Yasuaki Anami

A Mixed Population of Antagonist and Agonist Binding Conformers in a Single Crystal Explains Partial Agonism against Vitamin D Receptor: Active Vitamin D Analogues with 22R-Alkyl Group.

We are continuing to study the structural basis of vitamin D receptor (VDR) agonism and antagonism by using 22S-alkyl vitamin D analogues. Here we report the synthesis and biological evaluation of 22R-alkyl analogues and the X-ray crystallographic analysis of vitamin D receptor ligand-binding domain (VDR-LBD) complexed with a 22R-analogue. VDR-LBD complexed with the partial agonist 8a showed that 8a binds to VDR-LBD with two conformations, one of which is the antagonist/VDR-LBD complex structure and the other is the agonist/VDR-LBD complex structure. The results indicate that the partial agonist activity of 8a depends on the sum of antagonistic and agonistic activities caused by the antagonist and agonist binding conformers, respectively. The structural basis observed here must be applicable to the partial agonism of other ligand-dependent nuclear receptors. This is the first report describing the trapping of a conformational subset of the ligand and the nuclear receptor in a single crystal.

Development of vitamin D analogs modulating the pocket structure of vitamin D receptor.

The first determination of the X-ray crystal structure of the ligand binding domain (LBD) of the vitamin D receptor (VDR) complexed with 1α,25-dihydroxyvitamin D3 was reported in 2000. Since then several dozen crystal structures of VDR accommodating various ligands have been presented. Almost all of these complexes display the canonical active conformation observed in the VDR-LBD/1α,25- dihydroxyvitamin D3 complex, and all have quite similar ligand binding pocket (LBP) architectures. To develop new VDR ligands as therapeutic agents, it is important to separate the various biological activities of 1α,25- dihydroxyvitamin D3, such as calcium regulation, cell differentiation and anti-proliferation, and immune modulation. We focused on the structure of the LBP and discovered that vitamin D analogs with a branched side chain induce structural rearrangement of the amino acid residues lining the LBP. These analogs formed an additional cavity in the LBP for accommodation of the side chain and thus altered the structure of the LBP. Interestingly, the ligands showed agonistic, partial agonistic, or antagonistic activity depending upon the structure of the side chain. These results indicate that ligands which alter the pocket structure open a new perspective for the development of VDR ligands exhibiting a specific biological activity.

Fine tuning of agonistic/antagonistic activity for vitamin D receptor by 22-alkyl chain length of ligands: 22S-Hexyl compound unexpectedly restored agonistic activity.

1α,25-Dihydroxyvitamin D3 exerts its actions by binding to vitamin D receptor (VDR). We are continuing the study related to the alteration of pocket structure of VDR by 22-alkyl substituent of ligands and the relationships between the alteration and agonistic/antagonistic activity. Previously we reported that compounds 2 (22-H), 3 (22S-Et), and 4 (22S-Bu) are VDR agonist, partial agonist and antagonist, respectively. Here, we describe the synthesis and biological evaluation of 22S-hexyl analog 5 (22S-Hex), which was designed to be a stronger VDR antagonist than 4. Unexpectedly, 5 showed partial agonistic but not antagonistic activity when bound to VDR, indicating that it is not necessarily true that the bulkier the side chain is, the stronger the antagonistic activity will be. X-ray crystallographic analysis of the VDR-ligand-binding domain (VDR-LBD) accommodating compound 5 indicated that the partial agonist activity of 5 is dependent on the mixed population of the agonistic and antagonistic conformations. Binding of compound 5 may not bring the complex into the only antagonistic conformation due to the large conformational change of the VDR-LBD. From this study it was found that fine tuning of agonistic/antagonistic activity for VDR is possible by 22-alkyl chain length of ligands.

SRC2-3 binds to vitamin D receptor with high sensitivity and strong affinity

Vitamin D receptor (VDR) is a member of the nuclear receptor superfamily and regulates the expression of target genes through ligand binding. To express the target gene, coactivator binding to the VDR/ligand complex is essential. Although there are many coactivators in living cells, precise interactions between coactivators and VDR have not been clarified. Here, we synthesized two coactivator peptides, DRIP205-2 and SRC2-3, evaluated their affinity for the ligand-binding domain (LBD) of VDR using 1α,25-dihydroxyvitamin D3, partial agonist 1, and antagonist 2 by surface plasmon resonance (SPR), and assessed their interaction modes with VDR-LBD using X-ray crystallographic analysis. This study showed that the SRC2-3 peptide is more sensitive to the ligands (agonist, partial agonist, and antagonist) and shows more intimate interactions with VDR-LBD than DRIP205-2 peptide.

Glutamic acid–valine–citrulline linkers ensure stability and efficacy of antibody–drug conjugates in mice

Valine–citrulline linkers are commonly used as enzymatically cleavable linkers for antibody–drug conjugates. While stable in human plasma, these linkers are unstable in mouse plasma due to susceptibility to an extracellular carboxylesterase. This instability often triggers premature release of drugs in mouse circulation, presenting a molecular design challenge. Here, we report that an antibody–drug conjugate with glutamic acid–valine–citrulline linkers is responsive to enzymatic drug release but undergoes almost no premature cleavage in mice. We demonstrate that this construct exhibits greater treatment efficacy in mouse tumor models than does a valine–citrulline-based variant. Notably, our antibody–drug conjugate contains long spacers facilitating the protease access to the linker moiety, indicating that our linker assures high in vivo stability despite a high degree of exposure. This technology could add flexibility to antibody–drug conjugate design and help minimize failure rates in pre-clinical studies caused by linker instability.The valine-citrulline dipeptide, which is used as a cleavable linker for antibody-drug conjugates, is instable in mouse plasma. Here, the authors developed a glutamic acid–valine–citrulline tripeptide sequence as a stable alternative that still is susceptible to cathepsin-mediated cleavage.

Identification of the Histidine Residue in Vitamin D Receptor That Covalently Binds to Electrophilic Ligands

We designed and synthesized vitamin D analogues with an electrophile as covalent modifiers for the vitamin D receptor (VDR). Novel vitamin D analogues 1-4 have an electrophilic enone group at the side chain for conjugate addition to His301 or His393 in the VDR. All compounds showed specific VDR-binding potency and agonistic activity. Covalent bond formations of 1-4 with the ligand-binding domain (LBD) of VDR were evaluated by electrospray ionization mass spectrometry. All compounds were shown to covalently bind to the VDR-LBD, and the abundance of VDR-LBD corresponding conjugate adducts of 1-4 increased with incubation time. Enone compounds 1 and 2 showed higher reactivity than the ene-ynone 3 and dienone 4 compounds. Furthermore, we successfully obtained cocrystals of VDR-LBD with analogues 1-4. X-ray crystallographic analysis showed a covalent bond with His301 in VDR-LBD. We successfully synthesized vitamin D analogues that form a covalent bond with VDR-LBD.