1. Field of the Invention
Provided is a method for preparing hydroxyapatite, and uses therefor, including in tissue engineering and repair and in gene delivery.
2. Description of the Related Art
Calcium phosphate in its broadest sense is a ubiquitous material that naturally exists in a broad variety of places. It is typically an organic product that is found in bone, teeth and shells of a large variety of animals. It exists in a variety of forms as are well-known in the art, such as hydroxyapatite (Hydroxyapatite, Ca10(PO4)6(OH)2, Ca/P=1.67), tricalcium phosphate (TCP, Ca3(PO4)2, Ca/P=1.5) and brushite (CaHPO4.2H2O, Ca/P=1), as well as in amorphous form (ACP, Cax(PO4)y.NH2O) (Ca/P=˜1.5). The relative stability of the predominant forms of calcium phosphate are:ACP<TCP<<Hydroxyapatite.
Calcium phosphate, in the form of hydroxyapatite has been widely studied as a bone substitute due to its osteoconductive characteristics and its structural similarity to the mineralized matrix of natural bone (T. Kanazawa, Inorganic phosphate materials, 1989, 52, p15–20). Hydroxyapatite also has attracted much attention as a substitute material for damaged teeth over the past several decades and its biocompatibility has been experimentally proven to be superior to many other materials.
Nano-structured hydroxyapatite is believed to have several advantages in its use in bone tissue engineering due to its higher surface area and consequently higher reactivity which offers better cellular response. In addition, nano-sized hydroxyapatite is useful as an effective surface modification agent for binding numerous biological molecules.
There are several reported methods for the synthesis of hydroxyapatite. Some of the widely used processes include aqueous colloidal precipitation, sol-gel, solid-state and mechano-chemical methods. The solid state methods require elevated temperatures, which lead to grain growth and coarsening of the microstructure. Low temperature methods such as sol-gel, mechano-chemical synthesis and colloidal precipitation are performed at ambient temperatures and therefore provide the ability to synthesize nano-structured material with direct control of the particle and grain size.
Most widely used aqueous colloidal precipitation reactions to synthesize hydroxyapatite are as follows (R. Doremus. Review Bioceramics, Journal of Materials Science, 1992, 27, p285–297):10Ca(OH)2+6H3PO4→Ca10(PO4)6(OH)2+18H2O  (Formula I)10Ca(NO3)2+6(NH4)2HPO4+2H2O→Ca10(PO4)6(OH)2+12NH4NO3+8HNO3  (Formula II)
Current popular methods of gene delivery include viral gene delivery, chemical methods, such as calcium phosphate precipitation methods and liposome delivery. Non-viral plasmid gene delivery methods have certain advantages, including transient expression of the delivered gene, low systemic toxicity and are typically relatively simple to manufacture. However, there procedures typically result in low transfection efficiency. Calcium phosphate has long been recognized as a useful transfection agent, with many commercially available kits (Graham, F. L., et al. (1973); Wigler, M., et al., (1978)). Both of the reactions shown in Formulas I and II require the continuous maintenance of pH in excess of 11.0 during the entire duration (at least 12–24 hours of the reaction) to ensure the formation of stoichiometric quantities of hydroxyapatite. This is a major disadvantage for gene delivery. Furthermore this pH range is not suitable for cell stability and growth. Therefore, there is a need for a much more effective and a biocompatible synthesis approach.
