Potential of targeted drug delivery system for the treatment of bone metastasis

Abstract

Bone metastasis is a devastating complication of cancer that requires an immediate attention. Although our understanding of the metastatic process has improved over the years, yet a number of questions still remain unanswered, and more research is required for complete understanding of the skeletal consequences of metastasis. Furthermore, as no effective treatments are available for some of the most common skeleton disorders such as arthritis, osteoarthritis, osteosarcoma and metastatic bone cancer, there is an urgent need to develop new drugs and drug delivery systems for safe and efficient clinical treatments. Hence this article describes the potential of targeted delivery platforms aimed specifically at bone metastasized tumors. The review gives a brief understanding of the proposed mechanisms of metastasis and focuses primarily on the targeting moieties such as bisphosphonates, which represent the current gold standard in bone metastasis therapies. Special focus has been given to the targeted nanoparticulate systems for treating bone metastasis and its future. Also highlighted are some of the therapeutic targets that can be exploited for designing therapies for bone metastasis. Some of the patented molecules for bone metastasis prevention and treatment have also been discussed. Recently proposed HIFU-CHEM, which utilizes High Intensity Focused ultrasound (HIFU) guided by MRI in combination with temperature-sensitive nanomedicines has also been briefed. The study has been concluded with a focus on the innovations requiring an immediate attention that could improve the treatment modality of bone metastasis.

• Bisphosphonates
• HIFU-CHEM
• hydroxyapatite
• metastatic bone cancer
• polymeric carriers
• targeted nanoparticles

Introduction

Bone metastases are among the most debilitating secondary complications occurring in up to 70% of patients with advanced breast or prostate cancer and in approximately 15–30% of patients with carcinoma of the lung, colon, stomach, bladder, uterus, rectum, thyroid or kidney. The fatal condition is responsible for claiming the lives of 350 000 patients annually in the United States. Furthermore, the consequences have been found to be so drastic that only 20% of patients with breast cancer are still alive five years after the discovery of bone metastasis (Roodman, 2004). Wong & Pavlakis (2011) have reported it as an incurable disease, which is also associated with skeletal-related morbidities including bone pain, pathological fractures, spinal cord compression and hypercalcemia.

The current therapeutic scenario for some of the common skeleton disorders such as arthritis, osteoarthritis, osteosarcoma and metastatic bone cancer has been found to be ineffective. Hence there is a pressing need to develop new drugs and drug delivery systems for safe and efficient clinical treatments (Gu et al., 2013). However Suva et al. (2009) states that the treatment regimen of bone metastases should not only be stressed on the inhibition or arrest of the tumor cells that have metastasized to the bone but should also be focused on the bone degeneration occurring as a result of tumor burden. Furthermore, the clinical outcome can be enhanced by possibly reducing the chances of further metastasis of the tumor cells to the healthy bones and by reducing the chances of relapse. These achievements can really prove to be a ray of hope for the ailing patients with cancer.

Gu et al. (2013) report that targeted nanomedicine in bone therapeutics has attracted a lot of attention due to its various applications. Nanoparticles (NPs) can offer many unique features for the potential targeted delivery of treatments for bone diseases. The advantages of using NPs may include the following: (1) carrying the drug to its target while keeping the drug concentrated so that once endocytosed by cells, the drug can maximize its effect for better therapeutic outcomes; (2) protecting the drug from being dispersed or degraded by the harsh body fluids and increasing the circulation time or retention time in the body; (3) carrying multiple drug molecules for effective chemotherapy of resistant tumors; (4) increasing the solubility of some hydrophobic drugs due to the large surface area of NPs; and (5) providing flexibility for grafting of various targeting molecules to achieve specific delivery via surface modification of NPs.

Low & Kopevcek (2012) and Thamake et al. (2012) have explored the hydroxyapatite (HAp) mineral portion of the bone by which bone diseases can be targeted using molecules such as bisphosphonate (BP) and acidic oligopeptides. Nanomedicines can be easily conjugated to several ionic targeting ligands, which would enhance their binding affinity to the bone. HAp crystal structures or HAp exposures are disease specific and can be utilized for specific targeting opportunities. In addition to targeting, imaging of the affected regions can be effectively approached by the use of nanomedicines.

Treatment of bone metastasis with multiple drugs by employing nanomedicines has also drawn certain attention recently (Low & Kopevcek, 2012; Thamake et al., 2012). In this review, we discuss the mechanisms of bone metastasis in brief and the importance of targeted drug delivery in its treatment. The analysis also includes current treatment approaches and the future of bone targeted therapy (Table 1).

Table 1. Targets responsible for bone metastasis and novel agents developed for them with a focus on adverse effects and stage of development.

