Spot on: Bacterial transglutaminase enables the site‐specific modification of Gln side chains of tumor‐targeting antibodies with various probes containing lysine or lysine surrogates. The method yields completely homogeneous immunoconjugates with a defined stoichiometry. In comparative in vivo studies with xenografted mice the pharmacological profiles for enzymatically conjugated antibodies were better than those of chemically modified analogues.

Click chemistry has been employed for the assembly of novel and efficient triazole-based multidentate chelating systems while simultaneously attaching them to molecules of biological interest. The "click-to-chelate" approach offers a powerful new tool for the modification of (bio)molecules with metal chelators for potential diagnostic and therapeutic applications.

The folate receptor (FR) has proven a valuable target for nuclear imaging using folic acid radioconjugates. However, using folatebased radiopharmaceuticals for therapy has long been regarded as an unattainable goal because of their considerable renal accumulation. Herein, we present a novel strategy in which a DOTA-folate conjugate with an albumin-binding entity (cm09) was designed with the aim of prolonging circulation in the blood and therewith potentially improving tumor-to-kidney ratios. Methods: The folate conjugate cm09 was radiolabeled with 177 LuCl 3 , and stability experiments were performed in plasma. Cell uptake studies were performed on FR-positive KB tumor cells, and an ultrafiltration assay was used to determine the plasma protein-binding properties of 177 Lu-cm09. In vivo, 177 Lu-cm09 was tested in KB tumor-bearing mice using SPECT/CT. The therapeutic anticancer effect of 177 Lu-cm09 (20 MBq) applied as a single injection or as fractionated injections was investigated in different groups of mice (n 5 5) by monitoring tumor size and the survival time of treated mice, compared with untreated controls. Results: Compound cm09 was radiolabeled at a specific activity of 40 MBq/nmol, a radiochemical yield of more than 98%, and a stability of more than 99% over 5 d in plasma. Ultrafiltration revealed significant binding of 177 Lu-cm09 to serum proteins (;91%) in plasma, compared with folate radioconjugate without an albumin-binding entity. Cell uptake and internalization of 177 Lu-cm09 was FR-specific and comparable to other folate radioconjugates. In vivo studies resulted in high tumor uptake (17.56 percentage injected dose per gram [%ID/g] at 4 h after injection), which was almost completely retained for at least 72 h. Renal accumulation was significantly reduced (28 % ID/g at 4 h after injection), compared with folate conjugates that lack an albumin-binding entity (;70 %ID/g at 4 h after injection). These circumstances enabled SPECT imaging of excellent quality. Radionuclide therapy (1 · 20 MBq) revealed complete remission of tumors in 4 of 5 cases and a significantly prolonged survival time, compared with untreated controls. Conclusion: The modification of a folate radioconjugate with an albuminbinding entity resulted in a significant increase of the tumor-tokidney ratio of radioactivity, enabling for the first time, to our knowledge, the preclinical application of folic acid-targeted radionuclide therapy in mice.