Single-chain Fv/folate conjugates mediate efficient lysis of folate-receptor-positive tumor cells

A Bibliometric analysis of folate receptor research

BACKGROUND

The objective of this study was to conduct a bibliometric analysis of the entire field of folate receptor research. Folate receptor is expressed on a wide variety of cancers and certain immune cells.

METHODS

A Web of Science search was performed on folate receptor or folate binding protein (1969-to June 28, 2019). The following information was examined: publications per year, overall citations, top 10 authors, top 10 institutions, top 10 cited articles, top 10 countries, co-author collaborations and key areas of research.

RESULTS

In total, 3248 documents for folate receptor or folate binding protein were retrieved for the study years outlined in the methods section search query. The range was 1 per year in 1969 to 264 for the last full year studied (2018). A total of 123,720 citations for the 3248 documents retrieved represented a mean citation rate per article of 38.09 and range of 1667 citations (range 0 to 1667). Researchers in 71 countries authored publications analyzed in this study. The US was the leader in publications and had the highest ranking institution. The top 10 articles have been cited 7270 times during the time frame of this study. The top cited article had an average citation rate of 110 citations per year. Network maps revealed considerable co-authorship among several of the top 10 authors.

CONCLUSION

Our study presents several important insights into the features and impact of folate receptor research. To our knowledge, this is the first bibliometric analysis of folate receptor.

Redirecting cytotoxic T cells with chemically programmed antibodies

T-cell engaging bispecific antibodies (T-biAbs) mediate potent and selective cytotoxicity by combining specificities for target and effector cells in one molecule. Chemically programmed T-biAbs (cp-T-biAbs) are precisely assembled compositions of (i) small molecules that govern cancer cell surface targeting with high affinity and specificity and (ii) antibodies that recruit and activate T cells and equip the small molecule with confined biodistribution and longer circulatory half-life. Conceptually similar to cp-T-biAbs, switchable chimeric antigen receptor T cells (sCAR-Ts) can also be put under the control of small molecules by using a chemically programmed antibody as a bispecific adaptor molecule. As such, cp-T-biAbs and cp-sCAR-Ts can endow small molecules with the power of cancer immunotherapy. We here review the concept of chemically programmed antibodies for recruiting and activating T cells as a promising strategy for broadening the utility of small molecules in cancer therapy.

PSMA-targeted bispecific Fab conjugates that engage T cells

Bioconjugate formats provide alternative strategies for antigen targeting with bispecific antibodies. Here, PSMA-targeted Fab conjugates were generated using different bispecific formats. Interchain disulfide bridging of an αCD3 Fab enabled installation of either the PSMA-targeting small molecule DUPA (SynFab) or the attachment of an αPSMA Fab (BisFab) by covalent linkage. Optimization of the reducing conditions was critical for selective interchain disulfide reduction and good bioconjugate yield. Activity of αPSMA/CD3 Fab conjugates was tested by in vitro cytotoxicity assays using prostate cancer cell lines. Both bispecific formats demonstrated excellent potency and antigen selectivity.

Bioconjugation – using selective chemistry to enhance the properties of proteins and peptides as therapeutics and carriers

Both peptide and protein therapeutics are becoming increasingly important for treating a wide range of diseases. Functionalisation of theseviasite-selective chemical modification leads to enhancement of their therapeutic properties.

Tumor detection using folate receptor-targeted imaging agents

Folate receptors are up-regulated on a variety of human cancers, including cancers of the breast, ovaries, endometrium, lungs, kidneys, colon, brain, and myeloid cells of hematopoietic origin. This over-expression of folate receptors (FR) on cancer tissues can be exploited to target folate-linked imaging and therapeutic agents specifically to FR-expressing tumors, thereby avoiding uptake by most healthy tissues that express few if any FR. Four folate-targeted therapeutic drugs are currently undergoing clinical trials, and several folate-linked chemotherapeutic agents are in late stage preclinical development. However, because not all cancers express FR, and because only FR-expressing cancers respond to FR-targeted therapies, FR-targeted imaging agents have been required to select patients with FR-expressing tumors likely to respond to folate-targeted therapies. This review focuses on recent advances in the use of the vitamin folic acid to target PET agents, gamma-emitters, MRI contrast agents and fluorescent dyes to FR(+) cancers for the purpose of diagnosing and imaging malignant masses with improved specificity and sensitivity.