Target^a	Function or role of target in metastasis^a	Drug^a	Developer^b	Recent stage of development^a	Reported adverse effects^b
RANK-L	Osteoclast development, activity and survival	Denosumab	Amgen	Xgeva^TM-approved by FDA for prevention of skeletal-related events in patients with bone metastases from solid tumors.	Infections of the urinary and respiratory tracts, cataracts, constipation, rashes and joint pain
Src kinase enzyme	Dual Src/abl kinase inhibitor. Src kinase-mediated osteoclast bone resorption inhibition	Saracatinib	Astra Zeneca	AZD 530 – phase II (ovarian, prostate, osteosarcoma, melanoma, colon cancer) ongoing	–
Non-specific inhibitor of Src	Form ruffled border of osteoclast; cross-talk with multiple pathways (HER2, ER)	Dasatinib	Bristol myers squibb co	Phase II (breast CA): 2 completed, 3 ongoing, in use in chronic myelogenous leukemia	–
Microtubule stabilization	Inhibits tumor growth and bone resorption	Sagopilone	Bayer Schering Pharma AG	In vivo, ex vivo completed Phase II (breast CA) completed	Neuropathy and neutropenia
Inhibitor of cathepsin K	Inhibition of osteoclast-mediated bone degradation	Odanacatib (MK-0822)	Merck & Co	Phase III clinical trial for this compound was stopped early after a review showed it was highly effective and had a good safety profile.	–
CXCR4	Prevents tumor migration to bone	CTCE-9908	–	Phase I/II completed	No toxicity except mild erythema at the site of inoculation
Endothelin A receptor	Stimulates osteoblast proliferation	Atrasentan (ABT-627) (Xinlay™)	Abbott Laboratories	Phase III (prostate carcinoma) Atrasentan and Zometa for Men with prostate cancer metastatic to bone – Phase 2	Rhinitis, headache and peripheral edema
Proteasome inhibitor	Promotes osteoclast differentiati-on through nuclear NF-κB	Bortezomib	Myogenics later Leukosite	Approved in the United States for treating relapsed multiple myeloma and mantle cell lymphoma	Peripheral neuropathy, gastro-intestinal (GI) effects and asthenia, shingles

^aSee Wong & Pavlakis, 2011.

^bExtracted from references Oakervee et al., 2005; Chiappori et al., 2008; Le Gall et al., 2008; Galmarini, 2009; Richert et al., 2009; Herper, 2012; Nam et al., 2013; Amgen.com, 2014; Fda.gov, 2014, Cancer.gov, 2014a,b; http://www.cancerresearchuk.org/cancer-help/about-cancer/treatment/cancer-drugs/denosumab; http://www.ncats.nih.gov/files/AZD0530.pdf; and http://www.cancer.gov/drugdictionary?cdrid=453588.

Mechanisms of bone metastasis

Site-specific metastasis of tumor cells to in the skeleton is not a random event but a complex multistep process with a dependence on anatomical factors, cancer cell properties and suitability of bone microenvironment to support metastasis. The initiation of metastatic process by detachment or “shedding” of cancer cells from the primary site results is followed by many steps resulting in entry to the systemic circulation. In the circulation, the tumor cell must endure several stresses, including evasion of the host immune responses, while seeking a distant hospitable site, which can support tumor growth. Following arrest in the distant capillary bed and subsequent extravasation into the tissue, the tumor cell manipulates host-derived factors and cellular processes to get firmly established in the foreign environment, which is bone in this case. The skeletal integrity of the body is maintained mainly by two types of cells: osteoblasts, which are bone-forming cells and osteoclasts associated with bone degradation. The fundamental physiology of the bone is dependent on a balance between the two processes regulated by these cells. However, under pathologic conditions such as rheumatoid arthritis, osteoporosis or bone metastases, this tightly regulated balance between bone formation and degeneration is severely disrupted (Guise, 2010).

This can be best described by the “seed-and-soil hypothesis”, which was first proposed by Stephan Paget in 1889. The prominent reason of metastasis to the bone is the high blood flow to the red marrow accounting for the tendency of metastases for those sites. Furthermore, tumor cells produce adhesive molecules that aid in binding to the marrow stromal cells and bone matrix. These adhesive interactions accounts for the increased production of angiogenic factors and bone-resorbing factors that further enhances tumor growth in bone. Bone is also a large repository for immobilized growth factors, including transforming growth factor, insulin-like growth factors I and II, fibroblast growth factors, platelet-derived growth factors, bone morphogenetic proteins and calcium. Upon being released and activated during bone resorption, they provide fertile ground in which tumor cells can grow (Roodman, 2004).

