Cloned Cardiac Ion Channel Data Not Predictive of Human Tissue Responses
In a recent study, the cardiac safety margin was established based on hERG, as well as voltage-gated sodium and calcium channel data. The safety margin was estimated to be more than 30-fold the expected unbound therapeutic plasma concentration.
AnaBios performed a pro-arrhythmia risk assessment in a human ex vivo cardiac action potential assay and identified the potential for serious cardiac problems at a concentration as low as the expected therapeutic level. In fact, the drug candidate was predicted to have no margin of safety. Analysis indicated the problem was most likely related to the inhibition of the inward sodium current.
By focusing on lowering sodium channel inhibitory activity, a new compound series was created and tested in the human ex vivo cardiac action potential assay. These molecules were found to have no toxic effects at 50-fold above the expected therapeutic concentration.
This case study illustrates the difficulty and complexity in measuring drug action against cardiac ion channels. Due to the frequency and voltage dependence of drug-induced inhibition, it is incredibly problematic to measure accurate IC50 values—a problem which often leads to erroneous safety margin estimates.
The use of fully-integrated physiological systems, such as adult human cardiac tissues or cardiomyocytes, provides the most effective strategy to assess ion channel-related risks.
Predicting Heart Rate Effects on Ex Vivo Human SA Node
A drug in preclinical development for atrial fibrillation had no measurable activity in hERG, Nav1.5 and Cav1.2 channels. Preliminary tests in guinea pigs appeared to confirm lack of potential cardiac risks, while a dog telemetry study confirmed the absence of drug-induced pro-arrhythmia markers. However, a significant increase in heart rate found in the dog telemetry study halted the program due to concerns about blood clotting and stroke risk in patients.
Utilizing Phase X® technology, AnaBios evaluated the heart rate-related risk of the compound in a human ex vivo sino-atrial (SA) node model. This unprecedented preparation detects both positive and negative chronotropic effects of drugs by recording the pacemaker activity of the human SA node. The molecule exhibited no measurable chronotropic effects, while positive (isoproterenol) and negative (carbachol) chronotropes exhibited the expected activities.
Unfortunately, species differences in drug-induced chronotropic effects are often observed in drug development programs. Ultimately, this study suggests animal models pose significant challenges in predicting drug-induced heart rate changes in humans. The availability of the human ex vivo SA node preparation opens up new opportunities for assessing this class of toxicities at an early stage in drug development.
Cardiac Contractility Effects and Translational Cross-Species Challenges
As part of a pre-clinical development program focusing on heart failure, which specifically targeted phosphodiesterase activity, a set of five molecules were tested in an ex vivo contractility assay that utilized canine ventricular trabeculae to measure the inotropic effect of the compounds.
Two compounds had significant positive inotropic effects, the third had a mild positive inotropic effect, while the remaining two drugs exhibited negative inotropic activity. These ex vivo results were confirmed during an in vivo dog study that monitored left ventricular pressure.
The same five compounds were subsequently tested in Phase X® utilizing human ventricular trabeculae in a contractility assay. Surprisingly, only one molecule had a similar effect (small positive inotropic effect) across the two species. The two molecules that were found to be negative inotropes in dog had a positive inotropic effect in human ventricular tissue. The two that were positive inotropes in dog had a small negative inotropic effect in humans. Consequently, the prioritization of these compounds was modified by taking into account the human contractility data.
This study exemplifies the commonly-observed discordance in inotropic effects of drugs across species and validates the importance of assessing inotropic effects on human tissues and cells at the preclinical stage.