Retroviral transduction of cancer cell lines with the gene encoding Drosophila melanogaster multisubstrate deoxyribonucleoside kinase

Nucleoside kinases from several species are investigated as "suicide genes" for treatment of malignant tumors by combined gene/chemotherapy. We have recently cloned a multisubstrate deoxyribonucleoside kinase of Drosophila melanogaster (Dm-dNK), and we have shown that the enzyme phosphorylates cytotoxic pyrimidine and purine nucleoside analogs. The broad substrate specificity of the enzyme, as well as its very high catalytic rate, makes it a unique member of the nucleoside kinase enzyme family. In the present study, we evaluated Dm-dNK as a suicide gene by constructing a replication-deficient retroviral vector that expresses the enzyme. The human pancreatic adenocarcinoma cell line MIA PaCa-2 and a thymidine kinase-deficient osteosarcoma cell line were transduced with the recombinant virus. We showed that Dm-dNK can be expressed in human cells, that the enzyme retained its enzymatic activity, and that it is localized in the cell nuclei due to a nuclear localization signal in its C-terminal region. The cells expressing Dm-dNK exhibited increased sensitivity to several cytotoxic nucleoside analogs, such as 1-beta-d-arabinofuranosylcytosine, 1-beta-d-arabinofuranosylthymine, (E)-5-(2-bromovinyl)-2'-deoxyuridine, 2-chloro-2'-deoxyadenosine, and 2',2'-difluorodeoxycytidine. These findings suggest that Dm-dNK may be used as a suicide gene in combined gene/chemotherapy of cancer.     

Detecting and targeting mesenchymal-like subpopulations within squamous cell carcinomas

Curative eradication of all cells within carcinomas is seldom achievable with chemotherapy alone. This limitation may be partially attributable to tumor cell subpopulations with intrinsic resistance to current drugs. Within squamous cell carcinoma (SCC) cell lines, we previously characterized a subpopulation of mesenchymal-like cells displaying phenotypic plasticity and increased resistance to both cytotoxic and targeted agents. These mesenchymal-like (Ecad-lo) cells are separable from epithelial-like (Ecad-hi) cells based on loss of surface E-cadherin and expression of vimentin. Despite their long-term plasticity, both Ecad-lo and Ecad-hi subsets in short-term culture maintained nearly uniform phenotypes after purification. This stability allowed testing of segregated subpopulations for relative sensitivity to the cytotoxic agent cisplatin in comparison to salinomycin, a compound with reported activity against CD44⁺CD24⁻ stem-like cells in breast carcinomas. Salinomycin showed comparable efficacy against both Ecad-hi and Ecad-lo cells in contrast to cisplatin, which selectively depleted Ecad-hi cells. An in vivo correlate of these mesenchymal-like Ecad-lo cells was identified by immunohistochemical detection of vimentin-positive malignant subsets across a part of direct tumor xenografts (DTXs) of advanced stage SCC patient samples. Cisplatin treatment of mice with established DTXs caused enrichment of vimentin-positive malignant cells in residual tumors, but salinomycin depleted the same subpopulation. These results demonstrate that mesenchymal-like SCC cells, which resist current chemotherapies, respond to a treatment strategy developed against a stem-like subset in breast carcinoma. Further, they provide evidence of mesenchymal-like subsets being well-represented across advanced stage SCCs, suggesting that intrinsic drug resistance in this subpopulation has high clinical relevance.Key words: EMT, squamous cell carcinoma, head and neck cancer, esophageal cancer, chemotherapy resistance, salinomycin, tumor heterogeneity     

Telomerase-directed molecular therapeutics

The management of malignant disease remains one of the most challenging areas of modern medicine. The lifetime risk of developing cancer in the western world is estimated to be as high as 1 in 3. Traditionally, surgery, chemotherapy and radiotherapy have been the primary choice of treatment for patients with malignant tumours. Despite advances in the use and development of conventional cytotoxic agents, the cure rate remains disappointing in most patients with advanced disease of the common solid tumours. Consequently, the development of novel anti-cancer therapies is a high priority in cancer medicine. In recent years, a new generation of cancer therapies has emerged, based on a growing understanding of the molecular events that contribute to malignant transformation. A major difference between normal and cancer cells is the ability of cancer cells to multiply in an unrestricted and ungoverned fashion. In this context, there is considerable interest in elucidating the mechanisms that allow this unrestricted proliferation and that ultimately result in immortal cancer cells. It is now clear that the enzyme telomerase confers immortality on cells in most types of cancer. With the cancer cell reliant on telomerase for its survival, telomerase represents an extremely attractive mechanism-based target for the development of new cancer therapeutics.     

