Biocompatibility of fully biodegradable polylactic acid and its copolymers: present and future applications

doi:10.3969/j.issn.2095-4344.0734

Abstract

Abstract:

BACKGROUND: Polylactic acid and its copolymers have been widely used in biomedical fields because of their good biocompatibility and biodegradability, which have become a hotspot in the research of biomaterials.

OBJECTIVE: To review the biocompatibility of fully degradable polylactic acid and its copolymers.

METHODS: A computer-based online retrieval of PubMed and CNKI was performed to search relevant papers published from 2006 to 2016, with the key words of ”polylactide, polylactic acid, copolymer, biodegradability, biocompatibility, animal” in English and Chinese, respectively.

RESULTS AND CONCLUSION: Polylactic acid and its copolymers as polyester compounds are approved by the US Food and Drug Administration for use in the biomedical field because of their good biocompatibility mainly as drug delivery carriers and temporary implants. Moreover, their side effects in clinical application have attracted attentions. Polylactide copolymers can cause some adverse reactions when used in drug delivery systems, orthopedic and skin care, and other clinical medical fields. These copolymers are deemed to have no impact on the central nervous system, eyes, cardiovessels and other tissues and organs. They also have no virtually genotoxic and carcinogenic effects. Currently, the polylactide copolymer implant mainly results in local reactions in the surrounding tissues, and no systemic reactions have been found.

中国组织工程研究杂志出版内容重点：生物材料；骨生物材料; 口腔生物材料; 纳米材料; 缓释材料; 材料相容性；组织工程

Key words: Biocompatible Materials, Biomedical Engineering, Tissue Engineering

CLC Number:

R318

Shen Xin, Liu Xue, Su Feng, Chen Zuo-sheng, Li Su-ming. Biocompatibility of fully biodegradable polylactic acid and its copolymers: present and future applications[J]. Chinese Journal of Tissue Engineering Research, 2018, 22(14): 2259-2264.

