ci9b01085_si_001.pdf (1.48 MB)
When Does the IC50 Accurately Assess the Blocking Potency of a Drug?
journal contribution
posted on 2020-03-10, 18:10 authored by Julio Gomis-Tena, Brandon M. Brown, Jordi Cano, Beatriz Trenor, Pei-Chi Yang, Javier Saiz, Colleen E. Clancy, Lucia RomeroPreclinical assessment
of drug-induced proarrhythmicity is typically
evaluated by the potency of the drug to block the potassium human
ether-à-go-go-related gene (hERG) channels, which is currently
quantified by the IC50. However, channel block depends
on the experimental conditions. Our aim is to improve the evaluation
of the blocking potency of drugs by designing experimental stimulation
protocols to measure the IC50 that will help to decide
whether the IC50 is representative enough. We used the
state-of-the-art mathematical models of the cardiac electrophysiological
activity to design three stimulation protocols that enhance the differences
in the probabilities to occupy a certain conformational state of the
channel and, therefore, the potential differences in the blocking
effects of a compound. We simulated an extensive set of 144 in silico IKr blockers with different kinetics and affinities
to conformational states of the channel and we also experimentally
validated our key predictions. Our results show that the IC50 protocol dependency relied on the tested compounds. Some of them
showed no differences or small differences on the IC50 value,
which suggests that the IC50 could be a good indicator
of the blocking potency in these cases. However, others provided highly
protocol dependent IC50 values, which could differ by even
2 orders of magnitude. Moreover, the protocols yielding the maximum
IC50 and minimum IC50 depended on the drug,
which complicates the definition of a “standard” protocol
to minimize the influence of the stimulation protocol on the IC50 measurement in safety pharmacology. As a conclusion, we
propose the adoption of our three-protocol IC50 assay to
estimate the potency to block hERG in vitro. If the IC50 values obtained for a compound are similar, then the IC50 could be used as an indicator of its blocking potency, otherwise
kinetics and state-dependent binding properties should be accounted.