The phase of cell growth most suitable for drug testing is:
Pharmacological testing and chemosensitivity evaluation are most predictive during logarithmic exponential growth phase when cell population is actively cycling, metabolically robust, and homogeneous. During lag, cells are still recovering and fraction in S phase low, causing underestimation of antimetabolite efficacy. In log phase, DNA synthesis enzymes including topoisomerase II, thymidylate synthase, and kinases targeted by many drugs are highly expressed, mitochondrial activity high, ATP levels stable. Dose-response curves generated in this window show reproducible IC50 values. Plateau or quiescent cultures activate stress pathways, upregulate drug efflux transporters such as P-glycoprotein, increase glutathione detoxification, and exhibit reduced incorporation of nucleoside analogs leading to false resistance. For agents specific to S or M phases such as 5-fluorouracil, doxorubicin, paclitaxel, presence of cycling cells is essential. Therefore standard protocols seed cells 24 hours before dosing to allow attachment and entry into log before drug addition, measure viability after 48-72 hours using MTT or ATP luminescence, ensuring correlation with in vivo proliferative tumor tissue sensitivity.
Ref: Freshney Ch.21 Drug testing log phase; NIH Assay Guidance Manual 2021 Chemosensitivity assays log phase optimal viability.