On the Road towards Small-Molecule Programmed Cell Death 1 Ligand 1 Positron Emission Tomography Tracers: A Ligand-Based Drug Design Approach

Abstract

PD-1/PD-L1 immune checkpoint blockade for cancer therapy showed promising results in clinical studies. Further endeavors are required to enhance patient stratification, as, at present, only a small portion of patients with PD-L1-positive tumors (as determined by PD-L1 targeted immunohistochemistry; IHC) benefit from anti-PD-1/PD-L1 immunotherapy. This can be explained by the heterogeneity of tumor lesions and the intrinsic limitation of multiple biopsies. Consequently, non-invasive in vivo quantification of PD-L1 on tumors and metastases throughout the entire body using positron emission tomography (PET) imaging holds the potential to augment patient stratification. Within the scope of this work, six new small molecules were synthesized by following a ligand-based drug design approach supported by computational docking utilizing lead structures based on the (2-methyl-[1,1′-biphenyl]-3-yl)methanol scaffold and evaluated in vitro for potential future use as PD-L1 PET tracers. The results demonstrated binding affinities in the nanomolar to micromolar range for lead structures and newly prepared molecules, respectively. Carbon-11 labeling was successfully and selectively established and optimized with very good radiochemical conversions of up to 57%. The obtained insights into the significance of polar intermolecular interactions, along with the successful radiosyntheses, could contribute substantially to the future development of small-molecule PD-L1 PET tracers.

Development and In Vivo Evaluation of Small-Molecule Ligands for Positron Emission Tomography of Immune Checkpoint Modulation Targeting Programmed Cell Death 1 Ligand 1

“…This model encompassed three hydrophobic features and a positive ionizable area (Figure B). Hydrophobic features represent the 2-methylbiphenyl core substructure situated at the base of the hydrophobic pocket formed within the interplay of two PD-L1 monomers. , …”

“…Hydrophobic features represent the 2-methylbiphenyl core substructure situated at the base of the hydrophobic pocket formed within the interplay of two PD-L1 monomers. 21,31 To identify novel potential structures substituting the 2methylbiphenyl moiety, the generated pharmacophore model underwent screening against a data set of 34,207 low molecular weight compounds (≤200 g/mol) including bioactive molecules with drug-like properties, marking the positive ionizable area as an optional feature. A total of 2695 in silico hits (7.9%) were acquired, exhibiting Pharmacophore-Fit Scores spanning from 34.73 to 38.89.…”

“…This work represents a significant advancement in binding capabilities when compared to commercially available small molecules, such as PD-1/PD-L1 Inhibitor 1 (BMS-1; IC 50 = 202 nM), PD-1/ PD-L1 Inhibitor 2 (BMS-202; IC 50 = 101 nM) and PD-1/PD-L1 Inhibitor 3 (a macrocyclic peptide; IC 50 = 113 nM) (Table Table 3 S2), as well as small molecules described in our previously reported ligand-based drug design approach. 31 Cell Viability and Cell-Based Competitive Binding Assay. Compounds 5a and 5c, exhibiting high affinities (IC 50 ) of 6.2 and 10 nM, respectively, were chosen as promising candidates and underwent further in vitro evaluation using cellbased assays.…”

“…Experiments were repeated for a total of three times. 10-fold dilution series of the small molecules were prepared at a constant final DMSO concentration of 0.2%, as it is recommended to keep DMSO below 0.5% . 10-fold dilution series with constant DMSO concentration was used for the PD-1/PD-L1 Inhibitor 1 (Selleck Chemicals Llc, Houston, USA) and PD-1/PD-L1 Inhibitor 2 (Selleck Chemicals Llc, Houston, USA).…”

“…Nonetheless, this inves-tigation provided valuable insights into the effects of structural modifications. 31 Our primary objective was to design and develop small molecules for noninvasive PET imaging targeting PD-L1, with the overarching goal of enhancing patient stratification within the framework of personalized medicine. This endeavor was rooted in the identification of promising substructures through rigorous in silico investigations and an extensive review of existing literature 18,32−34 (Figure 1).…”