Evaluating the proarrhythmic risk of delayed-action compounds in serum free cell culture conditions; serum-starvation accelerates/amplifies the effect of probucol on the KCNQ1 + KCNE1 channel

In vitro testing procedures for evaluating acute effects of compound on ion channels, utilizing heterologous expression systems (HES), are well-established, while slowly manifesting delayed effects remain challenging to detect. For this, immortalized HES are exposed to the compounds for a longer time, in general 24 h. As these cells proliferate every 12–20 h, we evaluated if the proliferation status, and by extension cell metabolism, influences the delayed compound response. The intervention of halting cell proliferation by excluding serum from the culturing medium was evaluated on CHO cells, stably expressing the KCNQ1 + KCNE1 channel complex that mediates the slow delayed rectifier potassium current (I_ks). No abnormal changes in KCNQ1 + KCNE1 current were observed upon serum-starvation, except for a negative shift in the voltage dependence of channel activation (GV-curve) after 72 h. The delayed effect of probucol, a compound reported to interfere with I_ks expression, was evaluated after 24 and 72 h of incubation. In serum-free conditions the inhibitory effect of probucol was increased fourfold after 24 h, compared to serum supplemented conditions. After 72 h, the current inhibition was similar between both culture conditions. Besides decreasing current expression, probucol shifted the GV-curve more positive combined with a shallower voltage response, changes that were more pronounced in serum-depleted conditions. The results indicated that serum-starvation had no substantial effect on the KCNQ1 + KCNE1 current in the tested CHO cells, but it amplified or accelerated the response to probucol, suggesting that halting cell proliferation is a method for enhancing the detection of delayed compound effects in HES.