Polymer-doxorubicin Conjugates Research Articles

Self-assembly of polymer-doxorubicin conjugates to form polyprodrug micelles for pH/enzyme dual-responsive drug delivery

Functionality of polymers is crucially fundamental to polymers. However, the application of polymers faces great challenges due to the insufficient function of hydrophobic chain, especially in drug delivery. One novel approach to solve this dilemma involves the use of medicated hydrophobic chains. Herein, the polymer-doxorubicin conjugates are synthesized from hydrophilic copolymer and hydrophobic chain composed of doxorubicin (DOX). DOX, a common hydrophobic anticancer drug, is used to serve as a hydrophobic pendant for inducing self-assembly of polymer-doxorubicin conjugates. The self-assembly of polymer-drug conjugates is employed to encapsulate the unreacted drug for the first time and form drug-loaded polyprodrug micelles that possess a low critical micelle concentration (CMC), which improves the use efficiency of expensive drug. The detachment of DOX moieties from polymer-doxorubicin conjugates caused by amidase facilitates the disintegration of DOX-loaded polyprodrug micelles to trigger the enhanced release of DOX in a slightly acidic medium. It is demonstrated that the DOX-loaded polyprodrug micelles have a higher suppression against cancer cells in in vitro investigations. This work thus proposes a strategy to modify the poor functionality of polymer-based nanostructures, through designing the medicated hydrophobic chains. This strategy facilitates the potential application of polymer-drug conjugates and provides profound insights into the effect of medicated polymer on drug delivery system.

Polymer nanomedicines based on micelle-forming amphiphilic or water-soluble polymer-doxorubicin conjugates: Comparative study of in vitro and in vivo properties related to the polymer carrier structure, composition, and hydrodynamic properties

The study compared the physico-chemical and biological properties of a water-soluble star-like polymer nanomedicine with three micellar nanomedicines formed by self-assembly of amphiphilic copolymers differing in their hydrophobic part (statistical, block and thermosensitive block copolymers). All nanomedicines showed a pH-responsive release of the drug, independent on polymer structure. Significant penetration of all polymer nanomedicines into tumor cells in vitro was demonstrated, where the most pronounced effect was observed for statistical- or diblock copolymer-based micellar systems. Tumor accumulation in vivo was dependent on the stability of the nanomedicines in solution, being the highest for the star-like system, followed by the most stable micellar nanomedicines. The star-like polymer nanomedicine showed a superior therapeutic effect. Since the micellar systems exhibited slightly lower systemic toxicity, they may exhibit the same efficacy as the star-like soluble system when administered at equitoxic doses. In conclusion, treatment efficacy of studied nanomedicines was directly controlled by the drug pharmacokinetics, namely by their ability to circulate in the bloodstream for the time needed for effective accumulation in the tumor due to the enhanced permeability and retention (EPR) effect. Easy and scalable synthesis together with the direct reconstitution possibility for nanomedicine application made these nanomedicines excellent candidates for further clinical evaluation.

HPMA Polymeric Nanocarriers for Anticancer Drugs with Tumor Microenvironment-Responsive Extracellular Biodegradation and Intracellular Drug Release.

The balance between systemic toxicity and circulation time for a polymeric nanocarrier to deliver antitumor drugs has been trialed. A new approach to break the balance was proposed in this study by significantly improving its biosafety and prolonging the circulation time, hence, to enhance its anti-tumor efficacy. A matrix metalloproteinases (MMPs)-sensitive peptide (PVGLIGK) was introduced to cross-link the N-(2-hydroxypropyl) methylacrylamide polymer-doxorubicin conjugates (HPMA-Dox) conjugate to construct a nano-size polymeric nanocarrier-Dox assembly (PMD) with a molecular weight (MW) of 73 kDa and this modification has resulted in a prolonged circulation time (a half-time of 20.1 h) and enhanced accumulation of PMD at the tumor site, while negligible systemic toxicity and excellent biocompatibility were displayed after injection of PMD into the mice. The cross-linked nanoassembly was unpacking in the presence of MMPs in the extracellular microenvironment, and the conjugated Dox was released from the nanoassembly in the lysosome/endosome due to an intracellular low pH microenvironment. The released Dox from PMD inhibited tumor cells very efficiently with a tumor growth inhibition of around 70%. The outstanding performance of the dual stimuli-responsive biodegradable polymeric nanocarriers may open a door for other hydrophobic anti-tumor drugs.

