Case Study Scenarios:

• hERG positive, LQT negative (due to block of compensatory ion channels)
• hERG negative, LQT positive (due to non-hERG block; non-human models)
• hERG negative, LQT positive (due to non-hERG channel block)
• hERG negative, LQT positive (due to hERG trafficking inhibition)

Scenario: hERG positive, LQT negative results due to block of compensatory ion channels
Resolved by: Cardiac Channel Panel™

Vanoxerine blocks hERG at nanomolar concentrations but does not prolong the QT interval because of compensatory blockade of sodium and calcium channels. Screening against the full cardiac ion channel panel provided a more complete assessment of the compound’s profile.

Click here for additional examples of Cardiac Channel Panel™ assay results.

See how additional TSP assays were used to augment Cardiac Channel Panel™ data (verapamil).

Scenario: hERG negative, LQT (non-human) negative, LQT (human) positive
Resolved by: SC-hCM APD Assay, Cardiac Channel Panel™

Chromanol 293B prolongs action potential duration and selectively blocks KvLQT1/minK, a slow potassium current that contributes to action potential repolarization, with little effect on hERG or other cardiac channels. KvLQT1/minK block can cause long QT in humans, but KvLQT1/minK blockers are not detected in conventional dog or rabbit Purkinje fiber action potential assays. Action potential prolongation is detected in stem cell-derived human cardiomyocytes (SC-hCM).

Representative trace of Action Potential Prolongation in a Human Heart Cell induced by IKs Channel Blocker Chromanol 293B

Scenario: hERG negative, LQT positive results induced by non-hERG channel block
Resolved by: Cardiac Channel Panel™ and SC-hCM APD Assay

Alfuzosin and pentobarbital prolong the QT interval by non-hERG mechanisms. Alfuzosin prolongs the action potential through an increase in a slowly inactivating sodium current. Pentobarbital blocks the KvLQT1/minK channel responsible for IKs. Action potential duration testing against human cardiomyocytes would have confirmed involvement of other cardiac ion channels with subsequent screening against the full cardiac ion channel panel revealing the specific channels affected by the compound.

Alfuzosin increases frequency of opening in “late” cardiac sodium channels (Lacerda et al. 2008 JPET, 324: 427-433)

Scenario: hERG negative, LQT positive results induced by hERG trafficking inhibition
Resolved by: HERG-Lite® hERG Trafficking Assay

Acute application of these drugs has no effect on hERG currents in patch clamp assays, but overnight exposure causes a 2 - 3 fold decrease in the number of hERG channels at the cell surface and is the probable mechanism for QT prolongation. hERG channel trafficking inhibition was detectable with the HERG-Lite® Assay.