EP0843548A2

EP0843548A2 - Stabilized steroid compositions

Info

Publication number: EP0843548A2
Application number: EP96919250A
Authority: EP
Inventors: Chong Min Won; Christianna L. Strohbeck; Sheng-Yuh Tang; Bih-Hsiung Hsu
Original assignee: Rhone Poulenc Rorer Pharmaceuticals Inc
Current assignee: Aventis Pharmaceuticals Inc
Priority date: 1995-06-07
Filing date: 1996-06-05
Publication date: 1998-05-27
Also published as: WO1996040042A3; AU6164196A; CA2223385A1; EP0843548A4; MX9709431A; JPH11507359A; BR9608721A; WO1996040042A2

Abstract

The degradation of triamcinolone acetonide in aqueous solutions is studied under accelerated storage conditions of 70 °C over the pH range 1-10. Trace level metal ions can markedly accelerate the oxidative degradation in neutral and basic solutions, but pH conditions and/or the inclusion of metal-sequestering agents, such as ethylenediamine tetra acetate (EDTA), in a preparation depresses the metal catalysis.

Description

STABILIZED STEROID COMPOSITIONS Field of the Invention The present invention relates to stabilized liquid compositions of steroidal compounds, particularly adrenocorticosteroids. More particularly, the present invention relates to stabilized aqueous steroidal compositions.

Background of the invention Many of the adrenocorticosteroids share a common structural feature, namely, the dihydroxy acetone side chain at C-17. A number of studies has demonstrated that the dihydroxy acetone side chain is prone to oxidative and hydrolytic degradation in aqueous solutions. Kinetic studies germane to this discussion include degradation in aqueous solutions of prednisolone, described by Guttman, D.E. and Meister, P.D., "The kinetics of the base- catalyzed degradation of prednisolone," J. Am. Pharm. Assoc. 47 (1958) 773- 778 and Oesterling, T.O. and Guttman, D.E., "Factors influencing stability of prednisolone in aqueous solution," J. Pharm. Sci. 53 (1964) 1189-1192, hydrocortisone, described by Bundgard, H. and Hansen, J., "Studies on the stability of corticosteroids. IV. Formation and degradation kinetics of 21- dehydrocorticosteroids, key intermediates in the oxidative decomposition of 21- dehydrocorticosteroids, key intermediates in the oxidative decomposition of 21- hydroxy corticosteroids," Arch. Pharm. Chem.. Sci. Edn. 8 (1980) 187-206, Hansen, J. and Bundgard, H., "Studies on the stability of corticosteroids. I. Kinetics of degradation of hydrocortisone in aqueous solution," Arch. Pharm. Chem.. Sci. Edn. 7 (1979) 135-146, Hansen, J. and Bundgard, H., "Studies on the stability of corticosteroids. II. Kinetics and mechanism of the acid-catalyzed degradation of corticosteroid," Arch. Pharm. Chem.. Sci. Edn. 8:5-14 (1980), Hansen, J. and Bundgard, H., "Studies on the stability of corticosteroids. V. The degradation pattern of hydrocortisone in aqueous solution," Int. J. Pharm. 6 (1980) 307-319 and Pitman, I.H., Higuchi, T., Alton, M. and Wiley, Ft., "Deuterium isotope effects on degradation of hydrocortisone in aqueous solution," J. Pharm. Sci. 61 (1972) 918-920, cloprednol, described by Johnson, D.M., "Degradation of cloprednol in aqueous solution. The enolization step," 1 ^Qrg. Chem. 47 (1982) 198-201 , and triamcinolone acetonide, described by Gupta, V.D., "Stability of triamcinolone acetonide solutions as determined by high-performance liquid chromatography." J. Pharm. Sci. 72 (1983) 1453-1456 and Timmins, P. and Gray, E.A., 'The degradation of triamcinolone acetonide in aqueous solution: influence of the cyclic ketal function," J. Pharm. Pharmacol. 35 (1982) 175-177. Autoxidation has been reported to be the primary degradation pathway under aerobic conditions in neutral and alkaline aqueous solutions.

