Osteotropic nanoscale drug delivery systems based on small molecule bone-targeting moieties

Bone-targeted drug delivery is an active research area because successful clinical applications of this technology can significantly advance the treatment of bone injuries and disorders. Molecules with bone-targeting potential have been actively investigated as promising moieties in targeted drug delivery systems. In general, bone-targeting molecules are characterized by their high affinity for bone and their predisposition to persist in bone tissue for prolonged periods, while maintaining low systemic concentrations. Proteins, such as monoclonal antibodies, have shown promise as bone-targeting molecules; however, they suffer from several limitations including large molecular size, high production cost, and undesirable immune responses. A viable alternative associated with significantly less side effects is the use of small molecule-based targeting moieties. This review provides a summary of recent findings regarding small molecule compounds with bone-targeting capacity, as well as nanoscale targeted drug delivery approaches employing these molecules.

High-molecular weight star conjugates containing docetaxel with high anti-tumor activity and low systemic toxicity in vivo

The star polymer-docetaxel conjugates exert a much higher therapeutic activity and yet a lower systemic toxicity than free DTX.

Biological rationale for the design of polymeric anti-cancer nanomedicines

Understanding the biological features of cancer is the basis for designing efficient anti-cancer nanomedicines. On one hand, important therapeutic targets for anti-cancer nanomedicines need to be identified based on cancer biology, to address the unmet medical needs. On the other hand, the unique pathophysiological properties of cancer affect the delivery and interactions of anti-cancer nanomedicines with their therapeutic targets. This review discusses several critical cancer biological properties that challenge the currently available anti-cancer treatments, including cancer heterogeneity and cancer stem cells, the complexcity of tumor microenvironment, and the inevitable cancer metastases. In addition, the biological bases of the enhanced permeability and retention (EPR) effect and tumor-specific active targeting, as well as the physiological barriers for passive and active targeting of anti-cancer nanomedicines are covered in this review. Correspondingly, possible nanomedicine strategies to target cancer heterogeneity, cancer stem cells and metastases, to overcome the challenges related to tumor passive targeting and tumor penetration, and to improve the interactions of therapeutic payloads with the therapeutic targets are discussed. The focus is mainly on the designs of polymeric anti-cancer nanomedicines.

Selective inhibitory effect of HPMA copolymer-cyclopamine conjugate on prostate cancer stem cells

Improved treatments for prostate cancer are in great need to overcome lethal recurrence and metastasis. Targeting the tumorigenic cancer stem cells (CSCs) with self-renewal and differentiation capacity appears to be a promising strategy. Blockade of the hedgehog (Hh) signaling pathway, an important pathway involved in stem cell self-renewal, by cyclopamine leads to long-term prostate cancer regression without recurrence, strongly suggesting the connection between Hh pathway and prostate CSCs. Here we designed an HPMA (N-(2-hydroxypropyl)methacrylamide)-based cyclopamine delivery system as a CSC-selective macromolecular therapeutics with improved drug solubility and decreased systemic toxicity. To this end, HPMA and N-methacryloylglycylphenylalanylleucylglycyl thiazolidine-2-thione were copolymerized using the RAFT (reversible addition-fragmentation chain transfer) process, followed by polymer-analogous attachment of cyclopamine. The selectivity of the conjugate toward CSCs was evaluated on RC-92a/hTERT cells, the human prostate cancer epithelial cells with human telomerase reverse transcriptase transduction. The use of RC-92a/hTERT cells as an in vitro CSC model was validated by stem cell marker expression and prostasphere culture. The bioactivity of cyclopamine was retained after conjugation to the polymer. Furthermore, HPMA polymer-conjugated cyclopamine showed anti-CSC efficacy on RC-92a/hTERT cells as evaluated by decreased stem cell marker expression and CSC viability.

Clinical experience with anthracycline antibiotics-HPMA copolymer-human immunoglobulin conjugates

This paper reviews an early clinical experience with anthracycline (epirubicin; Epi or doxorubicin; Dox) containing an N-(2-hydroyxypropyl)methacrylamide copolymer carrier targeted with autologous or commercial human immunoglobulin in six patients aged 28-55 suffering from therapy-resistant metastatic cancer. More than 100 biochemical, hematological and immunological parameters, including nine tumor markers, were tested in blood samples taken 24 h after the first and up to 10 months after the last application. The intravenous application proceeded without serious adverse or side effects and did not require hospitalization. Cardiotoxicity was not observed. Four of six monitored patients attained stabilization of disease (liver ultrasound scan and bone computer tomography) with a very good quality of life lasting from seven up to 18 months. Positive response to the treatment was, among others, evaluated as decreased CA 15-3 and CEA tumor markers. In three of five tested patients the serum level of C-reactive protein was temporarily increased 72 h after the treatment. A stable or elevated number of peripheral blood reticulocytes together with activation of natural killer (NK) cells and lymphokine-activated killer (LAK) cells supports the data previously obtained in experimental animals pointing to a dual role, i.e. the cytotoxic and immunomobilizing character of doxorubicin-HPMA conjugates.

