Researchers at Tufts University School of Engineering have discovered a way to maintain the potency of vaccines and other drugs that otherwise require refrigeration for months and possibly years at temperatures above 110 degrees F, by stabilizing them in a silk protein made from silkworm cocoons. Importantly, the pharmaceutical-infused silk can be made in a variety of forms such as microneedles, microvesicles and films that allow the non-refrigerated drugs to be stored and administered in a single device.
The Tufts findings address a serious obstacle to the effective use of life-saving pharmaceuticals: keeping them cold. Most vaccines, enzymes and antibodies and many antibiotics and other drugs require constant refrigeration from manufacture to delivery to maintain their effectiveness.
International health experts estimate that nearly half of all global vaccines are lost due to breakdowns in the “cold chain.” Even in industrialized nations, loss of drug efficacy at body temperature is a serious problem for advanced pharmaceutical delivery systems such as implantable drug-coated devices.
The research will be published before print in the Proceedings of the National Academy of Sciences (PNAS) Online Early Edition the week of July 9.
Tufts biomedical engineers, led by David L. Kaplan, found that silk stabilization preserved the efficacy of the measles, mumps and rubella (MMR) vaccine, as well as penicillin and tetracycline, at a wide range of temperatures (at least up to 60 degrees C or 140 F) significantly better than other options such as collagen encapsulants, dried powders and solutions.
“Silk protein has a unique structure and chemistry that makes it strong, resistant to moisture, stable at extreme temperatures and biocompatible, all of which make it very useful for stabilizing antibiotics, vaccines and other drugs. The fact that we can also make silk into microneedles to deliver a vaccine is an enormous added advantage that can potentially provide a lot of useful solutions to stabilization, distribution and delivery,” says Kaplan, who has been studying silk for two decades.
Nanoscale Bubble Wrap
Protein function depends on chains of amino acids folding into specific shapes. At higher temperatures or in the presence of water, the chains tend to unfold, then clump together, which renders them inactive. Silk fibroin is composed of interlocked crystalline sheets with numerous tiny hydrophobic pockets. The pockets trap and immobilize bioactive biomolecules—keeping them from unfolding—and also protect them from moisture. The end result is like enveloping a fragile material in a nanoscale Bubble Wrap.
According to the paper’s first author, Jeney Zhang, who is pursuing a Tufts doctorate in chemical and biological engineering, silk stabilization has “the potential to significantly change the way we store and deliver pharmaceuticals, especially in the developing world.”
Measles is one of the leading killers of children worldwide. Without refrigeration, the MMR vaccine rapidly loses potency. But after six months of storage in freeze-dried silk films at body temperature (37 degrees C) and at 113 F (45 degrees C), all components of the vaccine retained approximately 85 percent of their initial potency.
Silk-stabilized antibiotics also retained high activity. Storage in silk films at body temperature resulted in no activity loss for tetracycline, compared with an 80 percent loss within four weeks of storage in solution. Even for films stored at 140 degrees F (60 degrees C), tetracycline activity loss was only 10 percent after two weeks, compared with 100 percent loss after two weeks of storage in solution. No activity loss was observed for penicillin stored in silk films at 60 degrees C for 30 days; in contrast, total activity loss was observed within 24 hours when penicillin was stored in solution at the same temperature.
Silk stabilization also protected the tetracycline against degradation by light, a benefit that the researchers did not anticipate, according to co-author and research assistant professor Bruce Panilaitis, G01. Panilaitis earned his Ph.D. in biology at the Graduate School of Arts and Sciences before joining Kaplan’s lab in 2001 as a postdoctoral fellow.
So far, Panilaitis adds, the researchers haven’t found any pharmaceutical that they have been unable to stabilize. This could be a “universal storage and handling system.”
Additional authors on the paper include Eleanor Pritchard, who earned her doctorate in biomedical engineering at Tufts and is now a postdoctoral fellow at St. Jude Children’s Research Hospital in Memphis; Xiao Hu, a biomedical engineering postdoctoral fellow who will join Rowan University as an assistant professor in September; Thomas Valentin, a biomedical engineering master’s degree student; and Fiorenzo Omenetto, professor of biomedical engineering.
The research was supported by a grant from the National Institute of Biomedical Imaging and Bioengineering at the National Institutes of Health. It builds on a significant body of work previously published by Tufts biomedical engineers seeking to tap the potential of silk for a wide range of applications. In December 2011 researchers from Kaplan’s lab announced development of a silk-based microneedle system able to deliver precise amounts of drugs over time and without need for refrigeration.