The autoxidation is strongly catalyzed by trace metal ions especially copper and the incorporation of a sequestering agent eliminates the metal catalysis. The oxidative degradation products have been characterized for hydrocortisone and flurandrenolide in cream base. The steroidal glyoxals (21- dehydro steroid derivatives) were found to be the key intermediates in the oxidative decomposition of the steroids. Triamcinolone acetonide is a known pharmaceutically active ingredient used for the treatment of a variety of topical, nasal, bronchial and other inflammation conditions as described in US Patent Nos. 3,897,779 and 4,767,612, the disclosures of which are incorporated herein by reference.

Brief Summery of the Invention The present invention relates to a pharmaceutical composition comprising a therapeutically effective amount of triamcinolone acetonide in admixture with an aqueous pharmaceutical acceptable carrier providing said composition with properties resistant to triamcinolone acetonide degradation in the presence of contaminants. A special embodiment of the invention relates to a pharmaceutical composition comprising a therapeutically effective amount of triamcinolone acetonide in admixture with an aqueous pharmaceutically acceptable carrier providing said composition with properties resistant to triamcinolone acetonide degradation in the presence of contaminants, wherein the pH of the composition is between about 4.9 and 5.1 , comprises an effective degradation inhibiting amount of EDTA.

Brief Description of the Drawings Figure 1 shows HPLC chromatograms of degraded triamcinolone acetonide in aqueous solutions at 70°C for 22 hours, (a) pH 4.0, (b) pH 6.1 , (c) pH 7.4, (d) pH 8.6

Figure 2 shows time-courses for triamcinolone acetonide, I (o), the glyoxal hydrate, IV (o), the glycolic acid, V (Δ) and the etianic acid, VI (♦) during

the oxidative degradation of I in borate buffer of pH 8.9 at 70°C. Buffer concentration, 0.032 M. Figure 3 shows the effect of borate buffer concentration on the rate of degradation of I in the presence (o) and absence (o) of EDTA at 70°C. pH, 9.2.

Ionic strength, 0.1. EDTA concentration, 5X10^*4 M.

Figure 4 shows the effect of EDTA concentration on the rate of degradation of I in carbonate buffer of pH 9.4 at 70°C.

Figure 5 shows the effect of CuSO₄ concentration on the rate of degradation of I in borate buffer of pH 8.9 at 70°C.

Figure 6 shows the log k-pH profiles for the degradation of I in aqueous

solutions at 70°C in the absence (o) and in the presence of 1x10^"5 M CuS0₄ (Δ)

or 5X10^"4 M EDTA (o).

Figure 7 shows a scheme of degradation products of triamcinolone acetonide (I).

Detailed Description of Preferred Embodiments

Reference is made to the following non-limiting examples. These examples utilize the following materials, equipment and analytical procedures.

Materials Triamcinolone acetonide was obtained from Upjohn (Kalamazoo, Ml). The purity of the drug substance was greater than 99 % as determined by HPLC analysis. Cupric acetate (Fisher, Pittsburgh, PA), periodic acid (Fisher), EDTA disodium salt (Fisher) and all other chemicals were of ACS reagent grade and used as received. Acetonitrile was HPLC grade.

HPLC Analysis The chromatography system consisted of a pump (Perkin Elmer 410), an automatic injector (Perkin Elmer ISS 100), a photo diode array detector (Perkin Elmer 480), and a networking computer data acquisition system (Waters 860). The HPLC method employed a 250 mmx 4.6 mm i.d., 5 urn particle size, octyl- bonded silica stationary phase column which is sterically protected (Zorbax Rx- C8) and a mobile phase consisting of acetonitrile:water:trifluoroacetic acid (320:680:0.68, v/v/v). The flow rate was 1.5 mlJminute and the detector wavelength for UV absorbance detection was 238 nm. Kinetic method

Stock solutions of triamcinolone acetonide (4 mg/mL) in methanol and buffers (0.2 M) in deionized water were prepared. An aliquot (0.5 mL) of the triamcinolone acetonide stock solution, an appropriate amount of buffer stock solution, hydrochloric acid (pH 1.1-2.0), chloroacetate (pH 3.0), acetate (pH 4.0-5.2), phosphate (pH 6.1-7.4), borate (pH 8.6-8.9) or carbonate (pH 9.0- 10.0) buffer stock solution and an appropriate amount of 1 M NaCl to maintain an ionic strength of 0.1 were transferred to a 100 mL volumetric flask and filled to volume with water. A low buffer concentration (0.02 M) was used to minimize possible catalysis by buffer species. To study the influence of cupric ion or EDTA on the oxidative degradation rate, an appropriate amount of CuS0₄ (5 x 10^*4 M) or Na₂ EDTA (1.1 x 10^"2M) stock solution was added to the flask. No attempts were made to control the oxygen concentration in the system.

