Title:  Dissolvable oxides for biological applications

United States Patent:  6,764,690

Issued:  July 20, 2004

Inventors:  Ahola; Manja (Turku, FI); Fagerholm; Heidi (Parainen, FI); Kangasniemi; Ilkka (Turku, FI); Kiesvaara; Juha (Littoinen, FI); Kortesuo; Pirjo (Turku, FI); Kurkela; Kauko (Espoo, FI); Saarinen; Niilo (Turku, FI); Yli-Urpo; Antti (Littoinen, FI)

Assignee:  DelSiTech Oy (Turku, FI)

Appl. No.:  178718

Filed:  June 25, 2002

Abstract

Controllably dissolvable silica-xerogels prepared via sol-gel process and their use. A delivery device including controllably dissolvable silica-xerogel into which structure a biologically active agent is incorporated. Pharmaceutical preparations including this device. Medical devices for orthopedic and surgical purposes which contain controllably dissolvable silica-xerogels, which may further include a biologically active agent.

SUMMARY OF THE INVENTION

An object of the present invention is to provide controllably dissolvable silica-xerogels prepared via a sol-gel process. A further object of the invention is to provide controllably dissolvable silica-xerogel particles of small diameter prepared via sol-gel process, where the gelation of the sol and evaporation of the solvent occur simultaneously. Specifically, the present invention provides controllably dissolvable silica-xerogel particles of small diameter prepared via sol-get process, where the gelation of the sol and evaporation of the solvent occur by a spray drying method or by a fiber spinning or drawing technique.

A further object of the invention is to provide sustained and/or controlled release delivery devices for biologically active agents, especially medicines, proteins, or hormones, which are made of controllably dissolvable sol-gel produced silica-xerogel, and pharmaceutical preparations comprising said devices. Specifically, the present invention provides sustained and/or controlled release delivery devices for biologically active agents, which are made of controllably dissolvable silica-xerogel particles of small diameter prepared via sol-gel process, where the gelation of the sol and evaporation of the solvent occur simultaneously, and pharmaceutical preparations comprising said devices.

A further object of the present invention is to provide a method of administering a biologically active agent to a human or animal body, which comprises implanting, injecting, or transmucosally attaching to a human or animal body a delivery device made of a sol-gel produced, controllably dissolvable silica-xerogel according to the present invention, in which structure a biologically active agent is incorporated.

A further object of the present invention is to provide an implantable medical device comprising controllably dissolvable sol-gel-produced silica-xerogel, which may further comprise a biologically active agent.

DISCRIPTION OF THE INVENTION

Applicants have discovered that silica-xerogels prepared via a sol-gel process, and silica-xerogel particles of small diameter prepared via sol-gel process where the gelation of the sol and evaporation of the solvent occur simultaneously, dissolve controllably for a long (more that 24 hours) period of time. Further, the biologically active agents incorporated into the silica-xerogel structure are also released controllably for a long period of time. Therefore, the silica-xerogels of the invention can be used for a long-term delivery of biologically active agents. Thus, they can be used for delivery devices or pharmaceutical preparations that are, for example, implanted or injected into, or transmucosally attached to a human or animal body. Administration into any tissue, soft tissues or bone, is possible. This allows local application so that targeting of the biologically active agent release site is possible. Therefore, the maximum effect from the agent is received.

A delivery device or a pharmaceutical preparation is implantable subcutaneously; intramuscularly; intraosseously; in oral, sinuidal, and uteral cavities; and into any diseased tissue. Transmucosally attached delivery devices or pharmaceutical preparations can be, e.g., particles, such as spheres, administered as a spray into sinuidal or lung tissue where they will dissolve and release the biologically active agent. Similarly, small particles can be injected in a carrier fluid in the tissues.

It has also been found that the silica-xerogels of the invention can be used for implantable medical devices. A medical device of the invention can be implanted into any human or animal tissue. Silica-xerogels of the invention dissolve totally during the period desired when they are in contact with body fluids. Thus, delivery devices and medical devices of the invention dissolve totally and controllably.

