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.

Nanomedicine is an emerging form of therapy that focuses on alternative drug delivery and improvement of the treatment efficacy while reducing detrimental side effects to normal tissues. Cancer drug resistance is a complicated process that involves multiple mechanisms. Here we discuss the major forms of drug resistance and the new possibilities that nanomedicines offer to overcome these treatment obstacles. Novel nanomedicines that have a high ability for flexible, fast drug design and production based on tumor genetic profiles can be created making drug selection for personal patient treatment much more intensive and effective. This review aims to demonstrate the advantage of the young medical science field of nanomedicine for overcoming cancer drug resistance. With the advanced design and alternative mechanisms of drug delivery known for different nanodrugs including liposomes, polymer conjugates, micelles, dendrimers, carbon-based, and metallic nanoparticles, overcoming various forms of multi-drug resistance looks promising and opens new horizons for cancer treatment.

Brain glioma treatment with checkpoint inhibitor antibodies to cytotoxic T-lymphocyteassociated antigen 4 (a-CTLA-4) and programmed cell death-1 (a-PD-1) was largely unsuccessful due to their inability to cross blood-brain barrier (BBB). Here we describe targeted nanoscale immunoconjugates (NICs) on natural biopolymer scaffold, poly(β-L-malic acid), with covalently attached a-CTLA-4 or a-PD-1 for systemic delivery across the BBB and activation of local brain anti-tumor immune response. NIC treatment of mice bearing intracranial GL261 glioblastoma (GBM) results in an increase of CD8+ T cells, NK cells and macrophages with a decrease of regulatory T cells (Tregs) in the brain tumor area. Survival of GBM-bearing mice treated with NIC combination is significantly longer compared to animals treated with single checkpoint inhibitor-bearing NICs or free a-CTLA-4 and a-PD-1. Our study demonstrates trans-BBB delivery of tumor-targeted polymer-conjugated checkpoint inhibitors as an effective GBM treatment via activation of both systemic and local privileged brain tumor immune response.

Effective treatment of brain neurological disorders such as Alzheimer's disease, multiple sclerosis, or tumors should be possible with drug delivery through blood–brain barrier (BBB) or blood–brain tumor barrier (BTB) and targeting specific types of brain cells with drug release into the cell cytoplasm. A polymeric nanobioconjugate drug based on biodegradable, nontoxic, and nonimmunogenic polymalic acid as a universal delivery nanoplatform was used for design and synthesis of nanomedicine drug for i.v. treatment of brain tumors. The polymeric drug passes through the BTB and tumor cell membrane using tandem monoclonal antibodies targeting the BTB and tumor cells. The next step for polymeric drug action was inhibition of tumor angiogenesis by specifically blocking the synthesis of a tumor neovascular trimer protein, laminin-411, by attached antisense oligonucleotides (AONs). The AONs were released into the target cell cytoplasm via pH-activated trileucine, an endosomal escape moiety. Drug delivery to the brain tumor and the release mechanism were both studied for this nanobiopolymer. Introduction of a trileucine endosome escape unit resulted in significantly increased AON delivery to tumor cells, inhibition of laminin-411 synthesis in vitro and in vivo, specific accumulation in brain tumors, and suppression of intracranial glioma growth compared with pH-independent leucine ester. The availability of a systemically active polymeric drug delivery system that passes through the BTB, targets tumor cells, and inhibits glioma growth gives hope for a successful strategy of glioma treatment. This delivery system with drug release into the brain-specific cell type could be useful for treatment of various brain pathologies.

A new prototype of nanoconjugate, Polycefin, was synthesized for targeted delivery of antisense oligonucleotides and monoclonal antibodies to brain tumors. The macromolecular carrier contains: 1. biodegradable, nonimmunogenic, nontoxic β-poly(L-malic acid) of microbial origin; 2. Morpholino antisense oligonucleotides targeting laminin α4 and β1 chains of laminin-8, which is specifically overexpressed in glial brain tumors; 3. monoclonal anti-transferrin receptor antibody for specific tissue targeting; 4. oligonucleotide releasing disulfide units; 5. L-valine containing, pHsensitive membrane disrupting unit(s), 6. protective poly(ethylene glycol); 7. a fluorescent dye (optional). Highly purified modules were conjugated directly with N-hydroxysuccinimidyl esteractivated β-poly-(L-malic acid) at pendant carboxyl groups or at thiol containing spacers via thioether and disulfide bonds. Products were chemically validated by physical, chemical, and functional tests. In vitro experiments using two human glioma cell lines U87MG and T98G demonstrated that Polycefin was delivered into the tumor cells by a receptor-mediated endocytosis mechanism and was able to inhibit the synthesis of laminin-8 α4 and β1 chains at the same time.Inhibition of laminin-8 expression was in agreement with the designed endosomal membrane disruption and drug releasing activity. In vivo imaging showed the accumulation of intravenously injected Polycefin in brain tumor tissue via the antibody-targeted transferrin receptor-mediated endosomal pathway in addition to a less efficient mechanism known for high molecular mass biopolymers as enhanced permeability and retention effect. Polycefin was nontoxic to normal and tumor astrocytes in a wide range of concentrations, accumulated in brain tumor, and could be used for specific targeting of several biomarkers simultaneously.