Stimuli-responsive polymer-doxorubicin conjugate: Antitumor mechanism and potential as nano-prodrug

Polymer-drug conjugates has significantly improved the anti-tumor efficacy of chemotherapeutic drugs and alleviated their side effects. N-(1,3-dihydroxypropan-2-yl) methacrylamide (DHPMA) copolymer was synthesized via RAFT polymerization and polymer-doxorubicin (DOX) (diblock pDHPMA-DOX) were formed by conjugation, resulting in a self-aggregation-induced nanoprodrug with a favorable size of 21 nm and great stability. The nanoprodrug with a molecular weight (MW) of 95 kDa released drugs in response to tumor microenvironmental pH variations and they were enzymatically hydrolyzed into low MW segments (45 kDa). The nanoprodrug was transported through the endolysosomal pathway, released the drug into the cytoplasm and some was localized in the mitochondria, resulting in disruption of the cellular actin cytoskeleton. Cellular apoptosis was also associated with reduction in the mitochondrial potential caused by the nanoprodrug. Notably, the nanoprodrug had a significantly prolonged blood circulation time with an elimination half time of 9.8 h, displayed high accumulation within tumors, and improved the in vivo therapeutic efficacy against 4T1 xenograft tumors compared to free DOX. The tumor xenograft immunohistochemistry study clearly indicated tumor inhibition was through the inhibition of cell proliferation and antiangiogenic effects. Our studies demonstrated that the diblock pDHPMA-DOX nanoprodrug with a controlled molecular structure is promising to alleviate adverse effects of free DOX and have a great potential as an efficient anticancer agent. Statement of SignificanceIn this work, we prepared a biodegradable diblock DHPMA polymer-doxorubicin conjugate via one-pot of RAFT polymerization and conjugate chemistry. The conjugate-based nanoprodrug was internalized by endocytosis to intracellularly release DOX and further induce disruption of mitochondrial functions, actin cytoskeleton alterations and cellular apoptosis. The nanoprodrug with a high molecular weight (MW) (95 kDa) showed a long blood circulation time and achieved high accumulation into tumors. The nanoprodrug was degraded into low MW (∼45 kDa) products below the renal threshold, which ensured its biosafety. Additionally, the multi-stimuli-responsive nanoprodrug demonstrated an enhanced antitumor efficacy against 4T1 breast tumors and alleviated side effects, showing a great potential as an efficient and safe anticancer agent.

Effective doxorubicin-based nano-therapeutics for simultaneous malignant lymphoma treatment and lymphoma growth imaging

In this study, we report the in vivo anti-lymphoma efficacy and diagnostic potential of newly designed near-infrared fluorescent dye containing polymer-doxorubicin conjugates using murine models of malignant lymphomas including one cell line-derived xenograft (RAJI) and two patient-derived lymphoma xenografts (VFN-D1 and VFN-M2). Two types of passively targeted conjugates differing in architecture of the polymer backbone were synthesized. One of the conjugates was designed using a single linear polymer chain, and the second was more sophisticated with a star-shaped high-molecular-weight (HMW) polymer employing a dendrimer core. The linear HPMA copolymers were linked to the dendrimer core via a one-point attachment, thus forming a hydrophilic polymer shell. Both polymer-doxorubicin conjugates were long-circulating with reduced side effects. Both polymer prodrugs were designed as stimuli-sensitive systems in which the anti-cancer drug doxorubicin was attached to the hydrophilic copolymers via a pH-labile hydrazone linkage. Such polymer prodrugs were fairly stable in aqueous solutions at pH 7.4, and the drug was readily released in mildly acid environments at pH 5–6.5 by hydrolysis of the hydrazone bonds. In addition, polymers were labelled with near-infrared fluorescent dye enabling long term in vivo visualization. Malignant lymphomas represent the most common type of haematological malignancies. Therapy for the majority of malignant lymphomas consists of multi-agent chemotherapy based on an anthracycline doxorubicin, the most prominent side effect of which is cardiotoxicity. We have demonstrated significant anti-lymphoma efficacy of the polymer-doxorubicin conjugates when compared to equally toxic doses of conventional (unbound) doxorubicin in all tested models. Favourable pharmacokinetics for carried drug and labelled polymer carrier was observed, showing predominant uptake of the drug and polymer itself in the tumour mass. In addition, we have observed a promising diagnostic potential of fluorescently labelled polymer prodrugs. Dynamically analyzed fluorescence intensity over subcutaneously xenografted lymphomas closely corresponded to changes in the lymphoma tumour volumes, thereby enabling a non-invasive assessment of treatment efficacy

