DTP’s Natural Products Repository is the world’s largest storehouse of natural products. It houses close to 170,000 extracts from samples of more than 70,000 plants and 10,000 marine organisms collected from more than 25 countries, plus more than 30,000 extracts of diverse bacteria and fungi. The natural products stored in DTP’s repository are screened against the NCI human tumor cell line assay for potential anticancer activity shortly after their collection. So far, about 4,000 natural-source extracts have shown in vitro activity against human cancer cells, making them worthy of further study by DTP researchers.
The Natural Products Repository, administered by the Natural Products Branch, provides extramural researchers with natural products extracts for testing against any human disease.
Since this repository began about 50 years ago, more than 500,000 proprietary and nonproprietary compounds have been submitted to the program. In addition to being a repository for NCI screens, the repository distributes compounds for research purposes both as specific vialed compounds and in plated sets for high-throughput screening.
DTP’s plated sets have been instrumental in the discovery of compounds that enhance antilymphoma activity, nucleic acid antagonists with anti-HIV activity, and inhibitors of angiogenin—to name a few important advances.
The Synthetic Products Repository has recently developed a plated set to help evaluate drugs in combination. NCI’s Pediatric Drug Development Group will be one of the first groups to use the new plated sets; the results of the studies will be posted on DTP’s open-use Website at: http://dtp.nci.nih.gov.
Since the early 1960s, DCTD has maintained this low-temperature repository, which holds transplantable in vivo-derived tumors and in vitro-established tumor cell lines from an assortment of species. The repository serves as a resource for viable, contaminant-free experimental tumor lines, many of which are not obtainable elsewhere.
Researchers can access these materials under a material transfer agreement.
DTP’s Biological Testing Branch oversees animal-production facilities that produce inbred, outbred, and hybrid strains of rats and mice. This program provides researchers nationwide with genetically defined, pathogen-free laboratory animals, as well as animal-related services such as jugular vein cannulations, vasectomies, ovariectomies, and castrations. In 2007, the branch distributed 951,566 rodents to about 1,750 investigators at 250 institutions.
In 1985, the hypothesis was put forward that a human tumor cell line screen could help investigators discover cell type-specific agents with clinical activity against solid tumors. The emerging reality was that while correlation of in vitro histology to clinical activity is poor, the pattern of cellular sensitivity and resistance of the cell lines to a drug may correlate with molecular target expression.
Since April 1990, DTP has used the human tumor cell line in vitro screen as its primary assay, with follow-up in vivo evaluation in mouse models. The screen is currently composed of 60 human tumor cell lines (NCI60), representing leukemia, melanoma, and cancers of the lung, colon, brain, ovary, breast, prostate, and kidney. These cell lines were selected partly on pragmatic terms: those selected behaved best under typical assay conditions. The screen was designed so that for each compound tested, both the absolute and the relative sensitivities of individual cell lines were reproducible to the extent that a characteristic profile or fingerprint of cellular response was generated.
Although the particular inhibitory response of a single cell line might be relatively uninformative, the pattern of response of the cell lines as a group can be used to rank a compound according to the likelihood of sharing common mechanisms. The COMPARE algorithm (a computer program) qualifies this pattern and searches an inventory of screened agents to compile a list of the compounds that have the most similar patterns of cellular sensitivity and resistance.
Extramural researchers who wish to access this service should complete the online submission form at http://dtp.nci.nih.gov/compsub/index.html. Pure compounds must be of known molecular structure, and the investigator is required to enter the molecular structure on the online submission form before sending samples of the test compound. Additional information is available on DTP’s Website at http://dtp.nci.nih.gov/docs/misc/common_files/submit_compounds.html.
In September 2005, DTP’s human tumor cell line in vitro screening assay was reviewed by a panel of extramural experts. Because of reproducibility issues, DTP’s standard operating procedures were evaluated and a series of recommendations was made to improve quality control.
DTP has also instituted, as the first step in the screening process, a one-dose NCI60 screen. This screen has a higher throughput and thus more rapid turn-around time for suppliers. Compounds that pass criteria are screened against the NCI60 at five doses.
Although NCI’s human tumor cell line screen remains the benchmark, DTP’s Screening Technologies Branch is developing and operating new in vitro screening technologies, including high-throughput molecularly targeted screens, computational tools for new approaches to data mining, dynamic data visualization tools, and cell-free biophysical assays of macro-molecular interactions.
The branch’s labs and offices are located on the NCI-Frederick campus and are operated under a contract with Science Applications International Corporation (SAIC)-Frederick.
In 1995, DTP implemented a new way to test the activity of potential anticancer compounds using cells grown inside biocompatible hollow fibers. The hollow fiber assay, developed by Dr. Melinda Hollingshead, chief of DTP’s Biological Testing Branch, has the ability to provide quantitative indices of drug efficacy in heterogeneous tumors with minimal expenditures of time and materials. This system currently is being used as the initial in vivo experience for agents found to have reproducible activity in the in vitro anticancer drug screen.
The hollow fiber assay has several advantages over standard animal efficacy models. First, demonstrating that potential anticancer agents have in vivo efficacy in one or more animal models of neoplastic disease can require considerable investments in laboratory animals and quantity of test compound. Second, conducting studies in animal models requires substantial amounts of time and resources. Even when such studies can be conducted, it is possible that the experimental agent or series of agents will exhibit only minimal antitumor activity. Third, cancer treatments that appear promising in tissue culture are often less effective in solid tumors, in part because of the proliferative and microenvironment heterogeneity that develops in these tumors as they grow.
The hollow fiber assay at full capacity allows screening of 50 or more compounds per 10-day assay. In addition to requiring less than two weeks to complete, the assay requires at most only 450 mg of material, as opposed to the multigram quantities required for many xenograft studies. Compounds that retard the growth of the selected tumor cell lines are recommended for the next level of testing.
As part of DTP’s Molecular Targets Program, samples of protein, DNA, and RNA from human tumor cell lines are distributed to the intramural and extramural research communities. Cell lines also are sent to extramural researchers who measure the expression levels of various proteins or determine the status (e.g., wild type or mutant) of certain oncogenes. By using these measurements, DTP can determine whether the sensitivity of the set of human tumor cell lines is related to the expression levels of the compound that was measured.
Once the results are collected, the molecular target data are posted to the DTP Website. The program’s goal is to correlate anticancer activity with molecular target measurements to identify cell lines with desired characteristics and to perform this work at a low cost. Since the early 1990s, nearly 300 research groups have contributed to the molecular characterization of the NCI60 panel of human tumor cell lines, which the DTP uses to screen potential new agents for anticancer activity. To date, over 300,000 measurements have been completed, with 132,000 available to the public.