Spectroscopy The ¹H and ¹³C NMR spectra were recorded on a Varian VXRS 200 NMR spectrometer using CDCI₃ or DMSO-d₆ as the solvent. The electron impact (El) mass spectra were obtained using a Finnigan 4500 mass spectrometer via direct inlet. The electron energy was 70 eV. The FAB mass spectra were obtained using a VG 70 SE mass spectrometer and nitrobenzyl alcohol as a matrix. Example 1 Degradation product in acidic solution. Triamcinolone acetonide (200 mg) was suspended in 200 mL of 0.1 N HCl and the suspension was refluxed for 24 hours. At the end of this time period, the solution became clear. Upon cooling the solution, a white solid material precipitated from the solution. The solid was filtered and the product was recrystallized from 20% methanol in water. The crystalline material was dried under vacuum at 60°C for two hours.

The El mass spectrum of the isolated product (II in Fig. 7) showed a molecular ion at m/z 394 (C₂₁H₂₇FO6) and a peak at m/z 374(M⁺-HF). The carbon NMR spectrum showed the absence of peaks at 25,26 and 110 ppm which correspond to the carbons of the cyclic ketal group of triamcinolone acetonide. Likewise, the proton NMR spectrum showed the absence of peaks at 1.0 and 1.3 ppm corresponding to the methyl protons of the ketal group. The mass and NMR spectra were identical to those of an authentic sample of triamcinolone (II).

Example 2 The steroidal glvoxal hydrate (IV in Fiα. 7^ To a solution of 1 g of triamcinolone acetonide in 125 ml of methanol was added a solution of 250 mg of cupric acetate in an equal volume of methanol. The solution was stirred at room temperature for one hour. HPLC analysis of the solution showed that the reaction was complete with only one product. The methanol was removed under vacuum using a rotary evaporator. The residue was suspended in 500 ml of water and the product was extracted with 200 ml of ethyl acetate. The ethyl acetate layer was washed with water and evaporated to dryness under vacuum. The residue was dissolved in a minimum amount of acetone. To the acetone solution, water was added carefully until the solution became slightly turbid. The solution was kept in a refrigerator overnight. The crystallization from aqueous acetone gave fine needles. The material was filtered and dried at 60°C under vacuum for 2 hours. The fast atom bombardment (FAB) mass spectrum of the compound showed a protonated molecular ion (M+H)⁺ at m/z 451 and a peak at 431 (M+H-HF)⁺. The El mass spectrum did not show the molecular ion but contained peaks at 432 (M-H₂0)+ and 412 (432-HF)⁺. The carbon NMR spectrum showed a resonance of C-21 at 85 ppm (doublet) in place of 66 ppm (triplet) in I. The theoretical elemental analysis values calculated for C₂₄H₃₁FO₇ are C 63.98, H 6.94; found, C 62.70, H 7.06. The spectral information agreed with the structure IV in Fig. 7.

Example 3 The steroidal glycolic acid (V in Fig. 7) The glyoxal hydrate (IV) prepared from 1 g of triamcinolone acetonide was suspended in 250 mL of 0.1 N NaOH. The suspension was stirred at room temperature for two hours. HPLC analysis showed that the glyoxal hydrate was completely converted to the glycolic acid (V). The solution was filtered and the filtrate was acidified by adding 1 N HCl dropwise until the pH of the solution was approximately 3. The product was extracted with 250 mL of ethyl acetate and the ethyl acetate layer was washed with water. The ethyl acetate was removed under vacuum using a rotary evaporator. The residue was dissolved in a minimum amount of methanol. To the solution was added water slowly until no further precipitation occured. The solid material was filtered and dried under vacuum at 60°C for two hours. The FAB mass spectrum showed a protonated molecular ion (M+H)⁺ at m/z 451. The El mass spectrum also showed (M+H)⁺ at 451 and peaks at 435 (M-CH3)⁺ and 430 (M-HF)⁺. The carbon NMR spectrum of the compound showed C-20 and C-21 at 71 (doublet) and 173 ppm(singlet), respectively. The proton NMR spectrum showed an acid proton at 12.4 ppm and C-20 non- exchangeable proton at 4.3 ppm. The mass and NMR spectra agreed with the structure V.