Cytotoxicity and immunostimulation: double attack on cancer cells with polymeric therapeutics

The successful treatment of cancer with conventional drugs is frequently complicated by the resistance of tumor cells to such a non-specific therapy. Over the last few years, immunotherapy has gained attention as a tumor-specific approach. Recent findings demonstrated that some conventional cytostatics stimulate local anticancer responses. New anticancer drugs, including their polymeric derivatives, are currently being developed with the aim of destroying tumors more effectively and more specifically. Among these, the water-soluble conjugates of doxorubicin with N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer carrier have emerged as efficient therapeutics because they are able to not only directly destroy cancer cells but also elicit systemic tumor-specific anticancer responses. Here, we discuss new insights into their mechanisms of immune surveillance, which could suggest novel approaches to cancer therapy.

Treatment with HPMA copolymer-based doxorubicin conjugate containing human immunoglobulin induces long-lasting systemic anti-tumour immunity in mice

Linkage of doxorubicin (Dox) to a water-soluble synthetic N-(2-hydroxypropyl)methacrylamide copolymer (PHPMA) eliminates most of the systemic toxicity of the free drug. In EL-4 lymphoma-bearing C57BL/6 mice, a complete regression of pre-established tumours has been achieved upon treatment with Dox-PHPMA-HuIg conjugate. The treatment was effective using a range of regimens and dosages, ranging from 62.5 to 100% cured mice treated with a single dose of 10-20 mg of Dox eq./kg, respectively. Fractionated dosages producing lower levels of the conjugate for a prolonged time period had substantial curative capacity as well. The cured mice developed anti-tumour protection as they rejected subsequently re-transplanted original tumour. The proportion of tumour-protected mice inversely reflected the effectiveness of the primary treatment. The treatment protocol leading to 50% of cured mice produced only protected mice, while no mice treated with early treatment regimen (i.e. starting on day 1 after tumour transplantation) rejected the re-transplanted tumour. Exposure of the host to the cancer cells was a prerequisite for developing protection. The anti-tumour memory was long lasting and specific against the original tumour, as the cured mice did not reject another syngeneic tumour, melanoma B16-F10. The immunity was transferable to naïve recipients in in vivo neutralization assay by spleen cells or CD8(+) lymphocytes derived from cured animals. We propose an effective treatment strategy which eradicates tumours without harming the protective immune anti-cancer responses.

Nanomedicine

The intricate problems associated with the delivery and various unnecessary in vivo transitions of proteins and drugs needs to be tackled soon to be able to exploit the myriad of putative therapeutics created by the biotechnology boom. Nanomedicine is one of the most promising applications of nanotechnology in the field of medicine. It has been defined as the monitoring, repair, construction and control of human biological systems at the molecular level using engineered nanodevices and nanostructures. These nanostructured medicines will eventually turn the world of drug delivery upside down. PEGylation (i.e. the attachment of polyethylene glycol to proteins and drugs) is an upcoming methodology for drug development and it has the potential to revolutionise medicine by drastically improving the pharmacokinetic and pharmacodynamic properties of the administered drug. This article provides a total strategy for improving the therapeutic efficacy of various biotechnological products in drug delivery. This article also presents an extensive analysis of most of the PEGylated proteins, peptides and drugs, together with extensive clinical data. Nanomedicines and PEGylation, the latest offshoots of nanotechnology will definitely pave a way in the field of drug delivery where targeted delivery, formulation, in vivo stability and retention are the major challenges.

Polymeric prodrugs

In 1975 Prof. H. Ringsdorf proposed a model for rational design of polymeric prodrugs [J. Polym. Sci. Symp. 51 (1975) 135]. The model has been the most important basis for research in the field, since it was the first model that took into account both the chemical and biological aspects needed for the design of polymeric prodrugs. This paper deals with the most important properties that were discovered by designing polymeric prodrugs: prolongation of action of the drug, controlled release of the drug, passive tumor accumulation by the EPR-effect and alteration of body distribution and cell uptake. Over the years, other objectives have been formulated and other properties of polymer-drug conjugates were discovered. One recent example, the immunoprotective ability of polymeric prodrugs, is described in more detail in this paper.

HPMA copolymer-bound doxorubicin targeted to tumor-specific antigen of BCL1 mouse B cell leukemia

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymer carrier containing the anticancer drug doxorubicin and targeted with B1 monoclonal antibody (mAb) to BCL1 leukemia cells was synthesised and tested in vitro and in vivo. BCL1 leukemia growing in syngenic Balb/c mice was selected as a tumor model system. B1 mAb recognising the idiotype of surface IgM on BCL1 cells was used as a targeting moiety. Both B1 mAb and doxorubicin were conjugated to HPMA copolymer carrier by aminolysis through a tetrapeptidic Gly-Phe(D,L)-Leu-Gly spacer to ensure the intracellular delivery and controlled release of the drug. B1 mAb-targeted conjugate was shown to possess strictly tumor-specific binding capacity to target BCL1 cells in vitro. A similar conjugate, but containing human nonspecific Ig (HuIg) instead of B1 mAb, failed to bind to BCL1 cells. In vitro, B1 mAb-targeted conjugate demonstrated 40-fold higher cytotoxic effect than nontargeted or human nonspecific Ig-containing HPMA copolymer-bound doxorubicin. Conjugate targeted with B1 mAb was also shown to bind to target BCL1 cells in vivo. B1 mAb-targeted conjugate was shown to be more efficient in the treatment of established BCL1 leukemia than free doxorubicin, nontargeted and human nonspecific Ig-containing conjugate. Antibody-targeted polymeric drugs are thus promising conjugates for cancer treatment.

