Abstract: An attempt has been made to enhance the solubility and stability of two poorly-soluble drugs, clarithromycin and prednisolone by forming their combinations with three widely consumed non-prescription drugs: paracetamol, caffeine and ibuprofen. For solubility enhancement, three methods; Solution phase studies, solid dispersions and physical mixtures were used. It is seen that paracetamol produced significant enhancement in solubility of both clarithromycin and prednisolone by all the three methods. A comparison of different methods showed that solid dispersion method produced maximum enhancement for both the drugs. However, as compared to clarithromycin, the magnitude of enhancement was smaller in the case of prednisolone. With increase in ionic strength of the medium, a small decrease in solubility was observed in each case. Paracetamol was also found to be most effective combination drug in enhancing the stability of clarithromycin as well as prednisolone.
INTRODUCTION
Solubility and stability issues are two major formulation obstacles hindering the development of therapeutic agents (Cappello et al., 2006; Yang et al., 2007). Although a number of solubilizing and stabilizing agents are available, each of these has a number of significant disadvantages (Nuijen et al., 2001; Yalkowsky et al., 1998). The idea of combining two or more drugs with complementary mode of action is to produce additivity of the desired therapeutic effect, but not of the side effects (Tallaride et al., 1997). Combination Therapy also plays an important role in the development of improved drug delivery devices (Ammar et al., 1997; Lim and Go, 2000).
Clarithromycin (6-O-methylerythromycin), is a semi-synthetic macrolide antibiotic and prednisolone is a synthetic adrenal corticosteroid with potent anti-inflammatory properties. Both the drugs exhibit poor aqueous solubility. Solubility enhancement has broad implications in the delivery of poorly-soluble drugs (Zhao et al., 1999; Strickley, 2004; Millard et al., 2002). Potential absorption problems occur if the aqueous solubility of a drug is less than 1 mg mL-1. Stability studies are important since drug degradation not only leads to decrease in the therapeutic activity, it may also result in the appearance of a toxic degradation product upon storage of formulation (Carstensen, 1990). Yonemochi et al. (1999) have studied the physico-chemical properties of amorphous clarithromycin produced by grinding and spray drying. Cyclodextrin-complexation and liposome-encapsulation are some of the other approaches reported in the literature for solubility enhancement of clarithromycin (Salem and Duzqunes, 2003). Increase in the solubility of prednisolone in the presence of gelatin has been reported (Kallinteri and Antimisiaris, 2001). In this work efforts have been made to enhance the solubility and stability of poorly-soluble drugs, clarithromycin and prednisolone by combination with widely consumed non-prescription drugs: paracetamol, caffeine and ibuprofen.
MATERIALS AND METHODS
The study was conducted at the Department of Chemistry, Panjab University, Chandigarh, India during the year 2006. Clarithromycin, prednisolone, caffeine, paracetamol and ibuprofen were obtained as gift samples from various manufacturers. All other reagents were of analytical grade and were used without further purification. Water used was double distilled in an all glass apparatus. 0.1 M phosphate buffer (pH 7.4) was prepared by mixing 19 mL of 0.2 M NaH2PO4 with 81 mL of 0.2 M Na2HPO4 and diluting the mixture to 200 mL with water.
Drug Analysis
Estimation of clarithromycin was based on the formation of a red colored
chromogen with ferric chloride and 1,10-phenanthroline (Reddy et al.,
2003; Sivasubramanian et al., 2004). Three hundred μM stock solution
of clarithromycin was prepared in standard 0.1 M HCl. Five dilutions of the
drug stock solution in the concentration range 10-300 μM were used for
analysis. Three milliliter aliquot of each standard drug solution was mixed
with 1.5 mL of ferric chloride (0.033 M) and 1.5 mL of 1,10 phenanthroline (0.1
M). The solutions were maintained at 40°C for 15 min. They were then cooled
and 1 mL of ortho-phosphoric acid was added to each solution. The absorbance
of the red colored chromogen was measured at 510 nm against a reagent blank.
The absorbance of the reaction mixture remained stable for 4 h. The extinction
coefficients, obtained from the absorbance versus drug concentration plots were
used for drug estimation. Prednisolone could be analyzed directly by preparing
standard drug solutions in phosphate buffer (pH 7.4) and using ultraviolet absorption
spectrophotometric method for drug analysis.
Solubility Enhancement
An attempt has been made to enhance the solubility of clarithromycin and
prednisolone by forming their combinations with paracetamol, caffeine and ibuprofen,
using the following three methods: i) Solution phase studies, ii) Solid dispersions,
iii) Physical mixtures.
