Technical Field of the Invention
The present invention relates to stable parenteral formulations of tigecycline and process of preparation thereof.
Background of the Invention
Tigecycline is the first antibiotic belonging to the glycylcycline class, and the first new tetracycline analogue launched since over 30 years. It acts by binding to the bacterial 30S ribosome and thereby blocking the entry of amino-acyl t-RNA molecules into the A site of ribosome. This ultimately prevents protein synthesis by halting the incorporation of amino acids into peptide chains and thus limits bacterial growth. Chemically, it is known as (4S,4aS,5aR,12aS)-9-[2-(tert-butylamino)acetamido]-4,7-bis(dimethylamino)- 1, 4, 4a, 5, 5a, 6,11,12a-octahydro-3,10,12,12a-tetrahydroxy-1,11 -dioxo-2-naphthacene carboxamide and structurally represented as follows:
(Structure Removed)
The US Patent Number 5,494,903 discloses tigecycline and the pharmaceutically acceptable organic and inorganic salts or metal complexes. Further, the US Patent Number RE40, 086 describes the use of tigecycline in treating and controlling bacterial infections in warm blooded animals. Owing to its poor oral bioavailability, it is advantageous to administer tigecycline parenterally. In India, it is commercialized by Wyeth Limited in strengths of 50mg/vial as an orange lyophilized powder or cake for intravenous administration under the proprietary name TYGACIL®. It is indicated for treatment of the following infections in adults: (a) complicated skin and skin structure infections, including those with methicillin-resistant Staphylococcus aureus (MRSA) and (b) complicated intra-abdominal infections. The standard dosage regimen for tigecycline is an initial dose of 100mg, followed by 50mg every 12 hours.
The prior art reports several documents that disclose formulations containing tigecycline and process of formulating thereof. For example, the PCT application WO/ 2006/138641 refers to a process for manufacturing a reconstitutable lyophilized composition of tigecycline, wherein the process comprises of admixing tigecycline with water for injection (while reducing the oxygen level and maintaining the said reduced oxygen level in the water for injection); lyophilizing the said composition containing tigecycline and water for injection and sealing
the vials containing lyophilized composition under nitrogen. Another PCT application, WO/2007/075794 describes an oral composition containing tigecycline with at least one enteric coating. It further discloses that calcium binds to tigecycline, which reduce its water solubility and this may lead to a 30% to 40% loss of tigecycline due to precipitation of the calcium complex at pH 7.4. Thus, calcium binding and subsequent precipitation of the calcium/tigecycline salt may be at least partially responsible-for low oral bioavailability. Hence, the disclosed oral compositions may further contain a chelating agent, like EDTA which would help in solving the problem of poor oral bioavailability of tigecycline by chelating any calcium present.
As per the TYGACIL® Prescribing Information (Wyeth, India), tigecycline lyophilized powder Should be reconstituted with 5.3ml: of 0.9% sodium chloride injection, USP, or 5% dextrose injection, USP, to achieve a concentration of 10mg/ml_ of tigecycline. The reconstituted solution should be yellow to orange in colour, otherwise it should be discarded. At the concentration of 10mg/ml_, tigecycline expressly degrades in solution and therefore, it becomes essential that these reconstituted solutions are immediately diluted to about 1 mg/mL with saline or other pharmaceutically acceptable carriers into intravenous bags for administration to patients. However, once reconstituted it may be stored at room temperature for/up to 6 hours, or refrigerated at 2°C to 8°C for up to 24 hours. Hence, it becomes obvious that the concerned individual, like doctors, attendants, other hospital staff, or the like needs to immediately act upon and administer the reconstituted formulation of tigecycline to the person in need1 of the said medicament. Thus, this makes the whole process of administering tigecycline to a person in need of, extremely inconvenient and non-patient-friendly.
The above-discussed oxidative degradations of tigecycline are decreased to a great extent by reducing the pH of tigecycline formulations. This, however in turn hinders stability by epimer formation which is an inherent property of tetracyclines, and which is severe in case of tigecycline. Thus degradation by oxidation and/or epimerization exists to be a major drawback while administering tigecycline to a person in need of. The PCT application WO/2006/099258 addresses this problem, and describes a formulation of tigecycline comprising a carbohydrate and an acid or buffer, which does not require immediate use and also helps in providing stability from both oxidation and epimerization for a maximum of 24 hours of admixture/dilution with a suitable carrier. However, in the present case, it has been observed that formulating together tigecycline, an edetate, and a pH-modifying agent or an antioxidant gives a stable parenteral formulation which would provide stability from degradation by oxidation as well as epimerization at least for 45 hours.

