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-tobackground-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. 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-tobackground ratios. No specific binding was observed in a HER2-negative model. High specific tumor uptake combined with the lowest background uptake was measured using the 1B4M-DTPA-based conjugate.