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Biomarkers: An Invaluable Tool for Therapeutic Drug Monitoring (TDM)

Therapeutic drug monitoring (TDM) is utilized in a variety of clinical settings to assess the safety and efficacy of a particular medication or drug. The goal of TDM is to enable the individualization of therapeutic regimens to provide safe and effective therapy to patients with optimal benefit. For some drugs, the plasma level of the drug and its clinical effect are closely related; in these instances, therapeutic drug monitoring is a powerful tool for effective patient care management. Additionally, when there is difficulty defining a precise therapeutic end point, monitoring drug levels with TDM can provide valuable insight and help to determine dosage regimens. Criteria for TDM include:

  • Drugs with a narrow therapeutic index (therapeutic range);
  • Long-term therapies;
  • Therapies that have a correlation between clinical response and serum concentration;
  • Drugs with variable pharmacokinetics;
  • Drugs without a suitable biomarker associated with the desired therapeutic outcome, and
  • Therapies that are co-administered with potentially interacting drugs

A variety of samples can be used for therapeutic drug monitoring, including plasma, serum, whole blood, and dried bloodspot samples.

Drugs Commonly Monitored Using Therapeutic Drug Monitoring

A number of drugs are commonly monitored using therapeutic drug monitoring, including:

  • Antiepileptics
  • Antiarrhythmics
  • Antibiotics
  • Antidepressants
  • Antineoplastics
  • Bronchodilators
  • Cancer chemotherapy
  • Cardio active drugs
  • Immunosuppressives

Therapeutic Concentration and Therapeutic Range/Therapeutic Window

TDM is particularly helpful in managing patient care and dosage regimens for drugs that have a narrow therapeutic range or therapeutic window, which means they are only effective over a small range of dosages. In addition, not all patients have the same response at similar doses, thereby adding even more complexity. In optimal dosing regimens, the blood concentration should not fall below the minimum effective concentration (MEC) and the peak blood concentration should not exceed the maximum safe concentration (MSC). If a patient is under-dosed and their concentration is primarily at the subtherapeutic level, there is a risk of treatment failure. Alternatively, if the patient is over-dosed and their concentration is primarily above the target range, there is a risk of toxicity.

Figure 1. Therapeutic range (also known as therapeutic window), the concentration range of drug in plasma where the drug is efficacious without causing toxic effects in most people

The Importance of Pharmacokinetics and Pharmacodynamics (PK/PD) & ADME Assays in Therapeutic Drug Monitoring

While the therapeutic range/window provides valuable insight into plasma drug concentration, target concentration is directly linked to individual dosage regimens, rather than a range of doses as illustrated in therapeutic range graph. In order to select a target concentration, the concentration/effect relationship must be understood to enable the evaluation of desired and unfavorable effects. Pharmacokinetics (PK) and pharmacodynamics (PD) help clinicians to understand how the body reacts to the drug, providing insight into pharmacological effects and therapeutic effects and outcome. Specifically, there are five particular pharmacokinetic parameters that are important to therapeutic drug monitoring:

  • Bioavailability;
  • Volume of distribution and distribution phases;
  • Clearance
  • Half-life, and
  • Protein binding of drugs

In summary, PK/PD studies help to inform the target concentration to optimize the balance between desired and unfavorable effects. ADME assays help to measure adherence, absorption, distribution, metabolism, and elimination of drugs in patients.

Figure 2. Processes involved in drug handling and therapeutic drug monitoring

Finding the Right Lab Partner for Therapeutic Drug Monitoring Studies

TDM is, at its core, a multidisciplinary function and requires collaboration and effective, open communication between scientists, clinicians, nurses, and pharmacologists. Finding the right scientific partner and lab for therapeutic drug monitoring studies is vital to obtaining actionable results and data. Avomeen’s Ph.D-led teams of consultative scientists are knowledgeable in biomarker assay development, PK/PD, DMPK, and ADME assays. Our flexible teams can meet your needed turnaround time, delivering quality results on tight timelines, and our dedicated team of Project Managers facilitate consistent, open communication between clients and scientists. Partner with Avomeen today for therapeutic drug monitoring to:

  • Maximize therapeutic efficacy;
  • Identify therapeutic failure;
  • Facilitate dosage adjustments;
  • Facilitate the therapeutic effect of drugs by achieving target concentration, and/or
  • Identify drug toxicity and abuse

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Additional Resources

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Our featured expert is Kevin Gorman, Ph.D., Avomeen’s Manager of Bioanalytics. Kevin’s extensive knowledge of protein biochemistry and molecular biology enables him to provide clients with expert strategic and tactical guidance to help them reach their goals. Kevin has experience in analytical method development, validation, and optimization of test methods in cGMP settings per ICH guidelines. In addition, Kevin has experience developing in-vitro diagnostics, including a serology assay for COVID-19. He is listed as a co-inventor on three patents, and is highly skilled in several research techniques including LC-MS, HPLC, ddPCR, and ELISA.