Synthesis, spectroscopic and thermal characterization of some transition metal complexes of folic acid

Compounds having general formula: [M(FO)(Cl)x(H2O)y]x zH2O, where (M=Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II), FO=folate anion, x=2 or 4, y=2 or 4 and z=0, 1, 2, 3, 5 or 15) were prepared. The obtained compounds were characterized by elemental analysis, infrared as well as electronic spectra, thermogravimetric analysis and the conductivity measurements. The results suggested that all folate complexes were formed by 2:1 molar ratio (metal:folic acid) as a bidentate through both of the two carboxylic groups. The molar conductance measurements proved that the folate complexes are electrolytes. The kinetic thermodynamic parameters such as: E*, DeltaH*, DeltaS* and DeltaG* were estimated from the DTG curves. The antibacterial evaluation of the folic acid and their complexes was also done against some Gram positive/negative bacteria as well as fungi.

Ligand based dendritic systems for tumor targeting

Medications that can selectively target tumors at the same time avoid access of the drug to nontarget areas, employ utilization of homing devices termed as ligands, that can bind to specific epitopes expressed on the surface of the necrotic mass of cells. Molecular signatures for transferrin, Epidermal Growth Factor, Sialic Lewis and folic acid are expressed on the surface of these cells. Dendrimers are nanosized, non-immunogenic, and hyper-branched vehicles that can be efficiently tailored for spatial distribution of bioactives, thereby reducing untoward cytotoxicity on normal cells. These nanoparticulate drug delivery vehicles provide a unique platform that has precisely placed functional groups so that multiple copies of ligands can be attached to it and facilitate targeting to the tumor surface or neo-vascularizing vessels proliferating around these cells. The article reviews the scope of ligand based dendritic system as a prospective for delivery of anti-cancer drugs, via active targeting with interception of minimal side effects.

Folate Receptor-Mediated Drug Targeting: From Therapeutics to Diagnostics

Folate targeted drug delivery has emerged as an alternative therapy for the treatment and imaging of many cancers and inflammatory diseases. Due to its small molecular size and high binding affinity for cell surface folate receptors (FR), folate conjugates have the ability to deliver a variety of molecular complexes to pathologic cells without causing harm to normal tissues. Complexes that have been successfully delivered to FR expressing cells, to date, include protein toxins, immune stimulants, chemotherapeutic agents, liposomes, nanoparticles, and imaging agents. This review will summarize the applications of folic acid as a targeting ligand and highlight the various methods being developed for delivery of therapeutic and imaging agents to FR-expressing cells.

Nanoparticle Targeting of Anticancer Drug Improves Therapeutic Response in Animal Model of Human Epithelial Cancer

Prior studies suggested that nanoparticle drug delivery might improve the therapeutic response to anticancer drugs and allow the simultaneous monitoring of drug uptake by tumors. We employed modified PAMAM dendritic polymers <5 nm in diameter as carriers. Acetylated dendrimers were conjugated to folic acid as a targeting agent and then coupled to either methotrexate or tritium and either fluorescein or 6-carboxytetramethylrhodamine. These conjugates were injected i.v. into immunodeficient mice bearing human KB tumors that overexpress the folic acid receptor. In contrast to nontargeted polymer, folate-conjugated nanoparticles concentrated in the tumor and liver tissue over 4 days after administration. The tumor tissue localization of the folate-targeted polymer could be attenuated by prior i.v. injection of free folic acid. Confocal microscopy confirmed the internalization of the drug conjugates into the tumor cells. Targeting methotrexate increased its antitumor activity and markedly decreased its toxicity, allowing therapeutic responses not possible with a free drug.

Vitamin-mediated targeting as a potential mechanism to increase drug uptake by tumours

