To improve delivery of protein drugs, researchers have developed a way to make highly concentrated, injectable suspensions of proteins (ACS Nano, DOI: 10.1021/nn204166z.
When patients need a high dose of a protein drug, they usually receive an intravenous drip. Some rheumatoid arthritis patients, for example, must go to a clinic for regular intravenous treatment, which is expensive, stressful, and time-consuming. Simple, quick injections under the skin often aren’t feasible, because highly concentrated protein solutions become too viscous.
In solution, proteins start to unfold and clump at concentrations around 100 mg/mL. But inside cells, proteins abound at concentrations four times as high and don’t normally form clumps. By mimicking the crowded environment inside cells, Keith P. Johnston of the University of Texas, Austin, and colleagues can pack a high dose of protein drugs in a syringe.
To re-create the crowding in a cell, the researchers encouraged the proteins to assemble into nanosized clusters. First they adjusted the pH of the proteins’ solution so that the molecules had almost no net surface charge. Then they added trehalose, a nontoxic sugar, in sufficient quantities to push the proteins together. With no surface charge, the proteins don’t repel one another, and they form nanoclusters measuring a few hundred nanometers across.
The scientists characterized nanocluster suspensions of three different proteins, including an antibody that binds the pertussis toxin, at concentrations as high as 700 mg/mL. When they injected the antibody nanoclusters under the skin of mice, the nanoclusters took longer to diffuse into the bloodstream than did conventional protein solutions, but once in the bloodstream, both protein formulations had similar activities and lifetimes.
Theodore W. Randolph of the University of Colorado, Boulder, says that to demonstrate that the nanocluster method is practical, the researchers will have to show that the nanoclusters have at least a two-year shelf life and that the method works for other proteins as well. The Texas group is working on further tests.