Rapid synthesis of RGD mimetics with isoxazoline scaffolds on solid phase: identification of alphavbeta3 antagonists lead compounds

The challenges and opportunities of αvβ3-based therapeutics in cancer: From bench to clinical trials

Integrins are main cell adhesion receptors serving as linker attaching cells to extracellular matrix (ECM) and bidirectional hubs transmitting biochemical and mechanical signals between cells and their environment. Integrin αvβ3 is a critical family member of integrins and interacts with ECM proteins containing RGD tripeptide sequence. Accumulating evidence indicated that the abnormal expression of integrin αvβ3 was associated with various tumor progressions, including tumor initiation, sustained tumor growth, distant metastasis, drug resistance development, maintenance of stemness in cancer cells. Therefore, αvβ3 has been explored as a therapeutic target in various types of cancers, but there is no αvβ3 antagonist approved for human therapy. Targeting-integrin αvβ3 therapeutics has been a challenge, but lessons from the past are valuable to the development of innovative targeting approaches. This review systematically summarized the structure, signal transduction, regulatory role in cancer, and drug development history of integrin αvβ3, and also provided new insights into αvβ3-based therapeutics in cancer from bench to clinical trials, which would contribute to developing effective targeting αvβ3 agents for cancer treatment.

Synthesis and Evaluation of a Non-Peptide Small-Molecule Drug Conjugate Targeting Integrin αVβ3

An integrin α_Vβ₃-targeting linear RGD mimetic containing a small-molecule drug conjugate (SMDC) was synthesized by combining the antimitotic agent monomethyl auristatin E (MMAE), an enzymatically cleavable Val-Ala-PABC linker with a linear conjugable RGD mimetic. The structure proposal for the conjugable RGD mimetic was suggested upon the DAD mapping analysis of a previously synthesized small-molecule RGD mimetic array based on a tyrosine scaffold. Therefore, a diversifying strategy was developed as well as a novel method for the partial hydrogenation of pyrimidines in the presence of the hydrogenolytically cleavable Cbz group. The small-molecule RGD mimetics were evaluated in an ELISA-like assay, and the structural relationships were analyzed by DAD mapping revealing activity differences induced by structural changes as visualized in dependence on special structural motifs. This provided a lead structure for generation of an SMDC containing the antimitotic drug MMAE. The resulting SMDC containing a linear RGD mimetic was tested in a cell adhesion and an in vitro cell viability assay in comparison to reference SMDCs containing cRGDfK or cRADfK as the homing device. The linear RGD SMDC and the cRGDfK SMDC inhibited adhesion of α_Vβ₃-positive WM115 cells to vitronectin with IC₅₀ values in the low µM range, while no effect was observed for the α_Vβ₃-negative M21-L cell line. The cRADfK SMDC used as a negative control was about 30-fold less active in the cell adhesion assay than the cRGDfK SMDC. Conversely, both the linear RGD SMDC and the cRGDfK SMDC are about 55-fold less cytotoxic than MMAE against the α_Vβ₃-positive WM115 cell line with IC50 values in the nM range, while the cRADfK SMDC is 150-fold less cytotoxic than MMAE. Hence, integrin binding also influences the antiproliferative activity giving a targeting index of 2.8.

Total Synthesis of the Interleukin-1β Converting Enzyme Inhibitor EI-1941-2 Using Tandem Oxa-electrocyclization/Oxidation1

[reaction: see text] The total synthesis of the interleukin-1beta converting enzyme inhibitor EI-1941-2 was achieved utilizing tandem oxidation/oxa-electrocyclization/oxidation to access a key alpha-pyrone intermediate. Support for the tandem reaction mechanism was obtained by evaluation of a stepwise oxidation protocol.