Figures/Tables 2

2.2 聚乳酸及其共聚物应用于骨科的生物相容性临床上，骨科植入物导致的延迟炎症反应会在愈合伤口周围出现疼痛、红斑和肿胀等症状。这种反应显然与植入物的降解速率有关，例如聚乙交酯植入后12周可观察到炎症反应，然而聚乳酸的降解速率要慢得多，会在手术植入两到三年后才会出现反应[11]。除了材料本身，其降解速率也可能影响不良反应的形成。降解速率不仅取决于植入物的尺寸、设计和表面结构，还与植入部位、血流和患者的年龄等因素有关。植入物承受的压力增大会导致其断裂，这会增加表面积，使其降解加快。这反过来可能导致局部组织中的结晶物质浓度过高，并增强巨噬细胞介导的FBRs[12-13]。报道显示左旋聚乳酸可吸收螺钉在多种情况下以胫骨和胫前黏液性囊肿、溶骨性股骨隧道变宽、侧股骨上髁异物反应以及关节内炎症反应延迟等形式引起异物反应，异物反应主要发生在螺钉断裂后，并且囊肿和螺钉碎片的消除会使症状得到缓解[14]。左旋聚乳酸颗粒仅造成了轻微的细胞损伤，不会引起严重的细胞损伤或细胞死亡。然而，这些组织病理学观察却不能完全解释患者中出现的组织肿胀现象，其可能是由多种因素共同造成的，例如左旋聚乳酸降解成较小的颗粒，导致渗透压增加，同时皮下组织的阻力相对于骨更小。组织病理学结果还显示左旋聚乳酸颗粒从纤维基质中的吞噬细胞外进入细胞内，这会导致成纤维样细胞的活化。囊肿本身由非细胞液体组成，且机体无疼痛感。囊肿的形成伴随着聚合物器械的降解和清除，这表明炎症反应与聚合物的降解产物密切相关。生物可降解材料也被应用于上唇或肩袖损伤的治疗。有报道显示使用左旋聚乳酸类生物材料出现了并发症。例如4例患者的关节窝骨溶解和盂肱关节病进一步恶化，这不仅是由于软骨细胞的机械损伤，还可能是细胞介导的反应造成的[15]。在另一项研究中，22例患者中有3例出现软骨溶解症状，这可能是由与降解材料相关的炎症反应引起的[16]。最近的一项大型调查分析表明，植入左旋聚乳酸材料后有44例出现了并发症，84%的病例出现巨细胞反应。在所有病例中都观察到了晶体材料的存在，而分别有79%和70%的病例出现了乳头状滑膜炎和软骨损伤[17]。目前主要通过对聚乳酸改性，制备复合材料来克服其在骨科应用时产生的不良生物医学反应。将羟基磷灰石[18]、β-磷酸三钙和珍珠层等与聚乳酸复合，中和聚乳酸降解的酸性微环境，提高材料的骨诱导性和骨传导性，达到减少非炎症反应的目的，并辅以血小板衍生生长因子等生物活性因子构建骨修复材料(图1)[19-20]。 2.3 聚乳酸及其共聚物应用于皮肤的生物相容性聚乳酸在欧洲和美国被广泛用于软组织填充。皮肤治疗时由于炎症反应，会形成纤维结缔组织和胶原纤维再生组织，这反而有益于脂肪萎缩或面部老年性改变的患者[21]。值得注意的是在治疗早期不良反应的发生率很高，主要表现为丘疹和结节。材料分布不均匀和不恰当重构都会引起这些不良反应[22]。在一项研究中，Vleggaar等[23]发现在接受7 185个疗程的2 131例患者中，大约3.2%的患者在面部发现了肉眼看不见的皮下丘疹，其中26.9%的患者丘疹在3个月内会自发消退，26例(1.2%)患者出现可见的皮下丘疹，组织学检查显示3例(0.1%)患者出现了肉芽肿瘤。 12例患者用聚乳酸制成的填充剂进行面部注射5-36个月后，出现了口内病变[24]。大剂量和高压力注射、肌肉运动或重力都能造成例如移位或错位等现象。病理组织学检查这些口内病变发现包括卵形、米粒状或纺锤状折射结构组成的边缘清晰小叶，具有非坏死性肉芽肿反应的巨细胞细胞质中也经常能观察到这些结构。所有病例都伴随着极微到中等程度的慢性炎症浸润和纤维变性，其中2例患者出现细胞坏死，2例患者观察到肌肉炎。正如其他报告中观察到的，在5个病例的肉芽肿处也发现了星状小体[25]。注射填充剂的体积较大，反复注入微液滴，填料中含有杂质，注射深度不当和粒子粒径大等因素都可能引发炎症形成。聚乳酸是牛津循证医学中心(the Oxford Centre for Evidence-based Medicine)惟一推荐等级为B的填充剂，其他填充剂均为C或D[26]。在聚乳酸类填充剂得到广泛使用的同时，注射技术的提高显著降低了不良反应发生的数量。No等[27]通过将聚乳酸注入58例患者的鼻唇沟来研究提高注射技术的安全性，其中30例患者持续观察了24个月，仅有4例患者在注射部位出现了轻微的不良反应，但均在2个月后恢复，并且24个月后没有观察到与聚乳酸有关的不良反应。因此，聚乳酸填充剂能够安全有效地修复鼻唇沟。 2.4 聚乳酸及其共聚物在其他应用中的生物相容性生物可降解材料可应用于中枢神经系统。Lewitus等[28]评价了将聚丙交酯乙交酯共聚物植入SD大鼠脑实质内1-4周后对中枢神经系统的影响，结果表明聚丙交酯乙交酯共聚物植入导致神经胶质细胞和星形胶质细胞的持续活化，并伴随持续的神经衰退。