Preparation and Evaluation of Microwave-Assisted Acetylation of Plectranthus edulis (Ethiopian Potato) Starch as a Sustained Release Excipient in Tablet Formulations

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Addis Abeba University


Starch has been the subject of intensive research over many decades due to the fact that native starches are diverse, biodegradable, applications are enormous and modifications to starch are numerous. In this study, Plectranthrus edulis (P.edulis) starch has been chemically modified under microwave heating with the aim of determining its potential for sustained release application. It was chemically modified by microwave-assisted acetylation using acetic anhydride as acetylating and sodium hydroxide as a catalyst. The reaction conditions were optimized for maximizing the degree of substitution (DS) and the DS selected were 1.77 and 0.44. The prepared starch acetate (SA) were evaluated and compared with the native P. edulis starch and standard excipients in terms of various physicochemical characteristics including the chemical structural changes, powder properties, swelling power (SP) and solubility, moisture sorption pattern, compactibility and drug release properties. The Fourier transform infrared (FTIR) spectra of the modified starches verified the structural change in the starch molecules resulting from acetylation. The x-ray diffraction (XRD) and differential scanning calorimetry (DSC) studies shows acetylation reduced the formation of intermolecular hydrogen bonds and thereby resulted in the slight destruction of the ordered crystalline structure. The results of the powder properties (Carr’s index and Hausner ratio, angle of repose and Kawakita analysis) indicated that acetylation improved the flow property of P. edulis starch. Moreover, microwave-assisted acetylation appreciably increased starch SP at a lower DS 0.44 than higher DS 1.77. Modification also increased solubility of native P.edulis starch. The water uptake of the P.edulis SA powders slightly increased with relative humidity (RH) at lower values but increased significantly at about 100% RH. The effect of DS on the compactibility and drug release retardant effect were also evaluated by using anhydrous theophylline as model drug. The plain tablets of SA DS 1.77 shows higher tensile strength than the SA DS 0.44 tablets and it also shows significant tablet properties than native P.edulis starch tablet which reflected the better compactibility of the former modified starch. Plain tablets of SA DS 0.44 and microcrystalline cellulose (MCC) (Avicel PH 101) were completely disintegrated within few min while those made of SA with DS 1.77 and ethyl cellulose (EC), did not disintegrate at all during the measurement time of 2 h. The tensile strength (19.81 Kg/cm ) of tablets containing 20% of theophylline and 79.5% P. edulis SA DS 1.77 with crushing strength of 150.2 N was significantly higher than tablets with crushing strength of 51.1 N because the tablets with crushing strength of 51.1 N has the highest porosity when compared to tablet with crushing strength of 150.2 N. The tablets of SA DS 1.77 has the highest tensile strength and the lowest porosity than tablets of SA DS 0.44 at the same percentage composition of theophylline and P. edulis SA which indicate the effect of DS. The in vitro release rate was found to be significantly related to the DS and crushing strength, hence more sustained release (SR) of the drug was observed with the matrix incorporated with SA of higher DS and highest crushing strength. The drug release rate of theophylline changed from rapid release to sustained release as the DS increased from 0.44 to 1.77 and as crushing strength increased from 51.1 N to 150.2 N. The drug release rate also increased as the SA DS 1.77 percentage decreased from 79.5 to 59.5% (w/w). Further the drug release data were fitted to five kinetic models. Among the various drug release kinetic models applied, the best linearity was found with Higuchi’s plot (R 2 = 0.9988) indicating the release of drug from the matrix as a square root of time dependent process based on Fickian diffusion and based on Korsemayer–Peppas model the release mechanism observed was anomalous diffusion (diffusion coupled with swelling). Therefore, it was concluded that P. edulis SA could have a potential for use as a sustained release excipient.



Degree of Substitution, Microwave-assisted Acetylation, P. edulis Starch, Starch Acetate, Sustained Release