The Effect of Core and Coating Composition on Drug Release from Directly Compressed Time-Controlled Release Tablets
The authors prepared and tested press-coated tablets with various weight ratios of ethylcellulose to hydroxypropylcellulose (HPC) and various ratios of two different batches of HPC as an outer coating shell and fillers in core tablets. The tablets were examined for changes in time lag and release patterns of salbutamol sulfate.
Press-coated tablets containing Starlac as a filler in the core tablet showed drug release after the coating layer eroded
because upon contact with the dissolution medium, Starlac starts to erode and the Ac-Di-Sol present was unable to generate
sufficient internal pressure to cause a bursting of the outer-coating layer. Press-coated tablets containing Avicel PH 101
as a filler in the core tablet showed drug release after some erosion of outer shell and when the dissolution medium hydrates
the core tablet. The Ac-Di-Sol in the core tablet swelled and generated sufficient internal pressure to burst the outer shell.
Obviously, the erosion of the outer coating shell of core tablet containing Starlac as a filler was higher than that of tablets
containing Avicel PH 101 as the filler (see Figure 6). The order of the time lag changed according to the weight ratio of
EC N 22 F to HPC EXF mixture containing Starlac as a filler as follows: 87.5:12.5% (9.5 h), 75:25% (8.5 h), 50:50% (7 h),
25:75% (6.5 h), and 12.5:87.5% (5.5 h). The order of the time lag changed according to the weight ratio of EC N 22 F to HPC
EXF mixture containing Avicel PH 101 as filler as follows: 87.5:12.5% (8.5 h), 75:25% (7.5 h), 50:50% (6.5 hrs), 25:75% (6
hrs), and 12.5:87.5% (5 h).
Gellable polymer (HPC HF) combined with erodible polymer (HPC EXF). Figure 7 and Figure 8 show the increase in lag time and decrease in the release rate of salbutamol with increasing weight
ratio of HPC HF to HPC EXF. These figues confirm barrier layer effectiveness and core composition effectiveness in delaying
the release starts. The press-coated tablet with erodible shells (HPC EXF) show the same release kinetics of the cores once
the time lags are omitted. When the gellable shells are used instead, the release start can be delayed, and the release rate
can be decreased dramatically (12). Tablets containing a 12.5:87.5% w/w ratio of HPC HF to HPC EXF formed a gel-like structure
upon contact with dissolution medium. Because of the high percentage of HPC EXF, however, this structure was not very firm
and eroded and gelled simultaneously. Tablets containing Avicel PH 101 as a core-tablet filler showed drug release by a bursting
of the inner core followed by a rupturing of the outer shell. The pressure generated in the core tablet was enough to rupture
the outer shell after some erosion of the outer shell and led to a 7-h lag time followed by a release lasting 1 h.
Tablets containing Starlac as a filler in the inner core showed a slight delay in drug release and release rate. Upon contact
with the dissolution medium, Starlac present in the core tablet eroded, and the pressure generated in the inner core was unable
to rupture the outer shell. The drug was released only upon complete erosion of the outer shell. Tablets containing Starlac
also showed a slightly longer lag time compared with those containing Avicel PH 101 as a filler in the core tablet. Tablets
containing a combination of HPC HF and HPC EXF in a 25:75% w/w weight ratio showed almost the same release pattern of salbutamol
sulfate after a time lag. The difference was only in the delay in the time lag to salbutamol sulfate release. The erosion
of the outer coating shell of the core tablet containing Starlac as a filler was greater than that of the tablets containing
Avicel PH 101 as a filler (see Figure 9).