Types of bone metastasis

In cancer with bone metastases, the delicate balance between bone formation and resorption is disrupted based on which the type of metastasis is identified. In osteolytic lesions, the bone resorption rate exceeds that of bone formation, whereas in osteoblastic lesions, the bone formation rate is faster (Wong & Pavlakis, 2011).

Patients can have both osteolytic and osteoblastic metastasis or mixed lesions containing both elements. Most patients with breast cancer have predominantly osteolytic lesions. Patients suffering from osteolytic metastases experience severe pain, pathological fractures, life-threatening hypercalcemia, spinal cord compression and other nerve compression syndromes. However, in contrast, the lesions in prostate cancer are predominantly osteoblastic. Patients with osteoblastic metastases develop bone pain and pathological fractures resulting from the poor quality of bone produced by the hyper activated osteoblast (Roodman, 2004; Guise, 2010).

Designing-targeted delivery for the prevention and treatment of bone metastases

Bisphosphonates

Treatment of bone-related disorders achieved a significant boost by the introduction of BPs and its first clinical use in the 1960s, which highlighted on their use as agents for bone scanning, based on their ability to adsorb to bone mineral. Other potential uses of BPs are attributed to their striking effectiveness in clinical disorders associated with increased bone resorption, initially in Paget's disease of bone, then in hypercalcemia of malignancy, myeloma and bone metastases, and much later in osteoporosis. As a result, several BPs (e.g. etidronate, clodronate, pamidronate, alendronate and tiludronate) now are licensed as drugs for various indications and more will follow (risedronate (RIS), ibandronate, zoledronate (ZOL), etc.). Furthermore, it has been also reported that BPs bound to bone mineral also inhibit the adhesion and spreading of breast carcinoma cells to bone matrix and therefore could prevent metastasis of tumor cells to bone.

BPs posses a strong affinity to HAp mineral of the bone, which explains its targeting efficiency to bone. Furthermore, upon administration in vivo, BPs are rapidly cleared from the circulation and bind to areas of exposed bone mineral (e.g. around resorbing osteoclasts in bone metastasis). This is in addition that BPs retain much of the binding affinity after conjugation to other molecules or carriers. This property makes it ideal candidate for developing targeted delivery systems to bone in addition to its inherent affinity to bone.

BPs are of the following two types: (1) Non-nitrogen containing and (2) nitrogen containing. Non-nitrogen containing BPs incorporates into AMP resulting in a modified ATP, which cannot be hydrolyzed and hence energy supply to the cells is ceased. Buildup of this modified ATP analog inside osteoclasts leads to apoptosis and reduced bone turnover. Nitrogen-containing BPs (nBP) are much more potent. They inhibit farnesyl pyrophosphate synthase activity and by so doing disrupt the mevalonic acid pathway. Inhibition of farnesyl diphosphate synthase prevents protein prenylation of small GTPases, such as Ras, Rho and Rab, which are important signaling proteins that regulate cell survival in osteoclasts resulting ultimately in apoptosis of osteoclasts and reduced bone resorption (Rogers et al., 1997; Low & Kopevcek, 2012).

Recent research studies have highlighted the importance of BPs for targeted delivery of anticancer agents encapsulated in a nanoparticulate system. Thamake et al. (2012) co-encapsulated curcumin and bortezomib in the poly(lactic-co-glycolic acid) (PLGA) NPs to further enhance the therapeutic efficiency and overall clinical outcome of the research directed at active targeting of metastatic breast cancer. The study was based on the understanding that high affinity and rapid mobilization of alendronate – a BP – along with NPs could lead to higher delivery of NPs to bone along with longer retention and prolonged release the encapsulated chemotherapeutic agents. In vivo non-invasive bioimaging showed higher localization of alendronate-coated NPs to the bone, which was further confirmed by histological analysis (Table 2). Furthermore, results also showed that aminobisphosphonate alendronate (ALN)-coated NPs protected bone resorption and decreased the rate of tumor growth as compared to control groups in an intraosseous model of bone metastasis.

Table 2. Novel therapies targeted at specific genes or enzymes responsible for bone metastasis.