Targeted chemotherapy for cancer treatment offers a great potential advantage in tumour treatment due to greater specificity of delivery which leads to increased dose of the cytotoxin delivered to the tumour relative to the rest of the body. In order to achieve such selective targeted delivery one needs to identify generic markers that are over-expressed on the surface of tumour cells but are not over-expressed on normal tissue. Work of several authors has shown that some cells, such as those of rapidly dividing, aggressive tumours, over-express surface receptors involved in the uptake of vitamin B(12) [B. Rachmilewitz, M. Rachmilewitz, B. Moshkowitz, J. Gross, J. Lab. Clin. Med. 78 (1971) 275-279; B. Rachmilewitz, A. Sulkes, M. Rachmilewitz, A. Fuks, Israel J. Med. Sci. 17 (1981) 874-879] or folate [P. Garin-Chesa, I. Campbell, P.E. Saigo, J.L. Lewis Jr., L.J. Old, W.J. Rettig, Am. J. Pathol. 142 (1993) 557-567; O.C. Boerman, C.C. van Niekerk, K. Makkink, T.G.J.M. Hanselaar, P. Kenemans, L.G. Poels, Int. J. Gynecol. Pathol. 10 (1991) 15-25; G. Toffoli, C. Cernigoi, A. Russo, A. Gallo, M. Bagnoli, M. Boiocchi, Int. J. Cancer 74 (1997) 193-194; J.A. Reddy, D. Dean, M.D. Kennedy, P.S. Low, J. Pharm. Sci. 88 (1999) 1112-1118; J.A. Reddy, P.S. Low, Crit. Rev. Ther. Drug Carrier Syst. 15 (1998) 587-627; G.J. Russell-Jones, K. McTavish, J.F. McEwan, in: Proceedings of the 2nd International Symposium on Tumor Targeted Delivery Systems, 2002]. Furthermore the degree of over-expression has been found to correlate with the stage of tumour growth, with the highest levels found on stage IV carcinomas. Using fluorescently-labelled polymers to which are linked the targeting agents, vitamin B(12), folate or biotin, the relative uptake of these polymers into various types of tumour cell lines grown both in vitro and in vivo has been examined. These studies have shown that while some tumour types do NOT over-express receptors involved in vitamin uptake, most tumour types over-express receptors for folate, or vitamin B(12). In either case there is also a greatly increased expression of a yet to be identified biotin receptor. In cases of receptor over-expression, binding of the targeted fluorochrome leads to rapid internalization of these molecules within the cells to levels that are two to thirty times higher than with non-targeted polymers. Using a number of cancer models, these studies were extended further and it was found that the increased expression of receptors also leads to increased levels of killing with targeted cytotoxins. Thus the preliminary data described suggests that the use of vitamins as targeting agents has enormous potential for use in cancer diagnosis and chemotherapy.

Recombinant fusion proteins for haemagglutination-based rapid detection of antibodies to HIV in whole blood

Recombinant fusion proteins, consisting of a monovalent anti-human RBC monoclonal antibody B6, and conserved immunodominant peptide of HIV-1 envelope glycoprotein gp41 or HIV-2 envelope glycoprotein gp36, have been designed and purified after over-expression in E. coli. These fusion proteins are Fab-based and were obtained by assembling the light chain with Fd (variable domain and the first constant domain of the heavy chain) or Fd fusions containing HIV-derived peptide, and following a protocol of in vitro denaturation of inclusion bodies and subsequent renaturation to assemble functional Fab. Using a multistep column chromatographic procedure, monomeric Fab and Fab fusion proteins containing HIV-derived peptide were purified to high degree, free of aggregates. The yield of various proteins on the laboratory scale (1-2 l of shake flask culture) was in the range of tens of milligram. Purified anti-human RBC Fab fusion proteins containing sequences derived from HIV-1 gp41 and HIV-2 gp36 were highly specific for detection of antibodies to HIV-1 and HIV-2, respectively. The described design, expression and purification protocols will make it possible to produce specific recombinant reagents in large quantities for agglutination-based rapid detection of antibodies to HIV in whole blood.

Folate-mediated targeting: from diagnostics to drug and gene delivery

The covalent attachment of the vitamin folic acid to almost any molecule yields a conjugate that can be endocytosed into folate receptor-bearing cells. Because folate receptors are significantly overexpressed in the majority of human cancers, this methodology is currently being investigated for the selective delivery of imaging and therapeutic agents to tumor tissue. Phase I and II clinical studies for the first folate-containing imaging agent were initiated in 1999, and clinical trials of folate-targeted therapeutic agents should soon follow. This review will summarize folate-mediated drug delivery and highlight those techniques undergoing active preclinical or clinical investigation.

