Darin Gustin

Structure-based design of novel inhibitors of the MDM2-p53 interaction.

Structure-based rational design led to the discovery of novel inhibitors of the MDM2-p53 protein-protein interaction. The affinity of these compounds for MDM2 was improved through conformational control of both the piperidinone ring and the appended N-alkyl substituent. Optimization afforded 29 (AM-8553), a potent and selective MDM2 inhibitor with excellent pharmacokinetic properties and in vivo efficacy.

Structure guided design of a series of sphingosine kinase (SphK) inhibitors.

Sphingosine-1-phosphate (S1P) signaling plays a vital role in mitogenesis, cell migration and angiogenesis. Sphingosine kinases (SphKs) catalyze a key step in sphingomyelin metabolism that leads to the production of S1P. There are two isoforms of SphK and observations made with SphK deficient mice show the two isoforms can compensate for each others loss. Thus, inhibition of both isoforms is likely required to block SphK dependent angiogenesis. A structure based approach was used to design and synthesize a series of SphK inhibitors resulting in the identification of the first potent inhibitors of both isoforms of human SphK. Additionally, to our knowledge, this series of inhibitors contains the only sufficiently potent inhibitors of murine SphK1 with suitable physico-chemical properties to pharmacologically interrogate the role of SphK1 in rodent models and to reproduce the phenotype of SphK1 (-/-) mice.

Sphingosine Kinase Activity Is Not Required for Tumor Cell Viability

Sphingosine kinases (SPHKs) are enzymes that phosphorylate the lipid sphingosine, leading to the formation of sphingosine-1-phosphate (S1P). In addition to the well established role of extracellular S1P as a mitogen and potent chemoattractant, SPHK activity has been postulated to be an important intracellular regulator of apoptosis. According to the proposed rheostat theory, SPHK activity shifts the intracellular balance from the pro-apoptotic sphingolipids ceramide and sphingosine to the mitogenic S1P, thereby determining the susceptibility of a cell to apoptotic stress. Despite numerous publications with supporting evidence, a clear experimental confirmation of the impact of this mechanism on tumor cell viability in vitro and in vivo has been hampered by the lack of suitable tool reagents. Utilizing a structure based design approach, we developed potent and specific SPHK1/2 inhibitors. These compounds completely inhibited intracellular S1P production in human cells and attenuated vascular permeability in mice, but did not lead to reduced tumor cell growth in vitro or in vivo. In addition, siRNA experiments targeting either SPHK1 or SPHK2 in a large panel of cell lines failed to demonstrate any statistically significant effects on cell viability. These results show that the SPHK rheostat does not play a major role in tumor cell viability, and that SPHKs might not be attractive targets for pharmacological intervention in the area of oncology.

Discovery and molecular basis of potent noncovalent inhibitors of fatty acid amide hydrolase (FAAH)

Fatty acid amide hydrolase (FAAH), an amidase-signature family member, is an integral membrane enzyme that degrades lipid amides including the endogenous cannabinoid anandamide and the sleep-inducing molecule oleamide. Both genetic knock out and pharmacological administration of FAAH inhibitors in rodent models result in analgesic, anxiolytic, and antiinflammatory phenotypes. Targeting FAAH activity, therefore, presents a promising new therapeutic strategy for the treatment of pain and other neurological-related or inflammatory disorders. Nearly all FAAH inhibitors known to date attain their binding potency through a reversible or irreversible covalent modification of the nucleophile Ser241 in the unusual Ser-Ser-Lys catalytic triad. Here, we report the discovery and mechanism of action of a series of ketobenzimidazoles as unique and potent noncovalent FAAH inhibitors. Compound 2, a representative of these ketobenzimidazoles, was designed from a series of ureas that were identified from high-throughput screening. While urea compound 1 is characterized as an irreversible covalent inhibitor, the cocrystal structure of FAAH complexed with compound 2 reveals that these ketobenzimidazoles, though containing a carbonyl moiety, do not covalently modify Ser241. These inhibitors achieve potent inhibition of FAAH activity primarily from shape complementarity to the active site and through numerous hydrophobic interactions. These noncovalent compounds exhibit excellent selectivity and good pharmacokinetic properties. The discovery of this distinctive class of inhibitors opens a new avenue for modulating FAAH activity through nonmechanism-based inhibition.

