Tracy R. Daniels

The transferrin receptor and the targeted delivery of therapeutic agents against cancer

BACKGROUND Traditional cancer therapy can be successful in destroying tumors, but can also cause dangerous side effects. Therefore, many targeted therapies are in development. The transferrin receptor (TfR) functions in cellular iron uptake through its interaction with transferrin. This receptor is an attractive molecule for the targeted therapy of cancer since it is upregulated on the surface of many cancer types and is efficiently internalized. This receptor can be targeted in two ways: 1) for the delivery of therapeutic molecules into malignant cells or 2) to block the natural function of the receptor leading directly to cancer cell death. SCOPE OF REVIEW In the present article we discuss the strategies used to target the TfR for the delivery of therapeutic agents into cancer cells. We provide a summary of the vast types of anti-cancer drugs that have been delivered into cancer cells employing a variety of receptor binding molecules including Tf, anti-TfR antibodies, or TfR-binding peptides alone or in combination with carrier molecules including nanoparticles and viruses. MAJOR CONCLUSIONS Targeting the TfR has been shown to be effective in delivering many different therapeutic agents and causing cytotoxic effects in cancer cells in vitro and in vivo. GENERAL SIGNIFICANCE The extensive use of TfR for targeted therapy attests to the versatility of targeting this receptor for therapeutic purposes against malignant cells. More advances in this area are expected to further improve the therapeutic potential of targeting the TfR for cancer therapy leading to an increase in the number of clinical trials of molecules targeting this receptor. This article is part of a Special Issue entitled Transferrins: molecular mechanisms of iron transport and disorders.

Inhibition of Yin Yang 1-Dependent Repressor Activity of DR5 Transcription and Expression by the Novel Proteasome Inhibitor NPI-0052 Contributes to its TRAIL-Enhanced Apoptosis in Cancer Cells

TRAIL promotes apoptotic tumor cell death; however, TRAIL-resistant tumors need to be sensitized to reverse resistance. Proteasome inhibitors potentiate TRAIL apoptosis in vitro and in vivo and correlate with up-regulation of death receptor 5 (DR5) via an unknown mechanism. We hypothesized that the proteasome inhibitor NPI-0052 inhibits the transcription repressor Yin Yang 1 (YY1) which regulates TRAIL resistance and negatively regulates DR5 transcription. Treatment of PC-3 and Ramos cells with NPI-0052 (≤2.5 nM) and TRAIL sensitizes the tumor cells to TRAIL-induced apoptosis. By comparison to bortezomib, a 400-fold less concentration of NPI-0052 was used. NPI-0052 up-regulated DR5 reporter activity and both surface and total DR5 protein expression. NPI-0052-induced inhibition of NF-κB activity was involved in TRAIL sensitization as corroborated by the use of the NF-κB inhibitor dehydroxymethylepoxyquinomicin. NPI-0052 inhibited YY1 promoter activity as well as both YY1 mRNA and protein expression. The direct role of NPI-0052-induced inhibition of YY1 and up-regulation of DR5 in the regulation of TRAIL sensitivity was demonstrated by the use of YY1 small interfering RNA. The NPI-0052-induced sensitization to TRAIL involved activation of the intrinsic apoptotic pathway and dysregulation of genes that regulate apoptosis. The NPI-0052 concentrations used for TRAIL sensitization were not toxic to human hematopoetic stem cells. The present findings demonstrate, for the first time, the potential mechanism by which a proteasome inhibitor, like NPI-0052, inhibits the transcription repressor YY1 involved in TRAIL resistance and DR5 regulation. The findings also suggest the therapeutic application of subtoxic NPI-0052 concentrations in combination with TRAIL/agonist DR4/DR5 mAbs in the treatment of TRAIL-resistant tumors.

Antibody-cytokine fusion proteins: applications in cancer therapy

Background: Antibody–cytokine fusion proteins consist of cytokines fused to an antibody to improve antibody-targeted cancer immunotherapy. These molecules have the capacity to enhance the tumoricidal activity of the antibodies and/or activate a secondary antitumor immune response. Objective: To review the strategies used to develop antibody–cytokine fusion proteins and their in vitro and in vivo properties, including preclinical and clinical studies focusing on IL-2, IL-12 and GM-CSF. Methods: Articles were found by searching databases such as PubMed and Clinical Trials of the US National Institutes of Health. Results/conclusion: Multiple antibody–cytokine fusion proteins have demonstrated significant antitumor activity as direct therapeutics or as adjuvants of cancer vaccines in preclinical studies, paving the way for their clinical evaluation.