Tumor Specific and Renal Excretable Star-like Triblock Polymer-Doxorubicin Conjugates for Safe and Efficient Anticancer Therapy.

Efficient tumor accumulation and body clearance are two paralleled requirements for ideal nanomedicines. However, it is hard for both to be met simultaneously. The inefficient clearance often restrains the application of drug delivery systems (DDSs), especially for high-dosage administration. In this study, the star-like and block structures are combined to enhance the tumor specific targeting of the parent structures and obtain additional renal excretion property. The influences of polymer architectures and chemical compositions on the physicochemical and biological properties, particularly the simultaneous achievement of tumor accumulation and renal clearance, have been investigated. Among the tested conjugates, an eight-arm triblock star polymer based on poly(ethylene glycol) (PEG) and poly( N-(2-hydroxyl) methacrylamide) (PHPMA) is found to simultaneously fulfill the requirements of superior tumor accumulation and efficient renal clearance due to the appropriate micelle size and reversible aggregation process. On the basis of this conjugate, 60 mg/kg of Dox equivalent (much higher than the maximum tolerated dose (MTD) of Dox) can be administered to efficiently suppress tumor growth without causing any obvious toxicity. This work provides a new approach to design polymer-drug conjugates for tumor specific application, which can simultaneously address the efficacy and safety concerns.

Ability of polymer-bound P-glycoprotein inhibitor ritonavir to overcome multidrug resistance in various resistant neuroblastoma cell lines.

Polymer prodrugs can considerably improve the treatment of tumors with multidrug resistance, often caused by overexpression of P-glycoprotein (P-gp). Here, we present the effect of the N-(2-hydroxypropyl) methacrylamide-based polymer conjugate with P-gp inhibitor ritonavir (RIT) on the increase of free doxorubicin (DOX) and polymer-bound DOX cytotoxicity in the human neuroblastoma 4 cell line and its resistant clones to different cytostatics. The increase in cytotoxicity after polymer-RIT conjugate pretreatment was higher for the lines overexpressing P-gp and less pronounced for those with decreased P-gp levels. Moreover, the effect of polymer conjugate containing inhibitor and DOX on the same polymer chain was lower than that of two individual polymer conjugates used sequentially. In conclusion, the polymer-RIT conjugate can significantly increase the cytotoxicity of free DOX and polymer-DOX conjugates in cells with various multidrug resistance origins and can thus be considered a suitable therapeutic enhancer of polymer prodrugs.

Legumain-cleavable 4-arm poly(ethylene glycol)-doxorubicin conjugate for tumor specific delivery and release.

Herein we describe the construction of a novel tumor environment-sensitive delivery system through the instruction of a legumain-cleavable linkage to a polymer-DOX conjugate (4-arm PEG-AANL-DOX). This particular design strategy allows for polymer-DOX conjugates to be delivered in a tumor-specific manner and selectively activable in tumor microenvironment so that it can combine the advantages of tumor-specific delivery and tumor intracellular microenvironment-triggered release systems.

PH-Switch Nanoprecipitation of Polymeric Nanoparticles for Multimodal Cancer Targeting and Intracellular Triggered Delivery of Doxorubicin.

