Evaluation of in vitro chemosensitivity of antitumor drugs using the MTT assay in fresh human breast cancer

Practical criteria were developed in this paper for the purpose of evaluating chemosensitivity of fresh human breast cancer by the MTT assay. The survival rates at maximum inhibition (Imax %) and the concentrations of drugs which caused fifty percent reduction in absorbance compared to baseline values (IC50) of 175 samples of 10 anti-tumor drugs were evaluated by logistic analyses of the dose-response curves. Distributions of Imax% appeared as normal curves, while those of the IC50 significantly deviated from normal distribution (p < 0.0001). We assessed the in vitro chemosensitivity by comparing the Imax % of each drug on individual samples with the mean Imax % + SD which was obtained from the Imax% of 175 samples. If the individual Imax % > mean Imax % + SD, we thought the tumor sample was resistant to this drug. If the Imax % ≤ mean Imax % + SD, we would compare its IC50 with Q50 which was used as a cutoff point for in vitro chemosensitivity of anti-tumor drugs. The in vitro chemosensitivity could be graded as sensitive (Q1–Q25), intermediate (Q26–Q75), and resistant (Q76–Q100) by means of percentile method. If the individual IC50 ≥ Q76, the tumor sample would be defined as resistant. If the individual IC50 ≤ Q25, it would be defined as sensitive. In the range of Q26–Q75, we used Q50 as a cutoff point between relative sensitivity and relative resistance. Preliminary results showed that the in vitro chemosensitivity to different anti-tumor drugs determined by these criteria were consistent with the clinical response in 83 advanced breast cancer patients.