Example 4 The etianic acid derivative (VI) To a solution of 2 g of triamcinolone acetonide in 300 mL of methanol was added a solution of 4 g of periodic acid in 400 mL of water. The aqueous methanolic solution was left at room temperature for two days in the dark. The methanol was removed under vacuum using a rotary evaporator and the residue was suspended in 200 mL of water. To the aqueous solution was added 1 N NaOH dropwise until the pH of the solution was 8-9. The solution was filtered and the filtrate was shaken with ethyl acetate (2x30 mL). The ethyl acetate layer was discarded. The aqueous layer was acidified by dropwise addition of 1 N HCl until the pH of the solution was 2-3. The product was extracted with ethyl acetate (3x100 mL). The ethyl acetate layer was dried over 200 mg of anhydrous Na₂SO₄ and removed under vacuum using a rotary evaporator. The residue was recrystallized from methanol. The white solid was dried under vacuum at 60°C for two hours.

The El mass spectrum of the compound showed a molecular ion atm/z 420 and peaks at 405 (M-CH3)⁺ and 400 (M-HF)⁺. The carbon NMR spectrum showed a resonance of C-20 at 174 ppm in place of 210 ppmin I and loss of C- 21 at 66 ppm in I. The proton NMR spectrum showed an acid proton at 12.8 ppm and loss of C-21 protons in I. The mass and NMR spectra agreed with the structure VI. The stability-specific HPLC method was used to follow the extent of the degradation of triamcinolone acetonide in aqueous solutions (Fig. 1). Because of the low solubility of the drug in aqueous solutions, the concentrations of the degradation products were not enough for isolation and identification for most of the degradates. Therefore the approach taken in elucidating the degradation profile of the steroid had two stages. The first stage involved the partial identification of the degradates in the degraded sample solutions by molecular weight determination using an LC-MS technique. The second stage required the synthesis and characterization of potential degradation products followed by identification of such compounds in the degraded solutions by comparing their molecular weights and HPLC retention times.

The glyoxal synthesized from I was characterized as a hydrate(IV) by elemental analysis, NMR and FAB mass spectra. However, the El mass spectrum of the compound yielded the highest m/z peak corresponding to the non-hydrated aldehyde, due to the loss of water during ionization of the sample. The co-injection of a degraded sample of I and the synthetic compound displayed one peak at 8.3 minutes. The ion-spray mass spectra of the synthetic and degraded samples produced identical peaks at m/z 451 (M+H)⁺ and 492 (MH⁺+CH₃CN) in the mobile phase. Thus the degradate was identified as the steroidal glyoxal. It exits in the hydrated form (IV) in aqueous solutions as well as in the solid state. The glyoxal hydrate peak appears first in degrading solutions of the drug in neutral and alkaline pH regions (Fig. 1 b, c, d).

In neutral and basic solutions, the primary degradation pathway is autoxidation of the primary alcoholic group at C-21 as in other corticosteroids. The major degradation product is the steroid glyoxal hydrate (IV) as shown before (Fig. 2). The product further degrades to V in alkaline solutions. As the pH of the solution decreases below 4, this oxidative degradation pathway is absent (Fig. 1a). Instead, the cyclic ketal of triamcinolone acetonide is cleaved yielding triamcinolone (II).

The rate of disappearance of triamcinolone acetonide exhibited a dependency on the buffer concentration at constant pH and ionic strength (Fig. 3). In the absence of EDTA, a plot of the rate constant against the buffer concentration is curved and the rate constant levels off at high buffer concentrations. In the presence of EDTA, the rate constant is independent of the buffer concentration. The results strongly indicate that the buffer components themselves have no catalytic influence, but that the rate increase is due to the catalytic effect of trace-metal contaminants present in buffer components. Similar observations were made in the degradation of prednisolone (Oesterling and Guttman, 1964) and hydrocortisone (Hansen and Bundgard, 1979). The effect of EDTA concentration on the degradation rate constant is shown in Fig. 4. The results show that EDTA even in a very low concentration has a profound inhibitory effect, reaching the maximum inhibition level at the concentration of approximately 1x10^'5 M.

