Darlene K Taylor & Uddhav Balami. American Scientist. Volume 102, Issue 2. Mar/Apr 2014.
Although we rarely think about it as we fill a prescription at the drugstore or refill an order through an online service, delivering medicine to its intended site inside the human body is an elaborate problem in materials science.
The challenge begins the moment a drug enters the digestive tract or the bloodstream. The body naturally strives to defend itself against foreign invasion, even if the invasive substance is a lifesaving medicine. Some of our inner defenses work strategically to destroy invading agents at the molecular level, such as the antibodies launched by the immune system that attack unfamiliar entities in the bloodstream. Others act more like physical walls to tightly regulate intruding agents, such as the blood-brain barrier, a dense network of capillaries that separate the brain from circulating blood.
Even if a therapeutic compound clears these hurdles, it may still fail in its mission if it is prevented from dissolving properly, or even if it dissolves too quickly and is cleared from the body before it can act. When an effective drug reaches the correct location in the body, at the right concentrations, and continues to work for the optimal amount of time, this represents the finish line of a very long obstacle course.
Some drugs manage to travel smoothly to their target site, whereas others are beset with challenges all the way. Unfortunately, this second category includes drugs that are vital to preventive care for some of the most lethal diseases, such as cancer. To develop more effective therapeutic agents, researchers need to come up with not only viable drug candidates but also adaptive materials for drug delivery-materials that could respond to changes in the immediate environment by altering their own physical properties.
Thanks to major advances in molecular engineering, scientists are now able to design materials so that they respond to specific kinds of external stimuli, including temperature, acidity, mechanical force, pressure, electrical potential, solvents, chemical agents, and even magnetic field, ionic strength, or certain wavelengths of light. The nature of the response can vary as well: Modifications of a material’s shape or surface characteristics, self-assembly, the formation of more intricate molecules, and transitions in physical state are all possible under the right conditions.
Different materials address different types of problems. One type turns into a gel when it warms up, then becomes a liquid again on cooling. Another material can be woven into a textile that can break and re-form chemical bonds in response to photostimulation, enabling it to act as a light sensor. These and other so-called smart materials are being developed right now in labs around the world. Such materials offer applications ranging from the delivery of therapeutics, tissue engineering, and bioseparation to intelligent optical systems, sensors, and coatings.
Collectively, the new smart materials fall into the class that chemists call polymers. The defining trait of polymers is that they consist of repeated chains of smaller molecules, like perfectly matched beads on a string. This versatile structure occurs throughout the natural world, forming the basis of materials as varied as rubber, beetle shells, and the silken threads of a spider’s web.
The beads in a polymer (known, in chemical terms, as monomers) are all exactly the same and are bonded to one another-an arrangement that gives polymers strength and flexibility. Moreover, many different monomers can be built up into polymers. These compounds have long been an important part of the chemist’s bag of tricks, but it required molecular-level engineering to make the polymers “smart,” allowing them to adapt in desirable ways to changes in the environment.
Molecules That Feel the Heat
A primary focus of the work in our lab is developing smart polymers that respond to changes in temperature. If the chemical structure of a polymer resembles a long string of identical beads, thermo-responsive materials are made of polymers whose beads, suspended in a solution, change their behavior when the temperature of the solution reaches a certain threshold. Above that critical temperature, the individual molecules of the polymer are transformed from a hydrophilic to a hydrophobic, or water-repellant, state-that is, they alter their shape, allowing less of their surface area to come in contact with surrounding water molecules. This change disrupts the hydrogen-bonded network between the polymer chains and the water molecules that are in the solution. The water is rapidly expelled and the polymeric chains collapse into globules, which causes the thermo-responsive material to precipitate out of the solution. If the temperature drops back below the critical point, the polymeric molecules change back from hydrophobic to hydrophilic, absorbing a significant amount of water and transforming the material from a solid back to a solution.
Even though this phase transition has been widely observed to occur in both directions and has been documented in many different polymeric materials, scientists continue to debate exactly why it occurs. According to one theory, disruption of the polymerto-water interactions is the main factor that controls the phase transition. Another theory holds that the most significant change takes place in the local structure of the water molecules surrounding the hydrophobic groups of the polymer. A third theory offers a compromise of sorts: Perhaps, on reaching a critical temperature, the polymeric chains undergo a combination of changes occurring in both the hydrogen bonding and the hydrophobic interactions within the solution.
