The lack of tumor cell specificity often results in life-threatening toxic effects for patients undergoing traditional chemotherapy for cancer. To overcome this problem and improve the selectivity of cancer therapy, cytotoxic drugs should be delivered to tumor-specific sites. To accomplish this, biochemical synthesis of ligand-linked drug conjugates play an important role. For instance, monoclonal antibodies (mAb) that bind to specific markers on the surface of tumor cells have been covalently linked to drugs for such targeted cancer therapy. These mAb immunoconjugates can selectively deliver drugs to tumors and, hence, improve antitumor efficacy, while reducing the systemic toxicity of otherwise beneficial therapies. In addition, immunoconjugates have been created using drugs that have a wide range of functions and potencies. So far, only the immunoconjugates which incorporate drugs with much higher potencies demonstrated impressive results in preclinical models, and they are currently being evaluated in clinical trials.[1
] An example of one of these advanced agents is the humanized anti-CD33 antibody-calicheamicin conjugate Mylotarg, which has already been approved for the treatment of Acute myeloid leukemia (AML).
In recent years, a new type of single-stranded oligonucleotides called aptamers has emerged as a novel class of molecules that rivals antibodies in both therapeutic and diagnostic applications.[3
] Aptamers not only combine the advantages of antibodies, such as high affinity, excellent specificity and low toxicity or immunogenicity, but they are also stable and easy to synthesize, modify and manipulate. Aptamers which can bind their specific targets are selected from a process called SELEX (s
volution of l
igands by ex
] Following several selection cycles, aptamers from a DNA or RNA pool can be selected and enriched by repetitive binding of their target molecules. Recently, cell-SELEX has been developed for the generation of aptamers for specific recognition of target tumor cells such as T-cell acute lymphoblastic leukemia (T-cell ALL), small-cell lung cancers and liver cancers.[9
] These aptamers can be generated relatively easily. They are highly specific for different types of tumor cells and have excellent affinity. Since they can provide specificity at the molecular level, we believe that these aptamers can be used to enhance the efficacy of experimental and/or commercial drugs in clinical applications. Molecular engineering is thus needed to create molecular conjugates that have specificity and drug potency.
In this report, we explored the usage of DNA-based aptamers, selected from cell-SELEX, for the molecular engineering of a ligand-drug conjugate for targeted drug therapy applications. Specifically, we used an aptamer, which was selected for human T-cell ALL CCRF-CEM cell lines,[11
] as a drug carrier for targeting specific tumor cells. Sgc8c can recognize the protein tyrosine kinase 7 (PTK7), a transmembrane receptor highly expressed on CCRF-CEM cells[14
] with high binding affinity (Kd
~ 1 nM). Its high specificity and well-characterized DNA structure[15
] give sgc8c the capacity to distinguish between target leukemia cells and normal human bone marrow aspirate, as well as identify cancer cells closely related to the target cell line in clinical specimens.[11
] Recently, we also demonstrated that sgc8c can internalize into the target cells after the binding to its target protein.[17
] For these reasons, sgc8c is considered a good candidate for proof-of-concept. Doxorubicin (Dox) is the most utilized anticancer drug against a range of neoplasms, including acute lymphoblastic and myeloblastic leukemias, as well as malignant lymphomas.[18
] However, its efficacy in cancer treatment is impeded by such toxic effects as myelosuppression, mucositis, alopecia, and, most concerning, cumulative cardiac damage.[19
] Therefore, we have molecularly assembled Dox into our aptamer probe through a simple conjugation method in order to demonstrate the feasibility of this target-specific approach in intracellular drug delivery.