CL656 (cAIM(PS)2 Difluor (Rp/Sp)) - STING Agonist

cAIMP bisphosphorothioate and difluorinated (Rp/Sp) - STING ligand

InvivoGen's cAIM(PS)₂ Difluor (Rp/Sp) (also known as CL656) is a synthetic cyclic dinucleotide (CDN) and potent agonist of the endoplasmic reticulum-resident receptor STING (stimulator of interferon genes) [1-3]. It is composed of Rp/Sp-isomers of the bisphosphorothioate derivative of cAIMP, an analog of the bacterial cyclic dinucleotide (CDN) 3'3'-cGAMP [1-2]. The binding of cAIM(PS)₂ Difluor (Rp/Sp) to STING triggers the expression of interferon-β (IFN-β) and nuclear factor-κB (NF-κB) dependent inflammatory cytokines [1]. 
STING has been a privileged target for developing immunomodulatory therapeutics [3, 4]. Of note, cAIM(PS)₂ Difluor (Rp/Sp) has entered clinical trials for treating advanced/metastatic, recurrent, solid tumors, with emphasis on squamous cell carcinoma of the head and neck, triple-negative breast cancer, anaplastic thyroid carcinoma, and cutaneous squamous cell carcinoma [5]. CL656 can be considered a clinically relevant safe STING agonist since it has been successfully administered in humans in combination with exosomes (ExoSTING) [3,5].  

Key Features

• Unlike natural CDNs, whose constituent nucleosides are guanosine and/or adenine, cAIMP and its derivatives contain one adenine nucleoside and one inosine nucleoside. cAIM(PS)₂ Difluor (Rp/Sp) is composed of two 2’-deoxynucleosides with a fluorine atom at the 2’ position of each nucleoside for improved stability [5]. 
   
• A clinically relevant safe STING agonist successfully administered in humans to treat advanced solid tumors [3,5].
   
• In addition, this cAIMP analog contains two phosphorothioate diester linkages, which prevent its degradation by the phosphodiesterases that are present in host cells or in the systemic circulation [6].
   
• 2’3’‑cGAMP, cAIM(PS)₂ Difluor (Rp/Sp) (referred to as compound 53 by Lioux et al. [1]) is more potent than 2’3’‑cGAMP for STING activation [1].

Note: InvivoGen has developed stable STING reporter cells in two well-established immune cell models: THP-1 human monocytes and RAW 264.7 murine macrophages. These cells express inducible SEAP and/or Lucia luciferase reporter genes under the control of an IRF-inducible or NF-κB-inducible promoter.

References:

1. Lioux T. et al., 2016. Design, synthesis, and biological evaluation of novel cyclic adenosineinosine monophosphate (cAIMP) analogs that activate stimulator of interferon genes (STING). J Med Chem. 59:10253-10267.
2. Cyclic dinucleotides for cytokine induction, patent US10011630B2 and foreign equivalents.
3. Jang S., et al. 2021. ExoSTING, an extracellular vesicle loaded with STING agonists, promotes tumor immune surveillance. Commun. Biol. 4:497.
4. Zhang H., et al. 2020. Targeting Stimulator of Interferon Genes (STING): a medicinal chemistry perspective. Journal of Medicinal Chemistry. 63(8):3785.
5. https://clinicaltrials.gov/ct2/show/NCT04592484
6. Böhm H.J. et al., 2004. Fluorine in medicinal chemistry. Chembiochem. 5:637-43.
7. Yan H. et al., 2008. Synthesis and immunostimulatory properties of the phosphorothioate analogs of cdiGMP. Bioorg. Med. Chem. Lett. 18, 5631–5634.