Manipulation of cellular redox parameters for improving therapeutic responses in B-cell lymphoma and multiple myeloma

Developing novel combined-modality therapeutic approaches based on understanding of the involvement of redox biology in apoptosis of malignant cells is a promising approach for improving clinical responses in B-cell lymphoma and multiple myeloma. Therapeutic modalities that generate reactive oxygen species (i.e., radiation, photodynamic therapy, and specific chemotherapeutic drugs) have been shown to be selectively cytotoxic to malignant B-cells. In this review, we will discuss agents that induce apoptosis in B-cell tumors by oxidative stress. Subsequently, a novel biochemical rationale (based on fundamental differences in cancer vs. normal cell oxidative metabolism) for combining oxidative stressors with radiotherapy and chemotherapy, that may lead to designing of more effective treatment strategies for B-cell malignancies, will be discussed. Besides providing potential curative benefit, such novel therapies could also selectively target and inhibit the emergence of drug-resistance in tumor cells, which is a major determinant of treatment failure in many B-cell malignancies.     

Enhancement of cancer chemotherapy by simultaneously altering cell cycle progression and DNA-damage defenses through global modification of the serine/threonine phospho-proteome

Despite improvements in the therapeutic efficacy of rationally designed cancer treatment regimens, most cancers remain incurable once spread beyond their sites of origin. Failure to achieve sustained control or eradication of cancers arises in large part because a sub-population of quiescent “cancer stem cells” is insensitive to drugs targeting cell growth and replication and because defense mechanisms critical to survival of the normal cell also protect the cancer cell from cytotoxic injury. Global alteration of signal transduction by inhibition of serine/threonine dephosphorylation has recently been shown to markedly potentiate cancer cell killing by the DNA-methylating drug, temozolomide. Inhibition of the multifunctional protein phosphatase 2A appears to drive quiescent cancer cells into cycle and simultaneously inhibits cycle arrest, permitting cancer cell entry into mitosis despite the presence of chemotherapy induced DNA-damage. Absence of toxicity in animal models suggests that multi-site mutations in pathways regulating cell cycle in cancer cells make them more vulnerable than normal cells to large changes in the balance of phosphorylation-regulated signaling. Global modulation of the serine-threonine phospho-proteome may be a general method for enhancing the effectiveness of cytotoxic cancer therapy.     

Targeting leukemia stem cells: The new goal of therapy in adult acute myeloid leukemia

The most popular view of hematopoietic cell lineage organization is that of complex reactive or adaptative systems. Leukemia contains a subpopulation of cells that display characteristics of stem cells. These cells maintain tumor growth. The properties of leukemia stem cells indicate that current conventional chemotherapy, directed against the bulk of the tumor, will not be effective. Leukemia stem cells are quiescent and do not respond to cell cycle-specific cytotoxic agents used to treat leukemia and thus contribute to treatment failure. New strategies are required that specifically target this malignant stem cell population.     

Involvement of stromal p53 in tumor-stroma interactions

p53 is a major tumor-suppressor gene, inactivated by mutations in about half of all human cancer cases, and probably incapacitated by other means in most other cases. Most research regarding the role of p53 in cancer has focused on its ability to elicit apoptosis or growth arrest of cells that are prone to become malignant owing to DNA damage or oncogene activation, i.e. cell-autonomous activities of p53. However, p53 activation within a cell can also exert a variety of effects upon neighboring cells, through secreted factors and paracrine and endocrine mechanisms. Of note, p53 within cancer stromal cells can inhibit tumor growth and malignant progression. Cancer cells that evolve under this inhibitory influence acquire mechanisms to silence stromal p53, either by direct inhibition of p53 within stromal cells, or through pressure for selection of stromal cells with compromised p53 function. Hence, activation of stromal p53 by chemotherapy or radiotherapy might be part of the mechanisms by which these treatments cause cancer regression. However, in certain circumstances, activation of stromal p53 by cytotoxic anti-cancer agents might actually promote treatment resistance, probably through stromal p53-mediated growth arrest of the cancer cells or through protection of the tumor vasculature. Better understanding of the underlying molecular mechanisms is thus required. Hopefully, this will allow their manipulation towards better inhibition of cancer initiation, progression and metastasis.     

