Effect of the buffer system on stability of tetracaine hydrochloride 0 . 5 % eyedrops

The estimation of stability is an important segment of the process of development of pharmaceutical dosage forms. The choice of optimal formulation is supported by good stability performance. Also stability tests are required for defining the expiry period and storage conditions (1,2). Tetracaine hydrochloride, as a local anaesthetic agent, is indicated in ophthalmology for diagnostic purposes as well as for surgery and post surgery treatment (3). Eyedrop formulations contain 0.25-1%(w/v) of tetracaine hydrochloride in aqueous media. It should be prepared with respect of adjusting pH. The pH value is a critical formulation parameter not only because of the physiological tolerance of the eye (4), but also becouse of the pH-dependent stability of tetracaine hydrochloride (5,6). Due to its ester structure, tetracaine hydrochloride is stable at a lower pH (7). As an ester-type compound, tetracaine hydrochloride undergoes hydrolysis, which results in the formation of n-buthyldimethylaminobenzoic acid and dimethylaminoethanol (5,8,9). Different buffer systems could be used for adjusting the pH value of aqueous solutions of tetracaine hydrochloride intended to be used as eyedrops in a range from 3.7 to 6.5 (5). Generally, the selection of buffer agent should be made with respect not only to adjusting and maintaining the pH of the solution, but also to the type and concentration of buffer agent. Since the hydrolytic degradation of the drug is acid and base catalysed, it is obvious that the buffer concentration should be kept as low as possible to diminish this catalytic effects (1,10). Therefore, in the development stage of the liquid pharmaceutical dosage form, it is necessary to evaluate the stability of the drug substance in dosage form in terms of the catalytic effect of the buffer system. For this purpose, the drug degradation rate should be followed as a function of the type of buffer system in the formulation, while the pH and ionic strength are kept constant. The aim of this study was to determine whether the type of buffer system (phosphate, acetate, and borate) affects the stability of tetracaine hydrochloride in 0.5%(w/v) eyedrop formulation with pH 5.4 and a buffer concentration of 0.06M. Effect of the buffer system on stability of tetracaine hydrochloride 0.5% eyedrops


Introduction
The estimation of stability is an important segment of the process of development of pharmaceutical dosage forms.The choice of optimal formulation is supported by good stability performance.Also stability tests are required for defining the expiry period and storage conditions (1,2).
Tetracaine hydrochloride, as a local anaesthetic agent, is indicated in ophthalmology for diagnostic purposes as well as for surgery and post surgery treatment (3).Eyedrop formulations contain 0.25-1%(w/v) of tetracaine hydrochloride in aqueous media.It should be prepared with respect of adjusting pH.
The pH value is a critical formulation parameter not only because of the physiological tolerance of the eye (4), but also becouse of the pH-dependent stability of tetracaine hydrochloride (5,6).Due to its ester structure, tetracaine hydrochloride is stable at a lower pH (7).As an ester-type compound, tetracaine hydrochloride undergoes hydrolysis, which results in the formation of n-buthyldimethylaminobenzoic acid and dimethylaminoethanol (5,8,9).Different buffer systems could be used for adjusting the pH value of aqueous solutions of tetracaine hydrochloride intended to be used as eyedrops in a range from 3.7 to 6.5 (5).Generally, the selection of buffer agent should be made with respect not only to adjusting and maintaining the pH of the solution, but also to the type and concentration of buffer agent.Since the hydrolytic degradation of the drug is acid and base catalysed, it is obvious that the buffer concentration should be kept as low as possible to diminish this catalytic effects (1,10).Therefore, in the development stage of the liquid pharmaceutical dosage form, it is necessary to evaluate the stability of the drug substance in dosage form in terms of the catalytic effect of the buffer system.For this purpose, the drug degradation rate should be followed as a function of the type of buffer system in the formulation, while the pH and ionic strength are kept constant.
The aim of this study was to determine whether the type of buffer system (phosphate, acetate, and borate) affects the stability of tetracaine hydrochloride in 0.5%(w/v) eyedrop formulation with pH 5.4 and a buffer concentration of 0.06M.

