胰腺癌納米遞藥系統靶向性設計研究進展

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(1復旦大學附屬中山醫院介入科,上海 200032; 2上海市影像醫學研究所 上海 200032)

胰腺導管腺癌(pancreatic ductal adenocarcinoma,以下簡稱胰腺癌)是一種常見的惡性腫瘤,具有極高的致死性,患者的5年生存率僅為6%,且近40年來胰腺癌的治療無明顯進展[1-2]。胰腺癌早期無典型癥狀,缺乏早期診斷的標記物及成像檢測方法,手術根治性切除的患者不足20%。胰腺癌放射治療效果不確切,最新的研究證實中晚期胰腺癌化療4個月后再行同步放化療,與單行化療生存時間無明顯差異[3]。因此,化療是提高胰腺癌患者生存質量與延長生存時間的主要治療方法[4-5]。

化療是基于化療藥物的細胞毒性而抑制代謝旺盛的細胞生長[6]。由于大多數癌細胞比正常細胞代謝旺盛,因此會攝取更多的化療藥物,從而發揮藥物的治療作用;但化療藥物本身不具有選擇性,代謝旺盛的正常細胞也會受到藥物的影響,如毛囊細胞、骨髓造血細胞及胃腸黏膜上皮細胞等,故化療藥物又會產生不良反應[7]。然而胰腺癌不同于大多數癌癥,其血供相對匱乏,癌細胞代謝活度較低,化療藥物不能在胰腺癌組織內高濃度富集并到達其有效治療濃度,因此腺癌化療藥物使用強度高,不良反應明顯,預后不理想[8]。

20世紀90年代末期,基于對癌細胞分化與增殖相關信號通路網絡及機制的深入研究,通過靶向特定的生物分子以阻斷癌細胞信號網絡而控制腫瘤生長的治療方式,癌癥治療進入靶向治療的新時代[9]。2005年,美國FDA批準了中晚期胰腺癌治療的首個也是目前唯一的靶向藥物厄爾替尼(erlotinib),其與吉西他濱聯合用藥較吉西他濱單藥可以顯著延長受試者的中位生存時間(實驗組與對照組分別為6.4與6.0個月,校正風險比HR=0.81,P=0.028)[10]。但是,傳統細胞毒性藥物在胰腺癌的藥物治療中仍然占據重要地位,其無選擇地抑制癌細胞與正常細胞,在體內的快速降解失效,從而具有較高的毒性作用和不良反應[11]。

面對胰腺癌化療的困境,靶向性納米載體遞送化療藥物治療胰腺癌成為近年來的研究熱點。納米載體具有納米級的尺寸(粒徑為1 ～ 1 000 nm),可以靶向遞送抗腫瘤藥物達到靶組織,響應腫瘤微環境并實現智能控釋的功能,提高藥物在靶細胞的利用率,減少不良反應[12]。因此,靶向胰腺癌的納米遞藥有望改變傳統化療的遞藥方式,提高藥物治療胰腺癌療效,成為近年來的研究熱點。

被動靶向性設計癌組織內通常含有大量不成熟的新生腫瘤血管,使腫瘤具有高灌注高滲透性,且瘤體內淋巴系統引流功能不完善 ,即通過EPR效應(enhanced permeability and retention effect)使藥物更多富集于癌組織內。納米載體經長循環修飾后在體內的半衰期更長,且不易從組織中廓清,因而可以被動靶向聚集在癌組織內[16]。EPR效應在眾多癌癥納米遞藥中發揮了重要作用,但部分腫瘤并不具有明顯的EPR效應,其中包括胰腺癌[17]。

胰腺癌具有獨特的腫瘤微環境,包括豐富的腫瘤基質、匱乏的血供以及較高的瘤內組織液壓強,限制了納米遞藥系統通過EPR效應被動靶向提升腫瘤組織的藥物濃度(圖1)[17-18]。因此,克服胰腺癌腫瘤微環境對藥物EPR效應的限制,是設計被動靶向納米遞藥系統的重要策略。這包括:

重塑腫瘤血供 受腫瘤微環境的影響,胰腺癌的毛細血管纖細扭曲,是造成胰腺癌血供匱乏的重要因素之一,限制了藥物的遞送效率[12]。已經有研究報道,利用血管緊張素Ⅱ受體抑制劑氯沙坦(losartan),可以重塑腫瘤血管,提升血管內灌注,從而提升藥物遞送效能[19];同時,通過siRNA抑制胰腺癌組織內血管緊張素II受體表達也觀察到上述現象[20]。

降解腫瘤基質 透明質酸酶(hyaluronidase)是一種可以降解透明質酸的生物蛋白酶,其PEG化的納米尺度復合物全身性給藥后可以降解胰腺癌腫瘤基質,提高藥物在瘤體內的分布[21-22]。但基礎研究發現,打破胰腺癌腫瘤基質在提高藥物(如吉西他濱)滲透濃度的同時,使癌細胞趨向分化為更高的惡性程度及更強的轉移傾向,荷瘤小鼠生存時間更糟糕[23-24]。臨床研究也發現,在基礎研究獲得良好結果的納米藥物,在臨床研究中并未得到令人樂觀的數據,進一步說明了胰腺癌的復雜性及改變腫瘤微環境所產生的潛在風險[17]。

