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Title:  Biodegradable low molecular weight triblock poly(lactide-co- glycolide) polyethylene glycol copolymers having reverse thermal gelation properties

United States Patent:  6,201,072

Inventors:  Rathi; Ramesh C. (Salt Lake City, UT); Zentner; Gaylen M. (Salt Lake City, UT); Jeong; Byeongmoon (Salt Lake City, UT)

Assignee:  MacroMed, Inc. (Sandy, UT)

Appl. No.:  396589

Filed:  September 15, 1999

Abstract

A water soluble, biodegradable ABA- or BAB-type tri-block polymer is disclosed that is made up of a major amount of a hydrophobic A polymer block made of a biodegradable polyester and a minor amount of a hydrophilic polyethylene glycol(PEG) B polymer block, having an overall average molecular weight of between about 2000 and 4990, and that possesses reverse thermal gelation properties. Effective concentrations of the tri-block polymer and a drug may be uniformly contained in an aqueous phase to form a drug delivery composition. At temperatures below the gelation temperature of the tri-block polymer the composition is a liquid and at temperatures at or above the gelation temperature the composition is a gel or semi-solid. The composition may be administered to a warm-blooded animal as a liquid by parenteral, ocular, topical, inhalation, transdermal, vaginal, transurethral, rectal, nasal, oral, pulmonary or aural delivery means and is a gel at body temperature. The composition may also be administered as a gel. The drug is released at a controlled rate from the gel which biodegrades into non-toxic products. The release rate of the drug may be adjusted by changing various parameters such as hydrophobic/hydrophilic component content, polymer concentration, molecular weight and polydispersity of the tri-block polymer. Because the tri-block polymer is amphiphilic, it functions to increase the solubility and/or stability of drugs in the composition.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide low molecular weight triblock copolymer drug delivery systems that are biodegradable, exhibit reverse thermal gelation behavior, namely, exist as a liquid solution at low temperatures, reversibly form gels at physiologically relevant temperatures, and provide good drug release characteristics.

A fuirther object of this invention is to provide a drug delivery system for the parenteral administration of hydrophilic and hydrophobic drugs, peptide and protein drugs, hormones, genes/nucleic acids, oligonucleotides and anti-cancer agents.

Yet another object of this invention is to provide a method for the parenteral administration of drugs in a biodegradable polymeric matrix resulting in the formation of a gel depot within the body, from which the drugs are released at a controlled rate.

These and other objects are accomplished by means of a biodegradable ABA- or BAB-type block copolymer having an average molecular weight of between about 2000 and 4990 consisting of about 51 to 83% by weight of an hydrophobic A polymer block comprising a biodegradable polyester and about 17 to 49% by weight of a hydrophilic B polymer block consisting of polyethylene glycol(PEG). Preferably, the biodegradable polyester is synthesized from monomers selected from the group consisting of D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, .epsilon.-caprolactone, .epsilon.-hydroxy hexonoic acid, .gamma.-butyrolactone, .gamma.-hydroxy butyric acid, .delta.-valerolactone, .delta.-hydroxy valeric acid, hydrooxybutyric acids, malic acid, and copolymers thereof. More preferably, the biodegradable polyester is synthesized from monomers selected from the group consisting of D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, .epsilon.-caprolactone, .epsilon.-hydroxy hexonoic acid, and copolymers thereof

Most preferably, the biodegradable polyester is synthesized from monomers selected from the group consisting of D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, and copolymers thereof.

Polyethylene glycol (PEG) is also sometimes referred to as poly(ethylene oxide) (PEO) or poly(oxyethylene) and the terms can be used interchangeably for the purposes of this invention.

In the hydrophobic A-block, the lactate content is between about 20 to 100, preferably between about 20 to 80 mole percent and most preferably between about 50 to 80 mole percent. The glycolate content is between about 0 and 80 mole percent, preferably between about 20 to 80 mole percent and most preferably between about 20 to 50 mole percent.

Additional objects and advantages of this invention will become apparent from the following summary and detailed description of the various embodiments making up this invention.

As used herein the following terms shall have the assigned meanings:

"Parenteral" shall mean intramuscular, intraperitoneal, intra-abdominal, subcutaneous, and, to the extent feasible, intravenous and intraarterial.

"Gelation temperature" means the temperature at which the biodegradable block copolymer undergoes reverse thermal gelation, i.e. the temperature below which the block copolymer is soluble in water and above which the block copolymer undergoes phase transition to increase in viscosity or to form a semi-solid gel.

The terms "gelation temperature" and "reverse thermal gelation temperature" or the like shall be used interchangeably in referring to the gelation temperature.

