Evaluation of a Radiolabeled Macrocyclic Peptide as Potential PET Imaging Probe for PD-L1.

The interaction between the immune checkpoint PD-1 and PD-L1 promotes T-cell deactivation and cancer proliferation. Therefore, immune checkpoint inhibition therapy, which relies on prior assessment of the target, has been widely used for many cancers. As a non-invasive molecular imaging tool, radiotracers bring novel information on the in vivo expression of biomarkers (e. g., PD-L1), enabling a personalized treatment of patients. Our work aimed at the development of a PD-L1-specific, peptide-based PET radiotracer. We synthesized and evaluated a radiolabeled macrocyclic peptide adapted from a patent by Bristol Myers Squibb. Synthesis of [68 Ga]Ga-NJMP1 yielded a product with a radiochemical purity>95 % that was evaluated in vitro. However, experiments on CHO-K1 hPD-L1 cells showed very low cell binding and internalization rates of [68 Ga]Ga-NJMP1 in comparison to a control radiopeptide (WL12). Non-radioactive cellular assays using time-resolved fluorescence energy transfer confirmed the low affinity of the reported parent peptide and the DOTA-derivatives towards PD-L1. The results of our studies indicate that the macrocyclic peptide scaffold reported in the patent literature is not suitable for radiotracer development due to insufficient affinity towards PD-L1 and that C-terminal modifications of the macrocyclic peptide interfere with important ligand/receptor interactions.

Unexpected transformation of n.c.a. [111In]InCl3 in stock solutions into an unreactive [111In]In-species.

While performing multiple indium-111 labeling of DOTA-modified peptides from a single batch of [111In]InCl3, inconsistent radiochemical yields were observed. We found that the formation of a radioactive impurity in the [111In]InCl3 stock solution hampered the reactivity of the indium-111 during radiolabeling reactions. The formation of this unknown 111In-species could be successfully suppressed by increasing the concentration of chloride ions in the stock solution and [111In]InCl3 was "recovered". Radiolabeling of DOTA-peptides with the stabilized [111In]InCl3 resulted again in acceptable radiochemical yields. In addition, we report convenient iTLC systems that allow distinguishing between [111In]InCl3, the formed unknown 111In-species, radiocolloids, and radiolabeled peptides (DOTANOC).

Sorbitol as a Polar Pharmacological Modifier to Enhance the Hydrophilicity of 99mTc-Tricarbonyl-Based Radiopharmaceuticals.

The organometallic technetium-99m tricarbonyl core, [99mTc][Tc(CO)3(H2O)3]+, is a versatile precursor for the development of radiotracers for single photon emission computed tomography (SPECT). A drawback of the 99mTc-tricarbonyl core is its lipophilicity, which can influence the pharmacokinetic properties of the SPECT imaging probe. Addition of polar pharmacological modifiers to 99mTc-tricarbonyl conjugates holds the promise to counteract this effect and provide tumor-targeting radiopharmaceuticals with improved hydrophilicities, e.g., resulting in a favorable fast renal excretion in vivo. We applied the "Click-to-Chelate" strategy for the assembly of a novel 99mTc-tricarbonyl labeled conjugate made of the tumor-targeting, modified bombesin binding sequence [Nle14]BBN(7-14) and the carbohydrate sorbitol as a polar modifier. The 99mTc-radiopeptide was evaluated in vitro with PC-3 cells and in Fox-1nu mice bearing PC-3 xenografts including a direct comparison with a reference conjugate lacking the sorbitol moiety. The glycated 99mTc-tricarbonyl peptide conjugate exhibited an increased hydrophilicity as well as a retained affinity toward the Gastrin releasing peptide receptor and cell internalization properties. However, there was no significant difference in vivo in terms of pharmacokinetic properties. In particular, the rate and route of excretion was unaltered in comparison to the more lipophilic reference compound. This could be attributed to the intrinsic properties of the peptide and/or its metabolites. We report a novel glycated (sorbitol-containing) alkyne substrate for the "Click-to-Chelate" methodology, which is potentially of general applicability for the development of 99mTc-tricarbonyl based radiotracers displaying an enhanced hydrophilicity.

Mini-review: Targeted radiopharmaceuticals incorporating reversible, low molecular weight albumin binders.

The combination of low molecular weight, reversible human serum albumin (HSA) binders with targeted radiopharmaceuticals in dual-targeted radioconjugates holds great promise, in particular for endoradiotherapy. Attachment of HSA-binders to radiopharmaceuticals extends their blood circulation time and results in an enhanced tumour uptake as well as often in an improved pharmacokinetic profile. In this mini-review, an overview of currently pursued approaches of this novel strategy is provided.

Quantitative Ligand Affinity Scales for Metal Triflate Salts: Application to Isomer Differentiation.

Owing to the importance of metal triflates in catalysis, the affinity of the cationic center for a selection of organic ligands was explored for InIII and ZnII triflates. The organic Lewis bases include a variety of carbonyls (amides, unsaturated ketones, a lactone) and cyclic 1,2-diols. The relative affinity of the ligands for the cationic center in triflates was quantitatively determined on the basis of relative ion concentrations determined by electrospray-ionization mass spectrometry. The affinity scales were discussed with reference to gas-phase proton basicity and Lewis basicity scales. Structural isomers and stereoisomers display significant affinity differences in several cases. In the case of isomer mixtures, a model describing the relative peak intensities in the mass spectra was developed. On this basis, an isomer titration method was set up. Remarkably, this MS-based method overcame the blindness of mass spectrometry to isomers without the need for isotope labeling or MS/MS experiments. This model may prove to have applications in analytical chemistry and catalysis.