Patent Publication Number: US-8118926-B2

Title: Self-foaming cement for void filling and/or delivery systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/449,047, Jun. 8, 2006, the entirety of which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Bone and other connective tissue generally derive their structure from an extensive matrix structure. Fibrous bundles that are composed of collagen make up the extensive network that provides bone with tension-resistant behavior. Other materials appear in bone matrixes such as proteoglycans, noncollagenous proteins, lipids and acidic proteins. These materials are associated with a mineral phase consisting primarily of hydroxyapatite and the combination of the materials with hydroxyapatite tend to be poorly crystallized. In other words, bone and tooth minerals are impure forms of hydroxyapatite. In general, the crystals of pure synthetic apatites, geological apatites and many impure synthetically produced apatites are larger and more crystalline than the biological crystals of bone, dentin, cementum and cartilage. The crystals of bone, dentin and cementum are very small, irregularly shaped, very thin plates whose rough average dimensions are approximately 10 to 50 angstroms in thickness, 30 to 150 angstroms in width, and 200 to 600 angstroms in length. 
     When bone undergoes fracture or degradation, bone tissue undergoes remodeling, which is a process that occurs in mammals wherein bone tissue is continuously renewed throughout the life of those mammals. The process of remodeling occurs through the interplay of osteoblasts (bone forming cells) and osteoclasts (a large multinucleate cell found in growing bone that resorbs bony tissue, such as in the formation of canals and cavities). 
     There are diseases that affect the remodeling of bone such as osteoporosis. Osteoporosis is a systemic disease of the whole organism, which is essentially expressed by an imbalance of bone formation (i.e., the catabolic pathways of the osteoclasts predominates over the metabolic pathway of the osteoblasts). In other words, the anabolic and catabolic bone restructuring processes are reversed, and more bone material is decomposed by an osteoclastic activity, than is grown by the osteoblastic activity. 
     One means of attempting to control this reversal of bone formation rate is to deliver systemically effective substances. These include, for example, bisphosphonates and hormone preparations, which may aid in accelerating (or at least maintaining) the bone formation rate, but in the process may also lead to adverse side effects at other parts of the patient. Accordingly, to limit the use of these potentially threatening therapies, it is desirable to find bone substitute materials that not only acts as a bone substitute substance or filler, but that also operate upon surrounding bone cells in such a way that it increases and/or induces the metabolic processes (while slowing catabolic processes), so that the excessive osteoclastic activity is attenuated and osteoblastic activity (the in-growth of bones) is increased. 
     Thus, much research has been performed on developing different bone filling cement formulations to aid in treating bone diseases such as osteoporosis. The use of these bone cement formulations may be used when performing surgery. There are many types of surgery in which bone cement formulations may be used. For example, back surgery, which includes but is not limited to vertebroplasty and/or kyphoplasty, is a type of surgery where bone cement(s) is/are used. 
     Researchers and physicians have developed cement formulations that contain calcium phosphate. Calcium phosphate is a material that is used in bone cement that is known to enhance the accretion (growth) of bone to a non-biological surface. When bone cements containing calcium phosphate are used as bone void fillers, the calcium phosphates replace living bone through the bone cascade and remodeling process. Although calcium phosphate cement formulations increase accretion, there are instances where it is desirable to further increase accretion to reverse bone catabolism or wherein a bone injury has occurred. 
     The rate of replacement and resorption in bone is a function of a plurality of factors, including but not limited to the crystallinity of the bone cement formulations as well as its porosity. 