In this connection, a delivery device is a silica-xerogel incorporated with a biologically active agent into the structure. A pharmaceutical preparation, such as a granulate or capsule, in this context is a preparation that comprises the delivery device and possibly additional excipients useful in pharmaceutical preparations. A medical device of the invention is also useful for orthopedic and surgical purposes and need not contain a biologically active agent incorporated into the structure of the silica-xerogel. A medical device may be, e.g., a woven or nonwoven mat made of silica-xerogel fibers.

The silica-xerogel material of the invention has been found to be very biocompatible. In other words, it does not adversely affect the surrounding tissue, e.g., by causing an inflammation reaction.

The silica-xerogel of the invention dissolves controllably, and the release of the biologically active agent from the silica-xerogel of the invention is based on this dissolution, which allows constant local release of the biologically active agent into the tissue. The release rate of the biologically active agent can be controlled via processing parameters of the gelation conditions such as spray drying temperature. Also factors such as the surface area/volume ratio of the material, the elemental composition of the silica-xerogel, and the dimension of the gel, which allows faultless silica-xerogels to be produced, control the release rate of the biologically active agent.

The silica xerogel matrix and the incorporated biologically active agent are released slowly when diameter of the xerogel particles is in the order of about 1-500 .mu.m. When the diameter of the particles is increased, the release rates of the matrix and the active agent are also increased.

The biologically active agent can be any organic or inorganic agent that is biologically active. The biologically active agent can be, e.g., a medicine, a protein, a hormone, a living or dead cell, a bacteria, a virus or a part thereof. Biologically active agents include those especially useful for long-term therapy, such as hormonal treatment, e.g., contraception and hormone replacement therapy and for the treatment of osteoporosis, cancer, epilepsy, Parkinson's disease, pain, and cognitive dysfunction. The suitable biologically active agents may be, e.g., anti-inflammatory agents, anti-infectives (e.g., antibiotics and antiviral agents, such as glindamycin, miconazole), analgesics and analgesic combinations, antiasthmatic agents, anticonvulsants (e.g., oxycarbazepine), antidepressants, antidiabetic agents, antineoplastics, anticancer agents (e.g., toremifene, tamoxifene, taxol), antipsychotics, antispasmodics, anticholinergics, sympatomimetics, cardiovascular preparations, antiarrythmics, antihypertensives, diuretics, vasodilators, CNS (central nervous system) drugs such as antiparkinsonism dugs (e.g., selegiline), steroidal hormones (e.g., estradiol, progesterone, nestorone), sedatives (e.g. atipamezole, dexmedetomidine, levomedetomidine), tranquilizers, and cognitive dysfunction drugs. The medicine can be in the form of a salt, such as selegiline hydrochloride, (-)-4-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole hydrochloride, 4-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole hydrochloride, dexmedetomidine hydrochloride and toremifene citrate. The medicine can also be in the form of a free acid, such as ibuprofen; a free base, such as coffein or miconatzole; or a neutral compound, such as Z-2-(4-(4-chloro-1,2-diphenyl-but-1-enyl)phenoxy) ethanol. A peptide can be e.g. levodopa, and a protein can be e.g., an enamel matrix derivative or a bone morphogenetic protein. An effective amount of a biologically active agent can be added to the reaction mixture at any stage of the process. However, it is preferable to add the biologically active agent to the reaction mixture at the sol-stage before polycondensation reaction takes place or mix it with the starting materials. The precise amount employed in a particular situation is dependent upon numerous factors, such as the method of administration, type of mammal, the condition for which the biologically active agent is administered, the particular biologically active agent used, the desired duration of use, etc. The amount of toremifene citrate in the silica-xerogel may vary from about 1 w-% to about 40 w-%.

