On-Demand Drug Release Possible with Thermo-Responsive Polymers

Controlled-release formulations are attractive to pharmaceutical manufacturers and patients alike. For drug companies, the branded life of a drug can be extended through reformulation as a controlled-release version. For patients, the steady release of active pharmaceutical ingredient (API) over time into the bloodstream ensures more consistent results. A potential further advance on controlled-release technology is on-demand drug release, which has the potential to further reduce side effects associated with over-dosages, particularly for highly potent anticancer drugs. Such a system would be based on stimuli-responsive materials, and the drug would only be released when the system was in the “on” state, according to Svetlana Sukhishvili, a professor in the Department of Chemistry, Chemical Biology, and Biomedical Engineering at the Stevens Institute of Technology (SIT).

Responding to temperature
Sukhishvili, former doctoral student Zhichen Zhu, and professor Hongjun Wang at SIT recently reported the development of an on-demand drug-release system that responds to changes in temperature (1). “While other environmental stimuli could be used for these systems, such as changes in light, sound, pH, and ionic concentrations, organic tissues are fairly tolerant to changes in temperature, particularly when they occur below physiological temperature, and thus, this type of stimulus is favorable for the initiation of on-demand drug release,” Sukhishvili notes.

Binding for stability
The system employed by the SIT researchers was based on the attachment of API-loaded block copolymer micelles (BCMs) comprised of a thermo-responsive polymer onto a film of tannic acid formed using layer-by-layer (LbL) assembly (1). “Free BCMs have the potential to accumulate in the body, whereas BCMs permanently attach or embed in a film are used to provide localized release,” says Sukhishvili. The BCMs consisted of the neutral diblock copolymer poly(N-vinylpyrrolidone)-b-poly(N-isopropylacrylamide) (PVPON-b-PNIPAM), which undergoes temperature-controlled swelling/deswelling transitions just below physiological temperature. The PNIPAM was located in the core while the PVPON formed a corona. Tannic acid was used to construct the film because it forms stable hydrogen-bonded assemblies with neutral homopolymers, including PVPON. In this particular system, the tannic acid stabilized the micelles by forming multiple hydrogen bonds with the PVPON (1).

To form the micelles, the PVPON-b-PNIPAM block copolymer, synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization, was simply heated above the lower critical solution temperature (LCST) of 32–34 °C (1). The BCMs were then bound to layers of tannic acid via hydrogen bonding with the PVPON in the coronae of the micelles, and the surface of the film was completely covered with micelles when more than three bilayers were deposited. Importantly, according to Sukhishvili, these film-bound micelles were stable under neutral and slightly basic conditions, confirming the benefits of the hydrogen bonding between tannic acid and PVPON. In addition, the bound micelles underwent the expected swelling and shrinking at temperatures below (20 °C) and above (40 °C) the LCST, respectively, due to water uptake and release, and thus binding to the tannic acid did not prevent the desirable thermo-responsive behavior of the block copolymer (1).

Biocompatible films
Next, the researchers loaded the bound micelles with doxorubicin (DOX) in order to investigate their thermo-responsive drug release behavior. “DOX is a widely used, hydrophobic anticancer drug with known adverse side effects. The on-demand release of this drug could potentially minimize these side effects,” observes Sukhishvili. The DOX was loaded into the micelles by simply dializing the films against water in a solution of DOX at 37 °C, and a relatively high loading content of slightly more than 9% based on the mass of the film was achieved (1). The biocompatibility of unloaded and DOX-loaded films was then tested using MCF-7 cancer cells, and it was found that cell proliferation on the films, regardless of whether the top layer was tannic acid or the micelles, was similar to that on glass coverslip controls.

Reloadable release
“These results were important, because they confirmed that the micelle-embedded films were compatible with human tissue, and encouraged us to examine the ability of the micelles to release DOX on-demand,” Sukhishvili says. The release of DOX from the bound micelles was slower than the release from free micelles at 20 °C, and for both types of micelles, the release was significantly faster below the LCST than above it. Notably, according to Sukhishvili, the release behavior correlated strongly with the swelling transitions of the films, which confirmed that the retention and release of DOX was due to partitioning of the DOX in the collapsed and hydrated PNIPAM cores of the micelles.

Pulsed release of DOX was also achieved using the BCM-embedded films. The DOX-loaded films were subjected to cyclic thermal stimulation above and below the LCST, and a rapid release of the drug was observed each time the micelles were exposed at 20 °C although the released amount declined slightly after each cycle due to the overall decrease in the DOX content (1). Reloading of the BCM/TA films was possible as well, and the researchers demonstrated that the DOX-reloaded films exhibited similar drug-release profiles as the originally loaded film for as many as 15 swelling/deswelling cycles.

Demonstrated cell death
Finally, breast cancer cells (MCF-7) were exposed to the DOX-loaded films, which were subjected to three temperature cycles (37 °C for one day followed by 20 °C for 30 minutes), and it was confirmed that while the toxicity of the film at 37 °C was minimal, the viability of the cells was significantly reduced (90% killing efficacy) after the temperature was reduced below the LCST, and that the films maintained high efficiency even after being subjected to multiple swelling/deswelling cycles. “Because it is very important that the drug only be released when in the ‘on’ states, we were pleased to see that the amount of DOX released at 20 °C was well below the therapeutic dose and led to minimal cell death even after several days,” Sukhishvili notes.

With these promising initial results in hand, the researchers are exploring ways to tune the behavior of these thermo-responsive systems by changing the size and chemical structure of the block copolymer (i.e., adjusting the LCST) and varying the number of BCM layers within the films. “We believe that these hydrogen-bonded, BCM-containing films have real potential for use as reusable, highly responsive, temperature-controlled on-demand drug delivery vehicles,” she concludes.

Reference

1. Z. Zhu et al., J. Control. Rel. 171 (1) 73-80 (2013).