     The formulations that are presently in use as bone cements may contain either one or the other of the requisite crystallinity or porosity or neither. However, the bone cements that are used for replacement and resorption tend to lack both adequate crystallinity and porosity. That is, the bone cement formulations may have adequate crystallinity but inadequate porosity or may have adequate porosity but inadequate crystallinity. However, to date, bone cements that have both adequate crystallinity and porosity have not yet been developed. Having adequate porosity means having adequate macroscopic pores of varying sizes and densities that allow for efficient bone remodeling. Moreover, having adequate macroscopic pores of varying sizes can increase bone accretion. Without being bound by a particular theory regarding increased porosity, it is believed that increasing surface area of the bone cement formulation may allow osteoblastic cells to better perform their metabolic function. In particular, an increase in variably sized macroscopic pores may result in enhanced accretion. Moreover, without being bound to a particular theory, it is believed that the bony defects that are created by the macroscopic pores also allow blood to better absorb and provide avenues for the entrance of growth factors and BMP (bone morphogenic protein). 
     Accordingly, bone filling cement with improved crystallinity and/or porosity may be desirable in various medical applications. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to bone filing cement. In one aspect of the invention, the bone cements comprise a self setting calcium phosphate cement formulation which contains macroscopic pores of varying sizes and densities with sufficient surface area to provide substantial regions for bone turnover. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Calcium-strontium-hydroxyphosphate cement (strontium-apatite) preparations are described herein. In one embodiment, the cement comprises a powder mixture (i.e., the dry element), which contains components sodium (Na), potassium (K), calcium (Ca), strontium (Sr), phosphates (PO 4 ) and/or carbonates (CO 3 ), together with an alkali salt or an ammonium salt of phosphoric acid or some other anion, and with water and/or an aqueous solution (i.e., the wet element). In one variation, the powder mixture contains, as a Na-component, NaHCO 3 , as K-components KHCO 3  and/or K 2 HPO 4 , as a Ca-component, Ca 3 (PO 4 ) 2  (i.e., TCP), and as Sr-components SrHPO 4 , and/or strontium carbonate (SrCO 3 ) and/or Sr 3 (PO 4 ) 2 . Moreover, the powder component optionally further contains hydroxyapatite (Ca 5 (PO 4 ) 3 (OH)). As the aqueous mixing solution for the formation of the strontium-apatite cement, an aqueous solution of an alkali salt or an ammonium salt of the phosphoric acid is suitable. 
     Calcium phosphates enhance the accretion of bone to a non-biological surface, and are able to fill voids in bone created after surgery or disease. When used as a bone void filler, calcium phosphates are replaced by living bone through the bone cascade and remodeling processes. The rate of replacement or resorption is a function both of the crystallinity of the material and its porosity. 
     Thus, the present invention relates to bone cement compositions or formulations as well as methods of preparing self-setting calcium phosphate materials which contain macroscopic pores of varying size and density, thereby creating a foamed structure with a highly increased surface area which provides substantially more regions for bone turnover than the non-foamed analogue. In a variation of one embodiment, pore sizes may typically be in the range of 4 to 200 microns in diameter. In another variation, pore sizes may be in the range of 10 to 100 microns in diameter. It should be understood that other pore sizes are contemplated. In an embodiment, there is a distribution of varying pore sizes in the above mentioned ranges. 
     A typical composition of the dry element of the present invention includes one or more of the following components: tricalcium phosphate (TCP), strontium hydrophosphate (SrHPO 4 ), strontium phosphate (Sr 3 (PO 4 ) 2 ), strontium carbonate (SrCO 3 ), precipitated hydroxyapatite (pHA), sodium phosphate dibasic (Na 2 HPO 4 ), K 2 HPO 4 , and/or a cohesion promoter. The sodium phosphate dibasic and potassium phosphate dibasic may act as a setting accelerator when present in the dry element. Alternatively, these dibasic salts may be to keep a solution acidic, for example, when a buffering capacity is overwhelmed. 
     The cohesion promoters can be added to either of the wet element or the dry element of the cement prior to mixing. Typical cohesion promoters include soluble starch and/or hyaluronic acid. 
     Additional components that can be added include (NH 4 ) 2 CO 3 , (NH 4 )HCO 3 , and/or M 2-y H y CO 3  (wherein M is a metal in a +1 oxidation state and y is 0, 1, or 2). Metals that can be added include lithium, sodium, potassium, rubidium, cesium, and/or francium. 