Selective induction of apoptosis in various cancer cells irrespective of drug sensitivity through a copper chelate, copper N-(2 hydroxy acetophenone) glycinate: crucial involvement of glutathione

Drug induced toxicity and drug resistance are the major impediments to successful application of cancer chemotherapy. Therefore, selective targeting of the key biochemical events of the malignant cells may have a great therapeutic potential in specifically kill the cancer cells. We have evaluated in vitro the cytotoxic efficacy of a previously reported copper complex viz. copper N-(2-hydroxy acetophenone) glycinate (CuNG) on different drug sensitive and resistant cancer cell lines by MTT, annexin V positivity and caspase 3 activation assays. We have also investigated the underlying signalling events in CuNG mediated apoptosis of cancer cells by Western blotting technique. We have found that CuNG preferentially induces apoptosis to malignant cells irrespective of drug sensitivity and spares the normal cells. Our studies disclose that CuNG causes cellular redox imbalance in cancer cells through depletion of intracellular GSH level. CuNG mediated depletion of intracellular GSH level induces mitochondrial superoxide generation, which detaches cyto C from mitochondrial membrane through lipid peroxidation. The detached cyto C then release into the extra mitochondrial milieu in Bax mediated pathway where CuNG facilitates the binding of Bax through dissociation of hexokinase II from mitochondrial membrane. The present study opens the possibility of developing effective chemotherapeutic drugs by synthesizing numerous chemical compounds capable of targeting cellular redox environment and thus specifically kills cancer cells of broad spectrum.     

Chemoimmunotherapy: reengineering tumor immunity

Cancer chemotherapy drugs have long been considered immune suppressive. However, more recent data indicate that some cytotoxic drugs effectively treat cancer in part by facilitating an immune response to the tumor when given at the standard dose and schedule. These drugs induce a form of tumor cell death that is immunologically active, thereby inducing an adaptive immune response specific for the tumor. In addition, cancer chemotherapy drugs can promote tumor immunity through ancillary and largely unappreciated immunologic effects on both the malignant and normal host cells present within the tumor microenvironment. These more subtle immunomodulatory effects are dependent on the drug itself, its dose, and its schedule in relation to an immune-based intervention. The recent approvals of two new immune-based therapies for prostate cancer and melanoma herald a new era in cancer treatment and have led to heightened interest in immunotherapy as a valid approach to cancer treatment. A detailed understanding of the cellular and molecular basis of interactions between chemotherapy drugs and the immune system is essential for devising the optimal strategy for integrating new immune-based therapies into the standard of care for various cancers, resulting in the greatest long-term clinical benefit for cancer patients.     

De novo ceramide synthesis is responsible for the anti-tumor properties of camptothecin and doxorubicin in follicular thyroid carcinoma

Doxorubicin and camptothecin are two cytotoxic chemotherapeutic agents triggering apoptosis in various cancer cells, including thyroid carcinoma cells. Recent studies revealed a critical role of ceramide in chemotherapy and suggested that anti-cancer drugs may kill tumor cells through sphingomyelinase activation. However, in comparison to sphingomyelin hydrolysis, the relative involvement of de novo ceramide synthesis remained poorly explored and highly controversial. Here, we evidenced that both doxorubicin and camptothecin triggered ceramide accumulation in thyroid carcinoma cells. We demonstrated that ceramide increase occurred via the de novo pathway without neither acidic nor neutral sphingomyelinase contribution. Interestingly, de novo ceramide generation was responsible for the drug-induced malignant cell apoptosis through a caspase-3-dependent pathway and a decrease of thrombospondin amount. Furthermore, blocking ceramide metabolism by inhibiting glucosylceramide synthase strengthened the camptothecin and doxorubicin-dependent effects. Altogether, we evidenced that de novo ceramide synthesis mediates the anti-tumor properties of doxorubicin and camptothecin in thyroid carcinoma and suggested that glucosylation of ceramide may contribute to the drug-resistance phenotype in thyroid malignancies.     