"Polymer solution," "aqueous solution" and the like, when used in reference to a biodegradable block copolymer contained in such solution, shall mean a water based solution having such block copolymer dissolved therein at a functional concentration, and maintained at a temperature below the gelation temperature of the block copolymer.

"Reverse thermal gelation" is the phenomena whereby a solution of a block copolymer spontaneously increases in viscosity, and in many instances transforms into a semisolid gel, as the temperature of the solution is increased above the gelation temperature of the copolymer. For the purposes of the invention, the term "gel" includes both the semisolid gel state and the high viscosity state that exists above the gelation temperature. When cooled below the gelation temperature, the gel spontaneously reverses to reform the lower viscosity solution. This cycling between the solution and the gel may be repeated ad infinitum because the sol/gel transition does not involve any change in the chemical composition of the polymer system. All interactions to create the gel are physical in nature and do not involve the formation or breaking of covalent bonds.

"Drug delivery liquid" or "drug delivery liquid having reverse thermal gelation properties" shall mean a polymer solution that contains drug (the drug per se can either be dissolved or colloidal) suitable for administration to a warm-blooded animal which forms a gelled drug depot when the temperature is raised to or above the gelation temperature of the block copolymer.

"Depot" means a drug delivery liquid following administration to a warm-blooded animal which has formed a gel upon the temperature being raised to or above the gelation temperature.

"Gel" means the semi-solid phase that spontaneously occurs as the temperature of the "polymer solution" or "drug delivery liquid" is raised to or above the gelation temperature of the block copolymer.

"Aqueous polymer composition" means either a drug delivery liquid or a gel comprised of the water phase having uniformly contained therein a drug and the biodegradable block copolymer. At temperatures below the gelation temperature the copolymer may be soluble in the water phase and the composition will be a solution. At temperatures at or above the gelation temperature the copolymer will solidify to form a gel with the water phase, and the composition will be a gel or semi-solid.

"Biodegradable" means that the block copolymer can chemically break down or degrade within the body to form nontoxic components. The rate of degradation can be the same or different from the rate of drug release.

"Drug" shall mean any organic or inorganic compound or substance having bioactivity and adapted or used for a therapeutic purpose. Proteins, hormones, anti-cancer agents, oligonucleotides, DNA, RNA and gene therapies are included under the broader definition of drug.

"Peptide," "polypeptide," "oligopeptide" and "protein" shall be used interchangeably when referring to peptide or protein drugs and shall not be limited as to any particular molecular weight, peptide sequence or length, field of bioactivity or therapeutic use unless specifically stated.

"Poly(lactide-co-glycolide)" or "PLGA" shall mean a copolymer derived from the condensation copolymerization of lactic acid and glycolic acid, or, by the ring opening polymerization of .alpha.-hydroxy acid precursors, such as lactide or glycolide. The terms "lactide," "lactate," "glycolide" and "glycolate" are used interchangeably.

"Poly(lactide)" or "PLA" shall mean a polymer derived from the condensation of lactic acid or by the ring opening polymerization of lactide. The terms "lactide" and "lactate" are used interchangeably.

"Biodegradable polyesters" refer to any biodegradable polyesters, which are preferably synthesized from monomers selected from the group consisting of the group consisting of D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, .epsilon.-caprolactone, .epsilon.-hydroxy hexonoic acid, .gamma.-butyrolactone, .gamma.-hydroxy butyric acid, .delta.-valerolactone, .delta.-hydroxy valeric acid, hydrooxybutyric acids, malic acid, and copolymers thereof.

Therefore, the present invention is based on the discovery of ABA- or BAB-type block copolymers, where the A-blocks are relatively hydrophobic A polymer block comprising a biodegradable polyester, and the B-blocks are relatively hydrophilic B polymer block comprising polyethylene glycol (PEG), having a hydrophobic content of between about 51 to 83% by weight and an overall block copolymer molecular weight of between about 2000 and 4990, and which exhibit water solubility at low temperature and undergo reversible thermal gelation at mammalian physiological body temperatures.

With such high hydrophobic content it is unexpected that such block copolymers would be water soluble. It is generally taught that any polymer having a hydrophobic content in excess of 50% by weight is substantially insoluble in water and can only be made appreciably soluble in aqueous systems, if at all, when a certain amount of an organic co-solvent has been added.

Therefore, basic to the present invention is the utilization of a block copolymer having hydrophobic or "A" block segments and hydrophilic or "B" block segments. Generally the block copolymer will be ABA- or BAB-type tri-block copolymers. However, the block copolymer could also be a multiblock copolymer having repeating BA or AB units to make A(BA)n or B(AB)n copolymers where n is an integer of from 2 to 5.

