Analysis of antiarrhythmic drug concentrations determined during electrophysiologic drug testing in patients with inducible tachycardias

Stereoselective inhibition of serotonin transporters by antimalarial compounds

The serotonin (5-HT) transporter (SERT) is an integral membrane protein that functions to reuptake 5-HT released into the synapse following neurotransmission. This role serves an important regulatory mechanism in neuronal homeostasis. Previous studies have demonstrated that several clinically important antimalarial compounds inhibit serotonin (5-hydroxytryptamine, 5-HT) reuptake. In this study, we examined the details of antimalarial inhibition of 5-HT transport in both Drosophila (dSERT) and human SERT (hSERT) using electrophysiologic, biochemical and computational approaches. We found that the cinchona alkaloids quinidine and cinchonine, which have identical stereochemistry about carbons 8 and 9, exhibited the greatest inhibition of dSERT and hSERT transporter function whereas quinine and cinchonidine, enantiomers of quinidine and cinchonine, respectively, were weaker inhibitors of dSERT and hSERT. Furthermore, SERT mutations known to decrease the binding affinity of many antidepressants affected the cinchona alkaloids in a stereo-specific manner where the similar inhibitory profiles for quinine and cinchonidine (8S,9R) were distinct from quinidine and cinchonine (8R,9S). Small molecule docking studies with hSERT homology models predict that quinine and cinchonidine bind to the central 5-HT binding site (S1) whereas quinidine and cinchonine bind to the S2 site. Taken together, the data presented here support binding of cinchona alkaloids to two different sites on SERT defined by their stereochemistry which implies separate modes of transporter inhibition. Notably, the most potent antimalarial inhibitors of SERT appear to preferentially bind to the S2 site. Our findings provide important insight related to how this class of drugs can modulate the serotonergic system as well as identify compounds that may discriminate between the S1 and S2 binding sites and serve as lead compounds for novel SERT inhibitors.

Formulation substitution and other pharmacokinetic variability: underappreciated variables affecting antiarrhythmic efficacy and safety in clinical practice

The process of treating a patient with an antiarrhythmic drug only begins when a physician chooses the drug to be employed. In the given patient, not only must the drug be chosen, but so must the dose, the formulation, and the method of follow-up. Choosing the proper dose requires an understanding of clinical trial efficacy and safety data for the agent chosen in a population of patients resembling the individual to be treated. It also requires a detailed understanding of pharmacologic principles of drug kinetics (e.g., absorption, distribution, metabolism, and excretion) that might affect the dose needed for the specific patient. The physician must be familiar with subsequent changes in clinical circumstances that might indicate a need for a change in dose or drug. Many circumstances determining drug pharmacokinetics are not under the immediate control of physicians, such as genetic patterns, organ function, and disease circumstances. One, however, is-or should always be-the selection of the drug formulation used. Although generic versions of innovator drugs exist for many agents and often are clinically acceptable, most physicians are unaware of the meager degree of testing that is necessary for the release of a generic drug, and the wide range of attained serum levels that are called bioequivalent by the US Food and Drug Administration (FDA) when one formulation is compared with another. In patients with cardiac arrhythmias, arrhythmia recurrence, proarrhythmia, and death have been reported in association with antiarrhythmic drug formulation substitution. Despite their reported bioequivalence, the generic agents involved were clearly not therapeutically equivalent. Accordingly, this article was written to educate physicians further about the above-noted important pharmacokinetic variables that can affect a patient's outcome when an antiarrhythmic drug is employed, and to provide information on the generic drug approval process and guidelines for the use of formulation substitution.

Generic antiarrhythmics are not therapeutically equivalent for the treatment of tachyarrhythmias

The consequences of antiarrhythmic drug formulation substitution were assessed by survey of 130 experts on arrhythmias. Fifty-four arrhythmia recurrences, 7 proarrhythmic events, and 3 deaths resulting from generic substitution are reported, thus raising serious concerns about both antiarrhythmic drug substitution and the adequacy of the generic drug approval process.

Practical optimisation of antiarrhythmic drug therapy using pharmacokinetic principles

The optimisation of antiarrhythmic drug therapy is dependent on the definitions and methods of short term efficacy testing and the characteristics of those drugs used for rhythm disturbances. The choice of an initial antiarrhythmic drug dosage is highly empirical, and will remain so until the measurement of free concentrations, enantiomeric fractions and genetic phenotyping becomes routine. However, the clinician can devise an efficient initial dosage for efficacy testing procedures based on pharmacokinetic principles and disposition variables in the literature. In this regard, a nomogram for commonly used agents and dosages was constructed and is offered as a guide to accomplish this goal. Verification of the accuracy and usefulness of this nomogram in a prospective manner in patients with symptomatic tachyarrhythmias is still required. On a long term basis, dosage regimens can be modified by the use of pharmacokinetic principles and patient-specific target concentrations, in accordance with the methods used to monitor arrhythmia recurrence and drug-related side effects.

Clinical toxicology of cardiovascular drugs

Drugs used to treat cardiovascular diseases have low therapeutic indices, may produce the very pathophysiologic effects that it is hoped they will reverse, and are used commonly in aged animals with multisystemic diseases for which they are receiving other compounds. These factors predispose to undesirable drug reactions and interactions. It is difficult to determine, a priori, which animals will respond with profound toxic manifestations; therefore, resuscitative measures, including drugs and devices, must be available at all times, and the clinician must be schooled in their use. In particular, class IA antiarrhythmics, digitalis glycosides, and antineoplastic compounds, all used relatively frequently, have great potential for producing toxicosis. An important role of the clinician with regard to cardiovascular toxicology lies in providing consultation to both the pharmaceutical industry and governmental regulatory agencies. Because the worst aspects of cardiovascular toxicosis lie in electrical disturbances of the heart, and because electrocardiography is the best method for studying these electrical properties, the clinician and adviser to the pharmaceutical industry and the FDA must be well schooled in electrocardiography.