[Expanding therapeutic reference ranges using dose-related reference ranges]

[Drug-drug interactions-How can we protect ourselves from the flood of information?]

Physicians look at drug-drug interactions (DDI), or more correctly xenobiotic interactions, in an emotional mixture of fear and interest, due to the apparently countless number of adverse drug effects (ADE) that can occur. The interactions as such are seen as errors; however, interactions cannot be avoided and are an inevitable part of the normal work of physicians. The problem is how to recognize interactions and how to handle them. A xenobiotic interaction can often even improve the effectiveness of a pharmacotherapy and minimize the risks. If all examples of what can possibly happen are not necessarily counted, the flood of information becomes relatively manageable. There are only seven different classes of interactions, four pharmacodynamic and three pharmacokinetic interactions. Currently, there are hotlines and both analogue as well as digital databanks to answer questions and address uncertainties, unfortunately of markedly different quality! Aids, such as therapeutic drug monitoring (TDM) supplement these offers. Every pharmacokinetic interaction can be recognized by determination of the concentration of the active agent. The comprehensive clinical pharmacological TDM report explain the information contained in the concentration of the active agent about the individual patient from whom the blood was drawn. All physicians can learn how to compile the clinical pharmacological TDM report by themselves or they can request it in interdisciplinary cooperation via a council. Medical expertise in handling xenobiotic interactions not only opens the door to adaptation of the pharmacotherapy to the needs of the individual patient but also saves huge budget resources for the healthcare system.

Dose-Related Reference Range as a Tool in Therapeutic Drug Monitoring

BACKGROUND

Therapeutic drug monitoring (TDM) aims to individualize drug therapy. This systematic review provides a state-of-the-art overview of the benefits of adding the dose-related reference range (DRR) as a second reference range to the set of tools used by TDM for measurement and evaluation. It discusses alternative pharmacokinetic approaches for individualization of drug therapy.

METHODS

Literature was searched in PubMed. Textbooks provided Bateman transformations for calculating expected drug concentrations at various times after drug application in "normal patients," that is, the population of phase II clinical trials. The review compiles conditions and prerequisites for these transformations to be valid.

RESULTS

Relating a measured drug concentration to the orienting therapeutic reference range provides pharmacodynamic information for improving the benefit-to-risk ratio of desired drug effects versus adverse drug effects. The discriminating DRR considers a patient's individual pharmacokinetic situation. DRR is statistically based on the pharmacokinetic parameters total clearance, time to reach maximal concentrations, and elimination half-life. Relating the measured drug concentration to a range rather than a particular value, DRR determines if individual patients do or do not belong to the population of "normal patients." Once a patient is identified to be outside the population of "normal patients," the clinical-pharmacological TDM report elaborates the cause. It consists of the measured value, the TDM 9-field-board, the elimination pathways table, and a medication recommendation taking into account clinical information. The internet-based platform KONBEST supports editing of the clinical-pharmacological TDM report. It is personally signed and send to the therapist.

CONCLUSIONS

The DRR embedded into a clinical-pharmacological TDM report allows adjusting a patient's medication to the patient's individual needs (individualization of drug therapy).

Quantification of Methylphenidate, Dexamphetamine, and Atomoxetine in Human Serum and Oral Fluid by HPLC With Fluorescence Detection

BACKGROUND

For psychostimulants, a marked individual variability in the dose-response relationship and large differences in plasma concentrations after similar doses are known. Therefore, optimizing the efficacy of these drugs is at present the most promising way to exploit their full pharmacological potential. Moreover, it seems important to examine oral fluid as less invasive biological matrix for its benefit in therapeutic drug monitoring for patients with hyperkinetic disorder.

METHODS

A high-performance liquid chromatography method for quantification of methylphenidate (MPH), dexamphetamine (DXA), and atomoxetine in serum and oral fluid has been developed and validated. The analytical procedure involves liquid-liquid extraction, derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl)benzoyl chloride as a label and chromatographic separation on a Phenomenex Gemini-NX C18 analytical column using gradient elution with water-acetonitrile. The derivatized analytes were detected at 330 nm (excitation wavelength) and 440 nm (emission wavelength). To examine the oral fluid/serum ratios, oral fluid samples were collected simultaneously to blood samples from patients with hyperkinetic disorder.

RESULTS

The method allows quantification of all analytes in serum and oral fluid within 16 minutes under the same or similar conditions. Oral fluid/serum ratios for MPH and DXA were highly variable and showed an accumulation of these drugs in oral fluid.

