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Biomaterials offer a promising approach to repair bone defects. Whereas traditional studies predominantly focused on optimizing the osteogenic capacity of biomaterials, less focus has been on the immune response elicited by them. However, the immune and skeletal systems extensively interact, a concept which is referred to as 'osteoimmunology'. This realization has fuelled the development of biomaterials with favourable osteoimmunomodulatory (OIM) properties, aiming to modulate the immune response and to support bone regeneration, thereby affecting the success of an implant. Given the plethora of in vitro assays used to evaluate the OIM properties of biomaterials, it may be challenging to select the right methods to produce conclusive results. In this review, we aim to provide a comprehensive and practical guide for researchers interested in studying the OIM properties of biomaterials in vitro. After a concise overview of the concept of osteoimmunology, emphasis is put on the methodologies that are regularly used to evaluate the OIM properties of biomaterials. First, a description of the most commonly used cell types and cell culture media is provided. Second, typical experimental set-ups and their relevant characteristics are discussed. Third, a detailed overview of the generally used methodologies and readouts, including cell type-specific markers and time points of analysis, is given. Finally, we highlight the promise of advanced approaches, namely microarrays, bioreactors and microfluidic-based systems, and the potential that these may offer to the osteoimmunology field. STATEMENT OF SIGNIFICANCE: Osteoimmunology focuses on the connection and communication between the skeletal and immune systems. This interaction has been recognized to play an important role in the clinical success of biomaterials, which has resulted in an increasing amount of research on the osteoimmunomodulatory (OIM) properties of biomaterials. However, the amount of literature makes it challenging to extract the information needed to design experiments from beginning to end, and to compare obtained results to existing work. This article intends to serve as a guide for those aiming to learn more about the commonly used experimental approaches in the field. We cover early-stage choices, such as cell types and experimental set-ups, but also discuss specific assays, including cell markers and time points of analysis.

There is an urgent need of synthetic bone grafts with enhanced osteogenic capacity. This can be achieved by combining biomaterials with exogenous growth factors, which however can have numerous undesired side effects, but also by tuning the intrinsic biomaterial properties. In a previous study, we showed the synergistic effect of nanostructure and pore architecture of biomimetic calcium deficient hydroxyapatite (CDHA) scaffolds in enhancing osteoinduction, i.e. fostering the differentiation of mesenchymal stem cells to bone forming cells. This was demonstrated by assessing bone formation after implanting the scaffolds intramuscularly. The present study goes one step forward, since it analyzes the effect of the geometrical features of the same CDHA scaffolds, obtained either by 3D-printing or by foaming, on the osteogenic potential and resorption behaviour in a bony environment. After 6 and 12 weeks of intraosseous implantation, both bone formation and material degradation had been drastically affected by the macropore architecture of the scaffolds. Whereas nanostructured CDHA was shown to be highly osteoconductive both in the robocast and foamed scaffolds, a superior osteogenic capacity was observed in the foamed scaffolds, which was associated with their higher intrinsic osteoinductive potential. Moreover, they showed a significantly higher cell-mediated degradation than the robocast constructs, with a simultaneous and progressive replacement of the scaffold by new bone. In conclusion, these results demonstrate that the control of macropore architecture is a crucial parameter in the design of synthetic bone grafts, which allows fostering both material degradation and new bone formation. Statement of Significance 3D-printing technologies open new perspectives for the design of patient-specific bone grafts, since they allow customizing the external shape together with the internal architecture of implants. In this respect, it is important to design the appropriate pore geometry to maximize the bone healing capacity of these implants. The present study analyses the effect of pore architecture of nanostructured hydroxyapatite scaffolds, obtained either by 3D-printing or foaming, on the osteogenic potential and scaffold resorption in an in vivo model. While nanostructured hydroxyapatite showed excellent osteoconductive properties irrespective of pore geometry, we demonstrated that the spherical, concave macropores of foamed scaffolds significantly promoted both material resorption and bone regeneration compared to the 3D-printed scaffolds with orthogonal-patterned struts and therefore prismatic, convex macropores.

