Photonics continues to improve drug discovery

Thanks to research that began in the 1980s on spectrochemical instrumentation, laser miniaturization, biotechnology, and fiber optics, scientists are now able to combine their knowledge of molecular biology, photonics, and nanotechnology to increase the speed and effectiveness of drug discovery and development.

Breakthroughs in chemistry and molecular biology have resulted in a rate of drug discovery that exceeds the rate of drug development. While new drugs are being discovered rapidly, it still takes 15 years on average and $800 million before a drug is ready for market. But with imaging and nanotechnology, scientists are better able to determine in pre-clinical and early clinical testing if a drug will fail, allowing resources to be redirected from failing samples to potentially successful drugs.

To increase the quality of drugs and the efficiency of development, scientists are examining many emerging optical technologies, including in vitro imaging, nanobiosenors, quantum dots (QDs), spectral analysis, nanoprobes, Raman imaging, and high-resolution sensitive imaging.

"The need to identify the functional impact of drugs on cellular function is clearly a driving force. Imaging is one of the most direct and efficient techniques available," says J. Paul Robinson, chair of the Optics East Life Sciences symposium (see sidebar) and director of Purdue University Cytometry Laboratories (West Lafayette, IN).

Mostafa Analoui, senior director at Pfizer Global Research and Development (Groton, CT), emphasizes that imaging technology is "non-invasive, cost-effective, high resolution, and fast."

Overall, imaging's increased quantity and accuracy of information leads to shorter and smaller experiments, increasing the speed with which drugs go on the market and reducing their cost, Analoui explains.

Tuan Vo-Dinh, the Optics East Sensors and Industry Applications chair and director of Duke University's Fitzpatrick Institute for Photonics (Durham, NC), believes that many promising advances in drug development can be found in single-cell research using nanosensors, nanoprobes, and molecular sentinels.

"So what's the big deal about going into the single cell? First, when you compare this to standard techniques, as people are doing in drug discovery, the first thing they do is use animals. We kill hundreds of thousands of animals. Now people try to go into cell space; of course with cell space, the standard theory is so basic. Take hundreds of cells or thousands of cells, grind them, and then see what's going on by separation tomography," Vo-Dinh explains. "So when you treat the drug, or look at the pathway, you don't know where which cells are, and you have to go through a lot of statistics. Here with the single cells, you can just go and look at the cells [for results]."

Along with needing fewer animal subjects and requiring less laborious statistical analysis, nanosensors do not allow photons to escape from the tip of the fiber and become absorbed during the excitation process, as conventional optical biosensors do. Instead, the photons travel down the fiber, and evanescent fields continue to move down through the remainder of the tip, providing evanescent excitation in the layer of interest. Additionally, a nanoprobe only excites species extremely close to its tip, leaving fluorescent species within other locations of the sample undisturbed.

"Because the wavelength of the light is 10 times bigger than the aperture of the tip, you excite only the reaction at the tip, so you know where it is, localized, you can get a lot of information," Vo-Dinh says.

"With that grand combination of photonics and nanotechnology," Vo-Dinh explains, researchers can look at single cells "to detect early effects of drugs or trace amounts of drugs at levels never imagined before."

A related technology, nano-based contrast agents such as QD are emerging as another imaging method to keep an eye on, says Analoui, chair of the Optical Methods for Drug Discovery and Development conference at Optics East.

These semiconductor colloidal crystals are only a few nanometers in diameter, and can act as fluorescent beacons with narrow emissions, high photostability, and broad UV excitation. Adhering to quantum behavior, QDs also offer a better absorption range, do not bleach rapidly, and have a longer emissions lifetime when compared to traditional organic fluorescent molecules.

Nanoprobes using QDs can be slipped into a living cell, affecting only .2% of the human genome, and allows biologists to watch cellular processes and drug effectiveness for hours and even days at a time.

QDs also have many applications for disease detection and monitoring, including the ability to track enzymatic activity within a cell and the capability to function as in vivo tags for localizing tumors.

Like QDs, many optical technologies have a wide range of application in addition to speeding the rate of drug development, improving quality of information, and reducing the costs. Originally, Vo-Dinh's work on molecular sentinels was designed to improve HIV diagnosis. Vo-Dinh explains that while traditional HIV testing methods are very sensitive, they require complex instrumentation and elaborate sample preparation. But molecular sentinels allow room temperature testing using a simpler homogeneous assay format without sacrificing sensitivity or molecular specificity.

"We put DNA loops on the nanoparticles and when the DNA sees a gene or a molecule, the loop opens. When the loop opens up, the label is separate from nanoparticles and the signal disappears," Vo-Dinh says. If the signal disappears because the molecular sentinels have extinguished their lights, it is a sign that something has gone wrong: in drug development, a sign that the drug didn't function properly; in HIV diagnosis, a sign of infection; and, in Vo-Dinh's latest application, breast cancer detection, a sign of a cancerous tumor.

Despite the high praises many have been singing about these technologies, Robinson warns that speed of cellular analysis is still a concern. "Automated classification is always the holy grail—there are many problems in this area—almost all of which require that you first identify a cell accurately before a classification routine is activated. Issues such as automated focus are difficult. Despite numerous manufacturers claiming that they have 'solved' the problem, few have. Speed is clearly an issue in analysis."

Robinson also believes that end users will see results from the current imaging technology. "Seeing impact on in vitro and eventually in vivo systems provides an incredibly rich data source that allows quality decisions to be made in very complex environments," he says.

Combining this rich data with careful attention to the challenges and strengths of these new cellular technologies may someday help us to diagnosis and treat our most devastating diseases even before they become anatomically apparent.