Researchers report the development of a technique that allows them to measure how well cancer drugs reach their targets inside the body. It shows individual cancer cells in a tumor in real time, revealing which cells interact with the drug and which cells the drug fails to reach, according to the scientists.
In the future, the findings (“Heterogeneity in tumor chromatin-doxorubicin binding revealed by in vivo fluorescence lifetime imaging confocal endomicroscopy”), published in Nature Communications, could help clinicians decide the best course and delivery of treatment for cancer patients, note the researchers.
“We present an approach to quantify drug–target engagement using in vivo fluorescence endomicroscopy, validated with in vitro measurements. Doxorubicin binding to chromatin changes the fluorescence lifetime of histone-GFP fusions that we measure in vivo at single-cell resolution using a confocal laparo/endomicroscope. We measure both intra- and inter-tumor heterogeneity in doxorubicin chromatin engagement in a model of peritoneal metastasis of ovarian cancer, revealing striking variation in the efficacy of doxorubicin–chromatin binding depending on intra-peritoneal or intravenous delivery,” write the investigators.
“Further, we observe significant variations in doxorubicin–chromatin binding between different metastases in the same mouse and between different regions of the same metastasis. The quantitative nature of fluorescence lifetime imaging enables direct comparison of drug–target engagement for different drug delivery routes and between in vitro and in vivo experiments. This uncovers different rates of cell killing for the same level of doxorubicin binding in vitro and in vivo.”
Failure of chemotherapy to reach all cancer cells in a tumor is a major cause of poor treatment outcomes. To study this problem in detail, the researchers say, requires a technique for accurately measuring how well drugs bind their targets in the body. They add that existing methods can't show which cells have been targeted by cancer drugs because measurements are taken from liquefied cancer biopsies, so material from different cells gets mixed together.
The scientists, from the Francis Crick Institute and Imperial College London, have developed a way to measure and visualize drug-target engagement of individual cells within a tumor, using a miniature fluorescent microscope. They mapped out how doxorubicin (Adriamycin) targeted ovarian cancer cells in living mice and found significant variation in drug-target engagement between cells within a single tumor, and between different tumors. The team also discovered that drug-target engagement was better when doxorubicin was administered via abdominal injection rather than intravenously, which is the currently preferred method for doctors treating patients in many clinics.
“Our findings show that in a mouse model delivery of doxorubicin through the blood does not reach all its target cells in the body, which could help explain why this chemotherapy drug is only partially effective in some cancer patients. In contrast, delivering the drug directly into the abdomen adjacent to ovarian tumors improved its target engagement, but this was still not sufficient to kill the cancer cells,” says Erik Sahai, Ph.D., senior author of the paper and group leader at the Francis Crick Institute.
“If we know that a specific cancer drug isn't reaching all of the cells within a tumor, it might be that we need to find ways to improve drug delivery throughout the whole tumor. Conversely, if we know the drug does engage its target but is still not sufficiently effective, it might be that different drugs or drug combinations should be explored.”
The technique works by imaging the interaction between two light-sensitive molecules. In this study, the team labeled the DNA inside cancer cells with green fluorescent protein (GFP), which can transfer energy to doxorubicin (which is intrinsically light-sensitive) when it is sufficiently close by. Calculating the efficiency of this energy transfer was used to determine the binding between the drug and DNA of different cancer cells in real-time.
The team believes they can adapt their technique to work with other chemotherapy drugs, and in combination with engineered biosensors that fluorescently tag cancer cells, so that they can measure drug-target engagement in various preclinical scenarios.