Summary of the Invention
In one general aspect, it relates to a stable parenteral formulation comprising (a) tigecycline; (b) an edetate; (c) a pH-modifying agent or an antioxidant; and (d) optionally, other pharmaceutically acceptable excipients.
In another general aspect, it relates to a stable parenteral formulation comprising (a) tigecycline; (b) an edetate; (c) a pH-modifying agent or an antioxidant; and (d) optionally, other pharmaceutically acceptable excipients, wherein the said formulation does not degrade at least for 45 hours.
In another general aspect, it relates to a process of preparation of a stable parenteral formulation comprising (a) tigecycline; (b) an edetate; (c) a pH-modifying agent or an antioxidant; and (d) optionally, other pharmaceutically acceptable excipients, wherein said process comprises
(i) dissolving tigecycline, edetate, pH modifying agent or an antioxidant and optionally other pharmaceutically acceptable excipients in a suitable solvent;
(ii) filtering the solution in step (i) under aseptic conditions and filling it in vials;
(iii) optionally, lyophilizing the solution in step (ii); and
(iv) stoppering and sealing the solution in step (ii) or the lyophilizate in step (iii) under vacuum.
Detailed Description of the Invention
The terms "stable parenteral formulation" encompasses sterile solutions, as well as sterile or lyophilized powders and cakes which may be reconstituted into solutions by admixing with pharmaceutically acceptable carriers, and wherein the said formulation would achieve stability against degradation of tigecycline by epimerization and/or oxidation. Such formulations may be administered via intravenous route. In one embodiment, the term "stable parenteral formulation" refers to a formulation that remains stable from degradation at least for 45 hours.
The term "tigecycline" as described herein, means a therapeutically effective amount of tigecycline or pharmaceutically acceptable salts, enantiomers, solvates, hydrates, polymorphs and complexes thereof.
The term "edetate" as referred to herein, includes ethylenediamine tetraacetic acid (EDTA) and derivatives thereof, for example, disodium edetate, trisodium edetate, tetrasodium edetate, disodium calcium edetate and the like.
The pH-modifying agents that may be used includes acids and or/buffers that is required to prevent degradation from oxidation and/or epimerization of the stable parenteral formulation as described herein, and that which helps in modifying the pH of the said formulation from about 3.0 to about 5.0. Any acid/buffer which does not adversely affect the effectiveness of the drug formulations may be employed. Acids may be exemplified as hydrochloric, succinic, L-(+)-lactic or L-tartaric acid and the like. Buffers may be selected from citrate, acetate or phosphate buffer and the like. In one particular embodiment, 1.0N hydrochloric acid/ 1.0N sodium hydroxide is used.
Examples of antioxidants include without limitation, sodium metabisulphite, acetone sodium metabisulphite, sodium formaldehyde sulfoxylate, citric acid, d, I - a-tocopherol, butylated hydroxy anisole, butylated hydroxy toluene, monothioglycerol, ascorbic acid, propyl gallate, or the like.
Other pharmaceutically acceptable excipients may also be employed in the stable parenteral formulations as described herein, and may be exemplified as diluents, like sugars (e.g. lactose), sugar alcohols or other complex carbohydrates (e.g. cyclodextrins) and the like.
Further, preservatives, surfactants, antimicrobial agents, tonicity modifiers and the like, which are conventionally used for parenteral solutions, and which are compatible with tigecycline and/or edetate and which will not interfere with the manufacture, storage or use of the final formulation may be incorporated in the stable parenteral formulations referred to herein.
The pharmaceutically acceptable carriers that may be employed for admixing with the sterile or lyophilized powders or cakes include vehicles customarily used for administering parenteral solutions such as water, sodium chloride solution, Ringer's solution, dextrose solution, dextrose and sodium chloride solution, lactated Ringer's solution and the like.
The stable parenteral formulations may be stored in sealed containers, e.g. glass vials, having vacuum headspaces. The active ingredient, i.e. tigecycline is mixed with an edetate and dissolved in a suitable solvent, which for example may be sterile, pyrogen-free water for injection. The said water for injection may be purged with an inert gas through the said solution, to remove gaseous impurities and reduce the amount of residual oxygen therein. Further, the temperature may be maintained below 10°C, preferably below 8°C. In one embodiment, the purging gas is nitrogen.
Lyophilization of the solutions may be done by any available conventional pharmaceutical methods.
In one embodiment, the stable parenteral formulation comprises (a) tigecycline; (b) disodium edetate; and (c) hydrochloric acid.