A versatile targeting system with lentiviral vectors bearing the biotin-adaptor peptide.

Targeted gene transduction in vivo is the ultimate preferred method for gene delivery. We previously developed targeting lentiviral vectors that specifically recognize cell surface molecules with conjugated antibodies and mediate targeted gene transduction both in vitro and in vivo. Although effective in some experimental settings, the conjugation of virus with antibodies is mediated by the interaction between protein A and the Fc region of antibodies, which is not as stable as covalent conjugation. We have now developed a more stable conjugation strategy utilizing the interaction between avidin and biotin.

Enhanced cytotoxicity of an anti-transferrin receptor IgG3-avidin fusion protein in combination with gambogic acid against human malignant hematopoietic cells: functional relevance of iron, the receptor, and reactive oxygen species.

The human transferrin receptor (hTfR) is a target for cancer immunotherapy due to its overexpression on the surface of cancer cells. We previously developed an antibody-avidin fusion protein that targets hTfR (anti-hTfR IgG3-Av) and exhibits intrinsic cytotoxicity against certain malignant cells. Gambogic acid (GA), a drug that also binds hTfR, induces cytotoxicity in several malignant cell lines. We now report that anti-hTfR IgG3-Av and GA induce cytotoxicity in a new broader panel of hematopoietic malignant cell lines. Our results show that the effect of anti-hTfR IgG3-Av is iron-dependent whereas that of GA is iron-independent in all cells tested. In addition, we observed that GA exerts a TfR-independent cytotoxicity. We also found that GA increases the generation of reactive oxygen species that may play a role in the cytotoxicity induced by this drug. Additive cytotoxicity was observed by simultaneous combination treatment with these drugs and synergy by using anti-hTfR IgG3-Av as a chemosensitizing agent. In addition, we found a concentration of GA that is toxic to malignant hematopoietic cells but not to human hematopoietic progenitor cells. Our results suggest that these two compounds may be effective, alone or in combination, for the treatment of human hematopoietic malignancies.

Conjugation of an anti–transferrin receptor IgG3-avidin fusion protein with biotinylated saporin results in significant enhancement of its cytotoxicity against malignant hematopoietic cells

We have previously developed an antibody fusion protein composed of a mouse/human chimeric IgG3 specific for the human transferrin receptor genetically fused to avidin (anti-hTfR IgG3-Av) as a universal delivery system for cancer therapy. This fusion protein efficiently delivers biotinylated FITC into cancer cells via TfR-mediated endocytosis. In addition, anti-hTfR IgG3-Av alone exhibits intrinsic cytotoxic activity and interferes with hTfR recycling, leading to the rapid degradation of the TfR and lethal iron deprivation in certain malignant B-cell lines. We now report on the cytotoxic effects of a conjugate composed of anti-hTfR IgG3-Av and biotinylated saporin 6 (b-SO6), a toxin derived from the plant Saponaria officinalis that inhibits protein synthesis. Conjugation of anti-hTfR IgG3-Av with b-SO6 enhances the cytotoxic effect of the fusion protein in sensitive cells and also overcomes the resistance of malignant cells that show low sensitivity to the fusion protein alone. Our results show for the first time that loading anti-hTfR IgG3-Av with a biotinylated toxin enhances the cytotoxicity of the fusion protein alone. These results suggest that anti-hTfR IgG3-Av has great potential as a therapeutic agent for a wide range of applications due to its intrinsic cytotoxic activity plus its ability to deliver biotinylated molecules into cancer cells. [Mol Cancer Ther 2007;6(11):2995–3008]

Disruption of HOX activity leads to cell death that can be enhanced by the interference of iron uptake in malignant B cells

The HOX genes encode a family of transcription factors that are dysregulated in several malignancies and have been implicated in oncogenesis and cancer cell survival. Disruption of HOX protein function using the peptide HXR9 has shown anti-tumor effects against melanoma, lung cancer and renal cancer. In this report, we evaluated the expression of all 39 HOX genes in a panel of six malignant B-cell lines, including multiple myeloma cells and found different levels of expression of HOX family members suggesting that they also have a role in malignant B-cell survival. We show that disrupting HOX function using the peptide HXR9 induces significant cytotoxicity in the entire panel of cell lines. Importantly, we found that the cytotoxic effects of HXR9 can be enhanced by combining it with ch128.1Av, an antibody-avidin fusion protein specific for the human transferrin receptor 1 (CD71). Iron starvation induced by the fusion protein contributes to the enhanced effect and involves, at least in part, the induction of a caspase-independent pathway. These results show the relevance of HOX proteins in malignant B-cell survival and suggest that our therapeutic strategy may be effective in the treatment of incurable B-cell malignancies such as multiple myeloma.

