Initial performance evaluation of a preclinical PET scanner available as a clip-on assembly in a sequential PET/MRI system

We evaluated the performance characteristics of a prototype preclinical PET scanner available as an easy clippable assembly that can dock to an MRI system. The single ring version of the PET system consists of eight detectors, each of which comprises a 12  ×  12 silicon photomultipliers (SiPMs) array coupled with a dual layer of offset scintillation crystals to measure depth of interaction. The crystal arrays have 29  ×  29 (30  ×  30 for the outer layer) 4 mm long LYSO crystals (6 mm for the outer layer). The ring diameter is 119.2 mm and the axial field of view is 50.4 mm. The NEMA NU 4-2008 protocol was followed for studying the PET performance. Temperature stability of SiPMs was also investigated. The peak system absolute sensitivity was 4.70% with an energy window of 250-750 keV. The spatial resolution was 1.28/1.88/1.85 mm FWHM (radial/tangential/axial) at a distance of 5 mm from the center. Peak noise equivalent counting rate and scatter fraction for mouse phantom were 61.9 kcps at 14.9 MBq and 21.0%, respectively. The uniformity was 6.3% and the spill-over ratios in the images of the water-and air-filled chambers were 0.07 and 0.17, respectively. Recovery coefficients ranged from 0.13 to 0.96. Change in sensitivity as a function of ambient temperature was 0.3%/°C. These first results indicate excellent spatial resolution performance for use with animal studies. Moreover, the clippable assembly can be upgraded to accept a second ring of SiPMs modules, leading to improved sensitivity and axial coverage.

Radiolabeled F(ab')2-cetuximab for theranostic purposes in colorectal and skin tumor-bearing mice models

PURPOSE

This study aimed to investigate theranostic strategies in colorectal and skin cancer based on fragments of cetuximab, an anti-EGFR mAb, labeled with radionuclide with imaging and therapeutic properties, ¹¹¹In and ¹⁷⁷Lu, respectively.

METHODS

We designed F(ab')₂-fragments of cetuximab radiolabeled with ¹¹¹In and ¹⁷⁷Lu. ¹¹¹In-F(ab')₂-cetuximab tumor targeting and biodistribution were evaluated by SPECT in BalbC nude mice bearing primary colorectal tumors. The efficacy of ¹¹¹In-F(ab')₂-cetuximab to assess therapy efficacy was performed on BalbC nude mice bearing colorectal tumors receiving 17-DMAG, an HSP90 inhibitor. Therapeutic efficacy of the radioimmunotherapy based on ¹⁷⁷Lu-F(ab')₂-cetuximab was evaluated in SWISS nude mice bearing A431 tumors.

RESULTS

Radiolabeling procedure did not change F(ab')₂-cetuximab and cetuximab immunoreactivity nor affinity for HER1 in vitro. ¹¹¹In-DOTAGA-F(ab')₂-cetuximab exhibited a peak tumor uptake at 24 h post-injection and showed a high tumor specificity determined by a significant decrease in tumor uptake after the addition of an excess of unlabeled-DOTAGA-F(ab')₂-cetuximab. SPECT imaging of ¹¹¹In-DOTAGA-F(ab')₂-cetuximab allowed an accurate evaluation of tumor growth and successfully predicted the decrease in tumor growth induced by 17-DMAG. Finally, ¹⁷⁷Lu-DOTAGA-F(ab')₂-cetuximab radioimmunotherapy showed a significant reduction of tumor growth at 4 and 8 MBq doses.

CONCLUSIONS

¹¹¹In-DOTAGA-F(ab')₂-cetuximab is a reliable and stable tool for specific in vivo tumor targeting and is suitable for therapy efficacy assessment. ¹⁷⁷Lu-DOTAGA-F(ab')₂-cetuximab is an interesting theranostic tool allowing therapy and imaging.

Abstract P5-01-03: HER2 imaging by SPECT-CT using 111In radiolabeled pertuzumab-fab DOTAGA-conjugate: A proof of concept study in a preclinical model of breast cancer

Introduction: HER2 is positive in approximately 20-30% of all breast cancer and is associated with poor prognosis, higher mortality and higher metastatic incidence. Current diagnosis of HER2 expression relies on invasive methods requiring tissue biopsy which can lead to variable results due to inter/intra-metastatic and intratumoral heterogeneity in breast cancers. It has been recently demonstrated that HER2 molecular imaging based on pertuzumab, an antibody targeting HER2, could represent a more accurate non-invasive method to assess HER2 expression and evaluate its spatial and temporal heterogeneity. Aim: In the present study, we aimed at developing radiolabeled pertuzumab Fab fragments for HER2 imaging. Tumor uptake of radiolabeled Fab was evaluated in an animal model of HER2 breast cancer by SPECT-CT, and the impact of trastuzumab on HER2 imaging was assessed. The objective of this study was to validate the feasibility of HER2 imaging with a pertuzumab-derived probe and to evaluate the possible translation of such a probe for clinical use in patients treated or not with anti-HER2 therapy. Methods: Fab fragments of pertuzumab have been generated by papain digestion and bioconjugated with the bifunctional chelating agent DOTAGA for incorporation of Indium 111 to generate an HER2-specific probe for SPECT (111In-DOTAGA-pertuzumab-Fab). The functionality of both radiolabeled Fab and whole pertuzumab was evaluated by in vitro assays using HER2-overexpressing human breast cancer cell line (HCC1954). Tumor uptake of pertuzumab-Fab and whole pertuzumab (111In-DOTAGA-Pertuzumab) has been evaluated in Balb/c nude mice bearing BT-474 tumors with or without pre-treatment with trastuzumab for 24h. Results: In the current study we demonstrate that Fab fragments of pertuzumab keep similar HER2 binding properties than whole pertuzumab and can therefore be a suitable HER2-targeted probe. In vivo, 111In-DOTAGA-pertuzumab-Fab showed better pharmacokinetics than whole pertuzumab with faster tumor uptake and blood clearance allowing faster imaging with better tumor/blood ratio. In addition, tumor uptake of 111In-DOTAGA-pertuzumab-Fab is not modified by pre-treatment with trastuzumab. Conclusion: We assume that radiolabeled pertuzumab-Fab should be of great interest if the intention to treat is based on anti-HER2 monoclonal antibodies-based therapies rather than small molecules (e.g. lapatinib). Interestingly, pertuzumab has been shown to be also beneficial in combination with trastuzumab and chemotherapy in non-metastatic patients through its approval for the neoadjuvant treatment and very recently in the APHINITY clinical trial aiming for an approval in the adjuvant treatment of early breast cancer. These data suggest that pertuzumab will be extensively used in various HER-2 positive breast cancers underlining the interest to perform molecular imaging of HER2 expression as a predictive biomarker of efficacy.