In another embodiment, the stable parenteral formulation comprises (a) tigecycline; (b) disodium edetate; and (c) sodium metabisulphite.
In another embodiment, the stable parenteral formulation comprises (a) tigecycline; (b) disodium edetate; (c) sodium metabisulphite; and (d) lactose.
In another embodiment, the stable parenteral formulation may be prepared by the process, wherein the process comprises of
(i) dissolving tigecycline, disodium edetate in sterile, pyrogen-free water for injection
which is maintained at temperature below 10°C and purged with nitrogen gas; (ii) adjusting the pH of the solution in step (i) using 0.1N hydrochloric acid/0.1N
sodium hydroxide; (iii) filtering the solution in step (ii) under aseptic conditions, filling it in vials and
partially stoppering the said vials ; (iv) lyophilizing solution in vials in step (iii); and (v) stoppering and sealing the lyophilizate in step (iv) under vacuum.
In another embodiment, the stable parenteral formulation may be prepared by the process, wherein the process comprises of
(i) dissolving tigecycline, disodium edetate in sterile, pyrogen-free water for injection
which is maintained at temperature below 10°C and purged with nitrogen gas; (ii) adding sodium metabisulphite in the solution of step (i); (iii) filtering the solution in step (ii) under aseptic conditions, filling it in vials and
partially stoppering the said vials ; (iv) lyophilizing solution in vials in step (iii); and (v) stoppering and sealing the lyophilizate in step (iv) under vacuum.
In another embodiment, the stable parenteral formulation may be prepared by the process, wherein the process comprises of
(i) dissolving tigecycline, disodium edetate in sterile, pyrogen-free water for injection which is maintained at temperature below 10°C and purged with nitrogen gas;
(ii) adding sodium metabisulphite and lactose in the solution of step (i);
(iii) filtering the solution in step (ii) under aseptic conditions, filling it in vials and
partially stoppering the said vials ; (iv) lyophilizing solution in vials in step (iii); and (v) stoppering and sealing the lyophilizate in step (iv) under vacuum.
The stable parenteral formulations comprising (a) tigecycline; (b) an edetate; (c) a pH-modifying agent or an antioxidant; and (d) optionally, other pharmaceutically acceptable excipients, and process of preparation thereof described herein is further illustrated by the following examples, but, these should not be construed as limiting the scope of invention.
(Table Removed)
Procedure:
Comparative Example: Tigecycline was dissolved in water for injection (cooled to a temperature of 2-8°C) under continuous stirring. The solution was filtered and filled into USP type I glass vial (1.5mL). The glass vials were partially stoppered and lyophilized. The lyophilized vials were finally stoppered and sealed under inert conditions.
Example 1: Disodium edetate was dissolved in water for injection (cooled to a temperature of 2-8°C and oxygen content lowered by continuous purging of nitrogen gas). In the solution so obtained, tigecycline was added under continuous stirring. The pH of this solution was adjusted to 4.0-5.0 with 1.0N hydrochloric acid/1.0N sodium hydroxide solution. The final solution was filtered and filled into USP type I glass vials (1.5mL). The glass vials were partially stoppered and lyophilized. The lyophilized vials were finally stoppered and sealed under vacuum.
Example 2: Disodium edetate was dissolved in water for injection (cooled to a temperature of 2-8°C and oxygen content lowered by continuous purging of nitrogen gas). In the solution so obtained, lactose monohydrate, sodium metabisulphite and tigecycline were added under continuous stirring. The final solution was filtered and filled into USP type I glass vials (1.5ml_). The glass vials were partially stoppered and lyophilized. The lyophilized vials were finally stoppered and sealed under vacuum.
The lyophilized vials of Comparative Example, Examples 1 and 2 were stored at 80°C for 3 days, at 60°C for 15 days, and at accelerated stability conditions of 40°C and 75% relative humidity for 30 days, 60 days and 90 days. Each of the stored vials were then reconstituted separately with (a) 5.3 mL 0.9% sodium chloride solution; and (b) 5.3 mL 5% dextrose solution and compared with the reconstituted solutions of the marketed TYGACIL® lyophilized powder (Batch No 30819) available from Wyeth Limited, India (Manufactured at UK) (a) 5.3 mL 0.9% sodium chloride solution; and (b) 5.3 mL 5% dextrose solution. The reconstituted solutions were stored at room temperature. The colour of the reconstituted solutions was observed in daylight at 6 and 48 hour intervals, and the observation as recorded is given in Tables 1-5.
