Cancer drugs are typically developed to interfere with specific molecular targets, but their effects inside cells can extend well beyond those targets. Researchers at the University of Cincinnati have now used advanced microscopy to track the cancer drug sunitinib inside living cells, revealing how it disrupts several structures that are essential for normal cellular function.
The study provides a detailed picture of how the drug accumulates inside cells and affects lysosomes, mitochondria and the endoplasmic reticulum. The findings also illustrate how super resolution microscopy could support drug research by revealing cellular effects that conventional imaging cannot capture.
Tracking sunitinib inside living cells
Sunitinib is used to treat several types of cancer, including kidney cancer. Its therapeutic effect has traditionally been associated with inhibiting enzymes involved in tumor growth. However, the location of a drug within a cell may also influence both its effectiveness and potential side effects. Researchers at the Diao Laboratory combined cell viability tests with structured illumination microscopy, or SIM. This super resolution imaging technique makes it possible to visualize structures that are too small to be clearly distinguished using conventional light microscopy.
Sunitinib has an important property for this type of research: the drug is naturally fluorescent. As a result, researchers could track its movement through living cells without first chemically modifying the molecule. Instead of examining only whether cells survived treatment, the team studied the drug’s effects on three interconnected cellular systems. These were lysosomes, which process and recycle cellular materials, mitochondria, which play a central role in energy production, and the endoplasmic reticulum, which is involved in protein production and cellular signaling.
Lysosomes become primary destination
Initial experiments confirmed that sunitinib reduced cell viability in a dose dependent manner, with higher concentrations resulting in progressively greater cell death. SIM imaging subsequently showed that the drug accumulated primarily in lysosomes. Quantitative analysis found a considerably stronger association with lysosomes than with mitochondria.
The higher imaging resolution also enabled the researchers to measure structural changes in the organelles. Healthy mitochondria normally form elongated, interconnected networks. Following treatment with sunitinib, these networks became increasingly fragmented. Longer structures broke down into shorter and more isolated segments, suggesting disruption of the mechanisms responsible for maintaining mitochondrial organization and energy production.
Lysosomes themselves also underwent substantial changes. Although sunitinib continued to accumulate inside them, their number declined as the remaining lysosomes became larger and more irregularly shaped. According to the researchers, these changes indicate increasing stress within the lysosomal system as drug concentrations rise.
Multiple cellular systems disrupted
Changes were also observed in the endoplasmic reticulum, or ER. In untreated cells, the ER forms a continuous network throughout the cell. Exposure to sunitinib caused this network to progressively fragment into disconnected structures. To quantify these changes, the researchers developed topological analyses that measured ER connectivity. They assessed both the number of separate network segments and an organelle connectivity index. The results demonstrated a clear loss of ER organization as the concentration of sunitinib increased.
Together, the observations suggest that the effects of sunitinib extend beyond the inhibition of specific enzymes. The drug appears to cause coordinated disruption across several interconnected cellular systems, which may ultimately contribute to declining cell function and survival.
Imaging as a tool for drug development
The study is also significant because of the methodology used. Rather than relying exclusively on biochemical measurements, the researchers were able to directly observe where a therapeutic molecule traveled inside living cells and how cellular structures responded. By combining optical engineering, image analysis and cell biology, the approach generates quantitative information about the architecture and connectivity of organelles. This could provide researchers with a more detailed understanding of how medicines work at the cellular level.
Super resolution imaging may therefore have broader applications in drug development and cellular engineering. By visualizing interactions that remain hidden with conventional microscopy, researchers could gain greater insight into why particular drugs are effective, why others fail and how unwanted side effects arise. Ultimately, such knowledge could contribute to the development of more targeted and effective cancer therapies.
Tumor analysis
Two years ago, researchers at Caltech developed an imaging technology that could allow surgeons to determine during an operation whether a tumor has been completely removed. The technique, called parallel ultraviolet photoacoustic microscopy (PUV PAM), could significantly accelerate the examination of tissue samples. Conventional intraoperative histology requires tissue to be frozen, stained and cut into thin slices before microscopic examination. PUV PAM eliminates these steps and can directly image relatively thick samples with irregular surfaces. This could enable pathologists to identify remaining cancer cells while the patient is still in the operating room, potentially preventing repeat surgery.
The technology uses laser light to make tissue vibrate and measures the resulting ultrasonic waves. Differences in cell nuclei and cell density can help distinguish cancerous tissue. To increase imaging speed, the researchers divided one laser beam into eight parallel spots and combined two scanning methods. As a result, PUV PAM is approximately 40 times faster than previous advanced techniques developed in the same laboratory.
References
Biophotonics Discovery (research)
