Size matters: effects of PLGA-microsphere size in injectable CPC/PLGA on bone formation

Poly(lactic-co-glycolic acid) microsphere production based on quality by design: a review

Poly(lactic-co-glycolic acid) (PLGA) has garnered increasing attention as a candidate drug delivery polymer owing to its favorable properties, including its excellent biocompatibility, biodegradability, non-toxicity, non-immunogenicity, and mechanical strength. PLAG are specifically used as microspheres for the sustained/controlled and targeted delivery of hydrophilic or hydrophobic drugs, as well as biological therapeutic macromolecules, including peptide and protein drugs. PLGAs with different molecular weights, lactic acid (LA)/glycolic acid (GA) ratios, and end groups exhibit unique release characteristics, which is beneficial for obtaining diverse therapeutic effects. This review aims to analyze the composition of PLGA microspheres, and understand the manufacturing process involved in their production, from a quality by design perspective. Additionally, the key factors affecting PLGA microsphere development are explored as well as the principles involved in the synthesis and degradation of PLGA and its interaction with active drugs. Further, the effects elicited by microcosmic conditions on PLGA macroscopic properties, are analyzed. These conditions include variations in the organic phase (organic solvent, PLGA, and drug concentration), continuous phase (emulsifying ability), emulsifying stage (organic phase and continuous phase interaction, homogenization parameters), and solidification process (relationship between solvent volatilization rate and curing conditions). The challenges in achieving consistency between batches during manufacturing are addressed, and continuous production is discussed as a potential solution. Finally, potential critical quality attributes are introduced, which may facilitate the optimization of process parameters.

Sustained-release of sclerostin single-chain antibody fragments using poly(lactic-co-glycolic acid) microspheres for osteoporotic fracture repair

Osteoporotic fracture is one of the most common bone diseases in middle and old age, as the most serious consequence of osteoporosis. Sclerostin single‐chain antibody fragments (SCL‐scFv) have been proven to promote bone formation by binding to scleroprotein, a natural antagonist of the Wnt pathway, but it is difficult to rule alone due to the weak permeability and immunogenicity. Herein, we prepared poly(lactic‐co‐glycolic acid) microspheres as a sustained‐release vehicle to prolong the activity of SCL‐scFv. The morphology of microspheres were uniform and nearly sphere, loading efficiency and encapsulation efficiency of SCL‐scFv microspheres were 6.28 ± 1.04% and 48.37 ± 8.11%, respectively. Approximately 90% of the SCL‐scFvs were released from the microspheres over 28 days with initial burst releasing (38%) in the first 4 days. Sustained‐release of active SCL‐scFv from microspheres promoted bone marrow mesenchymal stem cells osteogenic differentiation in vitro and enhanced fracture healing in ovariectomized rats by improving bone mass and bone formation in the fracture region. All these findings demonstrate that the microspheres are able to simultaneously achieve localized long‐term SCL‐scFv controlled release and effectively promote bone formation, which provides a promising approach for osteoporotic fracture.

Multimodal porogen platforms for calcium phosphate cement degradation

Calcium phosphate cements (CPCs) represent excellent bone substitute materials due to their biocompatibility and injectability. However, their poor degradability and lack of macroporosity limits bone regeneration. The addition of poly(d,l-lactic-co-glycolic acid) (PLGA) particles improves macroporosity and therefore late stage material degradation. CPC degradation and hence, bone formation at an early stage remains challenging, due to the delayed onset of PLGA degradation (i.e., after 2-3 weeks). Consequently, we here explored multimodal porogen platforms based on sucrose porogens (for early pore formation) and PLGA porogens (for late pore formation) to enhance CPC degradation and analyzed mechanical properties, dynamic in vitro degradation and in vivo performance in a rat femoral bone defect model. Porogen addition to CPC showed to decrease compressive strength of all CPC formulations; transition of the crystal phase upon in vitro incubation increased compressive strength. Although dynamic in vitro degradation showed rapid sucrose dissolution within 1 week, no additional effects on CPC degradation or bone formation were observed upon in vivo implantation. © 2019 The Authors. journal Of Biomedical Materials Research Part A Published By Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1713-1722, 2019.