Active pharma-ceutical ingredient	Condition treated or claim of the patent	MOA or target of the drug	Patent no.
Degarelix (Persson, 2013)^a	Metastatic prostate cancer	GnRH antagonist	EP20130168404
Anti-IGF-IR and/or anti-PDGFRα antibodies (Fatatis et al., 2009)^a	Metastatic bone cancer	Inhibition of IGF-IR and PDGFRα receptors resulting in cellular apotosis	PCT/US2006/023856
Phosphatidylinositol 3-kinase inhibitor compound (Novartis et al., 2013)^a	Treatment of bone cancer or for preventing metastatic dissemination primary cancer cells into the bone	Inhibits Phospho-inositide 3-kinase enzyme resulting in inhibition of cellular growth	PCT/EP2012/067019
Cathepsins S and K	Treatment of bone cancer and bone cancer pain	Inhibitors of cysteine proteases	PCT/US2012/036184
PTH (l–34) (Booth, 2012; Chung et al., 2013)^a	Reduces bone loss, reduces bone fracturing,or reduces pain in bone metastasized cancer	Parathyroid hormone receptor agonist	PCT/US2005/001139
miRNA inhibitors (Boender et al., 2010)^a	Treatment of prostate cancer, bone metastasis and drug-resistant lung cancer	Inhibits messenger RNA translation (targets MIR-409-5P, MIR-379 AND MIR-154) resulting in tumor suppression	PCT/US2012/067403
Gefitinib (Sands, 2011)^a	Treatment and diagnosis of metastatic prostate cancer	Epidermal Growth Factor Receptor (EGFR) inhibitors	EP20080749545
Tryptophan hydroxylase inhibitors (Rabbani et al., 2005)^a	Treatment of metastatic bone disease		PCT/US2011/024141
Prostrate secretory protein (PSP-94) (Adwan et al., 2005)^a	Treatment of hypercalcemia and bone metastasis		US 10/857,358
Antisense oligo-nucleotides (Zhang et al., 2007)^a	Prevention of metastasis formation of cancer cells	inhibit the expression of gene encoding of osteopontin, bone sialoprotein II and/or osteonectin	PCT/EP2004/009738

^aReferences are extracted from patents.

Another research carried out by Chaudhari RK et al. (2011) wherein ZOL anchored PLGA NP loaded with docetaxel was developed for targeting bone metastasis. The results of the study made it clear that ZOL-conjugated PLGA NPs depicted more cellular uptake than PEGylated PLGA NPs. In vitro studies on MCF-7 and BO2 cell line revealed that ZOL anchored PLGA-polyethylene glycol (PEG) NPs showed enhanced cell cytotoxicity, increase in cell cycle arrest and more apoptotic activity. In vivo animal studies using technetium-99 m radiolabeling indicated a prolonged blood circulation half-life, reduced liver uptake and significantly higher retention of ZOL tagged NPs at the bone site with enhanced tumor retention.

The use of BPs as polymer/NP targeting moieties has several advantages. nBPs contain a primary amine, which can be exploited for conjugation to carboxylic acids. Second, as with tetracycline, if they are conjugated to nanomedicines via a degradable linker such as a pH-sensitive hydrazone bond, they are pharmacologically active and may produce synergistic effects when coupled with appropriate drugs. The efficacious treatment of bone metastasized tumors is a fine example of this. Finally, the patent protection of many BPs will expire soon, which makes them an economic option for a targeting moiety (Low & Kopevcek, 2012).

Tetracyclines

Tetracycline, which was derived from Streptomyces rimosus, was introduced as a broad-spectrum antibiotic in 1947. Soon after its incorporation into medicine, however, tetracycline was found to bind strongly to the bone due to which its usage was discontinued in pediatric medicine as the high affinity to HAp caused children's teeth to stain yellow. Furthermore, other early studies indicated that tetracycline may inhibit skeletal growth in children. However, its use in adults for bone targeting has been attracting attention of researchers.

The type of tetracycline to be used for targeting, however, is debated. Some researchers suggest that tetracycline must be in the correct orientation in order to bind HAp. Oxygens bound to C2, C10 and C12 are among the primary HAp-binding atoms. As such, modifications around the 5, 6 and 7 carbons can be made with minimal effects on binding and biological activity. However, other studies have suggested that bone binding and antimicrobial activity may be retained even with a simplified tetracycline molecule moiety (Low & Kopevcek, 2012).

As reported by Amgen use of chemically modified non-antimicrobial tetracyclines as multifunctional drugs against advanced cancers. Tetracyclines are potent inhibitors of matrix metalloproteinases (MMPs), key components for the degrading enzymes in bone metastases. The potent MMP inhibitory activities of tetracyclines, especially their chemically modified analogs, combined with their relatively well-tolerated pharmacological profile, has inspired several researchers to investigate their anticancer potential in a variety of cancers, including melanoma, lung, breast and prostate cancers. Chemically modified non-antibiotic tetracyclines (CMT or COL) were tested using tumors of prostate, breast and melanomas. Some of these CMTs, notably, CMT-3 and CMT-308, significantly inhibited not only invasive potential and MMP activity but also inhibited cell proliferation by inducing cell cycle arrest and apoptosis.