The capacity of calcium phosphates to be replaced by bone is tightly linked to their resorbability. However, the relative importance of some textural parameters on their degradation behavior is still unclear. The present study aims to quantify the effect of composition, specific surface area (SSA), and porosity at various length scales (nano-, micro- and macroporosity) on the in vitro degradation of different calcium phosphates. Degradation studies were performed in an acidic medium to mimic the osteoclastic environment. Small degradations were found in samples with interconnected nano- and micropores with sizes below 3 µm although they were highly porous (35–65%), with maximum weight loss of 8 wt%. Biomimetic calcium deficient hydroxyapatite, with high SSA and low crystallinity, presented the highest degradation rates exceeding even the more soluble β-TCP. A dependence of degradation on SSA was indisputable when porosity and pore sizes were increased. The introduction of additional macroporosity with pore interconnections above 20 µm significantly impacted degradation, more markedly in the substrates with high SSA (>15 m 2 /g), whereas in sintered substrates with low SSA ( 2 /g) it resulted just in a linear increase of degradation. Up to 30 % of degradation was registered in biomimetic substrates, compared to 15 % in β-TCP or 8 % in sintered hydroxyapatite. The incorporation of carbonate in calcium deficient hydroxyapatite did not increase its degradation rate. Overall, the study highlights the importance of textural properties, which can modulate or even outweigh the effect of other features such as the solubility of the compounds. Statement of Significance The physicochemical features of calcium phosphates are crucial to tune biological events like resorption during bone remodeling. Understanding in vitro resorption can help to predict the in vivo behavior. Besides chemical composition, other parameters such as porosity and specific surface area have a strong influence on resorption. The complexity of isolating the contribution of each parameter lies in the close interrelation between them. In this work, a multiscale study was proposed to discern the extent to which each parameter influences degradation in a variety of calcium phosphates, using an acidic medium to resemble the osteoclastic environment. The results emphasize the importance of textural properties, which can modulate or even outweigh the effect of the intrinsic solubility of the compounds.

The design of synthetic bone grafts to foster bone formation is a challenge in regenerative medicine. Understanding the interaction of bone substitutes with osteoclasts is essential, since osteoclasts not only drive a timely resorption of the biomaterial, but also trigger osteoblast activity. In this study, the adhesion and differentiation of human blood-derived osteoclast precursors (OCP) on two different micro-nanostructured biomimetic hydroxyapatite materials consisting in coarse (HA-C) and fine HA (HA-F) crystals, in comparison with sintered stoichiometric HA (sin-HA, reference material), were investigated. Osteoclasts were induced to differentiate by RANKL-containing supernatant using cell/substrate direct and indirect contact systems, and calcium (Ca++) and phosphorus (P5+) in culture medium were measured. We observed that OCP adhered to the experimental surfaces, and that osteoclast-like cells formed at a rate influenced by the micro- and nano-structure of HA, which also modulate extracellular Ca++. Qualitative differences were found between OCP on biomimetic HA-C and HA-F and their counterparts on plastic and sin-HA. On HA-C and HA-F cells shared typical features of mature osteoclasts, i.e. podosomes, multinuclearity, tartrate acid phosphatase (TRAP)-positive staining, and TRAP5b-enzyme release. However, cells were less in number compared to those on plastic or on sin-HA, and they did not express some specific osteoclast markers. In conclusion, blood-derived OCP are able to attach to biomimetic and sintered HA substrates, but their subsequent fusion and resorptive activity are hampered by surface micro-nano-structure. Indirect cultures suggest that fusion of OCP is sensitive to topography and to extracellular calcium. Statement of Significance The novelty of the paper is the differentiation of human blood-derived osteoclast precursors, instead of mouse-derived macrophages as used in most studies, directly on biomimetic micro-nano structured HA-based surfaces, as triggered by osteoblast-produced factors (RANKL/OPG), and influenced by chemistry and topography of the substrate(s). Biomimetic HA-surfaces, like those obtained in calcium phosphate cements, are very different from the conventional calcium phosphate ceramics, both in terms of topography and ion exchange. The role of these factors in modulating precursors’ differentiation and activity is analysed. The system is closely reproducing the physiological process of attachment of host cells and further maturation to osteoclasts toward resorption of the substrate, which occurs in vivo after filling bone defects with the calcium phosphate grafts.

A rapid and stereoselective access to novel polycyclic indolyldiamines is described. The key step involves simple chemoselective transformations of a common bicyclic aminal intermediate, easily available on a large scale from an enantiomerically pure cyano oxazolopiperidine precursor.

The synthesis of 2-(3-indolyl)-4-methylenepiperidines using an efficient formation of the piperidine ring by intramolecular reaction between an allylsilane group and an iminium ion is reported.