The phase transition in thermoresponsive polymers can also be explained from the point of view of thermodynamics. At some critical temperature, it is energetically more favorable for phase separation to occur. The entropy of the water is higher in the less ordered state, without polymer chains suspended in it.
Diving deeper into the thermodynamic interpretation: Initially the water molecules are trapped by hydrogen bonding with the polar groups of the polymer chain. These trapped water molecules form a thin shell of ordered structure around the hydrophilic part of the polymer. Meanwhile, elsewhere in the solution, water molecules interact with one another to form an ordered, ice-like structure around the hydrophobic part of the polymer chain. This hydrogen-bonded network contributes to a larger negative entropy change-in other words, a transformation to a more ordered state. Raising the temperature supplies enough energy to disrupt the hydrogen bonding within the waterpolymer interaction; as a result, the hydrated water shells around the polymers break down, and within seconds the water molecules are forced into the bulk of the water.
But the most important fact about thermo-responsive materials is that they work.
Thermo-Responsive Drug Delivery
With their unprecedented ability to alter their physical properties in response to temperature changes, thermo-responsive polymers make good building blocks for intelligent drug delivery systems, either on their own or in combination with other materials. Medical scientists, biotechnologists, and pharmaceutical researchers have been quick to seize the opportunity. A growing number of polymer-based drug delivery systems are already under development. In some cases, a particular polymer is engineered to serve a specific purpose. ReGel® is a broadly used delivery platform made by Allergan and marketed in two formulations: OncoGel® when coupled with chemotherapeutic agent paclitaxel for esophageal cancer, and Cytoryn(TM) when coupled with interleukin-2 (IL-2) for cancer immunotherapy. In similar fashion, Timoptic-XE® (Merck & Co.) is used for slow-release delivery of the drug timolol to treat hypertension within the eye.
At North Carolina Central University, our laboratory has pioneered the development of LiquoGel, another synthetic thermo-responsive polymer for targeted drug delivery. (We have filed for a patent on this work, but have not yet begun to commercialize it.) At room temperature, LiquoGel forms an aqueous solution that can be administered by syringe to specific sites in the body. Once in place, the solution warms up to body temperature and assumes the form of a gel. The therapeutic drug suspended in the solution becomes trapped within the gel, to be released gradually at the site of administration as the gel degrades.
The physical changes of LiquoGel on exposure to mild heat make it suitable for a variety of medical applications. In one of its most extensively tested applications so far, collaborators at Duke University have identified targets in the treatment of uterine fibroids, for which LiquoGel can be pivotal in delivering insoluble, highly effective medicines.
Fibroid growths in the uterus, composed of muscle and fibrous tissue, affect 70 to 80 percent of all American women by the age of 50. Although benign, uterine fibroids pose a significant public health problem when they cause pain or excessive menstrual bleeding, or problems in pregnancy. As the leading indication for hysterectomy in the United States today, this condition entails healthcare costs of more than $2 billion each year. Nevertheless, the current treatment options for uterine fibroids are unsatisfactory.
Surgical procedures (myomectomy, uterine artery embolization, and MRI-guided focused ultrasound) can destroy fibroids while preserving the uterus, but they are lengthy and expensive, and the fibroids frequently recur afterward. Medical therapies, too, are often short-lived or accompanied by significant side effects, or their clinical trial results have been disappointing. Targeted drug delivery by means of a smart polymer may offer a new way to treat uterine fibroids. The polymer is injected directly into the fibroids (a), where it releases the drug upon reaching body temperature ib, c), thereby avoiding the risk of systemic side effects or the need for surgery (d).
Effective drug molecules exist-selective progesterone receptor modulators, for example, have demonstrated significant shrinkage of fibroids and symptomatic improvement-but have not yet received FDA approval, owing to concerns over the effect of these agents on the endometrium.
