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.

MANOTA: a promising bifunctional chelating agent for copper-64 immunoPET.

Improved bifunctional chelating agents (BFC) are required for copper-64 radiolabelling of monoclonal antibodies (mAbs) under mild conditions to yield stable, target-specific imaging agents. Four different bifunctional chelating agents (BFC) were evaluated for Fab (Fragment antigen binding) conjugation and radiolabelling with copper-64. Two DOTA- (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and two NOTA- (1,4,7-triazacyclononane-1,4,7-triacetic acid) derivatives bearing a p-benzyl-isothiocyanate group were conjugated to Fab-trastuzumab - which targets the HER2/neu receptor - and the average number of chelators attached ranged from 2.4 to 4.3 macrocycles per Fab. Labelling of the immunoconjugate with copper-64 was achieved in high radiochemical yields after 45 min at 37 °C, and the radiochemical purity of each 64Cu-BFC-Fab-trastuzumab reached 97% after purification. The affinity of each 64Cu-BFC-Fab-trastuzumab ranged between 10 and 50 nM as evaluated by in vitro saturation assays using the HCC1954 breast cancer cell line. PET-MR imaging and biodistribution studies were performed in mice bearing breast cancer BT-474 xenografts. BT-474 tumours were clearly visualized on PET images at 4 and 24 hours post-injection. The tumour uptake of 64Cu-BFC-Fab-trastuzumab reached 8.9 to 12.8% ID g-1 24 hours post-injection and significant differences in non-specific liver uptake were observed depending on the BFC conjugated, the lowest being observed with MANOTA. These results show that MANOTA is a valuable tool for copper-64 radiolabelling.

Minor changes in the macrocyclic ligands but major consequences on the efficiency of gold nanoparticles designed for radiosensitization.

Many studies have been devoted to adapting the design of gold nanoparticles to efficiently exploit their promising capability to enhance the effects of radiotherapy. In particular, the addition of magnetic resonance imaging modality constitutes an attractive strategy for enhancing the selectivity of radiotherapy since it allows the determination of the most suited delay between the injection of nanoparticles and irradiation. This requires the functionalization of the gold core by an organic shell composed of thiolated gadolinium chelates. The risk of nephrogenic systemic fibrosis induced by the release of gadolinium ions should encourage the use of macrocyclic chelators which form highly stable and inert complexes with gadolinium ions. In this context, three types of gold nanoparticles (Au@DTDOTA, Au@TADOTA and Au@TADOTAGA) combining MRI, nuclear imaging and radiosensitization have been developed with different macrocyclic ligands anchored onto the gold cores. Despite similarities in size and organic shell composition, the distribution of gadolinium chelate-coated gold nanoparticles (Au@TADOTA-Gd and Au@TADOTAGA-Gd) in the tumor zone is clearly different. As a result, the intravenous injection of Au@TADOTAGA-Gd prior to the irradiation of 9L gliosarcoma bearing rats leads to the highest increase in lifespan whereas the radiophysical effects of Au@TADOTAGA-Gd and Au@TADOTA-Gd are very similar.

The use of theranostic gadolinium-based nanoprobes to improve radiotherapy efficacy.

A new efficient type of gadolinium-based theranostic agent (AGuIX®) has recently been developed for MRI-guided radiotherapy (RT). These new particles consist of a polysiloxane network surrounded by a number of gadolinium chelates, usually 10. Owing to their small size (<5 nm), AGuIX typically exhibit biodistributions that are almost ideal for diagnostic and therapeutic purposes. For example, although a significant proportion of these particles accumulate in tumours, the remainder is rapidly eliminated by the renal route. In addition, in the absence of irradiation, the nanoparticles are well tolerated even at very high dose (10 times more than the dose used for mouse treatment). AGuIX particles have been proven to act as efficient radiosensitizers in a large variety of experimental in vitro scenarios, including different radioresistant cell lines, irradiation energies and radiation sources (sensitizing enhancement ratio ranging from 1.1 to 2.5). Pre-clinical studies have also demonstrated the impact of these particles on different heterotopic and orthotopic tumours, with both intratumoural or intravenous injection routes. A significant therapeutical effect has been observed in all contexts. Furthermore, MRI monitoring was proven to efficiently aid in determining a RT protocol and assessing tumour evolution following treatment. The usual theoretical models, based on energy attenuation and macroscopic dose enhancement, cannot account for all the results that have been obtained. Only theoretical models, which take into account the Auger electron cascades that occur between the different atoms constituting the particle and the related high radical concentrations in the vicinity of the particle, provide an explanation for the complex cell damage and death observed.