Solution Phase Studies
0.1 g portions of clarithromycin/prednisolone, taken in sealed conical flasks,
were mixed with 10 mL portions of combination drug (caffeine, paracetamol, ibuprofen)
solutions of increasing concentrations prepared in 0.1 M phosphate buffer (pH
7.4). The mixture was stirred on a magnetic stirrer at 25°C for 24 h. Preliminary
experiments showed that this time interval was sufficient for attainment of
equilibrium. The solutions were filtered through 0.45 μm filter and the
clarithromycin concentration in the filtrate was determined spectrophotometrically
after appropriate dilution. The concentration range for combination drugs was
50-4000 μM in the case of caffeine, 50-200 μM in the case of paracetamol
and 50-500 μM in the case of ibuprofen. The concentration range was lower
in the case of paracetamol and ibuprofen, due to the limited solubility of these
drugs in phosphate buffer.
Solid Dispersions
For preparation of solid-dispersions, 0.05 g of clarithromycin/prednisolone
and 0.05 g of the combination drug were dissolved in 5 mL of chloroform, the
contents were stirred for 1 h and the solvent was evaporated at 40°C using
rotary vacuum evaporator. The solubility of clarithromycin/prednisolone in the
solid dispersion was determined, as described earlier.
Physical Mixtures
For preparation of physical mixtures, equal weights of two drugs were taken
in a passel mortar and thoroughly mixed. The mixture was used for solubility
determination in the same way as described earlier.
Effect of Ionic Strength on Solubility
To study the effect of ionic strength on solubility, the data in solution
phase was also obtained in the presence of 0.15 M NaCl. All other conditions
were same.
Stability Studies
For stability studies, drug solutions were kept at room temperature in dark.
The amount of drug degraded and drug remaining was determined spectrophotometrically
at different time intervals for a period of 42 days. Single drugs as well as
their combinations were studied. The final concentration of clarithromycin/prednisolone
as well as the combination drugs was 100 μM in each case. The solvent used
was 0.1 N HCl in case of clarithromycin and its combinations and phosphate buffer
(pH 7.4) in the case of prednisolone and its combinations.
RESULTS AND DISCUSSION
An attempt has been made to enhance the solubility and stability of poorly soluble drugs, clarithromycin and prednisolone by forming their combinations with three non-prescription drugs: paracetamol, caffeine and ibuprofen.
Solubility Enhancement
For solubility enhancement, three methods; Solution phase studies, solid
dispersions and physical mixtures were used.
Solution Phase Studies
Clarithromycin-drug Combinations
Solubility of clarithromycin alone and in the presence of increasing concentrations
of caffeine (50-4000 μM), paracetamol (50-2000 μM) and ibuprofen (50-500
μM) was determined at 25°C in phosphate buffer (pH 7.4). The maximum
drug concentration used was lower in the case of paracetamol and ibuprofen due
to the limited solubility of these drugs in phosphate buffer. It was found that
all the three drugs, caffeine, paracetamol and ibuprofen enhanced the solubility
of clarithromycin. Solubility was found to increase with increase in the concentration
of combination drug in each case. Significant enhancement in solubility of clarithromycin
was observed in the case of paracetamol. The increase was relatively smaller
in the case of caffeine and least in case of ibuprofen. As compared to clarithromycin
only, the solubility could be enhanced by 20.3, 4.74 and 1.156 times in the
case of paracetamol, caffeine and ibuprofen, respectively (Fig.
1, 2).
| Fig. 1: | Solubility of clarithromycin in the presence of caffeine and paracetamol |
| Fig. 2: | Solubility of clarithromycin in the presence of ibuprofen |
| Fig. 3: | Solubility of prednisolone in the presence of caffeine and paracetamol |
The solubilization efficiency of a solvent should be a function of the relative magnitudes of the solute-solvent and various inter- and intra-molecular solvent-solvent interactions. Since clarithromycin as well as the combination drugs used, contain a large number of hydrogen bond donors and acceptors, hydrogen bonding interactions between clarithromycin and the combination drugs appear to be responsible for the enhancement of aqueous solubility of clarithromycin. The solubility increase in each case was significant up to an optimum concentration of the combination drug required for hydrogen bond formation. For example, in the case of paracetamol, solubility enhancement was significant only up to 500 μM concentration.