Neale et al. tried to reduce potential side effects caused by tetracycline's biological activity by minimizing tetracycline structure so that it retains no biological activity yet is still able to bind to HAp. They findings suggested that 3-amino-2,6-dihydroxybenzamide retains 50% of the ability to bind to HAp when compared to native tetracycline. To achieve bone anabolism, the new targeting ligand was bound to estradiol via a succinate linker. Following conjugation, the compound had a binding affinity of 105% over tetracycline alone. As estradiol alone did not bind to HAp, the increased affinity may be attributed to the addition of a succinate linker (Low & Kopevcek, 2012).

Acidic oligopeptides

Bone sialoprotein is one of several naturally occurring proteins with a strong affinity for HAp. Glutamic acid and aspartic acid oligopeptides with 4–10 amino acids long, which are modeled after the sialoprotein, provide a more biocompatible option when adequate blood flow and bone turnover are needed. Furthermore, D peptides, which are not easily recognized by the body's immune system, provide a better biocompatible option to replace L peptides. Similar to BPs, the HAp-binding capabilities of these oligopeptides are retained after conjugation to a nanomedicine carrier via the peptide's alpha amino group (Low & Kopevcek, 2012).

Estradiol analogs

Estrogen replacement therapy is being used for the treatment of osteoporosis related to low estrogen levels. Crooks and coworkers (Nasim et al., 2010a,b) developed estradiol analogs that would confine only in bone tissue but are lacking estrogenic properties. They designed bone-targeting compounds by attaching calcium chelators to an estradiol moiety through succinoyl or carboxyethyl linkers. To further enhance the targeting potential, they prepared a series of phosphate esters of the carboxyethyl linker-containing compound that possessed similar bone-targeting properties as tetracycline (Low & Kopevcek, 2012).

Monoclonal antibodies

The formation of osteoclasts is dependent on the production of macrophage colony-stimulating factor and receptor activation of nuclear factor k B (RANK) ligand (RANKL) by stromal cells or osteoblasts. RANKL binds the RANK receptor on osteoclast precursors and by signaling through the nuclear factor k-B and Jun N-terminal kinase pathways induces the formation of osteoclasts. Studies have indicated that patients with breast cancer with bone metastases are more likely to express RANK in tumor cells and osteoclasts, and RANK-L in stromal cells and osteoblasts in comparison to patients without bone metastases indicating the significance of targeting RANK-L (Roodman, 2004).

Denosumab, a fully human monoclonal antibody to RANK-L, has been shown to inhibit osteoclast-mediated bone destruction in various preclinical and clinical trials. Recently, in comparison to zoledronic acid, denosumab was found to be more effective in delaying skeletal-related events in both metastatic breast cancer and prostate cancer. Furthermore, its fewer acute-phase reactions, such as fever, myalgia or arthralgia, and lesser renal toxicity makes it favorable candidate for the treatment of bone metastases and opens wide opportunities for developing efficient drug delivery systems in this direction (Wong & Pavlakis, 2011).

Potential targets or mediators within bone metastatic breast cancer and bone microenvironment

In recent years, gene expression profiling has become a standard technique used to identify genes that are deregulated in cancer. By coupling gene expression profiling with pre clinical mouse models of breast cancer metastasis to bone, a better understanding of the various stages of metastatic progression has evolved. Extensive research in this field has revealed the molecular complexity of bone metastatic breast tumors and also the fact that not a single gene or pathway dictates the metastasis of tumors to bone. Furthermore, a careful scrutinization of the primary tumors for specific gene expressions or pathways that may be ultimately responsible for the metastasis of the primary tumor. Moreover, studies suggest that the most effective strategies to eradicate bone metastases in the future will be those that combine therapies to target several molecular pathways (Table 2) with conventional interventions, such as surgery and radiotherapy (Rose & Siegel, 2010).

Polymers used as carriers for targeted delivery to bone

Polyethylenimine

Polyethylenimine (PEI) has proved to be a promising candidate for delivering negatively charged DNA in vitro and in vivo because PEI has a high positive charge density in aqueous solutions. PEI forms complexes with DNA via cooperative electrostatic interactions. In a research, it was found that PEI could effectively bind in vitro to the Hap portion of the bone. The hypothesis was that as Hap was negatively charged, it could attract the positively charged PEI (Shiels et al., 2012).

Shiels et al. studied the effectiveness of using polyethyleneimine (PEI) and a PEG tether to bind human recombinant bone morphogenetic protein-2 (rhBMP-2) to HAp. Using human fetal osteoblast cells, the PEI- and PEG-tethered BMP-2 was also observed to increase cellular attachment by 10-fold when compared with uncoated HAp and adsorbed rhBMP-2. It was also concluded that the rhBMP-2 conjugation to PEI and PEG tether promoted an increase in cellular attachment to the HAp surface (Zhang et al., 2008).

