For many cancer patients, it is not clear at the outset which treatment will have the highest chance for success. Many cancer treatments have significant side effects, which can occur whether the drug provides any benefit to the patient or not. If it is possible to know at the outset the likelihood of treatment success for the various available drugs, the patient and doctor can choose a treatment course with the highest chance for success while minimizing the side effects and costs of ineffective treatments. Sometimes patients can be stratified on the basis of a marker from a biopsy of their cancer, but often they cannot, and it is not clear which treatments should be undertaken. Therefore, the process of finding the right drug is often trial and error.
Ideally, a patient's cells could be tested ex vivo against the possible drugs to see which is most effective. When the patient's cells are initially removed and tested, the cells may be sick or dying and results of drug screening at this point could be misleading. The authors* describe a method of expanding and stabilizing the human patient's cancer cells in a modified mouse (hypoxanthine phosphoribosyl transferase [hprt]–null immunodeficient mouse) and then removing those cells and testing the cells for drug sensitivity in an in vitro cell assay (see Figure 1).
One of the important aspects of their research was that their modified mouse allows the noncancer fibroblasts of the human patient to be replaced by biochemically defective mouse cells, which can be selectively eliminated prior to the cell assay (by means of hypoxanthine, aminopterin, and thymidine [HAT]–containing media). In culture, the noncancer fibroblasts can overtake the cancer cells, so this method of stromal cell removal allows for the malignant cancer cells to be retained. This method was successful in seven of the nine cell lines attempted (six pancreatic ductal adenocarcinoma cell lines and one ovarian cancer cell line). One of the pancreatic lines (Panc502) was isolated from the mouse and used to test the in vitro sensitivity to a drug panel containing over 3,000 drugs (Johns Hopkins drug library panel, see Figure 2).
The authors identified 10 drugs for further study that had not been shown to be active in pancreatic cancer cells previously. Nogalamycin and digitoxin were selective for Panc502 over untransformed or another transformed pancreatic duct line. Xenograft mice were then used to test the effect of the drugs, and there was a correlation between the in vitro response and the in vivo response. The authors point out that the current shortfall with this method was the time it took from initiation to completion of this study (8 months). They indicate that this timeline could be greatly reduced in the future by implanting additional mice, additional optimization of the procedure, and more automation of the drug screening process. It remains to be seen whether this timeline can be reduced to the point where it will be clinically useful for deciding treatment courses in a timely fashion. It can, though, be potentially useful even with the current timeline for generating cell lines for cancer types that currently do not have available lines.
We know that there can be patient-to-patient variability on the effectiveness of drug treatments, so having more lines available for a given cancer type should be helpful for scientists trying to better understand the different disease drivers.
*Abstract from Clinical Cancer Research 2013, Vol. 19: 1139–1146
Purpose. High-throughput chemosensitivity testing of low-passage cancer cell lines can be used to prioritize agents for personalized chemotherapy. However, generating cell lines from primary cancers is difficult, because contaminating stromal cells overgrow the malignant cells.
Experimental Design. We produced a series of hypoxanthine phosphoribosyl transferase (hprt)–null immunodeficient mice. During growth of human cancers in these mice, hprt-null murine stromal cells replace their human counterparts.
Results. Pancreatic and ovarian cancers explanted from these mice were grown in selection media to produce pure human cancer cell lines. We screened one cell line with a 3,131-drug panel and identified 77 FDA-approved drugs with activity, including two novel drugs to which the cell line was uniquely sensitive. Xenografts of this carcinoma were selectively responsive to both drugs.
Conclusion. Chemotherapy can be personalized using patient-specific cell lines derived in biochemically selectable mice.