Both ABA and BAB type triblock copolymers may be synthesized by ring opening polymerization, or condensation polymerization according to reaction schemes disclosed in U.S. Pat. No. 5,702,717, and copending U.S patent application Ser. No. 08/943,167, filed on Oct. 3, 1997, and Ser. No. 09/164,865 filed on Oct. 1, 1998, and fully incorporated herein by reference.

The block copolymers that have utility as disclosed in this invention meet the criteria summarized in Table 1, namely having compositional make-up within the indicated ranges that result in block copolymers that demonstrate the desired reverse thermal gelling behavior. For purposes of disclosing molecular weight parameters, all reported molecular weight values are based on measurements by NMR or GPC (gel permeation chromatography) analytical techniques. The reported average molecular weights and number average molecular weights were determined by GPC and NMR respectively. The reported lactide/glycolide ratio was calculated from NMR data. GPC analysis was performed on a Styragel HR-3 column calibrated with PEG using RI detection and chloroform as the eluent, or on a combination of Phenogel, mixed bed, and Phenogel, 500 .ANG. columns calibrated with PEG using RI detection and tetrahydrofuran as the eluent. NMR spectra were taken in CDCl3 on a Bruker 200 MHz instrument.

TABLE 1
Total weight average molecular weight: 2000 to 4990
PEG content:             17 to 49% by weight
Total polyester content: 51 to 83% by weight
Lactate content:         20 to 100 mole percent
Glycolate content:       0 to 80 mole percent
Behavior:                (character pullout}  water 
                         soluble below the gelation temperature;
                        {character pullout}  gels above the
                        gelation temperature
The biodegradable, hydrophobic A polymer block comprising a polyester synthesized from monomers selected from the group consisting of D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, .epsilon.-caprolactone, .epsilon.-hydroxy hexonoic acid, .gamma.butyrolactone, .gamma.hydroxy butyric acid, .delta.-valerolactone, .delta.-hydroxy valeric acid hydrooxybutyric acids, malic acid, and copolymers thereof. Calculating from the values for total molecular weight and percent by weight of A and B polymer blocks as given in Table 1, and assuming that the weight average molecular weight of each of the A-blocks in an ABA triblock copolymer or the B-blocks in a BAB triblock copolymer are essentially the same, the average molecular weight of polymeric A block is between about 600 and 3000.

By similar calculations, the hydrophilic B-block segment is preferably polyethylene glycol (PEG) having an average molecular weight of between about 500 and 2200.

Both ABA and BAB type triblock copolymers may be synthesized by ring opening polymerization, or condensation polymerization according to reaction schemes disclosed in U.S. Pat. No. 5,702,717 and U.S. patent application Ser. No. 08/943,167, hereby fully incorporated by reference. For example, the B(PEG) blocks may be coupled to the A blocks(polyesters) by ester or urethane links and the like. Condensation polymerization and ring opening polymerization procedures may be utilized as may the coupling of a monofinctional hydrophilic B block to either end of a difunctional hydrophobic A block in the presence of coupling agents such as isocyanates. Furthermore, coupling reactions may follow activation of functional groups with activating agents such as carbonyl diimidazole, succinic anhydride, N-Hydroxy succinimide and p-nitrophenyl chloroformate and the like.

The hydrophilic B-block is formed from PEG of appropriate molecular weights. PEG was chosen as the hydrophilic, water-soluble block because of its unique biocompatibility, nontoxicity, hydrophilicity, solubilization properties, and rapid clearance from a patient's body.

The hydrophobic A-blocks are utilized because of their biodegradable, biocompatible, and solubilization properties. The in vitro and in vivo degradation of these hydrophobic, biodegradable polyester A-blocks is well understood and the degradation products are naturally occurring compounds that are readily metabolized and/or eliminated by the patient's body.

Surprisingly, the total weight percentage of the hydrophobic polyester A-block, relative to that of the hydrophilic PEG B-block, is high, e.g. between about 51 to 83% by weight, and most preferably between about 65 to 78% by weight, yet the resulting triblock polymer retains the desirable water solubility and reverse thermal gelation properties. It is an unexpected discovery that a block copolymer with such a large proportion of hydrophobic component would be water soluble below normal room temperatures, such as refrigerator temperatures (5oC.). It is believed that this desirable solubility characteristic is made possible by maintaining an overall low molecular weight of the entire triblock copolymer of between about 2000 and 4990. Thus, water soluble biodegradable block copolymers possessing thermally reversible gelation properties are prepared wherein the hydrophilic B-block or blocks make up about 17 to 49% by weight of the copolymer and the hydrophobic A-block or blocks make up about 51 to 83% by weight of the copolymer. In a preferred embodiment, the A-blocks(polyesters) may comprise between about 65 to 78% by weight of the copolymer and the PEG B-blocks may comprise between about 22 to 35% by weight of the copolymer. Furthermore, the preferred overall average molecular weight of the entire triblock copolymer will be between about 2800 and 4990.