Cupric ion has been known to catalyze the oxidative degradation of 21- hydroxy corticosteroids. The addition of cupric salt to a borate buffer increased the degradation rate (Fig. 5), with the maximum rate at the concentration of 5x10^"6 M CuS0₄. Ferric and nickel ions exhibited negligible catalytic effects. The degradation of triamcinolone acetonide was studied in aqueous solutions over the pH range of 1-10 at 70°C and ionic strength of 0.1. At constant pH and temperature, the degradation followed an apparent first-order process under all experimental conditions. The results are seen as the plot of logarithm of the rate constant versus pH (Fig. 6). In the pH region below 3, the log k-pH profile shows a straight line with a slope of approximately -1 indicating that the degradation appears to be a specific-acid-catalyzed process. The same straight line was observed when the degradation proceeded in the presence of cupric ion or EDTA. In this pH region, the cleavage of the cyclic ketal is the dominant reaction yielding triamcinolone(ll). The non-oxidative cleavage reaction is not dependent on metal catalysis. Therefore, it is expected that incorporation of cupric ion or EDTA into the solutions would have no effect on the degradation rate as shown in Fig. 6. At pH above 4, the predominant degradation product was the oxidation product (IV). Between pH 4 and 7, the profile shows a straight line with a slope of approximately +1 indicating specific-base catalysis. In this pH region, incorporation of 1 x10^'5 M of CuSO₄ into the solutions did not have any effect on the degradation rate, whereas 5 x 10^"4 M EDTA decreased the degradation rate two orders of magnitude. This observation indicates that the trace metal ions present in the buffer components catalyze the degradation to the maximum and, therefore, additional cupric ion has no further catalyzing effect.

In the pH region above 7, a pH-independent plateau is reached, followed by a straight line portion with a slope of approximately +1 between pH 8 and 10. Between pH 7 and 10, the experimental points are more scattered. Fig. 3 shows that the rate constant (1.2x10^'5 sec^'1) extrapolated to zero buffer concentration coincide with that obtained in the presence of EDTA. Thus, on eliminating the buffer catalysis (trace-metal catalysis in buffer components),the log k-pH profile would be superimposable on that determined in the presence of EDTA.

The log k-pH profile in the present study shows no plateau when CuSO₄ or EDTA is incorporated into the solutions and the log k increases with increasing pH with a slope of +1. Cupric ion enhances the rate, whereas EDTA retards the rate. This observation strongly indicates that the plateau is not due to the ionization of steroid molecules but to a different degree of catalysis by trace-metal-ion contaminants present in the buffer components. The rate expression for the copper-catalyzed and the metal-sequestered reactions is given by k = k_H [H + k_o + ko_H [OH-] where k is the observed rate constant, k_H and k_0H are the respective second- order rate constants and k₀ is the water-catalyzed or spontaneous reaction rate

constant. The values of k_H, k_0H and k₀ were estimated from Fig. 6 to be 3.0x10^'4 sec^"1 M^'1, 15.9 sec^"1 M^'1 and 4.6x10^"8 sec^"1, respectively, for the copper- catalyzed degradation reaction, and 3.0x10^"4 sec^"1 M^'1, 0.11 sec^"1 M^"1 and 2.6x10^"8 sec^"1 , respectively, for the metal-sequestered reaction. It is noteworthy that the cupric-ion-catalyzed degradation is 150 times as fast as that of the metal-sequestered degradation in neutral and alkaline pH regions. The steroidal glyoxal (III) undergoes further degradation to the corresponding glycolic acid (V) in alkaline solutions. It is seen that the formation of V goes through an induction period (Fig. 2).A small amount of the corresponding etianic acid (VI) was observed in the degradation of I in alkaline solutions (Fig. 2). This result indicates that a small amount of the steroid undergoes cleavage between C-20 and C-21 during the oxidation. It is likely that VI could have been formed by oxidative cleavage of the glyoxal (III). The experimental data described above and shown in the figures demonstrate the stability and degradation-resistant properties of embodiments and preferred embodiments according to the present invention under accelerated laboratory conditions. These properties provide long term stability to the aqueous triamcinolone acetonide compositions of the present invention under normal use, at ambient temperature for storage times awaiting use by the distributor, pharmacist and patients. It is expected that the shelf life, required for the commercial acceptability of the present compositions, will be at least 6 months to one or more years, at room temperature, that is about 25 degrees C.