The synthesis of chiral 2-(2-indolyl)- and 2-(3-indolyl)-4-piperidones by reaction of N,N-dimethyl-4-oxo-2-[1-(phenylsulfonyl)-indolyl] piperidinium iodide and (R)-(-)-phenylglycinol is reported

The photocyclization of enaminoester 5a gave indoline 8 which was used in the synthesis of tricyclic spiro compound 4a.

The Zn 2 + catalyzed reaction of the 2-cyano-Δ 3 -piperidine (9a X = SO 2 C 6 H 5 and 9b X =CN ) with indole gave the C-7 indole-substituted aminonitriles 37a,b and 38, respectively. These intermediates were converted to Δ 2 -piperidine 40 on reaction with sodium dimethyl malonate and AgBF 4 . Stereoselective hydrogenation of the enamine doyble bond in 40 furnished the required cis 3,4-disubstituted piperidine 41, which was cyclized under acidic conditions to the target molecule, 20-deethylsilicine (20).

A neat and high yield chemoselective epoxidation of alkene moieties present in tertiary amines is accomplished by treatment of the corresponding amine–boron trifluoride adduct with dimethyldioxirane or methyl(trifluoromethyl)dioxirane.

The {Trp-phenylglycinol} pseudodipeptide (αR,3R∗,4R∗)-N-(2-acetoxy-1-phenylethyl)-3-amino-4-indolyl-2-piperidone 33 has been synthezised by condensation of the 3-amino-substituted anhydride 30 with (R)-(-)-phenylglycinol followed by reduction of the resulting imide.

We describe a stereoselective preparation of 3-amino-2-piperidone 1 , a new conformationally constrained Ser-Leu surrogate. The key steps of the synthesis of compound 1 are the lactamisation of the secondary aminolactone 4 and the amination of the 3-position via the sulfite 2 .

In order to study the regioselectivity of thermal cyclocondensation, aminoimidazo[1,2-a]pyridines (AIP) 5a–e were prepared, further converted into iminophosphoranes 7a–e , and ultimately converted regioselectively in angular annulated imidazonaphthyridines (IN) 8a , 10a , 11a , 12a or linear annulated dipyridoimidazole (DPI) 17a . From 2-substituted derivative 23 , the peri annulated product 24a was obtained. The starting amines 5a–f reacted with aldehydes to yield regioselectively IN 8a–c , 10a–c , 11a–c , 12a,b , DPI 16a–e , 17a–d and TIBO like structures (±)-13 and 24a–c , as proved by X-ray analysis. The 1,2- or 1,4-addition between amines and α,β-unsaturated aldehydes concerning the pyridine and imidazole moieties is discussed in the light of these results.

The preparation of chiral 3,4-aziridinopiperidin-2-ones 1 and their reactivity with a range of nucleophiles has been studied. In all cases, nucleophilic attack on compounds 1 occurred exclusively at the C(3) centre, leading to the asymmetric synthesis of various β-pseudodipeptides.

3-Amino-delta-valerolactams trans-11a-c were synthesized through conjugate addition and Curtius rearrangement and converted into Fmoc-[Trp-Gly], Fmoc-[Ile-Gly], and Fmoc-[Phe-Gly] pseudodipeptides. Conformational analyses of tripeptide analogues Ac-[Trp-Gly]-Leu-NH(2) 17a and 17b by NMR experiments and molecular modeling calculations showed that diastereomer 17a adopted a gamma-turn/distorted type II beta-turn structure, whereas diastereomer 17b adopted mainly a gamma-turn structure.

In order to determine the absolute configuration of the new chiral auxiliary trans-1,2-cyclo­pentane­diyl­bis(2-benzene­thiol), one of the diastereomeric bis-esters with (−)-menthol, used in the resolution of the auxiliary, was subjected to X-ray study. This (−)-diastereomer proved to be (1S,2S)-1,2-bis{2-[(−)-3-p-menthyl­oxy­carbonyl­thio]­phenyl}­cyclo­pentane, C39H54O4S2.

A new synthesis of 9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-benzo aquinolizin-2-one ( 1a and its 3-ethyl derivative 1b via the corresponding 2-(3,4-dimethoxyphenyl)-4-piperidone ethylene acetals 7 is reported. Alkylation of 2-arylpipendires 7 with 2-bromoethanol followed by oxidation of the resulting amino alcohols 9 with oxalyl chloride and dimethyl sulfoxide afforded the aldehydes 10 , which were cyclized with hydrochloric acid to give 7-hydroxybenzo[α]quinolizidines 11 . The reduction of 1 with trinethylsilane and subsequent acid hydrolysis led to benzo aquinolizidin-2-ones 1 .