Tricyclic pharmacophore-based molecules as novel integrin alpha(v)beta3 antagonists. Part III: synthesis of potent antagonists with alpha(v)beta3/alpha(IIb)beta3 dual activity and improved water solubility

In order to optimize our novel integrin alpha(v)beta3/alpha(IIb)beta3 dual antagonists, spatial screening at the N-terminus was performed. The alpha(v)beta3 antagonistic activity varied depending on the space that was occupied by the N-terminus, but high potency against alpha(IIb)beta3 was well maintained. The (3S)-aminopiperidine analogue had the strongest activity against alpha(v)beta3, and the S isomer at piperidine was more potent than the R isomer. Compounds selected on the basis of SAR analysis of a novel lead compound showed acceptable early absorption, distribution, metabolism, excretion, and toxicity (ADMET) profiles and sufficient water solubility for use as infusion drugs. Docking studies with the alpha(v)beta3 receptor were performed to confirm the SAR findings.

RGD mounted on an L-proline scaffold

The construction of an L-proline scaffold that enforces a defined beta-turn loop for RGD is reported. A key feature was the use of SASRIN (super acid sensitive resin) that allowed solid-phase synthesis of the tetrapeptide. A HATU-induced cyclization of the sequence was successful, followed by a single acid-promoted deprotection of the final product.

Highly potent and selective alphaVbeta3-receptor antagonists: solid-phase synthesis and SAR of 1-substituted 4-amino-1H-pyrimidin-2-ones

Solid-phase synthesis and SAR of alpha(V)beta(3)-receptor antagonists based on a N(1)-substituted 4-amino-1H-pyrimidin-2-one scaffold are described. The most potent compounds exhibited IC(50) values towards alpha(V)beta(3) in the nano- to subnanomolar range and high selectivity versus related integrins like alpha(IIb)beta(3). For selected examples efficacy in functional cellular assays was demonstrated.

Synthesis of highly potent and selective hetaryl ureas as integrin alpha(V)beta3-receptor antagonists

Solid-phase synthesis and SAR of integrin alpha(V)beta3-receptor antagonists containing a urea moiety as non-basic guanidine mimetic are described. The most potent compounds exhibited IC(50) values towards alpha(V)beta3 in the nanomolar range and high selectivity versus related integrins like alpha(IIb)beta3. For selected examples efficacy in functional cellular assays is demonstrated.

Solid-phase synthesis of a nonpeptide RGD mimetic library: new selective alphavbeta3 integrin antagonists

The solid-phase synthesis of a low molecular weight RGD mimetic library is described. Activities of the compounds in inhibiting the interaction of ligands, vitronectin and fibrinogen, with isolated immobilized integrins alphavbeta3 and alphaIIbbeta3 were determined in a screening assay. Highly active and selective nonpeptide alphavbeta3 integrin antagonists with regard to orally bioavailability were developed, based on the aza-glycine containing lead compound 1. An important variation is the substitution of the aspartic amide of 1 by an aromatic residue. Furthermore, different guanidine mimetics have been incorporated to improve the pharmacokinetic profile. Exchange of the beta-amino acid NH by a methylene moiety in one set of RGD mimetics leads to the azacarba analogue compounds representing a novel peptidomimetic approach, which should increase the metabolic stability.

Identification and in vivo efficacy of small-molecule antagonists of integrin alphavbeta3 (the vitronectin receptor)

The integrin alphavbeta3 is thought to play a key role in the initiation and/or progression of several human diseases, including osteoporosis, restenosis following percutaneous transluminal coronary angioplasty (PTCA), rheumatoid arthritis, cancer and ocular diseases. Antagonism of integrin alphavbeta3 is therefore expected to provide an approach for the treatment and/or prevention of these diseases. A variety of potent, small-molecule alphavbeta3 antagonists have been identified, several of which are active in disease models, thereby demonstrating the therapeutic potential of alphavbeta3 antagonism. This review will focus on recent advances in the identification of small-molecule alphavbeta3 antagonists, with an emphasis on those studies where small-molecule alphavbeta3 antagonists have been used in proof-of-concept studies in vivo.

