An efficient synthesis of a 4'-phosphonodifluoromethyl-3'-formyl-phenylalanine containing SRC SH2 ligand

[Synthesis of phosphonic acid and phosphinic acid derivatives for development of biologically active compounds]

This paper covers recent publications from our laboratory on the synthesis of a variety of phosphonate and phosphinate derivatives. New methods for the enantioselective synthesis of alpha-hydroxyphosphonates were established by Lewis acid-mediated cleavage of homochiral 1,3-dioxaneacetals with P(OEt)(3) and chiral metal ligand-mediated hydrophosphonylation of aldehydes. Two diastereomers of HPmp derivatives were prepared by an application of these methods. The HPmp derivatives were convered to FPmp derivatives but with low diastereoselectivity. Hydrophosphonylation of alpha-aminoaldehydes afforded threo- and erythro-beta-amino-alpha-hydroxyphosphonates under chelation and nonchelation controlled conditions, respectively. The asymmetric dihydroxylation of alpha, beta-, and beta, gamma-unsaturated phosphonates with AD-mix-alpha and AD-mix-beta reagents gave alpha, beta- and beta, gamma-dihydroxyphosphonates with high enantioselectivity. The method was applied to the kinetic resolution of racemic alpha-oxygetated beta, gamma-unsaturated phosphonates. Treatment of allyloxymethylphosphonates with the base afforded alpha-hydroxyphosphonates via the [2,3]-Wittig reaction. Threo- and erythro-beta-amino-alpha-hydroxyphosphinates were obtained with high diastereoselectivity by phosphinylation of alpha-aminoaldehydes in the presence of (R)- and (S)-ALB, respectively. The phosphinylation of alpha-oxygenated aldehydes afforded the corresponding alpha, beta-dioxygenated phosphinates, but with low diastereoselectivity. Sphingomyelin analogues containing CF(2)PO(OH)(2) were synthesized starting from (S)- and (R)-Garner aldehyde for the purpose of obtaining potent sphyngomyelinase inhibitors. A useful method for the synthesis of alpha, alpha-difluorobenzylphosphonates was established based on the cross coupling reaction of an iodobenzene derivative with ZnCuBr(2)CF(2)PO(OEt)(2). The synthetic utility of ZnCuBr(2)CF(2)PO(OEt)(2) was examined to obtain alpha, alpha-difluoromethylenenphosphonates. The method was applied to a synthesis of PNP-inhibitory active compounds by combination of the purine base and alcohols containing difluoromethylenephosphonate. The methodology for the beta-selective N-glycosylation of 2,3-dideoxy glucoside was established by introducing phosphonothioates at the 3-position of glycosyl doners instead of phosphonate. Synthesis of new acylic nucleotide analogues designed based on the structural modification of ARS2267 is also described. Finally, kiral synthesis of some phosphonates was achieved using lipase through kinetic resolution.

The SH2 domain: versatile signaling module and pharmaceutical target

The Src homology 2 (SH2) domain is the most prevalent protein binding module that recognizes phosphotyrosine. This approximately 100-amino-acid domain is highly conserved structurally despite being found in a wide variety proteins. Depending on the nature of neighboring protein module(s), such as catalytic domains and other protein binding domains, SH2-containing proteins play many different roles in cellular protein tyrosine kinase (PTK) signaling pathways. Accumulating evidence indicates SH2 domains are highly versatile and exhibit considerable flexibility in how they bind to their ligands. To illustrate this functional versatility, we present three specific examples: the SAP, Cbl and SOCS families of SH2-containing proteins, which play key roles in immune responses, termination of PTK signaling, and cytokine responses. In addition, we highlight current progress in the development of SH2 domain inhibitors designed to antagonize or modulate PTK signaling in human disease. Inhibitors of the Grb2 and Src SH2 domains have been extensively studied, with the aim of targeting the Ras pathway and osteoclastic bone resorption, respectively. Despite formidable difficulties in drug design due to the lability and poor cell permeability of negatively charged phosphorylated SH2 ligands, a variety of structure-based strategies have been used to reduce the size, charge and peptide character of such ligands, leading to the development of high-affinity lead compounds with potent cellular activities. These studies have also led to new insights into molecular recognition by the SH2 domain.

Phosphotyrosyl mimetics in the development of signal transduction inhibitors

Phosphotyrosyl (pTyr) residues play important roles in cellular signal transduction by facilitating recognition and binding necessary for critical protein-protein interactions, and for this reason pTyr motifs represent attractive starting points in the development of signaling antagonists. Although the pTyr phosphoryl moiety is central in these phenomena, its incorporation into signaling inhibitors is contraindicated due to enzymatic lability and limited bioavailability associated with phosphate esters. To address these limitations, an entire field of study has arisen devoted to the design and utilization of pTyr mimetics. This Account provides a perspective on the roles of pTyr residues in signal transduction and approaches to pTyr mimetic development.

