Bone is a favorable microenvironment for tumor growth and a frequent destination for metastatic cancer cells. Targeting cancers within the bone marrow remains a crucial oncologic challenge due to issues of drug availability and microenvironment-induced resistance. Herein, we engineered bone-homing polymeric nanoparticles (NPs) for spatiotemporally controlled delivery of therapeutics to bone, which diminish off-target effects and increase local drug concentrations. The NPs consist of poly(
d,
l-lactic-co-glycolic acid) (PLGA), polyethylene glycol (PEG), and bisphosphonate (or alendronate, a targeting ligand). The engineered NPs were formulated by blending varying ratios of the synthesized polymers: PLGA-
b-PEG and alendronate-conjugated polymer PLGA-
b-PEG-Ald, which ensured long circulation and targeting capabilities, respectively. The bone-binding ability of Ald-PEG-PLGA NPs was investigated by hydroxyapatite binding assays and ex vivo imaging of adherence to bone fragments. In vivo biodistribution of fluorescently labeled NPs showed higher retention, accumulation, and bone homing of targeted Ald-PEG-PLGA NPs, compared with nontargeted PEG-PLGA NPs. A library of bortezomib-loaded NPs (bone-targeted Ald-Bort-NPs and nontargeted Bort-NPs) were developed and screened for optimal physiochemical properties, drug loading, and release profiles. Ald-Bort-NPs were tested for efficacy in mouse models of multiple myeloma (MM). Results demonstrated significantly enhanced survival and decreased tumor burden in mice pretreated with Ald-Bort-NPs versus Ald-Empty-NPs (no drug) or the free drug. We also observed that bortezomib, as a pretreatment regimen, modified the bone microenvironment and enhanced bone strength and volume. Our findings suggest that NP-based anticancer therapies with bone-targeting specificity comprise a clinically relevant method of drug delivery that can inhibit tumor progression in MM.The incidence of bone metastasis is common in 60–80% of cancer patients (
1). During bone metastasis, cancer cells induce a sequence of changes in the microenvironment such as secreting cytokines to increase the activity of osteoclasts via the parathyroid hormone-related protein (PTHrP), receptor activator of nuclear factor-κB ligand (RANKL), and interleukin-6 (IL-6), resulting in increased bone resorption and secretion of growth factors from the bone matrix (
2). This creates a “vicious cycle” of bone metastasis, where bone marrow becomes packed with cancer cells that develop resistance to conventional chemotherapy, and leads to devastating consequences of bone fractures, pain, hypercalcaemia, and spinal cord and nerve compression syndromes (
2,
3). Multiple myeloma (MM) is a plasma cell cancer that proliferates primarily in bone marrow and causes osteolytic lesions (
1). Antiresorption agents, such as bisphosphonates, may alleviate bone pain, but they are ineffective at inducing bone healing or osteogenesis in MM patients (
4).Bortezomib is a proteasome inhibitor that has shown marked antitumor effects in patients with MM. Proteasome inhibitors, such as bortezomib, are also effective at increasing bone formation, both preclinically and clinically (
5–
9). However, the major drawback of bortezomib use in early stages of MM development is its toxicity, specifically, peripheral neuropathy (
5). Therefore, we aimed to develop a method to deliver bortezomib with decreased off-target side effects by using bone-specific, bortezomib-loaded nanoparticles (NPs). The NP system was based on biodegradable, biocompatible, and Food and Drug Administration (FDA)-approved components, which are both clinically and translationally relevant. NPs derived from poly(
d,
l-lactic-co-glycolic acid) (PLGA), a controlled release polymer system, are an excellent choice because their safety in the clinic is well established (
10,
11). Polyethylene glycol (PEG)-functionalized PLGA NPs are especially desirable as PEGylated polymeric NPs have significantly reduced systemic clearance compared with similar particles without PEG (
12,
13). A number of FDA-approved drugs in clinical practice use PEG for improved pharmaceutical properties such as enhanced circulation in vivo (
12,
13). To target NPs to bone [rich in the mineral hydroxyapatite (HA)], the calcium ion-chelating molecules of bisphosphonates represent a promising class of ligands (
14). Bisphosphonates, upon systemic administration, are found to deposit in bone tissue, preferentially at the high bone turnover sites, such as the metastatic bone lesions, with minimal nonspecific accumulation (
14) and were used herein to deliver NPs to the bone.A few systems explored for MM treatment have been tested in vitro including the following: (
i) snake venom and silica NPs (
15); (
ii) thymoquinone and PLGA-based particles (
16); (
iii) curcumin and poly(oxyethylene) cholesteryl ether (PEG-Chol) NPs (
17), polyethylenimine-based NPs for RNAi in MM (
18), paclitaxel-Fe
3O
4 NPs (
19), and liposomes (
20). However, none of the above-mentioned systems have aimed to manipulate the bone marrow microenvironment rather than the myeloma cells directly (
21). To date, there are no reports of using bone-targeted, controlled release, polymeric NPs with stealth properties for MM therapy. In this study, we designed NPs bearing three main components: (
i) a targeting element that can selectively bind to bone mineral; (
ii) a layer of stealth (PEG) to minimize immune recognition and enhance circulation; and (
iii) a biodegradable polymeric material, forming an inner core, that can deliver therapeutics and/or diagnostics in a controlled manner. In this study, the physicochemical properties of a range of NPs was investigated (including NP size, charge, targeting ligand density, drug loading, and drug release kinetics) and an optimal formulation with ideal properties and maximal drug encapsulation was used for in vivo efficacy studies. We fine-tuned the NP targeting ligand density to optimize its bone-binding ability and further investigated its application for targeting myeloma in the bone microenvironment. We believe our NP system has the potential to increase drug availability by improving pharmacokinetics and biodistribution that can provide bone microenvironment specificity, which may increase the therapeutic window and most certainly decrease the off-target effects (
12,
13).
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