PT  - JOURNAL ARTICLE
AU  - Wu, Xiaoai
AU  - Pracitto, Richard
AU  - Pan, Lili
AU  - Li, Lin
AU  - Cai, Zhengxin
TI  - Synthesis and evaluation of novel PD-L1-targeted small molecules for <sup>18</sup>F-labeling and PET imaging
DP  - 2019 May 01
TA  - Journal of Nuclear Medicine
PG  - 338--338
VI  - 60
IP  - supplement 1
4099  - http://jnm.snmjournals.org/content/60/supplement_1/338.short
4100  - http://jnm.snmjournals.org/content/60/supplement_1/338.full
SO  - J Nucl Med2019 May 01; 60
AB  - 338Objectives: The immune checkpoint pathways play important roles for vertebrates to control the immune response. However, cancer cells also exploit these checkpoint pathways to evade antitumor immune responses, and hence immune checkpoint pathways are viable target for anti-cancer therapies and attracted much attention in recent years. At present, several immune checkpoint blocking antibodies have been approved by the U.S. Food and Drug Administration and used as effective treatment against metastatic melanoma, non-small-cell lung cancer (NSCLC) and renal cell carcinoma in the clinic. It is also important to measure the expression of immune checkpoint associated proteins before the applications of immune checkpoint blocking antibodies to optimize and predict the anti-cancer therapeutic effects. Though radiolabeled proteins/antibodies have been reported as promising PD-L1 imaging agents, the disadvantages of protein-based and antibody-based imaging agents, i.e., slow clearance rate, low brain penetration (for primary/ metastatic lesions in brain), call for small molecule-based PD-L1 imaging agents. Based on the structures revealed by Bristol-Myers-Squibb (BMS), we synthesized a small library of fluorine-containing PD-L1 ligands, to identify suitable PET tracers with high affinities to human PD-L1, and reasonable logP values for brain penetration. Methods: A total of 20 compounds were prepared from commercially available starting materials, using methods adapted from literatures with slight modifications. All the target compounds in this investigation were readily prepared using Mistunobu reaction or Buchwald-Hartwig cross coupling reactions as key steps to get the biphenyl intermediates, followed with sodium cyanoborohydride-mediated reductive amination or cesium carbonate-promoted O-alkylation before the reductive amination. Homogeneous Time-resolved Fluorescence (HTRF) human PD1/PD-L1 Binding Assay was performed to evaluate the IC50s of the newly synthesized PD-L1 ligands. Compound 9 showed most potent inhibitory and its organotin precursor was synthesized from the corresponding brominated analog using palladium catalyst. Results and Discussion: All compounds were successfully synthesized in 4-5 steps with overall yields ranging from 24% - 35%, and the organotin precursor was also synthesized via bromo intermediate. Compound 9 showed the most potent activity with IC50 being 4 nM, which is followed by compound 10 (9 nM), compound 7 (10 nM) and compound 11 (10 nM). Structure activity relationship analysis showed the influences of various substituents at the 1, 3, 4 and 6 position of the ’core ring’ of the lead compound from the BMS patent (see Figure). Studies indicate that the modification of R1 will slightly increase the activity, and the introduction of pyridine at the 3position (R3) of the benzene ring will also slightly increase the activity. Introduction of bromide at 6 position (R4) did not improve the biochemical potencies, but the chiral six membered substituent at 4 position (R2) showed good bioactivities. Importantly, the substitution with fluorine at the 6 position of the core ring did not significantly influence the binding affinity. And this allows for the radiofluorination to generate radiotracers for PET imaging. Conclusions: To develop small molecule-based PD-L1 PET imaging probes, we designed and synthesized a library of fluorinated PD-L1 ligands based on the biphenyl pharmacophore. In vitro binding assay showed that several of the compounds possess nano-molar affinities to human PD-L1, and are potential PD-L1 PET imaging probe candidates. The radiolabeling and in vitro and in vivo pharmacology and pharmacokinetics studies of these PET tracers are under the way. Figure 1. The structure of the lead compound and the regions for SAR