CONCLUSIONS

The developed method covers the determination of MPH, DXA, and atomoxetine concentrations in serum and oral fluid after the intake of therapeutic doses. Oral fluid samples are useful for the qualitative detection of MPH and DXA.

[Drug-drug interactions: interactions between xenobiotics]

Drug-drug interactions (DDI) are a major topic in programs for continuous medical education (CME). Many physicians are afraid of being trapped into charges of malpractice; however, DDI cannot be avoided in many cases. They belong to routine medical practice and it is often impossible to avoid them. Moreover, they do not just occur between drugs but between any kind of foreign substance (xenobiotica), such as food (e.g. grapefruit juice, broccoli, barbecue) as well as legal (e.g. tobacco smoke, caffeine and alcohol) and illegal drugs. Therefore, the medical challenge is not just to avoid any interaction. Instead the physician faces the question of how to proceed with drug treatment in the presence of such interactions. Based on the medical education a physician has to judge first of all whether there is a risk for interactions in the prescription being planned for an individual patient. The classification of interactions proposed in this article (PD1-PD4, PK1-PK3) might help as a sort of check list. For more detailed information the physician can then consult one of the many databases available on the internet, such as PSIAConline (http://www.psiac.de) and MediQ (http://www.mediq.ch). Pharmacokinetic interactions can be easily assessed, monitored and controlled by therapeutic drug monitoring (TDM). Besides these tools it is important to keep in mind that nobody knows everything; even physicians do not know everything. So take pride in asking someone who might help and for this purpose AGATE offers a drug information service AID (http://www.amuep-agate.de). Just good for nothing, without being based on any kind of medical approach are computer programs that judge prescriptions without taking into account a patient's individual peculiarities. In case these types of programs produce red exclamation marks or traffic lights to underline their judgment, they might even work in a contrapuntal way by just eliciting insecurity and fear.

The value of drug and metabolite concentration in blood as a biomarker of psychopharmacological therapy

Desirable and undesirable effects of a drug are related to its concentration at various sites of actions. For many psychotropic drugs, it has been shown that drug concentration in brain correlates with concentration in blood. The latter is also an available estimate of clearance and bioavailability. Its monitoring enables identification of multiple factors that have an impact on clinical outcomes, especially uncertain compliance and pharmacokinetic peculiarities. For this review we analysed for antidepressants if drug concentration in blood can be used as biomarker for psychopharmacological treatment. Systematic review of the literature revealed for new and old antidepressant drugs that drug and metabolite concentrations in blood are measures of the pharmacokinetic phenotype and related differentially to occupancy of primary target structures, therapeutic effects and unwanted anticholinergic, cardiac and other side effects. Drug concentration in blood can therefore be used as biomarker in clinical practice to guide psychopharmacological treatment with established antidepressant drugs. Monitoring of drug concentration is suitable to improve efficacy and safety of the pharmacotherapy, especially in elderly patients who require complex pharmacological therapies.

Therapeutic drug monitoring for drugs used in the treatment of substance-related disorders: literature review using a therapeutic drug monitoring appropriateness rating scale

BACKGROUND

The efficacy of drugs for the treatment of substance-related disorders is moderate at best. Therapeutic drug monitoring (TDM) could be an instrument to improve outcomes. Because TDM for most of those drugs is not established, the authors reviewed the literature and built a rating scale to detect the potential added value of TDM for these pharmacologic agents.

METHODS

A literature search was performed for acamprosate, bupropion, buprenorphine, clomethiazole, disulfiram, methadone, naltrexone, and varenicline. The rating scale included 22 items and was divided in five categories: efficacy, toxicity, pharmacokinetics, patient characteristics, and cost-effectiveness. Three reference substances with established TDM were similarly assessed for comparison: clozapine, lithium, and nortriptyline. The three reference substances achieved scores of 15, 12, and 14 points, respectively.

RESULTS

Drugs for treatment of substance-related disorders achieved 3 to 17 points, 17 for methadone, 11 for buprenorphine, 10 for disulfiram, also 10 for naltrexone for the indication opioid-dependence and 9 for the indication alcohol dependence as well as bupropion, 7 points for acamprosate, 6 points for clomethiazole, and 3 for varenicline.

CONCLUSIONS

It is concluded that systematic evaluation of drug- and patient-related variables with the new rating scale can estimate the appropriateness of TDM. Because their rating revealed similar scores as the three reference drugs, it is proposed that TDM should be established for bupropion, buprenorphine, disulfiram or a metabolite, methadone, and naltrexone. An objective rating of drug- and patient-related characteristics could help laboratories focus their method development on the most likely drugs to require TDM along with a thorough drug use evaluation.