Multimodal pore formation in calcium phosphate cements

Calcium phosphate cements (CPCs) are commonly used as bone substitute materials. However, their slow degradation rate and lack of macroporosity hinders new bone formation. Poly(dl-lactic-co-glycolic acid) (PLGA) incorporation is of great interest as, upon degradation, produces acidic by-products that enhance CPC degradation. Yet, new bone formation is delayed until PLGA degradation occurs a few weeks after implantation. Therefore, the aim of this study was to accelerate the early stage pore formation within CPCs in vitro. With that purpose, we incorporated the water-soluble porogen sucrose at different weight percentages (10 or 20 wt %) to CPC and CPC/PLGA composites. The results revealed that incorporation of sucrose porogens increased mass loss within the first week of in vitro degradation in groups containing sucrose compared to control groups. After week 1, a further mass loss was observed related to PLGA and CPC degradation. Macroporosity analysis confirmed that macroporosity formation is influenced by the dissolution of sucrose at an early stage and by the degradation of PLGA and CPC at a later stage. We concluded that the combination of sucrose and PLGA porogens in CPC is a promising approach to promote early stage bone tissue ingrowth and complete replacement of CPC through multimodal pore formation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 500-509, 2018.

Enhanced biocompatibility and osseointegration of calcium titanate coating on titanium screws in rabbit femur

This study aimed to examine the biocompatibility of calcium titanate (CaTiO₃) coating prepared by a simplified technique in an attempt to assess the potential of CaTiO₃ coating as an alternative to current implant coating materials. CaTiO₃-coated titanium screws were implanted with hydroxyapatite (HA)-coated or uncoated titanium screws into medial and lateral femoral condyles of 48 New Zealand white rabbits. Imaging, histomorphometric and biomechanical analyses were employed to evaluate the osseointegration and biocompatibility 12 weeks after the implantation. Histology and scanning electron microscopy revealed that bone tissues surrounding the screws coated with CaTiO₃ were fully regenerated and they were also well integrated with the screws. An interfacial fibrous membrane layer, which was found in the HA coating group, was not noticeable between the bone tissues and CaTiO₃-coated screws. X-ray imaging analysis showed in the CaTiO₃ coating group, there was a dense and tight binding between implants and the bone tissues; no radiation translucent zone was found surrounding the implants as well as no detachment of the coating and femoral condyle fracture. In contrast, uncoated screws exhibited a fibrous membrane layer, as evidenced by the detection of a radiation translucent zone between the implants and the bone tissues. Additionally, biomechanical testing revealed that the binding strength of CaTiO₃ coating with bone tissues was significantly higher than that of uncoated titanium screws, and was comparable to that of HA coating. The study demonstrated that CaTiO₃ coating in situ to titanium screws possesses great biocompatibility and osseointegration comparable to HA coating.

Long-term biological performance of injectable and degradable calcium phosphate cement

Enhancing degradation of poorly degrading injectable calcium phosphate (CaP) cements (CPCs) can be achieved by adding poly(lactic-co-glycolic acid) (PLGA) microparticles, generating porosity after polymer degradation. CPC-PLGA has proven to be biodegradable, although its long-term biological performance is still unknown. Optimization of injectability could be achieved via addition of carboxymethyl cellulose (CMC). Here, we evaluated the long-term in vivo performance of CPC-PLGA with or without the lubricant CMC in comparison to the devitalized bovine bone mineral (DBBM) predicate device Bio-Oss^®. Rabbit femoral bone defects were injected with a CPC-formulation or filled with Bio-Oss^® granules. Samples were retrieved at 6 and 26 weeks. Material degradation for Bio-Oss^® was marginal, starting with 57% material remnants at implantation, 49% at 6 weeks, and 35% at 26 weeks, respectively. In contrast, CPC-PLGA and CPC-PLGA-CMC showed significant material degradation, starting with 100% material remnants at implantation, 56 and 78% at 6 weeks, and 8 and 21% at 26 weeks. Bone formation showed to be rapid for Bio-Oss^®, with 24% at 6 weeks, and a similar value (27%) at 26 weeks. Both CPC-PLGA and CPC-PLGA-CMC showed a continuous temporal increase in bone formation, with 13 and 6% at 6 weeks, and 44 and 32% at 26 weeks. This study showed that CPC-PLGA induces favorable bone responses with  >90% degradation and  >40% new bone formation after an implantation period of 26 weeks.