NOS-2 signaling and cancer therapy

The role of NO and cGMP signaling in tumor biology has been extensively studied during the past three decades. However, whether the pathway is beneficial or detrimental in cancer is still open to question. We suggest several reasons for this ambiguity: first, although NO participates in normal signaling (e.g., vasodilation and neurotransmission), NO is also a cytotoxic or apoptotic molecule when produced at high concentrations by inducible nitric-oxide synthase (iNOS or NOS-2). In addition, the cGMP-dependent (NO/sGC/cGMP pathway) and cGMP-independent (NO oxidative pathway) components may vary among different tissues and cell types. Furthermore, solid tumors contain two compartments: the parenchyma (neoplastic cells) and the stroma (nonmalignant supporting tissues including connective tissue, blood vessels, and inflammatory cells) with different NO biology. Thus, the NO/sGC/cGMP signaling molecules in tumors as well as the surrounding tissue must be further characterized before targeting this signaling pathway for tumor therapy. In this review, we focus on the NOS-2 expression in tumor and surrounding cells and summarized research outcome in terms of cancer therapy. We propose that a normal function of the sGC-cGMP signaling axis may be important for the prevention and/or treatment of malignant tumors. Inhibiting NOS-2 overexpression and the tumor inflammatory microenvironment, combined with normalization of the sGC/cGMP signaling may be a favorable alternative to chemotherapy and radiotherapy for malignant tumors.     

Application of hypothermia to autologous stem cell purging

Autologous stem cell transplantation is used widely after high-dose chemotherapy for treating hematological and other malignancies. Bone marrow harvested for autologous bone marrow transplantation may contain residual malignant cells even when the cancer is judged to be in remission. Attempts to purge marrow of its putative residual malignant cells may delay hemopoietic reconstitution and are of uncertain efficacy. In this report, we demonstrate the possibility of applying hypothermia to autologous stem cell purging. Using clonogenic assay, we compared the surviving fraction of human leukemia (HL60, K562) and human small cell lung cancer (H69) cell lines with that of normal human bone marrow CFU-GM and BFU-E cells after incubation at 4 +/- 0.1 degrees C for 24 and 48 h. Hypothermia decreased the surviving fraction of HL60, H69, and K562 cells. In contrast, the surviving fractions of stem cells were not affected by the temperature shift. The surviving fraction of HL60 cells at 4 degrees C cooling was significantly lower than that at 22 degrees C cooling. These findings suggest that in vitro hypothermia may selectively purge residual malignant cells in stored remission bone marrow and may be applicable before autologous bone marrow transplantation. In addition, the method is very simple and cost effective.     

Combination treatment with arsenic trioxide and irradiation enhances autophagic effects in U118-MG cells through increased mitotic arrest and regulation of PI3K/Akt and ERK1/2 signaling pathways

Malignant gliomas are resistant to many kinds of treatments including chemotherapy, radiotherapy, and other adjuvant therapies. Autophagy is a novel response of cancer cells to ionizing radiation (IR) or chemotherapy, but its significance and underlying mechanism remains largely elusive. Induction of autophagy in glioma cells using irradiation and arsenic trioxide (ATO) have been reported separately. However, the combined effects of ATO and IR on the cell death processes of malignant glioma cells have not been thoroughly studied, especially in U118-MG cells. In the present study, we investigated the anticancer effect of IR combined with ATO and the underlying mechanisms on U118-MG human malignant glioma cells in vitro. We found that the enhanced cytotoxic effect of IR combined with ATO was through induction of more autophagy in U118-MG cells, which were characterized by the presence of acidic vascular organelle formation, determined by electron microscopic observation and immunoblotting of LC3. Combined treatment could induce more mitotic arrest compared to ATO or IR alone. In addition, we also found that the combined treatment-induced autophagy occurred through inhibition of PI3K/Akt and activation of ERK1/2 signaling pathways. These findings suggest a potential therapeutic strategy for malignant gliomas, which are resistant to various proapoptotic therapies.     

