Drug-drug interactions: proof of relevance (part I)

Drug-drug Interactions in Psychiatric Practice, Part 1: Reasons, Importance, and Strategies to Avoid and Recognize Them

This column begins a series exploring drug-drug interactions (DDIs) with a special emphasis on psychiatric medications. As explained in this column, this topic is important for multiple reasons. First, a large percentage of the population is receiving psychiatric medications. Second, these patients are likely to be on multiple medications which means that they are at risk for an adverse DDI. Third, DDIs may occur but not be recognized even though they have significant health care consequences for the patient. Fourth, these consequences can range from a catastrophic outcome to more everyday clinical problems involving a myriad of presentations as enumerated in this column. Also discussed in this column is the fact that all drugs, including psychiatric medications, interact on the basis of their pharmacodynamics and pharmacokinetics rather than their therapeutic use. Therefore, psychiatric medications may interact with medications prescribed for nonpsychiatric reasons as well as with other psychiatric medications. Tables are included that explain reasons for multiple medication use and principles to follow to minimize the risk of adverse DDIs.

Towards a foundational representation of potential drug-drug interaction knowledge

Inadequate representation of evidence and knowledge about potential drug-drug interactions is a major factor underlying disagreements among sources of drug information that are used by clinicians. In this paper we describe the initial steps toward developing a foundational domain representation that allows tracing the evidence underlying potential drug-drug interaction knowledge. The new representation includes biological and biomedical entities represented in existing ontologies and terminologies to foster integration of data from relevant fields such as physiology, anatomy, and laboratory sciences.

Drug-drug interactions in general hospital and psychiatric hospital in-patients prescribed psychotropic medications

Abstract Objectives. Drug-drug interactions (DDIs) present a serious, ever increasing clinical problem. Previous studies identified DDIs among psychiatric inpatients prescribed psychotropics, but none have focused on psychotropics prescribed to General Hospital inpatients. This study aimed to identify: putative drug-drug interactions; mechanisms; potential seriousness among patients prescribed psychotropes in both psychiatric and general hospital inpatients settings. We hypothesised that potential interactions per person would be greater in General Hospital inpatients on psychotropics, due to polypharmacy. Method. We surveyed psychotropic prescribing in hospital wards in a public sector mental health organisation and a 500-bed general hospital. Ward pharmacists collected drug prescription data. A computer based protocol evaluated DDIs. Results. A total of 7.4% of General Hospital inpatients and 100% of Psychiatric Unit inpatients surveyed were prescribed psychotropic medication. The General Hospital group had significantly more potential interactions per person (3.0) than Psychiatric inpatients (1.3) (P<0.05). There were significantly more potentially serious interactions in the general hospital group (P<0.025). Conclusions. DDIs affect those prescribed psychotropics in both General and Psychiatric Hospitals. The General Hospital patients had a higher number per person and more serious potential interactions, yet are often poorly served by psychiatric services, suggesting that liaison psychiatrists have a role in physician education and DDI assessment.

Effect of desvenlafaxine on the cytochrome P450 2D6 enzyme system

BACKGROUND

The cytochrome P450 2D6 (CYP2D6) enzyme is responsible for metabolizing approximately 25% of pharmaceutical agents. Individuals with impaired CYP2D6 metabolism and those concomitantly receiving agents that inhibit CYP2D6 can have variations in concentrations of such medications and their metabolites.

METHODS

Five studies assessing the interaction between desvenlafaxine and CYP2D6 are reviewed. Study 1 compared desvenlafaxine area under the plasma concentration-versus-time curve (AUC) in CYP2D6 extensive metabolizers (EMs) and poor metabolizers (PMs) after administration of 100 mg of desvenlafaxine or 75 mg of venlafaxine extended release (ER). Studies 2 to 5 assessed the effect of concomitant administration of desvenlafaxine 100 mg (studies 2, 4, and 5) or 400 mg (study 3), paroxetine (20 mg, study 4), and duloxetine (30 mg twice daily; study 5) on the CYP2D6 probe desipramine.

RESULTS

In study 1, there was no significant difference in mean desvenlafaxine AUC between the CYP2D6 EMs and PMs (-11%; P=0.641) who were administered desvenlafaxine. However, PMs receiving venlafaxine ER had significantly higher venlafaxine and lower desvenlafaxine AUCs compared with EMs (+350% and -74%, respectively; P<0.001 for each). In studies 2, 4, and 5, the mean increases in desipramine AUC with concomitant administration of desvenlafaxine 100 mg ranged from 17% to 36%; the increase with concomitant administration of desvenlafaxine 400 mg (study 3) was 90%. Paroxetine and duloxetine produced increases in mean desipramine AUC of 419% and 122%, respectively, which were significantly greater than the increases seen with desvenlafaxine 100 mg (P<0.001 for each comparison).

CONCLUSIONS

Based on the findings presented here, desvenlafaxine is expected to have a low risk for variability in efficacy and safety/tolerability resulting from CYP2D6 polymorphisms or drug-drug interactions when coadministered with CYP2D6 substrates or inhibitors.

Potential drug–drug interactions within Veterans Affairs medical centers

PURPOSE

This study assessed the prevalence of 25 clinically important drug-drug interactions (DDIs) in the ambulatory care clinics of the Department of Veterans Affairs medical centers (VAMCs).