Endogenous carriers and ligands in non-immunogenic site-specific drug delivery

Targeted drug delivery has gained recognition in modern therapeutics and attempts are being made to explore the potentials and possibilities of cell biology related bioevents in the development of specific, programmed and target oriented systems. The components which have been recognized to be tools include receptors and ligands, where the receptors act as molecular targets or portals, and ligands, with receptor specificity and selectivity, are trafficked en route to the target site. Although ligands of exogenous or synthetic origin contribute to the selectivity component of carrier constructs, they may impose immunological manifestations of different magnitudes. The latter may entail a continual quest for bio-compatible, non-immunogenic and target orientated delivery. Endogenous serum, cellular and extracellular bio-ligands interact with the colloidal carrier constructs and influence their bio-fate. However, these endogenous bio-ligands can themselves serve as targeting modules either in their native form or engineered as carrier cargo. Bio-regulatory, nutrient and immune ligands are sensitive, specific and effective site directing handles which add to targeted drug delivery. The present review provides an exhaustive account of the identified bio-ligands, which are not only non-immunogenic in nature but also site-specific. The cell-related bioevents which are instrumental in negotiating the uptake of bio-ligands are discussed. Further, a brief account of ligand-receptor interactions and the set of biological events which ensures ligand-driven trafficking of the ligand-receptor complex to the cellular interior is also presented. Since ligand-receptor interaction is a critical pre-requisite for negotiating cellular uptake of endogenous ligands and anchored carrier cargo, an attempt has been made to identify differential expression of receptors and bio-ligands under normal and etiological conditions. Studies which judiciously utilized bio-ligands or their analogs in negotiating site-specific drug delivery have been reviewed and presented. Targeted delivery of bioactives using endogenous bio-ligands offers enormous options and opportunities through carrier construct engineering and could become a future reality in clinical practice.

Bispecific agents target endogenous murine T cells against human tumor xenografts

A variety of immunological approaches to cancer treatment are currently being explored. These include strategies designed to enhance or redirect the activity of T cells against tumors. Bispecific antibodies comprise a class of agents capable of redirecting T cells by binding to a tumor antigen and the T-cell receptor (TCR). In vivo pre-clinical testing of bispecific antibodies against human tumors has to date been limited to the use of immunodeficient mice that receive the bispecific agent, activated human effector T cells, and human tumor cells. In this report, we show that TCR transgenic/RAG-1 knockout mice (TCR/RAG) serve as a unique model allowing endogenous T cells to be redirected against transplanted human tumors. The findings show that TCR/RAG mice (i) accepted transplants of human tumors, including the folate-receptor-positive tumor line KB; (ii) contained endogenous cytotoxic T lymphocytes that could be activated in vivo with an antigenic peptide recognized by the transgenic TCR; (iii) rejected human tumors after treatment with the activating peptide and bispecific agents that contained folic acid co-valently linked to an anti-TCR antibody. Successful rejection was achieved with folate conjugates of Fab or scFv fragments. Treatment with activating agents and bispecific conjugates resulted in the complete eradication of freshly transplanted tumors as well as significantly prolonging the survival of mice bearing established solid tumors. Our results highlight the importance of including T-cell-activating modalities in combination with bispecific antibodies. Additionally, we introduce a system that allows endogenous T cells to be redirected against human tumor xenografts and in which the T cells may be followed in vivo by use of a clonotypic marker.

Folic acid-PEO-labeled liposomes to improve gastrointestinal absorption of encapsulated agents

The design of targeted oral liposomes is anticipated to improve the systemic delivery of poorly absorbed agents, such as proteins and peptides. A poly(ethylene oxide) (PEO)-folic acid (FA) derivative was prepared and evaluated for improving liposome transport across a model gastrointestinal cell line (Caco-2). FA-PEO-cholesterol (Chol) derivatives were synthesized and adsorbed at liposome surfaces encapsulating Texas Red((R))-Dextran 3000 (TR-dex), a poorly-absorbed, neutral, hydrophilic, large molecular weight (M(w)) marker. Apparent permeabilities (P(app)) of Caco-2 cells to FA-PEO conjugates, TR-dex, uncoated TR-dex liposomes, and FA-coated TR-dex liposomes were compared at 2 h post-administration. Intracellular delivery of TR-dex was detected by fluorescence microscopy. An increase in intracellular accumulation of TR-dex associated with FA-PEO-coated liposomes, but not other formulations, was evidence of the potential of FA-targeted liposomes in the oral delivery of poorly absorbed, large M(w) agents.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Expression and Functional Characterization of the β-Isoform of the Folate Receptor on CD34+ Cells