Telomere dynamics in response to chemotherapy

The use of chemotherapy provides an essential arm in the treatment of a number of cancers. The biological feature common to all cancerous cells that sensitizes them to chemotherapeutic agents is their elevated division rate. Rapidly dividing cells, such as tumor cells, are more sensitive to chemotherapeutic agents that act to initiate pathways leading to cell death, a process enhanced in cells with compromised DNA damage responses. The toxicity accompanying chemotherapy is due to side-effects induced in normal regenerative tissues which also have relatively high replication rates, such as hair follicles, the hematopoietic system, the gastrointestinal system, the germline and skin cells. While the side-effects of chemotherapy may be tolerated by the patient, the long term impact of the cytotoxic effects of chemotherapy on healthy tissues is only now becoming apparent. Chemotherapy-induced cytotoxicity in regenerative tissues requires multiple cell divisions in order to reconstitute the affected tissues. At least in part as a consequence of these extra divisions, telomeres in individuals treated with chemotherapy are shorter than age-matched control individuals who have never been exposed to these drugs. Given the essential role of telomeres in regulating cellular aging and chromosomal stability, it is possible that the prematurely shortened telomeres that arise following chemotherapy may impact the long-term replicative potential of these tissues. This review is focused on how telomeres may be modulated, directly or indirectly, by anticancer drugs and the potential long-term consequences of accelerated telomere shortening in healthy tissue as a result of current cancer treatment protocols.     

Immune reactions with cytotoxic activity to "self" in natural surveillance of aberrant cells

Cytotoxic autoreactive lymphoid cells have been demonstrated to be a normal immunologic component in both normal and tumor-bearing organisms. The cells have a high antineoplastic potential. They are cytotoxic in vitro to non-immunogenic tumor cells, and a high in vivo activity of such cells may be positively correlated to in vivo regression of tumors. It is suggested that the cytotoxic activity of cytotoxic autoreactive cells reflects a natural mechanism to removal of all types of aberrant cells of which malignant cells are only a subset. It is further deduced that such mechanisms must be independent of cell surface neoantigens, and that cytotoxic autoreactive mechanisms with high probability therefore also are efficient against spontaneous malignant tumors.     

Targeting the NF-κB Pathway as a Combination Therapy for Advanced Thyroid Cancer

NF-κB signaling plays an important role in tumor cell proliferation, cell survival, angiogenesis, invasion, metastasis and drug/radiation resistance. Combination therapy involving NF-κB pathway inhibition is an attractive strategy for the treatment of advanced forms of thyroid cancer. This study was designed to test the efficacy of NF-κB pathway inhibition in combination with cytotoxic chemotherapy, using docetaxel and ionizing radiation in in vitro models of thyroid cancer. We found that while both docetaxel and ionizing radiation activated NF-κB signaling in thyroid cancer cells, there was no synergistic effect on cell proliferation and/or programmed cell death with either genetic (transduction of a dominant negative mutant form of IκBα) or pharmacologic (proteasome inhibitor bortezomib and IKKβ inhibitor GO-Y030) inhibition of the NF-κB pathway in thyroid cancer cell lines BCPAP, 8505C, THJ16T and SW1736. Docetaxel plus bortezomib synergistically decreased in vitro invasion of 8505C cells, but not in the other cell lines. Screening of a panel of clinically relevant targeted therapies for synergy with genetic NF-κB inhibition in a proliferation/cytotoxicity assay identified the histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA) as a potential candidate. However, the synergistic effect was confirmed only in the BCPAP cells. These results indicate that NF-κB inhibitors are unlikely to be beneficial as combination therapy with taxane cytotoxic chemotherapy, external radiation therapy or radioiodine therapy. There may be unique circumstances where NF-κB inhibitors may be considered in combination with docetaxel to reduce tumor invasion or in combination with HDAC inhibitors to reduce tumor growth, but this does not appear to be a combination therapy that could be broadly applied to patients with advanced thyroid cancer. Further research may identify which subsets of patients/tumors may respond to this therapeutic approach.     

Cell-mediated amplification and down regulation of cytotoxic immune response against autologous human cancer

The cytotoxic host immune response toward autologous human cancer may be regulated by the immunoregulatory network. Here we show that helper T cells, cloned from peripheral blood lymphocytes that were sensitized in vitro against an autologous human malignant paraganglioma, proliferated against and made interleukin 2 when cocultured with the tumor-associated antigen in the presence of autologous accessory cells. Furthermore, the helper cell clones amplified cytotoxic immune response by peripheral blood lymphocytes against the paraganglioma cells in coculture with the blood lymphocytes and the paraganglioma cells. An autologous T cell line bearing suppressor phenotype, established from a lymph node that had been infiltrated with the paraganglioma tumor cells, in contrast to the helper cells, selectively suppressed the cytotoxic immune response by the blood lymphocytes against the paraganglioma cells in identical coculture. These results, therefore, demonstrate the existence of cell-mediated immunologic regulations of the cytotoxic immune response (concurrent amplification and suppression in the same host) against an autologous human tumor.     

