Development of peptide-drug conjugates with a caged enzymatic cleavage site for improved drug delivery

脑肿瘤，如胶质母细胞瘤，是最致命的癌症类型之一。由于许多化疗药物的血脑屏障通透性差，这种高侵袭性癌症的治疗是有限的。为了克服这一问题和由于癌细胞选择性低而产生的副作用，经常使用多肽-药物结合物。已有研究表明，来自大自然的特定多肽序列可以穿越血脑屏障。此外，某些多肽与癌细胞膜上过度表达的受体具有高亲和力和选择性。这些有希望的特征将被用在由血脑屏障穿梭、多肽受体配体和化疗药物组成的结合物中。由此产生的特洛伊木马被设想携带药物穿过血脑屏障，并通过将药物运送到肿瘤细胞内来选择性地治疗癌症。为了进一步提高选择性，将在偶联物中引入可酶裂解的多肽连接物。特定的酶参与了肿瘤的生长，这意味着它们在恶性部位高水平地分泌。如果肽连接物被这样一种特定的酶切割，药物将被释放，主要作用于肿瘤生长和迁移的位置。为了稳定血液中的这个连接子，酶的裂解部位应该被一种新的机制暂时束缚(保护)，这种机制依赖于具有反向电子需求的Diels-Alder反应。这些多功能结构的合成和体外测试将有助于提高药物结合物的渗透性和选择性。此外，还计划建立和优化笼化酶切位点的应用，这将有助于开发一种新的前药方法。

Brain tumors such as glioblastoma are one of the most deadly cancer types. The treatment of this highly invasive cancer is limited due to the bad blood brain barrier permeability of many chemotherapeutics. To overcome this issue and the generation of side-effects as a result of low cancer cell selectivity, peptide-drug conjugates are often used. It has been shown that specific peptide sequences, derived from nature, can cross the blood brain barrier. Furthermore, certain peptides bind with high affinity and selectivity to receptors that are overexpressed in the membrane of cancer cells. These promising features will be utilized in a conjugate, consisting of a blood brain barrier shuttle, a peptidic receptor ligand and a chemotherapeutic agent. The resulting Trojan horse is envisioned to carry the drug across the blood brain barrier and to selectively treat cancer by shuttling the drug inside tumor cells. To further increase the selectivity, an enzymatically cleavable peptide linker will be introduced to the conjugate. Specific enzymes are involved in tumor growth, which means that they are secreted in high levels at the malignant site. If the peptide linker is cleaved by such a specific enzyme, the drug will be released to act mostly at a location where the tumor is growing and migrating. To stabilize this linker in the blood, the enzymatic cleavage site shall be temporally caged (protected) by a novel mechanism that relies on the Diels-Alder reaction with inverse electron demand. The synthesis and in vitro testing of these multifunctional constructs will help to improve permeability and selectivity of drug-conjugates. Furthermore, it is planned to establish and optimize the application of caging an enzymatic cleavage site, which can contribute to the development of a novel prodrug-method.