Most forms of diabetes are treated by administering insulin, to enable the body's cells to take up (and utilize) glucose. A complimentary approach may be to administer cyclic adenosine monophosphate (cAMP), a molecule that enhances insulin secretion from the pancreas.
In order for ths latter approach to work, cAMP molecules be delivered to pancreas cells by crossing the cell membrane (the outer barrier of cells). There is no reported method for achieving such delivery, with the goal of controlling insulin production.
Victor S.-Y. Lin (Iowa State University) and coworkers have combined both approaches, insulin administration and cAMP delivery, with the goal of treating diabetes. In response to glucose, their nanoparticles release insulin, are taken up by cells, and release cAMP.
The concept.
The scientists used porous silica nanoparticles to enable sequential insulin and cAMP delivery. Insulin molecules are affixed to the surface of the nanoparticles, by a chemical linkage that is readily broken by monosaccharides (single sugar units, such as fructose and glucose).
Additionally, cAMP is packed into the channels within the interior of the nanparticles. They are kept inside by the (channel-blocking) insulin molecules on the surface of the nanoparticles.
Thus, in the presence of an excessive quantity of glucose, insulin is released from the surface of the nanoparticles. Subsequently, cAMP is released from the interior of the nanoparticles.
The scientists evaluated their nanoparticles' insulin and cAMP release properties in detail, with a focus on those relevant for pharmacological applications. They specifically investigated the specificity of insulin release, and the time and pH dependence of insulin and cAMP release.
Specificity of insulin release.
The scientists' nanoparticles need to respond to high glucose levels in the blood. Other sugars should not interfere with molecular release from the nanoparticles.
Regarding monosaccharides other than glucose, the nanoparticles are more responsive to fructose than glucose, i.e., fructose releases insulin from the nanoparticle surface more readily than glucose. However, this is not a practical limitation, because diabetic patients have at least 100 times more glucose than fructose in their blood.
The nanoparticles are not responsive to disaccharides (multiple sugar units, such as lactose), i.e., disaccharides do not effectively release insulin from the nanoparticle surface. These results collectively demonstrate selectivity for glucose, on a practical level.
Time and pH dependence of insulin release.
In addition to a specific release of insulin, in response to abnormally high glucose levels, the nanoparticles must release insulin within a reasonable time frame. Ideally, it would be released at a rate similar to molecular secretion within normal cells.
The scientists found that insulin is released from the nanoparticles within 30 minutes. This is similar to normal insulin secretion within cells.
Furthermore, the nanoparticles must release insulin at a basic pH (one that is greater than 7). This is because pancreatic cells operate optimally under these conditions.
The scientists found that insulin is released from the nanoparticles (in response to glucose) only at pH 8 and above. Thus, insulin will be released where it is most needed, in the pancreas.
Time and pH dependence of cAMP release.
The nanoparticles must also release cAMP, in addition to insulin. The former must not be permanently trapped within the channels of the nanoparticles.
The scientists found that 67% of the total amount of cAMP is released within 30 hours, at pH 8.5, in response to a concentration of 50 millimolar glucose. Furthermore, roughly 80% of the total release takes place within 20 hours.
Uptake by and release within cells.
All of this data is promising, but the scientists also needed to ensure that their nanoparticles are taken up by cells. Although insulin does not need to be released within cells for nanoparticle efficacy, cAMP does, because it does not easily cross into cells on its own.
The scientists found that their nanoparticles are taken up by model cells twice as efficiently when insulin molecules are not affixed to the surface. This means that the nanoparticles are more likely to deliver their cAMP cargo (the longer-term modulator of blood glucose levels) to the interior of cells after the nanoparticles have released their insulin cargo (the shorter-term modulator of blood glucose levels).
Estimation of dosage requirements and toxicity.
The scientists further determined how much of their nanoparticles would be required for therapeutic benefit. A small amount is ideal, in order to mimimize the possibility of toxic side effects.
The scientists estimate that a nanoparticle concentration 10,000 times less than what they have evaluated in their research would still release enough insulin to keep blood glucose levels in check in diabetic patients. They then checked for nanoparticle toxicity, an obvious consideration for their possible use in living patients.
The nanoparticles did not appear to be toxic to cells, even at relatively high nanoparticle concentrations. The scientists observed greater than 90% cell viability and greater than 80% cell proliferation.
Implications and future development.
These scientists have developed a sequential, dual-release drug delivery system aimed at treating diabetes. However, clinical efficacy has not yet been investigated, and therefore its use in living patients is still a long way off.
Their approach is not limited to insulin and cAMP release, or treating diabetes. In principle, any two drug molecules can be released from the nanoparticles in a sequential manner, and therefore the general technique can be adapted for other purposes, when the timing of delivering multiple drugs is potentially of therapeutic benefit.
for more information:
Zhao, Y., Trewyn, B. G., Slowing, I. I., & Lin, V. S.-Y. (2009). Mesoporous Silica Nanoparticle-Based Double Drug Delivery System for Glucose-Responsive Controlled Release of Insulin and Cyclic AMP Journal of the American Chemical Society, 131 (24), 8398-8400 DOI: 10.1021/ja901831u