Table 1: The colour of reconstituted formulations of TYGACIL® (B.No30819) available from Wyeth (India), manufactured in UK; and Comparative Example, Example 1 and Example 2 which were previously stored at 80°C for 3 days, at different time intervals
(Table Removed)
Table 2: The colour of reconstituted formulations of TYGACIL® (B.No30819) available from Wyeth (India), manufactured in UK; and Comparative Example, Example 1 and Example 2 which were previously stored at 60°C for 15 days at different time intervals
(Table Removed)

Table 3: The colour of reconstituted formulations of TYGACIL® (B.No30819) available from Wyeth (India), manufactured in UK; and Comparative Example, Example 1 and Example 2 which were previously stored at 40°C and 75% relative humidity for 30 days at different time intervals
(Table Removed)
Table 4: The colour of reconstituted formulations of TYGACIL® (B.No30819) available from Wyeth (India), manufactured in UK; and Comparative Example, Example 1 and Example 2 which were previously stored at 40°C and 75% relative humidity for 60 days at different time intervals
Storage time Colour of the reconstituted formulations prepared in (a) 5.3mL 0.9% sodium
of the chloride solution; and (b) 5.3mL 5% dextrose solution for
(Table Removed)

Table 5: The colour of reconstituted formulations of TYGACIL® (B.No30819) available from Wyeth (India), manufactured in UK; and Comparative Example, Example 1 and Example 2 which were previously stored at 40°C and 75% relative humidity for 90 days at different time intervals
(Table Removed)
Example 1 and Tygacil® [(B.No30819) available from Wyeth, India; Manufactured at UK] vials were reconstituted separately with (a) 5.3 mL 0.9% sodium chloride solution; and (b) 5.3 mL 5% dextrose solution. From each vial 5mL of the reconstituted solution were separately withdrawn and added to a 100mL IV bag for infusion separately, containing (c)
0.9% sodium chloride solution, and (d) 5% dextrose solution. The reconstituted solutions were stored at room temperature for 6 and 24 hour intervals and also stored under refrigerated conditions (2°- 8°C) for 45 hours. The tigecycline content present in the reconstituted solutions was then assayed. The percent of initial tigecycline content remaining in reconstituted formulations as obtained are given in Table 6.
Table 6: Percent of initial tigecycline content remaining in reconstituted formulations of TYGACIL® [(B.No30819) available from Wyeth (India), manufactured in UK; and Composition in Example 1 at different time intervals
(Table Removed)

WE CLAIM:
1. A stable parenteral formulation comprising (a) tigecycline; (b) an edetate; (c) a pH-modifying agent or an antioxidant; and (d) optionally, other pharmaceutically acceptable excipient(s).
2. A stable parenteral formulation according to claim 1, wherein the said formulation does not degrade at least for 45 hours.
3. A stable parenteral formulation according to claim 1, wherein the edetate is selected from disodium edetate, trisodium edetate, tetrasodium edetate, disodium calcium edetate and the like.
4. A stable parenteral formulation according to claim 1, wherein the pH-modifying agent is an acid or buffer.
5. A stable parenteral formulation according to claim 4, wherein the pH-modifying agent is selected from hydrochloric acid, succinic acid, L-(+)-lactic acid, L-tartaric acid, citrate buffer, acetate buffer, phosphate buffer and the like.
6. A stable parenteral formulation according to claim 1, wherein the antioxidant is selected from sodium metabisulphite, acetone sodium metabisulphite, sodium formaldehyde sulfoxylate, citric acid, d, I - α-tocopherol, butylated hydroxy anisole, butylated hydroxy toluene, monothioglycerol, ascorbic acid, propyl gallate, and the like.
7. A stable parenteral formulation according to claim 1, wherein the other pharmaceutically acceptable excipient is a diluent.
8. A stable parenteral formulation according to claim 7, wherein the diluent is selected from sugars, sugar alcohols, complex carbohydrates and the like.
9. A process for preparing stable parenteral formulation according to claim 1, wherein the
process comprises the steps of
(i) dissolving tigecycline, edetate, pH modifying agent or an antioxidant and optionally other pharmaceutically acceptable excipients in a suitable solvent;
(ii) filtering the solution in step (i) under aseptic conditions and filling it in vials;
(iii) optionally, lyophilizing the solution in step (ii); and
(iv) stoppering and sealing the solution in step (ii) or the lyophilizate in step (iii) under vacuum.
10. The stable parenteral formulation comprising (a) tigecycline; (b) an edetate; (c) a pH-
modifying agent or an antioxidant; and (d) optionally, other pharmaceutically acceptable
excipients and process for the preparation thereof substantially as described and
illustrated by examples herein.