Using polymers to deliver a therapeutic agent directly into uterine fibroids is a novel approach that has the potential to reduce systemic side effects considerably. As an additional advantage, administering a drug directly to the fibroid can be used to deliver small peptides that would be degraded in the gastrointestinal tract if the drug were taken orally. Treating fibroids by means of a local injection-a procedure that can be accomplished in a doctor’s office-could significantly improve the feasibility of helping women who want to maintain fertility while seeking relief from troubling uterine symptoms. Moreover, by slowing the diffusion of the drug away from the injected fibroid, prolonging the release of the drug and delaying its inactivation, polymer-based treatments should reduce the need for repeated injections.
Other research teams have designed a different kind of thermo-responsive drug delivery system, one that uses a synthetic type of fluid-filled pouch called a liposome, for treating various medical conditions. Although liposomes do not occur in nature, they are derived from biological polymers: Their bilayer, the membrane that encloses them, is made of the same phospholipids found in all living cells.
Clinical trials now under way are testing these thermo-sensitive liposomes (TSLs), as they are known, in several applications. As an example, TSLs designed with a trigger temperature of 40 to 45 degrees C can deliver the antitumor drug doxorubicin (commercially known as ThermoDox®, made by Celsion Corporation) to specific sites. Currently in Phase II clinical trials for the treatment of breast cancer and colorectal liver metastasis, the same drug delivery system has recently entered Phase III trials for the treatment of primary liver cell carcinoma.
Yet another approach was described by the research team of Z. S. Al-Ahmady at the Centre for Drug Discovery, University College London, using a peptide-liposome hybrid with a coil of the amino acid leucine embedded in the lipid bilayer, like a plug in a hole. This system combines the TSL benefit of supramolecular assembly (the lipid bilayer plus the drug inside) with the property of responding to heat by unfolding its protein structure.. At cool temperatures, the peptide portion curls up on itself. But above the trigger temperature-typically about 40 to 45 degrees C-the peptide’s primary fold opens like the two halves of a zipper, unsealing the outer membrane of the bilayer and releasing the drug at the desired site. The ability to fine-tune the trigger temperature and the way the configuration of the molecules change in response to heat make leucine peptides attractive for the design of smart drug delivery vesicles.
The liposomes suitable for transporting drugs within the body must all be capable of releasing their contents, but not all liposomes are rendered permeable by the same means. In some instances the lipid bilayer takes on a gel-like state that then degrades in the body, gradually releasing the drug. In others, a peptide embedded in the bilayer breaks apart, leaving a hole.
Perhaps the most unexpected mechanism for opening a liposome at the right critical temperature is the use of bubbles. This innovative approach relies on creating permeable defects in the lipid bilayer with carbon dioxide bubbles. The carbon dioxide is generated (along with water and ammonia) as a result of the decomposition of ammonium bicarbonate-more commonly known as baking soda-at about 42 degrees C. (This is, in fact, the same principle used to leaven bread and cakes.) In this type of liposome delivery system, the gas bubbles created inside the membrane build up pressure and distort the arrangement of the membrane bilayers. As the carbon dioxide bubbles permeate the lipid bilayer, the compartmentalized drug is quickly released. The bubbles not only create the necessary defects in the lipid bilayer, enabling the drug to escape, they also help yield clearer results in ultrasound-guided tissue imaging.
A Path to Smarter Medicine
After taking several decades to come into widespread use, smart materials are now becoming an important tool in biomedicine and biotechnology. Combining polymers with other functional chemistries can lead to an almost unlimited variety of smart materials. In the near future, smart polymers will play a role in keeping track of changes in acidity in the kidneys to monitor homeostasis; taking their cue from enzymes in the blood vessels to repair embolisms; deploying biochemical compounds onsite to fix misshapen proteins in neurodegenerative diseases like Alzheimer’s, Parkinson’s, and Huntington’s; forming chemical and biochemical stimuli to regulate blood components such as glucose; and blocking chemical signals that can trigger melanoma.
Once a mere theoretical possibility, smart materials that respond to individual triggers like light and temperature are now a real possibility. Such materials are of immense interest to scientists searching for new ways to treat disease and bypass the body’s cunning, but often counterproductive, defenses.