A novel phosphotyrosine mimetic 4'-carboxymethyloxy-3'-phosphonophenylalanine (Cpp): exploitation in the design of nonpeptide inhibitors of pp60(Src) SH2 domain

The novel phosphotyrosine (pTyr) mimetic 4'-carboxymethyloxy-3'-phosphonophenylalanine (Cpp) has been designed and incorporated into a series of nonpeptide inhibitors of the SH2 domain of pp60(c-Src) (Src) tyrosine kinase. A 2.2 A X-ray crystal structure of 1a bound to a mutant form of Lck SH2 domain provides insight regarding the structure-activity relationships and supports the design concept of this new pTyr mimetic.

SH2 domain inhibition: a problem solved?

The past two years have witnessed a number of significant advances in the design of SH2 inhibitors of both Src and Grb2. For Src, several non-peptide templates have been developed with high affinity, and one case, in the context of bone-binding phosphotyrosine bioisostere, has yielded an in vivo active antiresorptive agent. Similarly, high-affinity Grb2 SH2 inhibitors with novel phosphotyrosine replacements have now been reported that demonstrate, for the first time, cellular activities consistent with an anticancer agent.

Selective inhibition of Src SH2 by a novel thiol-targeting tricarbonyl-modified inhibitor and mechanistic analysis by (1)H/(13)C NMR spectroscopy

Detailed analysis of Src SH2 binding by peptides containing a novel tricarbonyl-modified pTyr moiety is described. We envisaged that Src SH2 selectivity might be obtained by exploiting the thiol group of Cys188 present in the pTyr binding pocket of the protein at the betaC3 position. Peptidyl as well as non-peptidyl compounds 1-4 possessing a 4-alpha,beta-diketoester-modified pTyr mimic exhibited micromolar affinity to Src SH2. Furthermore, these tricarbonyl compounds were selective for Src SH2 to the extent they showed no significant affinity for either Cys188Ser or Cys188Ala Src SH2 mutants. Upon closer examination of the binding of these tricarbonyls to Src SH2 using NMR of 13C-labeled compounds (6a, 6b, and 6c), we found that after the initial binding event the molecule disproportionated in a 'retro-Claisen' fashion to provide benzoic acid 16 and, following hydrolysis of the methyl ester 17, the hemiketal adduct of glyoxalic acid 18.

Structure-based design of an osteoclast-selective, nonpeptide src homology 2 inhibitor with in vivo antiresorptive activity

Targeted disruption of the pp60(src) (Src) gene has implicated this tyrosine kinase in osteoclast-mediated bone resorption and as a therapeutic target for the treatment of osteoporosis and other bone-related diseases. Herein we describe the discovery of a nonpeptide inhibitor (AP22408) of Src that demonstrates in vivo antiresorptive activity. Based on a cocrystal structure of the noncatalytic Src homology 2 (SH2) domain of Src complexed with citrate [in the phosphotyrosine (pTyr) binding pocket], we designed 3',4'-diphosphonophenylalanine (Dpp) as a pTyr mimic. In addition to its design to bind Src SH2, the Dpp moiety exhibits bone-targeting properties that confer osteoclast selectivity, hence minimizing possible undesired effects on other cells that have Src-dependent activities. The chemical structure AP22408 also illustrates a bicyclic template to replace the post-pTyr sequence of cognate Src SH2 phosphopeptides such as Ac-pTyr-Glu-Glu-Ile (1). An x-ray structure of AP22408 complexed with Lck (S164C) SH2 confirmed molecular interactions of both the Dpp and bicyclic template of AP22408 as predicted from molecular modeling. Relative to the cognate phosphopeptide, AP22408 exhibits significantly increased Src SH2 binding affinity (IC(50) = 0.30 microM for AP22408 and 5.5 microM for 1). Furthermore, AP22408 inhibits rabbit osteoclast-mediated resorption of dentine in a cellular assay, exhibits bone-targeting properties based on a hydroxyapatite adsorption assay, and demonstrates in vivo antiresorptive activity in a parathyroid hormone-induced rat model.

A Src SH2 selective binding compound inhibits osteoclast-mediated resorption

BACKGROUND

The observations that Src(-/-) mice develop osteopetrosis and Src family tyrosine kinase inhibitors decrease osteoclast-mediated resorption of bone have implicated Src in the regulation of osteoclast-resorptive activity. We have designed and synthesized a compound, AP22161, that binds selectively to the Src SH2 domain and demonstrated that it inhibits Src-dependent cellular activity and inhibits osteoclast-mediated resorption.

RESULTS

AP22161 was designed to bind selectively to the Src SH2 domain by targeting a cysteine residue within the highly conserved phosphotyrosine-binding pocket. AP22161 was tested in vitro for binding to SH2 domains and was found to bind selectively and with high affinity to the Src SH2 domain. AP22161 was further tested in mechanism-based cellular assays and found to block Src SH2 binding to peptide ligands, inhibit Src-dependent cellular activity and diminish osteoclast resorptive activity.

CONCLUSIONS

These results indicate that a compound that selectively inhibits Src SH2 binding can be used to inhibit osteoclast resorption. Furthermore, AP22161 has the potential to be further developed for treating osteoporosis.