Design and development of colon specific microspheres for chronotherapy of hypertension.

The purpose of this research was to design, and evaluate multiparticulate systems for Chronotherapeutic delivery of beta blocker containing biodegradable polymers coated with pH sensitive polymers in hypertension. Chitosan was used as a carrier for drug delivery and Eudragit S100 was used as an enteric coating polymer. 3(2) full factorial design was employed to optimize the proper formulation for Chronotherapeutic drug delivery.

Atherapeutic system that would synchronize the drug delivery with the circadian variation in periods of increased risk is highly desirable for chronotherapy. This can be achieved by a bed time administration of a drug delivery system which, with a delayed start of drug release, can provide adequate protection in the early morning. In this context, Colon specific drug delivery systems have been used for chronotherapeutic drug administration.[1]

Recently much emphasis being laid on the development of multiparticulate dosage forms in comparison to single unit systems because of their potential advantages like increased bioavailability, reduction in the risk of systemic toxicity; local irritation, predictable gastric emptying and retention in the ascending colon for a relatively prolonged period of time. Moreover, smaller particle size enables these systems to pass through the GI tract easily, leading to less inter-and intra subject variability.[2] Following bedtime administration, microspheres are expected to maintain a low drug plasma concentration overnight when the cardiovascular risks are reported to be the minimum and release the optimal concentrations in the morning between 6 and 12 h when the ischemic risk is found to be the maximum. Carvedilol undergoes high pre-systemic metabolism (oral bioavailability is 25-35%) due to the inhibitory effects of cytochrome P-450 (CYP3A4) and P-glycoprotein, both found in the small intestine.[34]

Materials and Methods

32 full factorial design was used for the optimization. The levels for the selected independent variables were determined from the preliminary batches. A statistical model incorporating interactive and polynomial terms was utilized to evaluate the responses.

Y=b0 + b1X1 + b2X2 + b12X1X2 + b11X12 + b22 X22

On the basis of the preliminary trials a 32 full factorial design was employed to study the effect of independent variables i.e. drug-to-polymer-to-polymer (X1) and the stirring speed (X2) on dependent variables like particle size, drug entrapment efficiency, the time required for 50% drug release (t50) and drug released up to 5 hr (Y5) and 12 hr (Y12). The expected in vitro release pattern selected for the colonic delivery was not more than 10% of the drug release up to the end of small intestine (5 hr) and more than 85% of drug release up to 12 hrs. In factorial design batches C1 to C9, a chitosan concentration was fixed at 3% w/v. The microspheres were prepared using emulsion cross linking method. Microspheres thus obtained were recovered from the medium and dried subsequently and stored properly for the further studies. The concentration of cross linking agent and stirring speed were varied in batches C1 to C9. Potential variables such as concentration of chitosan solution, polymers to drug ratio (2:1) and amount of dispersion medium (500 ml) were kept constant. In vitro drug release studies in simulated gastro intestinal fluids showed a burst release pattern in the initial hour necessitating microencapsulation with Eudragit S100 by solvent evaporation technique. The effect of different coat/core ratio on particle size, drug entrapment efficiency and in vitro drug release were studied.

Results and Discussion

The chitosan microspheres were successfully prepared by emulsion cross linking method. The microspheres of all the batches of the factorial design were spherical and free flowing. The average particle size of different chitosan microsphere formulations was found to be in the range of 9.93 μm- 18.48 μm and showed good correlation co-efficient (0.9932). Results indicate that the effect of X1 (stirring speed) is more significant than X2 (amount of cross linking agent). Means, as the stirring speed increased, the particle size decreased. Similarly, when higher level of glutaraldehyde was used cross linking favoured and hence slower drug release was observed compared to the other batches. The entrapment efficiency of different formulations was found to be between 66.65 to 92.07% and showed good correlation co-efficient (0.9293). The effect of stirring speed on entrapment efficiency showed that optimum speed should be 1000 rpm. In vitro dissolution of all the batches indicates that the burst release pattern in the initial hour. Within 5 hr 70 to 90% of drug was released [Figure 1].

Figure 1

Cumulative in vitro drug release of carvedilol from formulations C1 to C9

Thus, only biodegradable polymers were not satisfactory for colonic delivery. The burst release may be due to solubility of chitosan in the acidic pH. In order to prevent the drug release in stomach and small intestine these chitosan microspheres were encapsulated with Eudragit S100, which shows solubility at a pH ≥ 7. Since formulation C4 showed high drug loading and drug release pattern, it is selected for microencapsulation process. Chitosan microspheres (C4) were microencapsulated with Eudragit S100 to achieve colonic delivery of carvedilol. The effect of core-coat ratio on Eudragit S100 microencapsulated chitosan microspheres was studied and found that the particle size was increased from 137.87 to 154.33 μm with increasing the core-coat ratio from 1:8 to 1:12. The entrapment efficiency of microencapsulated formulations varied between 90-95% with increasing core-coat ratio. The in vitro drug release studies of various Eudragit coated chitosan microspheres were performed in simulated gastro intestinal fluids. The effects of core:coat ratio on in vitro drug release were studied. Results showed that 8-12% of drug was released within initial 4 hrs and increased thereafter when the formulations were exposed to pH which is above solubility of Eudragit S100. The best formulation then subjected to in vitro drug release in presence of rat ceacal contents.

Conclusion

A result of release studies indicates that Eudragit S100 coating offers a high degree of protection from premature drug release in the stomach and small intestine. Eudragit coated chitosan microspheres deliver most of the drug load in the colon, an environment rich in bacterial enzyme that degrade the chitosan and allow drug release to occur at the desired site after proper transit time. Thus, the designed formulation is potential systems as multiparticulate for the chronotherapy of hypertension.