Immunomodulating activities of soluble synthetic polymer-bound drugs

The introduction of a synthetic material into the body always affects different body systems, including the defense system. Synthetic polymers are usually thymus-independent antigens with only a limited ability to elicit antibody formation or to induce a cellular immune response against them. However, there are many other ways that they influence or can be used to influence the immune system of the host. Low-immunogenic water-soluble synthetic polymers sometimes exhibit significant immunomodulating activity, mainly concerning the activation/suppression of NK cells, LAK cells and macrophages. Some of them, such as poly(ethylene glycol) and poly[N-(2-hydroxypropyl)methacrylamide], can be used as effective protein carriers, as they are able to reduce the immunogenicity of conjugated proteins and/or to reduce non-specific uptake of liposome/nanoparticle-entrapped drugs and other therapeutic agents. Recently, the development of vaccine delivery systems prepared from biodegradable and biocompatible water-soluble synthetic polymers, microspheres, liposomes and/or nanoparticles has received considerable attention, as they can be tailored to meet the specific physical, chemical, and immunogenic requirements of a particular antigen and some of them can also act as adjuvants.

Differences in the intracellular fate of free and polymer-bound doxorubicin

Internalization and subcellular fate of free doxorubicin or its polymeric conjugates based on poly N-(2-hydroxypropyl)methacrylamide (pHPMA), either non-targeted or targeted with anti-Thy1.2 or anti-CD71 monoclonal antibody was tested on EL-4 mouse T-cell lymphoma, SW620 human colorectal carcinoma and OVCAR-3 human ovarian adenocarcinoma. Doxorubicin fluorescence allowed us to follow the internalization and intracellular distribution of tested conjugates by laser scanning confocal microscopy and/or by fluorescent microscopy. Whereas free doxorubicin was always detectable only in the nuclei of treated cells, detectable fluorescence of doxorubicin bound to a polymeric carrier, targeted or non-targeted, was detectable up to 3 days of incubation only in the cytoplasmatic structures. While free doxorubicin causes apoptosis in the populations of tested cancer cell lines, significant number of apoptotic cells was never found in cell cultures exposed to targeted or non-targeted polymeric conjugates. In contrast to free doxorubicin, which is a strong inducer of p53 expression, increased p53 expression was never observed after the treatment with the polymeric drug. High-performance liquid chromatographic analysis shows that the percentage of cleaved doxorubicin is very low even after 48 h of incubation of tested cells with the polymeric conjugate, and cannot be the only reason for the toxicity of the conjugate. We suggest that: (a) after the treatment with pHPMA-bound drug, the cells die by necrosis and (b) the toxicity of pHPMA-based conjugates is a combination of the toxic effect of released doxorubicin and the toxic effect of doxorubicin in polymer-bound form directed against cell membranes.

Acquired and specific immunological mechanisms co-responsible for efficacy of polymer-bound drugs

We present data providing new evidence that poly[N-(2-hydroxypropyl)methacrylamide] (PHPMA)-bound drugs, unlike free drugs, have both cytostatic and immunomobilizing activity (CIA). Immediately after injection, due to the high level of the drug, the main activity of the polymeric conjugate is cytotoxic and cytostatic. Later on, long-term circulating PHPMA-bound drug, at concentrations lower than its minimal inhibitory levels, mobilizes the defense mechanisms of the host. Cytotoxic and cytostatic effects of drug-PHPMA were repeatedly confirmed. The following data support the concept of the immunomobilizing activity of the N-(2-hydroxypropyl)methacrylamide (HPMA) conjugates: (a) pre-treatment with free drugs (doxorubicin, cyclosporin A) accelerates the appearance of EL4 mouse T-cell lymphoma while a similar pre-treatment with doxorubicin-PHPMA induces limited but definitive mobilization of the host's defense mechanisms; (b) mice cured of EL4 mouse T-cell lymphoma, BCL1 mouse B-cell leukemia and 38C13 mouse B-cell lymphoma by injection of doxorubicin-PHPMA conjugate targeted with monoclonal antibodies (anti-Thy 1.2 for EL4, anti-B1 for BCL1 and anti-CD71 for 38C13) and re-transplanted with a lethal dose of the same cancer cells survive without any treatment considerably longer than control mice; (c) increased NK activity and anti-cancer antibody was detected only in animals treated with doxorubicin-PHPMA conjugate; and (d) considerably increased NK and LAK activity was seen in a human patient treated for generalized breast carcinoma with doxorubicin-PHPMA-IgG.