Drug-Drug Interactions (DDIs) in Psychiatric Practice, Part 8: Relative Receptor Binding Affinity as a Way of Understanding the Differential Pharmacology of Currently Available Antidepressants

Subtypes of Major Depressive Disorder Based on Pharmacological Responsiveness

Major depressive disorder (MDD) is a descriptive, syndromic diagnosis which will likely be discovered to be more than a single disorder when understood from a pathobiological or pathoetiological perspective. To date, attempts to divide this disorder into more homogenous phenotypes on the basis of signs and symptoms have not yielded more information on the pathobiological or pathoetiological factors that can cause a major depressive episode. This column proposes a new way of dividing MDD into 3 subtypes based on responsiveness to pharmacological treatments that are pharmacologically quite different from each other: type 1, which is responsive to treatment with biogenic amine antidepressants; type 2, which is not responsive to treatment with biogenic amine antidepressants but is responsive to antidepressants that work on the glutamine neurotransmitter system via the N-methyl-D-aspartate receptor; and type 3, which is not responsive to either of these 2 types of antidepressants. The goal of this formulation is to develop biologically meaningful subtypes that can be further studied to understand the pathobiology underlying these 3 types of MDD with the goal of developing newer treatments and earlier ways of diagnosing these conditions.

Drug-Drug Interactions (DDIs) in Psychiatric Practice, Part 9: Interactions Mediated by Drug-metabolizing Cytochrome P450 Enzymes

This column is the ninth in a series exploring drug-drug interactions (DDIs) with a special emphasis on psychiatric medications. The first 3 columns in this DDI series discussed why patients being treated with psychiatric medications are at increased risk for taking multiple medications and thus experiencing DDIs, how to recognize such DDIs, strategies for avoiding and/or minimizing adverse outcomes from such DDIs, and pharmacokinetic considerations concerning DDIs in psychiatric practice. The fourth and fifth columns in this series presented a pair of parallel tables, one of which outlined the primary, known mechanism(s) of action of all commonly used psychiatric medications and one of which summarized major types of pharmaco-dynamic DDIs based on mechanism of action. Clinicians can use these 2 tables together to predict pharmacodynamically mediated DDIs. The sixth column discussed key pharmacodynamic interactions involving ethanol, opioids, and monoamine oxidase inhibitors. The seventh and eighth columns presented the concept of relative receptor binding and included tables summarizing the relative receptor binding affinity of currently available antipsychotics and antidepressants, respectively. This ninth and final column in this series discusses pharmacokinetic DDIs with a focus on psychiatric medications and contains 3 tables. The first table is an abbreviated version of a table available online showing which drugs are substrates for which cytochrome P450 (CYP) enzymes and which drugs are inhibitors or inducers of specific CYP enzymes. The abbreviated version of the table presented in this column focuses on psychiatric medications. This table and the larger website version can allow prescribers to anticipate which drug combinations may pose the risk of a CYP enzyme-mediated DDI. The second table summarizes which antidepressants inhibit specific CYP enzymes and which antidepressants do not or are unlikely to inhibit specific CYP enzymes. The third table presents psychiatric medications whose clearance is not principally dependent on CYP enzyme-mediated oxidative metabolism as a necessary step in their clearance from the body. The latter 2 tables inform prescribers as to which drugs they may prefer to use to avoid CYP enzyme-mediated DDIs. The overall goal of this series of columns is to present a simple way of conceptualizing neuropsychiatric medications in terms of their pharmacodynamics and pharmacokinetics to allow prescribers to take these facts into consideration when they need to use ≥2 drugs in combination to optimally treat a patient.