Rational design of cytotoxic T-cell inhibitors

This study describes the use of the CD8/major histocompatibility complex (MHC) class I crystal structure as a template for the de novo design of low-molecular-weight surface mimetics. The analogs were designed from a local surface region on the CD8 alpha-chain directly adjacent to the bound MHC class I, to block the protein associations in the T-cell activation cluster that occur upon stimulation of the cytotoxic T lymphocytes (CTLs). One small conformationally restrained peptide showed dose-dependent inhibition of a primary allogeneic CTL assay while having no effect on the CD4-dependent mixed lymphocyte reaction (MLR). The analog's activity could be modulated through subtle changes in its side chain composition. Administration of the analog prevented CD8-dependent clearance of a murine retrovirus in BALB/c mice. In C57BL/6 mice challenged with the same retrovirus, the analog selectively inhibited the antiviral CTL responses without affecting the ability of the CTLs to generate robust allogeneic responses.

Parallel solid-phase synthesis of vitronectin receptor (alphavbeta3) inhibitors

A combinatorial approach for rapid optimization of a vitronectin receptor (alphavbeta3) inhibitor lead was accomplished by solid-phase synthesis. Orthogonally bis protected 2,3-diaminopropionic acid was used to immobilize the C-terminus of the molecule. Selective deprotection and functionalization of the alpha-amino group followed by acyl resorcinol scaffold attachment and N-terminus diversification was used to explore structure activity relationship (SAR).

RGD mimetics containing a central hydantoin scaffold: alpkha(v)beta3 vs alpha(IIb)beta3 selectivity requirements

The synthesis of a series of RGD mimetic alpha(v)beta3 antagonists containing a hydantoin scaffold is shown. The results demonstrate some of the structural requirements for the design of selective alpha(v)beta3 antagonists (vs alpha(IIb)beta3) in terms of the Arg-mimetic, the distance between N- and C-terminus and the lipophilic side chain.

Isoxazolines as potent antagonists of the integrin alpha(v)beta(3)

Starting with lead compound 2, we sought to increase the selectivity for alpha(v)beta(3)-mediated cell adhesion by examining the effects of structural changes in both the guanidine mimetic and the substituent alpha to the carboxylate. To prepare some of the desired aminoimidazoles, a novel reductive amination utilizing a trityl-protected aminoimidazole was developed. It was found that guanidine mimetics with a wide range of pK(a)'s were potent antagonists of alpha(v)beta(3). In general, it appeared that an acylated 2-aminoimidazole guanidine mimetic imparted excellent selectivity for alpha(v)beta(3)-mediated adhesion versus alpha(IIb)beta(3)-mediated platelet aggregation, with selectivity of approximately 3 orders of magnitude observed for compounds 3g and 3h. It was also found in this series that the alpha-substituent was required for potent activity and that 2,6-disubstituted arylsulfonamides were optimal. In addition, the selective alpha(v)beta(3) antagonist 3h was found to be a potent inhibitor of alpha(v)beta(3)-mediated cell migration.

Disubstituted indazoles as potent antagonists of the integrin alpha(v)beta(3)

A new series of indazole-containing alpha(v)beta(3) integrin antagonists is described. Starting with lead compound 18a, variations in a number of structural features were explored with respect to inhibition of the binding of beta(3)-transfected 293 cells to fibrinogen and to selectivity for alpha(v)beta(3) over GPIIbIIIa, another RGD-binding integrin. Indazoles attached to a 2-aminopyridine or 2-aminoimidazole by a propylene linker at the indazole 1-position and to a diaminopropionate derivative via a 5-carboxylate amide provided the best potency with moderate selectivity. Several differences in the SAR of the diaminopropionate moiety were observed between this series and a series of isoxazoline-based selective GPIIbIIIa antagonists. Compound 34a (SM256) was a potent antagonist of alpha(v)beta(3) (IC(50) 2.3 nM) with 9-fold selectivity over GPIIbIIIa.

Orally bioavailable nonpeptide vitronectin receptor antagonists with efficacy in an osteoporosis model

A new series of potent nonpeptide vitronectin receptor antagonists, based on a novel carbocyclic Gly-Asp mimetic, has been discovered. A representative of this series, SB 265123 (4), has 100% oral bioavailability in rats, and is orally active in vivo in the ovariectomized rat model of osteoporosis.