What do patients and doctors imagine when they think of the ideal treatment? Many treatments used for severe diseases today effectively turn the human body into a war zone. This means that much of the ammo targeted at disease agents—whether radiation or powerful antibiotics—catch a patient’s own cells in the crossfire. The ideal treatment would be easy to employ, effective against the enemy pathogen, efficient in its work, and quick to cure the patient of illness. Many diseases have no such treatment available, and worse still, even some of the most physically grueling treatments—such as chemotherapy—can become less effective over time. However, growing research on nanomachines offers new reasons for hope.
Researchers led by James Tour of Rice University, Robert Pal of Durham University, and Gufeng Wang of North Carolina State University recently developed and tested nano-sized molecular machines that may hold the key to treating diseases through mechanical attacks that leave healthy cells unharmed. The original design for these molecular motors was based on work by Nobel laureate Bernard Feringa.
Collaboration between these three groups of scientists lent these nanomachines biomedical applications, opening the door to an exciting new frontier in medical treatment. The researchers at Rice synthesized ten molecular motors with novel functions, the most significant being the ability to drill through the lipid bilayer membranes that encapsulate cells. The North Carolina State researchers then tested these new motors on artificial lipid vesicles, which are fluid-filled cavities surrounded by a fatty membrane, and finally, the Durham researchers conducted experiments on live cells.
The key difference between the molecular motors and traditional pharmaceuticals is their method of action. “It’s a whole new mechanism of treatment. We have a mechanical effect at the molecular level,” Tour said. The molecular motors, in effect, punch holes in the membranes of cells through pure force, an attack that is less vulnerable to cellular resistance than chemical attacks that interrupt cell processes like DNA replication. Unlike chemotherapy, which uses toxic chemicals to kill tumor cells, these nanomachines can target diseased cells more directly.
These molecular motors, except for the controls, each consist of a UV light-activated rotor and a larger portion of the motor that remains fixed during rotation and provides a magnetic pole around which the rotor turns. When hit with the correct wavelength of light, the motor will rotate at a frequency of up to two or three million rotations per second. If the motors have attached themselves to a cell, when UV light causes rapid spinning of the motor, it effectively drills through the cell’s lipid bilayer, causing the cell to spill its contents and die.
The sheer variety of motors developed provides a testament not only to the wide array of biomedical applications that these nanomachines can have, but also to the complex molecular interactions that govern the success of molecular motors. While the main components of each molecular motor are the same, each motor can vary and size and can be modified with different chemical groups to give it new capabilities. For example, the motors can be equipped with fluorophores, which are fluorescent chemicals that allow the motor to be tracked easily inside of cells. Several of the motors were also designed with attachments that bind specifically to protein groups found on cancer cells, thus testing the ability of the nanomachines to target specific cells.
To confirm whether the motors could open bilayers, the researchers tracked the release of dye from vesicles in the presence of activated motors. In the first set of experiments, the Wang group encapsulated both dye and molecular motors modified with fluorophores inside of synthetic vesicles. Application of UV light significantly decreased the fluorescence intensity within these vesicles, as the dye and molecular motors diffused out. In contrast, in the absence of motors, the vesicles released negligible amounts of dye when exposed to UV light—proving that the vesicles burst because of the motors themselves, not the UV light.
Next, the Pal group performed several experiments exposing live cells to the motors. The researchers first introduced the motors to cells without UV activation. Although cells internalize some of the motors through endocytosis—a normal cellular process by which cells take up materials from their extracellular environments—the experiments showed that cells remained healthy in the presence of the inactive motors, an important consideration for potential treatments.
Armed with the knowledge that the inactive nanomotors are non-toxic, the researchers then activated the motors with UV light to observe how quickly the motors would accelerate cell death. The results were remarkable: the most effective motors killed both mouse fibroblast cells (a common cell-type found in connective tissue) and prostate cancer cells fifty percent faster than UV light did alone. In absolute terms, cells died within a matter of minutes after UV-activation.
Perhaps most interestingly, the researchers were able to demonstrate that molecular motors could selectively target which cells they want to kill. In the experiments—performed on mammalian cells in petri dishes—the motors were modified with protein attachments and were able to target cancer cells that were overexpressing certain protein groups. The motors were able to kill only these desired targets, and cell death was completed in just two to three minutes. This finding could mark the beginning of a new kind of photodynamic, or light-based, therapy research that could offer new sources of hope for patient treatment.
Still, the molecular motors are not without their challenges when it comes to translating their functions into potential treatments. For example, UV light has poor penetration of human tissue, so the labs are actively investigating alternative sources of activation that have much deeper penetration. In the meantime, the labs expect the molecular motors to become viable options for diseases that occur at or near the surface of the skin, such as eczema and melanoma. Such treatments can have a strong impact: there are about 31.6 million people in the U.S. who live with eczema, and the American Cancer Society estimates that more than 87,000 new cases of melanoma will be diagnosed in 2017 alone.
The molecular motors may also have a flexible range of functions beyond killing cells, such as in drug delivery. “We are looking first at cancers because we have a good understanding of cancer cells, but any cell that you need to kill can be targeted in this way,” Tour said. “You can also use molecular motors as a treatment source. In other words, turn on the molecular motors momentarily and then drugs can enter the cell.” With these options, molecular motors have the potential to enhance everything from antibiotic delivery to gene therapy.
In the future, the labs will focus on applying the molecular motors to the issues of pancreatic cancer, carcinomas and eczema, and on developing different methods of activation that reach deep tissues within the body. Indeed, these motors provide hope for targeting nearly incurable diseases like pancreatic cancer in a way that minimizes side effects. As research progresses, a safe, quick, and effective photodynamic therapy could become a reality.