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Synthesis of radioactive adenosine(5')tetraphospho(Sf)adenosine in the presence of aminoacyl-tRNA synthetases of procaryotes or eucaryotes was measured by thin-layer chromatography on poly(ethy1eneimine)-cellulose sheets. Catalysis by the majority of aminoacyl-tRNA synthetases was observed. They were classified into a fast group (specificity for Lys, Phe, His), a slow group (Ser, Ile, Leu, Asp, Tyr, Val) and a non-catalytic group (Trp, Arg). The presence of the cognate amino acid is required.The mechanism was verified by direct synthesis of the tetraphosphate from preformed aminoacyladenylate and by its reverse reaction from chemically synthesized adenosine(5')tetraphospho(5')adenosine. Essentials are as follows. (1) Aminoacyladenylate is synthesized from a first molecule of MgATP. (2) A second ATP adds as substrate, Michaelis-Menten constants being 11 mM and 50 mM in the case of Escherichia coji lysine-specific and phenylalanine-specific enzymes, respectively; these values are two orders of magnitude higher than for the first ATP. (3) This is followed by the synthesis of adenosine(5')tetraphospho(5')adenosine from adenylate and the second ATP; the reaction is rate-determining in catalysis, rate constants being 0.5 s-' and 0.26 s-l respectively. (4) The equilibrium constant, K s 0.02, does not favour the synthesis of the tetraphosphate from ATP; removal of accumulating pyrophosphate by the action of added inorganic pyrophosphatase increases yields substantially. ( 5 ) The presence of Mg2+ ions is essential; in a second role, they can strongly activate the catalytic reaction. (6) Znz+ is a more potent activator, although it is not essential; its effect was to decrease the value of the Michaelis-Menten constant and to increase that of the catalytic rate constant; activation was observed only for a minority of aminoacyl-tRNA synthetases (Phe, Lys).The synthesis of adenosine(5')tetraphospho(5')adenosine and of adenosine(5')triphospho(5')adenosine and its reverse are discussed with regard to possible biological functions.

One of the major problems facing the treatment of neurological disorders is the poor delivery of therapeutic agents into the brain. Our goal is to develop a multifunctional and biodegradable nanodrug delivery system that crosses the blood–brain barrier (BBB) to access brain tissues affected by neurological disease. In this study, we synthesized a biodegradable nontoxic β-poly­(l-malic acid) (PMLA or P) as a scaffold to chemically bind the BBB crossing peptides Angiopep-2 (AP2), MiniAp-4 (M4), and the transferrin receptor ligands cTfRL and B6. In addition, a trileucine endosome escape unit (LLL) and a fluorescent marker (rhodamine or rh) were attached to the PMLA backbone. The pharmacokinetics, BBB penetration, and biodistribution of nanoconjugates were studied in different brain regions and at multiple time points via optical imaging. The optimal nanoconjugate, P/LLL/AP2/rh, produced significant fluorescence in the parenchyma of cortical layers II/III, the midbrain colliculi, and the hippocampal CA1-3 cellular layers 30 min after a single intravenous injection; clearance was observed after 4 h. The nanoconjugate variant P/LLL/rh lacking AP2, or the variant P/AP2/rh lacking LLL, showed significantly less BBB penetration. The LLL moiety appeared to stabilize the nanoconjugate, while AP2 enhanced BBB penetration. Finally, nanoconjugates containing the peptides M4, cTfRL, and B6 displayed comparably little and/or inconsistent infiltration of brain parenchyma, likely due to reduced trans-BBB movement. P/LLL/AP2/rh can now be functionalized with intra-brain targeting and drug treatment moieties that are aimed at molecular pathways implicated in neurological disorders.