Chapter 1: Cell Proliferation Assays
During the last dozen years, the cell proliferation assay which has been most heavily promoted and provided as a service to patients in the USA is the radioactive thymidine incorporation assay originally described by Tanigawa and Kern . In this assay, applied only to solid tumors and not to hematologic neoplasms, tumor cells suspended in soft agarose are cultured for 4 - 6 days in the continuous presence of antineoplastic drugs. At the end of the culture period, radioactive thymidine is introduced and differences in putative thymidine incorporation into DNA are compared between control and drug-treated cultures. Kern and Weisenthal analyzed the clinical correlation data and defined the concept of "extreme drug resistance," or EDR . This was defined as an assay result which was one standard deviation more resistant than the median result for comparison, database assays. Patients treated with single agents showing EDR in the assay virtually never enjoyed a partial or complete response. Kern and Weisenthal also defined "low drug resistance" (LDR) as a result less resistant than the median and "intermediate drug resistance" (IDR) as a result more resistant than the median but less resistant than EDR (in other words, between the median and one standard deviation more resistant than the median).
The principles and clinical correlation data with the thymidine "EDR" assay were reviewed in this journal 10 years ago . There have been only a few follow-up studies published since this time. One such study showed that EDR to one or more of the single agents used in a two drug combination is not apparently associated with a lower probability of response to the two drug combination in the setting of intraperitoneal chemotherapy of appendiceal and colon cancers . It is, however, possible that response to the high drug concentrations achievable with intraperitoneal chemotherapy may be more closely associated with drug penetration to the tumor than to intrinsic drug resistance of the tumor cells. It was also shown that EDR to paclitaxel does not appear to be a prognostic factor in ovarian cancer patients or in patients with primary peritoneal carcinoma treated with paclitaxel plus platinum [12,13]. However, it was recently reported that EDR to platinum in ovarian cancer may have prognostic implications (Fruehauf,J., et al Proc ASCO,v.20,Abs 2529, 2001). [Note added in proof]: It was also reported that previously-untreated breast cancer patients with tumors showing LDR (defined above) had superior times to progression and overall survivals than patients with tumors showing either IDR or EDR (Mehta,R.S., et al, Breast cancer survival and in vitro tumor response in the extreme drug resistance assay. Breast Cancer Res Treat 66:225-37, 2001).
Currently in the USA, the tritiated thymidine "EDR" assay is provided by two different national laboratories (Oncotech and Impath). Based on the publication validating the assay , it has a very high specificity (>98%) for the identification of inactive single agents, but a low sensitivity (<40%). In other words, a drug with assay-defined "EDR" is predicted to be almost certain to be inactive as a single agent (high specificity for identifying inactive drugs), but many drugs without "EDR" will also be inactive (low sensitivity for identifying inactive drugs).
A second form of cell proliferation assay currently provided as a service to patients (NuOncolology Labs, Houston, TX) is the adhesive tumor cell culture system, based on comparing monolayer growth of cells over a proprietary "cell adhesive matrix" . Positive clinical correlations were described with this system in 1987 , but confirmatory and follow-up studies have not been reported.
Return to HTAJ Home Page
Return to Title/Abstract/Chapter Table of Contents
Return to Previous Page (Introduction)
Proceed to Chapter 2: Cell Death Assays