Citation Format: Oudot A, Bellaye P-S, Vrigneaud J-M, Raguin O, Bernhard C, Dumont L, Brunotte F, Savina A, Bouquet F, Fumoleau P, Collin B. HER2 imaging by SPECT-CT using 111In radiolabeled pertuzumab-fab DOTAGA-conjugate: A proof of concept study in a preclinical model of breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-01-03.

Abstract P4-02-06: Molecular imaging with trastuzumab and pertuzumab of HER2-positive breast cancer in mice: a step towards personalized medicine

Introduction: The accurate and reliable assessment of HER2 status is an essential step in the diagnostic workup and selection of targeted treatment strategy in breast cancer patients. However, current ex vivo methods do not evaluate HER2 status possible discordance between primary tumor and distant metastases, nor immediate response to therapeutic intervention. Non-invasive imaging of HER2 expression could be a relevant alternative with additional benefits such as a better selection of patients for HER2-targeted therapies and the possible optimization of treatment strategies. Recent clinical trials suggest a better efficacy of trastuzumab/pertuzumab doublet but a lower activity of pertuzumab monotherapy over trastuzumab alone.

Aim: The aim of our study was therefore to evaluate preclinically the use of radiolabeled DOTAGA (2, 2′, 2″-(10-(2,6-dioxotetrahydro-2H-pyran-3-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid) conjugated trastuzumab and pertuzumab for SPECT/CT (Single Photon Emission Computed Tomography/Computed Tomography) HER2 imaging purposes.

Methods: Trastuzumab and pertuzumab were conjugated to a new bifunctional chelating agent: DOTAGA anhydride (DOTA analog) and subsequently radiolabeled with the gamma-emitter Indium 111. The functionality of [111In]-DOTAGA-antibodies analogs was evaluated by in vitro saturation assays using HER2-overexpressing human breast cancer cell line (HCC1954). In vivo biodistribution was studied by SPECT/CT imaging at 24h, 48h and 72h after intravenous injection of [111In]-DOTAGA-antibodies in subcutaneous BT474 tumor-bearing rodents.

Results: [111In]-DOTAGA-trastuzumab and [111In]-DOTAGA-pertuzumab showed respectively 2.6 and 2.7 DOTAGA/antibody. Both [111In]-DOTAGA-trastuzumab and [111In]-DOTAGA-pertuzumab achieved a radiochemical purity > 95%. Biological activity of [111In]-DOTAGA-trastuzumab and [111In]-DOTAGA-pertuzumab was maintained: the affinity determined on HER2 expressing HCC1954 cell lines, was 5.5 and 5.6 nM. SPECT/CT imaging experiments in tumor-bearing rodents and gamma-counting of organs both showed that at 72h post-injection, more than 60 % of the activity was localized in the tumor (66.9±0.9 %ID/g for [111In]-DOTAGA-trastuzumab and 63.8±6.3 %ID/g for [111In]-DOTAGA-pertuzumab) and that an excess of non-radiolabeled corresponding antibody significantly shifted down tumor-targeting (to 17.6±2.4 %ID/g for [111In]-DOTAGA-trastuzumab and 8.7±0.8 %ID/g for [111In]-DOTAGA-pertuzumab.

Conclusions: Our results demonstrate a similar HER2 binding of trastuzumab and pertuzumab. Both radiolabeled [111In]-DOTAGA-trastuzumab and [111In]-DOTAGA-pertuzumab should be considered and might be promising tools for molecular imaging diagnosis of HER2 overexpression in breast cancer.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-02-06.

Distribution of strand breaks produced by Auger electrons in decay of 125I in triplex DNA

In this study we investigate the possibility of using Auger electrons as a probing agent for the study of structures of nucleic acids. To this end, we present the distribution of breaks produced in strands of a DNA duplex and a triplex-forming oligonucleotide (TFO) carrying Auger emitting radionuclide 125I. The method of calculation includes use of a molecular model of plasmid DNA duplex with bound TFO carrying a labelled 125I at position C5 of a single deoxycytosine residue, a source of Auger spectra, Monte Carlo electron track structure and the ensuing chemistry codes, to simulate the distribution of breaks produced in both strands of a plasmid DNA. Frequencies of fragment length distributions were obtained for the TFO, the purine and the pyrimidine strands. The frequency of breaks in the purine strand showed good correlation with the published experimental results, while that for the pyrimidine strand is lower by a factor of 3. It is concluded that the true structure of triplex DNA may not be purely of B-form.