     Typically, the wet element of the present invention will contain a buffer solution that when mixed with the dry element is able to maintain the mixed components at an acidic pH. A buffer solution that has been found to be suitable is a phosphate buffer, such as dipotassium phosphate dibasic and potassium phosphate monobasic (which has a pKa 2  of 7.2). Phosphate buffer is a polyprotic acid that has one of its pKa&#39;s close to the physiological pH of a neutral solution (i.e., a pH of 7) so it can be readily made so that the buffer solution is acidic. Phosphate buffer is also a suitable buffer because some of the dry components that are being used in the cement contain phosphates in them. If the ratio of molarities of dipotassium phosphate dibasic to potassium phosphate monobasic (i.e., K 2 HPO 4 /KH 2 PO 4 ) is greater than about 5/3, then the buffer will have a pH that is acidic. Maintaining an acidic pH will allow foaming to occur. While it may be desirable to have an acidic pH for the buffer, in certain application, it may also be desirable that the pH be 5 or higher. 
     In an embodiment, sufficient buffer should be used so that the dry components do not overwhelm the buffering capacity. In an alternative embodiment, the powder can overwhelm the buffering capacity of the solution as long as there are components in the powder element that allows the solution to stay acidic. As an example, by having the acidic calcium phosphate salt, or the appropriate potassium phosphate salt or an organic acid in the powder, one can maintain the acidic nature of the solution even though the buffering capacity of the buffer would ordinarily be overwhelmed. Using this alternative embodiment should aid one in avoiding the use of too much buffer so that one obtains the requisite bone filling formulation consistency that allows for proper filling of bone. 
     Although phosphate buffer is suitable for the instant invention, it is contemplated, and therefore within the scope of the invention, that other buffers with pKa&#39;s that are somewhat acidic or close to a neutral solution are suitable for the present invention. Examples of these buffers include but are not limited to acetate, propionate, malate (pK2), pyridine, piperazine (pK1), cacodylate, succinate (pK2), MES (2-Morpholinoethanesulfonic acid), citrate (pK3), maleate (pK2), histidine, bis-tris, ethanolamine, ADA (N-[carbamoylmethyl]iminodiacetic acid), carbonate (pK1), ACES (N-(2-acetamido)-2-aminoethanesulfonic Acid), PIPES (Piperazine-1,4-bis(2-ethanesulfonic acid)), MOPSO (3-(N-morpholino)-2-hydroxypropanesulfonic acid), imidazole, BIS-TRIS propane, BES (N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic acid), MOPS (3-(N-morpholino)-propanesulfonic acid), HEPES (N-Cyclohexyl-2-aminoethanesulfonic acid), TES (N-Tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid), MOBS (4-(N-morpholino)butanesulfonic acid), DIPSO (3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxy-1-propanesulfonic acid), TAPSO (3-[N-Tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid), triethanolamine (TEA), pyrophosphate, HEPPSO (4-(2-Hydroxyethyl)piperazine-1-(2-hydroxypropane sulfonic acid)), tricine, hydrazine, glycylglycine (pK2), and Trizma (tris) buffers. 
     If a phosphate buffer is used as the wet element, an embodiment of the dry element includes components in the following amounts: 30-70 g tricalcium phosphate (TCP), 10-30 g strontium hydrophosphate (SrHPO 4 ), 3-15 g strontium carbonate (SrCO 3 ), 1-5 g precipitated hydroxyapatite (pHA), 0.5-5 g of sodium phosphate dibasic (Na 2 HPO 4 ) 10-30 g K 2 HPO 4 , and/or 0.5 to 5% w/w of a cohesion promoter. If the dry mixture is mixed with a 1 to 1 solution of potassium phosphate buffer containing 4M dipotassium phosphate dibasic and 1.5M potassium phosphate monobasic (K 2 HPO 4  (4M) KH 2 PO 4  (1.5M)), the pH will be approximately 6.8. Because of the relatively high molarity of both dipotassium phosphate dibasic and potassium phosphate monobasic, the buffer solution will have good buffering capacity and the amount of dry ingredients shown above will allow the buffer to stay acidic. 