Identification of potent, noncovalent fatty acid amide hydrolase (FAAH) inhibitors.

Starting from a series of ureas that were determined to be mechanism-based inhibitors of FAAH, several spirocyclic ureas and lactams were designed and synthesized. These efforts identified a series of novel, noncovalent FAAH inhibitors with in vitro potency comparable to known covalent FAAH inhibitors. The mechanism of action for these compounds was determined through a combination of SAR and co-crystallography with rat FAAH.

Imidazo-pyrazine derivatives as potent CXCR3 antagonists

A general way of improving the potency of CXCR3 antagonists with fused hetero-bicyclic cores was identified. Optimization efforts led to the discovery of a series of imidazo-pyrazine derivatives with improved pharmacokinetic properties in addition to increased potency. The efficacy of the lead compound 21 is evaluated in a mouse lung inflammation model.

Discovery of potent and specific CXCR3 antagonists.

The optimization of a series of 8-aza-quinazolinone analogs for antagonist activity against the CXCR3 receptor is reported. Compounds were optimized to avoid the formation of active metabolites and time-dependent-inhibitors of CYP3A4. In addition, antagonists showed potent against CXCR3 activity in whole blood and optimized to avoid activity in the chromosomal aberration assay. Compound 25 was identified as having the optimal balance of CXCR3 activity and pharmacokinetic properties across multiple pre-clinical species, which are reported herein.

Abstract LB-39: Sphingosine kinase activity is not required for tumor cell viability.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Sphingosine kinase activity is not required for tumor cell viability Holger Wesche, Matthew L. Brown, Timothy J. Carlson, Angela Coxon, Brendon Frank, Darin J. Gustin, Shawn Jeffries, Shyun Li, Yihong Li, Kurt Morgenstern, Matthew Plant, Karen Rex, Joanna Schmidt, Shanling Shen, Nigel Walker, Dineli Wickramasinghe, Mariwil Wong, Guifen Xu Contribution from the Departments of Oncology Research, Medicinal Chemistry, Molecular Structure and Characterization and Pharmacokinetics and Drug Metabolism, Amgen Inc., South San Francisco, 1120 Veterans Blvd., South San Francisco, Ca, 94080. Sphingosine kinases (SPHKs) are enzymes that phosphorylate the lipid sphingosine, leading to the formation of sphingosine-1-phosphate (S1P). In addition to the well established role of extracellular S1P as a mitogen and potent chemoattractant, SPHK activity has been postulated to be an important intracellular regulator of apoptosis. According to the proposed rheostat theory, SPHK activity shifts the intracellular balance from the pro-apoptotic sphingolipids ceramide and sphingosine to the mitogenic S1P, thereby determining the susceptibility of a cell to apoptotic stress. Despite numerous publications with supporting evidence, a clear experimental confirmation of the impact of this mechanism on tumor cell viability in vitro and in vivo has been hampered by the lack of suitable tool reagents. Utilizing a structure based design approach, we developed potent and specific SPHK1/2 inhibitors. These compounds completely inhibited intracellular S1P production in human cells and attenuated vascular permeability in mice, but did not lead to reduced tumor cell growth in vitro or in vivo . These results show that the SPHK rheostat does not play a major role in tumor cell viability, and that SPHK inhibition may not offer an advantage over S1P neutralization in the treatment of cancer. Citation Format: Holger Wesche, Matthew L. Brown, Timothy J. Carlson, Angela Coxon, Brendon Frank, Darin J. Gustin, Shawn Jeffries, Shyun Li, Yihong Li, Kurt Morgenstern, Kurt Morgenstern, Matthew Plant, Karen Rex, Joanna Schmidt, Shanling Shen, Nigel Walker, Dineli Wickramasinghe, Guifen Xu. Sphingosine kinase activity is not required for tumor cell viability. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-39. doi:10.1158/1538-7445.AM2013-LB-39