Theranostic nanoparticles are emerging as potent tools for noninvasive diagnosis, treatment, and monitoring of solid tumors. Herein, an advanced targeted and multistimuli responsive theranostic platform is presented for the intracellular triggered delivery of doxorubicin. The system consists of a polymeric-drug conjugate solid nanoparticle containing encapsulated superparamagnetic iron oxide nanoparticles (IO@PNP) and decorated with a tumor homing peptide, iRGD. The production of this nanosystem is based on a pH-switch nanoprecipitation method in organic-free solvents, making it ideal for biomedical applications. The nanosystem shows sufficient magnetization saturation for magnetically guided therapy along with reduced cytotoxicity and hemolytic effects. IO@PNP are largely internalized by endothelial and metastatic cancer cells and iRGD decorated IO@PNP moderately enhance their internalization into endothelial cells, while no enhancement is found for the metastatic cancer cells. Poly(ethylene glycol)-block-poly(histidine) with pH-responsive and proton-sponge properties promotes prompt lysosomal escape once the nanoparticles are endocyted. In addition, the polymer-doxorubicin conjugate solid nanoparticles show both intracellular lysosomal escape and efficient translocation of doxorubicin to the nuclei of the cells via cleavage of the amide bond. Overall, IO@PNP-doxorubicin and the iRGD decorated counterpart demonstrate to enhance the toxicity of doxorubicin in cancer cells by improving the intracellular delivery of the drug carried in the IO@PNP.

Anti-Lymphoma Efficacy Comparison of Anti-Cd20 Monoclonal Antibody-Targeted and Non-Targeted Star-Shaped Polymer-Prodrug Conjugates.

Here we describe the synthesis and biological properties of two types of star-shaped polymer-doxorubicin conjugates: non-targeted conjugate prepared as long-circulating high-molecular-weight (HMW) polymer prodrugs with a dendrimer core and a targeted conjugate with the anti-CD20 monoclonal antibody (mAb) rituximab (RTX). The copolymers were linked to the dendrimer core or to the reduced mAb via one-point attachment forming a star-shaped structure with a central antibody or dendrimer surrounded by hydrophilic polymer chains. The anticancer drug doxorubicin (DOX) was attached to the N-(2-hydroxypropyl)methacrylamide (HPMA)-based copolymer chain in star polymer systems via a pH-labile hydrazone linkage. Such polymer-DOX conjugates were fairly stable in aqueous solutions at pH 7.4, and the drug was readily released in mildly acidic environments at pH 5–5.5 by hydrolysis of the hydrazone bonds. The cytotoxicity of the polymer conjugates was tested on several CD20-positive or negative human cell lines. Similar levels of in vitro cytotoxicity were observed for all tested polymer conjugates regardless of type or structure. In vivo experiments using primary cell-based murine xenograft models of human diffuse large B-cell lymphoma confirmed the superior anti-lymphoma efficacy of the polymer-bound DOX conjugate when compared with the original drug. Targeting with RTX did not further enhance the anti-lymphoma efficacy relative to the non-targeted star polymer conjugate. Two mechanisms could play roles in these findings: changes in the binding ability to the CD-20 receptor and a significant loss of the immunological properties of RTX in the polymer conjugates.

Versatile preparation of intracellular-acidity-sensitive oxime-linked polysaccharide-doxorubicin conjugate for malignancy therapeutic

Recently, chemotherapy has been one of the most important therapeutic approaches for malignant tumors. The tumor tissular or intracellular microenvironment-sensitive polymer-doxorubicin (DOX) conjugates demonstrate great potential for improved antitumor efficacy and reduced side effects. In this work, the acid-sensitive dextran-DOX conjugate (noted as Dex-O-DOX) was synthesized through the versatile efficient oximation reaction between the terminal aldehyde group of polysaccharide and the amino group in DOX in the buffer solution of sodium acetate/acetic acid. The insensitive one, i.e., Dex-b-DOX, was prepared similarly as Dex-O-DOX with a supplemented reduction reaction. The DOX release from Dex-O-DOX was pH-dependent and accelerated by the decreased pH. The efficient intracellular DOX release from Dex-O-DOX toward the human hepatoma HepG2 cells was further confirmed. Furthermore, Dex-O-DOX exhibited a closer antiproliferative activity to free DOX·HCl as the extension of time. More importantly, compared with Dex-b-DOX, Dex-O-DOX exhibited higher antitumor activity and lower toxicity, which were further confirmed by the systemic histological and immunohistochemical analyses. Hence, the facilely prepared smart polysaccharide-DOX conjugates, i.e., Dex-O-DOX, exhibited great potential in the clinical chemotherapy of malignancy.

