Targeting polymer therapeutics to bone

Abstract

An aging population in the developing world has led to an increase in musculoskeletal diseases such as osteoporosis and bone metastases. Left untreated many bone diseases cause debilitating pain and in the case of cancer, death. Many potential drugs are effective in treating diseases but result in side effects preventing their efficacy in the clinic. Bone, however, provides a unique environment of inorganic solids, which can be exploited in order to effectively target drugs to diseased tissue. By integration of bone targeting moieties to drug-carrying water-soluble polymers, the payload to diseased area can be increased while side effects decreased. The realization of clinically relevant bone targeted polymer therapeutics depends on (1) understanding bone targeting moiety interactions, (2) development of controlled drug delivery systems, as well as (3) understanding drug interactions. The latter makes it possible to develop bone targeted synergistic drug delivery systems.

Figure 1

General osteoclast structure and function demonstrating unique environments, which can be utilized for site-specific release of drugs. Osteoclasts sequester portions of bone by sealing off areas called lacunae. The adjacent membrane to the bone ruffles and releases cathepsin K, and HCl, reducing the pH to 4-4.5 and dissolving the bone. Calcium from the bone is then transported to the secretory domain and released into the interstitial space. Although not specific to osteoclasts endosomes/lysosomes reduce pH to 5-6 and contain cathepsin B, two environmental specific attributes which can be used for the design of polymer-drug linkers.

Figure 2

General structure of bone-targeted polymeric nanomedicines.

Figure 3

Structures of several bone-targeting molecules including tetracycline as well as minimalized tetracycline (3-amino-2,6-dihydroxybenzamide) and its conjugate with estradiol [29]. Also shown, are the structures of several bisphosphonates in comparison with pyrophosphate. Acidic oligopeptides such as aspartic acid (shown) or glutamic acid 4-10 amino acids long are also excellent bone-targeting molecules.

Figure 4

MDP-Tc⁹⁹ scan demonstrates the targeting specificity of bisphosphonates. Shown here is a primary osteogenic sarcoma from a 72-year-old man with Paget's disease. Dark arrows indicate a metastasis to inguinal lymph node. Reprinted with permission from reference [36].

Figure 5

Atomic force microscopy histograms demonstrating rupture forces of (A) alendronate and (B) d-Asp₈ modified cantilever tips from a tooth enamel surface. (C) Binding ability of FITC labeled HPMA copolymer-ALN conjugate (P-ALN-FITC) and HPMA copolymer-d-Asp₈ conjugate (P-d-Asp₈-FITC) to hydroxyapatites with different crystallinity; black bars are high crystallinity and white bars exhibit low crystallinity. Adapted from reference [3].

Figure 6

Initial uptake and localization of FITC labeled HPMA copolymer-Asp₈ conjugate in bone compared with the uptake of tetracycline. (A) The conjugate preferentially incorporates in scalloped-appearing eroded surfaces in cancellous bone (white arrows); tetracycline (yellow label) is incorporated onto active bone mineralization surfaces. (B,C) Stained (B) and unstained (C) section of the same region of the proximal tibial growth plate and primary spongiosa. Tetracycline (yellow label) incorporated into the mineralizing zone of the growth plate (C, orange arrowhead) as expected, whereas HPMA copolymer-Asp₈ conjugate (green label) localized in the resorption areas of the primary spongiosa (C, white arrows). Magnifications: A=150x; B,C=125x. Reprinted with permission from reference [59].

Figure 7

Biodistribution in BALB/c mice of ¹²⁵I-labeled HPMA copolymer-Asp₈ conjugates (P-Asp₈) 24 h and ¹²⁵I-labeled HPMA copolymer-ALN conjugates (P-ALN) 48 h after i.v. administration. The impact of molecular weight and ALN content was evaluated. Adapted from references [58,60].

Figure 8

Scheme of release of unmodified prostaglandin E₁ (PGE₁) from HPMA copolymer-Asp₈-PGE₁ conjugate. Rate controlling cathepsin K cleavage is followed by fast 1,6 elimination.

Figure 9

General pathways affecting bone anabolism and cross talk between osteoclasts and osteoblasts. Most notably, PGE levels affect BMP-2 levels and vice versa, however, each contributes to bone anabolism by independent signaling pathways. Statins upregulate both BMP-2 and PGE through independent pathways. PGE and PTH1-34 upregulate cAMP however, stimulation of EP2 or EP4 by PGE will also trigger MAPK cascades. Not shown, PTH1-34 affects calcium levels in the body by regulating resorption in the kidneys and intestine. Wnt plays a critical role in bone turnover by production of osteoprotegerin and therefore inhibition of RANKL-RANK interactions. Also of note, COX-2 represents basic components PGE production rather than upregulation of COX-2 enzyme.

Figure 10

Treatment of mCherry-labeled MG-63-Ras osteosarcoma tumor-bearing mice with free ALN and TNP-470, HPMA copolymer-ALN-TNP-470 conjugate, or vehicle alone. (A) Structure of the HPMA copolymer-ALN-TNP-470 conjugate. (B) Intravital non-invasive fluorescence imaging of the tumor. (C) Tumor volume of mCherry-labeled MG-63-Ras tumor-bearing mice treated with free ALN and TNP-470 (open triangles), HPMA copolymer-ALN-TNP-470 conjugate (closed triangles), or vehicle alone (closed squares) [80].