Stability testing
Long-term testing at 26 0 C (II climate zone ) (2) and short-term, accelerated tests at three elevated temperatures 45, 50 and 60 0 C were used.Macroscopic evaluation (Ph.Eur.3),pH (Ph.Eur.3),sterility (Ph.Eur.3) and drug content (USP XX) were followed at certain time intervals over a period of 168 days.The obtained data were statistically processed.One way analysis of variance and the Scheffe method were used for processing of pH data.Rate constants of hydrolytic degradation of tetracaine hydrochloride at each temperature, the influence of temperature on the rate constant, as well as the t 90% at 26 0 C were estimated (1,2): In order to detect the degradation products, a HPLC method in accordance to the tetracaine hydrochloride eyedrops monograph (USPXXIII) was applied.

Results and discussion
Stability studies on prepared eyedrop formulations of tetracaine hydrochloride 0.5% (w/v) indicated certain changes in the parameters followed.
The long-term study at 26 0 C, did not show any macroscopic change in all three eyedrops formulations over a period of 168 days.Exposure to an elevated temperature (accelerated stability tests) caused very slight red-brownish colouring of the solutions (table 2).The coloration of formulation 1, containing phosphate buffer, was registered after 84 days of storage at 50 0 C and 56 days at 60 0 C, while in the presence of acetate buffer it was noticed after 56 days (45 0 C), 28 days (50 0 C) or 14 days (60 0 C).Formulation 3, prepared using a borate buffer, showed slight coloration only at the highest temperature of 60 0 C after 56 days.The red-brown colour is probably a result of decomposition of n-buthylaminobenzoic acid, induced by extreme temperature (5,11).
The follow up of the pH value is an important parameter since the decomposition of tetracaine hydrochloride results in pH decrease.Table 3 shows the statistical parameters obtained from pH data by use of one way analysis of variance and the Scheffe method.A statistically significant change of pH value within a storage period of 168 days at 26 0 C was registered only in the eyedrop solution containing borate buffer.Exposure to an elevated temperature facilitated the pH changes in all samples.During the accelerated stability tests, acetate buffer provided the most stable pH value.Taking into consideration the corresponding buffer capacities of the three buffer systems used (table 4), the 0.06M borate buffer has a very low buffer capacity (10 -6 M HCl), while the same molarity of acetate or phosphate buffer provides higher buffer capacities (10 -2 and 10 -4 M HCl respectively).Therefore, an acetate buffer enables the most efficient maintaining of the pH value, while the buffer capacity of borate buffer was insufficient.
Changes in the content of tetracaine HCl during the stability testing is presented in figure 1. Table 5 shows the rate constants of hydrolytic degradation of tetracaine hydrochloride at each temperature, as well as the t 90% at 26 0 C.
As can be seen, the slowest rate of hydrolytic degradation at each temperature occurred in the sample containing borate buffer (table 5).The time required for degradation of 10% of drug substance (t 90% ) at 26 0 C was 318 days.This could be explained by: (a) an insufficient buffer capacity of borate buffer (table 4) that allowed a decrease of pH value and (b), an increased stability of tetracaine hydrochloride at a lower pH on the other hand.The stability of tetracaine hydrochloride in formulations containing phosphate and acetate buffer is similar

Conclusion
The stability of the tetracaine 0,5%(w/v) eyedrop solution with pH 5,4 and a constant buffer concentration of 0.06M is a function of the buffer system used in formulation.Incorporation of a phosphate or acetate buffer 0.06M with pH 5.4 in formulation of 0,5%(w/v) tetracaine hydrochloride eyedrops provided satisfactory stability (t 90% 181 and 157 days respectively).Although the degradation rate of tetracaine hydrochloride in the formulation containing 0,06M of borate buffer was twice as slow (t 90% 318 days), this formulation is not acceptable because its poor buffer capacity was insufficient to maintain the pH value.

Table 3 .
Statistical analysis of pH value data -one way analysis of variance and Scheffe method

Table 2 .
Macroscopic changes of samples 1-3 exposed to different temperatures

Table 5 .
Degradation rate constant (k) and t 90% of tetracaine hydrochloride in eyedrop formulations 1-3 at different temperatures