The compositions of this invention are useful in the treatment of patients suffering from certain medical disorders. For example, compounds within the present invention are useful as bronchodilators and asthma-prophylactic agents, e.g. for the treatment of inflammatory airways disease, especially reversible airway obstruction or asthma, and for the treatment of other diseases and conditions characterized by, or having an etiology involving, morbid eosinophil accumulation. As further examples of conditions which can be ameliorated may be mentioned inflammatory diseases, allergic rhinitis, adult respiratory distress syndrome. A special embodiment of the therapeutic methods of the present invention is the treating of asthma.

In practice compositions of the present invention may generally be administered by inhalation and may be presented in forms permitting administration suitable for use in human or veterinary medicine. These compositions may be prepared according to the customary methods, using one or more pharmaceutically acceptable adjuvants or excipients. The adjuvants comprise, inter alia, diluents, sterile aqueous media and the various non-toxic organic solvents. The compositions may be presented in the form of aqueous solutions or suspensions, and can contain one or more agents chosen from the group comprising surfactants, flavorings, colorings, or preservatives in order to obtain pharmaceutically acceptable preparations. Suitable compositions containing the compounds of the invention may be prepared by conventional means. For example, compounds of the invention may be dissolved or suspended in a suitable carrier for use in a nebulizer or a suspension or solution aerosol. The percentage of active ingredient in the compositions of the invention may be varied, it being necessary that it should constitute a proportion such that a suitable dosage shall be obtained. Obviously, several unit dosage forms may be administered at about the same time. The dose employed will be determined by the physician, and depends upon the desired therapeutic effect, the route of administration and the duration of the treatment, and the condition of the patient. In the adult, the doses are generally from about 0.001 to about 50, preferably about 0.001 to about 5, mg/kg body weight per day by inhalation. In each particular case, the doses will be determined in accordance with the factors distinctive to the subject to be treated, such as age, weight, general state of health and other characteristics which can influence the efficacy of the medicinal product.

The products according to the invention may be administered as frequently as necessary in order to obtain the desired therapeutic effect. Some patients may respond rapidly to a higher or lower dose and may find much weaker maintenance doses adequate. For other patients, it may be necessary to have long-term treatments at the rate of 1 to 4 doses per day, in accordance with the physiological requirements of each particular patient. Generally, the active product may be administered orally 1 to 4 times per day. It goes without saying that, for other patients, it will be necessary to prescribe not more than one or two doses per day.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than the specification, as indicating the scope of the invention.

Claims

1. A pharmaceutical composition comprising a therapeutically effective amount of triamcinolone acetonide in admixture with an aqueous pharmaceutical acceptable carrier providing said composition with properties resistant to triamcinolone acetonide degradation in the presence of contaminants.

2. The composition according claim 1, wherein said contaminants comprise a trace metal.

3. The composition according to claim 2, wherein said trace metal comprises copper ion.

4. The composition according to claim 1, further comprising a metal- sequestering agent.

5. The composition according to claim 4, wherein the metal- sequestering agent comprises EDTA.

6. The composition according to claim 1 having a pH of about 3.5 to about 4.5.

7. The composition according to claim 6 having a pH of about 4.

8. The composition according to claim 4 having a pH of about 3.5 to about 6.5.

9. The composition according to claim 8 having a pH of about 5.

10. A method for treating inflammation comprising administering to a patient suffering from inflammation an effective amount of the composition according to claim 1.

11. A method for treating inflammation comprising administering to a patient suffering from inflammation an effective amount of the composition according to claim 4.

12. A method for treating inflammation comprising administering to a patient suffering from inflammation an effective amount of the composition according to claim 8.

13. A composition according to claim 1 wherein the ionic strength is about 0.1.

14. A pharmaceutical composition comprising a therapeutically effective amount of triamcinolone acetonide in admixture with an aqueous pharmaceutically acceptable carrier providing said composition with properties resistant to triamcinolone acetonide degradation in the presence of contaminants, wherein the pH of the composition is between about 4.9 and 5.1 , comprises an effective degradation inhibiting amount of EDTA.

15. A pharmaceutical composition according to claim 14 wherein the degradation inhibiting amount of EDTA is at least about 1 x 10^'5 M.