METHODS

This study was a retrospective, cross-sectional database analysis of pharmacy records to assess the prevalence of 25 clinically important DDIs. For each DDI, the object drug was defined as the medication that has its therapeutic effect modified by the drug interaction process. The precipitant drug was defined as the medication responsible for affecting the pharmacologic action or the pharmacokinetic properties of the object drug. Rates of interactions for each VAMC facility were calculated as the number of patients with a DDI divided by the total number of individual patients exposed to the object or precipitant medications. The 25 DDIs were categorized into four main categories on the basis of the therapeutic classification of the medications involved in the drug pairs.

RESULTS

The study population included 2,795,345 patients who filled prescriptions for medications involved in potential DDIs across 128 VAMCs. The highest DDI exposure rate was 129.2 per 1,000 recipients of monoamine oxidase inhibitors (MAOIs) that occurred with combinations of selective serotonin-reuptake inhibitors (SSRIs). The lowest DDI exposure rate was 0.01 per 1,000 warfarin recipients who had the warfarin and sulfinpyrazone combination.

CONCLUSION

The analysis of pharmacy records of veterans who filled prescriptions at the outpatient settings within VAMC found an overall rate of 2.15% for potential DDIs. Case-exposure rates were greatest for veterans receiving SSRIs and MAOIs, ganciclovir and zidovudine, anticoagulants and thyroid hormones, and warfarin and nonsteroidal antiinflammatory drugs.

Polypharmacy: when is it rational?

In this article, the authors discuss when it makes sense to consider using more than one medication to treat a single condition. They give a brief history of the use of polypharmacy in psychiatry and discuss how new discoveries in psychotropic drug development are making polypharmacy an increasingly important topic today. The authors then present a list of 10 criteria to guide the rational use of psychotropic polypharmacy and explain each in detail with examples drawn from clinical practice.

Pharmacogenomics, informatics, and individual drug therapy in psychiatry: past, present and future

The modern era of psychopharmacology began in the 10 year period from the late 1940s to the late 1950s. During this period, the first antidepressants, antipsychotics, anxiolytics and mood stabilizers were all discovered. In the 1960s, the pharmacology of these drugs was elucidated and theories about the mechanisms of action proposed. In the 1970s and 1980s, new, more selective compounds were developed based on improved structure-activity relationships derived from in vitro receptor binding studies and animal models. These compounds entered clinical testing in the 1980s and began to be marketed in the late 1980s and 1990s. All of these agents were approved to treat psychiatric syndromes which are conditions defined by a cluster of signs and symptoms. None of these agents was developed based on an understanding of the pathophysiology of the illnesses being treated. None of these agents are curative and virtually all have limited clinical efficacy. In the earliest days of the modern era, there were few drugs available to combine and many had such broad actions that they were often marginally tolerated or unsafe when used in combination (tricyclic antidepressants and monoamine oxidase inhibitors). With the advent of more medications, the frequency and extent of polypharmacy has exploded. In addition to simply having more drugs from which to select with different pharmacological profiles, many newer medications are also more selective in their pharmacological actions and thus are often better tolerated and safer when used in combination. In addition, there is the concern that the trade-off for more selective pharmacology may have been better tolerability at the expense of reduced efficacy, which clinicians then compensate for by using more medications in combination. For all of the above reasons, polypsychopharmacology has been present from the beginning of the modern era of psychopharmacotherapy and continues to be the rule rather than the exception. In fact, the frequency and the complexity of such polypsychopharmacology are both enormous and increasing. The percentage of patients being discharged from the Biological Branch of the National Institute of Mental Health on more than three psychiatric medications increased more than ten times between 1974-79, and 1990-95. The majority of patients seen in the Veterans Administration Medical System in the United States are on unique combinations of medications and the frequency and complexity of such polypharmacotherapy is increased in patients on psychiatric medications. Throughout the modern era, there have been attempts to determine whether there are populations of patients selectively responsible to specific agents (e.g. serotonin versus norepinephrine reuptake inhibitors). However, no compelling data have so far emerged. Instead, clinicians generally resort to combining drugs on the basis of symptoms such as psychosis and depression or anxiety and depression. Science has primarily informed the clinician about safety concerns rather than efficacy concerns when using such combinations. That will change in the future with a better understanding of the pathophysiology of psychiatric illnesses which in turn will lead to improved therapies and the potential for more rationally derived combination treatments.

Drug interactions and fluoxetine: a commentary from a clinician's perspective

This commentary suggests that clinicians require an additional perspective to that expressed in Wernicke's review. Clinicians must consider the relative risks, because even small differences may be a reason to adjust usage of a particular drug. As early as 1993, reviews noted a 'serious' interaction potential. Also, the product information requires supplementation because it does not provide the emphasis on predictable interactions that clinicians require. An extensive basis in experimental pharmacology provides the foundations for our knowledge of cytochrome P450 interactions which predict the effects on the levels of other drugs. This work has been confirmed in human studies. Adverse outcomes related to fluoxetine interactions are relatively frequent and sometimes fatal. Patients may suffer serious and irreversible ill-effects from the increased risk of many manifestations of toxicity, and the cost of resultant medico-legal settlements is considerable. A clinician who balances risks may use fluoxetine less frequently.