     In an embodiment, when the above dry components are in an acidic solution, carbon dioxide is generated. The underlying reaction to generate carbon dioxide is CO 2 +H 2 O⇄H 2 CO 3 . When the above system is acidic the reaction is driven to the left, CO 2  is generated and the CO 2  escapes into the cement. Because the buffer keeps the pH at a relatively constant level (unless or until the buffer capacity is exceeded), CO 2  will continue to be generated until the solution becomes more alkaline (causing the reaction to equilibrate or to move to the right). Likewise, if the buffer capacity is exceeded, the presence of organic acids and/or salts that maintain an acidic environment will allow the formation of CO 2  bubbles. 
     The formation of CO 2  in the cement will generate bubbles. Some of the bubbles will escape the cement and some of the bubbles will not escape. The formation of CO 2  bubbles will cause the cement to foam, leading to a plurality of different size macroscopic pores of varying sizes and densities. These pores will provide substantial regions for bone turnover leading to better accretion. 
     An example of mixing dry components with a wet element are described in, for example, U.S. Pat. No. 6,497,901 to Royer, which is incorporated herein by reference in its entirety for all purposes. Other mixing of dry components with a wet element are described in U.S. Pat. No. 5,462,356 to Murray, which is also incorporated herein by reference in its entirety for all purposes. 
     It is contemplated and therefore within the scope of the present invention that the bone cement may contain additional components. These additional components may include one or more antibiotics such as gentamicin, gentamicin sulfate, erythromycin, tobramycin, vancomycin, cefazolin, oxacillin, cefotaxime, colistin, clindamycin, and/or fusidic acid. In one variation, when the cement of the present invention foams, antibiotic that is added to the mixture will have the tendency to spread uniformly throughout the cement. This can lead to a more uniform release of the antibiotics when it is applied to bone. For example, gentamicin sulfate may be a particularly suitable antibiotic for incorporation into the foaming bone cement for certain applications, since it is wide spectrum antibiotic that can be used to attack a large variety of bacteria. 
     Alternatively, and/or additionally, additional components that can be added to the bone cement of the present invention include one or more radiopacifier compounds such as barium sulfate, 2-[2′,3′,5′-triiodobenzoyl]ethyl methacrylate (TIBMA), 3,5-diiodine salicylic methacrylate (DISMA), and/or zirconium (IV) oxide. It is contemplated that other compounds that can be seen under fluoroscopic guidance can be used as radiopacifier compounds. 
     Furthermore, anticancer agents can be added to the bone cement including, but not limited to, 6-mercaptopurine, methotrexate and/or cisplatin. 
     Other components that can be added include re-enforcing materials such as additional hydroxyapatite (HA) powder, K 2 O—Na 2 —CaO—MgO—SiO 2 —P 2 O 5  crystallized glass powder, other bioactive glasses, calcium phosphate, carbon, graphite, aramid, bone particles, bone chips, polyethylene, titanium, other metals, ultra high weight polyethylene, polymethylmethacrylate fibers in a cement matrix, tricalcium phosphate, and hydroxycarbonate apatite, and the like. 
     A common means of evaluating strength of bone filling cements is to use X-Ray Diffraction Spectroscopy (XRD). Alternatively, FTIR (Fourier Transform Infrared Spectroscopy) can be used. Measuring samples using XRD and FTIR are well known to one of ordinary skill in the art. For an example of XRD and FTIR measurements in bone cement, please see U.S. Pat. No. 5,914,356, which is herein incorporated by reference in its entirety for all purposes. A commonly used method in both XRD and FTIR involves taking spectroscopic samples of explanted bone material for observation. 