However, the cytotoxicity of PEI has been a matter of debate. Zhang et al. (2008) formulated PEI-coated albumin NPs for BMP-2 delivery. The bone-inducing growth factor BMP-2 was encapsulated in albumin NPs, which were coated with the cationic polymer, PEI, for better control of BMP-2 delivery kinetics. The electrostatic interaction between the anionic albumin and cationic PEI was favorable for creating an effective PEI coating on the NPs. The cytotoxicity of PEI was addressed using lower PEI concentration during coating process. The overall results indicated that BMP-2 encapsulated BSA NPs coated with 0.1 mg/mL PEI gave tolerable toxicity, retained a robust ALP induction activity in C2C12 cells and efficiently slowed the release of BMP-2 from the BSA NPs. These studies established the foundation for testing NP formulations for BMP-2 in animal models, and in particular evaluating the effect of sustained-release formulations on BMP-2-induced bone formation.

Poly-L-lysine

Poly(L-lysine) (PLL) are frequently used cationic polymers in drug and gene delivery systems; hence due to its cationic nature, it can also be exploited for delivering the drugs to the negatively charged Hap mineral of the bone (Low & Kopevcek, 2012).

Daubine et al. (2009) have reported supramolecular drug delivery systems based on a polyionic vector, which could improve the antitumor activity of BPs by lowering bone uptake and/or increasing cellular internalization of the drug. PLL covalently grafted with b-cyclodextrin as a polycationic vector for the antitumor BP drug RIS was proposed. The results indicated that the efficacy of the complexes on inhibiting cancer growth in vitro was greatly enhanced both in solution form as well as embedded in nanoarchitectures. Similar results were observed in vivo for prevention of metastasis in animals. The study pointed to a bright prospect of incorporating polyionic vector:drug complexes into PEM nanoarchitectures covering bone implants appears of interest for in situ prevention of bone metastasis after ablation.

Poly(lactic-co-glycolic acid)

PLGA is a biocompatible and biodegradable polymer, which has been widely utilized to entrap drugs in the form of NPs. Recently, PLGA NPs modified with bone-seeking agents have been investigated for potential application in bone metastasis treatment (Low & Kopevcek, 2012).

Salerno et al. (2010) developed the bone-targeted doxorubicin (DXR)-loaded NP as a tool for the treatment of skeletal metastases. The study involved conjugation between poly (D,L-lactide-co-glycolic) acid and alendronate for targeting the site of tumor-induced osteolysis. The NP were loaded with DXR and analyzed for the in vitro and in vivo activity of the drug encapsulated in the carrier system. After confirming the intracellular uptake of DXR-loaded NP, the antitumor effects were evaluated in human cell lines mimicking the bone metastatic tumors and in an orthotopic mouse model of breast cancer bone metastases. In vitro, DXR-loaded NP (58–580 ng/mL) determined a significant dose-dependent growth inhibition of all cell lines. Similarly, DXR-loaded NP also reduced the incidence of metastases in mice.

In another novel nanoapproch, Qiao et al. (2013) used PLGA microspheres to encapsulate plasmid of bone morphogenetic protein 2/PEI NPs to promote bone formation in vitro and in vivo. The study was designed such that effective BMP-2 levels were desired locally with a sustained release pattern for a sufficient period of time to allow osteoprogenitor cells to migrate to the target site and differentiate into osteoblasts. This objective was accomplished by formulating PEI NPs of pBMP-2/PEI plasmid DNA, which was entrapped in PLGA microspheres in order to effectively and efficiently promote bone formation locally. These types of multifunctional systems can also be exploited for targeting bone metastasis.