Gene delivery recently has been investigated for use in bone tissue engineering therapies to repair or heal challenging wound or defect sites. Tissue engineering approaches have typically involved implanting 3D biomimetic extracellular matrices (bECMs), seeded with cells or signaling molecules (SMs) or both, into defects to induce and guide the growth of new bone by host tissue ingrowth (Oldham, J. B., et al., “Biological activity of rhBMP-2 released from PLGA microspheres,” J Biomech Eng. 2000 June;122(3):289–92; Whang, K., et al., “Ectopic bone formation via rhBMP-2 delivery from porous bioabsorbable polymer scaffolds.” J Biomed Mater Res. 1998 Dec. 15; 42(4):491-9; Hollinger, J. O., et al., “Poly(alpha-hydroxy acid) carrier for delivering recombinant human bone morphogenetic protein-2 for bone regeneration,” J. Controlled Rel., (1996) 39:287–304 and Zegzula, H. D., et al., “Bone formation with use of rhBMP-2 (recombinant human bone morphogenetic protein-2),” J. Bone Joint Surg., (1997) 79–A(12):1778–1790). The bECMs are typically polymeric, biodegradable, and highly porous to mimic the microstructure of bone. Calcium phosphate and polymer/calcium phosphate composite bECMs have also been used (Lu, H. H., et al., “Three-dimensional, bioactive, biodegradable, polymer-bioactive glass composite scaffolds with improved mechanical properties support collagen synthesis and mineralization of human osteoblast-like cells in vitro,” J Biomed Mater Res. 2003 Mar. 1; 64A(3):465–74; Spitzer, R. S., et al. “Matrix engineering for osteogenic differentiation of rabbit periosteal cells using alpha-tricalcium phosphate particles in a three-dimensional fibrin culture,” J Biomed Mater Res. 2002 Mar. 15; 59(4):690–6; Marra K G, et al. <<In vitro analysis of biodegradable polymer blend/hydroxyapatite composites for bone tissue engineering,” J Biomed Mater Res. 1999 Dec. 5; 47(3):324–35). A gene therapy approach to tissue engineering incorporates DNA in the bECM. The DNA transfects local resident cells to secrete desired signaling molecules in a sustained fashion. When the bECM/DNA is implanted into the wound site, the structural matrix serves as an interactive support to wound repair cells that are naturally recruited during the granulation phase of bone wound repair. The cells migrate into the matrix and subsequently come in contact with the incorporated DNA. The ideal matrix would mimic the microstructure of the target tissue, optimize the activity of the expressed growth factors, enhance transfection efficiency of the DNA, provide stability in vivo, and degrade in a controlled fashion with minimal inflammatory response. Additional desirable attributes include controlled release of the gene and ability to promote conducive cell growth and differentiation.
The non-viral gene therapy approach to tissue engineering has been demonstrated by Fang, et al. and Bonadio, et al. (Fang, J., et aL, “Stimulation of new bone formation by direct transfer of osteogenic plasmid genes,” Proc. Natl. Acad. Sci. U.S.A., (1996) 93(12):5753–8 and Bonadio, J., et al., “Localized, direct plasmid gene delivery in vivo: prolonged therapy results in reproducible tissue regeneration,” Nat. Med., (1999) 5(7):753–9). Fang et al. and Bonadio et al. reported the use of a “Gene Activated Matrix” (GAM) to locally deliver pDNA to wound sites in rats. Fang et al. delivered either a bone morphogenetic protein-4 (BMP-4) plasmid or a plasmid coding for a fragment of parathyroid hormone (amino acids 1–34) (PTH1-34). In both cases, a biological response was shown. Both Fang et al. and Bonadio et al. used a collagen-based bECM. However, a major problem with the approach is that they had to use high concentrations of plasmid DNA to achieve a clinical result since collagen based scaffolds do not provide any properties to transfect DNA into the cells. The combination of a biodegradable polymer and plasmid DNA without the addition of a transfecting agent thus appears to yield an inefficient transfection of plasmid DNA into the cell. Hence there is a need for an optimum delivery system that would enhance in vivo gene transfer. To overcome these limitations, researchers are investigating various transfecting agents such as cationic lipids (liposomes or lipoplexes) and/or cationic polymers to incorporate into polymers for tissue engineering applications.