提升穿透腫瘤基質能力 通過優化納米粒的直徑、表面電荷及修飾特定配體,可以提高納米粒在腫瘤基質內的穿透性。腫瘤基質內的膠原蛋白呈正電性而硫酸黏多糖呈負電性[25],兩者與表面互補電荷的納米粒結合而使納米遞藥呈現分布異質性,降低遞藥效能,通過靶向的配體修飾(詳見主動靶向部分)可以克服上述限制,提高遞藥效能[26]。我們在前期研究中利用細胞穿膜肽修飾納米載體后局部瘤內注射納米粒,提高了其在瘤體內的擴散范圍和藥物在癌細胞內的分布[13]。

主動靶向性設計不同于被動靶向,納米載藥系統經恰當的配體修飾后,可以與目標細胞的受體特異性結合,從而具有主動靶向的功能,這構成了納米遞藥與小分子化療藥物的根本區別[27]。由于胰腺癌較弱的EPR效應,主動靶向性設計對提高納米系統胰腺癌遞藥性能具有重要意義。選擇適當的配體-受體(或抗體-抗原)對構建主動靶向的納米遞藥系統至關重要。一般而言,為實現納米粒的高度選擇性,靶細胞表面受體具有高度特異性,以區別正常細胞;因一方面,為保證納米粒具有較高識別效率,靶細胞表面的受體表達量一般高于105級別[28]。同時,部分受體與配體結合后可以啟動受體介導的吞噬過程,這適用于需要入胞后釋放藥物的納米粒,如基于CD20的靶向遞藥[29];而有的受體本身并不介入內吞過程,這對于實體腫瘤的靶向遞藥可能有幫助,可以實現靶向間質細胞和細胞外釋藥,作用于周圍的癌細胞[30](此時,癌細胞本身可能沒有合適的靶向標記物)。目前,在基礎研究中,除了已經廣泛報道的整合素-多肽[31-32]及葉酸-葉酸受體系統[33-34],近5年用于靶向胰腺癌的受體見表1。

A:The pathobiological barriers including:a dense desmoplastic stroma,excessive extracellular matrix deposition,increased interstitial fluid pressure,and compression of blood vessels.B:Vascular normalization;C:Normalizing the solid stress by reducing the desmoplastic stroma;D:Reduction of extracellular matrix.Figure was reprinted by permission from Macmillan Publishers Ltd:NatRevCliOncol[17],copyright 2016.

圖1胰腺癌腫瘤微環境與被動靶向遞藥設計策略
Fig1StrategiestoovercomethepathophysiologicalbarriersimpedingthepassivetargeteddeliveryforPDAC

智能響應納米遞藥系統的設計通過被動靶向及主動靶向設計,顯著提高了納米診療系統在目標組織內的分布。但是網狀內皮系統及分布在肝脾的巨噬細胞仍然是影響納米藥物分布的關鍵因素,目前已經進入臨床實驗的納米藥物也證實肝脾腎肺等組織內的分布多于腫瘤組織[42]。為進一步提高納米遞送藥物效率,研究者設計了多種局部智能響應的納米載體,它基于體內或體外特定的刺激源觸發并完成藥物的智能可控釋放[43]。

腫瘤內環境響應的納米遞藥系統 與大多數惡性腫瘤一樣,由于胰腺癌細胞的失控性生長,其內環境相對正常組織常表現出一些獨特的生理特征,如腫瘤微環境內及癌細胞溶酶體的弱酸性[43]、腫瘤細胞內的高還原性物質(如谷胱甘肽GSH)[45]等。

外環境觸發的納米遞藥系統 通過影像引導下,以熱、磁及機械波(含超聲波)為媒介觸發納米遞藥系統釋放負載藥物,可以實現更高的遞藥效率。Wang等[46]學者組合了腫瘤內外環境,設計的新型納米遞藥載體實現了三維響應觸發遞藥(超聲波、酸敏感及還原性物質谷胱甘肽)。

表1 用于胰腺癌主動靶向的受體藥物遞藥系統Tab 1 Receptors for active targeted drug delivery in pancreatic cancer

未來研究的啟示靶向性遞藥是胰腺癌納米遞藥系統必備要素,一個理想的胰腺癌靶向性納米遞藥系統可以有效克服胰腺癌微環境對藥物遞送的限制,提高納米藥物在目標細胞內的富積,并通過局部的智能控釋提高藥物的輸送能力。但胰腺癌的納米遞藥依然面臨巨大挑戰,突破胰腺癌獨特的腫瘤微環境對納米遞藥的限制,篩選胰腺癌高靈敏的靶向分子,設計構建智能響應的載藥釋藥系統,適應胰腺癌復雜的分子生物學特征,是未來提高納米靶向遞藥治療胰腺癌效果的關鍵。

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