Poly[N-(2-hydroxypropyl)methacrylamide] HPMA

Poly[N-(2-hydroxypropyl)methacrylamide] (HPMA) is one of the most studied polymer therapeutics to the bone. The research on the use of HPMA copolymers as drug carriers commenced in the early 1970s. Kopeček et al. (2010) describe HPMA copolymer–drug conjugates as nanosized (5–20 nm) water-soluble constructs with unique structural, physicochemical and biological properties. The concept of targeted polymer–drug conjugates was developed to address the lack of specificity of low molecular weight drugs for cancer cells. Furthermore, a new therapeutic strategy for bone neoplasms using combined targeted polymer-bound angiogenesis inhibitors was developed in which the ALN and the potent anti-angiogenic agent TNP-470 were conjugated with HPMA copolymer. A reversible addition–fragmentation chain transfer (RAFT) polymerization was utilized to synthesize a HPMA copolymer–ALN-TNP-470 conjugate bearing a cathepsin K-cleavable linker, a protease overexpressed in bone tissues. Free and conjugated ALN ALNTNP-470 demonstrated their synergistic anti-angiogenic and antitumor activity by inhibiting proliferation, migration and capillary-like tube formation of endothelial and osteosarcoma cells. The bi-specific HPMA copolymer conjugate reduced vascular hyperpermeability and remarkably inhibited human osteosarcoma growth in mice by 96%. These findings suggested that HPMA copolymer–ALN-TNP-470 was the first narrowly dispersed anti-angiogenic conjugate synthesized by RAFT polymerization that targets both the tumor epithelial and endothelial compartments warranting its use on osteosarcomas and bone metastases. Furthermore, a bone-targeted HPMA copolymer-conjugated with a well-established bone anabolic agent (prostaglandin E1; PGE1) was developed for the treatment of osteoporosis and other musculoskeletal diseases. The biorecognition of the conjugates by the skeleton was mediated by an octapeptide of D-aspartic acid or alendronate. HPMA copolymers have also been used in the design of micelles and dendrimers.

Polymers with potential for bone targeting

Poly (ε-caprolactone)

Poly (ε-caprolactone) (PCL), an aliphatic polyester, has been widely investigated as a biomaterial. The uncommon properties of PCL includes its high thermal stability with decomposition temperature of 350 °C. In bone engineering area, PCL can be attributed as a promising biocompatible and biodegradable polymer as it has been employed for bone growth and regeneration. Chen and Sun reported reinforcing PCL with HAp by melt blending technique. The method requires the exact melting temperature of the biodegradable polymers for ensuring that the polymer completely melts and provides fillers with spaces for fine dispersion in the polymer matrix. Smaller HA particles (3–8 µm) distributed higher compressive strength than HA (20–80 µm), indicating that size of particle has an impact on the improvement of the composites mechanical properties (Razak et al., 2012).

Chitosan

Chitosan, derived from chitin, is a natural biopolymer obtained from crustacean and cell wall of fungi. Chitosan is biodegradable, biocompatible and can be molded into porous structures (allows osteoconduction). Several studies have been focused on the use of chitosan/calcium phosphate composite for this rationale. Chitosan/nanocrystalline calcium phosphate scaffolds characterized by a relatively rough surface and approximately 20 times greater area/unit mass than chitosan scaffold indicated increased adsorption and enhanced cell attachment for bone regeneration. In the development of chitosan/nano-HA scaffold conducted by Thein-Han and his co-workers, high Mw chitosan scaffolds were examined by having better mechanical properties compared to medium Mw chitosan scaffolds.

Chitosan with incorporation of collagen (CCS) composite microgranules were fabricated as bone substitutes with the aim of obtaining high bone-forming efficacy. The microgranules have the flexibility to fill various types of defect sites with closer packing. The interconnected pores formed spaces between the microgranules, which allowed new bone growth and vascularization. Mechanical strength of CCS was significantly enhanced after adding HA or any other calcium phosphate components.

Other than composite scaffold, chitosan in the form of membrane was fabricated with silica xerogel. Their potential applications in guided bone regeneration were researched in terms of bone regeneration ability. The in vivo study in a rat calvarial model confirmed significantly enhanced bone regeneration using the chitosan/xerogel membrane in comparison to that of using the pure chitosan one. Histomorphometric analysis performed three weeks after implantation identified a fully closed defect in the hybrid membrane, whereas there was only 57% defect closure in the chitosan membrane (Razak et al., 2012).

Collagen

Approximately 30% of the protein contain in the human body, collagen is the most abundant protein as it is the major component of skin and bone. The repeating sequence is responsible for the helical structure and the intrinsic and predictable mechanical strength of collagen. As a biomaterial, collagen is biodegradable, biocompatible and osteoconductive. The composition of human bone is a well-known mineral/organic hybrid consisting of HA and organic mainly collagen constituents. Calcium phosphate/collagen composites are the most biomimetic system for osseous replacement. When coupled with calcium phosphate particles or any other filler forming composite, collagen prevents the filler dispersion in implants, resulting in an easily molded biomaterial (Razak et al., 2012).

Current and future of therapeutic approach for treatment of bone metastasis

At present, the paradigm for managing bone metastases is focused on symptom management and prevention of complications after bone metastases are diagnosed. Both the National Comprehensive Cancer Network and American Society of Clinical Oncology do not endorse routine blood tests, tumor markers or imaging studies for surveillance (Wong & Pavlakis, 2011).