Invading cancer cells are predominantly in G0/G1 resulting in chemoresistance demonstrated by real-time FUCCI imaging

Invasive cancer cells are a critical target in order to prevent metastasis. In the present report, we demonstrate real-time visualization of cell cycle kinetics of invading cancer cells in 3-dimensional (3D) Gelfoam® histoculture, which is in vivo-like. A fluorescence ubiquitination cell cycle indicator (FUCCI) whereby G₀/G₁ cells express a red fluorescent protein and S/G₂/M cells express a green fluorescent protein was used to determine the cell cycle position of invading and non-invading cells. With FUCCI 3D confocal imaging, we observed that cancer cells in G₀/G₁ phase in Gelfoam® histoculture migrated more rapidly and further than cancer cells in S/G₂/M phases. Cancer cells ceased migrating when they entered S/G₂/M phases and restarted migrating after cell division when the cells re-entered G₀/G₁. Migrating cancer cells also were resistant to cytotoxic chemotherapy, since they were preponderantly in G₀/G₁, where cytotoxic chemotherapy is not effective. The results of the present report suggest that novel therapy targeting G₀/G₁ cancer cells should be developed to prevent metastasis.     

EphB receptors trigger Akt activation and suppress Fas receptor-induced apoptosis in malignant T lymphocytes

Treatment of hematopoietic malignancies often requires allogeneic bone marrow transplantation, and the subsequent graft-versus-leukemia response is crucial for the elimination of malignant cells. Cytotoxic T lymphocytes and NK cells responsible for the immunoelimination express Fas ligand and strongly rely on the induction of Fas receptor-mediated apoptosis for their action. Although cancer cells are removed successfully by graft-versus-leukemia reactions in myeloid malignancies, their efficiency is low in T cell leukemias. This may be partially because of the ability of malignant T cells to escape apoptosis. Our work shows that Eph family receptor EphB3 is consistently expressed by malignant T lymphocytes, most frequently in combination with EphB6, and that stimulation with their common ligands, ephrin-B1 and ephrin-B2, strongly suppresses Fas-induced apoptosis in these cells. This effect is associated with Akt activation and with the inhibition of the Fas receptor-initiated caspase proteolytic cascade. Akt proved to be crucial for the prosurvival response, because inhibition of Akt, but not of other molecules central to T cell biology, including Src kinases, MEK1 and MEK2, blocked the antiapoptotic effect. Overall, this demonstrates a new role for EphB receptors in the protection of malignant T cells from Fas-induced apoptosis through Akt engagement and prevention of caspase activation. Because Fas-triggered apoptosis is actively involved in the graft-versus-leukemia response and cytotoxic T cells express ephrin-Bs, our observations suggest that EphB receptors are likely to support immunoevasivenes of T cell malignancies and may represent promising targets for therapies, aiming to enhance immunoelimination of cancerous T cells.     

Recent advances in tumor-targeting anticancer drug conjugates

Traditional cancer chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be a killed by cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing severe undesirable side effects. Therefore, various drug delivery protocols and systems have been explored in the last three decades. Tumor cells overexpress many receptors and biomarkers, which can be used as targets to deliver cytotoxic agents into tumors. In general, a tumor-targeting drug delivery system consists of a tumor recognition moiety and a cytotoxic warhead connected directly or through a suitable linker to form a conjugate. The conjugate, which can be regarded as 'prodrug', should be systemically non-toxic. This means that the linker must be stable in circulation. Upon internalization into the cancer cell the conjugate should be readily cleaved to regenerate the active cytotoxic agent. Tumor-targeting conjugates bearing cytotoxic agents can be classified into several groups based on the type of cancer recognition moieties. This review describes recent advances in tumor-targeting drug conjugates including monoclonal antibodies, polyunsaturated fatty acids, folic acid, hyaluronic acid, and oligopeptides as tumor-targeting moieties.