We have investigated the expression and functional competence of folate receptor (FR) isoforms on human hematopoietic cells. Using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR) methodology, we find that a substantial fraction of low-density mononuclear and CD34+ cells express both the β and γ isoforms of FR. The  isoform of FR (the form most commonly found on cancer cells) was surprisingly absent from all hematopoietic cells examined. Compared with KB cells (a human cell line known for its elevated expression of FR-), the abundance of FR-β on CD34+ cell surfaces was relatively low (≈8% of KB cell levels). Because many antifolates and folic acid-linked chemotherapeutic agents enter malignant cells at least partially via FR endocytosis, it was important to evaluate the ability of FR on CD34+ cells to bind folic acid (FA). Based on three FR binding assays, freshly isolated CD34+ cells were found to display no affinity for FA. Thus, regardless of whether steps were taken to remove endogenous folates before receptor binding assays, FR on primitive hematopoietic cells failed to bind 3H-FA, fluorescein isothiocyanate (FITC)-linked FA, or FA-derivatized liposomes. In contrast, analogous studies on KB cells showed high levels of receptor binding for all three FR probes. These studies show that although multipotent hematopoietic progenitor cells express FR, the receptor does not transport significant amounts of FA. Consequently, antifolates and FA-linked chemotherapeutic agents that can be engineered to enter malignant cells exclusively through the FR should not harm progenitor/stem cell function.

Expression and Functional Characterization of the β-Isoform of the Folate Receptor on CD34+ Cells

We have investigated the expression and functional competence of folate receptor (FR) isoforms on human hematopoietic cells. Using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR) methodology, we find that a substantial fraction of low-density mononuclear and CD34+ cells express both the β and γ isoforms of FR. The  isoform of FR (the form most commonly found on cancer cells) was surprisingly absent from all hematopoietic cells examined. Compared with KB cells (a human cell line known for its elevated expression of FR-), the abundance of FR-β on CD34+ cell surfaces was relatively low (≈8% of KB cell levels). Because many antifolates and folic acid-linked chemotherapeutic agents enter malignant cells at least partially via FR endocytosis, it was important to evaluate the ability of FR on CD34+ cells to bind folic acid (FA). Based on three FR binding assays, freshly isolated CD34+ cells were found to display no affinity for FA. Thus, regardless of whether steps were taken to remove endogenous folates before receptor binding assays, FR on primitive hematopoietic cells failed to bind 3H-FA, fluorescein isothiocyanate (FITC)-linked FA, or FA-derivatized liposomes. In contrast, analogous studies on KB cells showed high levels of receptor binding for all three FR probes. These studies show that although multipotent hematopoietic progenitor cells express FR, the receptor does not transport significant amounts of FA. Consequently, antifolates and FA-linked chemotherapeutic agents that can be engineered to enter malignant cells exclusively through the FR should not harm progenitor/stem cell function.

Targeting T cells against brain tumors with a bispecific ligand-antibody conjugate

High-affinity receptors expressed on the surface of some tumors can be exploited by chemically conjugating the ligand for the receptor and an antibody against immune effector cells, thus redirecting their cytolytic potential against the tumor. Ovarian carcinomas and some brain tumors express the high-affinity folate receptor (FR). In this report, a transgenic mouse model that generates endogenously arising choroid plexus tumors was used to show that folate/anti-T-cell receptor antibody conjugates can direct infiltration of T cells into solid brain tumor masses. An engineered single-chain Fv form of the anti-T-cell receptor antibody KJ16 was conjugated with folate, to produce a bispecific agent that was substantially smaller than most previously characterized bispecific antibodies. Folate conjugation to the antibody increased T-cell infiltration into the tumors by 10- to 20-fold, and significantly prolonged survival of the mice.

Targeting tumor cells with bispecific antibodies and T cells

It has been known for some time that mammalian immune systems are capable of eliminating large tumor burdens. Redirecting the immune response of a patient to an established tumor has now become the focus of various therapeutic strategies. In this report, two projects toward this goal are described. The first project involves the development of a transgenic mouse model for T cell directed therapeutics. These mice express specific T cell receptor alpha and beta transgenes on a background in which the recombinational-activating-gene-1 (RAG) has been knocked out. The mice express cytotoxic T cells but not either T helper cells or B cells. Despite these deficiencies, the animals are capable of eliminating tumors that express the appropriate peptide/major histocompatibility complex ligand that is recognized by the alphabeta transgenic T cell receptor. Human tumors grow as transplants in these mice, thereby allowing various agents that redirect the endogenous T cells against human tumors to be tested. The second project involves a description of such agents: bispecific antibodies that simultaneously bind to an immune effector cell and a tumor cell. The bispecific antibody described here consists of folate attached to anti-T cell receptor antibodies, or their fragments. A single-chain Fv coupled with folate can redirect the lysis of human tumor cells that bear the high affinity folate receptor. Preliminary in vivo data showed that the folate/antibody conjugates were also capable of mediating rejection of the human tumor. This transgenic mouse model should now allow the evaluation and optimization of bispecific agents that can redirect a patient's own T cell response.