The controllably dissolvable silica-xerogels of the invention can be prepared by allowing silica-alkoxide, such as tetraethylorthosilicate (TEOS), to react with water and optionally a solvent, e.g. ethanol or polyethylene glycol, or a combination of solvents, at low temperature, such as -20^oC. to 100^oC., preferably at room temperature, in the presence of an acidic, e.g. acetic acid, or a basic catalyst by hydrolyzation (sol is formed) and polycondensation (gel is formed). The catalyst should be chosen not harming the biologically active agent.

In contrast to the production of monolithic silica-xerogels and silica coatings, in producing silica-xerogel particles of small diameter, for example by a spray drying method or a fiber spinning or drawing method, the gelation of the sol and evaporation of the solvent occur simultaneously, forming controllably dissolvable particles of small diameter, such as spheres or fibers. When the gelation is allowed to be completed before evaporation of the solvent, the formed gel is a monolith extending from wall to wall of the container. In contrast, in the present invention where the gelation of the sol and evaporation of the solvent occur simultaneously, for example by a spray drying method or a fiber spinning or drawing method, the evaporation of the solvent from the sol forces the colloidal nano-sized gel particles already formed close to each other and forces them to react with each other thereby leading to the formation of silica-xerogel particles.

In the present invention, it has been shown that when the gel is produced in particles of small diameter, such as spheres and fibers, internal stresses of the gel formed during drying are avoided almost completely and the particles are slowly degradable.

Thus, slow release materials may now be produced at low temperatures without necessarily having to sinter at all, allowing for use of all organic substances as ingredients.

Dried and/or partially sintered gels, i.e., xerogels, comprise SiO₂ modified with OH-groups that break the continuous silica network. In order for these oxides to dissolve, hydrolyzation of the bonding between an oxygen atom and a metal atom must be broken, and a hydrogen atom takes the place of the metal. Thus, the metal oxide network becomes discontinuous. The hydrolyzation can advance all the way, breaking all metal to metal oxygen bonds untill the oxide has totally dissolved. The dissolution behaviour of xerogels depends on several parameters. The sintering or drying temperature is a parameter, which has an influence on the dissolution rate of the material. An increased sintering temperature increases the polycondensation reaction rate and final state. Other parameters that control the polycondensation reaction, such as TEOS:H₂ O molar ratio, pH of the silica sol, aging, gelation rate, shape, i.e., thickness of the gel, and, drying, have a minor influence on dissolution behaviour of gels sintered at low temperature (below 300^oC). Further, different additives, such as polyethylene glycol or sorbitol which are used as penetration agents, have also only a minor effect on the release rate of the bioactive agent. The composition of the gel also has an influence on the dissolution behaviour, especially on materials sintered at above 200^oC. The composition of the xerogel can be altered with elements such as Na, Ca, P, K, Mg, Cl, Al, B, Ti, N, Fe, and C.

Porosity and surface area of the silica-xerogel can be influenced by the sintering temperature and additives. When sintered at the same temperature, different additive compositions have a large influence to the porosity and surface area. However, this change has only a minor influence to the dissolution rate of the xerogels produced near room temperature. The dissolution rates of xerogels produced at high temperatures (500-1100^oC.) will be influenced stronly by these factors.

Instead, the diameter of the single gel-object and the production method seem to have a profound influence on the dissolution rate of the xerogel. Particles of silica gel may be produced in different ways. The traditional crushing results in particles that dissolve at the same rate as the bulk material per unit surface area. In WO 9603117, the release of vancomycin from crushed silica xerogel particles of 500-700 .mu.m is decribed. The release was very rapid and most of incorporated vancomycin (about 90%) released during the first day. In contrast, if for example the sol is spray dried into particles (below 200 .mu.m) at room temperature and kept in an exciccator for 2 months, dissolution of incorporated drug will be constant and total dissolution will last for 6 days. The dissolution rate of the spray dried particles seems to be over six times slower than the dissolution rate of the crushed particles in vitro.

