PubMed · 8058624
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L E Pena, B L Lee, J F Stearns. 1994. Videomicroscopy techniques for agglomeration studies.. https://doi.org/10.1023/a%3A1018943305717
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We previously outlined the fundamental principles that govern behavior of stabilized bubbles, such as the microbubbles being put forward as ultrasound contrast agents. Our present goals are to develop the idea that there are limits to the stabilization and to provide a conceptual framework for comparison of bubbles stabilized by different mechanisms. Gases diffuse in or out of stabilized bubbles in a limited and reversible manner in response to changes in the environment, but strong growth influences will cause the bubbles to cross a threshold into uncontrolled growth. Also, bubbles stabilized by mechanical structures will be destroyed if outside influences bring them below a critical small size. The in vivo behavior of different kinds of stabilized bubbles can be compared by using plots of bubble radius as a function of forces that affect diffusion of gases in or out of the bubble. The two ends of the plot are the limits for unstabilized growth and destruction; these and the curve's slope predict the bubble's practical usefulness for ultrasonic imaging or O2 carriage to tissues.
Tetanus toxoid (TT), the vaccine for tetanus, is an important protein antigen and candidate for sustained release from polymeric matrices. During administration from the latter, the solid (e.g., lyophilized) protein will be exposed to elevated levels of temperature and moisture, conditions which trigger its aggregation. To examine the connection between this aggregation and the structure of the TT molecule in the solid state, Fourier-transform infrared (FTIR) spectroscopy was employed to determine the secondary structure of TT in the presence of various excipients. We found that excipient-free TT undergoes a significant alteration (mostly reversible) in the secondary structure during lyophilization. Specifically, more than half the total alpha-helix content was lost with a concomitant increase in beta-sheet structure. The extent of structural alterations in the presence of 1:5 (g:g protein) NaCl, sorbitol, or poly-(ethylene glycol), did not correlate with stability conferred towards moisture-induced aggregation. These results suggest that the degree of retention of the native protein structure in the dry state is not a general predictor of stability for the "wetted" solid within polymer controlled-release vehicles.