     Typical strengths of the cements of the present invention were found to be between about 0.5 and about 2.5 MPa. The strengths of the cements can be increased by adding the above mentioned re-enforcing materials. The amount that is added is dependent on where the bone cement formulation is to be used and whether or not those bones are load bearing bones. For example, if it is to be used on vertebral bones, one might add sufficient re-enforcing materials to attain a strength of 2-12 MPa. 
     It is contemplated and therefore within the scope of the invention that the bone cement can be used in conjunction with one or more bone surgical screws, metal rods or plates (such as titanium rods or plates), NITINOL alloy structural devices, and/or other mechanical structural devices that add structural strength to the bone. When these structural devices are used, the cement may be used with or without one or more of the above-identified re-enforcing agents. When the bone cement formulation is used without one or more re-enforcing agents, the bone cement serves, in essence, as a lattice that allows accretion of bone into the macroscopic voids. The accretion of bone into the lattice (in combination with the bone cement formulation) may lead to bone that has greater structural strength. 
     Mixing antibiotics or other biologically useful compounds into the cement will prevent the development of infections or serve some other useful biological purpose when the bone undergoes accretion into the macroscopic voids. 
     EXPERIMENTAL 
     A dry mixture of 60 g tricalcium phosphate (TCP), 20 g strontium hydrophosphate (SrHPO 4 ), 10 g strontium carbonate (SrCO 3 ), 3 g precipitated hydroxyapatite (pHA), and 0.93 to 4.65 g of sodium phosphate dibasic (Na 2 HPO 4 ), is combined. 0.5 to 5% w/w (of dry constituents) of a cohesion promoter is optionally added to the dry mixture. The dry mixture is mixed with a 1 to 1 solution of potassium phosphate buffer containing 4M dipotassium phosphate dibasic and 1.5M potassium phosphate monobasic (K 2 HPO 4  (4M) and KH 2 PO 4  (1.5M)) pH≈6.8). The liquid to powder ratio is about 0.33 to 0.35 ml. liquid to 1.0 gram powder. The outgassing of CO 2  in the slightly acidic mixture from the carbonate generates internal bubbles in the mixture, yielding a foamed structure upon curing of the cement. This foamed structure is then ready for use as bone filler. 
     In an alternative embodiment, the following components are added in the dry element to generate the bone cement: 
     30 g TCP 
     10 g Sr 3 (PO 4 ) 2    
     10 g SrHPO 4    
     5 g SrCO 3    
     10 g K 2 HPO 4    
     added to a 1:1 mixture of K 2 HPO 4  (3M) KH 2 PO 4  (1M)) pH≈6.7. 
     In another embodiment, the following components are added in the dry element to generate the bone cement: 
     30 g TCP 
     10 g Sr 3 (PO 4 ) 2    
     10 g SrHPO 4    
     5 g SrCO 3    
     10 g K 2 HPO 4    
     added to a 1:1 mixture of K 2 HPO 4  (3M) KH 2 PO 4  (1M)) pH≈6.7. The L/P (liquid to powder) ratio is 0.22. 
     In another embodiment, the following components are added in the dry element to generate the bone cement: 
     60 g TCP 
     12 g Sr 3 (HPO 4 ) 2    
     6 g SrHPO 4    
     3 g SrCO 3    
     0.5 g NaHCO 3    
     added to a 1:1 mixture of K 2 HPO 4  (3.5M) KH 2 PO 4  (1M)) pH≈6.7 at a liquid to powder ratio. The L/P (liquid to powder) ratio is 0.35. 
     In another embodiment, the following components are added in the dry element to generate the bone cement: 
     60 g TCP 
     16 g Sr 3 (HPO 4 ) 2    
     3 g NaHCO 3    
     added to a 1:1 mixture of K 2 HPO 4  (3.5M) KH 2 PO 4  (1M)) pH≈6.7 containing monocalcium phosphate monohydrate (MCPM). A 4% solution of Na 2 HPO 4  and 5% solution of NaHCO 3  is used. 