Polymer conjugates of doxorubicin bound through an amide and hydrazone bond: Impact of the carrier structure onto synergistic action in the treatment of solid tumours

In this study, we describe the synthesis, physico-chemical characterisation and results of the in vitro and in vivo evaluation of the biological behaviour of N-(2-hydroxypropyl)methacrylamide-based (HPMA) copolymer conjugates bearing doxorubicin (DOX) partly bound via a pH-sensitive hydrazone and partly via enzymatically degradable amide bonds, each contributing to a different anti-tumour mechanism of action of the polymer–doxorubicin conjugate. The following two types of HPMA copolymer drug carriers designed for passive tumour targeting were synthesised and compared: the linear non-degradable copolymer and the biodegradable high-molecular-weight (HMW) diblock copolymer. The HMW diblock copolymer carrier containing a degradable disulphide bond between the polymer blocks showed a rapid degradation in a buffer containing glutathione within the first few hours of incubation. In contrast to the conjugate with the amide bond-bound DOX requiring the presence of lysosomal enzymes to release DOX, the polymer–drug conjugate with the DOX bound via a hydrazone bond released DOX by pH-sensitive hydrolysis, which was significantly faster in a buffer of pH 5.0 (intracellular pH) than pH 7.4, mimicking the conditions in the bloodstream. The significant and comparable in vivo anti-tumour activity of the diblock HMW conjugate and an equimolar mixture of the conjugates differing in the DOX attachment method along with the development of cancer resistance during treatment with these conjugates demonstrated the high potential of these compounds in the development of new nanomedicines suitable for the treatment of solid tumours.

A synergistic combination therapy with paclitaxel and doxorubicin loaded micellar nanoparticles

Doxorubicin was chemically conjugated to a biodegradable polymeric carrier as a polymer-doxorubicin (polymer-Dox) conjugate via an acid labile Schiff-base bond. Then, paclitaxel was physically encapsulated by the polymer-Dox conjugate to self assemble in water as micellar nanoparticles with both doxorubicin and paclitaxel in one nanoparticle. In this way, doxorubicin and paclitaxel were combined. The preparation of the polymer-doxorubicin conjugates, encapsulation of paclitaxel, characterization of nanoparticles was systematically studied and the biological evaluation of the free drug combination as well as the micellar platform combination in vitro was thoroughly detailed.

Micellar nanoparticles loaded with gemcitabine and doxorubicin showed synergistic effect

Gemcitabine and doxorubicin were separately chemically linked to biodegradable polymers to prepare polymer–gemcitabine and polymer–doxorubicin conjugates. Moreover, the two conjugates can self-assemble into micelles with both gemcitabine and doxorubicin. In this way, the two anticancer drugs were combined. The in vitro MTT assay with these combined drugs showed synergistic effect compared to single use of each drug.

Reduction/pH dual-sensitive PEGylated hyaluronan nanoparticles for targeted doxorubicin delivery

To minimize the side effect of chemotherapy, a novel reduction/pH dual-sensitive drug nanocarrier, based on PEGylated dithiodipropionate dihydrazide (TPH)-modified hyaluronic acid (PEG-SS-HA copolymer), was developed for targeted delivery of doxorubicin (DOX) to hepatocellular carcinoma. The copolymer was synthesized by reductive amination via Schiff's base formation between TPH-modified HA and galactosamine-conjugated poly(ethylene glycol) aldehyde/methoxy poly(ethylene glycol) aldehyde. Conjugation of DOX to PEG-SS-HA copolymer was accomplished through the hydrazone linkage formed between DOX and PEG-SS-HA, and confirmed by FTIR and 1H NMR spectra. The polymer–DOX conjugate could self-assemble into spherical nanoparticles (∼150nm), as indicated by TEM and DLS. In vitro release studies showed that the DOX-loaded nanoparticles could release DOX rapidly under the intracellular levels of pH and glutathiose. Cellular uptake experiments demonstrated that the nanoparticles could be efficiently internalized by HepG2 cells. These results indicate that the PEG-SS-HA copolymer holds great potential for targeted intracellular delivery of DOX.

