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Design, Synthesis and Biological Assessment of Novel RSK Inhibitors as Anti-Cancer Agents

Joel Kawakami, PhD headshot
Joel Kawakami, PhD

Project Summary (Background): RSK2 (Ribosomal Protein S6 Kinase 2), is an established regulator of proliferation and cell adhesion, and further, promotes invasion and metastasis processes (1-3). Dr. Ramos is a leader in establishing proof of principle for RSK2 as a therapeutic target in glioblastomas and neuroblastomas (4,5). Glioblastomas are one of the deadliest forms of cancer with no effective treatment currently available. In addition, Dr.  Ramos is currently pursuing RSK2 as a target in melanoma skin cancer as a potential co-therapeutic with PD-L1 checkpoint inhibitors (4). To date, no RSK inhibitor drug for cancer is available (1). Current leads, like most serine/threonine kinases, are non-efficacious in the clinic (i.e. PKB/Akt Inhibitors). A new drug discovery paradigm, especially for serine/threonine kinases, is the design of irreversible inhibitors for these intracellular kinase targets that must compete with high concentrations of intracellular ATP. Drug discovery with molecular modeling design, synthesis and biological evaluation in isolated protein has been performed in the laboratory of Dr. Kawakami but in the area of PKB/Akt kinases. This grant builds on that work by focusing on drug discovery with molecular design, synthesis, and biological evaluation of new RSK inhibitors as a therapeutic target in cancer, in collaboration with Dr. Ramos.

The significance is to address lack of effective therapies for glioblastoma which is an immediate and crucial unmet need (4). RSK2 has been shown to be a key molecular target for the treatment of glioblastomas as a potential co-therapeutic with current standard of care chemotherapeutic Temozolomide. The chemical series proposed in this grant provides a significant potential advantage for the discovery of a potent RSK2 inhibitor for use in in vivo models. The chemical series proposed herein are known to have minimal p-glycoprotein interaction to improve crossing the blood brain barrier (BBB), good metabolic stability (in vitro hepatocyte cells), and favorable chemical descriptors for oral bioavailability based on Lipinski’s rule of five. 

Dr. Kawakami will develop novel compounds that inhibit RSK2 as potential drugs for the treatment of cancer, the primary mediator of oncogenic Ras signaling to integrins and cell migration (1). Dr. Ramos will test these for efficacy in glioblastoma tumors. RSK2 inhibitors/irreversible-inhibitors will be developed through structure-based, rational drug design aided by Molecular Operating Environment (MOE), a molecular modeling software by Chemical Consulting Group (CCG). The design of this initial set of compounds will be based on the SAR of current non-irreversible inhibitors (6,7). The strategy is known in the art as Targeted Covalent Binding (TCB) or Targeted Covalent Inhibitors (TCI), an increasing trend in drug discovery research using small molecules (8-10).

The inhibitors will be designed and synthesized in the laboratory of Dr Kawakami at Chaminade University of Honolulu by a team of undergraduate researchers, most of whom are underrepresented minorities. The design will be based on ligand to protein docking interaction for the appropriate placement of these war-heads near nucleophilic amino acid residues on RSK. All compounds synthesized will be fully characterized (1H & 13C-NMR and High-Resolution Mass Spectroscopy). The laboratory of Dr Joe W. Ramos at University of Hawaii Cancer Center will evaluate the compounds for RSK inhibitory potency/efficacy in purified protein, kinase selectively and RSK Cell-Based Assays. 

Development of selective and potent RSK inhibitors that work within in vivo models will provide new therapeutic leads that can be tested and refined for treatment of cancers of the brain, melanoma and breast (triple negative) and new tool compounds to understand RSK function. 


  1. Casalvieri, K.A., Matheson, C.J., Backos, D.S. and Reigan, P., Selective Targeting of RSK Isoforms in Cancer, Trends in Cancer, 2017, 3(4), 302-312.
  2. Houles, T. and Roux, P.P., Defining the role of the RSK isoforms in cancer, Semin Cancer Biol, 2018, 48, 53-61.
  3. Poomakkath, N., Issa, A., Abdulrahman, N., Adelaziz, S.G. and Mraiche, F., p90 ribosomal S6 kinase: a potential therapeutic target in lung cancer, J. Transl Med, 2016, 14, 14.
  4. Sulzmaier, F.J. and Ramos, J.W., RSK Isoforms in Cancer Cell Invation and Metastasis, Can Res, 2013 73(20), 6099-6105.
  5. Fernandez-Gallardo, J., Elie, B.T., Sadhukha, T., Prabha, S., Sanau, M., Rotenberg, S.A., Ramos, J.W.and Contel, M., Heterometallic titanium-gold complexes inhiit renal cancer cells in vitro and in vivo,
    Chemical Science, 2015, 6(9), 5269-5283.
  6. A) Boyer, S.J., Burke, J., Guo, X., Kirrane, T.M., Snow, R.J., Zhang, Y., Sarko, C., Soleymanzadeh, L., Swinamer, A., Westbrook, J., Dicapua, F., Padyana, A., Cogan, D., Gao, A., Xiong, Z., Madwed, J,B., Kashem, M., Kugler, S., and O’Neill, M.M, Indole RSK Inhibitors. Part 1: discovery and initial SAR, Bioorg Med Chem Lett, 2012, 22(1), 733-737. B) Kirrane, R.M., Boyer, S.J., Burke, J., Guo, X, Snow, R.J., Soleymanzadeh, L., Zhang., S.A., Madwed, J.B., Kashem, M., Kugler, S. and O’Neill, M.M., Indole RSK inhibitors, Part 2: optimization of cell potency and kinase selectivity, Bioorg Med Chem Lett, 2012, 22(1), 738-742.
  7. Casalvieri, K.A., Matheson, C.J., Backos, D.S. and Reigan, P., Substituted pteridinones as p90 ribosomal S6 protein kinase (RSK) injioitors: A structure-activity study, Bioorg Med Chem, 2020, 28(5), 115303-115306.
  8. Lonsdale, R. and Ward, R.A., Structure-based design of targeted covalent inhibitors, Chem. Soc. Rev., 2018, 47, 3816-3830.
  9. Gehringer, M. and Laufer, S.A., Emerging and Re-Emerging Warheads for Targeted Covalent Inhibitors: Applications in Medicinal Chemistry and Chemical Biology, J. Med. Chem., 2019, 62, 12, 5673-5724.
  10. Ghosh, A.K., Samanta, I. Mondal A. and Liu, W.R., Covalent Inhibition in Drug Discovery, Chem Med Chem, 2019, 14, 9, 889-906.


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