Randall W. Hungate

Vascular endothelial growth factor receptor KDR tyrosine kinase activity is increased by autophosphorylation of two activation loop tyrosine residues.

Vascular endothelial growth factor is an important physiological regulator of angiogenesis. The function of this endothelial cell selective growth factor is mediated by two homologous tyrosine kinase receptors, fms-like tyrosine kinase 1 (Flt-1) and kinase domain receptor (KDR). Although the functional consequence of vascular endothelial growth factor binding to the Flt-1 receptor is not fully understood, it is well established that mitogenic signaling is mediated by KDR. Upon sequencing several independent cDNA clones spanning the cytoplasmic region of human KDR, we identified and confirmed the identity of a functionally required valine at position 848 in the ATP binding site, rather than the previously reported glutamic acid residue, which corresponds to an inactive tyrosine kinase. The cytoplasmic domain of recombinant native KDR, expressed as a glutathione S-transferase fusion protein, can undergo autophosphorylation in the presence of ATP. In addition, the kinase activity can be substantially increased by autophosphorylation at physiologic ATP concentrations. Mutation analysis indicates that both tyrosine residues 1054 and 1059 are required for activation, which is a consequence of an increased affinity for both ATP and the peptide substrate and has no effect on k cat, the intrinsic catalytic activity of the enzyme. KDR kinase catalyzes phosphotransfer by formation of a ternary complex with ATP and the peptide substrate. We demonstrate that tyrosine kinase antagonists can preferentially inhibit either the unactivated or activated form of the enzyme.

Synthesis and initial SAR studies of 3,6-disubstituted pyrazolo[1,5-a]pyrimidines: A new class of KDR kinase inhibitors

We have synthesized and evaluated the activity of 3,6-disubstituted pyrazolo[1,5-a]pyrimidines as a new class of KDR kinase inhibitors. Starting with screening lead 1, potency against isolated KDR was fully optimized with 3-thienyl and 4-methoxyphenyl substituents at the 6- and 3-positions (3g, KDR IC(50)=19 nM), respectively. The synthesis and SAR of these compounds are described.

Optimization of a pyrazolo[1,5-a]pyrimidine class of KDR kinase inhibitors: improvements in physical properties enhance cellular activity and pharmacokinetics.

We have introduced solubilizing functionality to a 3,6-disubstituted pyrazolo[1,5-a]pyrimidine series of KDR kinase inhibitors to improve the physical properties of these compounds. The addition of a basic side-chain to the 6-aryl ring, introduction of 3-pyridyl groups, and most significantly, incorporation of a 4-pyridinonyl substituent at the 6-position of the core are modifications that maintain and often enhance the intrinsic potency of this class of inhibitors. Moreover, the improvements in physical properties result in marked increases in cellular activity and more favorable pharmacokinetics in rats. The synthesis and SAR of these compounds are described.

Rapid X-ray diffraction analysis of HIV-1 protease-inhibitor complexes: inhibitor exchange in single crystals of the bound enzyme.

The ability to replace an inhibitor bound to the HIV-1 protease in single crystals with other potent inhibitors offers the possibility of investigating a series of protease inhibitors rapidly and conveniently with the use of X-ray crystallography. This approach affords a fast turnaround of structural information for iterative rational drug designs and obviates the need for studying the complex structures by co-crystallization. The replacement approach has been successfully used with single crystals of the HIV-1 protease complexed with a weak inhibitor. The structures of the complexes obtained by the replacement method are similar to those determined by co-crystallization.

Conformationally constrained HIV-1 protease inhibitors

Abstract The synsthesis and structure activity relationships of conformationally constrained analogs of the HIV-1 protease inhibitor L-685,434 are described. In addition, the X-ray crystal structure of a complex between L-700,497 and the HIV-1 protease is shown.

Design and synthesis of 1,5-diarylbenzimidazoles as inhibitors of the VEGF-receptor KDR

1,5-Diarylbenzimidazoles have been identified as potent inhibitors of KDR kinase activity. The series was developed with a goal of finding compounds with optimal drug-like properties. This communication describes structural modifications in the series that enhance solubility, lower protein binding, and provide compounds with excellent potency and pharmacokinetic profiles.

Discovery and evaluation of 3-(5-Thien-3-ylpyridin-3-yl)-1H-indoles as a novel class of KDR kinase inhibitors

We have discovered 3-(5-thien-3-ylpyridin-3-yl)-1H-indoles as potent inhibitors of KDR kinase activity. This communication details the evolution of this novel class from a potent screening lead of vastly different structure with an emphasis on structural modifications that retained activity and provided improvements in key physical properties. The synthesis and in-depth evaluation of these inhibitors are described.

New cytochalasins: Synthetic studies of a novel HIV-1 protease inhibitor

By using a benzyl copper reagent (e.g. 10→11) in combination with titanium enolate chemistry (e.g. 12→13) a concise sequence has been developed for the rapid introduction of functionality found in the cytochalasin HIV protease inhibitor (1).

Synthesis, antiviral activity, and bioavailability studies of γ-lactam derived HIV protease inhibitors

Incorporation of a gamma-lactam in hydroxyethylene isosteres results in modest inhibitors of HIV-1 protease. Additional structural activity studies have produced significantly more potent inhibitors with the introduction of the trisubstituted cyclopentane (see compound 20) as the optimum substituent for the C-terminus. This new amino acid amide surrogate can be readily prepared in large scale from (R)-pulegone. Optimized compounds (36) and (60) are potent antiviral agents and are well absorbed (15-20%) in a dog model after oral administration.

Aromatic P1 replacements for the highly potent HIV-1 protease inhibitor CRIXIVAN®

A series of analogs of CRIXIVAN® containing various aromatic P1 ligands were prepared and evaluated for inhibition of the HIV-1 protease enzyme. These new compounds were found to be effective inhibitors at nanomolar concentrations in the in vitro enzyme inhibition assay as well as the whole cell assay.