但聚丙交酯乙交酯共聚物比降解速度较快材料产生的影响更为轻微。在一项关于鼠气管滴注聚丙交酯乙交酯共聚物后的影响研究中，观察到了炎症反应[29]。但从多核中性粒细胞聚集的数量来看，聚丙交酯乙交酯共聚物比聚苯乙烯纳米微球所引发的炎症反应要轻微得多。同时新型聚丙交酯乙交酯共聚物衍生物——二乙基氨基丙胺-聚乙烯醇-聚丙交酯乙交酯共聚物会明显减少炎性递质。 Cantin等[30]评估了聚丙交酯乙交酯共聚物对薄壁组织的影响，将聚丙交酯乙交酯共聚物微粒注入SD大鼠的腮腺，随后进行了14 d的组织学检测。结果显示，薄壁组织的导管内腔和腺泡厚度等几项形态学参数发生改变。然而，这些变化都是轻微的，并且无明显的炎症反应。聚丙交酯乙交酯共聚物/聚乳酸微球可用作玻璃体内的药物递送系统。Rong等[31]通过将聚丙交酯乙交酯共聚物/聚乳酸微球注入新西兰兔玻璃体内，来评价这些生物可降解材料的生物相容性，并在注射后12周内进行了详尽地检查。结果表明，在视网膜中没有发现细胞死亡、神经胶质细胞活化或炎症反应，并且也没有观察到组织或细胞超微结构的变化。更重要的是，通过评估双眼视网膜电图没有发现视网膜功能损坏。因此，聚丙交酯乙交酯共聚物/聚乳酸微球是一种安全的药物递送系统，可用于玻璃体内注射来治疗眼后段疾病。类似的，Manna等[32]将包载甲氨蝶呤的聚乳酸微种植体植入到新西兰兔玻璃体内，未发现任何毒性迹象。并且在临床前和临床研究中，聚丙交酯乙交酯共聚物在眼内转运也是安全的。值得注意的是，左旋聚乳酸植入兔玻璃体内会导致眼内压大幅下降并伴随CD11b+炎性细胞在小梁网的累积。但左旋聚乳酸比聚苯乙烯和聚-N-异丙基丙烯酰胺等材料引起的炎症反应更小。聚乳酸可用作治疗粘连性中耳炎。在分别以豚鼠和南美栗鼠类啮齿动物为对象的两项研究中，聚乳酸对耳部没有明确的炎症反应[33-34]。而且，没有发现对听阈和波潜伏期等功能的影响。此外，将聚丙交酯乙交酯共聚物植入豚鼠的中耳时，主要在30 d后出现了中性粒细胞等一些急性炎症反应[35]。这些研究结果与阴性对照差异均统计学意义。在泌尿学领域，聚乳酸作为膨胀剂治疗膀胱输尿管回流和小便失禁。一些研究评价了聚乳酸和聚丙交酯乙交酯共聚物在小鼠、大鼠和兔体内的短期和长期影响，在这些研究中都未发现炎症反应[36-37]。生物可降解材料可用作血管支架，Pan等[38]在体外研究了左旋聚乳酸和聚丙交酯乙交酯共聚物的血液相容性。研究表明两种材料均不影响红细胞形态，而且溶血率正常。凝血活性测试表明左旋聚乳酸和聚丙交酯乙交酯共聚物会激活凝血途径，但引起的变化都在正常参考范围之内。因此，左旋聚乳酸和聚丙交酯乙交酯共聚物的生物相容性符合在循环系统中使用的要求。通过将聚乳酸及其共聚物与其他材料复合，负载药物和生物活性因子，并利用3D生物打印等技术手段，减少了不良反应的发生[39-42]。 2.5 聚乳酸及其共聚物的遗传毒性和致癌性由于大部分致癌物都有遗传毒性，因此在评价生物材料的生物相容性时必须进行适当的遗传毒性评估。Uzun等[43]通过中国仓鼠卵巢细胞株和胞质分裂阻滞微核测试法评价聚乳酸对DNA的潜在损伤，发现无遗传毒性。Setyawati等[44]在人类皮肤成纤维细胞系BJ的基础上，通过观察γH2AX的表达，从分子水平研究聚丙交酯乙交酯共聚物的潜在遗传毒性，证明聚丙交酯乙交酯共聚物纳米粒无遗传毒性。Kazimirova等[45]用TK6类淋巴母细胞系证明聚丙交酯乙交酯共聚物-聚氧化乙烯纳米粒没有遗传毒性。另外，用单核细胞检测聚丙交酯乙交酯共聚物-聚氧化乙烯纳米粒时会观察到微核数量增加。可降解材料植入啮齿动物体内较长时间后，会在植入部位附近形成肉瘤，鼠的发生率更可高达80%。这种在长期植入啮齿动物后形成肿瘤的现象，通常被归结为由于固态致癌性或者奥本海默效应[46]。聚乳酸类生物材料植入后也观察到此现象，这与在啮齿动物中植入没有毒性或致瘤性的固体材料(如惰性塑料、金属和其他材料)仍会形成肉瘤相类似[47]。将左旋聚乳酸植入70只鼠，在106周的观察期内发现3只鼠有纤维肉瘤，而植入50只雄性Wistar鼠，2年后发现22只鼠出现纤维肉瘤和恶性纤维组织细胞瘤[48]。此外，Manavitehrani等[49]将左旋聚乳酸-聚己内酯植入15只SD大鼠关节内，结果有1只鼠在植入部位出现肉瘤。在这些研究中，肿瘤都没有系统性影响。肉瘤形成的确切机制仍不清楚。肿瘤发生可能源于长期局部轻微炎症引发的一连串与生长因子和活性氧分泌有关的反应，导致Hh和Wnt信号通路激活和多重附加突变的积累，最终形成肿瘤[50]。局部肉瘤的形成与植入物的形状尺寸、植入时间和炎症程度等有关，与生物可降解聚合物的特定化学组分无关，同时人体出现局部肉瘤形成是极其罕见的。因此可得出结论，植入物引起的肉瘤具有物种依赖性，与植入物本身无关。 "

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