Wong & Pavlakis (2011) has also stated that the current model on bone metastasis detection, prevention and treatment is “reactive” rather than proactive. This reactive approach is based on the fact that rather than going for early detection for possible metastasis of primary tumor to bone, the assessment is initiated only on basis of early signs of any bone-associated problems such as pain, hypercalcemia, pathological fractures or spinal cord compression in patients with breast cancer. Following the assessment with imaging techniques such as bone scan, CT scan, MRI and X-ray, which confirms bone metastases, acute management is initiated with NSAIDS, BPs, surgery or radiotherapy. Furthermore, continuing management is preceded with chemotherapy or hormone therapy till the substantial decline in the tumor growth or spread is observed.

However, the proposed future model stresses on the importance of identifying certain group of patients with primary tumor with elevated levels of bone resorption markers are proposedly at a higher risk of bone metastasis. Monitoring of certain biomarkers helps in assessment of disease progression. For example, the prominence of N-telopeptide in predicting skeletal-related events for breast cancer bone metastases patients on zoledronic acid is being studied in BISMARK. If biomarker levels can be linked clearly to clinical outcome and treatment efficacy, it may become an important asset for monitoring and guiding duration of therapy with BPs or other bone cycle-targeted therapies (Wong & Pavlakis, 2011).

An advanced therapy proposed HIFU-CHEM as new treatment options for liver and bone metastases using High Intensity Focused Ultrasound (HIFU) guided by MRI (MRgHIFU) in combination with temperature-sensitive nanomedicines such as ThermoDox®. The proposed mechanism states that MRI will provide anatomical information for planning the therapeutic intervention and temperature mapping for local hyperthermia control, as well providing a means of monitoring drug release. In addition to inducing a local temperature raise, the interaction of ultrasound waves with tissue may also improve extravasation and membrane permeability due to local pressure fluctuations, leading to an additional increase of drug uptake. The major advantage will be the non-invasive (non-surgical) option for the treatment of solid cancer tumors by employing MRgHIFU in combination with ThermoDox®. This new field of MRI-guided drug delivery creates exciting opportunities for pharmaceutical companies and researchers to expand applications for existing drugs by altered (and often tunable) pharmacokinetics as well as new opportunities to monitor and validate drug delivery (ctmm.nl, 2014).

Future of targeted delivery in bone metastasis

The authors also propose that with the introduction of tailored delivery platforms, which can incorporate multiple therapeutic agents, the present scenario of acute management, which involves complicated surgery or radiotherapy and multiple chemotherapeutic agents, can be simplified. Furthermore, the therapy can be made more effective by the nanosystems targeted specifically to the tumor-infested bone by grafting the nanosystems with bone-seeking agents such as BPs. The reduction of drug side effects of the anti-neoplastic agent is an added advantage.

Furthermore, numerous attempts have been made to utilize dendrimers in drug delivery including targeting to bone. Clementi et al. (2011) synthesized PEG-based dendrimers from heterobifunctional PEG; they were decorated with H2N-PEG-β-Glu-(β-Glu)2-(COOH)4, exposing four carboxylic groups for the attachment of ALN and/or paclitaxel (PTX), which were specifically designed for targeting bone neoplasms. The function of the pH-sensitive linker was for rapid drug release. This tailored system was able to achieve nearly 100% HAp binding and nearly 80% HAp binding following conjugation to PTX. Recently a Canadian pharmaceutical company, Celator (http://www.celator.ca/) has developed a methodical approach to assess different drug ratios within their liposomal technology resulting in the development of different liposomal formulations that are now being assessed in phase II clinical trials, namely, CPX-1 (irinotecan:floxuridine) and CPX-351 (cytarabine:daunorubicin) (Lee & Nan, 2012). Such an approach needs to be exploited for other combination delivery systems such as dendrimers or polymer-drug conjugates.

Conclusion

Targeted drug delivery has directed us toward a brighter prospect in treating bone metastasis still an immense research is still required in efficiently developing this non-invasive yet effective approach. Development of newer targeting moieties is still a startup and understanding the bone metastasis condition on a molecular basis will help in evolving further newer technologies in this direction. Local delivery systems and multifunctional NPs with targeted delivery specific to bone tissues or cells, with better controlled release and evasion from endosomes, if drug delivery needs to occur in the cytoplasm (such as siRNA) have been promising in treating this fatal disease to great extent. More effective treatments, including the big improvement of current therapies for bone diseases, will be seen with the innovation of the targeting technology in the near future. Together with advances in predictive biomarkers, bone turnover markers, multimodality imaging these novel technologies will alter the way we manage bone metastasis patients in the future giving them a ray of hope in the adversity.

Declarations of interest

The authors report no declaration of interest.