The utility of cell-based assays for evaluating chemosensitivity depends on the quality of the quantitative processes, because the number of cells, replicates, and dilution series are critical for obtaining good measurement quality [8–10]. The 384-well microtiter plate used in the present study requires 5,000 to 10,000 cells per well, which allowed us to assess the differences and similarities in drug responses using multiple drug dose points. The drug concentration-cell growth (that is, dose–response) curve is generally not linear, and ideally should be logistic. To evaluate drug sensitivity, the curve must show a logistic curve that is considered to be a pharmacological drug response [12, 15]. We confirmed that hyperbolic dose–response curves are a sufficient indicator of drug resistance. Other curve types do not directly indicate any clinical applications, but did help with assay evaluation by multiple examiners in daily practice.
We determined the chemosensitivity rank based on response expectancy instead of binary nominal sensitivity (that is, sensitive or non-sensitive) [8–10]. Although the rank depends on how many drugs are tested in a given assay, PXL seems to be the most frequently highly-ranked drug. CIS and 5-FU (that is, S-1, an oral fluoropyrimidine), which had been the first choice for highly advanced gastric cancer chemotherapy, seemed to have lower ranks in post-treatment tumors, implying that the tumors may have acquired drug resistance during primary chemotherapy. In primary chemo-naive tumors, however, CIS showed good sensitivity, suggesting that CIS is a reasonable choice as a first line drug for primary tumors, as most regimens include this drug.
Although the number of cases is still limited, one of the major benefits of the present approach is the ability to see population changes in cancer cells at different time points for a given patient. For instance, GC-022 and GC-025 are samples from a patient who was treated with six cycles of 80 mg/m2 PXL weekly and showed a five-fold increase in GI50 values for PXL, while remaining resistant to CIS. It is reasonable to speculate that the acquisition of drug resistance would be due to changes in the tumor cell population . Recent reports demonstrated that genome-wide genetic and epigenetic events accumulate during cancer progression . Therefore, the phenotypic change we observed in the present case suggests that drug-induced phenotypic or genetic/epigenetic changes occurred during our routine cancer therapies .
The response rate of standard cancer therapies is still generally around 30% to 40% [1–3]. To determine standard therapies based on tumors of origin through epigenetic studies, large-scale clinical trials are important. However, continued efforts are necessary to minimize the fraction of patients who would not benefit from the standard therapy as well as to provide justification for each type of therapy. The biological properties of cancer cells are often drastically different depending on each patient, so treatments should be flexible based on tumor characteristics. Patients with ascites have very poor life expectancy from the date of diagnosis and many fail either standard or other subsequent therapies . Our chemotherapeutic response expectation rankings suggest that it would be possible to make better drug choices that may prolong survival and significantly decrease the cancer burden for these patients .