In the present invention, silica gel particles and spheres are produced by spray drying above the melting point of the silica sol. During spraying into air, the small droplets dry in the atmosphere sufficiently to result in gelation of the hydrolized silica ions and colloidal gel particles. If the droplets hit a surface before sufficient drying, they will form pseudo-spheres caused by surface energy differences between the droplet and the substrate. In that case, they will also gelate as pseudospheres. The gelated particles are heat treated or aged at room temperature which results in further polymerisation of the OH-groups. The heat or aging treatment slows the dissolution of the particles significantly. The particles can be incorporated with ions, such as Na, K, P, Ca, Mg, Al, and B, in order to produce dissolvable and/or bioactive bone bonding particles.

Spray drying of the gel particles without biologically active agent at the room temperature and aging them in an exciccator gives homogeneous, faultless particles with slow dissolution. These particles dissolve linearly at a rate of 1.9 w-% per week. From the at the room temperature spray dried particles with biologically active agent, silica released linearly at the rate of 22.4 w-% per week. Microspheres (<50 .mu.m) containing 10 w-% biologically active agent, prepared by mini spray dryer (Buchi, Switzerland) at 132^oC., dissolved at a rate of 77.3 w-% per week. Without a biologically active agent the release rate of 5.8 w-% per week was measured.

Controllably dissolvable silica-xerogel fibers can be produced by sol-spinning technique with further aging or treating with low temperature heat. The production temperature can be kept near room temperature. The fiber production techniques give homogeneous and faultless materials. Silica-xerogel fibers produced by a glass rod spinneret technique and kept in an exciccator for four months produced materials that dissolved 2.5 w-% per week. The fibres can be incorporated with ions, such as Na, K, P, Ca, Mg, Al, and B, in order to produce dissolvable and/or bioactive bone bonding fibers.

Vowen or nonvowen mats prepared from silica-xerogel fibers of the invention can be used to separate two or more types of tissues from each other. They can also be used as bone repair mats. It is advantageous if the tissue guide is dissolvable so that it does not need to be removed by second operation. The non-sintered and aged fibers of the invention were found to exhibit dissolution rates acceptable for such applications (10 w-% in 4 weeks).

A bone collecting filter is a medical device placed on a suction tube, which removes the debris and excess liquids from the operation site. When the surgeon is drilling, sawing, grinding or otherwise working on bony tissue the bone chips can be collected with the filter and placed back into the defect. So far, these filters are not dissolvable in the tissue. If these filters were made of sol-gel produced fibers or particles, they could be made dissolvable and loaded with a biologically active agent. Thus, the entire filter could be placed into the defect site with the bone chips.

The implants made of silica-xerogel fibermats are flexible and dissolvable.

Polylactic acid, polyglycolic acid and polykaprolacton are degradable polymers used in medical devices which, however, need to be reinforced to achieve and maintain sufficient strength long enough while the degradation reduces the strength of the matrix. Controllably dissolvable silica xerogel fibers and particles of the invention are ideal for this purpose since they have the sufficient strength and a controllable dissolution rate. They may also be used for strengthening plastic packing materials which may be made of polyiactic acid, starch or any other biodegradable polymer.

Sol-gel produced controllably dissolvable silica-xerogels according to the invention can be used as cell growth substrates in the form of for example, membranes and coatings made from spray dried particles or fibers. Cell growth assisting substances are released from the substrate with the dissolving silica.

Claim 1 of 12 Claims

What is claimed is:

1. A delivery device for the controlled release of a biologically active agent or biologically active agents, comprising

a dissolvable silica-xerogel particle having a particle size of less than 200 microns and having incorporated within its structure a biologically active agent or agents other than the silica-xerogel itself,

wherein the silica-xerogel particle is made by a sol-gel process, wherein gelation of the sol and evaporation of water or solvent occur simultaneously, and wherein the biologically active agent or agents are included in the reaction mixture by the sol-stage of the preparation of the silica-xerogel.

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