U.S. Pat. Nos. 5,258,044, 5,306,305, 5,543,019, 5,650,176, 5,676,976, 5,683,461, 5,783,217, 5,843,289, 6,027,742, 6,033,582, 6,117,456, 6,132,463 and 6,214,368 disclose methods of synthesizing calcium phosphate particles and a variety of biomedical uses for nanocrystalline calcium phosphate particles. These patents each describe to varying extents coating of substrates with calcium phosphates, including medical implants and medical devices. The implants and matrices formed from the calcium phosphate materials described in those patents are useful in tissue engineering and repair, especially bone engineering and repair. U.S. Pat. Nos. 5,258,044, 5,306,305, 543,019, 5,650,176, 5,676,976, 5,683,461, 5,783,217, 5,843,289, 6,027,742, 6,033,582, 6,117,456, 6,132,463 and 6,214,368 are incorporated herein by reference in their entirety for their teachings relating to uses for calcium phosphate.
U.S. Pat. Nos. 5,460,830, 5,441,739, 5,460,831, 5,462,750, 5,462,751, 5,464,634 and 5,639,505 describe a number of uses for brushite and TCP nanoparticles and various methods for preparing nanocrystalline brushite and TCP by standard methodology using acidic sodium phosphate as a precursors. Calcium phosphate is described in those patents as useful as a bioreactive particle, such as a transfection agent, that is complexed with nucleic acids, proteins and peptides (including antibodies) and pharmacological agents. Also described in those patent references are nanocrystalline calcium phosphate particles coated with viral proteins, useful as viral decoys for immunizing an animal, and nanocrystalline calcium phosphate particles coated with hemoglobin for use as red blood cell surrogates. U.S. Pat. Nos. 5,460,830, 5,441,739, 5,460,831, 5,462,750, 5,462,751, 5,464,634 and 5,639,505 are incorporated herein by reference in their entirety for their teachings relating to uses for calcium phosphate nanoparticles.
Dental Pulp Injury Model to Treat Carious Lesions—For many years, the treatment of dental carious lesions has been based on physical, chemical and biomechanical properties of the restorative materials. In recent years, we have seen the emergence of biological therapies that hold considerable promise to change the practice of dentistry.
The production of dentin under pathological inflammatory conditions often results in poor quality reparative dentin containing irregular deposition of collagen matrix, fewer and wider dentinal tubule and hypomineralization. Clinically, pulpal inflammation with minor exposure is treated with calcium hydroxide (for example Dycal®) using the direct pulp-capping technique. Some of the effects of calcium hydroxide treatment may include reparative dentin formation with preservation of tooth vitality, pulpal resorption, apical lesions and excessive reparative dentin formation. The mechanism of reparative dentin stimulation by Dycal® is that the inherent alkalinity (pH 11–12) of calcium hydroxide induces a focal necrosis upon contact with the pulp and neutralizes the acidity produced during the inflammatory response. This alkalinity may increase the risk of pulp morbidity and apical lesions. The surviving pulp may deposit excessive reparative dentin in response to Dycal®.
Reparative dentin is formed by matrix-mediated biomineralization. The predentin matrix is synthesized and secreted by odontoblasts and is subsequently mineralized to form dentin. After pulp exposure, cytokines are needed to induce cell division to replenish lost cells and transcription factor(s) are required to upregulate the genes encoding the extracellular matrix proteins(ECM). Nakashima has observed that several growth factors induces the proliferation and differentiation of pulp cells (Nakashima, M., “The effects of growth factors on DNA synthesis, proteoglycan synthesis and alkaline phosphatase activity in bovine dental pulp cells,” Arch. Oral Biol., (1992) 37(3):231–6 and Nakashima, M., et al., “Regulatory role of transforming growth factor-beta, bone morphogenetic protein-2, and protein-4 on gene expression of extracellular matrix proteins and differentiation of dental pulp cells,” Dev. Biol., (1994) 162(1):18–28). Rutherford, et al. have reported that osteogenic protein-1 (OP1 or BMP-7) induces formation of reparative dentin after pulp exposure in monkeys (Rutherford, R. B., et al., “Induction of reparative dentine formation in monkeys by recombinant human osteogenic protein-1,” Arch. Oral Biol., (1993) 38(7):571–6).
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