Three-dimensional porous HPMA-co-DMAEM hydrogels for biomedical application

The aim of this work is to develop a novel biocompatible drug delivery carrier and tissue engineering scaffold with the ability of controlled drug release and also tissue regeneration. We have synthesized N-(2-hydroxypropyl)methacrylamide and 2-(dimethylamino)ethyl methacrylate copolymer-based hydrogels loaded with doxorubicin and tested in vitro. The manifestation of temperature sensitivity is noted with a sharp decrease or increase in hydrogel optical transparency that happens with the temperature exceeding a critical transition value. The drug release profile exhibited pH-sensitive behavior of the hydrogel. The hydrolytic degradation of gel and in vitro studies of polymer–doxorubicin conjugate and doxorubicin release from hydrogel matrix indicated that hydrogels were stable under acidic conditions (in buffers at pH 4.64 and 6.65). In both drug forms, polymer–doxorubicin conjugate and free doxorubicin could be released from the hydrogel scaffold at a rate depending directly on either the rate of drug diffusion from the hydrogel or rate of hydrogel degradation or at rate controlled by a combination of the both processes. In vitro analysis showed homogenous cell attachment and proliferation on synthesized hydrogel matrix. In vivo implantation demonstrated integration of the gel with the surrounding tissue of mice within 2 weeks and prominent neo-angiogenesis observed in the following weeks. This multifunctional hydrogels can easily overcome biological hurdles in the in vivo conditions where the pH range changes drastically and could attain higher site-specific drug delivery improving the efficacy of the treatment in various therapeutical applications, especially in cancer therapy, and could also be used as tissue engineering scaffold due to its porous interconnected and biocompatible behavior.

Targeting and anti-tumor effect of folic acid-labeled polymer–Doxorubicin conjugates with pH-sensitive hydrazone linker

Multifunctional micelles were successfully prepared by co-assembling a Doxorubicin-conjugated copolymer (mPEG-b-P(LA-co-ME/Dox), for antitumor properties) and a Rhodamine B-conjugated copolymer (mPEG-b-P(LA-co-ME/RhB), for imaging) with a folic acid (FA)-conjugated copolymer (FA-PEG-b-PLA, for targeting), respectively. DLS showed that the sizes of these micelles were in the range of 150–300 nm. Fluorescent imaging analysis based on the RhB signals of the isolated visceral organs showed that the micelles with or without FA moieties were mainly accumulated in the liver and in the tumor from 2 h post drug administration, maximized at ca. 6 h, and decayed afterwards. More FA-carrying micelles were accumulated in the tumor for a longer time compared to the micelles without FA moieties. Dox-containing micelles were used for tumor inhibition experiments in vivo and the results showed that the micelles with FA moieties exhibited better antitumor efficacy than those without FA and free Dox.

Effect of cationic side-chains on intracellular delivery and cytotoxicity of pH sensitive polymer–doxorubicin nanocarriers

Fine-tuning the design of polymer-doxorubicin conjugates permits optimization of an efficient nanocarrier to greatly increase intracellular uptake and cytotoxicity. Here, we report synthesis of a family of self-assembled polymer-doxorubicin nanoparticles and an evaluation of the effects of various types of side-chains on intracellular uptake and cytotoxicity of the nanocarriers for lymphoma cells. Monomers with three different cationic side-chains (CA) and pK(a)'s, i.e., a guanidinium group (Ag), an imidazole group (Im), and a tertiary amine group (Dm), were comparatively investigated. The cationic monomer, poly(ethylene glycol) (PEG), and doxorubicin (Dox) were reacted with 1,4-(butanediol) diacrylate (BUDA) to prepare a poly(β-amino ester) (PBAE) polymer via Michael addition. All three polymer-Dox conjugates spontaneously formed nanoparticles (NP) through hydrophobic interactions between doxorubicin in aqueous solution, resulting in NP-Im/Dox, NP-Ag/Dox, and NP-Dm/Dox, with hydrodynamic sizes below 80 nm. Doxorubicin was linked to all 3 types of NPs with a hydrazone bond to assure selective release of doxorubicin only at acidic pH, as it occurs in the tumor microenvironment. Both NP-Im/Dox and NP-Ag/Dox exhibited much higher intracellular uptake by Ramos cells (Burkitt's lymphoma) than NP-Dm/Dox, suggesting that the type of side chain in the NPs determines the extent of intracellular uptake. As a result, NP-Im/Dox and NP-Ag/Dox showed cytotoxicity that was comparable to free Dox in vitro. Our findings suggest that the nature of surface cationic group on nanocarriers may profoundly influence their intracellular trafficking and resulting therapeutic efficacy. Thus, it is a crucial factor to be considered in the design of novel carriers for intracellular drug delivery.