The concentration at which the block copolymers are soluble at temperatures below the gelation temperature may be considered as the functional concentration. Generally speaking, block copolymer concentrations of as low as 3% and up to about 50% by weight can be used and still be functional. However, concentrations in the range of about 5 to 40% are preferred and concentrations in the range of about 10-30% by weight are most preferred. In order to obtain a viable gel phase transition with the copolymer, a certain minimum concentration, e.g. 3% by weight, is required. At the lower functional concentration ranges, phase transition may result in the formation of a weak gel. At higher concentrations, a strong gel network is formed.

The mixture of the biodegradable copolymer and peptide/protein drugs, and/or other types of drugs, may be prepared as an aqueous solution of the copolymer below the gelation temperature to form a drug delivery liquid where the drug may be either partially or completely dissolved. When the drug is partially dissolved, or when the drug is essentially insoluble, the drug exists in a colloidal state such as a suspension or emulsion. This drug delivery liquid is then administered parenterally, topically, transdermally, transmucosally, inhaled, or inserted into a cavity such as by ocular, vaginal, transurethral, rectal, nasal, oral, buccal, pulmonary or aural administration to a patient, whereupon it will undergo a reversible thermal gelation since body temperature will be above the gelation temperature.

This system will cause minimal toxicity and minimal mechanical irritation to the surrounding tissue due to the biocompatibility of the materials and pliability of the gel, and will be completely biodegraded to lactic acid, glycolic acid, and other corresponding monomers within a specific time interval. The drug release, gel strength, gelation temperature and degradation rate can be controlled by proper design and preparation of the various copolymer blocks, namely, through modifications of the weight percent of A-blocks and B-blocks, the mole percentages of lactate and glycolate, and the molecular weight and polydispersity of the ABA or BAB triblock copolymers. Drug release is also controllable through adjustment of the concentration of polymer in the drug delivery liquid.

A dosage form comprised of a solution of the block copolymer that contains either dissolved drug or drug as a suspension or emulsion is administered to the body. This formulation then spontaneously gels, due to the reverse thermal gelation properties of the block copolymer, to form a drug depot as the temperature of the formulation rises to body temperature. The only limitation as to how much drug can be loaded into the formulation is one of functionality, namely, the drug load may be increased until the thermal gelation properties of the copolymer are adversely affected to an unacceptable degree, or until the properties of the formulation are adversely affected to such a degree as to make administration of the formulation unacceptably difficult. Generally speaking, it is anticipated that in most instances the drug will make up between about 0.01 to 20% by weight of the formulation with ranges of between about 0.01 to 10% being highly common. These ranges of drug loading are not limiting to the invention. Provided functionality is maintained, drug loadings outside of these ranges fall within the scope of the invention.

A distinct advantage to the compositions of the subject of this invention lies in the ability of the block copolymer to increase the solubility of many drug substances. The combination of the hydrophobic A-block(s) and hydrophilic B-block(s) renders the block copolymer amphiphilic in nature. In that regard it functions much as a soap or surfactant in having both hydrophilic and hydrophobic properties. This is particularly advantageous in the solubilization of hydrophobic or poorly water soluble drugs such as cyclosporin and paclitaxel. What is surprising is the degree of drug solubilization of most, if not all, drugs since the major component of the block copolymer is the hydrophobic A-block content. However, as already discussed, even though the hydrophobic polymer block(s) are the major component, the block copolymer is water soluble and it has been found that there is an additional increase in drug solubility when combined in an aqueous phase of the block copolymer.

Another advantage to the composition of the invention lies in the ability of the block copolymer to increase the chemical stability of many drug substances. Various mechanisms for the degradation of drugs, that lead to a drug's chemical instability, have been observed to be inhibited when the drug is in the presence of the block copolymer. For example, paclitaxel and cyclosporin A are substantially stabilized in the aqueous polymer composition of the present invention relative to certain aqueous solutions of these same drugs in the presence of organic co-solvents. This stabilization effect on paclitaxel and cyclosporin A is but illustrative of the effect that can be achieved with many other drug substances.

