By Donald R. Cowsar (auth.), A. C. Tanquary, R. E. Lacey (eds.)
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It is interesting to note that the time to reach steady state only depends on D and £ and not on the amount of drug which must be sorbed or desorbed by a device. , D '" 1 X 10- 7 cm 2 /sec. In this case, the time lag (or-burst effect) is only a few minutes. On the other hand, a solution-diffusion membrane of the same thickness and with a diffusion coefficient of the order 10- 9 - 10- 10 cm 2 /sec has a time lag (or burst effect) lasting several hours or even days. Moreover, the extent of this effect increases with the square of the membrane thickness.
25) 00 Both of these approximations are plotted in Figure 10 which illustrates their different regions of applicability. For simplicity in presentin~ the results, it has been assumed that M~ 1 and D/~ 1. = = In general, the drug release rate at any time is of more interest than the accumulated total drug release. This is easily obtained by differentiating equations (24) and (25) which give: 2Moo ( D ) nR, 2 t ~ (26) for the early time approximation, and (27) for the late time approximation. Figure 11 shows a plot of these two approximations against time.
0 BURST EFFECT Eq w o m » ,.... , » OJ ~ 1'1' 00 39 CONTROLLED RELEASE: MECHANISMS AND RATES A good example of the burst-effect phenomenon can be found in data presented by Kincl and Rudel. 84 Presented in Figure 9 is a plot of their results of release rate of megestrol acetate from cylindrical polydimethylsiloxane capsules of varying length. The high initial release rate, which persisted for several days, is presumed to be the result of steroid accumulation in the membrane prior to the release rate measurements.
Controlled Release of Biologically Active Agents by Donald R. Cowsar (auth.), A. C. Tanquary, R. E. Lacey (eds.)