Novel star HPMA-based polymer conjugates for passive targeting to solid tumors

Novel star polymer-doxorubicin conjugates designed for passive tumor targeting have been developed and their potential for treatment of cancer has been investigated. In the present study the synthesis, physico-chemical characterization, drug release, bio-distribution and preliminary data of in vivo efficacy of the conjugates are described. In the water-soluble conjugates the core of a molecule formed by poly(amido amine) (PAMAM) dendrimers was grafted with semitelechelic N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers bearing doxorubicin (Dox) attached by hydrazone bonds enabling intracellular pH-controlled hydrolytic drug release, or by GFLG sequence susceptible to enzymatic degradation. The controlled synthesis utilizing semitelechelic copolymer precursors facilitated preparation of polymer conjugates in a broad range of molecular weights (1.1–3.0·105 g/mol). In contrast to free drug or linear conjugates the star polymer-Dox conjugates exhibited prolonged blood circulation and enhanced tumor accumulation in tumor-bearing mice indicating important role of the EPR effect. The star polymer-Dox conjugates showed significantly higher anti-tumor activity in vivo than Dox·HCl or its linear or graft polymer conjugates, if treated with a single dose 15 or 5 mg Dox eq./kg. Method of tumor initialization (acute or chronic experimental tumor models) significantly influenced effectiveness of the treatment with much lower success in treatment of mice bearing chronic tumors.

Biodegradable star HPMA polymer–drug conjugates: Biodegradability, distribution and anti-tumor efficacy

Herein, new biodegradable star polymer–doxorubicin conjugates designed for passive tumor targeting were investigated, and their synthesis, physico-chemical characterization, drug release, biodegradation, biodistribution and in vivo anti-tumor efficacy are described. In the conjugates, the core formed by poly(amidoamine) (PAMAM) dendrimers was grafted with semitelechelic N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers bearing doxorubicin (Dox) attached by hydrazone bonds, which enabled intracellular pH-controlled drug release. The described synthesis facilitated the preparation of biodegradable polymer conjugates in a broad range of molecular weights (200–1000 g/mol) while still maintaining low polydispersity (~ 1.7). The polymer grafts were attached to the dendrimers through either stable amide bonds or enzymatically or reductively degradable spacers, which enabled intracellular degradation of the high-molecular-weight polymer carrier to excretable products. Biodegradability tests in suspensions of EL4 T-cell lymphoma cells showed that the rate of degradation was much faster for reductively degradable conjugates (close to completion within 24 h of incubation) than for conjugates linked via an enzymatically degradable oligopeptide GFLG sequence (slow degradation taking several days). This finding was likely due to the differences in steric hindrance in terms of the accessibility of the small molecule glutathione and the bulky enzyme cathepsin B to the polymer substrate. Regarding drug release, the conjugates were fairly stable in buffer at pH 7.4 (model of blood stream) but released doxorubicin under mild acidic conditions that model the tumor cell microenvironment. The star polymer–Dox conjugates exhibited significantly prolonged blood circulation and enhanced tumor accumulation in tumor-bearing mice, indicating the important role of the EPR effect in its anti-cancer activity. The star polymer conjugates showed prominently higher in vivo anti-tumor activities than the free drug or linear polymer conjugate when tested in mice bearing EL4 T-cell lymphoma, with a significant number of long-term surviving (LTS). Based on the results, we conclude that a M w of HPMA copolymers of 200,000 to 600,000 g/mol is optimal for polymer carriers designed for the efficient passive targeting to solid tumors. In addition, an expressive therapy-dependent stimulation of the immune system was observed.