Development of 177Lu-nanobodies for radioimmunotherapy of HER2-positive breast cancer: evaluation of different bifunctional chelators.

Nanobodies show favourable pharmacokinetic characteristics for tumor targeting, including high tumor-to-background-ratios. Labelled with a therapeutic radionuclide, nanobodies could be used as an adjuvant treatment option for HER2-overexpressing minimal residual disease. The therapeutic radionuclide Lutetium-177 is linked to the nanobody using a bifunctional chelator. The choice of the bifunctional chelator could affect the in vivo behaviour of the radiolabeled nanobody. Consequently, we compared four different bifunctional chelators - p-SCN-Bn-DOTA, DOTA-NHS-ester, CHX-A"-DTPA or 1B4M-DTPA - in order to select the optimal chemical link between Lutetium-177 and a HER2 targeting nanobody. MS results revealed different degrees of chelator-conjugation. High stability in time was observed, together with nanomolar affinities on HER2-expressing tumor cells. Ex vivo biodistributions as well as SPECT/micro-CT analyses showed high activities in tumors expressing medium HER2 levels with low background activity except for the kidneys. The 1B4M-DTPA-coupled conjugate was further evaluated in a high HER2-expressing tumor model. Here, tumor uptake values of 5.99 ± 0.63, 5.12 ± 0.17, 2.83 ± 0.36 and 2.47 ± 0.38 %IA/g were obtained at 1, 3, 24 and 48h p.i., which coincided with exceptionally low background values, except for the kidneys, and unprecedented tumor-to-background ratios. No specific binding was observed in a HER2-negative model. In conclusion, the in-house developed anti-HER2 nanobody 2Rs15dHIS can be successfully labeled with (177) Lu using different bifunctional chelators. Both macrocyclic and acyclic chelators show high stability in time. High specific tumor uptake combined with the lowest background uptake was measured using the 1B4M-DTPA-based conjugate.

Increased tumor uptake of 3-(123)I-Iodo-L-alpha-methyltyrosine after preloading with amino acids: an in vivo animal imaging study.

UNLABELLED: 3-(123)I-Iodo-L-alpha-methyltyrosine (3-IMT) is an amino acid analog used for tumor imaging. Specific accumulation is mediated mainly by the system L amino acid transport system. System L activity is known to increase when cells are loaded with amino acids. The aim of our study was to measure the effects of amino acid preload on (123)I-3-IMT tumor uptake and image contrast in a rat tumor model using in vivo dynamic imaging. METHODS: Rhabdomyosarcoma (R1M) tumor-bearing rats underwent 2 dynamic (123)I-3-IMT studies on separate days: 1 baseline study and 1 after intraperitoneal injection (0.25 mmol/kg) of a single amino acid (arginine, proline, glutamate, asparagine, tryptophan, or phenylalanine) administered 30 min before intravenous injection of 18.5 MBq (123)I-3-IMT. A (99m)Tc-labeled human serum albumin study was performed on each rat for the calculation of the blood-pool activity inside the tumor. Time-activity curves were generated for tumor, contralateral background region, kidney, heart, and total body. Tumor uptake was corrected for blood-pool and background activity. Image contrast was calculated as the ratio between tumor and background activity. The rate (K(1)) of tracer entering the tumor was obtained using Patlak analysis. A displacement study was performed on a separate group of rats, in which a high dose of phenylalanine was administered 40 min after (123)I-3-IMT injection. RESULTS: (123)I-3-IMT accumulation in tumor reached a plateau 10 min after injection. Tumor uptake on the baseline scans correlated well with tumor size (r = 0.92). After preloading, tumor uptake and contrast increased in all conditions: arginine, +26% and +26%; proline, +15% and +13%; glutamate, +14% and +9%; asparagine, +19% and +15%; tryptophan, +36% and 11%; phenylalanine, +22% and + 13%. K(1) values also increased. Administration of an afterload with phenylalanine induced a significant displacement of (123)I-3-IMT tumor accumulation. CONCLUSION: Prior amino acid administration increases (123)I-3-IMT tumor accumulation and image contrast. This effect can be explained by the increased antiporter activity of the amino acid transport system L in preloaded conditions. Our results indicate that the fasted state might not be the optimal metabolic condition to study tumor accumulation of L-transported tracers such as (123)I-3-IMT. Amino acid administration before (123)I-3-IMT injection could improve tumor uptake and image contrast.