Design And Synthesis Of Bone-Targeting Polymer Vesicles

With the aging of the global population,the incidence of bone diseases is increasing rapidly.Among them,the diagnosis and treatment of bone tissue diseases such as osteoporosis,malignant bone tumors and bone metabolic disorder caused bone loss are of great concern.However,there are still many unsolved scientific problems at present:1)How to improve the efficiency and reduce side effects of drugs for osteoporosis?2)How to achieve precise medical treatment of malignant bone tumors?3)How to treat bone metabolic diseases without breaking bone homeostasis?In view of the above scientific problems,we set our eyes on nanomedicines and focused on the bone-targeting polymer vesicles.In terms of the selection of bone-targeting moieties,the design of bone-targeting polymers and the functional modification of bone-targeting polymer vesicles,a series of bone-targeting polymer vesicles with different functions were synthesized for the diagnosis and treatment of osteoporosis and malignant bone tumors,etc.The following are the main research results:1.A bone-targeting polymer vesicle capable of specifically delivering antiresorption drug estradiol to bone tissues was designed.First,PCL₂₈-b-P[Glu₇-stat-（Glu-ADA）₄]block copolymer with high bone affinity was synthesized by modifying alendronic acid（ADA）,a kind of bone-targeting moieties,on the side chain of PGA blocks of PCL₂₈-b-PGA₁₁block polymers.Then,estradiol was encapsulated in the self-assembly process of PCL₂₈-b-P[Glu₇-stat-（Glu-ADA）₄]block copolymers and estradiol-loaded bone targeting polymer vesicles were obtained.This bone-targeting polymer vesicle has two functional zones:The biodegradable PCL membranes can load and protect the structure of estradiol during body circulation,thus reducing the toxicity and side effects of estradiol to other organs;the P[Glu₇-stat-（Glu-ADA）₄]outer coronas with high bone affinity can get more estradiol to bone tissues,thus improving the therapeutic efficiency by increasing estradiol concentration in bone tissues.In vitro experiments showed that this estradiol-loaded,bone-targeting polymer vesicle had high bone affinity,excellent osteogenic promotion ability,low cytotoxicity and good stability.In vivo experiments showed that bone mineral density increased significantly and the missing bone structures were repaired after treatment,while no obvious side effects on major organs and reproductive system were observed.2.A theranostic nanoplatform based on bone-targeting polymer vesicle was developed to realize the precise medical treatment of malignant bone tumors.First of all,scintigraphic ^99mTc was chelated on the outer corona of bone-targeting polymer vesicles,which was self-assembled from PCL₆₇-b-P[Glu₆-stat-（Glu-ADA）₁₆]block copolymers,to realize real-time diagnosis based on single photon emission computed tomography/computed tomography imaging（SPECT/CT）.Afterwards,doxorubicin（DOX）,a kind of anticancer drug,was encapsulated in the lumen of this bone-targeting polymer vesicle:on the one hand,a high drug loading efficiency was achieved due to the electrostatic interaction between the-COO^-groups in the vesicle coronas and the-NH₃⁺of the of DOX;on the other hand,a responsive drug release behavior was observed as the acidic tumor microenvironment weakened this electrostatic interaction and promoted the release of DOX.Finally,the bone-targeting polymer vesicle based theranostic nanoplatform achieved precise medical treatment of malignant bone tumors by the integration of the real-time diagnostic and targeted therapy.In vitro experiments indicated that the drug loading efficiency was as high as50.5%,the drug release rate was up to 90%under acidic conditions,and high antitumor activity was achieved.In vivo experiments based on rabbit models demonstrated that real-time monitoring of drug distribution and bone tissue/malignant bone tumor imaging based on SPECT/CT was achieved after intravenous injection;meanwhile,the growth of malignant bone tumors was significantly inhibited after periodic treatment.3.A dual corona alkaline bone-targeting polymer vesicle was prepared to treat bone metabolic diseases by regulating bone tissue microenvironment.First,zoledronate（Zol）was selected as bone-targeting unit and polylysine was selected as alkaline blocks to synthesize PCL₃₂-b-P[Lys₁₆-stat-（Lys-Zol）₉]block copolymer.Subsequently,the above block copolymer was co-assembled with PCL₂₆-b-PEO₂₃block copolymer at a mass ratio of 1:1 to form dual corona alkaline bone-targeting polymer vesicles.The P[Lys₁₆-stat-（Lys-Zol）₉]coronas have good bone targeting ability and weak alkalinity,prompting the polymer vesicles to aggregate in bone tissues and neutralize the acidic bone environment caused by bone metabolic disorder and excessive osteoclasts activity;the PEO coronas can reduce the cytotoxicity and increase the circulation stability of the whole system.TEM and DLS analyses confirmed that this dual corona alkaline bone-targeting polymer vesicle had typical vesicle structure with narrow particle size distribution,and in vitro experiments verified its good bone affinity and acid neutralization capability.Besides,good stability and low cytotoxicity of this polymer vesicle system were also confirmed,which provided a good basis for its potential in vivo applications.In summary,we have designed and synthesized various bone targeted polymer vesicles,and applied these vesicles to the diagnosis and treatment of bone diseases such as osteoporosis and malignant bone tumors,which may be extended for other bone diseases.