     Accordingly, in an embodiment, the present invention relates to a bioactive agent or cement composition comprising a dry element and a wet element; wherein said dry element contains tricalcium phosphate, hydroxyapatite and one or more members selected from the group consisting of strontium carbonate and sodium bicarbonate; and said wet element contains a buffer sufficient to keep the bioactive agent or cement composition acidic while curing. The cement in one embodiment is a bone cement. 
     The composition may contain either of or both of strontium carbonate and sodium bicarbonate. The composition may further strontium hydrophosphate. 
     In an embodiment, the composition contains 30-70 g tricalcium phosphate (TCP), 10-30 g strontium hydrophosphate (SrHPO 4 ), 3-15 g strontium carbonate (SrCO 3 ), 1-5 g precipitated hydroxyapatite (pHA), 0.5-5 g of sodium phosphate dibasic (Na 2 HPO 4 ) 10-30 g K 2 HPO 4 . Further, the composition may further comprise 0.5 to 5% w/w of a cohesion promoter. In an embodiment, the wet element is a buffer that is a phosphate buffer. 
     The pH of the buffer is between 6.5 and 6.9 or alternatively, the buffer has a pH between 6.6 and 6.9, or alternatively, the buffer has a pH that is between 6.6 and 6.8. 
     The present invention also relates to methods of making a foamed self-curing cement comprising: 
     mixing together a dry element and a wet element; 
     wherein the dry element contains tricalcium phosphate, hydroxyapatite and one or more members selected from the group consisting of strontium carbonate and sodium bicarbonate; 
     and wherein said wet element contains a buffer sufficient to keep the cement composition acidic while curing to generate the foamed self-curing cement. 
     The dry element contains either or both of strontium carbonate and sodium bicarbonate, and optionally further comprises strontium hydrophosphate. 
     In an embodiment, the method of present invention has a dry element that contains 30-70 g tricalcium phosphate (TCP), 10-30 g strontium hydrophosphate (SrHPO 4 ), 3-15 g strontium carbonate (SrCO 3 ), 1-5 g precipitated hydroxyapatite (pHA), 0.5-5 g of sodium phosphate dibasic (Na 2 HPO 4 ) 10-30 g K 2 HPO 4 . The method optionally employs 0.5 to 5% w/w of a cohesion promoter. 
     In an embodiment, the method employs a buffer that is a phosphate buffer wherein the pH of the buffer is between 5.0 and 6.9 or alternatively, between 6.0 and 6.9 or alternatively, between 6.5 and 6.9 or alternatively, the buffer has a pH between 6.6 and 6.9, or alternatively, the buffer has a pH that is between 6.6 and 6.8. 
     In an embodiment, the present invention is also directed to kits containing the above mentioned compositions/bone cements. 
     An alternative embodiment is directed to a bone piece containing voids wherein the voids contain the above mentioned compositions/bone cements. 
     The present invention has been described above with regards to a plurality of different embodiments. It is contemplated and therefore within the scope of the present invention that any one or more of the elements discussed above can be combined with any other one or more elements discussed above. It should also be understood that minor modifications can be made to the invention without departing from the scope and spirit of the invention. In addition, where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modification are in accordance with the variations of the invention. Furthermore, certain steps may be performed concurrently in a parallel process when possible, as well as performed sequentially as describe above. Moreover, when a range is disclosed, it is contemplated and therefore within the scope of the present invention that any real number that falls within the scope of this range is a contemplated endpoint. For example, if a range of 6.5 to 6.9 is given, it is contemplated that an endpoint for the range can be 6.726 even though this real number is not explicitly mentioned. In any event, the present invention is not to be limited by the foregoing written description but is rather to be defined by the following claims.