Prednisolone-drug Combinations
Solubility of prednisolone alone and in the presence of increasing concentrations
of caffeine, paracetamol and ibuprofen was determined at 25°C in phosphate
buffer (pH 7.4) (Fig. 3, 4). Solubility
was found to increase with increase in the concentration of combination drug.
All the drugs, caffeine, paracetamol and ibuprofen enhanced the solubility of
prednisolone. Again maximum solubility enhancement was observed in case of paracetamol
and least in case of ibuprofen. As compared to prednisolone only, the solubility
could be enhanced by 2.149, 1.27 and 1.018 times in case of paracetamol, caffeine
and ibuprofen, respectively.
| Table 1: | Solubility of clarithromycin and prednisolone in solid dispersion with other drugs |
| *Reported solution phase values are at the highest drug concentration | |
| Fig. 4: | Solubility of prednisolone in the presence of ibuprofen |
Thus, solubility enhancement was lesser in the case of prednisolone as compared to clarithromycin
Solid Dispersion
An attempt has also been made to enhance the solubility of clarithromycin
and prednisolone by preparing solid dispersions of the drug with caffeine and
prednisolone in 1:1 weight ratio. Solid dispersion could not be prepared in
the presence of ibuprofen since a gelly like mass was obtained after the evaporation
of solvent It was found that the solubility of clarithromycin increased from
85 to 615 μM (7.2 times) in the case of clarithromycin-caffeine combination
and 85 to 1835 μM (21.6 times) in the case of clarithromycin-paracetamol
combination. The solubility of prednisolone increased from 82.08 μM 178.18
μM (2.17 times) in the case of prednisolone-caffeine combination and from
82.08 μM to 256.45 μM (3.12 times) in the case of prednisolone-paracetamol
combination (Table 1).
Physical Mixtures
The solubility of clarithromycin and prednisolone in physical mixtures with
the combination drugs, caffeine, paracetamol and ibuprofen was also determined
at 25°C in phosphate buffer (pH 7.4). The solubility enhancement data is
given in Table 1.
Comparison of the Three Methods
The solubility of clarithromycin and prdnisolone in the absence and presence
of combination drugs using the three methods has been compared. It is seen (Table
1) that paracetamol produced significant enhancement in solubility of both
clarithromycin and prednisolone by all the three methods. A comparison of different
methods showed that solid dispersion method produced maximum enhancement for
both the drugs.
| Table 2: | Effect of ionic strength on the solubility of poorly soluble drugs |
| Fig. 5: | Degradation of clarithromycin in the absence and presence of combination drugs |
However, as compared to clarithromycin, the magnitude of enhancement was smaller in the case of prednisolone.
Effect of Ionic Strength
The solubility of clarithromycin and prednisolone was also studied in solution
phase in the absence and presence of 0.15 M NaCl. A small decrease in the solubility
of the drug combination in the presence of sodium chloride appears to be a characteristic
of the ‘salting out’ phenomenon. With increase in ionic strength,
more salt would be competing with the solute for water molecules and this would
effectively reduce the solubility of the dissolved material (Table
2).
Enhancement of Stability
Stability of clarithromycin and prednisolone solutions (100 μM each)
was studied at 25°C in the absence and presence of combination drugs. Absorbance
data at different time intervals was used to calculate the concentration of
drug remaining. The stability of the combination was found to be more than that
of the individual drugs.
| Fig. 6: | Degradation of prednisolone in the absence and presence of combination drugs |
It is seen that both clarithromycin and prednisolone undergo significant degradation (Fig. 5, 6). In a time period of 42 days, degradation of clarithromycin was 77% and that of prednisolone was 87%. However, the combination of these drugs with caffeine, paracetamol and ibuprofen were found to enhance the stability of both clarithromycin and prednisolone. The percentage degradation was only 64, 57 and 35% in the presence of ibuprofen, caffeine and paracetamol, respectively as compared to 77% for clarithromycin only. Similarly in the case of prednisolone, the percentage degradation was 64, 46 and 41%, respectively in the case of ibuprofen, caffeine and paracetamol as compared to 87% for prednisolone only. Again paracetamol was found to be quite effective in enhancing the stability of both the drugs.
CONCLUSIONS
The solubility as well as stability of clarithromycin and prednisolone could be enhanced by combining them with three other drugs; paracetamol/ caffeine/ ibuprofen. Solubility increase was significant only up to a certain optimum concentration of the combination drug. Paracetamol was also found to be the most effective combination drug in enhancing both the solubility and stability of clarithromycin as well as prednisolone. The magnitude of solubility enhancement was relatively smaller in the case of prednisolone as compared to clarithromycin.