Targeting HER2/neu with a fully human IgE to harness the allergic reaction against cancer cells

Breast and ovarian cancer are two of the leading causes of cancer deaths among women in the United States. Overexpression of the HER2/neu oncoprotein has been reported in patients affected with breast and ovarian cancers, and is associated with poor prognosis. To develop a novel targeted therapy for HER2/neu expressing tumors, we have constructed a fully human IgE with the variable regions of the scFv C6MH3-B1 specific for HER2/neu. This antibody was expressed in murine myeloma cells and was properly assembled and secreted. The Fc region of this antibody triggers in vitro degranulation of rat basophilic cells expressing human FcεRI (RBL SX-38) in the presence of murine mammary carcinoma cells that express human HER2/neu (D2F2/E2), but not the shed (soluble) antigen (ECDHER2) alone. This IgE is also capable of inducing passive cutaneous anaphylaxis in a human FcεRIα transgenic mouse model, in the presence of a cross-linking antibody, but not in the presence of soluble ECDHER2. Additionally, IgE enhances antigen presentation in human dendritic cells and facilitates cross-priming, suggesting that the antibody is able to stimulate a secondary T-cell anti-tumor response. Furthermore, we show that this IgE significantly prolongs survival of human FcεRIα transgenic mice bearing D2F2/E2 tumors. We also report that the anti-HER2/neu IgE is well tolerated in a preliminary study conducted in Macaca fascicularis (cynomolgus) monkeys. In summary, our results suggest that this IgE should be further explored as a potential therapeutic against HER2/neu overexpressing tumors, such as breast and ovarian cancers.

An antibody-based multifaceted approach targeting the human transferrin receptor for the treatment of B-cell malignancies.

We previously developed an antibody-avidin fusion protein (ch128.1Av) targeting the human transferrin receptor 1 (TfR1, also known as CD71), which demonstrates direct in vitro cytotoxicity against malignant hematopoietic cells. This cytotoxicity is attributed to its ability to decrease the level of TfR1 leading to lethal iron deprivation. We now report that ch128.1Av shows the ability to bind the Fc&ggr; receptors and the complement component C1q, suggesting that it is capable of eliciting Fc-mediated effector functions such as antibody-dependent cell-mediated cytotoxicity and complement-mediated cytotoxicity. In addition, in 2 disseminated multiple myeloma xenograft mouse models, we show that a single dose of ch128.1Av results in significant antitumor activity, including long-term survival. It is interesting to note that the parental antibody without avidin (ch128.1) also shows remarkable in vivo anticancer activity despite its limited in vitro cytotoxicity. Finally, we demonstrate that ch128.1Av is not toxic to pluripotent hematopoietic progenitor cells using the long-term cell-initiating culture assay suggesting that these important progenitors would be preserved in different therapeutic approaches, including the in vitro purging of cancer cells for autologous transplantation and in vivo passive immunotherapy. Our results suggest that ch128.1Av and ch128.1 may be effective in the therapy of human multiple myeloma and potentially other hematopoietic malignancies.

Lethal iron deprivation induced by non-neutralizing antibodies targeting transferrin receptor 1 in malignant B cells

Abstract A number of antibodies have been developed that induce lethal iron deprivation (LID) by targeting the transferrin receptor 1 (TfR1/CD71) and either neutralizing transferrin (Tf) binding, blocking internalization of the receptor and/or inducing its degradation. We have developed recombinant antibodies targeting human TfR1 (ch128.1 and ch128.1Av), which induce receptor degradation and are cytotoxic to certain malignant B-cells. We now show that internalization of TfR1 bound to these antibodies can lead to its sequestration and degradation, as well as reduced Tf uptake, and the induction of a transcriptional response consistent with iron deprivation, which is mediated in part by downstream targets of p53. Cells resistant to these antibodies do not sequester and degrade TfR1 after internalization of the antibody/receptor complex, and accordingly maintain their ability to internalize Tf. These findings are expected to facilitate the rational design and clinical use of therapeutic agents targeting iron import via TfR1 in hematopoietic malignancies.