[ ¹¹ C]Cimbi-36 was recently developed as a selective serotonin 2A (5-HT_2A) receptor agonist radioligand for positron emission tomography (PET) brain imaging. Such an agonist PET radioligand may provide a novel, and more functional, measure of the serotonergic system and agonist binding is more likely than antagonist binding to reflect 5-HT levels in vivo. Here, we show data from a first-in-human clinical trial with [ ¹¹ C]Cimbi-36. In 29 healthy volunteers, we found high brain uptake and distribution according to 5-HT_2A receptors with [ ¹¹ C]Cimbi-36 PET. The two-tissue compartment model using arterial input measurements provided the most optimal quantification of cerebral [ ¹¹ C]Cimbi-36 binding. Reference tissue modeling was feasible as it induced a negative but predictable bias in [ ¹¹ C]Cimbi-36 PET outcome measures. In five subjects, pretreatment with the 5-HT_2A receptor antagonist ketanserin before a second PET scan significantly decreased [ ¹¹ C]Cimbi-36 binding in all cortical regions with no effects in cerebellum. These results confirm that [ ¹¹ C]Cimbi-36 binding is selective for 5-HT_2A receptors in the cerebral cortex and that cerebellum is an appropriate reference tissue for quantification of 5-HT_2A receptors in the human brain. Thus, we here describe [ ¹¹ C]Cimbi-36 as the first agonist PET radioligand to successfully image and quantify 5-HT_2A receptors in the human brain.

Tumor targeting using agents with slow pharmacokinetics represents a major challenge in nuclear imaging and targeted radionuclide therapy as they most often result in low imaging contrast and high radiation dose to healthy tissue. To address this challenge, we developed a polymer-based targeting agent that can be used for pretargeted imaging and thus separates tumor accumulation from the imaging step in time. The developed targeting agent is based on polypeptide-graft-polypeptoid polymers (PeptoBrushes) functionalized with trans-cyclooctene (TCO). The complementary 111In-labeled imaging agent is a 1,2,4,5-tetrazine derivative, which can react with aforementioned TCO-modified PeptoBrushes in a rapid bioorthogonal ligation. A high degree of TCO-loading (up to 30%) was achieved, without altering the physicochemical properties of the polymeric nanoparticle. The highest degree of TCO-loading resulted in significantly increased reaction rates (77-fold enhancement) compared to small molecule TCO moieties when using lipophilic tetrazines. Based on computer simulations, we hypothesize that this increase is a result of hydrophobic effects and significant rearrangements within the polymer framework, in which hydrophobic patches of TCO moieties are formed. These patches attract lipophilic tetrazines leading to increased reaction rates in the bioorthogonal ligation. The most reactive system was evaluated as a targeting agent for pretargeted imaging in tumor-bearing mice. After optimizing the set-up, sufficient tumor-to-background ratios were achieved as early as 2 h after administration of the tetrazine imaging agent, which further improved at 22 h enabling clear visualization of CT-26 tumors. These findings show the potential of PeptoBrushes to be used as a pretargeting agent when an optimized dose of polymer is used.