tumor but this was not correlated with counts obtained using the γ-probes. Surgical cavity counts were high and likely due to detection of γ-photons outside the surgical field.

CONCLUSION

We conclude that it was not feasible, at least at the administered amount of radioactivity used in this study, to reliably detect the margins of disease in patients with in situ or invasive ductal carcinoma intraoperatively using a hand-held γ-probe and (111)In-DTPA-trastuzumab Fab fragments due to low uptake in the tumor and involved margins.

Comparative binding of disulfide-bridged PEG-Fabs

Protein PEGylation is the most clinically validated method to improve the efficacy of protein-based medicines. Antibody fragments such as Fabs display rapid clearance from blood circulation and therefore are good candidates for PEGylation. We have developed PEG-bis-sulfone reagents 1 that can selectively alkylate both sulfurs derived from a native disulfide. Using PEG-bis-sulfone reagents 1, conjugation of PEG specifically targets the disulfide distal to the binding region of the Fab (Scheme 2 ). PEG-bis-sulfone reagents 1 (10-40 kDa) were used to generate the corresponding PEG-mono-sulfones 2 that underwent essentially quantitative conjugation to give the PEG-Fab product 4. Four Fabs were PEGylated: Fab(beva), Fab'(beva), Fab(rani), and Fab(trast). Proteolytic digestion of bevacizumab with papain gave Fab(beva), while digestion of bevacizumab with IdeS gave F(ab')(2-beva), which after reaction with DTT and PEG-mono-sulfone 2 gave PEG(2)-Fab'(beva). Ranibizumab, which is a clinically used Fab, was directly PEGylated to give PEG-Fab(rani). Trastuzumab was proteolytically digested with papain, and its corresponding Fab was PEGylated to give PEG-Fab(trast). Purification of the PEGylated Fabs was accomplished by a single ion exchange chromatography step to give pure PEG-Fab products as determined by silver-stained SDS-PAGE. No loss of PEG was detected post conjugation. A comparative binding study by SPR using Biacore with low ligand immobilization density was conducted using (i) VEGF(165) for the bevacizumab and ranibizumab derived products or (ii) HER2 for the trastuzumab derived products. VEGF(165) is a dimeric ligand with two binding sites for bevacizumab. HER2 has one domain for the binding of trastuzumab. Binding studies with PEG-Fab(beva) indicated that the apparent affinity was 2-fold less compared to the unPEGylated Fab(beva). Binding properties of the PEG-Fab(beva) products appeared to be independent of PEG molecular weight. Site-specific conjugation of two PEG molecules gave PEG(2×20)-Fab'(beva), whose apparent binding affinity was similar to that observed for PEG-Fab(beva) derivatives. The k(d) values were similar to those of the unPEGylated Fab(beva); hence, once bound, PEG-Fab(beva) remained bound to the same degree as Fab(beva). Biacore analysis indicated that both Fab(rani) and PEG(20)-Fab(rani) did not dissociate from the immobilized VEGF at 25 °C, but ELISA using immobilized VEGF showed 2-fold less apparent binding affinity for PEG(20)-Fab(rani) compared to the unPEGylated Fab(rani). Additionally, the apparent binding affinities for trastuzumab and Fab(trast) were comparable by both Biacore and ELISA. Biacore results suggested that trastuzumab had a slower association rate compared to Fab(trast); however, both molecules displayed the same apparent binding affinity. This could have been due to enhanced rebinding effects of trastuzumab, as it is a bivalent molecule. Analogous to PEG-Fab(beva) products, PEG(20)-Fab(trast) displayed 2-fold lower binding compared to Fab(trast) when evaluated by ELISA. The variations in the apparent affinity for the PEGylated Fab variants were all related to the differences in the association rates (k(a)) rather than the dissociation rates (k(d)). We have shown that (i) Fabs are well-matched for site-specific PEGylation with our bis-alkylation PEG reagents, (ii) PEGylated Fabs display only a 2-fold reduction in apparent affinity without any change in the dissociation rate, and (iii) the apparent binding rates and affinities remain constant as the PEG molecular weight is varied.

Block copolymer micelles target Auger electron radiotherapy to the nucleus of HER2-positive breast cancer cells

Intracellular trafficking of Auger electron emitting radionuclides to perinuclear and nuclear regions of cells is critical to realizing their full therapeutic potential. In the present study, block copolymer micelles (BCMs) were labeled with the Auger electron emitter indium-111 ((111)In) and loaded with the radiosensitizer methotrexate. HER2 specific antibodies (trastuzumab fab) and nuclear localization signal (NLS; CGYGPKKKRKVGG) peptides were conjugated to the surface of the BCMs to direct uptake in HER2 expressing cells and subsequent localization in the cell nucleus. Cell uptake and intracellular distribution of the multifunctional BCMs were evaluated in a panel of breast cancer cell lines with different levels of HER2 expression. Indeed cell uptake was found to be HER2 density dependent, confirming receptor-mediated internalization of the BCMs. Importantly, conjugation of NLS peptides to the surface of BCMs was found to result in a significant increase in nuclear uptake of the radionuclide (111)In. Successful nuclear targeting was shown to improve the antipoliferative effect of the Auger electrons as measured by clonogenic assays. In addition, a significant radiation enhancement effect was observed by concurrent delivery of low-dose MTX and (111)In in all breast cancer cell lines evaluated.

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.