In certain situations the drug loaded polymer may be administered in the gel state instead of as a solution. The gelation may be the result of raising the temperature of a drug laden polymer solution to above the gelation temperature of the polymer prior to administration, or may be caused by raising the concentration of the polymer in the solution to above the saturation concentration at the temperature of administration, or may be caused by addition of additives to the polymer solution which causes the solution to gel. In either event, the gel thus formed may be administered parenterally, topically, transdermally, transmucosally, inhaled or inserted into a cavity such as by ocular, vaginal, buccal, transurethral, rectal, nasal, oral, pulmonary or aural administration.

This invention is applicable to bioactive agents and drugs of all types including nucleic acids, hormones, anticancer-agents, and it offers an unusually effective way to deliver polypeptides and proteins. Many labile peptide and protein drugs are amenable to formulation into the block copolymers of the invention and can benefit from the reverse thermal gelation process described herein. While not specifically limited to the following, examples of pharmaceutically useful polypeptides and proteins may be selected from the group consisting of erythropoietin, oxytocin, vasopressin, adrenocorticotropic hormone, epidermal growth factor, platelet-derived growth factor (PDGF), prolactin, luliberin, luteinizing hormone releasing hormone (LHRH), LHRH agonists, LHRH antagonists, growth hormone (human, porcine, bovine, etc.), growth hormone releasing factor, insulin, somatostatin, glucagon, interleukin-2 (IL-2), interferon-.alpha.,.beta., or .gamma., gastrin, tetragastrin, pentagastrin, urogastrone, secretin, calcitonin, enkephalins, endorphins, angiotensins, thyrotropin releasing hormone (TRH), tumor necrosis factor (TNF), nerve growth factor (NGF), granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), macrophage-colony stimulating factor (M-CSF), heparinase, bone morphogenic protein (BMP), hANP, glucagon-like peptide (GLP-1), interleukin-11 (IL-11), renin, bradykinin, bacitracins, polymyxins, colistins, tyrocidine, gramicidins, cyclosporins and synthetic analogues, modifications and pharmacologically active fragments thereof, enzymes, cytokines, antibodies and vaccines.

The only limitation to the polypeptide or protein drug which may be utilized is one of functionality. In some instances, the functionality or physical stability of polypeptides and proteins can also be increased by addition of various additives to aqueous solutions or suspensions of the polypeptide or protein drug. Additives, such as polyols (including sugars), amino acids, surfactants, polymers, other proteins and certain salts may be used. These additives can be readily incorporated into the block copolymers which will then undergo the reverse thermal gelation process of the present invention.

Developments in protein engineering may provide the possibility of increasing the inherent stability of peptides or proteins. While such resultant engineered or modified proteins may be regarded as new entities in regards to regulatory implications, that does not alter their suitability for use in the present invention. One of the typical examples of modification is PEGylation where the stability of the polypeptide drugs can be significantly improved by covalently conjugating water-soluble polymers, such as polyethylene glycol, with the polypeptide. Another example is the modification of the amino acid sequence in terms of the identity or location of one or more amino acid residues by terminal and/or internal addition, deletion or substitution. Any improvement in stability enables a therapeutically effective polypeptide or protein to be continuously released over a prolonged period of time following a single administration of the drug delivery liquid to a patient.

In addition to peptide or protein based drugs, other drugs from all therapeutic and medically useful categories may be utilized. These drugs are described in such well-known literature references as the Merck Index, the Physicians Desk Reference, and The Pharmacological Basis of Therapeutics. A brief listing of specific agents is provided for illustration purposes only, and shall not be deemed as limiting: anti-cancer agents such as mitomycin, bleomycin, BCNU, carboplatin, doxorubicin, daunorubicin, methotrexate, paclitaxel, taxotere, actinomycin D and camptothecin; antipsychotics such as olanzapine and ziprasidone; antibacterials such as cefoxitin; anthelmintics such as ivermectin; antivirals such as acyclovir; immunosuppressants such as cyclosporin A (cyclic polypeptide-type agent), steroids, and prostaglandins.

Claim 1 of 77 Claims

We claim:

1. A biodegradable ABA- or BAB-type tri-block polymer, said ABA triblock comprises:

i) about 51 to 83% by weight of a biodegradable, hydrophobic A polymer block comprising a biodegradable polyester, and

ii) about 17 to 49% by weight of a biodegradable, hydrophilic B polymer block comprising a polyethylene glycol(PEG), and

wherein the tri-block copolymer having an average molecular weight of between about 2000 to 4990 and possessing reverse thermal gelation properties.



 

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